Successful Belt Maintenance. ADVANCED Maintenance Solutions

Transcription

1 Successful Belt Maintenance A research paper by ADVANCED Maintenance Solutions 7600 Delview Drive West Chester, OH Phone: Fax: Cell: [email protected]

2 Introduction It is generally recognized that power transmission systems operate more efficiently with less vibration, wear and failures when they are properly aligned. Typically when we discuss machinery alignment most people think of shafts and couplings. 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 improper alignment of the drive components. The definition of shaft alignment is that two (or more) shafts share a common center or rotation. When dealing with belts and sheaves, we must expand this definition slightly. What we want to accomplish is to get the rotational plane of the moveable sheave co-planar with the rotational plane of the stationary sheave. 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 belt travel. Any deviation in proper sheave alignment causes the belt to transmit forces axially on the belt grooves. This causes increased levels of vibration, increased wear of the belts and sheaves, higher operating temperatures and a decreased Mean Time Between Failures or MTBF. Often times this not only wears out belts and sheaves faster, but these forces are passed along to the seals and bearings supporting the shafts. Before we get into the specifics of sheave alignment, let s discuss sheave alignment terminology. Below are some graphics that describe the three types of sheave misalignment; Vertical Angularity (Twist), Horizontal Angularity (Pigeon- Toe) and Offset. 2

3 Poor drive train alignment can show up in many different ways. From a vibration perspective, poorly aligned drives will typically show high amplitude at multiples of belt speed as well as elevated levels if 1X Driver and/or Driven shaft rpm vibration. The 1X Driver and/or Driven vibration will typically present itself in the axial direction. The spectra and thermographic image below are from the Motor Drive End. Motor speed is RPM, Blower RPM is RPM and belt speed is RPM. 2X Belt Speed is 1192 CPM. PK Velocity in In/Sec F18 - P VACCUM PUMP P MIA MOTOR INBOARD AXIAL Analyze Spectrum 28-OCT-99 08:49:35 PK =.5376 LOAD = RPM = RPS = Frequency in CPM Freq: Ordr: Spec: *>127.1 F *<78.4 F 3

4 The data below is from the same machine following sheave alignment and proper tensioning. It is interesting to note that the blower shaft was found to have a significant bend. The sheaves were aligned to their best condition given and shaft bend and re-tensioned. The Blower vibration dropped from 0.5 in/sec to.19 in/sec. The motor temperature dropped from 120 F to 98.5 F F18 - P VACCUM PUMP P MIA MOTOR INBOARD AXIAL Analyze Spectrum 28-OCT-99 14:20:25 PK Velocity in In/Sec PK =.2596 LOAD = RPM = RPS = Frequency in CPM Freq: Ordr: Spec: *>98.9 F AFTER *<82.4 F 4

5 Sheave Alignment Theory The center of a belt groove on a sheave, when the sheave is rotated, will create a flat circular plane. The goal in sheave alignment is to position the plane created by the rotation of the moveable sheave to be co-planar with the plane created by the stationary sheave. A generally accepted alignment tolerance for belt drives recommends the following: As a general rule, sheave misalignment on V-belt drives should be less than ½ degree or 1/10 per foot of drive center distance. Misalignment for synchronous, Polyflex and Micro-V belts should be within ¼ degree or 1/16 per foot of drive center distance. It is also understood, however, that The greater the misalignment, the greater the 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 better performance with fewer failures and higher efficiency. With the current laser based measurement technology available today, these recommended tolerances are easy to improve upon. 5

