Check Calculation
Hi etse51815,
Check your shaft calculation on our free beam deflection spreadsheet on www.cospire.com / Engineering Tips / Quick Structures.
It all depends on your drive arrangement. On the pulley side the main consideration is bending stress limits. Deflection as such is not a major controlling component.
However, high shaft deflection can cause local stresses in your gearbox - hence high wear of bearings.
Use a stepped shaft to give you extra stiffness - resulting in reduced deflections. Or add an extra internal diaphragm in the drum, so the drum face provides the required stiffness.
Regards - John.Rz ■
Re: Pulley Shaft Deflection
Thanks for the advice Mr. John.Rz
I agree that for a drive pulley, the shaft size is normally governed by the stress limit. In my application I’ve got a 1650mm wide belt with relatively high tensions, so most of the bend pulley shaft sizes are governed by deflection.
To be more specific with my question, I’ve got a High Tension bend pulley before the drive. Its shaft size is also governed by deflection. Will it be overkill to select a shaft diameter for this HT Pulley based on the start-up tensions, which it will be exposed to for short periods? ■
Re: Pulley Shaft Deflection
Momentary belt tensions can be factored in a similar way to structural steel methods. We have written specification on the pulley stress analysis method that includes a load reduction for momentary forces of 25%. Thus, when the high momentary T1, T2, ect. values are divide by 1.25 and then are still larger than the high normal loaded operation, the resulting T1/1.25 will be used.
Care must be taken to include the combined T1 & T2 resulting force as they occur in operation and not from their separate maximums. ■
What Start Up Tensions?
Good point lads.
What do you mean by start up tensions? Is that real, or hypothetical?
1 - A drive motor has a certain maximum torque. This varies with the details of the motor windings, and the starter kit.
2 - Are we looking at a cold start empty, or from an emergency shut where the whole belt is full and overflowing?
3 - Do a What If Analysis http://www.cospire.com/browse.aspx?id=4317 . Is your mathematical catastrophe certain, probably, or only theoretically possible [belt, braces, and tin hat. What if a failure did occur? Would the sun stand still? Would you be sued?]
4 - Don't get tied up in knots by the politics of "Safety Factors". We are trying to assess realistic probablities of failure, and their monetary and safety consequencial damages.
5 - What can we control, or change? Starting time ramp on the PLC?
It is OK to use your engineering judgement at times - that's what you are paid for [just watch out for the legal bullies - they can be real snakes-in-the-grass].
Regards - John.Rz ■
Re: Pulley Shaft Deflection
Starting and stopping under full load is a reality. It might not happen regularly, but it happens. So I’m considering momentary tensions under these conditions.
Yes, you can control or change start-up to reduce these high momentary tensions, but what about braking? Let’s say you’ve got an emergency stop and you need to brake a long, fully loaded conveyor in a short period (to not flood the short receiving belt which stopped as well) with a brake on a low-tension pulley. If this pulley shaft were sized for normal run tensions, I would think that there is a realistic probability of failure during braking.
Thanks for the advice from both of you!
Regards ■
Re: Pulley Shaft Deflection
Not saying that it isnt possible, though I havent seen a relaxation on deflection criteria for conditions other than running (even "abnormal" - motor break down torque etc).
We would consider a fully loaded start an operational condition.
Put a flywheel on the receiving conveyor?...
Regards,
Lyle ■
Pulley shaft deflection
Hi,
I want to size a pulley shaft for deflection. The conveyor is designed for 6 starts per hour. Can anyone tell me if start-up tensions should be used to determine the minimum shaft diameter at the hub that will satisfy the deflection limit set by the locking device?
Start-up tensions result in a shaft diameter 20mm bigger than the required diameter to satisfy the deflection limit under normal run conditions. ■