Tuesday, August 3, 2010

Cam Belts

One of the silliest things the motor industry ever did, if you exclude customer-driven stuff like external sun visors & over-wide tyres, was to introduce belt drive for camshafts.

Previously, camshafts were driven by chains.
I don't think I have ever heard of a camshaft chain failing.

Starting slowly in the '60s, more & more manufacturers were attracted to internally-toothed rubber belts, instead of chains, to drive their camshafts.
Potential advantages included cost, weight, noise, packaging & sealing, though these advantages were not always realized.

The disadvantage is that rubber belts have a finite life.
And the finite life depends heavily on operating conditions, notably hot & cold temperatures, contamination by oil/dust/mud/stones, unscheduled overloads, long periods of non-use, etc.
And belt failure is always serious, usually very expensive & potentially fatal.
As a minimum, the car suddenly stops, which can be less than funny in the fast lane of a busy highway, or in Siberia or Death Valley.
Usually, the engine is severely damaged or destroyed, as pistons & valves crash.
See heading picture, from: http://www.robertsautorepair.com/faq.html
With older cars, that means not worth repairing.
A few engines have been designed so that cam belts can fail without pistons hitting valves, so a new belt can be fitted with no other costs, but that requires design compromises which are expensive or impossible with today's emissions legislation.

In view of the serious consequences, and after numerous court cases, manufacturers have had to recommend belt changes at intervals which should avoid any failures.
But because belt life is so heavily dependant on so many factors & hence so variable, the change interval is a sorry compromise.
Often 5 years or 60,000km, but sometimes 10 years or 240,000km.
Frequent enough to be a significant maintenance cost for most owners.
And a waste in 99% of cases.
Still allowing a (very, very small) number of motorists to suffer a failure inside that period.
And reserving nasty surprises for the forgetful or uninformed.

I think there is now a move back to chains.
There should be.

Parting thot: "The only reason for time is so that everything doesn't happen at once." – Albert Einstein

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