Machine Design Key 2014(1)

8/11/2019 Machine Design Key 2014(1)

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Machine Design 9

Machine Design 10


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Machine Design 11

Key-A key is defined as a machine element that is used to connect thetransmission shaft to rotating machine elements like pulley, gear,

sprocket, flywheel etc. .

Keys are used as temporary fastening of shaft and hub

Functions of Keys

The primary function of the key is to transmit the torque from the shaft to the hub of

connecting element or vice-versa

The another function of the key is to prevent relative rotational motion & axial

movement (except in case of feather key or splines) between the shaft & the joined m/c

elements like gear, pulley etc.

Keyed joint

Shaft

Hub

Key

Consisting of

Keyway is a slot or recess on a shaft and or hub to accommodate a key

Machine Design 12

Drawback

The keyway results in stress concentration in the shaft & the part becomes weak

Assembly procedure

For mounting a part at any intermediate location on the shaft,, first the key is firmly

placed in the keyway of the shaft & then the hub to be mounted is slide from one end of

the shaft till it is fully engaged with the key.

After mounting positioning the hub on the shaft, such that both the keyways areproperly aligned, the key is then driven from the end, resulting in a firm joint

Manufacturing process for keyways

Keyway is usually cut by vertical or horizontal milling cutter in case of shaft

Keyway is usually cut by slotting machine in case of hub

Materials Plain Carbon Steels like 45C8, 50C4 etc.

Key


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Machine Design 13

Types of Keyways

Machine Design 14

Splines

Sunk Keys

Saddle keys

Square sunk key

Rectangular sunk key or

Flat key

Gib- head key

Parallel Key

Taper key

Hollow Saddle Key

Flat Saddle Key

Special Keys

Woodruff key

Feather or kennedy key

Round key

Factors are considered for selecting of the type of key for a given application

Types of Keys

Power to be transmitted Tightness of fit Stability of connection Cost

Key


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Machine Design 15

Sunk Key

Square Rectangular

- Half the thickness of the key fits into the keyway on the shaft & the

remaining half in the keyway on the hub

- Power is transmitted due to shear resistance of the key. The relative

motion between the shaft & the hub is also prevented by the shear

resistance of key

Machine Design 16

Rectangular Sunk Key

-Sunk key with rectangular cross-section, is also called Flat Key

d=diameter of the shaft = diameter of the hole in the hub

b= width of key

h=height or thickness of key

l=length of key4

d=b

6

d=b

3

2=h

Usual proportions of dimensions of key

d5.1l

Square Sunk Key

-Width & thickness are equal

4

d=h=b

Usual proportions of dimensions of key

d5.1l

N.B: Flat key has more stability as compared with square key

Check: Check the dimensions considering mode of failure due to shear & crushing


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Machine Design 17

Dimensions of Square & Rectangular Sunk Keys (in mm) [IS : 2293]

4.3616 105850

5.5

5

5

4

3.5

3

2.5

1.8

1.2

In shaft

3.814 95044

3.312 84438

3.310 83830

3.38 73022

2.86 62217

2.35 51712

1.84 41210

1.43 3108

12 286

In hubUpto & includingAbove

Keyway depthKey size

Width Height

Shaft diameter

Machine Design 18

Parallel Sunk Key

- is a sunk key (with rectangular or square cross-section) which is uniform in width as

well as height throughout the length of key

IS: 2048

Taper Key

- is a sunk key which is uniform in width but tapered in height

-Bottom surface of the key is straight & the top surface is givena taper

- Standard taper is 1 in 100

IS: 2292

Designation of Parallel Sunk Keys

Width Height Length

Example: A parallel key of width 10mm, height 8 mm & a length 50 mm shall

be designated as : Parallel key 108 50 [IS: 2048]


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Machine Design 19

When the key is inserted in the keyways of shaft and the hub & pressed, it becomes

tight due to wedge action. This ensures tightness of the joint in operating condition

Due to taper, it is easy to remove the key & dismantle the joint

Taper is provide for following two reasons

As comp ared wi th parallel key , taper key has fo llowin g advantages

The taper surface results in wedge action & increases frictional force & the tightness of

the joint

The taper surface facilitates easy removal of the key, particularly with gib head

