MECHANICAL DRIVES 1 LEARNING ACTIVITY … 1 Describe the function and operation ... is also called a keyseat or sometimes a keyway. Figure 2. Parts ... • Saddle - The saddle key

LEARNINGACTIVITYPACKET

MECHANICALDRIVES 1

KEY FASTENERS

BB502-XD02AEN

BB502-XD02AEN KEY FASTENERSCopyright © 2012 Amatrol, Inc.

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LEARNING ACTIVITY PACKET 2

KEY FASTENERS

INTRODUCTIONIn the previous LAP you learned how to mount and operate the motor, but you didn’t

connect it to anything. In this LAP you will learn how a key fastener connects the motor shaft to other devices. On the job, you will encounter key fasteners in almost every rotating machine application.

In this LAP you will use a key fastener to connect the motor to a prony brake. The prony brake is a device that is used to place a load on a motor. It was chosen for two reasons: it is a simple device to connect to the motor because it does not require much alignment and it allows you to learn about motor torque and power. In later LAPs the prony brake will also be used to load the mechanical transmission system to demonstrate the effects of real world loads on the system.

ITEMS NEEDEDAmatrol Supplied 950-ME1 Mechanical Drives 1 Learning System

School Supplied Bench Vise Hacksaw Files

FIRST EDITION, LAP 2, REV. CAmatrol, AMNET, CIMSOFT, MCL, MINI-CIM, IST, ITC, VEST, and Technovate are trademarks or registered trademarks of Amatrol, Inc. All other brand and product names are trademarks or registered trademarks of their respective companies.Copyright © 2012 by AMATROL, INC.All rights Reserved. No part of this publication may be reproduced, translated, or transmitted in any form or by any means, electronic, optical, mechanical, or magnetic, including but not limited to photographing, photocopying, recording or any information storage and retrieval system, without written permission of the copyright owner.Amatrol,Inc., 2400 Centennial Blvd., Jeffersonville, IN 47130 USA, Ph 812-288-8285, FAX 812-283-1584 www.amatrol.com


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TABLE OF CONTENTS

SEGMENT 1 KEYSEAT FASTENERS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4OBJECTIVE 1 Describe the function and operation of a key fastenerOBJECTIVE 2 Describe the construction of six types of keys and give an application of eachOBJECTIVE 3 Describe how keys and keyseats are specifi ed

SKILL 1 Select a key size for a given application

SEGMENT 2 KEY ASSEMBLY . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22OBJECTIVE 4 Describe how to measure the actual size of a key and keyseatOBJECTIVE 5 Describe six types of set screws

SKILL 2 Measure the actual size of a key and keyseat given a sampleSKILL 3 Cut and fi le key stock to fi t a keyseat

OBJECTIVE 6 Describe how to assemble a hub to a shaft using a keySKILL 4 Assemble a hub to a shaft using a key fastener

SEGMENT 3 TORQUE AND POWER MEASUREMENT . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 50OBJECTIVE 7 Describe two methods of loading a mechanical drive system

SKILL 5 Use a prony brake to measure shaft torqueOBJECTIVE 8 Describe how to calculate rotary mechanical power

SKILL 6 Calculate rotary mechanical powerSKILL 7 Convert between U.S. Customary and S.I. units of motor power

SEGMENT 4 MECHANICAL EFFICIENCY . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 72OBJECTIVE 9 Describe how to calculate mechanical effi ciency and explain its importance

SKILL 8 Calculate mechanical effi ciencyOBJECTIVE 10 Describe two methods of measuring shaft torque and give an application of eachOBJECTIVE 11 Describe three methods of measuring electric motor current

SKILL 9 Measure electric motor current


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SEGMENT 1 KEYSEAT FASTENERS

OBJECTIVE 1 DESCRIBE THE FUNCTION AND OPERATIONOF A KEY FASTENER

A key fastener is used to secure a shaft to other devices such as couplings, sheaves, and gears, as shown in fi gure 1. Its job is to make sure that the drive shaft and the driven component are locked together and do not slip on each other.

Figure 1. Key Fastener Applications

COUPLING SHEAV EAR

KEY FASTENER


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A key fastener consists of up to three parts, as shown in fi gure 2: • Key• Keyseat - Shaft• Keyseat - Hub

A key is simply a piece of metal that is snugly fi tted between two grooves, which are machined in a shaft and the hub of a component to which it is to be connected. The groove in the drive shaft is called a keyseat. The groove in the hub is also called a keyseat or sometimes a keyway.

Figure 2. Parts of a Key Fastener

KEYSEAT:SHAFT

KEY

KEYSEAT:HUB


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In many cases, hubs have one or more set screws that can apply extra force to the key to lock the hub in place, as shown in fi gure 3.

Figure 3. Key Fastener and Set Screw

KEY

SET SCREW

HUB

SHAFT

SHEAVE


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OBJECTIVE 2 DESCRIBE THE CONSTRUCTION OF SIX TYPES OF KEYSAND GIVE AN APPLICATION OF EACH

There are many types of keys used in industry, depending on the application. Each key has a certain shape which is designed based on either ease of use, cost, or its ability to hold a high load. Six of these types, as shown in fi gure 4, are:

•Square - The square key is the most widely used because it is easy to make and provides a strong connection. Its cross section is square and its length can either be tapered or parallel. The parallel type is the most commonly used. The tapered type is used when there is a need to use the wedging action of the key to keep it in place.

•Rectangular - This key looks like a square key except that its width is greater than a square key. It can also be a parallel or tapered type. The rectangular key has a greater shear strength because of its larger cross-sectional area. Rectangular keys are preferred for shaft sizes of greater than 6-1/2 inches. For smaller sizes, a square key is preferred.

Figure 4. Key Fastener Shapes

SQUARE RECTANGLE GIB HEAD

OFFSET

OR

WOODRUFF SADDLE


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•Gib head - The gib head key is a tapered square key with a head on it. The head provides a way to easily remove the key if only one side of the assembly is accessible. A tapered key without a head is called a plain taper key.

•Woodruff - The woodruff key is shaped like a half moon. It is often used in light duty applications, because it gives more holding strength (more shear area) without requiring a large portion of the shaft to have a key seat machined in it. It is also used with tapered shafts because it reduces the tendency of the key to tip when a load is applied.

•Saddle - The saddle key is used when there is no keyseat of any kind. These are used in light duty applications.

•Offset - The offset or step key is type of square key that has a different width on one side of the key. This allows the key to connect a coupling hub and shaft which have different keyseat sizes. It is also used for repair and salvage of keyseats that have become larger through wear.


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OBJECTIVE 3 DESCRIBE HOW KEYS AND KEYSEATS ARE SPECIFIED

Keys are made from standard stock sizes, which are available from machine parts suppliers. The key stock is cut to the length needed and the burrs are fi led off. A key and keyseat are specifi ed, as shown in fi gure 5, by the following features:

• Nominal Width • Nominal Height • Width Tolerance • Height Tolerance • Length • Material Type

Figure 5. Key and Keyseat Features

WIDTH

WIDTH

HEIGHT

HEIGHT

HEIGHT

SHAFT

WIDTH

HUBKEY


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Nominal Width and Height of Keys and Keyseats

The nominal width is the width of both the key stock and the keyseat, without accounting for tolerance. Key stock is available in a variety of standard widths. It is usually matched to a certain shaft size with a table like the one shown in fi gure 6.

The keyseats are machined into the shaft and hub, usually by the manufac-turer of the equipment. However, many drive components can be ordered with no keyseat to allow the user to machine the keyseat themselves. This allows the user to select any type or size keyseat desired.

