Belt position control

Hello All

Recently I have broken two belts back-to-back. First one we figured was lean condition issue since we had just made some fuel changes, but after the second consecutive one, some forensics on the belt show where it is climbing the outer flange of the idler then subsequently overtighten and tearing the belt. We placed another belt on and ran it a little tighter and made a complete pass but there were whiteness marks indicating it was trying to climb the flange. Looking further, the belt was centered on the blower pulley but towards the leading edge of the crank pulley (both were centered when installed as well as the idler) Upon starting it in the pits and watching the belt position from idle up to 4000rpm, and as rpm increased it was noticeable that the belt was trying to move forward on the crank pulley and staying centered on the blower pulley idler squareness has been checked and is good, Blower snout/pulley run out looks good as well. There are no other noticeable conditions.

Tonight, I plan on removing the Blower and checking horizontal parallelism of the blower mounting surface on the intake to the crank snout and the Mounting surface of the blower to the blower snout. Additionally, the squareness of the Crank pulley diameter to the pulley mounting surface. This is a KB block, Venney head and Intake BTW.

Question is are there other things I should be looking at?

Pic included for reference

 

You may find broken or loose studs /nuts @ the rear of the blower or loose nuts @ the rear of the manifold.
If it was a touch lean and detonated at all, that would be common.
You had a correct mechanical relationship between the pullies. The belt position indicates that those parallel components are no longer parallel....the blower or manifold, being pulled down at the front as the belt tries to climb the pulley teeth can only tip up at the rear because it is under belt load downward at the front. I have had exactly this happen, of course in my case, I was never too lean, the rear blower studs simply failed.
If it is not the above the only other thing I can think of would be a bearing in the blower snout.
Jeff Johnsen

 

Thanks Jeff, I didnt mention that I had checked the blower studs and snout . They were tight/good. We will see about the intake tonight.

 

relationships all look good (crank to manifold, blower base to snout) no loose or broken fasteners, Do Pulleys wear out?? The bottom pulley although square (mounting surface to diameter) looks good the teeth have a wear pattern in the drive side of the teeth and the tops (addendum) of each tooth looks to be worn flat. So a new pulley is coming. Again thoughts from the more experiences are truly welcome.

 

Thanks Mike. My idler only has a washer under the head of the through bolt. The bearings are pressed on to an arbor that sits against the bracket. At idle speed (1800ish rpm) the belt runs evenly spaced front to back on both the upper and lower pulley, but as RPM increases it moves forward on the Crank Pulley putting pretty good force on the front flange of the Idler pulley.. Moving the Idler forward would allow the belt to run off the end of the Crank pulley until it rubs against the crank support,m moving the Idler back would facilitate climbing the flange. When warming the engine which is primarily idle speed the Idler get finger blistering hot from the force of the belt rubbing..
Giving it more thought as I sit here and type...I run a 75 mm belt and a 3.1" (78mm) Idler. Could the Idler be too narrow? Is the belt rubbing more on the front flange and building heat on the forward side of the belt allowing it to heat unevenly a crossed the width of the belt, making the front half of the belt longer than the rear by thermal growth? This would exacerbate the forward push of the belt as the rear tension would be greater than the front. Supporting this is that not only is my Idler only 3.1" wide it is also flat in a crossed the diameter leaving all fore and aft belt control to the flanges...I guess I'm gonna try a wider, crowned Idler as well.
See pics from last broken belt View attachment 4128 View attachment 4128

 

Rob Wendland has some blower belt and pulley setup recommendations on YouTube.

 

Thanks Graham

 

I have an support coming as well... thanks

 

Update, Seems I have resolved my issue (for now)...the crank is
parallel horizontaly to the manifold blower mounting surface. The mounting surface of the blower is horizontaly parellel to the blower snout axis. The Idler pulley Axis is horizontaly parellel to both.
I cut a barrel shape into the idler, replace the Magnesium Idler bracket with an Aluminum version, made a 1.375 deep T nut, replaced the Idler bolt with a stud/nut arrangment and added an outboard support. Additionaly I replaced the lower pulley.
I did find the outer edge of the belt to be in less tension than the inner edge. this indicates that one of the three axis are not veticaly parellel and once running you could see the whitness marks on the Idler...I will re-inspect the idler profile for size to confim symmetry of the barrel form (all other idler parts have been inspected as well). We were able to make 4 passes without and belt issues.