I am building a fatigue testing machine that includes the ability to induce stress from torsion, however I am stuck at how to convert the linear motion from the linear actuators we have chosen to torque efficiently. Because the cycles of the machine will be in the millions and involve high accelerations, a rack and pinion mechanism is not an option, so we have looked at a four bar linkage slider crank mechanism.

The problem with the slider crank is that we need to transmit the maximum torque at the extremes of the linear actuator position i.e. when the actuator is fully extended and fully retracted. The slider cranks transmit the most when the transmission angle is 90 degrees, so we have to try and create the motion in this video.

I understand most slider cranks that have the slider as the input and rely on inertia to keep going in a certain direction. Our setup needs to be able to go very slow and can't rely on inertia to keep the crank spinning. Does anyone know how we can achieve this type of motion?

  • $\begingroup$ To produce that rotation at point A with control have you considered a stepper motor or a hydraulic motor? $\endgroup$
    – Solar Mike
    Commented Sep 1, 2018 at 22:47
  • $\begingroup$ Yes but it complicates the system as it would need a motor and and linear actuator for 90% of the other tests. The linear actuator we are using is quite expensive so we are trying to develop a novel way of using what we already have $\endgroup$ Commented Sep 1, 2018 at 23:01
  • $\begingroup$ I'm thinking about some sort of lead screw but in reverse. Are they a thing? $\endgroup$ Commented Sep 1, 2018 at 23:14

1 Answer 1


a four-bar linkage is going to have a dead spot in its torque response where it will stall and stick at low speeds. My solution to this problem when I first encountered it years ago was to add a toothed belt drive that would pull the mechanism out of the stall point. In this case, the belt drive was exercising the 4-bar mechanism which idled along while the belt did the work.


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