Brock Institute for Advanced Studies


Design notes

Continuous rotary motion input to intermittent reciprocating linear motion output


These notes are intended to describe the design sequence and geometry underlying the class of mechanisms that convert continuous rotary input motion to an intermittent reciprocating linear motion such as the one animated at


The mechanism has the following parts with dimensions as described.  (Description or numerical values that are applicable to the example are shown in square brackets):


  1. A spoked input wheel that rotates [counterclockwise] at a uniform speed.  It has N [3] spokes each with a radius of R
  2. A perimeter frame mounted in tracks that constrain it to move in a straight [horizontal] line.  The frame has step features [at the top and bottom] that intermittently engage and release the input spokes [first at the top to produce frame motion to the left, then at the bottom to produce frame motion to the right to the return position]. 


The design proceeds by the following steps:

  1. Specify the number of complete linear output cycles, M, per complete input cycle.  For the class of mechanism described here, M must be an odd number and the number of input spokes N = M.
  2. Specify the linear stroke, L.
  3. Specify the desired linear position as a function of the angle of the input wheel.  The output profile will be characterized first by the number of complete cycles per input cycle, equal to M.  Each output cycle will then span 360/M degrees of the input cycle.  [For the example, M = 3, so that a complete output cycles spans 360/3 = 120 degrees of rotation of the input wheel.]  Then specify the input angle for the ramp up, U, top dwell, T, ramp down, D, and bottom dwell, B.  The sum U + T + D + B = 360/M.  [For the example, U = T = D = B = 30 degrees. Figure 1 shows the arrangement for the example.]


Figure 1.  Output vs. input profile.





  1. Set the input spoke radius, R,  and the frame step size, S.  To meet the travel requirements, the interface between spoke and frame step has to have the relationship shown in Figure 2


Figure 2. Interface between input spoke and output step.


and the geometry is specified in Figure 3.



Figure 3.  Calculating radius, R, and step, S. 


Given the stroke, L, and the ramp-up angle, U, the spoke radius, R, is determined.  With spoke radius, R, and ramp-up angle, U, the frame step, S, is determined. 


For a single input wheel with equal length spokes, the ramp-up angle, U, and the ramp-down angle, D, will be equal.  As a result, the return step on the bottom of the frame will have the same geometry as the one at the top.  If a more complex input / output profile is desired, it can be accommodated with correspondingly more complex mechanism arrangements.


Copyright 2003, Brock Engineering, Fred Klingener

May be reproduced for personal use with this notice.


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