Engine bay belts - who, what, when and why?

[widdowson2008]

May be wrong, but I don't think so - prepared to get shot down on this, but......
A belt drive relies ENTIRELY on the friction between belt and pulley, and that is entirely dependant on the contact area between the two.
If you take a look at the diagram, you will see that the 'wrap' over the water pump pulley is less than 90 degrees due to the limited space available and the close proximity and sizes of the pulleys, so the contact area at the pump pulley is relatively small.
To increase the frictional area required to drive the pump, the only option available to the designer is to double the number of belts.

Agree with munroman when he says 'replacing all your drive belts'. Particularly important when there is more than one on a particular pulley for the following reason - as belts are used, they 'stretch' as can be seen from the table showing belt deflection. A new belt will deflect between 7 and 9mm, but has a limit of 16mm. This is belt stretch in action. When the belt has reached the 16mm mark, the correct tension (and hence friction) cannot be achieved. If you put a new belt alongside an old one, yes they will work, but the old stretched one will not provide as much friction as the new one.

Haydn is right - it is the sides (V) that provides the drive, and it's the wear on the sides (+ a bit of stretch) that causes them to go slack and need re-tensioning.

[missfixit70]

Hi widdowson, I wondered about air con, but it still seems to be working OK. Scanner - I think you're onto something here! I'm going to hunt out some belt diagrams to see what belts run over what pulleys.

And as to "don't ask me why" it has two belts - so it keeps on working when one breaks???

Cheers
Dom

A Bongo has 3 belts - 2 identical and running side by side for the alternator and one different one for the air con.

As to why 2 for the alternator and me not knowing why - yes you are right, if one goes the other keeps things working.

But WHY? - other cars don't have 2, so why does the Bongo? and if one that size isn't strong enough, why not fit a stronger one like other cars?

The old Volvos used 2 belts too, I always thought it was a failsafe, so that if one snapped the other was still working, so that the vehicle could still keep going, and with a working heater, which could be a lifesaver in cold weather, given Japan has similarly cold weather in certain areas I thought the logic may have been similar.

More importantly it drives the water pump, sod the heating, no water pump = cooked engine in no time flat