I am designing a lifting machine that uses a roller ball screw (1" lead) driven by a 10:1 speed reducer (wormscrew type). The torque that the weight of the load will exert on the output shaft of the reducer is about 160 inlb (the gearbox is rated for 450 inlb @ 170 rpm). Do I need a brake motor to prevent the screw from back driving the motor?
I would appreciate any knowledgable input.
stan
Having ridden a basket to the ground (quite rapidly) in a very similar situation, (don't ask ) I would make allowance for a brake more to stop the download situation when lowering under power. Nothing (mine or it) got broken in the e-ticket ride, but there certainly was some wounded pride and embarrassment.
The ballscrew will happily run backwards under load but the load would need to be prodigious to make the worm drive reducer run backwards. Since the calculated torque is about 1/3 the rating of the reducer I would say none required for over-drive from a suspended load, but stopping it where you want it might be handy, so I'd opt for a brake for that reason if no other.
Whether a worm gear will back drive depends on the helix angle of the worm and the coefficient of friction between the worm and worm wheel. There is a significant difference between the static and dynamic friction, thus a gear that is locked and stationary may back drive in the presence of vibration, or if the input shaft is disturbed.
I've seen the cutoff ratio between self locking and back drivable gears quoted--I'd talk to the manufacturer of the reducer for a number. If safety is a factor, a brake on the motor may be a good idea unless the gear ratio is very high.
Ned Simmons
The ball screw will transmit torque in both directions. Worm gear drives take a bit more analysing. A 10:1 speed reducer could have a single lead worm and ten teeth on the worm gear. Or it could have a double lead worm and twenty teeth on the worm gear. I don't think the first one will turn from torque applied to the output shaft. I think the second case will.
I really am not a knowledgeable source, but think you need to provide a bit more info so some knowledgeable person can respond.
Dan
Thanks for your input. I called HubCity and they told me that they could not guarantee any ratio that would not backdrive.
stan
Cone Drive says ratios up to 15:1 will typically back drive freely. Ratios from 20:1 to 40:1 can be considered as overhauling with difficulty. Ratios above 40:1 may or may not overhaul depending on loading, lubrication, and vibration. They note that ratios as high as 100:1 have been known to back drive under certain conditions.
Note that they're speaking of single reduction gears.
You need to assess the safety consequences of the gear failing to hold the load in order to make a judgement as to whether a particular gear can be relied on to support your load without an auxiliary brake.
Ned Simmons
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