want a good belt

[chris w]

Peter

 

I take this to mean that the forces should be borne totally on the sides of the belt and the teeth should be kept clear of the pulley and they should not bear any load on their inner surfaces.

That's my understanding.

 

Chris

PS: The "teeth" are there to make the belt more flexible not to aid in gripping the pulley.

Edited November 18, 2006 by chris w

[Big COL]

Our Vetus certainly hammers drive belts. I would question whether the belts fitted on many engines are up to powering 90Amp plus alternators. The Vetus has just one 3/8inch drive belt. I'll probably have Vetus on my case again for posting that

 

 

Hi Catweasel

 

This probably applies to a lot of situations when the design is at its limitations. The problem with rubber vee belts is that when they warm up they start to melt bits start to stick to the pulleys and then go to a self destruct stage.

To lengthen the life of the belt I would use a Kevlar belt. the type used on ride on mowers and are capable of absorbing belt slip without destroying themselves. For this reason belt tension is critical to obtain the desired grip for the normal output, however should the belt start to slip the only thing that will happen is the output will fall off at this point you readjust the belt instead of replacing it if it was a rubber type. I understand the Kevlar belt has a chalk compound in its construction this is what allows the belt to slip without destroying it.

I await the experts to contradict this.

[anhar]

Hi Catweasel

 

This probably applies to a lot of situations when the design is at its limitations. The problem with rubber vee belts is that when they warm up they start to melt bits start to stick to the pulleys and then go to a self destruct stage.

To lengthen the life of the belt I would use a Kevlar belt. the type used on ride on mowers and are capable of absorbing belt slip without destroying themselves. For this reason belt tension is critical to obtain the desired grip for the normal output, however should the belt start to slip the only thing that will happen is the output will fall off at this point you readjust the belt instead of replacing it if it was a rubber type. I understand the Kevlar belt has a chalk compound in its construction this is what allows the belt to slip without destroying it.

I await the experts to contradict this.

These are the kind of belts used on the final drive of Harley Davidson motorcycles so you can imagine the harsh forces at work, yet they typically last for several tens of thousands of miles. However unlike engine belts used to drive alternators on boats etc., the Harley ones are toothed and engage sprockets at the gearbox and back wheel.

 

If belts of this material are available for boat engines, then it seems to me that their life would be particularly long and happy.

 

regards

Steve

Edited November 18, 2006 by anhar

[John Orentas]

John and Chris,

 

I take this to mean that the forces should be borne totally on the sides of the belt and the teeth should be kept clear of the pulley and they should not bear any load on their inner surfaces. The belt on the boat when we got it was quite narrow and I replaced it with like for like but from Halfords and it was sitting deeply in the pulleys - the new one is better but looking at the earlier link to an industrial supplier a wider belt may help to at lengthen belt lives by keeping the teeth away from the pulley but perhaps not cure the problem.

 

Will see what sort of temperature develops in the engine bay on next lengthy cruise.

 

Thanks,

 

peter.

 

Hi Peter.

 

As far as I know the teeth have no function other than allowing greater flexibility of the belt, when I bought my engine the belt that was fitted sat perhaps 2mm below the flanges, it lasted only a few months but then my alternator has an easy life, no 230 volts etc.

 

I consulted one of the manufactures catalogues, Fenner I think, the belts come in a number of widths in quite small increments, I found that the one that fitted perfectly (in spite of what our latest oracle thinks)when new sits 1 or 2 mm above the flanges, when you look at the cross section of a 'V' belt you will see that the taper does not extend to the outer major width anyway. The new belt lasted many times longer than the original.

 

We must always bear in mind with many of these marinised engines that much of this add on detail design is not carried out in the drawing office of a multinational company, more likely a purchasing clerk ordered what he thought was about right from his chap round the corner.

