poly-V belt tension - how much is too much?

[GreenV8S]

My blower drive belt was more or less OK at 0.3 bar but is slipping like a bastid at 0.6 bar. I suspect that the main problem is that I don't have a belt tensioner as such, I just have a fixed idler pulley which I can adjust to preload the belt.

Applying belt dressing and increasing the tension has helped a bit but I'm wary of going too far, especially since I don't actually know how much tension is on it. I can twist the belt through 90 degrees by hand which apparently means it is 'about right' but that doesn't seem a very precise way of measuring it.

So, I have two questions:

Is there any practical way to work out what the belt tension is? For instance I can measure how much torque it can apply to the pulley before it slips, or do a slightly more scientific version of the '90 degrees twist' test.

Second question, how much tension is too much? At the bottom of the belt is the crank pulley supported by the main crankshaft bearings. At the top is the blower drive which I think is ball raced. So perhaps the belt is the limiting factor. How much can they take?

In case it matters, it's a 6pk 3.4mm pitch 1200mm long.

Edited by GreenV8S on Thursday 6th November 21:19

[dilbert]

I don't know in an absolute sense, but I'd try technical data from companies like perhaps Fenner.

Some things that can help;

A longer looser belt is better than a short tight one.
A wider belt is better than a narrow one.
Bigger pulleys are better than smaller ones.
The more of the pulley that actually contacts with the belt the better.
Check the temperature of the pulleys, heat will transfer through bearings, and affect the belt performance.
Check the belt and pulleys for oil contamination.
If the belt is tight enough to stretch or wear, it's too tight. That's not actually very tight at all. Using a lever to apply tension is too much.

[GreenV8S]

dilbert said:

If the belt is tight enough to stretch or wear, it's too tight. That's not actually very tight at all. Using a lever to apply tension is too much.

Oh dear! I suspect I'm already pushing my luck there then since the idler pulley is already 'kin tight.

[dilbert]

GreenV8S said:

dilbert said:

If the belt is tight enough to stretch or wear, it's too tight. That's not actually very tight at all. Using a lever to apply tension is too much.

Oh dear! I suspect I'm already pushing my luck there then since the idler pulley is already 'kin tight.

Thankfully, you didn't go bonkers at what I said the other way. I mean I used to have an orion, and it was a pig to adjust the alternator belt. You had to use a lever because there was no adjuster screw, and the bolts to tighten the alternator were just sliding clamps. Not enough hands. In confined spaces, ideally you want an adjuster screw and a clamp screw.

I think the thing is that if you're using the lever to gain advantage, it's probably not right. If you're using the lever to *hold* a tension that you could otherwise apply with your hands, then it's OK.

"'kin tight" sounds a bit too much.

When I've seen superchargers using polyvee belts at shows, they're usually at least a couple of inches wide.

The other thing to bear in mind is that if the belt looks "glazed" then it's probably not doing all that a new one could. The more it slips, the more it will "glaze", but temperature and age, also make a difference.

I appreciate it's difficult because it's difficult to know the torque on input shaft to the SC. I'm pretty sure that most of the manufacturers will be quite explicit about the capabilities of their belts if you ask or look.

Perhaps another thing to look at might be a "timing style" belt. It probably won't get you much more in terms of the belt specification, but it will probably get you quite a bit more capability in practice. Clearly, unlike the timing application, it's not the end of the world if it fails.

I'm sure you've tried winding a camshaft over by hand, and you'll know the effort required. Unlike the camshaft, the input torque to the supercharger will climb exponentially with speed.

Edited by dilbert on Thursday 6th November 22:12

[bertelli_1]

Once the belt has slipped and 'glazed' it will then slip more easily. I reckon your adjustment method is sound & I'd take off the charger pulley, get it sandblasted (to roughen the surface), fit a new belt & try again with the same tension.

[GreenV8S]

I started with a new drive belt, and an old (but not noticeably worn) pulley on the blower.

I did clag quite a lot of belt dressing on to try to stop the squeal, and I wonder whether I may have overdone it. I've noticed that after running gently for a few minutes it will take full load without slipping for a second or so and then gradually slip more and more.

I assume that it is the blower pulley which is slipping, because this is much smaller than the crank pulley and intuitively this seems likely to have less grip.

The slip seems to get worse with revs and it usually won't slip below about 3000 rpm. Judging by the Eaton performance curves the torque goes up slightly with revs which may explain that.

I've estimated that at 3000 rpm the blower is using a bit less than 20 bhp, which corresponds to about 15-20 lbft, which corresponds to a force of about 135 lb at the pulley. Um, actually that's starting to sound like rather a lot. I think I'll get the torque wrench out tomorrow and see how much torque this little sucker is actually handling before it slips.

Edited by GreenV8S on Thursday 6th November 23:03

[dilbert]

Right at the end of that link there is a guide to belt tension.
http://www.fptgroup.com/downloads/friction_polydri...

[GreenV8S]

dilbert said:

Right at the end of that link there is a guide to belt tension.
http://www.fptgroup.com/downloads/friction_polydri...

Excellent stuff, thanks. Not only some numbers, but a practical way of measuring it. I've been digging around the GAM site all evening and they don't seem to have anything useful to say about it.

[350Matt]

Hello Pete

am I right in thinking that 6pc x 3.4 pitch = 20.4mm wide?

If so then thats not enough it needs to be about twice that

Matt

[GreenV8S]

That's how big the blower pulley is though.

In fact on the Jags they even fit smaller pulleys to raise the boost. I went the other way and fitted a bigger pulley on the crank because I thought this would give the least risk of belt slip problems.

[CrashTD]

I am not sure on specifics but it may be an idea to look at a toothed belt system.