NOMINAL SHAFTDIAMETER NOMINAL KEY SIZE NOMINAL KEYSEAT

DEPTH

Over To (Incl.) Width, WHeight, H H/2

Square Rectangular Square Rectangular

5/167/169/167/8

1-1/41-3/81-3/42-1/42-3/43-1/43-3/44-1/25-1/2

7/169/167/8

1-1/41-3/4

22-1/42-3/43-1/43-3/44-1/25-1/26-1/2

3/321/8

3/161/4

5/163/81/25/83/47/81

1-1/41-11/2

3/321/8

3/161/4

5/163/81/25/83/47/81

1-1/41-1/2

....3/321/8

3/161/41/43/8

7/161/25/83/47/81

3/641/163/321/8

5/323/16

5/163/8

7/161/25/83/4

....3/641/163/321/81/8

3/167/321/4

5/163/8

7/161/2

6-1/27-1/2

9

7-1/2911

3-3/42

2-1/2

1-3/42

2-1/2

1-1/2*1-1/21-34

7/81

1-1/4

3/43/47/8

All dimensions are given in inches. For larger shaft sizes, see ANSI Standard.Square keys preferred for shaft diameters above heavy line; rectangular keys, below.* Some key standards show 1-1/4 inches; preferred height is 1-1/2 inches.

Figure 6. Key Size Versus Shaft Diameter


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The nominal key height is the height of the key stock, without accounting for tolerance. For a square key, the nominal height is the same as the nominal width. The height sizes available are therefore determined by the nominal width. The nominal keyseat height, however, is not the same as the key height. The nominal keyseat height is normally chosen to be 1/2 the key height because the key must extend into the keyseats of both the hub and the shaft, as shown in fi gure 7.

Figure 7. Keyseat Height vs. Key Height

Width and Height Tolerances

The width and height tolerances are the allowable variations of the width and height dimensions of the key and keyseat. The exact variations of the width and height dimensions are very important. The key and keyseats should be of sizes so that the key is neither too loose nor too tight. If the key width is too small, making a loose fi t, it will shear off. If either the key width or the key height are too large, making a tight fi t, stress cracks can occur in the shaft or hub and cause it to fail.

The tolerances of the key and keyseat can have one of two types of fi ts as defi ned by ANSI: either class 1 or class 2. Class 1 is a looser fi t than class 2. In a normal application, the fi t you used with a key should be a class 1 fi t so it will be the only fi t discussed .

A class 1 fi t is a type of clearance fi t called a sliding fi t. This means that there is a slight clearance between the key and the keyseat (typically 0.001 to 0.002 inches clearance) but the clearance should not be detectable by touch. The key should be able to be pushed into the keyseat with your thumb.

KEYHEIGHT KEYSEAT

HEIGHT


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The class 1 tolerances for square keys and keyseats for various sizes of shafts are given in the following table in fi gure 8. This table, as well as the one for a class 2 fi t, can be found in the Machinery’s Handbook. You should make sure that the key stock you choose and the actual dimensions of the keyseats meet the tolerance specifi cations shown in this table for class 1 fi t before you attempt to assemble them.

Type of Key

KEY WIDTH SIDE FIT TOP AND BOTTOM FIT

OverTo

(Incl.)

Width Tolerance

Fit Range*

Depth Tolerance

Fit Range*Key

Key-seat Key

Shaft Key-Seat

Hub Key-Seat

Class I Fit for Parallel Keys

Square

…

1/2

3/4

1

1-1/2

2-1/2

1/2

3/4

1

1-1/2

2-1/2

3-1/2

+0.000 -0.002+0.000 -0.002+0.000 -0.003+0.000 -0.003+0.000 -0.004+0.000 -0.006

+0.002 -0.000+0.003 -0.000+0.003 -0.000+0.004-0.000+0.004 -0.000+0.004 -0.000

0.004 CL0.0000.005 CL0.0000.006 CL0.0000.007 CL0.0000.008 CL0.0000.010 CL0.000

+0.000 -0.002+0.000 -0.002+0.000 -0.003+0.000 -0.003+0.000 -0.004+0.000 -0.006

+0.000 -0.015+0.000 -0.015+0.000 -0.015+0.000-0.015+0.000

-0.0015+0.000 -0.015

+0.010 -0.000+0.010 -0.000+0.010 -0.000+0.010 -0.000+0.010 -0.000+0.010 -0.000

0.032 CL0.005 CL0.032 CL0.005 CL0.033 CL0.005 CL0.033 CL0.005 CL0.034 CL0.005 CL0.036 CL0.005 CL

Rectangular

....

1/2

3/4

1

1-1/2

3

4

6

1/2

3/4

1

1-1/2

3

4

6

7

+0.000 -0.003+0.000 -0.003+0.000 -0.004+0.000 -0.004+0.000 -0.005+0.000 -0.006+0.000 -0.008+0.000 -0.013

+0.002 -0.000+0.003 -0.000+0.003 -0.000+0.004 -0.000+0.004 -0.000+0.004 -0.000+0.004 -0.000+0.004 -0.000

0.005 CL0.0000.006 CL0.0000.007 CL0.0000.008 CL0.0000.009 CL0.0000.010 CL0.0000.012 CL0.0000.017 CL0.000

+0.000 -0.003+0.000 -0.003+0.000 -0.004+0.000 -0.004+0.000 -0.005+0.000 -0.006+0.000 -0.008+0.000 -0.013

+0.000 -0.015+0.000 -0.015+0.000 -0.015+0.000 -0.015+0.000 -0.015+0.000 -0.015+0.000 -0.015+0.000 -0.015

+0.010 -0.000+0.010 -0.000+0.010 -0.000+0.010 -0.000+0.010 -0.000+0.010 -0.000+0.010 -0.000+0.010 -0.000

0.033 CL0.005 CL0.033 CL0.005 CL0.034 CL0.005 CL0.034 CL0.005 CL0.035 CL0.005 CL0.036 CL0.005 CL0.038 CL0.005 CL0.043 CL0.005 CL

All dimensions are given in inches.* Limits of variation. CL = Clearance; INT = Interference§ To (Incl.) 3-1/2 inch Square and 7-inch Rectangular key widths.

Figure 8. ANSI Standard Class 1 Fits for Parallel Keys


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Key tolerances are specifi ed as either undersize, oversize, or over/undersize. Undersize key stock fi ts the tolerance specifi cation shown for a class 1 fi t. It has a zero upper tolerance. For example, a typical undersize tolerance is +0.000 to -0.001 inches. This tolerance type makes sure that there is always some clearance. It is what you should normally use. The undersize tolerance is also called a nega-tive tolerance or minus tolerance.

An oversize key tolerance means that the lower tolerance is zero. A typical oversize tolerance is +0.002 to -0.000 inches. Oversize key stock is used when the keyseats are worn and have therefore become larger than the normal specifi cation allows. The oversize key allows the shaft to still be used. Another term used for this tolerance type is a plus tolerance.

The third tolerance specifi cation, over/undersize, means that there is both an upper and lower tolerance. A typical example is +0.0005 to -0.0005 inches. This tolerance is used when you want a tighter fi t than normal fi t. An example of an application is with a reversing motor.

Keyseats are usually machined into the shaft and hub with class 1 tolerance, so that there is a clearance fi t. Many times the manufacturer of the shaft or hub machines the keyseat, so your only job is to select the key stock tolerance.


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Key and Keyseat Length

Another feature that must be specifi ed is the length of the key. The length of a key for an application is usually determined during the initial design stage. While the nominal length of the key is important, it is not a critical dimension that requires a close tolerance. The general guideline is to make the key long enough to fi t fl ush on one side of the hub and a little shorter than the length of the keyseat on the shaft, as shown in fi gure 9. This assures that the key cannot slide around in the keyseat.

