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    Discussion in 'The main mechanical design forum' started by proteanblue, Aug 25, 2011.

    1. proteanblue

      proteanblue Member

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      Hi All,

      I've been toying around with this transmission for some time. I'm offering it up to see if anyone has any ideas on how to improve it in a few key areas. I couldn't attach a drawing, so will try and describe it.

      Right now the transmission is composed of two parallel axles, which are connected by a spur gear set. The point of the transmission is to deploy and retract a "foot", attached to the driven axle. In short, by rotating the driving axle clockwise you engage the pinion and force the foot to rotate counterclockwise. Past a certain point the foot drives against a stopper. At this time the driven spur gear disengages by way of a slip clutch, and the foot is held in place via a ratchet with cam follower.

      To retract the foot, the driven axle is rotated counterclockwise. It drives a cam which forces the ratchet to retract. Built into the foot is a returning spring, which forces it back to its original position. I should note that both the pinion and cam follower have a built-in one-way clutch that causes them to disengage in clockwise and counterclockwise rotation of driven axle, respectively.

      Phew! Still following? Okay, in order for the design to work properly I need to do a few things, and this is where I'll need your help. I'll constrain the design space first: the mechanism is only controlled by clockwise or counterwise rotation of driving axle. Now, here goes:

      1) Axle Positioning. I need to push the axles farther apart without increasing thickness of assembly (for scale: the foot is about 3 inches long). My first thought is a belt. Now, the only problem is I would need to twist it, or consider an idler because it would no longer reverse rotation. Secondly, if I was to use a belt would it be reasonable to use the idler to tension it such that I could get rid of the slip clutch? The slip clutch currently disengages at 10 in-lbs, so a pretty small torque. The release torque doesn't have to be precise. Anything above 10 in-lbs would work fine. This is pretty mild application: probably less than 30 cycles per day, for a number of years; inside gear box; small torques; slow rotation.

      2) Deployment. Right now, anytime you turn the driven axle clockwise (in sideview) the foot is deployed. I want the system only to deploy if it passes through top dead center (the assembly is housed, and TDC could be set relative to the housing). I've been looking at single revolution clutches http://www.tinyclutch.com/a-series-roller-clutches/, but am unsure if/how they would do the job.

      3) Retracting. Right now, to retract the foot you must rotate the driver axle counterclockwise, and wait for the cam to retract the ratchet which then releases the foot. Sometimes you have to wait nearly an entire rotation for this to happen. I'd like to improve on this aspect of the design, but the more humps I add to the cam, the more likely it is the cam follower on the rachet will interefere during its deployment to engage the foot. Should my cam be spring-loaded, and slide along driving axle to engage with the rachet or vice-versa, or is there another way to go about this? I should add that the ratchet and cam follower are on the driving axle, directly across from the foot on the driven axle.

      Any help in any of these areas would be fantastic!

      Cheers,

      D
       
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    3. rajmech13

      rajmech13 New Member

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      sir i was out a half way through your description, so if it is possible plz mail me some drawings (subhashraj18@gmail.com) :geek:
       
    4. proteanblue

      proteanblue Member

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    5. johntargell

      johntargell Member

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      Hi Proteanblue

      I'm probably missing a lot of background but if all you want is for the foot to be deployed when you rotate the shaft CW and for it to retract when shaft goes ACW, couldn't you simply use a crank with limit stops and a slipping clutch.

      This allows instant reversal and retraction/deployment of the foot - wherever the driving shaft is rotationally.

      I guess there's a lot more to it, but I found the text description hard work and the drawing isn't very clear. so apologies if this is a useless idea...

      Cheers John
       
    6. proteanblue

      proteanblue Member

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      Hi John,

      Here's a video that shows the current device in action: wJ2pVyUYzsY. It'll give you some context, but not a better understanding of how it works. I really appreciate your working through the text and image. It's true, the explanation is tough to follow, and I'll see if I can get some better images up.

      I think you have the basic idea of the mechanism. In terms of deployment of the foot when one rotates the crank CW, I only want this to occur when the crank passes through TDC. This means that a cyclist could pedal in reverse without deploying the foot until they pass through TDC (or some other settable angle).

      Dustin
       
    7. proteanblue

      proteanblue Member

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      Two more images. Foot retracted.

      [​IMG]

      Foot deployed.

      [​IMG]
       
    8. pramod

      pramod New Member

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      as per my uderstanging u need a device which will deploy the foot while rotating driving shaft clockwise and retract the foot while driving shaft rotates anticlockwise.
      i have no idea about rpm of driving shaft but it operates 30 times /day so i assume it little lower...
      i ll suggest u to use worm gear to drive driving shaft. coz when the foot will heat the stop u can either stop power supply or use clutch.
      advantage of using worm gear is its a self locking gear so they will not allow the reverse rotation of driving gear in short it will hold the foot at deployed position and act as ratchet.(no need of ratchet in this case)
      and 2nd advantage is that when u will rotate driving shaft counterclockwise it will immediately retract the foot.



      please tell me whether it will work or not. if in any doubt please ask
      thanks
      pramod.
       
    9. Michael Davis

      Michael Davis Member

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      I may be missing something in the description, but why not just use a linear rack instead of a round one.
       
    10. proteanblue

      proteanblue Member

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      Hi Michael,

      Thanks for the reply. I hadn't considered a rack before. Because the rack would act as an idler, I'd have to think about how to reverse the driven gear's rotation. It might also be a bit tight in the housing (not shown).

      Dustin
       
    11. Michael Davis

      Michael Davis Member

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      The only other thing I can think of then would be to put a couple of gears between the drive gear and the pinion to get the spread out effect you mention, and make one of the gears a cam shape to exercise activation at tdc. I suppose it might affect the torque too. I am not an expert in mechanism design, but it is more like a puzzle problem, fun to think about.
       

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