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Linkage for Linear Motion from Servo. Answered

This was originally going to be a question on mechanical advantage and tinsnips but I've realised the answer during typing the question. So, my follow-up question is . . .

I'm looking to extend the throw of a servo and convert this to a roughly linear motion but still keep the whole mechanism compact.  I'd like around a 4 to1 gearing (with corresponding loss of force, of course).  Is there a simple linkage I should be looking at to get this?



Best Answer 6 years ago

There is a fairly comprehensive set of mechanical linkages on Rob Ives' website.

I think you're probably looking at some sort of crank mechanism.  If you join Rob's site, you can download a model of a crank for free here.  If you don't join, the model costs £2.50 to download.

Interesting site - I've signed up.

I find mechanical linkages hard to visualise for some reason, and the animations are a great help.  I think a variation on the third order lever operating a push-rod at right angles is what I'm looking for.

Your requirement for linear motion is ambiguous. needs more clarification.

For example this does what you want.


That's roughly what I was thinking of, Rick, but turning the servo arm by 90 degrees and having the fulcrum point on the servo, then a slotted arm (being the equivalent of your vertical arm from the fulcrum) operating a push-rod at the arm's end (the equivalent of your top horizontal arm), with a suitable guide at the top end.  I think my original mention of a 4 bar linkage was misleading - I don't actually need that.

Something like this is what I was thinking of.


Yep that's a version of a scotch yoke mechanism. will work although you need to attach the follower with another slot/pin to the upright to get reverse motion - or use a spring.

Gravity is what I had in mind for the return - The push-rod will actually be vertical.  However, now I'm considering your rack and pinion suggestion as using a decent size pinion does give a good throw in a small footprint and it would be easier to mount.  When you first mentioned it I was thinking steering rack ratios which would need multiple rotations. 

This is slightly more compact using a cam and gives a return motion.

cam 2.jpgcam.jpg

6 years ago

Rack and pinion comes to mind. . . . . . . .  A


Thanks Alex, but that doesn't give me the gearing I'm looking for.  It needs to be positionable, which would need a stepper rather than a servo. A typical servo only has around 270 degrees of rotation.

Certainly, but only over 270 degrees or so (360 for winch servos) which would only . . .
Just twigged : Bigger cog = higher gearing!

(I said I had trouble visualising mechanical linkages #;¬)

How about a "Lazy tong" action ?

Possibly, but I think the one I've mentioned above may be easier to implement and give a more linear response.