# Converting unidirectional linear motion into rotary motion?

by geekedsloth
Tags: converting, linear, motion, rotary, unidirectional
 P: 9 Does there exist a device to convert a unidirectional linear motion into a rotational motion? I have only been able to find devices which do the exact opposite. I am probably missing out on some kind of key point here but I don't have much of a background in physics so please don't slam me too hard for asking.
 PF Gold P: 8,964 Welcome to PF, Geekedsloth. Unless I misunderstand your question, a crankshaft does exactly what you want.
 P: 1,986 Most engines have this. The linear motion of the pistons is converted into circular motion of the axles and wheels. They use some system of cam shafts. Danger was faster...
 P: 9 Converting unidirectional linear motion into rotary motion? I am sorry I should have stated the question more clearly. Let me give an example: I am pulling back on a rope attached to a device, the device would convert the backwards pulling motion into a rotary(or any other motion for that matter) motion and for instance spin a wheel attached to the device. Am I missing something or is this physically impossible? I can not find any information on such a device and this is going to bother me until it is resolved one way or the other. I suppose another way to word the question would be is it possible to convert a linear motion that moves only in one direction into another motion? A good example would be pulling on an end of a spool of thread and unraveling it. The linear motion of pulling the string back is converted to a rotary motion as the spool rotates. However, this does not fit my purposes as once all the string is off the spool the device no longer functions. I would need the device to function as long as there is a linear energy input. I hope there is a really simple solution to this that I am not seeing.
 HW Helper P: 7,175 The linear motion could pull a wheel across a plane. The wheel would be moving linearly, but it would have rotational motion.
P: 1,986
 Quote by geekedsloth I suppose another way to word the question would be is it possible to convert a linear motion that moves only in one direction into another motion?
Are you sure you have clearly in your mind what you want?
Most of the devices converting a linear back-an forth motion will work even for motion in one direction (half the cycle). But how do you imagine a device with parts moving in one direction? For how long?
P: 9
 Quote by rcgldr The linear motion could pull a wheel across a plane. The wheel would be moving linearly, but it would have rotational motion.
While that would work it would become very impractical after a certain point. The device would need operate indefinetley as long as there is being energy put into it. That would require a very very long plane.
P: 9
 Quote by nasu Are you sure you have clearly in your mind what you want? Most of the devices converting a linear back-an forth motion will work even for motion in one direction (half the cycle). But how do you imagine a device with parts moving in one direction? For how long?
The device would need to operate as long as energy is being put into it. I thought a device for converting a linear motion would have to have a back and forth motion to continue operating without becoming impractical as demonstrated by the wheel moving across a plane. The device would need to continue to function without any input other than the linear motion(no resetting it by, for example, placing the wheel at the beginning of the plane). In the device suggested above would it be possible to harness some of the energy being put into the device to rotate the plane 180 degrees once the wheel reaches the end of the plane thus essentially "resetting" the device and putting the wheel back at its starting point?
PF Gold
P: 8,964
 Quote by geekedsloth I am pulling back on a rope attached to a device, the device would convert the backwards pulling motion into a rotary(or any other motion for that matter) motion and for instance spin a wheel attached to the device.
That is the perfect description of the recoil starter on a lawn mower or weed whacker.
 P: 9 I will have to look into that in more detail tomorrow, Danger. I am not familiar with the inner workings of the device you mentioned. However, if that does not meet specifications I am beginning to form a concept in my mind that I will make a sketch of tomorrow that I will upload. Any critique on it would be much appreciated if you feel so inclined. Edit: Is that device not limited in that once you have pulled the string out as far as it will go you must let it retract before it can perform its function again? The device I need would require a unidirectional linear force. In other words, the string can not be allowed to retract, a constant linear force must be applied to it. So, I believe one way of doing that would be to have the device itself rotate around the string by 180 degrees at a certain point. I know that is extremely vague but I will try to explain in more detail tomorrow with something visual.
PF Gold
P: 8,964
 Quote by geekedsloth the string can not be allowed to retract, a constant linear force must be applied to it.
Well, then... I'm gonna have to think upon that for a while. I have a couple of ideas, but even I consider them both to be the result of insanity. Catch you tomorrow.
 HW Helper Thanks P: 5,475 A waterwheel (or a modern turbine) converts the constant linear motion of river water into rotational motion. A windmill does the same for air. A pulley does the same for the linear motion of a chain passing over it.
 P: 9 Thanks for your contribution, Nascent Oxygen. The more responses I get the more I realize how vague I was with my question lol this is starting to get very specific. For my purposes, a modern turbine or windmill or such a device would not work. It does provide the function I asked for but that goes back to me being too vague. It's hard for me to be specific enough with my wording as I don't know many of the proper terms etc and would end up confusing even more, going to finish the sketch and find somewhere to upload it and ill post a link. It should help to explain a lot better.
 P: 9 First of all I would like to apologize for the crudeness of the picture, don't have many tools to work with or time at the moment so it is the best I could do. Of course this is just to illustrate the basic concept of what I am trying to get and there would be numerous flaws with this design, the most obvious being that the device will no longer function once it reaches the top of the plane, but again this is just in the hopes of more clearly explaining what it is that I have in mind. With that being said, would the basic idea here be plausible, and, if so, any suggestions on a way to reset the cylinder to its starting position once it reaches the top of the plane? Such as having the plane itself rotate 180 degrees with the cylinder at the top, essentially putting it back at the bottom? Of course I know that wouldn't work with the example I drew. And yes, I did make it in paint. I'm not Picasso. Edit: I'd also like to thank rcgldr, as the post made above is what helped me think of this. This is also where my skepticism comes into play. If the plane were to rotate, would I be correct in assuming it would need to overcome the force of the balloon pulling it up before rotating?
 HW Helper Thanks P: 5,475 You could have a screw thread inside of the cylinder, so as the balloon rose it dragged something up the thread causing the cylinder to rotate. There's a bit more to it, but that's the idea. It will take more energy to return the balloon to the ground than you can extract through it rising.
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Thanks
P: 5,475
 Quote by NascentOxygen It will take more energy to return the balloon to the ground than you can extract through it rising.
Unless you are prepared to wait until night time? I'm thinking it may be possible to arrange for the air in the balloon to cool faster than does the air at ground level, this causing the balloon to gradually descend of its own accord.
 P: 452 What about a simple cart? You pull it and the wheels rotate.

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