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I'm planning to build a machine which at some point tows a heavy weight on a rope in the air. Once the weight is heigh enough up in the air I want to keep it there for about an hour or so, before I want to put it down again in a controlled manner.

As far as I know, I can brake a DC motor, but I think this takes energy so I would like to avoid doing that for as long as an hour. I guess there should be other options, such as putting a lock of some sort on the rope, but I have no clue how something like that would work.

Could anyone give me some tips on what a usual/best way of blocking a rope for a long time is?

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  • $\begingroup$ "I can break a DC motor ...". You can break anything if you hit it hard enough. Maybe you want to brake it? $\endgroup$
    – Transistor
    Jun 28, 2017 at 22:29
  • $\begingroup$ Shorting the terminals of the DC motor makes it brake very strongly from movement, but the braking strength is proportional to RPM, so it won't hold the motor stopped - turning it slowly will be still perfectly possible. So yeah, you need an external brake. $\endgroup$
    – SF.
    Jun 30, 2017 at 11:29
  • $\begingroup$ @SF. - Thanks for the tip about braking a DC motor. I didn't know that. Do you think a worm gear is a good idea, as suggested by Dave Tweed in the answer below? $\endgroup$
    – kramer65
    Jun 30, 2017 at 14:09
  • $\begingroup$ @kramer65: Yes. It limits maximum speed, but increases torque. Generally any of gear solutions that change speed to torque a lot will work - simply, friction of the gearbox makes it movable only by turning the low-torque side; trying to turn the high-torque side would require torque so high it would sooner break the gearbox than turn the other axis - overcoming the friction from one side is vastly easier than from the other - to, and beyond the point of breaking the whole thing. So, yes, it will work perfectly well, but the speed of the tow rope spool will be much, much lower. $\endgroup$
    – SF.
    Jun 30, 2017 at 14:38

3 Answers 3

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You'll need a separate braking mechanism on the motor shaft, or somewhere in the gearing between it and the rope.

You can get brakes that are normally engaged, and release on the application of power, which you would do at the same time that you energize the motor.

You could also use something like a worm drive between the motor and the rope, which is normally self-locking — the motor can easily move the rope, but the rope cannot back-drive the motor.

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  • $\begingroup$ especially for the worm drive suggestion - zero energy to hold the load. $\endgroup$
    – Solar Mike
    Jun 28, 2017 at 22:47
  • $\begingroup$ Thanks for the tips. About the brake which releases when being powered; what are those called? I'm not sure what words I can search for. And a worm gear is also a very good idea. $\endgroup$
    – kramer65
    Jun 29, 2017 at 2:55
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The normal solution to this is called an Electric Winch or gear reduction motor with wired remote control and mounting bracket.

For any requirement there mut be specs or "requirements" which usualy stem from ; budget, time frame, skill (make or buy) then power, acceleration , top speed, mass, which are all related.

Thus every problem is easier to solved when you start by defining needs, or specs, without which, it becomes a "witch hunt".

The reason a Winch is preferred is because it has the mechanical advantages of transforming power by trading off speed for force and then this ratio with friction in the gears also provides a natural brake.

These are low cost and must be driven by a car battery with a trickle charger or equiv. A $100 unit weights 5kg and can lift up a weight of 500kg or more (750?1000?) I got mine on sale at half price. an you shop for DC gear motors DPDT 100A contactor switch and make your own fixtures but its better to buy one. You can imagine it runs 100x slower on RPM due to gear ratio.

Life all about tradeoffs and specs for Engineers. What's yours?

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One thing that I did to implement a brake system in one of my projects was to install a 20:1 worm-gear reduction. The worm gear, when fully metal and mounted in the proper housing, can act like a brake when stopped, since the interlocking gear cannot turn the worm, but the worm can turn the gear. For this, you will want the motor attached to the worm and whatever equipment you are moving/rotating/mounting/etc.

I would like to note, that while this DOES work - it can and will cause strain and wear on your gears if the load is very heavy. I would recommend that you get a larger gear (more expensive) for heavier loads.

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    $\begingroup$ There are jacks for lifting trucks that use the same worm gear method - and lift 10 tonnes or more. Also, what you put in your answer was already in Dave Tweed's answer from June 28th. $\endgroup$
    – Solar Mike
    Jul 31, 2017 at 8:49
  • $\begingroup$ Thank you for the clarification, Mike. I did not read previous comments before posting my information, and also did not know about the lifting trucks usage of the worm gear method - I thought it was something like a heavy-duty ballscrew. $\endgroup$
    – Jason
    Aug 1, 2017 at 1:08

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