Simple Techniques for Preventing Drive Belt Alignment Problems
Dan Parsons, Application Engineer, The Gates Rubber Co., Denver, CO
(Reprinted from Plant Engineering magazine)

     Belt drive misalignment is one of the most common causes premature belt failure. of This problem reduces belt drive performance and greatly increases wear and fatigue on belt. A belt can be destroyed within a matter of hours or days if the belt drives the have been aligned incorrectly during their installation.
     If the belt drives are correctly aligned when installed, the service life of the belt is greatly increased. Angular misalignment (Figure 1) results in accelerated belt/sheave wear and potential stability problems with individual related problem, V-belts. A uneven belt and cord loading, unequal load sharing with multiple belt drives results in and leads to premature failure.
(Figure 1)
     Angular misalignment has a severe effect on the of synchronous belt performance drives. Symptoms such tracking forces, uneven tooth/land wear, edge as high belt wear, high noise levels, and potential tensile failure due to uneven cord loading are possible. Also, wide belts are misalignment than narrow more sensitive to angular belts.
     Parallel misalignment (Figure 2) also results in belt/sheave wear and accelerated potential stability individual belts. Uneven belt and cord loading is not problems with as significant a concern as with angular misalignment. However, parallel misalignment is typically V-belts than with synchronous belts. V-belts run in more of a concern with fixed grooves and cannot free float between flanges to a limited degree as synchronous belts.
     Parallel misalignment is generally not a critical concern synchronous belt drives with as long as the belt is not pinched between flanges and the belt tracks trapped or completely on both sprockets. 
(Figure 2)
     Synchronous belt sprockets are designed with face widths greater than belt widths to prevent problems associated tolerance accumulation, and to allow for a small with amount (fractions of an inch) of mounting offset.
     As long as the width between opposite sprocket flanges exceeds the belt widths, the belt automatically aligns itself properly as it seeks a comfortable operating position on both sprockets. It is normal for a synchronous belt to lightly contact at least one sprocket flange in the system. 
Measuring Misalignment
     The most common tools for measuring misalignment are a straightedge or string (Figure 3). The improper use of tool, especially a string, can result in erroneous either conclusions.
(Figure 3)
     A straightedge or string should be used to project the orientation of one sheave or sprocket face with respect to the other.
     When preparing to measure paralleled misalignment, that edges of both sheaves verify or sprockets are of equal or quantify the difference in thickness. Align thickness, sheaf grooves or sprocket faces directly with respect to one another, rather than the outside surfaces of the be necessary to mount sheaves sheaves or sprockets. It may or sprockets with the outside surfaces offset with respect to each other in order to properly align grooves or surfaces on which belts operate.
     Sprocket flanges should be inspected. A bent flange could result in erroneous measurements if the straightedge or rests against the outside edge of a damaged string flange.
     To determine how much misalignment is acceptable and at what point it becomes excessive, alignment must be quantified and compared to the belt measured, manufacturer's recommendations for various drives.

 

 
Quantifying Misalignment
     Misalignment is quantified mathematically or compared to some general rules of thumb for quick and easy results.
     Angular misalignment is quantified into a real value by measurements. The taking actual angle of misalignment is the difference in clearance between the defined by straightedge or string and the outside surface of the sheave or sprocket across the diameter. The mathematical (X2-X1)/D), where A=angular relationship is: A=ArcTan misalignment, deg; D=diameter of sheave or sprocket, in.; X=distance from straight edge to sheave flange, in.
     The angle of parallel misalignment is defined by the in clearance between difference the straightedge or string surfaces of the two sheaves or sprockets and the outer across the length of the belt. The mathematical relationship is: P=ArcTan (Y/L), where P=parallel straightedge to sheave, n.; misalignment, deg; Y=distance from L=center distance between sheaves, in.
     The total allowable misalignment recommended for 1/2 deg. While V-belts is individual V-belts are capable of misalignment up to 6 deg before becoming handling unstable, maintaining the misalignment to within 1/2 deg maximizes belt live. Joined V-belts tolerate misalignment signification tieband damage occurs up to 3 deg before the total amount of misalignment recommended for synchronous, 60-deg, and V-ribbed belts is 1/4 deg. These drives are less tolerant of misalignment than conventional V-belt drives, and must be installed accurately.
     When determining if a V-type drive system is aligned these recommendations, within angular and parallel must be measured, quantified and added together. misalignment The total sum of angular and parallel misalignment is compared to the belt manufacturer?s of drive. Since synchronous belts are particularly sensitive to being pinched or trapped between opposite sprocket flanges, sprockets must be installed so there is clearance between belt and both flanges. This installation eliminates parallel misalignment, the and does not have to be quantified angular misalignment for a total and added to value.
Rules of Thumb
     It may not be practical or possible to accurately calculate misalignment in a total system while determining if it is in acceptable alignment. It is also difficult to visualize small fractions of an angle such as 1/4 or 1/2 deg. These angles are illustrated with the following rule of thumb:
  • For V-belt drives: 1/2 deg. = approximately 1/10-in. offset per foot.
  • For synchronous, 60-deg, angle, and V-ribbed drives: 14 deg = approximately 1/16-in. offset per foot.
Tips For Aligning Drives
     Dual plane drive alignment. The processes described above permit alignment checking in one plane only. Shafts may be misaligned in either of two different planes or both.
Parallel alignment. 
     Parallel misalignment is difficult to determine since an accurate common reference plane is not always available. Synchronous belt drives are checked by making sure the belt is not pinched between opposite flanges or does not track off any unflanged sprockets. If the shafts are horizontal, and one is located vertically above the other, a plumb bob or bubble level is used to determine if the sheaves or sprockets are in line with each other. A single V-belt could also be hung in an outside sheave groove from the upper shaft to indicate the proper position of the lower sheave.
Angular alignment. 
     After alignment and tension of a synchronous belt drive are set as accurately as possible a simple test is used to make sure the system is lined up properly.
     Carefully turn the drive over by hand and observe which direction and how fast the belt tracks toward one flange. The belt should move slowly enough that several revolution are required for the belt to move from one sprocket flange to the other. The drive should then be stopped and the rotation reversed. The belt should track in the opposite direction at about the same speed as before. Related components such as brackets and platforms should be checked for proper design and ability to withstand peak forces without flexing.



This article is provided courtesy of PTDA.

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