Need to reduce impact load due to gear backlash
Need to reduce impact load due to gear backlash
(OP)
I am designing a gear+motor arrangement that will start the driven equipment shaft train from rest ("Breakaway"). The motor is sized such that its 300% starting torque (locked rotor torque) is required to achieve breakaway. There is considerable backlash in the gearbox and inherent to its design I cannot reduce the backlash. This results in a high impact load when the motor is started DOL. I need to find a way to engage the gearbox slowly and/or softly to minimize or eliminate the impact load at start-up.
Here are the options I have considered so far:
1) Mechanical soft-start, i.e. fluid coupling- The fluid coupling rating would be equal to the full load rating of the motor, but we need to take advantage of the 300% starting torque of the motor to achieve breakaway, which the fluid coupling will not transmit.
2) VFD- A variable frequency drive would allow me to creep the gears into engagement, but I believe most VFDs will only allow ~150% rated motor torque which only gets me half way to where I need to be
3) Electrical soft-starter- We have tried these before and have found that as soon as the soft start module detects no load, it bypasses the soft-start function and allows full power to the motor. In our case, this is immediately upon start-up due to the backlash in the gears.
4) Pre-engagement motor- I have considered using a fractional-power motor mounted to a free shaft, or piggybacked onto my main motor, whose only function is to take up the backlash in the gears, but is not large enough to cause any damage or start the driven equipment turning. From my perspective this is the easiest solution, however it is not the most 'elegant' solution in that it causes the need for an additional motor starter and control logic to properly sequence the operation of the small pre-engagement motor and larger main drive motor.
5) Manual pre-engagement- Another simple solution, but the intent of the system is to be fully automatic and therefore not requiring manual operation. Plus the lock-out / tag-out procedures required to prevent accidental energizing of the motor while manually turning are cumbersome.
I am open to any suggestions you all might have! Thanks in advance for any help.
Here are the options I have considered so far:
1) Mechanical soft-start, i.e. fluid coupling- The fluid coupling rating would be equal to the full load rating of the motor, but we need to take advantage of the 300% starting torque of the motor to achieve breakaway, which the fluid coupling will not transmit.
2) VFD- A variable frequency drive would allow me to creep the gears into engagement, but I believe most VFDs will only allow ~150% rated motor torque which only gets me half way to where I need to be
3) Electrical soft-starter- We have tried these before and have found that as soon as the soft start module detects no load, it bypasses the soft-start function and allows full power to the motor. In our case, this is immediately upon start-up due to the backlash in the gears.
4) Pre-engagement motor- I have considered using a fractional-power motor mounted to a free shaft, or piggybacked onto my main motor, whose only function is to take up the backlash in the gears, but is not large enough to cause any damage or start the driven equipment turning. From my perspective this is the easiest solution, however it is not the most 'elegant' solution in that it causes the need for an additional motor starter and control logic to properly sequence the operation of the small pre-engagement motor and larger main drive motor.
5) Manual pre-engagement- Another simple solution, but the intent of the system is to be fully automatic and therefore not requiring manual operation. Plus the lock-out / tag-out procedures required to prevent accidental energizing of the motor while manually turning are cumbersome.
I am open to any suggestions you all might have! Thanks in advance for any help.





RE: Need to reduce impact load due to gear backlash
2) Use a wimpy VFD and separate contactor. The VFD drive to take the backlash out; then the contactor engages for full current.
RE: Need to reduce impact load due to gear backlash
I like this idea. We would essentially supply a control box along with the mechanical package that would include the VFD and the DOL motor starter contact.
RE: Need to reduce impact load due to gear backlash
There are rubber shaft couplings out there. Would one of these be resilient enough to protect your transmission?
Why is the impact load a problem?
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JHG
RE: Need to reduce impact load due to gear backlash
RE: Need to reduce impact load due to gear backlash
http://www.dodge-pt.com/products/pt_components/fle...
Mike Halloran
Pembroke Pines, FL, USA
RE: Need to reduce impact load due to gear backlash
Do you have any personal experience using these couplings?
RE: Need to reduce impact load due to gear backlash
Mike Halloran
Pembroke Pines, FL, USA
RE: Need to reduce impact load due to gear backlash
http://www.fluxdrive.com/smartcoupling.html
Walt
RE: Need to reduce impact load due to gear backlash
I read your OP and I'm unclear why backlash in the geartrain is an issue. If there is backlash present in the geartrain at startup, why would the motor "breakaway torque" be so high? If there is backlash in the geartrain when the motor is energized, there will be very little load created on the motor until the backlash is taken up. And then the dynamic loads produced on the gear flank contacts is a function of the motor inertia versus the driven load/inertia and drivetrain stiffness.
It seems to me that you have a motor starting load problem rather than a geartrain impact load problem. Even a commercial grade gearbox with several stages would only have around 20-30deg of total backlash at most.