Are there any established solutions for truning a gear (or wheel) half-circle in one direction, back in the opposite direction, and repeating? Mechanism must be driven by a motor and continuous rotary motion. Compact and simple mechanisms preferred. Below are two possible solutions I dreamed up. Which one is better and why? Or what should I do instead?

One idea I had was to use a motorized disk (orange) with a slotted shaft(yellow) to create reciprocating motion. Then by extending the shaft and connecting it to a second disk (Red) I could achieve it, I think.

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Another Idea I had was to use a full-square Scotch Yoke (Yellow) with gear teeth on the bottom. Properly sizing the connecting gear would allow for 180 degrees reciprocating motion.

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  • 1
    $\begingroup$ Rack and pinion wouldn't work? $\endgroup$
    – grfrazee
    Nov 19, 2015 at 17:09
  • $\begingroup$ Sounds like the desired motion is similar to the action of a windshield wiper, yes? $\endgroup$
    – Air
    Nov 19, 2015 at 17:13
  • $\begingroup$ @grfrazee That would require reversing the direction of the motor. I'm looking for something that works with continuous motion. $\endgroup$
    – skibulk
    Nov 19, 2015 at 17:43
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    $\begingroup$ Sorry, should have been more specific. I meant a reciprocating rack and pinion $\endgroup$
    – grfrazee
    Nov 19, 2015 at 17:45
  • $\begingroup$ @grfrazee But that's a full 360. I suppose I could add gear teeth to the bottom of that, then it would be pretty much the same as my second example. $\endgroup$
    – skibulk
    Nov 19, 2015 at 17:49

1 Answer 1


Selection of a mechanism highly depends on the end application. Here are some ways to evaluate a mechanism vs your application:

  1. Input (360 deg rotation in this case)
  2. Output (180 deg rotation motion in this case)
  3. Relationship between input and output (mechanical advantage, velocity, and acceleration for a given position). (linear, parabolic, sinusoidal, other crazy polar math, etc)
  4. Number of moving parts
  5. Number of rolling surfaces (rollers, gears)(expensive)
  6. Number of sliding surfaces (pins, slots)(reduced life)
  7. Number of rotation points (bearing locations)
  8. Can the mechanism be statically and dynamically balanced (for high speed)
  9. What is the footprint of the mechanism
  10. What loads does the mechanism have to endure (material required, fatigue stress, etc)
  11. Backlash (new and worn)

You may be able to use crank-rocker four bar linkage. I haven't sketched it out, but 180 degrees is likely its theoretical maximum. A spring at either end of the range would help avoid top dead center, but power transfer at those high angles would be difficult. In either example you could replace the orange pin connection with a four-bar. That would eliminate one pin and slot giving you a longer service life.

Four bar linkages are attractive because they employ only low cost rotation connections that have long service lives. Rack and pinion is well suited for when low backlash linear output is required, but can be expensive. Slots and linear bearings are less desirable because of service life, but sometimes reduce the number of components or provide the desired output.

  • $\begingroup$ When you say that rolling surfaces are expensive, do you mean in terms of money or energy or wear and tear? $\endgroup$
    – skibulk
    Dec 9, 2015 at 21:10
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    $\begingroup$ Rolling surfaces like gears or guide wheels have to be machined to high tolerance then hardened (quenched or the like); which is a more expensive process than just press fitting a bearing. When exposed to the environment during operation; the surface presses debris into the other surface, slowly eroding it away. Bearings also fail over time, but replacement is easier and much lower cost. Rolling surfaces and four bar links can both have very high energy efficiency. $\endgroup$
    – ericnutsch
    Dec 10, 2015 at 16:31

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