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Not too long ago, I was called out to balance a supply fan on an air handler at the local zoo.  I arrived on site and immediately noticed the heavy vibration coming from the air handler.  I collected some vibration readings, shut the machine down, and locked it out.  After looking through the vibration data and some visual inspection, I was able to rule out unbalance as the primary source of the vibration.  The motor and fan bases on this belt-driven machine were in rough shape: loose and missing hardware, isolators worn and out of adjustment, and machine mounted to a weak foundation.  These issues certainly made the vibration worse and needed to be addressed, but they were not the source of the vibration.

After performing a number of tests and checks, I found that the vibration was caused by a combination of sheave misalignment and an overly tensioned belt. After speaking with the customer, I found out that the belt and sheaves had just been installed the day before!  Unfortunately, many millwrights and mechanics fail to have the training, tooling, or desire to properly align and tension a belt-driven machine.  Taking the time to understand how and why to perform sheave alignment and how to properly tension a belt drive can save thousands on repair costs, increased efficiency, and unscheduled downtime.  In addition, reliability is especially critical when in a medical or veterinary application, such as was the case in the example above.  Below we will discuss how sheave alignment and belt tension affect the machine, as well as some best practices to get the job done and #keepitrunning.

Effects of Poor Sheave Alignment and Belt Tension

1. Belts Too Tight: An overly tensioned condition will cause excessive radial load on the shafts and bearings, as well as an added electrical load on the motor. It will lead to misalignment during operation (deflected shafts), decreased motor life, decreased shaft life, premature wear on bearings, and an overall loss of efficiency.
2. Belts Too Loose: A loose condition will cause belt slippage, and in turn, overheating. It will lead to a significant loss in efficiency, premature wear on belts and sheaves, and, through heat transfer, can decrease the life of the bearings and motor.
3. Sheave Misalignment: Sheaves out of alignment will cause many of the same issues as an improperly tensioned belt, including overheating, slippage, and excess radial load. More specifically, misaligned sheaves will cause heavy vibration, as well as premature and uneven wear on the belts and sheaves.

Best Practices for Sheave Alignment and Belt Tensioning

• Sheave alignment and belt tensioning should be done somewhat simultaneously. The alignment process will affect the belt tension and vice versa.  Don’t assume it’s in spec, check it!
• Install sheaves as close to the bearings as possible or reasonable. This reduces the load on the shaft and bearings by reducing the potential for shaft deflection.
• Check shafts and sheaves for runout before performing the alignment. Runout can cause many issues, including making alignment and belt tensioning more difficult.
• The closer the span between the motor sheave and the driven sheave, the more sensitive the machine will be to misalignment or improper belt tension. A longer span will be more tolerant.  Machine speed will also play a factor. The higher the RPM, the more wear over a shorter amount of time.  Keep this in mind.
• Check sheaves for wear using a sheave gauge.  Gaps 1/16″ or larger indicate a sheave is worn and should be replaced.  A belt will have a hard time gripping a worn sheave, which will translate into slippage and excessive heat.

• Check belt tension using a pencil gauge (with corresponding belt tension spec chart) or a frequency meter. Both methods are accurate and easy to perform if done correctly. In general, you should tension belts as follows: using a spring scale, press down on the belt in the approximate center of its span (on the tight side), to deflect the belt 1/64″ per inch of span length and observe the force required to do so. If you are not sure of the belt span length you may also use the center-to-center distance of the pulleys, which will be similar. Tension the belts until the force required for this deflection equals the belt manufacturer’s maximum recommended force values for the specific belts you are using. Make certain this force does not exceed the machinery’s design loads. The force values for all belts should fall within 10% of each other.
• You can align the sheaves using a cord, but the quickest and most accurate way is a laser pulley alignment tool such as the Easy-Laser XT190. The XT190 works for all types of belt drives, as well as chain drive applications.  The benefits of seeing offset, angularity, and twist simultaneously on the screen, while being able to make live adjustments, makes sheave alignment with the XT190 quick, easy, and accurate.

Does It Make a Difference?

In the scenario above the difference between a heavy vibration and a smooth running machine was a slight alignment adjustment and 2 lb. reduction in belt tension.  That’s it!  When it comes to installing and maintaining industrial machinery, the devil is in the details.  Don’t assume it’s right, know that it is using knowledge and the proper tools available.

Download our 5-Step Sheave Pulley Alignment Procedure for a simple and effective procedure for sheave pulley alignment of belt-driven equipment!

Good Practices for Sheave Alignment on Multi-Belt Drives