Keys can be purchased having various lengths. However, keys are usually cut to length from longer lengths of key stock. A typical stock length is 12 inches.

Figure 9. Length of Key

Key Stock Material Type

Keys are purposely chosen to be of a softer material than the shaft so that they will shear fi rst if the shaft is overloaded. Common key materials include:

• Cold rolled steel, e.g. C1018 • Zinc-Plated cold rolled steel • High carbon steel, e.g. C1095 • Brass

The most common material is cold rolled steel. This material may be zinc plated for corrosion resistance. Stainless steel, typically 316 or 18-8, is also used for the same reason. In marine applications, brass is also used.

For higher load applications, where tighter tolerances and higher strength are needed, a high carbon steel can be used. This steel is often annealed to make it easier to machine and has a tighter size tolerance.

KEYSEAT

KEY

K

S

KEYLENGTH

KEYSEATLENGTH


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SKILL 1 SELECT A KEY SIZE FOR A GIVEN APPLICATION

Procedure Overview

In this procedure, you will be given various scenarios and challenged to select the size and type of key stock that best fi ts the application. You will fi rst be given an example. Then you will do it yourself.

1. Examine the following application information, select the type and size of key for the application.

Given Information: • Application: Shaft to coupling connection for a hydraulic system• Shaft diameter: 0.500 inches• Shaft keyseat length: 1.500 inches• Hub keyseat length: 1.000 inches

Find:

FEATURE SPECIFICATION

Key Type

Nominal Width

Nominal Height

Width Tolerance

Height Tolerance

Length

Key Material

The solution is as follows:

The key type for most applications having a shaft diameter below 6-1/2 inches should be square.


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Key Type: Square

The nominal key width and height can be determined by either checking the shaft keyseat width and depth or by looking in the table of fi gure 6. Since the shaft keyseat size is not given, use the table. It shows that the width for a key for a shaft size of between 7/16 and 9/16 inches should be 1/8 inches. It also shows that the height should be 1/8 inches as well. You know this because it is a square key.

Nominal Key Width: 1/8 inches

Nominal Key Height: 1/8 inches

The tolerance on the width and height can be determined from the table in fi gure 8. A key that has a width and height of 1/8 inches has a width toler-ance of +0.000, -0.002 inches and a height tolerance of +0.000, -0.002 inches.

Key Width Tolerance: +0.000, -0.002 inches

Key Height tolerance: +0.000,-0.002 inches

The length should be determined by the length of the keyseat in the shaft and the hub. The length of the key should be at least as long as the length of the keyseat in the hub and slightly shorter than the keyseat in the shaft. Therefore, a length of 1-1/4 inches is an acceptable length.

Length: 1-1/4 (inches)

The key material is determined from the application of the system. Because this key will be used to connect a shaft and hub for a hydraulic system, high forces can be assumed. Therefore, a high carbon steel key should be used.

Key Material: High Carbon Steel


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2. Select the size and type of key given the following information. Given Information:

• Application: Shaft to coupling connection for an ore crusher used in mining.• Shaft diameter: 8.5 inches• Shaft keyseat length: 2.500 inches• Hub keyseat length: 2.250 inches

Find:


Key Type

Nominal Width

Nominal Height

Width Tolerance

Height Tolerance

Length

Key Material

The solution is as follows: In this case, the shaft size is above 6-1/2 inches, so you should use a rectangular

key. Key Type: Rectangular The nominal key width and height can be determined by either checking the

shaft keyseat width and depth or by looking in the table of fi gure 6. Since the shaft keyseat size is not given, use the table. It shows that the width for a key for a shaft size of between 7-1/2 and 9 inches should be 2 inches. It also shows that the height should be 1.5 inches.


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Nominal Key Width: 2 inches Nominal Key Height: 1.5 inches The width and height tolerance are obtained from the table in fi gure 8. The

width tolerance for a rectangular key having a width between 1.5 and 3 inches is +0.000, -0.005 inches. The tolerance for the height is +0.000, -0.005 inches.

Width Tolerance: +0.000, -0.005 inches Height Tolerance: +0.000, -0.005 inches The length of the key should be longer than the length of the keyseat in the

hub and shorter than the keyseat in the shaft. Therefore, a length of 2.375 is selected.

Key Length: 2.375 (inches) The material for the key should be high carbon steel because this is a heavy

duty application. Key Material: High Carbon Steel 3. Select the size and type of key given the following information. Given Information:

• Application: Shaft to gear connection• Shaft diameter: 3.0 inches• Shaft keyseat length: 2.125 inches• Hub keyseat length: 2.0 inches

Find:


Key Type

Nominal Width

Nominal Height

Width Tolerance

Height Tolerance

Length

Key Material

The solution is as follows: The key should be 3/4 inch square, having a width tolerance of +0.003 inch,

a height tolerance of +0.000 -0.003 inch, and a length of 2.0 inches. The key should be made from high carbon steel.


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• Application: Shaft connected to a belt drive to operate an exhaust fan.• Shaft diameter: 5/8 inch• Shaft keyseat length: 1.25 inches• Hub keyseat length: 1.0 inches

Find:


Key Type

Nominal Width

Nominal Height

Width Tolerance

Height Tolerance

Length

Key Material

The solution is as follows: The key should be 3/16 inch square, having a width tolerance of +0.000 -

0.002 inch, a height tolerance of +0.000 -0.002 inch, and a length of 1-1/8 inches. The key should be made from cold rolled steel.


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• Application: Shaft coupled to a pump used to supply cooling fl uid to a CNC Milling Machine.• Shaft diameter: 3/8 inch• Shaft keyseat length: 1 inch• Hub keyseat length: 3/4 inch

Find:


Key Type

Nominal Width

Nominal Height

Width Tolerance

Height Tolerance

Length

Key Material

The solution is as follows: The key should be 3/32 square, having a width tolerance of +0.000 -0.002

inch, a height tolerance of +0.000 -0.002 inch, and a length of 7/8 inch. The key should be made from cold rolled steel.


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SEGMENT 1 SELF REVIEW

1. A key fastener consists of up to three parts which are the key, keyseat -shaft, and ____________.

2. A key is a piece of metal that fi ts snugly between two ___________.

3. A(n) __________ key is a tapered square key with a head on it.

4. A(n) __________ key is shaped like a half moon.

5. A key is made from _________ which is cut to the length needed and the burs fi led off.


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SEGMENT 2KEY ASSEMBLY

OBJECTIVE 4 DESCRIBE HOW TO MEASURE THE ACTUAL SIZEOF A KEY AND KEYSEAT

Selecting the right size for a key is very important. It requires the keyseat to be accurately measured and the right size key stock to be selected. Three measuring tools that are used to measure the key and keyseat are:

• Dial Caliper • Micrometers • Rule

Dial Caliper

The dial caliper has the ability to measure the inside width of a keyseat and the depth of the keyseat, as shown in fi gure 10. These measurements allow you to determine the width and height of the key.

Figure 10. Measuring the Width and Height of a Keyseat

WIDTH

HEIGHT(DEPTH)


23

Micrometer

Either the micrometer or the caliper can be used to measure both the width and height of the key stock used to make the key. The micrometer is a more accurate measuring device than the dial caliper and is commonly used for this application.

A key stock which is purchased from a supplier has a specifi c tolerance. For example, square key stock (e.g. zinc-plated, cold-drawn C1018 steel key stock) is typically sold with a tolerance of +0.003, -0.000. Therefore, it is usually only necessary to verify that the nominal size of the key stock is correct. The tolerance of the key stock has already been chosen.

