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Serpentine belt versus timing belt

Serpentine belt versus timing belt — What’s the difference?

Serpentine belt versus timing belt — what they do

A serpentine belt drives accessory components like the alternator, water pump, power steering pump, smog pump, AC compressor. A serpentine belt is driven by the harmonic balancer located on the front of the engine. It is tensioned by an automatic tensioner.

A timing belt is mounted at the front of the engine and covered by a timing belt cover. It is used to rotate the camshafts in time with the crankshaft. In some applications the timing belt may also drive balance shafts in time with the crankshaft and camshafts. In still other applications, the timing belt may be used to drive a water pump. Timing belts are tensioned with spring loaded or hydraulic tensioners.

Serpentine belt versus timing belt — the implications if they wear or break

If a serpentine belt breaks, your engine will not longer generate power, cool the engine, run the hydraulic power steering or operate the AC. If you continue to drive with a broken/missing serpentine belt, your engine will most likely overheat.

If a timing belt breaks while you’re driving, your engine will stop immediately stop running. If the engine is an interference style the valves will hit the pistons if the belt breaks while running. That will bend the valves and most likely destroy the engine. That’s why timing belt replacement is so important.

Serpentine belt definition

serpentine belt tensioner

Automatic serpentine belt tensioner

To qualify as a serpentine belt, the belt must drive engine components from both the ribbed and backside of the belt. All serpentine belts are tensioned with an automatic belt tensioner that cannot be adjusted.

Difference between serpentine and polygroove belts

A polygroove belt looks just like a serpentine belt with multiple “V” shaped ribs on the drive side. However, polygroove belts are designed to drive components only from the ribbed side of the belt. Except in “stretch applications” polygroove belts are tensioned with a mechanical tensioner and the tension must be adjusted properly by the installer.

stretch belt tool

Stretch belt tool

A stretch polygroove belt does not need a tensioner. It is designed to stretch over the pulleys during installation using a special tool. It is designed for one-time use. Once it is removed, it cannot be reused, even if it is not worn out.

Serpentine belt construction

A serpentine belt is constructed using Ethylene Polypropylene Diene (EPDM) and other high-tech materials. EPDM is heat and crack resistant and provides higher resistance to vehicle fluids and oxidation than older neoprene elastomeric materials.

The belt is constructed with stretch resistant embedded tensile cords, usually made from aramid. The multi-ribbed side of the belt incorporates multiple “V’ profiles that mate with corresponding channels in the components pulleys. The “V” profiles provide the “traction” to rotate the driven components.

The flat backside of the belt has an embedded fabric material to drive flat pulleys and resist wear

serpentine belt construction


Example of a serpentine belt driving engine components from both the ribbed and flat sides of the belt
engine with serpentine and stretch belt

Timing Belt Purpose

A timing belt is a cogged belt designed to rotate the camshaftstiming belt in exact time with the crankshaft. The cogged side of the belt mates with corresponding cogged pulleys connected to the crankshaft, balance shaft(s), camshafts and idler rollers. The flat backside of the belt may be used to drive the engine’s water pump in some applications. Timing belts are tensioned with a spring loaded or hydraulic tensioner. Once set to the correct tension, the tensioner maintains that tension for the life of the belt.

Timing-belt-in-oil design

Carmakers have been under pressure to eliminate timing belts timing chain in oilfrom their engines due to the high cost of replacement. That has forced them to switch to timing chains in many applications. However, timing chains are not trouble-free. They’re extremely susceptible to stretching if the owner doesn’t adhere to the recommended oil change intervals. Even when new, timing chains make more noise than a traditional timing belt. When a timing chain wears, it can make even more noise

Enter the timing-belt-in-oil. This can be a single cob or double cog design. The belt is made from an oil resistant elastomeric material that performs the same job as a timing chain.

subaru timing belt routing

Subaru timing belt routing

Posted on by Rick Muscoplat

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