Neoprene vs. EPDM
Until recently, most belts were constructed of Neoprene material. Today, most serpentine belts feature EPDM construction, which stands for ethylene propylene diene M-class rubber. Visually, it's hard to tell the two types of belts apart, but EPDM belts far outlast and outperform any of their predecessors.
Older Neoprene belts have a life expectancy of 50-60K miles, and as they wear out, cracks and chunk-outs will occur, as shown below. EPDM belts rarely show these symptoms, even at very high mileage.
As EPDM belts age, they gradually lose rubber material—similar to the way tires wear out. Over a period of 100,000 miles, a belt can lose up to 10% of its rib material. While this may not seem like a lot, the consequences can be significant.
Belt wear after 100,000 miles
EPDM: Signs of Wear
The diagram below shows how EPDM belts wear as they age. Although the ribs have not become shorter, material has been lost in the valleys of the ribs (indicated in red), making the space between ribs wider. As more material is lost, the pulleys ride deeper into the belt valleys resulting in slip, noise and hydroplaning.
With sufficient material loss, the pulley ribs “bottom out” in the valleys and ride on the belt cord. This reduces the surface contact on the valley sides, where the traction is generated. Wear also increases the effective belt length, lowering the tension in the system, which also reduces traction.
Obvious Signs of Wear
Although EPDM belts do not tend to crack with age, they can still exhibit other symptoms that are caused by problems with the accessory drive—such as tensioner misalignment or failure, pulley misalignment, excessive heat, or bearing failure in one of the other components.
If the belt exhibits one or more of the symptoms depicted, it needs to be replaced. If it fails, it could damage other system components in addition to stranding the motorist. Many warranty-claim failures o alternators and other parts are actually caused by worn or improperly adjusted belts.
Other Belt Wear Symptoms
When a belt wears out, several problems can occur that reduce performance of the Accessory Belt Drive System.
BELT SLIP–LOSS OF TRACTION
Like a tire, a worn belt can lose traction (slip) on the pulleys, particularly in high-load and/or wet conditions. Slip can cause belt/pulley temperatures to rise by up to 50%—leading to early accessory bearing failure.
HYDROPLANING
This occurs when water cannot be effectively channeled away between a worn belt and the pulleys. The belt then “hydroplanes” on a film of water, resulting in loss of power transmission to the accessories. This can often result in the “Check Engine” or “Alternator Charging” warning lights to come on.
ELONGATION
Material loss can also cause a change in the effective length of the belt, which can move the tensioner beyond its take-up range. This will reduce overall system tension, also lowering accessory performance.
MISALIGNMENT
Misalignment wear can also be an indication that the internal components of the tensioner have failed. Material loss and subsequent changes to effective length on belts can also cause belt slip, resulting in noise, vibration and high heat, which can damage accessory bearings and cause accessories to fail.
