Seven Steps to Belt Maintenance_white Paper

7/28/2019 Seven Steps to Belt Maintenance_white Paper

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Successful Belt Maintenance

A research paper by

ADVANCED Maintenance Solutions

7600 Delview DriveWest Chester, OH 45069

Phone: 513-779-5880Fax: 513-779-5881Cell: 513-379-4574

[email protected]


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IntroductionIt is generally recognized that power transmission systems operate moreefficiently with less vibration, wear and failures when they are properly aligned.Typically when we discuss machinery alignment most people think of shafts andcouplings. This discussion is going to deal with the precision alignment of

flexible drives or belt driven machinery.

It is conceded that over half of all belt and drive failures are attributed to improperalignment of the drive components.

The definition of shaft alignment is that two (or more) shafts share a commoncenter or rotation. When dealing with belts and sheaves, we must expand thisdefinition slightly. What we want to accomplish is to get the rotational plane ofthe moveable sheave co-planar with the rotational plane of the stationarysheave. More specifically, the plane created by the groove the belt runs in.

Belts are designed to transmit power in only one direction, the direction of belttravel. Any deviation in proper sheave alignment causes the belt to transmitforces axially on the belt grooves. This causes increased levels of vibration,increased wear of the belts and sheaves, higher operating temperatures and adecreased Mean Time Between Failures or MTBF. Often times this not onlywears out belts and sheaves faster, but these forces are passed along to theseals and bearings supporting the shafts.

Before we get into the specifics of sheave alignment, lets discuss sheavealignment terminology. Below are some graphics that describe the three types ofsheave misalignment; Vertical Angularity (Twist), Horizontal Angularity (Pigeon-Toe) and Offset.


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Poor drive train alignment can show up in many different ways. From a vibrationperspective, poorly aligned drives will typically show high amplitude at multiplesof belt speed as well as elevated levels if 1X Driver and/or Driven shaft rpmvibration. The 1X Driver and/or Driven vibration will typically present itself in theaxial direction. The spectra and thermographic image below are from the Motor

Drive End. Motor speed is 1792.5 RPM, Blower RPM is 1462.5 RPM and beltspeed is 596.25 RPM. 2X Belt Speed is 1192 CPM.

F18 - P1-4357 VACCUM PUMP

P1-4357 -MIA MOTOR INBOARD AXIAL

Analyze Spectrum28-OCT-99 08:49:35

PK = .5376LOAD =100.0RPM = 1775.RPS = 29.58

0 4000 8000 12000

0

0.1

0.2

0.3

0.4

0.5

0.6

Frequency in CPM

PK

Velocity

inIn/Sec

1458.1

1191.0

1782.2

Freq:Ordr:Spec:

1458.1.821.497

120.1120.5

*>127.1F

*


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The data below is from the same machine following sheave alignment and propertensioning. It is interesting to note that the blower shaft was found to have asignificant bend. The sheaves were aligned to their best condition given andshaft bend and re-tensioned. The Blower vibration dropped from 0.5 in/sec to .19in/sec. The motor temperature dropped from 120 F to 98.5 F.

F18 - P1-4357 VACCUM PUMP

P1-4357 -MIA MOTOR INBOARD AXIAL

Analyze Spectrum28-OCT-99 14:20:25

PK = .2596LOAD =100.0RPM = 1775.RPS = 29.58

0 4000 8000 12000

0

0.06

0.12

0.18

0.24

0.30

Frequency in CPM

PK

VelocityinIn/Sec 1

458.2

1194.6

1783.2

Freq:Ordr:Spec:

1455.0.820.189

98.596.7

*>98.9F

*


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Sheave Alignment TheoryThe center of a belt groove on a sheave, when the sheave is rotated, will createa flat circular plane. The goal in sheave alignment is to position the planecreated by the rotation of the moveable sheave to be co-planar with the planecreated by the stationary sheave.

