Hey guys, new to the site and electronics in General so I wanted to ask this question in a place I know I will get a correct answer, and if not, then directions to an answer or alternative. Essentially I am making a project in which I need a motor to pull something that is under tension, at the apex of the pull I need the motor to stop pulling and I suppose, to release the load its pulling so that the tension it was working against can pull it in the opposite direction. God I hope that made sense. It's like a slingshot or a bow in which your hand is the motor pulling against the tension. My question is, is there a motor that would allow free movement in the opposite direction after pulling without providing resistance? Any help in the right direction would be appreciated. Thanks guys.

 

Can you not use a motor and an electromagnet, most dc motors run freely with no load connected.

 

If it really is for something like a slingshot beware that the motor will most likely be geared to have enough torque, which when you let it go will have large amount of inertia and will slowly accelerate under tension.

 

Lots of ways to accomplish the task.. More details are really required

Some potential mechanical solutions are
A motor with a snail cam (or similar) on its shaft.. The cam will release the object at a certain angle of rotation..
Or a "trigger" is used to free the motor from the gearing.. (pawl gear or similar)

 

How about a linear actuator?

Available with stroke length up to ~1.5m

You would have to provide a "release" for the object being pulled at one end of the stroke, then extend the actuator and "couple" to the object at the other end of the stroke.

 

It sounds to me as though at the end of the pull, you need to switch the motor off and allow the motor/mechanism to back feed under the its own steam by virtue of the pull created?
If so you need a mechanism that is capable of backfeeding with minimum resistance, which usually precludes most worm drive or mechanisms such as linear actuators.
It could be done in a few ways but a little more on the actual application would help.
Max.

 

Man, great feedback so far! Ok this is for a moving transport system. I'm using a silly on a track with resistance bands to pull the dolly from one end of the track to the other at a high rate of speed. I just need the motor to reset the dolly to the starting position of the track, and then release tension so the resistance bands can pull it back to the other side quickly. The motor has to be strong enough to pull the dolly plus the resistance bands, but the track is also 21 feet long! So I don't want it to take all day! Maybe a 10 second reset?

 

How about a linear actuator?

Available with stroke length up to ~1.5m

You would have to provide a "release" for the object being pulled at one end of the stroke, then extend the actuator and "couple" to the object at the other end of the stroke.

That's a great idea and I thought of that, but I need it to move 21 feet and thats a huge linear actuator!

 

Is it something heavy like a camera dolly? You might want to use one motor on each side, so that one will pull the dolly by a string in each direction.

 

You may want to look at a rack and pinion system if that long a pull, is it possible as has already been suggested that at the extreme end the load it can be released and then the motor follow it back to re-latch?
Max.

 

For that length you could always just use a simple motor with belt drive (ht type timing belt and a few pulleys)
Use some limit switches to indicate the end position and reverse the motors..
Use stepper motors and you could have all sorts of motion profiles,etc...