Figure 11. Micrometer Used to Measure Width and Height of Key

0

0

1

1

2

2

3

39

45

67

81514 13 12 1119 18 17 16

KEYWIDTH

KEYHEIGHT

HEIGHT

94 5 6 7 8

1514131211

19181716


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Rule

A rule is used to measure the length of a keyseat. This measurement cannot be easily obtained using the micrometer or the dial caliper. Because the length is not critical, a rule is the easiest and quickest method of determining the keyseats approximate length.

Figure 12. Rule Used to Measure Length of Keyseat1

2

3

4


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OBJECTIVE 5 DESCRIBE SIX TYPES OF SET SCREWS

A set screw is a threaded fastener used to hold components together. These fasteners generally do not have a head. There are various types of set screws avail-able. Six of these types are:

• Cup Point• Flat Point• Dog Point• Oval Point• Cone Point • Soft Tipped

Cup Point

Cup point set screws have a dished out area on its tip. This “cup” bites into the shaft for maximum locking strength.

Figure 13. Cup Point Set Screw


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Flat Point

Flat point set screws are used because they offer the least amount of shaft deformation. They are typically used on frequently dismantled components.

Figure 14. Flat Point Set Screw

Dog Point

Dog point set screws have a point that fi ts into a hole in the shaft. This provides not only locking strength, but also provides precise locating of the components in reference to each other.

Figure 15. Dog Point


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Oval Point

Oval point set screws do not create excessive indentations in the shaft. However, they are best used when the set screw will contact the shaft at an angle.

Figure 16. Oval Point

Cone Point

Cone point set screws are used for permanent mounting of components on shafts. The point bites into the shaft to create a high axial and torsional locking strength.

Figure 17. Cone Point Set Screw


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Soft Tipped

Soft tipped set screws have a different material on the point. This material conforms to the shape of the shaft. This provides adequate locking strength for many applications. However, it prevents damaging or scarring of soft shafts. Typical materials are nylon and brass.

Figure 18. Soft Tipped Set Screw


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SKILL 2 MEASURE THE ACTUAL SIZE OF A KEY AND KEYSEATGIVEN A SAMPLE

Procedure Overview

In this procedure, you will measure the size of a key and keyseat. You will then compare your measurements to the specifi cations for the key and keyseat to determine if they are in tolerance.

1. Obtain the following items from the tool crib.• Drum Brake Hub • Motor • Dial Caliper • 6-inch Rule

2. Perform the following substeps to measure the width, depth, and length of the keyseat on the motor shaft.

A. Use the inside jaws of the dial caliper to measure the width of the keyseat in the shaft, as shown in fi gure 19.

Shaft Keyseat Width: _________________________________ (inches)

The width of the keyseat should be approximately 3/16-inch (0.187 inches).

Figure 19. Measuring the Width of the Keyseat in the Shaft

WIDTH


30

B. Use the dial caliper to measure the depth of the keyseat, as shown in fi gure 20.

This is called the depth gauge of the dial caliper. The distance that the rod of the caliper extends into the keyseat is the same as the opening of the jaws. This is also shown in fi gure 20.

Shaft Keyseat Depth: _________________________________ (in/mm)

The depth of the keyseat should be approximately 3/32-inch (0.094 inches).

C. Use a 6-inch rule to measure the length of the keyseat in the shaft.

Figure 20. Measuring the Depth of the Keyseat in the Shaft

Shaft Keyseat Length: _________________________________ (inches)

NOTE

Some keyseats taper off at the end of the keyseat. Measure only the usable length of the keyseat.

The length of the keyseat should be approximately 1-3/4 inches.

HEIGHT(DEPTH

DEPTHGAUGE

)


31

3. Perform the following substeps to determine the width, depth, and length of the keyseat in the brake drum hub.

A. Use the inside jaws of the dial caliper to measure the width of the keyseat in the brake drum hub, as shown in fi gure 21.

Brake Drum Hub Keyseat Width: ________________________ (inches)

The width of the keyseat should be approximately 3/16-inches (0.187 inches).

Figure 21. Measurement of Keyseat Width

BRAKE DRUM HUB

KEYSEAT WIDTH


32

B. Use the dial caliper to measure the thickness of the hub wall, as shown in fi gure 22.

Hub Wall Thickness: __________________________________ (inches)

Figure 22. Measurement of Hub Wall Thickness

BRAKE DRUM HUB

HUB WALL THICKNESS


33

C. Use the dial caliper to measure the thickness from the outside of the hub wall to the bottom of the keyseat, as shown in fi gure 23.

Hub Wall to Keyseat: _________________________________ (inches)

Figure 23. Measurement from the Outside of the Hub Wall to the Bottom of the Keyseat

THICKNESSTO MEASURE

BOTTOMOF KEYSEAT


34

D. Calculate the depth of the hub’s keyseat by subtracting the Hub Wall to Keyseat measurement from the Hub Wall Thickness measurement.

Hub Keyseat Depth = _________________________________ (inches)

The depth of the keyseat should be 3/32-inch (0.094 inches).

E. Use a 6-inch rule to measure the length of the keyseat in the hub.

Shaft Keyseat Length: _________________________________ (inches)

The length of the keyseat is approximately 1½ inches. 4. Perform the following substeps to select the nominal size for a key.

A. Select the nominal width of the key.

The width should be the same width as the keyseat width in the motor shaft and brake drum hub.

Key Width: _________________________________________ (inches)

Therefore, the key width is 3/16-inch.

B. Select the nominal height of the key.

The height of the key is determined by adding the depth of the shaft keyseat to the depth of the hub keyseat.

Key Height: _________________________________________ (inches)

This is a 3/16-inch square key.

C. Determine the length of the key.

The length of the key should be at least as long as the keyseat in the hub and slightly shorter than the length of the keyseat in the shaft.

Key Length: ________________________________________ (inches)

The key length should be at least 1 inch.


35

SKILL 3 CUT AND FILE KEY STOCK TO FIT A KEYSEAT

Procedure Overview

In this procedure, you will learn how to cut key stock to the correct length and then use a fi le to prepare the key for assembly into a keyseat.

1. You will need the following items:• 3/16-inch square key stock • Hacksaw • Flat File- Single Cut, Smooth • Bench Vise with Soft Jaw Caps • 0-1 inch Micrometer • 6-inch Rule

2. Using the micrometer, measure the width and thickness of the square key stock to verify that it is 3/16-inch key stock.

Key Stock Width: _______________________________________ (in/mm)

Key Stock Height: ______________________________________ (in/mm)

3. Use the 6-inch rule to measure 1 inch of the key stock. Use a pencil to mark this measurement, as shown in fi gure 24.

Figure 24. Measuring 1 Inch of Key Stock


36

4. Perform the following substeps to cut the key stock to the required length.

A. Place the key stock in the bench vise, as shown in fi gure 25.

NOTE

Make sure that the soft jaw caps are in place to avoid marring the surface of the key stock.

Figure 25. Key Stock Placed in the Bench Vise

B. Use the hacksaw to cut off the 1 inch of required key stock.

C. Remove the keystock from the bench vise and return it to stock.

D. Retain the 1-inch piece for the next step.

KEY

BENCHVISE


37

5. Perform the following substeps to prepare the key for use.

A. Place the 1-inch key in the vise, as shown in fi gure 26.

Figure 26. Key Placed in the Bench vise

B. Use the fl at fi le to deburr the area where the cut was made.

KEY

BENCHVISE


38

C. Use the fl at fi le to lightly chamfer the edges of the key, as shown in fi gure 27.

You will need to turn the key over to chamfer both sides of the key. The key is ready for assembly at this time.