6 Degree Conversion: ½ degree sounds like a pretty small number, but keep this conversion in mind: 1 degree = 17 mils/1 ½ degree = 8.5 mils/1 ¼ degree = 4.25 mils/1 For a drive with a 36 distance between drive centers, the Gates Tolerance would be as follows: 8.5 mils/1 multiplied by 36 = 306 mils allowable offset! This also works for the 1/10 th inch per foot of center distance. 1/10 th inch = = 3, therefore the allowable misalignment would be 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 Offset applies 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 modern measurement techniques. All manufacturers of belt transmission products agree that alignment is critical. The Goodyear FAQ site states: Q A How critical is alignment and tensioning with power transmission belting? The primary causes of premature belt failure are incorrect tension and alignment of the belt during installation. Precision alignment is becoming more and more critical on belt drives with the increased use of joined belts and timing or synchronous drives. Joined belts or Power-Band V-belts are created by adding a common back or band to the top of two or more belts. The backing eliminates individual belt slippage however it significantly increases the transverse rigidity. Basically, the belt is harder to bend or twist in any direction other than the sheave wrap direction. Any offset misalignment at all will transmit large axial forces to the belt grooves resulting in high drive component temperatures, noise, increased vibration and premature drive/belt failures. Synchronous drives are a little more forgiving with the Offset Alignment, however the drive sprockets need to be absolutely parallel to each other. Horizontal Angular 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 the sprocket and causing high vibration, high drive component temperatures, noise and premature drive/belt failures. 6

7 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 System I will discuss each of the above methods below: Eye-Ball Alignment is when the alignment of the sheaves is evaluated by visual inspection of the belt grooves and possibly the straightness of the belt when the tension is adjusted. Obviously there are severe drawbacks to this method. This method depends greatly on the ability of maintenance personnel, distance between the sheaves, quality of the belts, etc. This method is also not very repeatable from operator to operator. Straight-edge alignment is just what it sounds like. A straight edge is placed on the outer faces of the sheaves. Any deviation in the alignment will present itself as a gap between the sheave face and the straight-edge. While this is an improvement in the alignment of the sheaves, this still leaves several areas for improvement. First, while this method addresses the amount of pigeon-toe and offset it does not address the twist or vertical angle measurement. Alignment is completed when the sheaves touch the straight edge at all edges of the sheaves. This method also has it s drawback over very long span drives. Precision straight edges are bulky, expensive and hard to manipulate. They often require great care when transporting and storing and require at least two people to measure. Another drawback of this measurement method is the quality of the sheave, or whether the sheaves on the driving and driven machine are made by the same manufacturer. As far as sheave quality is concerned, the manufacturing drawings have a very large tolerance for the distance from the sheave face to the center of the first belt groove. This value is not critical to the sheave manufacturers. This distance on some sheaves deviates by as much as 1/16 th inch. For two sheaves, the deviation has the potential to be doubled. That deviation could cause an offset error of 1/8 th inch. The factors above will cause the distance from the face of the sheave to the belt groove to vary from sheave to sheave. 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 if the 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 not precision machined surfaces. A bump or high spot on the sheave face will 7

8 force an angular error into the measurement. An error as small as on a 6 inch diameter sheave will result in a fairly significant error. The Tight-Wire method is similar to the straight edge. A string, length of dental floss 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 drawbacks regarding accuracy. 8

9 Laser Based Measurements Face mounted laser systems were the first real attempt to address the Vertical Angle or twist misalignment as well as the pigeon-toe and offset. This method uses a line laser transmitter magnetically mounted to the face of one of the sheaves. Three targets are mounted on the face of the other sheave. The amount of misalignment is interpreted by noting the difference height of the laser as it strikes the targets. Again, great care must be taken to measure the distance from the sheave face to the belt groove. Recent changes in this method are the addition of adjustable targets in an attempt to improve the accuracy of the face mounted laser based sheave alignment tools. Since the release of the BTA, several face mounted systems have flooded the market. They include but are not limited to: Easy Laser BTA-Digital PowerLine L-80 Ludeca DotLine Most of the face mounted systems also offer a low-cost or compact version of their systems. Alignment is completed with this type of system when the line laser strikes the monuments at their target center. The farther the monuments are apart, the better the results of the alignment. Damalini has gone the opposite direction. Utilizing 2 detectors a fixed distance apart, the BTA Digital can store and print out sheave alignment data. 9

