Some engineering thoughts on the lower QD pulley bolt

[Danbot]

I just lost my second lower QD pulley bolt, and am trying to wrap my head around why / how it fails. I would love to hear some educated insight on this, if anyone has a thought.
The first time, I found the washer floating around in the belly by my clutch, looking around I found the bolt broken off inside the driveshaft. I chalked it up to being over torqued and repaired it myself.
I just lost another bolt, this time the pulley slid out a bit and scraped the battery box, I "modified" the battery box for more room and limped it off the mountain.

-Both bolts broke at about the same spot (just inside the threads on the driveshaft).

-The bolt I installed was torqued to 29ft/lb ( a spec given to me by a sw user), with blue loctite

-I am still running my original driveshaft (clamped) with no problems at 1200 miles.


This time I will seek out a grade8 bolt to install, and I am thinking about lowering my torque. Anyone have any thoughts on the torque, or what forces could be causing this?
From what I can tell (haven't actually removed the pulley) The pulley must sit against a shoulder on the driveshaft. The outer pulley face sits out further than the tip of the shaft fully seated, so the flat washer installed with the bolt only makes contact with the pulley.

Here's my 3 theories:

1- Isolated incidents, the original bolt was overtourqed and snapped. My replacement bolt was inferior, flawed, and/or 29ft/lb is still overtorquing it.
To be honest I have no Idea what grade or brand the bolt I installed was.

2- Are we looking at the possibility that the splines have too much play and the pulley can actually "rock" on the shaft?
This could stress the bolt for sure, but I would expect to see additional problems witch I have not. My pulley feels very secure on the splines, although I have not removed the belt to check more thoroughly.

3- Are the pulleys tapered, or out of alignment causing a constant outward force on the lower pulley? Or Are the jackshaft and driveshaft not precisely parallel to each other?
I did notice that with no bolt holding the lower pulley secure, it had a tendency to ride about a 1/4" out on the shaft supporting this theory (and why I had to bend the rear strap tab on my battery box where the pulley had been rubbing). The sides of the belt were in contact with the shoulders on both QD pulleys so it seemed that this was what was actually keeping the pulley from walking right off the shaft.

 

[geo]

You got a funny one DB.

First my bolts were grade 8 stock (pretty sure 8.8 on the head means grade 8) and 29 ftlb is on the lower end of torque for a 10mm bolt.
Second, with no outer flange on the lower pulley I can`t see the belt causing enough outward force to snap a 10mm bolt.
The top flange would stop the belt from walking further out so both flanges touching without a lower bolt in place would seem normal to me. My belt (with both pulley tight) will position itself where it wants over about an 1/8th inch spacing in the pulley flanges. That seems normal to me and would allow a bit for tolerances.

Does your bolt actually thread in completely (without the pulley) or does it stop short abit?

If anything (for a third try) I would replace the wave washer with a heavyduty flat washer and locktite (that's what i did on my top pulley). The wave washer doesn't get close to flat at 29 ftlbs. This leaves just a thin edge doing the torque holding. That thin edge is also right at the edge of the aluminum to steel insert interface and with a little tolerance built in it may be slightly cocked when torqued.
If you do try the flat washer make sure it is the same OD as the original. I noticed on my pulleys that the steel insert (splines) is slightly recessed from the aluminum pulley. If you use a bigger OD washer you would be torquing on the aluminum (not good).

I'm not an engineer (disclaimer lol) just my thoughts.

 

[JMCX]

I discovered my bolt broken because the belt looked loose. I pulled off the belt and to me the splines felt too loose. The dealer said it was ok. With the bolt now repaired the belt is tighter like before.

That means the bolt is centering the pulley and probably means it will fail again like yours did. I'm sure it is fatigue.

 

[rmscustom]

I discovered my bolt broken because the belt looked loose. I pulled off the belt and to me the splines felt too loose. The dealer said it was ok. With the bolt now repaired the belt is tighter like before.

That means the bolt is centering the pulley and probably means it will fail again like yours did. I'm sure it is fatigue.

Interesting... I noticed how loose my pulley seemed on the stock driveshaft but when I put the steel shaft in it was much tighter and had to tap it on with a rubber mallet... Maybe another possible reason why some QD belts seem to last forever and some have had repeated failures.

 

[POLZIN]

This has to be a harmonics issue. The side force on the bolt should never be enough to snap the bolt not even the cheap junk polaris uses. Somehow high freqeuncy vibration has to be getting transfered to the shaft and the bolt causing it to break. I see this sometimes in pumps that have cavitation issues.

its hard to imagine how this is happening with a belt and a rubber track though. still thats my guess at the cause

 

[richracer1]

FWIW, in regards to bolt grades, 8.8 metric are NOT equivalent to grade 8 SAE bolts. Metric grade 8.8 bolts are equivalent to SAE grade 5 bolts. If you want the metric version of SAE grade 8, you need to find bolts that are stamped with 10.9. I hope this helps.

