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Why do engines use belts and not chains?

12 January 2019

Would an engine be stronger or better if it used a chain instead of a serpentine or fan belt?  Some engines use timing chains instead of timing belts, and chains are stronger - a timing chain shouldn't need to be replaced for the lifetime of the engine, while a timing belt definitely needs replacing and this can be costly.

No engines use chains to drive the alternator or AC pump.  Why not?  You'd think it would be less maintenance but it's not that simple.  Here are the reasons why chains aren't used everywhere in engines, and why the synthetic rubber drive belt is king.

Noise

Chains rattle: It's that simple - even a perfectly tensioned chain is noisier than a belt.  Adding more chians to an engine would add engine noise.  

Cost

A typical serpentine belt costs around $15.  While the cheapest timing chains are around $22.  That's not a huge difference, but there's also extra cost in using sprockets instead of pulleys, and keeping a chain lubricated.

Lubrication

Chains need to be lubricated and belts do not.  This adds complexity - both in getting the oil onto the chain, and keeping that oil from spraying all over the rest of the engine bay or, even worse, the road.

Pulleys are more reliable

The pulleys that are used to drive accessories like the power steering pump or alternator would need to be replaced with sprockets.  And pulleys deal with debris in the engine bay much better than sprockets.  Sprockets can also wear, while pulleys tend not to.  

Chains are not flexible

With today's super compact engines, belts snake their way around tight spaces and make sharp turns.  Not only is a chain deeper than a belt, but it can't make the sort of tight turn needed to wrap around all the engine components.  

Keeping the tension

Tensioning a chain is harder than a belt.  A typical belt is tensioned with a spring - being light this is easy enough.  But a chain requires a greater amount of adjustment in the way it is tensioned - it tends to expand as it heats up.  Chain tensioners are usually hydraulic - operating off the engine oil. 

Inertia

Chains weigh more.  This isn't a problem when an engine is up to speed - the chains momentum will carry it around.  But when changing engine speeds, this inertia will lead to a less responsive engine.

So why are chains used?

Chains are used where lots of force needs to be transmitted reliably.  A chain is very, very strong and that's why they are used to carry large forces from an engine - for example in a motorbike.  So there's a long life ahead for chains...just not in the engine bay.

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