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  • Semi Circle Reciprocating Motion Question

    Discussion in 'The main mechanical design forum' started by rsstansbury, Jun 11, 2012.

    1. rsstansbury

      rsstansbury New Member

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      Hello:
      I want to use a hydraulic cylinder to to rotate a shaft. This is a very slow speed motion. One end of the cylinder will be pinned to a fixed part of the structure. The rod end of the cylinder will be pinned to the end of a rotating shaft (offset from center). However, I only want the shaft to travel in a semi circle. For example, I want the shaft to go from 0 to 90 to 180 degrees as the cylinder rod is extended. I then want it to reverse and travel back the same path as the rod is retracted 180 to 90 to 0. I dont want the motion to "break over" at 180 and travel 180 to 270 to 0.

      Let's say for example that the hydraulic cylinder will be mounted in a horizontal plane. Lets also assume 0 degrees on the shaft is to the right of the center of the shaft, 90 degrees being top dead center, and 180 degrees being to the left of the shaft center. I thought about maybe limiting the angular motion of the hydraulic cylinder itself with an obsticale such that it could not fall below the horizontal centerline of the shaft. In other words, incorporating a fixed obstical in the structure such that the bottom side of the cylinder tube hits an obstical that is perpendicular to it when the rod is fully extended. Would this work or is there a better way?

      I hope I make sense....

      Thank you,
       
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    3. Dana

      Dana Well-Known Member

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      Instead of trying to reinvent the wheel, you might look at commercially available rotary actuators, which use a cylinder to drive a rack gear which in turn drives a spur gear. Any amount of rotation you want without the "break over" issue. Note also that your method would result in zero torque at the ends of stroke.
       
    4. rsstansbury

      rsstansbury New Member

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      yes but $$$

      Hello Dana,
      Thank you very much for taking the time to answer my question. I'm not setting out to reinvert the wheel. I have lived primarily in a world of hydraulic cylinders and motors. I am just not aware of all the other types of hydraulic devices that would be better suited for this application. Based on your response, I have looked into rotary actuators. This would definately work. Please corect me if I'm wrong, but they just don't seem to be as readily available and they look to be pretty pricey compared to a 2500 psi 12-inch stroke hydraulic cylinder that I could get for around $100. Do you think I can overcome the "break-over" and "zero-torque" problem if I can live with 175 degrees rotation? I dont really have a handle on what my torque requirements are at this time because I am in the design phase. I'm estimating 200 lb/ft or less provided my machine is relatively balanced. I have no doubt that most any hydraulic cylinder would have more than enough force provided I avoid a zero-torque condition (i..e 2 degrees to 178 degrees travel). Am I correct?

      Thank you...
       
    5. Dana

      Dana Well-Known Member

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      Depends on what torque you need, and what's available. If you use 175° total rotation that's \2.5° from horizontal at each end. To get 200 lb-ft at the end you would need 200/sin(2.5) or 9170 lbs assuming a 6" moment arm. That means that you'd have 4585 lb-ft of torque at mid stroke so that's what you'd have to size the rest of the mechanism for.

      A rack and pinion arrangement (whether you design your own or buy a rotary actuator) will give you constant torque throughout the stroke, no issues at the end of stroke.
       
    6. rsstansbury

      rsstansbury New Member

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      Dana,
      I would love to have a rack and pinion design if I can afford it. Designing my own sounds interesting. Do you know where I might look to see some basic designs for doing it? I found rack and pinion gear products on McMaster-Carr web site. My shaft diameter is 4 inches. They have a pinion that is 12-pitch, 20 deg pressure angle, 4-inch pitch dia, 48 teeth. They dont list the # of teeth per inch for the rack, so I haven't figured out what the linear distance I would need to move the rack to rotate 24 teeth on the pinion to achieve 180 degrees on the pinion. It doesnt look like it would be very far though. So I am curious how I would link the rack to a hydraulic cylinder and then limit the stroke of the cylinder.

      Thanks
       
    7. PWASS

      PWASS Well-Known Member

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    8. Dana

      Dana Well-Known Member

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      4" PD, 1/2 of the circumference of a 4" circle for 180° rotation or 6.28".

      If you're buying gear components look in a specialized catalog like Boston Gear... you will find a lot more technical information than in the McMaster catalog.
       
    9. Allfat

      Allfat Member

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      To link the rack to the cylinder, you could either have the rack threaded to utilize the threads on the ends of the cylinder rod end. Just screw off the yoke and screw on your rack. You could also utilize the yoke and have a adapter put onto the rack that will just pin it onto the cylinder. With the pin and yoke, you would have to support the rack more though as it would allow more freedoms of movement.

      For limiting the cylinder, calculate the linear movement you need, get a ram with at least that much throw. Lets say you need 9" of travel and you get a 10" cylinder. Then just make a 1" clamp spacer that clamps around the rod. This will limit you to the 9" of throw you need. Look up "Two Piece Clamp-On Shaft Collars" on McMaster for an idea on how to make your spacer.
       

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