Belt Conveyor Shaft Failure 2

[otball]

Hello all,

I just found this website/forum - seems to be a great resource of talented engineers.

My question relates to shaft failures that I've been experiencing on belt conveyors. We have several different HP motors ranging from 2HP to 7 1/2HP, all with C-Face motors and gear reducers. The output shaft of the reducers is mounted within 2" of the bearing, so the extended overhung load is minimal. However, some of the reducers are solidly mounted to the conveyor frame directly while others allow for some deflection due to the utilization of a torque arm. I'm not sure how true the shafts were prior to failure.

The trouble I'm having is that shafts have been failing, both between the bearing and gearbox, as well as between the bearing and squeeze-lock on the drive side. Other than torque loading and the compressive/tensive stresses caused on the rotating shaft, does anyone have any ideas on what else may cause the failures?

Additionally, to try to be proactive and identify the problem before failure, does anyone have any suggestions for determining the significant forces on the shaft while it's in operation other than some type of a rotary torque sensor?

We were also thinking about removing several shafts and doing some type of X-ray on the shafts as a spot check to see if there were internal cracks - any suggestions of additional testing methods?

Thanks.

 

[dvd]

Shaft steel selection comes to mind. Do you know what material the pulley shafts are made from? Were the pulleys all sourced from the same vendor? Are the conveyors uniformly loaded, or is there shock loading involved? It is unclear whether all of the reducers are shaft mounted, please clarify. Have you been experiencing reducer failures also?

 

[otball]

The shaft material is 1045 steel TG&P.

Yes, all of the pulleys were sourced from the same vendor (which made me consider that quality control could be an issue).

Shock loading has been minimal, but I'm glad you mentioned that - I hadn't really investigated that in too much detail, but it's worth a shot.

Yes, all of the reducers are shaft mounted. However, some are secured via direct mount to the conveyor frame and others are connected via torque arm. The ones that are failing are the secured connection type.

No reducer failures (yet!).

 

[Ross0684]

otball
I think you have answered your own question.
There is obviously mis-alignment of the rigid mounted drives.
From your initial question I perceive that there is a bearing between the drive motor and the drive pulley as well.
From experience this creates undue stresses unless the bearing mounting surface and the drive motor mountings are a fully machined arrangement.
I have built many conveyors without a bearing on the drive side ,just using the gearmotor as the shaft support, but due care needs to be taken to ensure correct alignment.
The answer is to have torque arm mounts in all cases, this is the very reason that they are available.
Consideration needs to be given though to shaft size to accomodate overhung loads due to the drive self weight.
Normal engineering practices should be maintained with regard to machining after welding, minimisation of stress concentration points at change in diameters, ends of keyways etc.

Another factor that can break pulleys is overtensioning of conveyor belts, as well, some belts shrink if they are hydroscopic and moisture is present, thus increasing the belt tension.( a lesson hard learnt from experience!!!)
Ross

 

[otball]

Thanks. I figured that the usual suspects were to blame, but figured I'd throw it out there to see if anyone else had any additional ideas. I've seen plenty of crack originations from the keyway slots, but didn't want to write this instance off as just another one.

 

[dvd]

See if someone has overtensioned the conveyor belts. This could cause the outboard shaft to move and give you misalignment at the reducer. Have you spark tested (or something) to see if the shafting is really 1045? I would consider un-bolting the rigidly mounted reducers to see where they want to reside. Since the reducers are shaft mounted already, you might consider weakening the rigid mounts in order to let them (and the reducers) flex a little.

 

[otball]

Overtensioning was one of the first things we checked. If not for component wear, the extra motor current draw from excessively tightened belts is reason enough to check that first thing every time.

We're investigating the shaft material from two fronts. First, we're getting the steel tested to verify that it is the steel that it's supposed to be. We're also trying to get verification from the manufacturer of the QC checks that they implemented before shipping out.

The rigid mounts are in the process of being redesigned - either to use a torque arm or a flange-type mount with some type of non-rigid material (i.e. rubber or similar). We're trying to go the opposite philosophy from what is currently designed - instead of ensuring alignment and secure fastening of the assembly, let it move and don't oppose the dynamic forces unnecessarily. I don't like to see the drive units wobble, but it sure beats having shafts break a lot.

Thanks for the ideas - if any more come to mind, please let me know!

 

[PeterCharles]

Perversely, I like to see a bit of wobble on shaft mounted gearboxes as it tells me they aren't locked up solid so producing a multi-bearing shaft with unknown additional loads that can lead to shaft or bearing failures. Belt conveyors aren't high precision things like an engine crankshaft. Oh, broken shafts, that's where this thread started....

 

[barron]

Can anybody in this forum assist with torque and friction drive calculations for an endless loop horizontal slat conveyor.
If so I will post full details.
Thanks and here's hoping
Roy

 

[MikeHalloran]

Roy, that's now how this site works.
Did you know you can rent engineers?
It's time for you to do so.



Mike Halloran
Pembroke Pines, FL, USA