10 The next improvement in sheave alignment technology came with the release of the first reflected beam system. While still a face mounted system, this system effectively doubles the measurement distance by reflecting the beam back to the transmitter. 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 to it s source the mil/in horizontal angularity is magnified. That is, the beam travels farther so a smaller amount of angular misalignment can be seen because it is projected over a longer distance. For example: consider a 20 tall metal rod placed exactly plumb. Movement of the rod of 1/32 nd inch 1 foot up from the bottom is hardly noticeable, however, at the top of the rod, that 1/32 nd inch is multiplied by a factor of 20, resulting in 5/8 movement which is very easy to see and measure. This system was originally offered in 2 versions; the Pulley Partner and the Pulley Pro. System specifications identified different sheave distances that each would be applicable for. Alignment was completed with this system when the laser 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, this system is face mounted and as such, is limited in accuracy by the same factors affecting the other face mounted systems. 10

11 Groove Mounted Laser Systems Groove Mounted Systems were the next, and most current development in sheave alignment technology. By definition, we are aligning the planes of power transmission, that s the belt grooves. Rather than assuming the face to groove distance is repeatable from sheave to sheave, groove mounted systems measure this alignment directly. Currently there are two versions of groove mounted measurement systems on the 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 grooves of one sheave and a 3 axis detector assembly mounted in the opposing sheave grooves. This system offers the advantage of a digital readout, however it can most easily be adapted to sheaves with 3 or more grooves. This is due to having 3 tooling balls required to solidly mount the laser transmitter and the detector heads. Different sizes of tooling balls are required to adapt this system properly 11

12 to different sizes of sheaves (A, B, C, D, etc). The Hamar S-600 utilizes many components to perform sheave alignment. Tooling balls for each sheave diameter, a laser transmitter, a detector assembly and a separate digital readout. The transmitter and detector heads are clamped onto the sheaves using a quick-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 two laser sources that are magnetically mounted in the opposing sheave belt grooves. Each laser source emits a line laser at the center surrounded by a target area. Changeable feet sized to accommodate different groove dimensions are available for the system. Since the Belt Hog is mounted in the center of the belt groove, the line laser transmitter projects a visible laser line coincident with the belt groove plane. The Belt Hog mounted on the stationary sheave projects a laser line coincident with the plane created by the stationary sheave. By positioning the moveable sheave until these planes are co-planar, we are aligning the planes created by the belt grooves, without regard to the errors in sheave face thickness. Because of the dual laser system, the horizontal angular resolution is at a maximum. 12

13 Transmission Inspection Checklist If it is determined that alignment is necessary, we recommend the following steps to help ensure a successful alignment of the drive components. 13

14 One thing common to all of the measurement methods (except the eye-ball method) is the order in which the corrections are made. Below is a list of steps required to achieve proper sheave alignment: 1. Pre Alignment Checks A. Inspect sheaves and belts for wear/deterioration. If excessive wear is noted, the component should be replaced. If a belt is worn, all the belts should be replaced. B. Generally speaking when using a Belt/Sheave gage, if wear in excess 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 how much 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 takeup bolts on the tapered bushing. Axial TIR should not exceed 0.5 mils/1 of sheave diameter. For sheaves larger than 10 inches in diameter, 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 Measure the sheave radial TIR. Sheave eccentricity will cause large values of radial TIR. Tolerances for radial TIR are similar to those for Axial TIR. Large amounts of radial TIR are often the result of a defect during manufacturing. A. Radial TIR, if excessive, will cause high 1X sheave rpm vibration in the direction of the belts, sometimes mistaken for unbalance of the sheave 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. 14

