Scotch Yoke Modification 4

[DavidLeech]

Can anyone suggest a modification to the Scotch Yoke Mechanism which will produce a uniform rate of motion of the reciprocating slider as the crank rotates. A standard Scotch Yoke does not produce a uniform movement.

 

[GregLocock]

You'll have to specify the curve of slider velocity vs angle more fully, at first sight you seem to be asking for a sawtooth wave of displacement which implies infinite accelerations, which may be a little difficult to achieve in practice.

You should probably check out the theory of 4 bar links, and consider extending it to 8 links, depending on what you really want.

There are numerous downloads of mechanism or linkage simulators available, perhaps the most versatile of the freebies is freecad,
available from

http://www.askoh.com/freecad/

. It has a steep learning curve, there are much simpler programs available.





Cheers

Greg Locock

 

[rmw]

With a normal scotch yoke mechanism, thinking of a jack screw type scotch yoke, the slider that the travelling nut is mounted in causes the fulcrum arm length to be constantly changing, giving the uneven movement, or nonlinear velocity. I am not going to propose how to construct it, but if you make the jack screw slide on a suitable mechanism, and fix the nut on the arm, then the fulcrum length stays constant, and the movement will be linear with shaft speed. Now it is up to you to figure out how to implement this modfication so that it will work.

rmw

 

[ornerynorsk]

The nature of converting rotary to linear is going to give you an inherant non-uniform acceleration/decceleration. I play around with scotch yokes and whitworth mechanisms quite extensively in animated mechanical display constructions, and if you want uniformity, you may have to go with a linear actuator (electric or pnuematic). Good luck on your project.

 

[notnats]

A heart shaped cam which moves the follower a constant distance for each degree of rotation will convert a rotary motion to a uniform linear motion. Of course at the end of the stroke some deviaton from uniform motion will be needed for acceleration. Such mechanisms were used in automatic lathes before NC control became available.