A generally accepted alignment tolerance for belt drives recommends thefollowing: As a general rule, sheave misalignment on V-belt drives should be lessthan degree or 1/10 per foot of drive center distance. Misalignment forsynchronous, Polyflex and Micro-V belts should be within degree or1/16 per foot of dr ive center distance.

It is also understood, however, that The greater the misalignment, the greaterthe chance of belt instability, increased belt wear and V-belt turnover.

This tolerance is designed to provide acceptable life for the belts and sheaves,not necessarily the rest of the machine components.

As with all transmission systems, better alignment typically results in betterperformance with fewer failures and higher efficiency. With the current laserbased measurement technology available today, these recommended tolerancesare easy to improve upon.


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Degree Conversion: degree sounds like a pretty small number, but keep this conversion in mind:1 degree = 17 mils/1 degree = 8.5 mils/1 degree = 4.25 mils/1

For a drive with a 36 distance between drive centers, the Gates Tolerance wouldbe as follows: 8.5 mils/1 multiplied by 36 = 306 mils allowable offset! This alsoworks for the 1/10th inch per foot of center distance. 1/10th inch = 0.100. 36 =3, therefore the allowable misalignment would be 0.300.For Synchronous drives the allowable misalignment would be 153 mils of offset.

The tolerance is also unclear as to whether or not this measurement of Offsetapplies to the sum of the offsets at both sheaves or is allowable at each sheave.Either way, the values can be greatly improved upon using proper modernmeasurement techniques.

All manufacturers of belt transmission products agree that alignment is critical.The Goodyear FAQ site states:

Q How critical is alignment and tensioning with power transmission belting?

A The primary causes of premature belt failure are incorrect tension and alignment of the beltduring installation.

Precision alignment is becoming more and more critical on belt drives with theincreased use of joined belts and timing or synchronous drives.

J oined belts or Power-Band V-belts are created by adding a common back orband to the top of two or more belts. The backing eliminates individual beltslippage however it significantly increases the transverse rigidity. Basically, thebelt is harder to bend or twist in any direction other than the sheave wrapdirection. Any offset misalignment at all will transmit large axial forces to the beltgrooves resulting in high drive component temperatures, noise, increasedvibration and premature drive/belt failures.

Synchronous drives are a little more forgiving with the Offset Alignment, howeverthe drive sprockets need to be absolutely parallel to each other. HorizontalAngular misalignment will cause the drive belt to ride to the tighter side of the

drive, therefore increasing the belt tension, riding on the guide at the edge of thesprocket and causing high vibration, high drive component temperatures, noiseand premature drive/belt failures.


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There are several methods currently employed to align sheaves.

eye-ball alignment

Straight-edge alignment

Tight wire alignment Face Mounted Laser System

Groove Mounted Laser SystemI will discuss each of the above methods below:

Eye-Ball Alignment is when the alignment of the sheaves is evaluated by visualinspection of the belt grooves and possibly the straightness of the belt when thetension is adjusted. Obviously there are severe drawbacks to this method. Thismethod depends greatly on the ability of maintenance personnel, distancebetween the sheaves, quality of the belts, etc. This method is also not veryrepeatable from operator to operator.

Straight-edge alignment is just what it sounds like. A straight edge is placed onthe outer faces of the sheaves. Any deviation in the alignment will present itselfas a gap between the sheave face and the straight-edge. While this is animprovement in the alignment of the sheaves, this still leaves several areas forimprovement. First, while this method addresses the amount of pigeon-toe andoffset it does not address the twist or vertical angle measurement. Alignmentis completed when the sheaves touch the straight edge at all edges of thesheaves.

This method also has its drawback over very long span drives. Precision

straight edges are bulky, expensive and hard to manipulate. They often requiregreat care when transporting and storing and require at least two people tomeasure.

