Alternator Belts: does "wrap" matter?

[nigelmercier]

I've got a problem with an alternator belt: it keeps shedding rubber and breaking. It's on my 12V 110A alternator which produces a large load.

If you look at the above photo, you see that the 12V alternator is on the same belt as the water pump. This means that the “wrap” around the 12V alternator is only about 120 degrees. Contrast with the 24V alternator, which has a 180 degree wrap. Most of the load is on the 12V alternator, so it would seem sensible to move the water pump belt so that it shares with the 24V alternator. Obviously this depends on our ability to move the alternators or pulleys fore and aft to align to the new belts.

Note that I've considered other options, such as: 12mm belts, twin belts, alignment etc. The purpose of this post is to get opinions on the validity of the above only

 

[Marsupial]

Alternator is too big

your alternator @ 110A is too powerful for a single vee belt, you need to consider a different drive arrangement which will mean changing at least three pulleys.

 

[nigelmercier]

your alternator @ 110A is too powerful for a single vee belt, you need to consider a different drive arrangement which will mean changing at least three pulleys.

Sigh! I am not looking for opinions on this criterion, see last paragraph. The 110A alternator is using the pulley it was fitted with by the manufacturer, I assume they knew what they were doing.

 

[Marsupial]

Sigh! I am not looking for opinions on this criterion, see last paragraph. The 110A alternator is using the pulley it was fitted with by the manufacturer, I assume they knew what they were doing.

bigger sigh! NO THEY DID'NT! pulley wrap has nothing to do with the fact that a standard VEE belt is going to be over spec if you put a 110A alternator on it!

 

[VicS]

Fit a spacer behind the pump pulley to bring it in line with the pulley on the 24 volt alternator. Fit a longer belt to that BUT you would have to be careful not to over tension and put too much load on the waterpump bearings.

Fit a shorter belt between just the crankshaft pulley and the 12 volt alternator.

if you are not happy with the spacer idea on the water pump the you will have to make new mountings to move the 24 volt alternator back if there is room behind it and the 12 volt alternator forward.


Or would it be possible to swap the positions of the 12 and 24 volt alternators.

yet another sigh,and no comment on more sensible solutions as you have asked for those not to be considered

Even the 55 amp alternator on my car has twin belts!

 

[nigelmercier]

Fit a spacer behind the pump pulley...

Yes, this is what I had in mind, but wasn't 100% sure it would help. Any idea where I can get a spacer?

I'm open to other suggestions of this nature, just not those I have considered already (as listed in my original post).

 

[Marsupial]

Nigel,

with the best will in the world, what ever you do that involves a single vee belt WONT WORK!

 

[earlybird]

I would agree with the others. For fairly small changes, power capability will be roughly proportional to the change of the wrap angle. Judging from your picture, 180 deg will only give 25% or so improvement. Also, a spacer behind the water pump pulley will load pump the bearing more.
IMHO, a 10mm belt is way too small for your alternator.

 

[nigelmercier]

Power capability will be roughly proportional to the cosine of the wrap angle.

Good some maths, and I guess you mean (wrap angle)/4 to limit it to a quarter of a wavelength. But surely the cosine decreases as the angle increases towards 90 degrees?

 

[VicS]

Any idea where I can get a spacer?

Either make one if you have the workshop facilities or get one made by an small engineering firm to your dimensions and specifications.

If you do this and the waterpump bearing fails don't say you have not been warned

 

[Salty John]

You need to transmit about 4 HP. Very borderline for any single belt, even a Gates X or VX range, on that small pulley. At least get the best belt you can, perhaps a Gates 5VX, before you try messing around with belt wrap. You might want to find a belt selection guide and see what you need to transmit 4HP (1HP per 25 amps) with that size pulley at the speed at which the maximum load comes on.
I think you are in danger of damaging the pump bearing if you create too much overhang or use excessive tension to try and overcome what is, actually, a problem of borderline belt rating.

 

[nigelmercier]

I think you are in danger of damaging the pump bearing if you create too much overhang or use excessive tension to try and overcome what is, actually, a problem of borderline belt rating.

Ideally I would like to move the alternators such that one belt powers the 24V alternator and water pump from the original larger (green) pulley, which leaves the 12V alternator and the smaller retrofit (silver) pulley for me to play around with. This would also help the 12V alternator revs a bit, as it is currently running at 3x engine speed, and the rev counter is reading high by about 15%.