 

If my engine had the complex systems many of you describe I would think seriously about converting to toothed timing belts, a bit of lathe work is required but the pulleys and belts are quite cheap you will probably get 3 pulleys and a belt for less than £60. If you find a Power Transmission Company you will find they have a vast range of this stuff. I looked in the RS book for reference.

[DHutch]

These are the kind of belts used on the final drive of Harley Davidson motorcycles...

Yeah, the lufbra 'formualar studernt' car also uses a large timing belt as the final drive down to the diff.

 

 

Daniel

[Crazy Scheme]

Hi Crazy.

 

In your case tightening the belt would help, nothing wears a belt faster that it slipping on it's pulley. When this topic last came up several people said buying good quality belts from Halfords partially cured their problems. Make sure you get the correct width too, the correct size belt should sit a little above the pulley flanges.

 

Also take a look at my previous post re-overheating I am certain that is a major factor.

 

I know, we have to adjust it EVERY time we run the engine. We buy the belt recommended for our engine from French Marine (Yanmar dealer). We will go down Halfords and see whats on offer. When we had the engine serviced we got one new pulley as the engineer seemed to think that was the problem. It was an improvement on what we had before (yes it really was that bad!). I don't think we are overheating, but will check. Also will consider a slightly wider belt - we do seem to wear the teeth out quickly.

Edited November 18, 2006 by Crazy Scheme

[Norman Bond]

An excellent thread with lots of good contributions. If no one takes offence I am going to summarise what we have and try to add some explanations to some of the comments. And if anyone does take offence - too late, I've done it now

 

I am not going to keep quoting from the original posts, as I think this will be long enough without that, but it will mainly be based on John's and Tony's comments. My knowledge is mainly of the materials involved and is a good 20 years out of date so if anyone has more recent information please feel free to correct me. This is the way we all learn. And some of this will be greatly simplified and in doing so some of the details will not be strictly accurate. Live with it.

 

While there is the possibility of mechanical damage the causes of this are usually easy to spot and correct - damaged pulleys, the belt rubbing against part of the engine block etc. So this leaves, as John pointed out right at the start, heat. A hot belt is more easily damaged and if the heat build up is too much the belt delaminates - ie the inner rubber "teeth" strip away. When you see lorry tire treads at the side of the motorway this is the usual reason.

 

I can think of 4 mechanisms by which the belt will get hot. Friction (mainly due to slippage); Extension (belt stretch due to loading); Torsion (twisting due to pulleys out of alignment); and Hysteresis ( bending).

 

So, as usual, lets take them in reverse order.

 

If you bend anything backwards and forwards fast enough it will get hot. This energy loss mechanism in a rubber belt is known as hysteresis. The rubber layer above the polyester reinforcement will stretch, but as the layer is thin will not heat up very much. The layer below it has to be deep as it is the contact with the sides of the pulley that actually drives the pulley round. This layer in a lot of modern belts is moulded as a set of "saw teeth" for a number of reasons. There is no rubber to compress so there is no heat build up and it bends much more easily around a small pulley. The increased surface area also means it looses heat more readily when running in the air between the pulleys. Less useful on a boat as there is a lower airflow than a car. But as Tesco used to say "Every little helps". The only problem I can see with these types of belts is that there could be more scuffing than with a non-toothed belt. This could be one reason for rubber powder around the engine. Steve (Split pin) could probably give some advice on this. (By the way, great first post Steve.)

 

Torsion is fairly easily dealt with. Make sure all the pulleys are properly lined up. Use a piece of straight steel inside the pulley to check. Don't do it on the outside as the pulleys may be different thicknesses. You may need to add washers to the alternator mountings to align them properly.

 

Stretching and relaxing (going from load to no-load as it revolves) causes heating, but it is small and there is sod all you can do about it. That stretch, however leads to the big problem...