Another drawback of this measurement method is the quality of the sheave, orwhether the sheaves on the driving and driven machine are made by the samemanufacturer. As far as sheave quality is concerned, the manufacturingdrawings have a very large tolerance for the distance from the sheave face to thecenter of the first belt groove. This value is not critical to the sheavemanufacturers. This distance on some sheaves deviates by as much as 1/16thinch. For two sheaves, the deviation has the potential to be doubled. That

deviation could cause an offset error of 1/8th

inch. The factors above will causethe distance from the face of the sheave to the belt groove to vary from sheave tosheave. Remember the goal above, to align the planes of power transmission,i.e. the belt grooves. Aligning the faces of the sheaves is counter productive ifthe distance from the face of the sheave to the belt grooves is inconsistent.Another quality issue is the surface finish of the face. Typically, these are notprecision machined surfaces. A bump or high spot on the sheave face will


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force an angular error into the measurement. An error as small as 0.010 on a 6inch diameter sheave will result in a fairly significant error.

The Tight-Wire method is similar to the straight edge. A string, length of dentalfloss or the like is stretched across the faces of the sheaves. While this method

is a lot less expensive than the straight edge, it has the same drawbacksregarding accuracy.


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Laser Based MeasurementsFace mounted laser systems were the first real attempt to address the VerticalAngle or twist misalignment as well as the pigeon-toe and offset. This methoduses a line laser transmitter magnetically mounted to the face of one of thesheaves. Three targets are mounted on the face of the other sheave. The

amount of misalignment is interpreted by noting the difference height of the laseras it strikes the targets. Again, great care must be taken to measure the distancefrom the sheave face to the belt groove. Recent changes in this method are theaddition of adjustable targets in an attempt to improve the accuracy of the facemounted laser based sheave alignment tools. Since the release of the BTA,several face mounted systems have flooded the market. They include but arenot limited to:

Easy Laser BTA-Digital

Ludeca DotLine

PowerLine L-80

Most of the face mounted systems also offer a low-cost or compact version oftheir systems. Alignment is completed with this type of system when the linelaser strikes the monuments at their target center. The farther the monumentsare apart, the better the results of the alignment. Damalini has gone the oppositedirection. Utilizing 2 detectors a fixed distance apart, the BTA Digital can storeand print out sheave alignment data.


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The next improvement in sheave alignment technology came with the release ofthe first reflected beam system. While still a face mounted system, this systemeffectively doubles the measurement distance by reflecting the beam back to thetransmitter. This results in better horizontal angular resolution but does not affect

the accuracy of the Vertical Angle or the Offset. By reflecting the beam back toits source the mil/in horizontal angularity is magnified. That is, the beam travelsfarther so a smaller amount of angular misalignment can be seen because it isprojected over a longer distance.For example: consider a 20 tall metal rod placed exactly plumb. Movement ofthe rod of 1/32nd inch 1 foot up from the bottom is hardly noticeable, however, atthe top of the rod, that 1/32nd inch is multiplied by a factor of 20, resulting in 5/8movement which is very easy to see and measure.

This system was originally offered in 2 versions; the Pulley Partner and thePulley Pro. System specifications identified different sheave distances that eachwould be applicable for. Alignment was completed with this system when thelaser line struck the target line on both the reflecting target and the transmitter.

As with straight-edge, tight-wire, and monument based single laser systems, thissystem is face mounted and as such, is limited in accuracy by the same factorsaffecting the other face mounted systems.


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Groove Mounted Laser SystemsGroove Mounted Systems were the next, and most current development insheave alignment technology.

By definition, we are aligning the planes of power transmission, thats the belt

grooves. Rather than assuming the face to groove distance is repeatable fromsheave to sheave, groove mounted systems measure this alignment directly.Currently there are two versions of groove mounted measurement systems onthe market; the Belt Hog (aka P.A.T) and the Hamar S-600.