15 If you are using a face mounted system relying on monuments, it is very important to reverse the fixture setup and verify the alignment looking the other direction. The reason is simple; small amounts of Horizontal Angularity may not be seen until the distance traveled by the laser beam is increased. Let s consider the following example: Sheave A is 10 diameter Sheave B is 20 diameter Distance between sheave centers is 36 With the monuments mounted on sheave A and the Transmitter mounted on sheave B a difference in laser position on the monuments of may be undetectable to the naked eye. That is 2 mils/1 of Horizontal Angularity. By reversing the set-up, that 2 mils/1 angularity is projected over 46 (center distance + Sheave B radius). Now the laser line will appear (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 tension the belts. Over-tensioning of the belts significantly increases the loading on the bearings of the machines. Variable Pitch Sheaves Basically, there are two types of Variable Pitch Sheaves, single belt Variable Pitch Sheaves and Multiple Belt Variable Pitch Sheaves. Variable Pitch means that the Pitch Diameter (effective drive diameter) can be adjusted to fine tune the speed of the output shaft. Typically these are found on air handling units in buildings where air balancing is critical to maintain building pressure. Originally, these sheaves were designed to be replaced with fixed pitch sheaves after the proper pitch diameter was found during the air balance procedure. Variable Pitch sheaves are commonly found to be source of high drive vibration in air handling units. The sides of a variable pitch sheave are adjustable, therefore the belt groove center will change each time the pitch diameter is changed. Single Belt Variable Pitch Sheaves can be aligned using groove mounted measurement devices, provided that the pitch diameter is not going to be changed at a later date without realignment. Multiple belt variable pitch sheaves cannot be properly aligned. As the belt groove halves are moved closer together increasing the pitch diameter, the belt groove center changes by half of the adjustment amount. When dealing with multiple belts, it is only possible to align one groove on the variable pitch sheave with the corresponding groove on the fixed pitch sheave. For the case of a 3 belt variable pitch sheave, one groove can be properly aligned, however 2 grooves will have significant amounts of offset misalignment. 15

16 Unless constant changes are made to the output shaft speed, variable speed sheaves should be replaced with fixed pitch sheaves as soon as possible following the identification of the proper pitch diameter. Belt Tension Belt tension is as important to the long life and superior performance of a flexible drive system as the alignment. As a general rule, belts that exceed their tension by a little as 15 lbs can add 300 lbs of preload to the machine bearings. Here is an example of a recommended belt tension table. 16

17 The correct tension is the lowest tension at which belts will run and not slip when the drive is under full load. Experienced mechanics may claim to check belt tension with their thumb. But why take chances when there is a simple and more accurate 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 flexible drive system. They range from spring mounted plungers to sonic belt tension measurement devices. Spring based tensioning devices are relatively simple to use. The belt drive configuration is looked up in the supplied manual. This configuration includes the belt 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 of deflection force while the scale at the other end is marked in inches of span between the sheaves. When the belts are properly tensioned, the O-Ring on the plunger will just make contact with the housing while the O-Ring on the housing will be level with the surface of the other belts on the drive. 17

18 The Sonic Belt tension device from Gates functions very much like a guitar string tuner. When a belt with a given cross section and with a given span length is plucked it will emit a specific sound frequency. This frequency is picked up by the tensioning device s 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. 18

19 Goodyear also has released a vibration based belt tensioning device that works similar to the sonic meter above. It measures the vibration frequency across the free span of the belt when struck or plucked. The results are displayed in force unites of newtons. Another innovative belt tensioning device comes from Goodyear Tire and Rubber Company. This device is called the Tensionrite Strip. It sticks to the belt surface like a piece of tape when the belt is in a relaxed state. As the tension is increased on the drive and the belt begins to stretch, the tape stretches. The amount of stretch is indicated as a yellow line appearing behind a series of numbers. These are recommended for single use only and are priced to be disposable. 19

20 20

21 21

22 22

23 23

24 24

25 25

26 26

27 Note: throughout this troubleshooting guide, you might notice that Belt Alignment was referenced as a possible cause for drive failure 19 times. This does not include the failures caused by sheave wear that can also be caused by a misaligned drive! 27