 

Imagine an inverted triangle. Top left corner is alternator A, top right is alternator B and bottom is crankshaft pulley C. If the belt runs clockwise the load on the belt caused by the resistance of alternator B will cause the belt to stretch. On a car engine this will not be much, but a boat engine with a large output alternator there will be a lot more. This increase in belt length has to go somewhere and the only place is between C and back to B across pulley A. There is now a loss of belt tension across A and the belt will slip. With a new belt the amount of slip may be negligible, but there will be some heat build-up and after a while the belt will set (i.e. some stretch will become permanent) and the slippage and heat build-up will increase. Add a fourth pulley and the 1/3 contact with pulley A may now decrease to 1/4. It will now slip even more. When Tony says only a prat would try to run a V belt over 4 pulleys I think he is being too kind.

 

So. How can we deal with it? The closer alternator B is to the crankshaft the less the belt will stretch, so that would be one design point, but one that an owner would probably not be able to change. Using a longer belt and adding a jockey pulley against the outside of the belt just before alternator A so that the belt has a longer time in contact with the pulley may help. You will get more heat loss due to bending but less due to friction. That should be a good trade off (and you could even get better charging!!!). Tony's 2 suggestions should also be considered as part of this paragraph.

 

Away from mechanical changes to the engine layout, Colin's suggestion of a Kevlar reinforced belt is a good one. Kevlar stretches much less than polyester. It is not used on car V belts because most people tend to overtighten them and it is cheaper and easier to replace a belt than it is to replace your water pump and alternator bearings. (And polyester is cheaper so the car manufacturer saves money ). So if you did go to using Kevlar you should take great care over getting the tension just right.

 

(And Col, "chalk" is a major ingredient of most rubber compounds. It is a cheap filler!)

 

John's substitution of a timing belt would be the ultimate answer, but probably well beyond what most of us would contemplate. Steve's multivee belts are probably much easier. For those who haven't seen one of these imagine a very narrow (and shallow) v belt and join a number of them side-to-side to get a belt about an inch wide. They have been standard on Nissans for a long time (back to the days of the Bluebird at least) and I have yet to have one fail or even slip. You may even be able to get compatible pulleys from a scrap yard.

 

 

And to go right back to Gaggle's original question, yes there is a possibility that the belt could have been frozen to the pulley. Not very likely, but severe wear due to slip would probably be noticed by the screeching coming from your engine or by your batteries going flat. Keep going long enough and the belt will jump off the pulleys. Either way a broken belt would be unlikely before you noticed other symptoms. Of course there is the possibility that it snagged when jumping the pulleys and that snapped it!

 

 

 

Norman

[DHutch]

Here is a photo of a Multi-Vee (although im not sure of itd conection to the opjects in the foreground)

 

Daniel

[Yoda]

An excellent posting Steve.

[Norman Bond]

Thanks Daniel,

 

I had a horrible feeling that someone was going to ask for a photo. I'm sure I've got an old Bluebird alternator in my garage. Somewhere. With the state of my garage it would probably take ages to find it!!!

 

 

Norman

[Crazy Scheme]

This is all really good advice folks, food for thought - we'll let you know how we get on!

[DHutch]

I had a horrible feeling that someone was going to ask for a photo...

http://images.google.com/

- Your one-stop photo provider!

 

 

Daniel

[tony collins]

, Colin's suggestion of a Kevlar reinforced belt is a good one. Kevlar stretches much less than polyester. It is not used on car V belts because most people tend to overtighten them and it is cheaper and easier to replace a belt than it is to replace your water pump and alternator bearings. (And polyester is cheaper so the car manufacturer saves money ). So if you did go to using Kevlar you should take great care over getting the tension just right.

 

(And Col, "chalk" is a major ingredient of most rubber compounds. It is a cheap filler!)

 

 

 

I would re-inforce the above, Kevlar (or Aramid which is the generic name for the product) is much more brittle than polyester. The earlier suggestion of using white or "mower" belts would not in my opinion be a good move. These belts are designed for use either in a very wet, hot environment for example, in a food factory where they are used to drive jars in a bottling factory (they don't mark the jars), or on a drive where you want to promote slip, i.e. as a clutch in a mower drive. Most vee belt drives are designed to avoid slip.