The Hamar S-600 system uses a single line laser source mounted in the groovesof one sheave and a 3 axis detector assembly mounted in the opposing sheavegrooves. This system offers the advantage of a digital readout, however it canmost easily be adapted to sheaves with 3 or more grooves. This is due to having3 tooling balls required to solidly mount the laser transmitter and the detectorheads. Different sizes of tooling balls are required to adapt this system properly


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to different sizes of sheaves (A, B, C, D, etc). The Hamar S-600 utilizes manycomponents to perform sheave alignment. Tooling balls for each sheavediameter, a laser transmitter, a detector assembly and a separate digital readout.The transmitter and detector heads are clamped onto the sheaves using aquick-grip clamp or something similar. This clamp can also be bulky and require

the entire guard to require disassembly.

The Belt Hog also mounts in the belt grooves. This system incorporates twolaser sources that are magnetically mounted in the opposing sheave beltgrooves. Each laser source emits a line laser at the center surrounded by atarget area. Changeable feet sized to accommodate different groove dimensionsare available for the system. Since the Belt Hog is mounted in the center of thebelt groove, the line laser transmitter projects a visible laser line coincident withthe belt groove plane. The Belt Hog mounted on the stationary sheave projects alaser line coincident with the plane created by the stationary sheave.

By positioning the moveable sheave until these planes are co-planar, we arealigning the planes created by the belt grooves, without regard to the errors insheave face thickness. Because of the dual laser system, the horizontal angularresolution is at a maximum.


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Transmission Inspection Checklist

If it is determined that alignment is necessary, we recommend the following stepsto help ensure a successful alignment of the drive components.


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One thing common to all of the measurement methods (except the eye-ballmethod) is the order in which the corrections are made. Below is a list of stepsrequired to achieve proper sheave alignment:

1. Pre Alignment Checks

A. Inspect sheaves and belts for wear/deterioration. If excessive wearis noted, the component should be replaced. If a belt is worn, allthe belts should be replaced.

B. Generally speaking when using a Belt/Sheave gage, if wear inexcess of 1/32 is noted, the component should be replaced.

2. Measure sheave groove in the axial direction for TIR.A. By mounting a dial indicator with the plunger directed toward one

side of the belt groove, you will be able to accurately measure howmuch the sheave is cocked on the shaft.

B. Excessive axial runout can also be caused by excessive sheave

wear. Often, this axial TIR can be corrected by adjusting the take-up bolts on the tapered bushing. Axial TIR should not exceed 0.5mils/1 of sheave diameter. For sheaves larger than 10 inches indiameter, Axial TIR should not exceed 5 mils.

3. If excessive axial TIR is noted, measure the runout of the machine shaft.Shaft TIR should be less that 0.002.

4. Measure the sheave radial TIR. Sheave eccentricity will cause largevalues of radial TIR. Tolerances for radial TIR are similar to those forAxial TIR. Large amounts of radial TIR are often the result of a defectduring manufacturing.

A. Radial TIR, if excessive, will cause high 1X sheave rpm vibration in

the direction of the belts, sometimes mistaken for unbalance of thesheave or fan rotor.

5. Check and correct motor soft foot.6. Mount the measurement system and evaluate the results.7. Alignment corrections should be made in the following order:

A. Correct Vertical Angle or Twist.B. Correct Horizontal Angle or Pigeon-Toe.C. Reposition sheaves axially on the shaft to correct Offset.


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If you are using a face mounted system relying on monuments, it is veryimportant to reverse the fixture setup and verify the alignment looking the otherdirection. The reason is simple; small amounts of Horizontal Angularity may notbe seen until the distance traveled by the laser beam is increased. Letsconsider the following example:

Sheave A is 10 diameterSheave B is 20 diameterDistance between sheave centers is 36

With the monuments mounted on sheave A and the Transmitter mounted onsheave B a difference in laser position on the monuments of 0.020 may beundetectable to the naked eye. That is 2 mils/1 of Horizontal Angularity. Byreversing the set-up, that 2 mils/1 angularity is projected over 46 (centerdistance + Sheave B radius). Now the laser line will appear 0.086 (over 1/16)from the center of the monument, very easy to identify with the naked eye.

