A bit of mathematics, or lateral thinking ...

[nigelmercier]

This is the general layout of my 12V alternator belt, with the alternator pulley, Adjuster and Pivot on the right:

The belt is currently 975mm. If I increase it by 25mm to 1000m, my question is what will the movement be:

[1] Of the bolt along the adjuster
[2] Of the pulley centre to the right

 

[RobbieW]

If the adjuster arm is fixed (unlikely) then both A and P will change. If the adjuster arm changes angle at its pivot, A may stay in the same place if you want it to and P will change. No ??

 

[Dave100456]

http://www.bbc.co.uk/bang/handson/teaser-string.shtml

The above will lead you along the correct route. In respect of the pivot relocating to a new position, simple locus drawings will allow you to determine these.

 

[reeac]

Re. Question 2, if you regard the extra 25 mm. Of belt length as being shared equally between the two belt runs to and from the alternator then that's 12.5mm. Added to each run with the angle between the runs being (by my measurement) 40 degrees so that's 12.5 cosine 20 of movement along the centre line I.e. Ca. 11.7mm. That's slightly in error due to changes of belt wrap around the pulleys but those errors will be very small. Question 1 requires knowledge of more dimensions than are given. You could get an idea by physically adjusting the alt. pulley to be 12mm. CLOSER and seeing how much bolt movement that entailed. Not accurate but would give a ballpark figure.

 

[theoldsalt]

The alternator pully will move 12.5mm further away from the other two pullies in an arc with a centre on the alternator pivot. Just redraw the diagram to measure the adjuster movement (easier than using trig to calculate)

 

[CreakyDecks]

It depends whether your proposed new belt is the same width as the old one

 

[Lakesailor]

They'll move a bit. Just do it.

 

[Spirit of Glenans]

You could get a piece of cord and tie it into a loop of the required dimensions, take off the existing belt and loosen out the adjustment to take up the slack. IMO this would give a fairly empirical answer to the questions.

 

[Rowana]

Why do you need to move it in the first place?

If it ain't broke, don't fix it!

 

[Lakesailor]

You could get a piece of cord and tie it into a loop of the required dimensions, take off the existing belt and loosen out the adjustment to take up the slack. IMO this would give a fairly empirical answer to the questions.

That requires the cord sits at the same depth in the pulley as the belt.

 

[ghostlymoron]

You need dimensions on your drawing to calculate it by trig. As suggested by Lakesailor 'JFDI'
Why do you want to do it anyway?

 

[Graham_Wright]

You need dimensions on your drawing to calculate it by trig. As suggested by Lakesailor 'JFDI'
Why do you want to do it anyway?

There's a prize for the best answer.

 

[prv]

Why do you want to do it anyway?

Presumably he has a source of 1000mm belts that are cheaper than 975mm ones.

Pete

 

[nigelmercier]

Why do you want to do it anyway?

Because I want to increase the distance at the adjuster, but not so far that the pulley moves out by more than 30mm. The belts are sold in 25mm increments.

 

[CreakyDecks]

The centre of the alternator can only rotate in a circle about P. If it rotates about 13mm then it will move vertically by only a couple of mm, meaning that the two parts of the belt pointing towards the alternator will both increase by about 13mm. I'd say your alternator will move out about half an inch in real money.

 

[nigelmercier]

The centre of the alternator can only rotate in a circle about P. If it rotates about 13mm then it will move vertically by only a couple of mm, meaning that the two parts of the belt pointing towards the alternator will both increase by about 13mm. I'd say your alternator will move out about half an inch in real money.

Lateral thinking, good. In fact it may not move much at all vertically, as the pivot may be a bit more under the centre than my drawing.

I believe that if the alternator moves by, say, 13mm outwards, then the adjuster bolt will move more than this.

 

[Lakesailor]

In a manufacturing environment where the next 200,000 units would be affected by the outcome it would be wise to work this out mathematically.
On a single boat with a single engine just fit the bugger.

 

[VicS]

I believe that if the alternator moves by, say, 13mm outwards, then the adjuster bolt will move more than this.

Yes it will. If your diagram is accurately to scale then, because the distance from P to A is slightly more than 1½ times the distance from P to the alternator centre, A will move slightly more than 1½ times the distance that the alternator centre moves.

 

[GHA]

Download sketchup and draw it

 

[cliff]

Pivot point P to alternator is fixed length. Increasing the length of belt by 25mm will result in more of the 25mm being taken by the top section of the belt as the alternator swings to the right to take up the slack. It is all to do with angles but at a guess I would think more than ½" but less than ¾" to the right. (Pully cannot rise vertically as much as it can move to the right. as for the other bit - insufficient information - as LS said JFDI.