Belt slip is the enemy of belt life.

The sequence goes like this.

Belts slip,

slip causes heat to be generated, usually (but not always) the slipping belts stretch

this generates even more heat,

rubber carbonises and breaks.

 

Just one point most Vee belts (Z, A B, C, D, E. are made in a very cheap rubber (SBR) Styrene Butiedene Rubber. Whereas Wedge belts (SPZ, SPA, SPB, SPC, are made in Chloroprene rubber (neoprene is a chloroprene) the most popular filler in any commercial rubber is carbon black (soot )

Toothed wedge belts allow, as has been said, the use of smaller pulleys. If any of the above touch the bottom of the groove, either pulley is knackered, or the belt is either the wrong belt or worn out.

Most industrial belts are re-infored with polyester and provided the drive is designed correctly are very tolerant of misalignement. Using four pulleys in a single belt drive is very bad engineering practice unless (at least) one of the pulleys is an inside idler.

 

 

PolyVee belts are a better proposition for high ratios, (have a look in the back of the washing machine at home) They are less tolerant of misalignment. Changing a crankshaft pulley for another design is more difficult than you would think. It doesn't matter if a vee type belt is proud of the pulley top as long as reinforcing cord (about a quarter of the way down the belt is inside the pulley outside diameter.

 

I'll shut up now.

 

Tony

Edited November 19, 2006 by tony collins

[blackrose]

Several contributors have referred to external alternator controllers putting additional stress on belts, but can someone explain how this is possible? I thought alternator speed (& belt speed) was controlled by the throttle. Does the controller add greater turning resistance to the alternator to produce the extra output?

[chris w]

Without an alternator controller, let's say an alternator is delivering say 20 amps charging current at a certain rpm and battery state.

 

The power involved is 12v x 20 amps = 240 watts = ~ 0.3 HP

 

Right, now install an alternator controller, which will charge the batteries at a higher current, let's say 60 amps at the same rpm.

 

The additional load put on the engine is now 12v x 40 amps (40 amps is the additional current)

 

Therefore the additional load put on the engine is 480 watts = ~ 0.7 HP

 

It doesn't matter that the figures may be more or less in practice, the principle is that if you feed additional current into your batteries that additional power has to come from the engine. It will load the engine and will cause the revs to fall. The alternator, when delivering higher current, becomes harder to turn due to increased magnetism and so additional stress is put on the belt.

 

Some alternator controllers (eg: Sterling) have a "soft start" whereby, when you start the engine, the alternator controller doesn't cut in for about 2 minutes to allow the engine to start and get itself warmer and more stable before it is asked to deliver the additional power.

 

Chris

Edited November 20, 2006 by chris w

[blackrose]

Without an alternator controller, let's say an alternator is delivering say 20 amps charging current at a certain rpm and battery state.

 

The power involved is 12v x 20 amps = 240 watts = ~ 0.3 HP

 

Right, now install an alternator controller, which will charge the batteries at a higher current, let's say 60 amps at the same rpm.

 

The additional load put on the engine is now 12v x 40 amps (40 amps is the additional current)

 

Therefore the additional load put on the engine is 480 watts = ~ 0.7 HP

 

It doesn't matter that the figures may be more or less in practice, the principle is that if you feed additional current into your batteries that additional power has to come from the engine. It will load the engine and will cause the revs to fall. The alternator, when delivering higher current, becomes harder to turn due to increased magnetism and so additional stress is put on the belt.

 

Some alternator controllers (eg: Sterling) have a "soft start" whereby, when you start the engine, the alternator controller doesn't cut in for about 2 minutes to allow the engine to start and get itself warmer and more stable before it is asked to deliver the additional power.

 

Chris

 

I see, it's interesting to know the knock-on effects of adding one of these devices, thank you.