Back-driven Worm Gear
Back-driven Worm Gear
(OP)
Hi,
I'm hoping a more experienced ME can shed some light on my problem.
The projet I'm working on involves a simple gear train (all spur gears) that is being driven by a DC motor with a worm gear. The last gear in the train (which is the output gear) is connected to a shaft in the center, but also has a handle attached to that shaft so the user can manually turn that gear and rotate the shaft. All gears here are plastic. This is an electronically operated deadbolt -- same as ones you'd have in your house -- as pictured below.

My problem is this:
As far as I know worm gears can't be back driven, or at least not without seriously damaging the gear directly connected to them, so how do I design my system so that the deadbolt can be turned by the DC motor but also turned by the user manually?
I even bought an electrically operated deadbolt and took it apart. They have a DC motor, with a worm gear, and a gear train very very similar to the design I have in mind. How is it that their product lasts in the field without the gears being destroyed? Their spur gears are very standard plastic, straight cut. Their worm gear seems to be standard as well. I can include pictures if needed.
Was I misinformed? Are worm gears allowed to be back driven without any trouble if they are not helical? Is there something here that is dictated by the height of the threads on the worm gear and the depth of the spacing between the spur gear connecting to it?
Thank you in advance.
I'm hoping a more experienced ME can shed some light on my problem.
The projet I'm working on involves a simple gear train (all spur gears) that is being driven by a DC motor with a worm gear. The last gear in the train (which is the output gear) is connected to a shaft in the center, but also has a handle attached to that shaft so the user can manually turn that gear and rotate the shaft. All gears here are plastic. This is an electronically operated deadbolt -- same as ones you'd have in your house -- as pictured below.

My problem is this:
As far as I know worm gears can't be back driven, or at least not without seriously damaging the gear directly connected to them, so how do I design my system so that the deadbolt can be turned by the DC motor but also turned by the user manually?
I even bought an electrically operated deadbolt and took it apart. They have a DC motor, with a worm gear, and a gear train very very similar to the design I have in mind. How is it that their product lasts in the field without the gears being destroyed? Their spur gears are very standard plastic, straight cut. Their worm gear seems to be standard as well. I can include pictures if needed.
Was I misinformed? Are worm gears allowed to be back driven without any trouble if they are not helical? Is there something here that is dictated by the height of the threads on the worm gear and the depth of the spacing between the spur gear connecting to it?
Thank you in advance.





RE: Back-driven Worm Gear
There is another possibility. Gears normally produce a separating force under load. I have seen a few mechanisms where one gear is mounted elastically, e.g. on springs or on a structure that is intentionally flexible in the direction of the separating force, so at a particular torque level the teeth just 'ride over' each other without damage. It usually makes a soft ratcheting noise. ... sometimes not so soft.
I _think_ I have seen a few mechanisms that do the same thing without the ratcheting noise. I'm not sure how that works, but I'd guess axial travel of a worm, or maybe some funny business with cams and clutches.
Mike Halloran
Pembroke Pines, FL, USA
RE: Back-driven Worm Gear
RE: Back-driven Worm Gear
RE: Back-driven Worm Gear
So I guess my question now is: Is this o-ring a special type?? Would any o-ring work? I'm guessing this is a common scheme?
Sorry I'm an EE by training and trade, I'm doing this for a personal project so I appreciate any guidance on where to find this o-ring or how to spec it, from my fellow MEs.
Thanks.
RE: Back-driven Worm Gear
Mike Halloran
Pembroke Pines, FL, USA
RE: Back-driven Worm Gear
RE: Back-driven Worm Gear
Walt
RE: Back-driven Worm Gear