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Old 09-09-2006, 09:36 PM
72NOVA454
 
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Belt Keeps Flying Off at High RPM - Why?

what the heck is going on?

I've spent hours on this. My single groove v-belt keeps coming off my BB Chevy at high rpm (above 5000 RPM). Why?

The belt ties three pulleys together: the crankshaft pulley, the water pump pulley and the alternator pulley. All are old-style single groove V-belt type. The belt is new, the pulleys are new.

At first I thought it was an alignment problem. the alternator pulley was not perfectly aligned with the other 2 pulleys. after a half dozen pulley alignments and adjustments and subsuquent road tests at high rpm, the frickin thing keeps coming off. Now all three pulleys are perfectly aligned when viewed from both sides of the vehicle. It's like there's a little gremlin in there that pushes the belt off at high rpm just to upset me.

I tried different tensions on the belt, from mildly tight to very tight. Still the same problem. Keeps coming off. I checked it with the engine running and it looks very good, perfectly aligned, smooth operation - no obvious problems. when I open the throttle and watch the engine run faster, still no problem. smooth operation. I can't figure it out

Question: How much deflection should there be in the longest portion of the belt (between the crank pulley and alternator (about 15 inches)

Question: this car is a big block 72 nova. the crank pulley and belt is only 3/16" from contacting the chassis frame crossmember. Very close to hitting it. Is it possible that at high rpm the crank pulley and belt make contact with the frame and that's kicking it off? If that was the case the top of the belt would be damaged and it's not.

Please send in your thoughts. I'm totally stumped on this one. I'm in chicago west suburbs. Come on over and help me fix it. We'll go for a ride.

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Old 09-09-2006, 10:45 PM
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Throwing belts was a big problem back in the day of the high winding 302,s and 327,s thats why chevy went to deep groove pulleys. I doubt you're hitting the crossmember, it would show on the pulley. I always figured about 1" of deflection on the long side but everyone has their own idea on that. If you make them too tight then you start wearing things out. Chevy still sells the deep groove pulleys, #3858533 for the crank and #3770245 for the short water pump. BUT these are 2 groove pulleys and probably pricey from GM but its a start if thats your problem. I think Moroso made deep groove pulleys also...
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Old 09-09-2006, 10:46 PM
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The belt is growing from centrifical force at higher rpm's.
It may be as simple as getting a new belt. If that doesn't work you may need deep groved pulleys.
Good luck.
Kim
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Old 09-09-2006, 11:14 PM
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Why did it start throwing the belt? Did you change the heads, intake?

At 5000 RPM it's not a matter of needing deep grove pulleys. 99% of the time the alignment is off. You can't see it but if you centered a laser pointer in the groove of the pulley and shot it towards the other pulleys you'd see it.
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Old 09-10-2006, 01:51 AM
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Rick, The man said he aligned the pulleys several times and now he has them perfect. Dont know if he used a lazer or not, I just took his word for it that he in fact had them aligned and that was'nt the problem. Thats why I offered the deep pulley info because maybe thats what he needs. And yes maybe his alignment is off a shade but if thats as good as he going to get it then the deep pulleys will work, thats why chevy designed them. Dont be so quick to dismiss other members suggestions because thats what is was, a suggestion.
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Old 09-10-2006, 05:52 AM
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You could go with a smaller crank pulley. That would help with your fear of it hitting the frame, which isn't likely, but would also slow the belt down somewhat at the alternator. Problem with that is your charging wont be as good.

Also, are you using the short or long water pump? The short has the alternator way outside of the drivers side head, whereas the long pump has it tucked in on the other side in front of the head.....much shorter belt needed. The belt tossing issue of the 60s was mainly with the short pump and long belt.
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Old 09-10-2006, 06:18 AM
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Why does a belt come off at high RPM? Not aligned properly. Not tight enough. Too much stretch under load. Belt is too large for grooves. Pulley is bent/warped. Excessive run between pulleys. You seem to have covered all these. Your set up is one that has been around for almost as long as the SBC/BBC has been around and it has a good track record. You may need to rig up something to really check alignment other than depending on the eyeball. Also, the belt has experienced some big stress every time it has slung so it may need tossing and start with a new one. This is far fetched, but check to see if the water pump shaft has end play that would let it move forward as the fan is pulling the air thru the rad.

