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  • Continuous Change of Direction of Rotation

    Discussion in 'The main mechanical design forum' started by SachinHB, Feb 6, 2016.

    1. SachinHB

      SachinHB New Member

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      Hi guys,
      I need your help and suggestion on a machine I am designing.
      [​IMG]
      The attached drawing shows the part of a machine I am designing. In this the motor is rotating at a constant speed of 1000rpm and is connected to a rack which drives a pinion. This pinion is fixed on a shaft which rotates at 2000rpm and it is supported with ball bearings. The linear reciprocating movement of the rack makes the pinion to change its direction of rotation continuously. The moment of inertia of the shaft is 0.040Kg-cm2. My doubt is is this possible. Will there be any problem caused due to continuous change of rotation direction. Will there be power loss??
       
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    3. Dana

      Dana Well-Known Member

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      The shaft is rotating continuously at 2000 rpm and the the pinion is oscillating at 1000 cycles/minute? Doesn't make sense. Or do you mean you're driving the shaft in oscillating motion at a max of 2000 rpm in either direction?
       
    4. s.weinberg

      s.weinberg Well-Known Member

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      Haven't thought about it for more than a few seconds, but I think, to address Dana's point, you won't be getting a steady RPM and a sudden change of direction, as it seems you may have thought. You'll get a speed profile following a sinusoidal pattern, maxing when the driving wheel is at the 'top' and 'bottom', and reaching 0 at the 'sides'.

      Given that that is acceptable, nothing obviously wrong jumps out at me.
       
    5. Bill @ ERG

      Bill @ ERG Member

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      Yes there will be be power required to drive this mechanism. There are losses in all the bearings, friction losses in the rack to roller contacts, and losses between the rack and pinion. As mentioned above there will a relatively smoothly varying rack velocity with two maximum acceleration areas in each stroke (acceleration/deceleration). Gear DP and material will have to be selected to suit these high load zones, not average loads for suitable rack and gear life. Design of the crank/flywheel will be interesting if the machine is expected to be smooth running. Good luck!
       
    6. le tant

      le tant New Member

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      I need more details but i think there is no huge power loss but you need a sliding frame with good lubrication for rack
       
    7. SCIYER

      SCIYER Well-Known Member

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      Not a good mechanism. The motion being sinusoidal, you will get infinite jerk at the direction reversals. I would prefer a grooved face cam with roller follower (attached to the rack) in the groove.
      By using a groove cam, one can define the profile of the motion thus ensuring "Finite" Jerk (Rate of change of acceleration) at the direction reversals.
       
      Last edited: Feb 24, 2016
    8. robertjeffery

      robertjeffery Active Member

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      Sciyer.

      I would have thought that the crank effect of the direction changes would work in its favour. Can you explain what you mean by a "infinite jerk"
       
    9. s.weinberg

      s.weinberg Well-Known Member

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      I think Sciyer has it backwards. At the point of direction reversal, there will be (theoretically) 0 velocity, thus no jerk.
       
    10. Bill @ ERG

      Bill @ ERG Member

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      I believe that the maximum acceleration is at each end of the stroke, but is not infinite, and can be calculated quite easily. For 1000 rpm and a 2" stroke it would be approximately 28.4 std gravity or 10,970 in/sec². This varies slightly with the rod/stroke ratio and which end of the stroke you are at because of the geometry of the crank / connecting rod. There are free online calculators for sinusoidal motion that will provide the information.
       
    11. robertjeffery

      robertjeffery Active Member

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      Bill @ erg, surely the max acceleration will be when the crank is at 90deg to the main journal bearing. Therefore middle of the stroke. It will be of a sinusoidal wave form. The changes in direction will be the peaks and troughs. And max acceleration will be where it crosses the x axis. now I have written this, I am starting to question myself. Does the acceleration stay constant...... And the displacement is sinusoidal.... Still I don't think their will be any "jerk" just like a piston in a car engine.
       

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