I may indeed add twin belts after this stage, but first I wanted to establish if the interim solution was a sensible upgrade; hence my reluctance to discuss it originally

 

[pvb]

From my experience, moving the alternator bracketry could be problematical. Would it be possible to simply swap the alternators?

I'd agree with the suggestion that you first get a high-quality belt, and Gates are some of the best (although the 5VX suggested is a multi-V which obviously wouldn't be suitable). My second alternator is a 90A Leece-Neville and the Gates belt I use has extended belt life and reduced belt slip.

If you only use the 24v alternator for the bowthruster, you could replace it with a 12v unit which would increase domestic charging efficiency and reduce the load on your existing 12v alternator. Then charge the 24v thruster batteries from the 12v supply.

 

[Salty John]

No, a Gates 5VX is a moulded notched vee belt.

 

[Salty John]

It's the section I'm referring to. Obviously you would use a notched vee belt on this application.
For example:http://www.royalsupply.com/store/pc/5VX-Super-HC-Molded-Notch-Belts-c1065.htm

 

[pvb]

No, a Gates 5VX is a moulded notched vee belt.

Sorry, you're right, I was thinking of the 5VX Powerband. But the 5VX appears to be 5/8" wide, which isn't going to work too well on a 10mm pulley.

 

[DownWest]

Since the water pump requires very little power, might you not fit an extra pulley on the front of the crank and a longer spacer to the water pump and a third belt. Tension could be by a pulley with spacers between the halves(a la VW Beetle) or an idler. At least it would take the w/p out of the tension equation.
A

 

[boatmike]

I've got a problem with an alternator belt: it keeps shedding rubber and breaking. It's on my 12V 110A alternator which produces a large load.

If you look at the above photo, you see that the 12V alternator is on the same belt as the water pump. This means that the “wrap” around the 12V alternator is only about 120 degrees. Contrast with the 24V alternator, which has a 180 degree wrap. Most of the load is on the 12V alternator, so it would seem sensible to move the water pump belt so that it shares with the 24V alternator. Obviously this depends on our ability to move the alternators or pulleys fore and aft to align to the new belts.

Note that I've considered other options, such as: 12mm belts, twin belts, alignment etc. The purpose of this post is to get opinions on the validity of the above only

I have total sympathy with your question and the fact that you are seeking a very specific answer. Therefore let me say that with regards to other posts the insistence that a single belt will not drive a 110A alternator is entirely wrong. While I could give you chapter and verse on the calculations I won't as you have not asked for this. I have a similar setup myself though and while twin belts would be better, a single belt is totally adequate if maintained correctly.

Wrap angle does in fact matter too, and your suggestion to change would indeed improve matters. However, from your photo it appears that the belt is riding high on all pulleys except the alternator. Do bear in mind that there are in fact several belt widths and also both metric and imperial standards. When I first fitted mine with the existing belt I missed this and the belt slipped because it was very close to the bottom of the vee and very quickly bottomed out. Belts need to be well clear of the bottom and grip on the flanks only. Sorry if this sounds obvious but it's very easy to assume the pulley is the same width as the others. Mine wasn't. It was about 1mm wider. Yours looks in fact identical to mine and you may have the same problem! Please at least check....

Edit. If that's not entirely clear, I believe your Alternator is a Prestolite. If so it's American and has an imperial sized pulley. Your engine pulleys are probably metric widths as mine are. The solution in my case was to remove the existing pulley and fit the one off my old alternator...... Simples!

 

[sarabande]

The 12v alternator pulley looks a bit small. What about getting a larger diameter one:-

1 to increase belt contact area
2 to decrease rpm.

 

[2Tizwoz]

Lots of suggestions are put forward. Your original question concerned the wrap around the pulley and that is one of the limiting factors.

An extra pulley between the water pump and the alternator could give more wrap.

http://www.roymech.co.uk/Useful_Tables/Drive/Vee_belts.html This is a link to a website with v belt design tables.

Have you considered putting a larger pulley on the alternator? You would effectively reduce the speed of the alternator as well as increasing the belt life so a calculation of the output of the alternator at the reduced speed would be necessary. The ratio of the pitch circle diameters of the pulleys would give the speed reduction. You might get one from a car scrap yard.

Which belt have you in place at present? Could you replace an existing 'Z' section with an 'SPZ' section? The smallest recommended pulley which the deeper section belt would run around would be larger with the 'SPZ' belt.

How worn is the small pulley?

http://www.directbearings.co.uk/downloads/pix_belt_manual.pdf This is a link to a manual which gives more information enabling production of a design life.