WARNING

Chamfering means to only remove the sharp corners from the key. Do not round off the edges of the key. This will reduce the holding power of the key and create a potential hazard.

Figure 27. Chamfering the Edges of the Key

6. Return all tools to the tool crib. The key you made will be used in the next skill.


39

OBJECTIVE 6 DESCRIBE HOW TO ASSEMBLE A HUB TO A SHAFTUSING A KEY FASTENER

Assembling a shaft and hub with a key fastener is a very easy task if the compo-nents are sized correctly. The steps are as follows:

•Step 1 - Check to see if the hub has a set screw hole drilled in its side, as shown in fi gure 28. A set screw is sometimes used to provide extra holding force on the key to hold it in position. If there is a set screw, make sure to back it out so that it is not extending into the shaft hole.

Figure 28. Set Screw and Hub

•Step 2 - Clean the shaft keyseat and the hub keyseat with a wire brush to make sure that no dirt or burrs are in the keyseats.

•Step 3 - Slide the key onto the keyseat of the shaft. The key should fi t into the keyseat without forcing it. If it is too tight, take it out and measure it to see which part is out of tolerance. You can either replace the key, machine the keyseat, or sand the key. Sanding the key is usually not recommended because it is hard to do it evenly. If you choose to sand it, use a belt sander, not a grinder.

Also, check the key for play when it is in the keyseat by wiggling it. There should be no play. If there is, replace the key.

•Step 4 - Remove the key from the shaft keyseat and insert it into the hub keyseat. It also should slide in without forcing it and have no play.


40

•Step 5 - Remove the key from the hub and insert into the shaft keyseat. Line it up fl ush with the end of the shaft, as shown in fi gure 29.

Figure 29. Key Positioned on Shaft

•Step 6 - Pick up the hub in your hand and line it up in front of the shaft so that the hub’s keyseat is in line with the key on the shaft, as shown in fi gure 30.

Figure 30. Aligning the Shaft and Hub

MOTOR

KEY

SHAFT

KEY

MOTOR

HUBKEYSEAT


41

•Step 7 - Then slide the hub onto the shaft until the end of the hub is fl ush with the end of the shaft, as shown in fi gure 31. The hub should slide without using tools. If it doesn’t, pull it off and check the dimensions.

Figure 31. Hub Slides onto Shaft

•Step 8 - Tighten the set screw onto the key. Sometimes you may use two set screws, as shown in fi gure 32, to keep the fi rst one from backing out when the shaft is turning.

Figure 32. 2-Set Screw Key Assembly

HUB FLUSHWITH SHAFT

MOTOR

SET SCREWACCESS HOLE

KEY

MOTORSHAFT

SECONDSET

SCREW

FIRSTSET

SCREW


42

Hub Removal

The best way to remove a hub from a shaft is to use a bearing puller, as shown in fi gure 33. This unit pushes on the end of the shaft while it pulls on the hub. This method will remove the hub without damaging the components.

Figure 33. Removing a Coupling Hub with a Bearing Puller

Another method of removing a hub is to use a key punch and soft hammer to tap the key out. The coupling can then be removed by hand. In no case, however, should the coupling itself be knocked out by a hammer. This will destroy the coupling.

KEY

COUPLING

MOTOR

PULLER


43

SKILL 4 ASSEMBLE A HUB TO A SHAFT USING A KEY FASTENER

Procedure Overview

In this procedure, you will assemble onto the motor shaft a hub called a brake drum, which is used by the prony brake to load the motor. This will teach you the basic steps of assembling a key fastener. You will apply this process in later LAPs by assembling other types of hubs, including those used in couplings.

1. Perform the following safety checkout to prepare for working with power transmission equipment. Make sure that you are able to answer yes to each item before proceeding.

YES/NO SAFETY CHECKOUT

Wearing safety glasses

Wearing tight fi tting clothes

Ties, watches, rings, and other jewelry are removed

Long hair is tied up or put in a cap or under shirt

Wearing heavy duty shoes

Wearing short sleeves or long sleeves are rolled up

Floor is not wet

2. Perform a lockout/tagout on the Motor Control Unit’s safety switch. 3. Place Shaft Panel 1 on the work station’s overhead rack. 4. Perform the following substeps to mount and level the Constant Speed Motor.

A. Locate the Constant Speed Motor and place it on the work surface.

B. Select four Constant Speed Motor Risers from Shaft Panel 1.

C. Make sure that the motor base, risers, and mounting area of the work surface shown in fi gure 34 are free of dirt, rust, and burrs.


44

D. Position the Constant Speed Motor over the set of holes on the 950-ME work surface, as shown in fi gure 34.

The outlines of the other components to be mounted are also shown.

Figure 34. Location of Components on 950-ME Work Surface

E. Place one Constant Speed Motor Riser under each of the motor feet.

F. Locate four bolts with the specifi cations 5/16-18UNC-2A x 1-1/2 Hex Head, along with compatible fl at washers, lock washers, and nuts.

G. Fasten the motor and risers to the work surface by assembling bolts, washers, and nuts.

Use a criss-cross pattern to tighten the bolts.

PRONYBRAKE

MOTOR


45

H. Check the shaft for run-out. Record below the amount of run-out.

Run-out: ____________________________________________ (in/mm)

The run-out should be less than 0.002 inches.

I. Check for motor shaft end fl oat.

End Float ___________________________________________ (in/mm)

It should be less than 0.002 inches.

J. Check the level of the motor shaft. Shim the motor feet as needed.

Feeler Gage Leaf Thickness_____________________________ (in/mm)

Effective Level Length _________________________________ (in/mm)

Mounting Bolt Distance ________________________________ (in/mm)

Shim Ratio _________________________________________________

Shim Thickness ______________________________________ (in/mm)

5. Locate the components for the prony brake, as shown in fi gure 35. This unit consists of a brake drum, load unit, and water spray bottle. The

spray bottle is used to spray water into the drum to keep it cool while the unit is loaded. The brake drum attaches to the shaft of the motor.

Figure 35. Components of Amatrol’s Prony Brake


46

6. Perform the following substeps to assemble the prony brake drum to the shaft of the motor using a key fastener.

WARNING

Your instructor must be present for the completion of this skill. This is because you will be using the key you made in an earlier skill. The instructor must be satisfi ed that the key fi ts correctly into this application before the motor can be started.

A. Locate the two cap screws that hold together the two halves of the brake drum hub, as shown in fi gure 36.

Note that these are not set screws. These screws clamp the two sections of the brake drum together against the key fastener on the shaft to hold the drum hub in place.

Figure 36. Cap Screws on Hub

B. Use a hex key wrench to back out the cap screws so the two halves of the drum hub can be separated enough for the hub assembly to slide over the motor shaft and key.

C. Clean the shaft keyseat and the hub keyseat with a wire brush to make sure that no dirt or burs are in the keyseats.

D. Obtain the 3/16-inch square key you made in the last skill.

CAP SCREWS


47

E. Slide the key into the keyseat of the motor shaft.

The key should fi t into the keyseat without forcing it. If it is too tight, take it out and measure it to see which part is out of tolerance. Select a correct sized key from the tool crib and continue.

F. Check the key for play when it is in the keyseat by wiggling it.

There should be no play. If there is play, replace the key.

G. Remove the key from the shaft keyseat and insert it into the brake drum keyseat.

It also should slide in without forcing it and have no play.

H. Remove the key from the brake drum and insert it into the motor shaft keyseat. Line it up fl ush with the end of the shaft, as shown in fi gure 37.

Figure 37. Key Positioned on Shaft

I. Pick up the brake drum in your hand and line it up in front of the shaft so that the drum’s keyseat is in line with the key on the shaft.

