In order to give you a good idea of what I want to accomplish suppose you have a telescope mounted on a single rod extruding from the ground. The telescope is free to move such that you can tilt it to point at a particular altitude but can never point towards the ground. In order to move the telescope you have a wheel which can only be rotated in a single direction (say clockwise). Is there a gear configuration that would make a point alternate rotations every 180 degrees? and also have it configured such that the power loss is consistent throughout the entire cycle so that no half a cycle takes more power than the other?

Here is an illustration



1 Answer 1


A crank (or 4 bar) type linkage can be forced to do this. In a crank mechanism there is a a point at bottom dead centre and top dead centre where the rotation could go in either direction regardless of the direction of the driving wheel/arm.

So you could have a stop which prevents the driven arm from rotating past 180 degrees, at which point it will naturally reverse direction. Note that steam trains have opposite wheels driven by cranks 90 degrees out of phase precisely to prevent this.

You could also use half gears (ie gears with no teeth on half of their circumference) along with an idler gear on one to achieve a similar effect.

There are also lots of different cam and multi-bar linkages which will produce reciprocating motion depending on your exact requirements.

The difference between the myriad of possible solutions will depend to a large extent on the function which relates input to output, particularly how linear it is throughout the range of movement.

For this sort of application it is also well worth looking through old patents as there are thousands and thousand of mechanical linkage patents out there for almost every imaginable mechanical application, many of which have now expired.


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