Gib- head KeyIS: 2293

- It is a rectangular sunk key with a head at one end & taper at top surface to

facilitate removal

At large end, b=d/4; h=(2/3).b=d/6

Machine Design 20

Gib Head Key


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Machine Design 21

Saddle Key

Hollow Saddle

KeyFlat Saddle Key

- Is a key that fits in the keyways of the hub only

-There is no such keyway on the shaft

Hollow Saddle

Key

-Fits in a keyway in the hub &

the bottom of the key is

concave shaped to match the

circular/curve surface of the

shaft

Flat Saddle Key

-Fits in a keyway in the hub &

the bottom of the key sits on

the flat surface machined on

the shaft

Machine Design 22

Friction between shaft, key & hub prevents relative motion between the shaft & the

hub. Therefore power is transmitted by means of friction

Saddle keys are suitable for light duty & low power transmission as compared with

sunk keys

The resistance to slip in case of flat saddle key is slightly more than that of hollow

saddle key. Therefore flat saddle key is slightly superior to hollow saddle key as far as

power transmitting capacity is concerned

Saddle Key

-Requires keyway only on the hub

Cost is less

- It is necessary to cut keyways both on the

shaft & the hub.

Cost is more

-Is liable to slip around the shaft whensubjected to heavy torque

-It can not be used in medium & heavy duty

applications

-There is no possibility of the key to sliparound the shaft.

-It can be used in medium & heavy duty

applications

Saddle KeySunk Key


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Machine Design 23

Feather Key - Is a parallel sunk key that is fixed either to the shaft or to the hub

& that permits relative axial movement between them

-There is a clearance fit

between the key & the keyway

in the hub.

-The hub is free to slide over

the key, at the same time,

there is no relative rotational

movement between the shaft

& the hub

- It transmits torque & permits

some axial movements of hub

N.B: It is an alternative to splined connection

Machine Design 24

Woodruff Key - is a piece from cylindrical disc having segmental cross-section (in

the form of an almost semi-circular disk of uniform thickness)

-Keyway in the shaft is in the form of a semi-circular recess with the same curvature as

that of the key. The bottom portion of the key fits into circular keyway in the shaft.

-The projecting part fits in the keyway in the hub

- Once placed in position, the

woodruff key tilts & aligns itself

on the shaft

The key is largely used on

tapered shafts in Automobile &

machine tool construction


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Machine Design 25

Woodruff Key

Can be used on tapered shaft because it can be aligned by slight rotation in the

seat

The extra depth of key in the shaft prevents its tendency to turn over the shaft

Advantages

The extra depth of keyway in the shaft increase stress concentration & reduces its

strength

The key does not permit axial movement between the shaft & the hub

Disadvantages

Machine Design 26

Round Key

-are circular in section & fit into holes drilled partly in the shaft & partly in the hub

-Sometimes the tapered pin is held in tapered holes

It has the advantage that their keyways may be drilled after the mating parts have

been assembled


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Machine Design 27

Splines -Splines are keys that are made integral with the shaft. Such shafts

are known as SplinedShaft-These types of shafts usually have 4, 6, 10 or 16 splines

-They are used when there is a relative axial motion

between the shaft & the hub and are also used when the

force to be transmitted is large in proportion to the size

of the shaft as in Automobile transmission & sliding gear

transmission.

-These types of shafts usually have 4, 6, 10 or 16 splines

-Manufacturing Method:Splinesare cut on the shaft by Milling

the hub by Broaching

Machine Design 28

Serrations

Straight Sided Splines

Involute SplinesTypes of Splines

- Stub teeth with

pressure angle 30

- Are specified by

module

- Greater strength

- Are used inapplications where it

is important to keep

overall size of

assembly as smallas possible

- Used as interferance

joint

- Used in gear shiftingmechanism in

Automobile gear

boxes & machine

tool gear boxes


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Machine Design 29

Forces acting on a sunk key

Step 1

Step 2

Design of Sunk Keys

- Function: Key is used in transmitting torque from a shaft to a hub.

-The distribution of the forces along the length of the key are not uniform because

the forces are concentrated near the torque-input end. Therefore, the stresses are

not uniform along the key in the axial direction.

-The non-uniformity of distribution is caused by the twisting of the shaft within the

hub.

The following two types of forces act on the key:

Forces due to fit of the key in its keyway. These forces produce compressive

stresses in the key which are difficult to determine its magnitude and distribution.

Force (P) due to the torque transmitted by the shaft.

Assumption

Forces due to fit of the key are neglected.

The distribution of forces along the length of key is uniform.

