Torque to Back drive a worm and wheel gearbox. 1

[MattieG]

Hi All

I am having difficulty in determining the torque required to back drive a worm and wheel gearbox. I have calculated the efficiency of the gearbox when the worm is driving and when the wheel is driving.
I understand that when the efficiency of the wheel driving is less than zero the gearbox will not backdrive, however there must be an input torque at which you can force the wheel to turn the worm.
Does anyone know how to calculate this? Any help would be appreciated.

 

[Dennyd]

Unless you have an unusual worm drive you probably will not be able to back drive it. This is actually one of the reasons for their use. Compared to inline gear trains, which have transmission efficiencies in the 90+% range, worm drives are in the 70% range so they are undesirable from that standpoint. However, since worms typically cannot be backdriven they eliminate a need for a brake or some other device, which is advantageous for the overall system.

What are the particular conditions that would need the back drive capability?

- - -Dennyd, P.E.

 

[MattieG]

The reason for the question is that we have a 60:1 worm gearbox fitted to an application, with around 25Nm load on it. My feeling was that it should not back drive, which would eliminiate the need for a brake etc. However, in service the system creeps down. I have considered modifying the geometry of the worm but would like to determine the torque required to back drive the wheel, in order to access whether it would resolve the problem.

 

[Dennyd]

Typically ratios above 30:1 are considered non-back drivable. At 60:1 your system should be solid. However, if your system is creeping down I suspect you have something else that is contributing, such as a vibration condition. If you are trying to prevent this creeping then you have several options, such as eliminate the vibration, put a brake on the output side of the drive (strong brake torque) or a brake on the input side of the worm drive (1/60th the required brake torque) or "lock" the input drive so that it does the braking.

Is the drive motor electric or hydraulic? If it is hydraulic you can close the inlet and exhaust ports with a valve and that may provide adequate braking. If the motor is DC electric you may be able to switch its terminals to where they are shorted, which does a surprisingly good job of braking. If the motor is AC electric you might have some other electronic braking options, but I'm not knowledgeable enough about electric motors for that.

As I proof this and reread your comments it sounds more like your problem is one of output positional creep and not one of backdriving torque. Is that correct?

- - -DennyD, P.E.

 

[simongee]

MattieG,
In order to be able to help you in your quest for an answer it would be best to submit information on the drive:
What type of motor are you using AC, DC, Pneumatic or Hydraulic?
What is the power and speed of the motor?
How is the motor controlled?
Is the motor directly coupled or via another drive system ie belt & pulley, chain & sprocket, if so what is it?
What size is the worm gear reducer? Metric or Imperial?
What is the load?
What is the operating cycle?
What is the mounting position?
What is the application?
With this information I may be able to assist you.
Regards
simongee

 

[MattieG]

Thanks Dennyd & Simongee for your assistance.
The system is driven by a DC motor, with max power of 1kW @ 4800RPM. It is controlled through simple control gear, but the motor does get shorted out when stationary. The motor and Gearbox are coupled to the driven device through a sprocket & chain. The ratio is 60:1 (metric), which is holding a load on a slope equal to 25Nm. But we are finding that the load still moves, backdriving the gearbox. Although I can not reproduce this on the bench, by enducing a similar torque into the gearbox.

 

[beloka]

My experience with worm boxes has been that under a reverse load condition they will almost certainly creep backwards. They cannot perform the function of a holdback or holding brake safely. Vibration as noted by Denny D makes all the difference. Reverse efficiency calculations do not provide the answer. Refer Machinery's Handbook Page 797 of the 22nd Edition sums up the situation.

 

[MattieG]

Many Thanks to everyone for your help. I agree the only way to be certain is to fit a brake. We shall continue testing.
Thanks Again
Matt

 

[strokersix]

MattieG,

Can you change to a different or eliminate the lubricant? If this is an intermittent use or few total hours expected life it may be possible to run without lubricant.

I'm not saying this is a good idea, just something to consider.