belt tension

[Guest 000]

Tensioning an alternator/water pump belt should be simple....shouldn't it?

As far as I can figure it, the rule of thumb is that there should be 10mm of deflection on the longest section of V belt between pulleys. This seems a bit rough and ready because surely the amount of belt deflection must be dependent on the distance between pulley centres. For example, because of my particular engine setup my alternator is very close to the engine ie short belt length between pulleys whereas the freezer compressor has a long distance between pulleys. They can't both have the standard 10mm give. There must be a formula for calculating belt deflection based on pulley centres but unfortunately I don't know what it is. I don't even know if its important to be so fussy. I broke a V belt last summer and didn't enjoy it much, so I want to do it right.

[erice 732]

there's a pretty big range between so tight it broke

 

and so loose it wouldn't turn everytnig and lashed around

 

90 degree twist at mid of longest run seems as reliable and as easy as anything

 

[Guest 000]

Not bad.

The video shows a flat ribbed belt. I'm running An 11mm V belt.I wonder if the same rule applies?

[Island Time 1,286]

Yep. it does

[wheels 544]

There is a way to work it out and it is complex. There are calculators that work out what you can drive re Hp and then belt type and section and so on. But it is easy for simple applications like the Alternator belt to simple use the 10mm rule. A good belt should not stretch much, so you will find that a bit of movement either way of 10mm is close enough. The belt tension would not have been what broke the belt. It would have been wear or a cheap belt. The biggest issue and reason for correct tension is so you don't load bearings too much.

[wheels 544]

I should add, a belt drives by it's edge, not the bottom face. Making sure you have a proper section to fit the pulley is important.

[Guest 000]

OK, I reckon I'll go with the 90° twist method. Wheels points out the importance of running the right sized belt - well I am now. The boat came with a 9mm belt and I never really looked at it until it shredded. Should have been an 11mm. Two years ago I had the alternator rebuilt and just today I reinstalled the water pump after a rebuild. Both had rattly bearings. Hence my preoccupation with correct belt placement.

[Guest 000]

And just found some info that suggests a belt deflection of 1/64th of an inch for every inch of length between pulley centres.

It's in REAL measurement so it must be good!

[Island Time 1,286]

I'm amazed that the old belt lasted that long! When they bottom out in the V, they normally break pretty fast! As wheels said, they should contact only on the sides. I bet this new one lasts a LOT longer!

[John B 108]

If you ever had the joy of owing a mid '90's v8 holden like I did, the holden tech way of belt tightening was to put a 3 ft long crow bar on it and tension it until it sang a Roy orbison song. I queried it, but it was 'the fix' for an engine at the end of its life, overloaded with junk it never had originally and it was the only way they could stop it squealing.

 

But won't it run water pump and alternator bearings ? I said.

 

OOOO nooooo said Mr Holden.

50 k . waterpump and alternator bearings toast.