KEY


48

J. Then slide the drum onto the shaft until the end of the drum meets the step on the motor shaft, as shown in fi gure 38.

The drum should slide on without using tools. If it doesn’t, pull it off and check the dimensions.

NOTE

You may have to hold the key in place as you do this to prevent it from sliding out of the motor shaft keyseat.

Figure 38. Drum Slid Onto Shaft

K. Tighten the cap screws with the hex hand wrench so that the key is compressed between the drum hub and the motor shaft.

7. Pull on the drum to see if the drum is securely fastened to the shaft. You should fi nd that it is. 8. Leave the motor and hub set up and proceed directly to the Self Review. In

the next segment, you will use this setup.


49


1. Checking to see if the hub has a set screw hole drilled in its side is the _________ step to assembling a shaft and a hub using a key fastener.

2. Cleaning the shaft keyseat and the hub keyseat with a wire brush is the ______ step to assembling a shaft and a hub using a key fastener.

3. The length of a keyseat can be measured using a(n) _________.

4. A key stock which is purchased from a supplier has a(n) __________________.

5. The depth of the keyseat on the shaft can be measured using a(n) ________.


50

SEGMENT 3TORQUE AND POWER MEASUREMENT

OBJECTIVE 7 DESCRIBE TWO METHODS OF LOADING A MECHANICAL DRIVE SYSTEM

In some cases, certain mechanical devices are loaded by an external device in order to measure the performance characteristics at various loads.

There are two common methods used to load a mechanical drive system:

• Prony Brake • Dynamometer

In the next skill, you will use the prony brake to load the motor. This device will be used throughout this module to demonstrate the effects of loads on various types of mechanical drive systems.

Prony Brake

The prony brake is one device that is used to load a motor. This device also has the ability to tell you how much load is applied to the motor.

The prony brake drum, as fi gure 39 shows, is coupled to the motor shaft and rests inside a canvas friction belt. As the canvas belt is tightened against the brake drum using the wingnut, the load on the motor is increased. This applies a force to the pivot arm.

The force is measured by a spring scale that is placed at a specifi c distance from the center of the motor shaft. This is the radius distance. You can then use the force reading from the scale and the radius distance can then be used to calculate the torque.


51

Figure 39. Prony Brake

Dynamometer

A dynamometer is another type of device that places a load on a motor and measures the amount of power that the motor can produce. Race car builders use dynamometers to tune their engines.

Figure 40. Typical Dynamometer

RADIUS

SCALE

BRAKE DRUM

WINGNUT PIVOT POINT

6" (15.24cm)

CANVASFRICTION

BELT

PIVOTARM

FORCE

COUNTERWEIGHT

SCALE

COUPLINGDEVICE

GENERATOR


52

SKILL 5 USE A PRONY BRAKE TO MEASURE SHAFT TORQUE

Procedure Overview

In this procedure, you will attach the prony brake to the electric motor and load it. This prony brake will used in other skills to demonstrate that the torque output changes as the power is transmitted through various types of mechanical drives. Also, it will allow you to show the power effi ciency of mechanical drives. In this skill, you will measure the speed of the motor as its load is increased. This is simply an exercise to reinforce your ability to measure motor speed. However, you will also learn how AC motors react to load changes.

1. Perform the following safety checkout to prepare for working with power transmission equipment. Make sure that you are able to answer yes to each item before proceeding.








Floor is not wet

2. Perform a lockout/tagout on the Motor Control Unit’s safety switch.


53

3. If you are continuing from the previous Skill, proceed directly to step 4. If not, repeat all steps in Skill 4 to set up the motor, level it, and attach it to the brake drum using the square key. When complete, your setup should appear as shown in fi gure 41.

Figure 41. Setup from Skill 4


54

4. Perform the following substeps to mount the prony brake to the work surface and to connect it to the motor.

A. Loosen the load nut on top of the prony brake by turning it counterclock-wise (looking down on it from above). Continue loosening until the screw threads are fl ush with the hand knob.

Figure 42. Load Nut on Prony Brake

CCW DECREASESSETTING

LOAD NUT

PRONYBRAKE


55

B. Position the prony brake on the work surface so that its mounting holes line up the holes on the work surface shown in fi gure 43.

Figure 43. Location of Prony Brake

C. Make sure that the friction band rests under the drum, as shown in fi gure 44.

Figure 44. Friction Band Positioned Under Brake Hub

PRONYBRAKE

MOTOR


56

D. Obtain four bolts with the specifi cation 5/16-18UNC-2A x 1-1/4 hex head, along with compatible fl at washers, lock washers, and nuts.

E. Mount the prony brake to the work surface by assembling the bolts, nut, and washers, as shown in fi gure 45.

Make sure the prony brake belt wraps evenly around the brake drum.

Figure 45. Prony Brake Mounted to Work Surface

F. Loosen the fasteners on the motor mount and adjust the position of the motor until the prony brake belt is even with the brake drum hub.

G. Retighten the motor mounts.


57

5. Fill the brake drum with water using the water bottle so that the water level is about 1/4” (0.635 cm ), as shown in fi gure 46.

Figure 46. Brake Drum Water Level and Tape Location

6. Attach a 3-inch strip of black tape and a small piece of refl ective tape on the brake drum, as is also shown in fi gure 46.

1/4"

WATER LEVEL

BLACKTAPE

REFLECTIVETAPE


58

7. Perform the following substeps to install the mechanical system’s guard.

WARNING

Do not operate the mechanical drive system without the guard in place. Also, do not attempt to open or bypass the guard at any time during operation. Performing any of these actions will create a hazardous situation.

A. Locate both halves of the guard, shown in fi gure 47.

Figure 47. Mechanical System’s Guard


59

B. Place both hatch links in the back position. An example is shown in fi gure 48.

This prevents the latch links from interfering with the placement of the guard halves.

Figure 48. Latch Link in the Back Position

C. Place one half of the guard on the work surface, as shown in fi gure 49. Slide the guard in until the tabs are against the side of the work surface.

Figure 49. First Half of Guard in Position

LATCHLINK

WORKSURFACE

TAB


60

D. Place the other half of the guard on the work surface from the opposite side. Make sure that the interlock tabs, located on the sides of the guard halves, are positioned to the inside of the guard, as shown in fi gure 50. Also, ensure that the motor’s power cord runs through the cut-out in the guard.

Figure 50. Interlock Tab and Motor Cord

INTERLOCK TABTO INSIDE

MOTORCORD


61

E. Lift the latch handles and rotate the latch link over the top of the strike plate as shown in fi gure 51. Then, press the latch handles down.

This will draw the guard halves together and lock them in position.

Figure 51. Latch Operation

LATCHHANDLE STRIKE

PLATE


62

F. Verify that there is a hole in the guard directly over the prony brake.

This hole allows you to adjust the prony brake’s setting while protecting you from the moving components of the drive system.

Figure 52. Prony Brake Adjustment Hole in Guard

8. Perform the following substeps to start the motor.

A. Clear all tools and loose components from the work surface and put them in their proper storage.

B. Make sure that the Motor Control Unit’s power cord is plugged into a wall outlet.

C. Make sure the Motor Power switch is in the OFF or down position.

D. Connect the Constant Speed Motor’s power cord to the Motor Port.

E. Remove the lockout/tagout.

F. Turn on the safety switch.

The Main Power Indicator on the Motor Control Unit should turn on.

G. Make sure that no one is near the motor.

H. Turn on the Constant Speed Motor by moving the Motor Power switch to the ON up position.

The motor should accelerate to full speed quickly and run at a constant speed.

