Torque to Belt Pull relationship? 1

[MuskieBoy]

We have drive pulleys failing in the feild where the hub of the pulley is welded to the shaft.

I have been asked for a formula for a relationship between Torque on a shaft and belt pull. Is the torque the difference between the tension side and slack side times the radius? This sounds to simple to be correct. Specifically because it does not take into account the moments on the shaft itself.

Any insights on this analysis would be appreciated.

 

[CESSNA1]

MUSKIEBOY: Yes it is, but I do not know how you determine of measure the actual belt tension while operating. Just remember that the drive belt/pulley supplies whatever is called for by the load. It is the load that determines the belt tensions. I would start with the load first and then look at the belt tensions. You may want to have the pulley failure analyzed to determine if is fatigue, overload, design defect, etc.

Regards
Dave

 

[dwt]

Without knowing the details, my first thought is that this is a fatigue failure resulting from a "problem" with the weld. Typically there is some stress riser resulting from undercut, improper penetration, etc that initiated a crack which propogates under the cyclical loading. If the failure shows the beachmarks or striations that's your problem.

 

[MuskieBoy]

Thanks Dave,

We have pulleys being analyzed at the University of Iowa's metalurgy deopartment now. My boss asked for the formula. He's in meetings with the President, Purchasing Manager, and Quality Control. (Notice the absence of an engineer in that group?) I can formula the belt pull for maximum load per the job specs. I am just concerned if I analyze torque and moment seperately I will not get a correct answer.

Thanks again,
Dan

 

[MuskieBoy]

dwt,

Yes there are weld issues going on. I have information coming in from the feild from other units where there are stress fractures at the centerline of the weld about the diameter. We are also looking at the thickness of the hub.

Basically we have been specifying the same pullies for years and seem to have fianlly crossed a line when we switched to direct drives. (The shaft extended through the reducer.) I am doing a design analysis from scratch to determine where the miminum specifications are to our current designs.

 

[sbi]

A few questions?
Are you breaking the driven pulley or the driver?
which one is has the larger OD?

the reason, I work with a lot of drive pulleys on Diesel engine were th drivei the larger and along the way they forgot that you figure HP and torque transferd on hte smallist pulley, which if the belts have been grossly over tighten will lead to the demize of the smaller (driven) pulley

Bill L

SBI
Central Ne.,USA

 

[MuskieBoy]

Hi Bill,

The Drive Pulley is the one giving us problems. Yes it is the larger of the pulleys in the system. We are confident there is a welding problem that we are looking into with the manufacturer. I have a couple of engineers working on verifying the overall pulley design. That is where this question is coming from.

Word from the feild is not as bad as feared. It looks like we have between 5-10% with visible cracking in the weld.

Thanks,
Dan

 

[sbi]

OK is the crack around were the weld was started or ended. this is the coldist part of the weld, that is why alot of us go over the beginning ny a inch or so

after assembly, is there a noticable wobell? or a out of ballance to help create a vibration to cause the crack to start?
One of my other favorite's how tight was the belt? alot of people like to make bow srings out of drive belts, adding to much side load adding to the smaller problem
Since we do alot of cast iron pulley for industrial engines, I have seen many a good engine bite the dust from to tight belts that pulled frount of the crank of the engine

SBI
Central Ne.,USA

 

[Cockroach]

Have you checked out other threads on this website? There was a discussion recently on this very issue.

Kenneth J Hueston, PEng
Principal
Sturni-Hueston Engineering Inc
Edmonton, Alberta Canada

 

[Philrock]

One school of thought is that unless a weld is stress relieved, virtually all of the material in the weld is at its yield point - half of it in compression, half in tension. This being the case, it will not do well at all in a fatiigue application.

I've heard a recommendation that if a weld is used in a fatigue application without being stress relieved, the calculated nominal maximum stress should not exceed 2,000 psi.

Shot peening is widely used to do some stress relieving without heat.

 

[dvd]

We had a number of pulley failures due to poor welding procedures followed by our pulley manufacturer, especially when the shaft material was 1045. Based on the number of problems encountered, we finally converted most of our pulleys to a shaft locking device (Ringfeder, B-Loc, etc.) and eliminated the welded shaft. That was very beneficial for us from a reliability and cost standpoint.

Are your gear boxes shaft-mounted or are they base-mounted with a hollow shaft? I ask this to determine if there is a possibility of a large induced load in the shaft due to misalignment of a base-mounted reducer and the pulley center (assuming the pulley is supported by bearings on each side). Depending on your previous belt or chain drive, I would think that the belt pull would exceed the load from a shaft mounted reducer, especially if you used a poly chain drive. But this will depend on size of reducer and motor, etc. So you may have overloaded the shaft due to weight of the shaft mounted reducer. Another possibility is that when you went to the shaft mounted reducer you moved the overhung load further out away from the bearing and thus increased the bending moment on the shaft even though the load may not have increased.

 

[diamondjim]

I would be just a little concerned about how
much sulphur is in the material. I know that this
can cause cracking in the welding process.