Following the proper alignment of the sheaves, it is important to properly tensionthe belts. Over-tensioning of the belts significantly increases the loading on thebearings of the machines.

Variable Pitch Sheaves

Basically, there are two types of Variable Pitch Sheaves, single belt VariablePitch Sheaves and Multiple Belt Variable Pitch Sheaves. Variable Pitch meansthat the Pitch Diameter (effective drive diameter) can be adjusted to fine tune thespeed of the output shaft. Typically these are found on air handling units inbuildings where air balancing is critical to maintain building pressure.

Originally, these sheaves were designed to be replaced with fixed pitch sheavesafter the proper pitch diameter was found during the air balance procedure.Variable Pitch sheaves are commonly found to be source of high drive vibrationin air handling units.

The sides of a variable pitch sheave are adjustable, therefore the belt groovecenter will change each time the pitch diameter is changed. Single Belt VariablePitch Sheaves can be aligned using groove mounted measurement devices,provided that the pitch diameter is not going to be changed at a later date withoutrealignment.

Multiple belt variable pitch sheaves cannot be properly aligned. As the beltgroove halves are moved closer together increasing the pitch diameter, the beltgroove center changes by half of the adjustment amount. When dealing withmultiple belts, it is only possible to align one groove on the variable pitch sheavewith the corresponding groove on the fixed pitch sheave. For the case of a 3 beltvariable pitch sheave, one groove can be properly aligned, however 2 grooveswill have significant amounts of offset misalignment.


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Unless constant changes are made to the output shaft speed, variable speedsheaves should be replaced with fixed pitch sheaves as soon as possiblefollowing the identification of the proper pitch diameter.

Belt TensionBelt tension is as important to the long life and superior performance of a flexibledrive system as the alignment. As a general rule, belts that exceed their tensionby a little as 15 lbs can add 300 lbs of preload to the machine bearings. Here isan example of a recommended belt tension table.


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The correct tension is the lowest tension at which belts will run and not slip whenthe drive is under full load. Experienced mechanics may claim to check belttension with their thumb. But why take chances when there is a simple and moreaccurate method available. This is very important as belts with different

construction will exhibit a different feel when properly tensioned.

There are several methods available to measure the belt tension on a flexibledrive system. They range from spring mounted plungers to sonic belt tensionmeasurement devices.

Spring based tensioning devices are relatively simple to use. The belt driveconfiguration is looked up in the supplied manual. This configuration includes thebelt type, sheave diameters, span and rpm. The tool has a spring loaded plunger

on one end and a scale at the other end. The plunger scale reads pounds ofdeflection force while the scale at the other end is marked in inches of spanbetween the sheaves.

When the belts are properly tensioned, the O-Ring on the plunger will just makecontact with the housing while the O-Ring on the housing will be level with thesurface of the other belts on the drive.


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The Sonic Belt tension device from Gates functions very much like a guitar stringtuner. When a belt with a given cross section and with a given span length isplucked it will emit a specific sound frequency. This frequency is picked up bythe tensioning devices microphone and is displayed. If the belt is over-tightened,the pitch will be too high, if the belt is under-tensioned the pitch will be too low.


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Goodyear also has released a vibration based belt tensioning device that workssimilar to the sonic meter above. It measures the vibration frequency across thefree span of the belt when struck or plucked. The results are displayed in forceunites of newtons.

Another innovative belt tensioning device comes from Goodyear Tire and RubberCompany. This device is called the Tensionrite Strip. It sticks to the beltsurface like a piece of tape when the belt is in a relaxed state. As the tension isincreased on the drive and the belt begins to stretch, the tape stretches. Theamount of stretch is indicated as a yellow line appearing behind a series ofnumbers. These are recommended for single use only and are priced to bedisposable.


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Note: throughout this troubleshooting guide, you might notice that Belt Alignment

was referenced as a possible cause for drive failure 19 times. This does notinclude the failures caused by sheave wear that can also be caused by amisaligned drive!

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Seven Steps to Belt Maintenance_white Paper