PRONY BRAKEADJUSTMENT HOLE


63

9. Perform the following substeps to measure the motor speed at various load settings.

A. Observe the reading on the prony brake scale.

It should be zero. If not, turn the prony brake’s load nut counterclockwise one or more times until it is.

B. Measure the speed of the motor using the tachometer. Record your reading in column 2, row 1. This is the unloaded speed of the motor. It should be approximately 1790 RPM.

SCALE READING(Ounces)

SPEED(RPM)

TORQUE(In-ounces)

0

4

8

12

16

20

24

C. While running, increase the load on the prony brake to 4 ounces by turning the load nut clockwise until the scale reads 4 ounces.

D. Measure the speed of the motor at this load setting. Record it in column 2, row 2 of the table.

E. Repeat substeps C and D for each of the other load settings in the table.

As you increase the load on the motor, you should notice that the motor slows down some but still maintains a speed that is close to its unloaded speed. This is a characteristic of AC motors.

F. Reduce the load on the motor to zero by turning the load nut counterclock-wise until the scale reads zero.

10. Turn off the motor. The motor should coast to a stop. 11. Perform a lockout/tagout. 12. Calculate the torque delivered by the motor for each load setting in the table

of step 8. To do this, multiply the scale reading by the radius of the prony brake, which is 6.0 inches (15.24 cm). Record your calculations in column 3 of the table.

13. Leave the setup in place. In the last skill of this LAP, you will use it.


64

OBJECTIVE 8 DESCRIBE HOW TO CALCULATE ROTARYMECHANICAL POWER

Rotary mechanical power is defi ned as the rate or speed at which the rotating power transmission system turns the load. Since work is defi ned as Force x Distance, work in a rotating system is actually torque.

This means that the power output at a motor’s shaft is found by multiplying the torque by the speed (rate) as follows:

Rotary Power = Shaft Torque × Shaft Speed

Figure 53. Motor Torque and Speed

TO LOAD

SPEED(RPM)

TORQUE(ft-lb OR N-m)

MOTOR


65

The units of rotary power are expressed in horsepower (Hp) in the U.S. customary system and Kilowatts (kW) in the Systems International (S.I.) system. They are calculated as shown in the following formulas:

MOTOR POWER FORMULAS

U.S. Customary Units:

where Pout = Output power (Hp) T = Torque (ft-lb) S = speed (RPM)

S.I. Units:

where Pout = Output power (kW) T = Torque (N-m) S = speed (RPM)

out

T SP

5252×=

out

T SP

9549×=


66

SKILL 6 CALCULATE ROTARY MECHANICAL POWER

Procedure Overview

In this procedure, you will calculate the power output of various power transmissions based on given information.

1. Calculate the power delivered by a belt drive system shown in fi gure 54.

Pout

= ____________________________________________________ (Hp)

Figure 54. Belt Drive System

The solution is as follows:

TORQUE = 50 ft - lbsSPEED = 1200 RPM

DRIVEN

TORQUE = 25 ft - lbsSPEED = 2400 RPM

DRIVER

out

out

out

out

T SP

5252

50 1200P

5252

60000P

5252

P 11.42 Hp

×=

×=

=

=


67

2. Calculate the power in kW delivered by the gear drive system shown in fi gure 55. The formula for converting Hp to kW is:

PkW = PHp × 0.746

Pout

= ___________________________________________________ (kW)

Figure 55. Gear Drive System

It should be approximately 1.72 kW. 3. Calculate the power delivered by the motor given the information shown in

fi gure 56.

Pout

= ____________________________________________________ (Hp)

It should be approximately 9.52 Hp.

Figure 56. Electric Motor

INPUTGEAR

TORQUE OUT = 22 ft - lbsSPEED OUT = 550 RPM

SPEED IN = 400 RPM

SPEED OUT = 150 RPM

R1 R2

F = 1000 lbs.

OUTPUTGEAR

INPUTGEAR

WHEELR = 1.5 inR = 4 in

12


68

4. Calculate the power in kW at a gear shaft given the following information.

Force = 3.75 N Radius Distance = 1.2 m Speed = 1200 RPM

Pout

= ___________________________________________________ (kW)

The solution is 0.57 kW.

5. Calculate the maximum torque that can be generated by a motor given the following information. Show all calculations on the data sheet. Round off to the nearest 0.1 N-m.

Speed = 1140 RPMPout = 0.25 kW

Maximum Torque = _______________________________________(N-m)

The solution is 2.1 N-m.

6. Calculate the maximum speed of a motor based on the following information. Show all calculations on the data sheet. Round off to the nearest RPM.

Maximum Torque = 425 ft-lb. Pout = 100 Hp

Maximum Speed = _______________________________________ (RPM)

The solution is 1236 RPM.

7. Solve the following design problem: Select the best small DC motor from the table based on the following

information. Choose the rating closest to your calculated value. However, be careful not to undersize your selection.

Torque required = 1.25 N-mSpeed required = 1200 RPM

STANDARD POWER RATINGS FOR SMALL DC MOTORS

Hp 1/20 1/12 1/8 1/6 1/4 1/3 1/2 3/4 1

kW 0.037 0.083 0.093 0.124 0.187 0.250 0.373 0.560 0.746

Calculated Power = ________________________________________ (kW)

Selected Motor’s Power = ___________________________________ (kW)

The solution is 0.157 kW, 0.187 kW.


69

SKILL 7 CONVERT BETWEEN U.S. CUSTOMARY AND S.I. UNITSOF MOTOR POWER

Procedure Overview

In this procedure, you will convert motor power values from US customary to SI units, and from SI to US customary units. This is a common task you will do when you work with products made in the United States and other countries.

1. Convert 1.78 KW to PHp. The conversion formula is as follows:

PkW = PHp × 0.746

PHp

= ____________________________________________________ (Hp)

It should be approximately 2.39 Hp. 2. Convert the PkW values in the table below to P

Hp.

MOTOR POWER RATINGS

(kW)

U.S. CUSTOMARY MOTORPOWER RATINGS

(Hp)

15.7

10.5

9.2

1.7

0.65


70

3. Solve the following design problem: Calculate the S.I. output power value (P

kW) of the motor in fi gure 57. Show

all calculations on the data sheet.

PkW

= ___________________________________________________ (kW)

The solution is 1.94 kW.

Figure 57. Design Problem

TO LOADMOTORSPEED =1750 RPM

TORQUE =7.8 ft-lb

MOTOR


71


1. On a prony brake, torque is computed by multiplying the load times ________ __________

2. A prony brake is a device used to apply a ______ to a motor.

3. Work is defi ned as force times _________________.

4. A dynamometer is used to _______________ a mechanical drive system.

5. _________ mechanical power is the rate or speed at which the rotating power transmission system turns a load.


72

SEGMENT 4 MECHANICAL EFFICIENCY

OBJECTIVE 9 DESCRIBE HOW TO CALCULATE MECHANICAL EFFICIENCY AND EXPLAIN ITS IMPORTANCE

One of the problems with power transmission equipment of any kind is that the power output is always less than the power input. This is because there are frictional effects in the transmission that cause some of the power to be lost to heat. The ratio of the Power Out to the Power In is called the Power Effi ciency. If it is describing the power lost through a mechanical drive system, it is called the Mechanical Power Effi ciency. It can be stated in a formula as follows:

The mechanical power effi ciency is important to any machine. The goal of any designer is to make it as high as possible, so that the machine uses as little energy as possible to perform its task. Maintenance technicians also affect the effi ciency of the machine by how they align and lubricate it. The mechanical effi ciency will also go down as the machine wears. This means that monitoring the effi ciency can tell you when a machine needs servicing.

