Alternator belts

[Dave Bebb]

Hi all a was just looking round the engine bay checking oil and and I thought I would check the alternator belts ,and both were very tight with no movement at all hardley ,so slacked them off a tutch now  I can push on the belts and they give about 1ins is that about right ,as I have read that it can cause the bearings to fail if belts are to tight .

 

[David Schweizer]

I have heard that advice as well, but I always kept mine tight, in order to prevent screamuing belt slippage when a high demand was called for by low batteries. In twenty years, I never had a bearing failure.

Edited October 11, 2017 by David Schweizer

[nicknorman]

17 minutes ago, Dave Bebb said:

Hi all a was just looking round the engine bay checking oil and and I thought I would check the alternator belts ,and both were very tight with no movement at all hardley ,so slacked them off a tutch now  I can push on the belts and they give about 1ins is that about right ,as I have read that it can cause the bearings to fail if belts are to tight .

 

It depends on how hard you push and the length of the belt run! An inch is perhaps slightly on the loose side but if it is not slipping, it's fine. Slipping is indicated by a squeal at low revs after starting with low batteries, and/or the appearance of fine black dust thrown from the belt.

[Machpoint005]

I agree with Nick. My engine alternator V belt slips at 1 inch slack, I keep it to half an inch or less if I can

[cuthound]

As others have said 1/2 inch on the longest run for a vee belt. With a flat multigroove belt you should just about to be able to twist the longest run through 90°.

Edited October 11, 2017 by cuthound
To unmangle the effects of autocorrect.

[john6767]

I would also say an inch on the longest side is probably too loose particular for the domestic alternator belt.  I believe the Beta engine handbook says1/2 and inch on longest side which sounds right to me.  For the domestics the key time will be when you start up in the morning and the domestic batteries are at their lowest.

[Dave Bebb]

Just been to our boat and started engine up no squeals of slipping belts and charging at 14.7 volts I will keep an eye on it should be going out for a day or two soon before weather gets to bad 

[Sir Nibble]

When I am presented with an alternator to test I obviously inspect it visually first and make sure it turns. One of the things I have learned to look for because it is so common is that reddish rust you get from friction of belt slip burning the paint/plating off the pulley and encouraging oxidisation. it's often the case that a "failed" alternator comes in like this and works fine on test. As for bearing failure, yes we are all told that over tensioned belts can cause bearing failure. I've never seen any evidence of it and to be honest it tends to be the slip ring end bearing that sh1ts itself big time. Drive end bearings do go of course but I don't see it often at all and I wonder if the old advice dates back to little 15mm shaft dynamos or screw down cup lubricated water pumps. There's a serious bearing in the front of a high output alternator.

[Machpoint005]

Glad to hear it. I would have hoped the alternator drive end bearings were always man enough for the job -- it could hardly be a surprise to the designer that the bearing would have to resist a fair old lateral force. Perhaps a case of traditional wisdom being as out of date as double-declutching on an all-synchromesh gearbox.     

[Tony Brooks]

FWIW

The other week I had a question about new belts from Barrus that do not fit properly. It seems the Barrus preferred supplier stopped making the belt required and supplied one of a different profile. The boater got the correct size from Alf at Beltingonline.

Might help someone.