Trees
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Old 09-10-2006, 07:28 AM
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I would replace the belt, after tossing it a few times it could have some stretched or broken cords in it that you may not see. Also use a quality belt Gates, Dayco or equiv.
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Old 09-10-2006, 07:41 AM
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A pic of your setup might help. DEFINETLY replace the belt.Are all the crank sheaves the same size? If not, do you have two of different diameters driving the same thing? If the drive or driven diameters are not equal, one belt will be trying to drive faster than the other inducibg stretch in the other that will cause it to derail. Make sense?
Went back and reread your original post. This only applies to multiple belt setups. I had a setup like your that did the same thing. I put a slide over the belt that kept it from riding up in the alt pulley. At high rpm, the alt is putting the highest load on the belt so any slack in the belt is built up between the crank and the alt on the waterpump side and it will climb out of the pulley where it makes the shortest bend. Another thing to look at is the groove itself. A rough spot will make the belt climb.

Last edited by 61bone; 09-10-2006 at 07:54 AM.
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Old 09-10-2006, 07:45 AM
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I had the same problem when I changed over from electric water pump to belt. I found something I'm going to try, its a cog belt drive set up on ebay. The sellers id is unlimitedparts. It looks like a blower drive. I'm using factory 454 pulleys, and new belts and I still throw them if I am running someone and I get over 5600. Good luck with it.
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Old 09-10-2006, 09:30 AM
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327, not dismissing anything in disrespect. After being around this stuff for 20-30 years though I try to look at it a couple feet away as well as from experience in similar situations.

A belt will not fly off at or approaching 5000 RPM if the pulleys are aligned and in good shape. It just won't happen. Something changed and that's what needs to be answered by the original poster.
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Old 09-10-2006, 12:58 PM
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wow - you guys are great.

I got 10 responses overnight. Very Impressive.

thanks to all you guys - I really appreciate it. Each and every response was helpful and informative.

Let me add a few additional comments on this.

It is a "long" water pump assembly for those of you wondering.

First: I said that the belt is new - what I should have said is that the belt is new but has been "thrown" a few times from the vehicle - it's a little raggy. I would imagine during the "throw" period it is stretched, compressed, and generally beat up. I just bought a perfectly new one and will put it on there. By the way, it's one of those "notched" type V-belts. Does that make a difference?

Secondly: when I said the alignment is perfect, what I should have said is that by "eye-balling" the alignment it is perfect. I never used a laser or other device to actually check it. Chances are it is not "perfect". Although I have seen other cars where the alignment is noticeably off and they don't complain (maybe because they never run their motor at redline).

Third: I have attached a few pics. Please study them. If there is something that catches your eye please respond back to me. The pics are not the best, as it is difficult to photograph this type of problem. The alternator pulley is deep groove, the other pulleys are medium depth - not as deep as the alternator groove, but the belt still seats well enough in those 2 pulleys I believe.

thanks.

Lee
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Old 09-10-2006, 02:45 PM
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To me (kinda hard to tell in the pictures) it looks like like the width of the belt is a little too big and is not seating all the way in the water pump/crank pulleys and you said yourself that their grooves are different in size compared to the deep groove alternator pulley. Just try swapping out the belts for a slightly smaller one in width so it'll sit deeper in the pulleys. Can you get a closer picture of the belt in the crank/water pump pulley so we can get a better idea of how far it's actually seating in there?
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Old 09-10-2006, 03:07 PM
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Maybe its the angle the picture was taken at, but the 1st pic...it seems like the alternator is a bit too far back and a little cockeyed.
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Old 09-10-2006, 03:19 PM
72NOVA454
 
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Quote:
Originally Posted by poncho62
Maybe its the angle the picture was taken at, but the 1st pic...it seems like the alternator is a bit too far back and a little cockeyed.
it's the angle of the picture.

thanks.
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