Linkage for Linear Motion from Servo.

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?

Picture of Linkage for Linear Motion from Servo.
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Kiteman6 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.
AndyGadget (author)  Kiteman6 years ago

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.
rickharris6 years ago
Your requirement for linear motion is ambiguous. needs more clarification.

For example this does what you want.
AndyGadget (author)  rickharris6 years ago
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.
AndyGadget (author)  AndyGadget6 years ago
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.
AndyGadget (author)  rickharris6 years ago
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
iceng6 years ago
Rack and pinion comes to mind. . . . . . . .  A
AndyGadget (author)  iceng6 years ago

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.

Err.. Aren't servos postionable ?
AndyGadget (author)  steveastrouk6 years ago
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 ?
AndyGadget (author)  steveastrouk6 years ago
Possibly, but I think the one I've mentioned above may be easier to implement and give a more linear response.