 

[rsp111]

I also just had this issue this past weekend

 

[Danbot]

Ok, I just finished extracting the broken bolt, all went smoothly. She is ready to have a new bolt and washer installed.
Just to touch base on the comments so far:
-The bolt that failed was actually the OEM bolt from the upper pulley, I had installed the replacement up top.
-My belt feels like similar tension whether or not the bolts are installed in the pulleys.
-Can anyone see any benefit or problem in using a spacer on the end of the shaft to take up the space to match to the outer face of the pulley? what purpose does the cupped washer serve?

Kreg... re: your comment on harmonics. If this is the case, is a high grade bolt the proper application? or is a softer bolt more suitable?

Right now I'm thinking high grade bolt, with a very thick washer, or 2 stacked if I cant find what I'm looking for.
You can see how contact with the battery box lightened my lower pulley even more!

 

[JMCX]

Belleville washer

How long did the second bolt last?

 

[POLZIN]

after reading more closley I wonder why there is a "wave" washer ? that could be the problem if there is any looseness what so ever that could explain the broken bolt. the sprocket doesnt float does it? Assuming it doesnt I would definetly try a flatwasher and a high grade bolt . the stock polaris fateners are crap and I doubt they would pass grade 5 spec .

 

[skibreeze]

With repeated bolt breakage, I'd seriously wonder what the runout on the shaft is. The splines should be holding the pulley square to the shaft, not the bolt.

 

[Danbot]

Belleville washer

How long did the second bolt last?

First one I found at about 400 miles. This one at 1300 miles

after reading more closley I wonder why there is a "wave" washer ? that could be the problem if there is any looseness what so ever that could explain the broken bolt. the sprocket doesnt float does it? Assuming it doesnt I would definetly try a flatwasher and a high grade bolt . the stock polaris fateners are crap and I doubt they would pass grade 5 spec .

My pulley fits very tight on the splines, and has no play in any direction. No the pulley does not float, in fact the pulley is thicker thn the available shaft for it to slide on. Even a flat washer will only sit against the pulley.... the shaft end sits in a little, maybe a 1/16" or less.

With repeated bolt breakage, I'd seriously wonder what the runout on the shaft is. The splines should be holding the pulley square to the shaft, not the bolt.

My runout has never been checked, but I suspect it's reasonable since I am at 1300 hard miles on the stock shaft and belt, My shaft was problem free naked up to 600 miles when I clamped it as well.

 

[whoisthatguy]

Maybe it is the heat issue again, expanding the lower pulley beneath the bolt head, and adding extra tension to the bolt, which could be combined with reduction in bolt steel strength due to heat softening the steel. Perhaps an HDPE plastic washer below a metal washer, would absorb some of the expansion of the pulley, and insulate the bolt somewhat. The exact same bolt for the quick drive, is used for the chain case standard RMK. The oil in the chain case cools that bolt and lower sprocket. Any reports of that bolt failing in the standard RMK's?

 

[winter brew]

30lbs doesn't seem very tight for that bolt. I have had a few of these apart but didn't pay close attention to the Bellville washer....is this enough to flatten the washer ?
Is the driveshaft not flush with the face of the pulley?...I ask this because up to 80% of a bolts shear strength can be lost if there are any shanks between parts and not at the correct designed torque for that fastener. If the bolt can't apply clamp force to the end of the shaft then (as said by Polzin) you have the harmonics and a bolt that is prone to breakage.

 

[Danbot]

30lbs doesn't seem very tight for that bolt. I have had a few of these apart but didn't pay close attention to the Bellville washer....is this enough to flatten the washer ?
Is the driveshaft not flush with the face of the pulley?...I ask this because up to 80% of a bolts shear strength can be lost if there are any shanks between parts and not at the correct designed torque for that fastener. If the bolt can't apply clamp force to the end of the shaft then (as said by Polzin) you have the harmonics and a bolt that is prone to breakage.

The shaft is not flush with the face of the pulley. The shaft ends up in-set 1/16" or less (didn't measure)

There is no way that 29lbs is flattening this bellville washer.... I doubt it budges at all. You can clearly see on either pulley where the outer edge of the washer is the only contact point.

As for the comment about heat.... I never noticed any excessive heat from either pulley, or the belt.

This bolt is M10 x 1.25 right?

 

[Danbot]

From the Carls Poo parts fiche
OEM QD pulley bolt (upper and lower):

SCR-M10X1.25X20 8.8 HH ZOD P30 7519004

So 8.8 Metric grade is pretty shoddy

 

[Danbot]

I just finished up, couldn't find high grade bolts in my town. Used a pair of M10x1.25 25 8.8 bolts, installed with very large flat washers.
For those of you familiar with the 866 Liberty.... I used the washers for the rear engine mounts. I will post pics later when I get home, hopefully a test ride today!

Next time I get out of town I will replace the bolts with high grade units.

 

[Danbot]

 

[rmscustom]

"If you do try the flat washer make sure it is the same OD as the original. I noticed on my pulleys that the steel insert (splines) is slightly recessed from the aluminum pulley. If you use a bigger OD washer you would be torquing on the aluminum (not good)."

From Geo's post, Just wondering if you missed it or if your not concerned about it?

 

[Hawkster]

Sorry , didn't mean to change the subject but this is a really good picture . The timing of the gears and belt are off a bit , the teeth on the left are against the rotation and on the right with the rotation to the point that it is bending the lugs in to different directions and the one on the right that is engaging is pushing on the lug.