The mechanical effi ciency of a power transmission can be determined by measuring the shaft speed and torque at the input and the output. In some cases, the power loss may be caused by a loss of speed due to slip in the drive components. In others, the power loss may be caused by lost torque from friction. Lost torque is the most common source of power loss.

FORMULA: MECHANICAL POWER EFFICIENCY

where Eff = Mechanical Effi ciency in % Pout = Output Power (Hp or Kw) Pin = Input Power (Hp or Kw)

out

in

PEff 100

P= ×


73

In actual application, measuring the mechanical power at either the input shaft or the output shaft is hard to do because it is not easy to measure the torque. Measuring the torque can be done by either using a torque transducer, an electronic device that attaches to the shaft, or a dynamometer.

In most cases, you can more easily monitor the effi ciency of the system by measuring the electric power drawn by the motor. If it increases over time, you know that the mechanical drive is losing effi ciency.

SKILL 8 CALCULATE MECHANICAL EFFICIENCY

Procedure Overview

In this procedure, you will calculate the mechanical effi ciency of a number of power transmissions.

1. Calculate the effi ciency of the gear train shown in fi gure 58 given the following information.

Horsepower of Motor = 20 hp Horsepower of Gearbox Output shaft = 17 hp

Effi ciency = ______________________________________________ (%)

Figure 58. Effi ciency of a Gearbox Connected to a Motor Shaft

The solution is 85%.


74

2. Calculate the effi ciency of the belt drive system shown in fi gure 59 given the following information.

Motor Power = 4 hp

Output Shaft Speed = 3500 rpm

Output Shaft Torque = 54 in-lb

Effi ciency = _______________________________________________(%)

Figure 59. Effi ciency of a Pulley System Connected to a Motor Shaft

The solution is 75%. 3. Calculate the effi ciency of the chain drive shown in fi gure 60 given the

following information.

Motor Shaft Speed = 1750 rpm Motor Shaft Torque = 12 in-lbOutput Shaft Speed = 875 rpm Output Shaft Torque = 18 in-lb

Effi ciency = ______________________________________________ (%)

Figure 60. Effi ciency of a Chain Drive Connected to a Motor Shaft

The solution is 75%.


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OBJECTIVE 10 DESCRIBE TWO METHODS OF MEASURING SHAFT TORQUE AND GIVE AN APPLICATION OF EACH

Measuring the load on the mechanical drive system is useful because it allows you to determine how the system is operating. A problem in the drive system will often cause a change in the load. For example, excessive tension in a v-belt will cause a higher load.

These are two methods you can use to measure the load on a shaft:• Current Measurement• Torque Transducer

Current Measurement

Torque is related to the electrical current supplied to the motor. As motor torque increases, so does electrical current. Most motor manufacturers have already tested this relationship and include a graph with the specifi cations of the motor, like the one in fi gure 61, that shows the torque vs. current characteristics. By using this graph and measuring input current, it is possible to determine the torque.

For example, from the torque vs. current graph in fi gure 61 you can see that if the measured current is 2 amps, the torque delivered by the motor is approximately 150 in-oz.

Figure 61. Torque vs. Current Curve for a Motor

0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0

20

40

60

80

100

120

140

160

180

200

220

240

CURRENT (AMPS)

TOR

QU

E (

in-o

z)


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Current measurement is often used to measure motor torque in the fi eld because the motor is already connected to a load.

In a later skill, you will observe the current as load on the motor is increased. This relationship will be used in later LAPs to prove various design formulas and to show the efforts of poor adjustment of the system.

Torque Transducer

A torque transducer is a device which is directly coupled to the shaft. As the shaft load increases the transducer generates an electrical signal which can be received by an ammeter or a controller.

The form of the signal is usually either a ±10 VAC or 4-20 ma signal. This signal is proportional to the torque, as shown in fi gure 62.

Figure 62. Electrical Signal vs. Torque

0

2

4

6VOLTS

8

10

12

0 10 20 30 40 50 60TORQUE (in - oz)


77

OBJECTIVE 11 DESCRIBE THREE METHODS OF MEASURINGELECTRIC MOTOR CURRENT

The effi ciency of a mechanical drive system can be monitored by measuring the electric motor’s input current. As the effi ciency goes down, the motor’s current will increase. This shows that the load of the drive has increased.

There are three methods by which motor current can be measured:• Clamp-on ammeter• Hand-held ammeter• Built-in ammeter

A clamp-on ammeter, shown in fi gure 63, can be opened and placed around a wire in which you want to measure the current. This is very convenient because it allows you to measure current without disconnecting the circuit and connecting the meter in series. This is particularly important for AC power applications where the current level is often quite high and very dangerous.

Figure 63. Clamp-On Ammeter Used to Measure Current


78

SKILL 9 MEASURE ELECTRIC MOTOR CURRENT

Procedure Overview

In this procedure, you will use a built-in ammeter to measure the current to the motor as load is applied using the prony brake.

1. Verify that the motor and prony brake are set up as you did in Skill 5. If the setup has been disassembled, return to Skill 5 and reconnect it. 2. Perform the following safety checkout to prepare for working with power

transmission equipment. Make sure that you are able to answer yes to each item before proceeding.








Floor is not wet

3. Verify that the guard is installed.

WARNING

Do not operate the mechanical drive system without the guard in place. Also, do not attempt to open or bypass the guard at any time during operation. Performing any of these actions will create a hazardous situation.

4. Remove the lockout/tagout. 5. Perform the following substeps to start the motor.

A. Observe the reading on the prony brake.

It should be zero. If not, turn the prony brake’s load nut counterclockwise one or more times until it is.

B. Add water to the brake drum if necessary.

The water level should be about 1/4 inch (0.635 cm) below the lip of the drum opening. Do not let the drum run dry.

C. Turn on the motor and allow it to accelerate to full speed.


79

6. With the motor running at zero load, turn on and hold the Meter Read Switch on the Motor control Unit, as shown in fi gure 64. Record the current reading on the Constant Speed Motor Load Meter in row 1 of the table below. Also record the motor speed.

SCALE READING(Ounces)

CURRENT(Amps)

SPEED(RPM)

0

4

8

12

16

20

24

28

32

36

40

44

Figure 64. Meter Read Switch


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7. Set the load on the prony brake to 4 ounces.

CAUTION

Remember to check the water in the brake drum frequently and add water as needed.

The ammeter should immediately show the current in amps being drawn by the motor.

Record the current reading and speed in row 2 of the table of Step 6. 8. Measure and record the current drawn by the motor and speed for each of the

other loads specifi ed in the table. 9. Does the current increase as the load on the motor is increased?

_____________________________________________________ (Yes/No)

You should observe that it does. This proves that current input can be used as a monitor of motor load. In later LAPs, you will use this ability to monitor the effects of your adjustments.

10. Release all load on the motor by turning the prony brake’s load nut counterclockwise.

11. Turn off the motor power switch.

WARNING

The heavy loads and high speeds applied to the brake drum cause it to get very hot. Avoid touching any part of the motor and brake drum assembly until they have had ample time to cool off.

12. Turn off the safety switch. 13. Perform a lockout/tagout. 14. Disassemble the components and store them. 15. Remove the lockout/tagout.


81


1. The output of power transmission equipment is always _______ than the power input.

2. The formula for mechanical power effi ciency is ________________.

3. The mechanical effi ciency of a power transmission can be determined by measuring the shaft __________ and ________ at the input and output.

4. In most cases you can more easily monitor the effi ciency of a power transmission by measuring the __________ drawn by the motor.

5. Current measurements are compared to a(n) ____________ supplied by the motor manufacturer to determine the torque of a motor.

6. The torque delivered by a motor can be measured by using a(n) _________________ or a dynamometer.

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