I have a small DC gear motor which spools a plastic line. Once I've engaged the motor to tighten the line, I want to lock it in place so that the line doesn't unspool. I would then like it to remain in this position mechanically so that I do not have to apply holding current. When I'm ready to release the line, I'd then like to be able to electrically reverse the process and disengage the lock. Is there a standard design that fits this requirement?
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\$\begingroup\$ Huh, what? ---- \$\endgroup\$– Olin LathropCommented Nov 26, 2013 at 13:28
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8\$\begingroup\$ I felt really discouraged when I initially got down-voted by asking this question, but thanks to Scott I think he put my question in terms most engineers can understand. I try my best to communicate, but I don't always hit the mark. I guess that's why we're engineers. Being social isn't really our cup of tea or else we'd...probably be managers. \$\endgroup\$– AlexandruCommented Nov 26, 2013 at 14:13
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4\$\begingroup\$ This is a great example of how users here can re-word and edit questions to not only clarify the meaning, but also show how to make a question really shine. \$\endgroup\$– JYeltonCommented Nov 27, 2013 at 0:02
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3 Answers
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A common way of doing this is with a self-locking worm drive The worm drive has a gear ratio that provides high mechanical advantage and depending on the helix angle of the gear, the output can't backdrive the input, so it is self locking. You can find one here at Servo city or try to DIY with hardware store threaded rod and an off the shelf gear.
Another suggestion is a ratchet and pawl mechanism that may be easier to DIY than a worm drive and much cheaper than purchasing it. Note that the rachet can only be driven in one direction.
There are other ideas I can think of such as using a solenoid to lock/unlock a catch that holds the belt in place when the motor is stopped, but they are more complex than what I describe above.
Please post back here if you do build one of these. I love mechanisms but find that few people are interested in building them.
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\$\begingroup\$ I really like your answer, and I want to thank you very much for helping me out! I'm going to try and make use of the self-locking worm drive in a prototype for a project idea I have, so if it works out I'll post back here with a kickstarter link! I'll take advantage of this mechanism's abilities: The worm can easily turn the gear, but the gear cannot turn the worm. This is because the angle on the worm is so shallow that when the gear tries to spin it, the friction between the gear and the worm holds the worm in place. \$\endgroup\$ Commented Nov 26, 2013 at 14:42
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\$\begingroup\$ Worm drives hold because an increase in load torque will increase the bearing friction; depending upon the slope of the worm and nature of the bearing, its friction may be engineered to always exceed the load torque. Unfortunately, many worm drives will when driven waste a lot of power on bearing friction. A drive scheme may be designed so that the mechanism driving the worm also tries to push it away from the bearings, but I don't know how common that is. \$\endgroup\$– supercatCommented Nov 26, 2013 at 16:48
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1\$\begingroup\$ If you want a wormdrive 'out of the box', look for a 2nd hand windshield wiper motor from a scrapyard. It might be overkill depending on what torque you need, but you'll have a motor + wormdrive all in one package. \$\endgroup\$– RJRCommented Nov 27, 2013 at 1:58
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\$\begingroup\$ @RJR It was right under our noses. Thanks for pointing that out; I bet a lot of people will find that useful for larger applications. In my case, I need it to be pretty small. Do you have any ideas on what might contain a very small (even plastic) worm drive in the range of a few millimeters in size? Right now, I'm thinking Lego's...but I bet there must be something smaller and even more robust... \$\endgroup\$ Commented Nov 27, 2013 at 3:00
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3\$\begingroup\$ Funny, I was going to say legos. However, you might want to tear apart an old floppy drive (5 1/4'' or 3.5'') - the head is driven by a wormwheel. from memory, it's about 3mm diameter, 5cm long or so (but you can cut it down ofcourse) and the motor (stepper) is similar in size to a small DC motor (2x2cm or so). \$\endgroup\$– RJRCommented Nov 27, 2013 at 5:19
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You want a DC motor with a mechanical brake.
You can electrically brake (permanent magnet) motors by shorting the contacts together, but the braking force of this isn't very strong for small motors.
For your cord application, I'd look into some sort of clamp applied to the cord itself with a servo. Especially if it's safety-critical; I can't tell from the description whether this is the sort of cord used for opening curtains or the sort that might have heavy objects or people hanging from it.
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2\$\begingroup\$ I'd look at a mechanical pawl/ratchet/clutch that can be released with a solenoid, so you only have to power it when you want to slip backwards. \$\endgroup\$– John UCommented Nov 26, 2013 at 13:32
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If you don't mind intermittent motion, you may find a Geneva Drive mechanism to be suitable, although you may want to use the roller kind instead of the usual sort, because they are somewhat smoother. (I can't find an image right now, so I will add it later when I do find it.)
