Belt And Gear Driven Accessory Vibrations

Belt-Driven Accessories 

The vibration order of belt-driven components is not usually a simple multiple of engine RPM. Example: An alternator may rotate 2.8 times faster than the crankshaft. This would result in a 2.8-order engine-speed-related vibration. If the engine speed were 1000 RPM, the alternator speed would be 2800 RPM.

Belt-driven accessories can also cause engine speed related vibrations. Belt-driven accessory speed related vibrations can be isolated by removing the drive belt where possible. These vibrations are usually caused by:

• An out-of-balance or out-of-round pulley, pulley shaft, or the part the pulley shaft turns.
• A dirty pulley or excessively thick paint on the pulley.
• Failed or failing bearings or gears in any of these belt-driven components could also cause an engine speed related vibration.
• The drive belt itself can cause a vibration if it is coming apart.

Removing the drive belt can isolate all of the pulleys from the engine. If the vibration is gone with the belt removed, either the belt or one of the belt-driven accessories is the cause of the vibration.

Gear-Driven Accessories 

The vibration order of these gear-driven components is not usually a simple multiple of engine RPM. Example: A water pump may rotate 1.5 times faster than the crankshaft. This would result in a 1.5-order engine-speed-related vibration. If the engine speed were 1000 RPM, the water pump speed would be 1500 RPM.

Gear-driven accessory speed related vibration can be difficult to diagnose because it often requires disassembly of some components. Gear driven accessories can also cause engine speed related vibrations.

These vibrations are usually caused by an out-of-balance or out-of-round gear, pulley shaft, or the part the pulley shaft turns. Uneven gear tooth backlash or clearance can also cause this type of vibration. Failed or failing bearings or gears in any of these gear-driven components could also cause an engine speed related vibration.