Checking VMAC Belt Drive Systems

Checking for belt misalignment and belt drive design requirements.

VMAC compressor systems use serpentine belts, also known as a micro-v, poly-v or multi-rib belts, which are continuous rubber belts with k-type cross section typically with 6 ribs but can vary between 4 and 8 ribs. Belt drive systems are designed to take into account many different requirements to allow continuous smooth running with minimal maintenance. Some of these requirements are:

  • Acceptable belt load and life.
  • Acceptable bearing load and life.
  • Consistent belt tension
  • Routing belt between driven and driving components
  • Minimizing engine crankshaft side load.
  • Quiet operation
  • Correct belt alignment

Serpentine belts are commonly used for their power transmission ability and flexibility of design when routing around engine components.  By using idler pulleys, the belt routing can be designed to clear OEM engine components as well as keeping the belt span between pulleys to a reasonable length. This helps to prevent “belt flap” and tensioner “bounce”.  Idlers can also be positioned to obtain the desired angle of wrap around the drive and driven pulleys. The use of an automatic spring loaded belt tensioner ensures the correct amount of belt load necessary for power transmission and system durability and eliminates the periodic tension adjustments required on v-belt systems.

Serpentine belts need to run in close alignment, with no more than 0.5° of misalignment between pulleys. Maintaining this alignment is primarily dependent on idler bearing wear and tensioner pivot wear or damage.  Belt misalignment will typically cause the serpentine belt to become damaged (usually front or rear edge) from the belt jumping a rib off one or more of the ribbed idlers. Worst case would be for the belt to come off the pulleys entirely or break which could subsequently damage other engine components.


To check for misalignment.

  • Run the engine and listen for any belt squeal or chirping as this is often evidence of slight misalignment, belt tension too low or components seizing
  • Inspect the alignment visually by sighting down the belt line. Any serious variation in pulley alignment can often be noticed this way
  • Check the belt tensioner pivot point with the belt on by slowly removing tension on the belt and look for any twisting movement between the arm and base.
  • Remove the serpentine belt and repeat the same check with the belt off and the tensioner resting against the stop
  • Inspect the ribs of the belt for damage such as fraying, missing ribs, chunks, cracks or oil contamination.
  • Check all the idlers for any wear/play in the bearings or damage to the idler run surface. Some small movement is normal due to bearing clearance.
  • Check the idler bearing seals for signs of grease staining. This may indicate the seal or bearing is worn and bearing grease has leaked out. If this has occurred bearing failure is usually not far behind. Don’t forget to also check the idler on the tensioner.
  • Check the crank pulley for any signs of damage, run out, poor fit or loose fasteners.
  • Check compressor clutch for misalignment, bearing wear, damage or loose fasteners.

If any components require replacement, always replace the serpentine belt at the same time. The belt will likely have suffered damage while it was running misaligned. After servicing has been completed run the system without any load for 10 minutes while varying the engine rpm to let the belt settle in.

Once running unloaded is satisfactory, apply load by turning on and operating the compressor system. Observe the belt tensioner arm as load comes off and on; any movement should be relatively smooth. Allow the engine to elevate the rpm a few times and ensure the belt operation is smooth and the belt is noise free.

As a final check, road test the vehicle and listen for any noise or vibration at highway speeds and especially during shifting.

With proper care and attention, you will find that your serpentine belt driven compressor system will give you years of trouble free-service.

If you have any questions about this article or anything mobile compressor related, please contact us.