Machine Design 30

Forces acting on Key

A

B

C

D

P

P

P

P-The transmission of torque from the

shaft to the hub results in two equal

& opposite forces denoted by P.

-The torque (T) is transmitted by

means of a force P acting on the left

surface (AC) of the key.

-The equal & opposite force (P),

acting on the right surface (DB) ofthe key is reaction of the hub on the

key.

- It is observed that force (P) on left surface AC and its equal & opposite reaction P on

right surface DB is not in same plane. Therefore, forces P (=P) act as resisting couple

preventing the key to roll in the keyway.

-The exact location of force (P) on surface (AC) is unknown.


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Machine Design 31

Engineers commonly assume that the entire torque is carried by a tangential force(P) located at the shaft surface.

T

T : Torque transmitted by the shaft (N-mm).

P : Tangential force acting at the circumference

of the shaft (N).

d : Diameter of the shaft (mm).

dT P

2=

2T

P d=

Designation of Parallel Sunk Keys

Width (b) Height (h) Length (l)

Forces acting on Key

Machine Design 32

Due to the power or torque transmitted by the shaft, the key may fail due to

shearing or crushing.

Design of sunk key is based on two crit eria:

Failure due to shear.

Failure due to crushing.

T

Design Analysis


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Machine Design 33

- Shear failure will occur in plane AB.

Shear stress induced in plane AB =

Area resisting shearing: As = bL

s

P P

A b L = =

key

2T

L d.b.[ ]

T

Failure due to Shear

[ ]key

2T

d.b.L =

L= Effective length of the Key

A B

Design Analysis

Machine Design 34

- Crushing failure will occur on surface AC or DB.

Crushing stress induced = c

Area resisting crushing: Ac =Lh/2

c

c

P P

hAL

2

= =

c min

4TL

d.h.[ ]

c

4T

d.h.L =

Failure due to Crushing

min. of ( [c]key, [c]shaft, [c]hub)

AB

C

D

Design Analysis


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Machine Design 35

Step 1

Step 2

Step 3

- Either shaft diameter (d) is given or estimate shaft diameter (d).

Main Steps in Design Analysis of Key

- Select widthheight of the key from IS 2293:1963 (Data Book)

- Calculate force acting on key.

- Calculate effective length of the key (L) based on two design criteria

(shear failure & crushing failure) & recommend larger of the above two

dimensions.

Step 4

Design Analysis

Machine Design 36

Prob#1: It is required to design a sunk key for fixing a gear on a shaft (made of plain

C-steel 50C4) of 25 mm diameter. The shaft is transmitting 10 KW power at 720

RPM to the gear (made of same material of shaft). The drive is subjected to

medium shocks for which a service factor of 1.5 is to be considered. The key is

made of steel 45C8 and factor of safety is 3.

Solution Diameter of the shaft (d)=25 mm

Power transmitted by the shaft (Pow)=10 KW.

N=720 RPM

02 NT

Pow ;60

= 0

60 10000T 132.63 N m 132630 N mm

2 720

= = =

s 0T C T 1.5 132630 N mm 198943.6 N mm= = =

Material of the key Plain C-steel: 45C8, Yield stress Syt=380 MPa, Factor of safety=3

Allowable tensile stress [t]key=Syt/FOS=127 N/mm2,

Allowable shear stress []key=0.5[t]key=63 N/mm2,

Allowable crushing stress [c]key=1.25[t]key=158 N/mm2,


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Machine Design 37

Material of shaft & hub Plain C-steel: 50C4, Yield stress Syt=460 MPa, Factor of safety=3

Allowable tensile stress [t]s=Syt/FOS=153 N/mm2,

Allowable crushing stress [c]s=1.25[t]s=191 N/mm2.

Selection of widthheight of the key from IS 2293:1963 (Data Book)

For shaft diameter d=25 mm: width (b) height (h)=87

Width of the key (b) =8 mm; Height of the key (h)=7 mm.

Shear stress induced in plane AB =

s

P P;

A b L = =

key

2TL

d.b.[ ][ ]key

2T;

d.b.L =

Failure due to Shear

L 31.58 mm

Crushing stress induced = c

c

c

P P

hAL

2

= =

c min

4TL

d.h .[ ]

c

4T

d.h.L =

Failure due to Crushing

min. of ( [c]key, [c]shaft, [c]hub)

L 28.78 mm

Effective length of the key=32 mm

Documents

Machine Design Key 2014(1)