Mechanism

In class we talked about mechanisms.  Mechanisms are ways to transform import forces and movements into desired set of output forces and movements. 

In this blog I would like to talk about this Helical Rack and Pinion Gear Pair

A rack and pinion is one of the most straightforward means of converting rotational motion into linear motion, at a 90 degree angle from the axis of rotation by means of gears on the rack and on the pinion.  Of course, it can also be used the other way around to convert linear motion into rotational motion.  

which is the rack and which is the pinion you ask?  the rack is the rectangle with the linear back and the pinion is the cylinder with the gears. 

This picture example is called a helical rack because notice that the gears don't point exactly in the same direction as the axis of rotation but actually curve around it (kind of like a helix - you should know what a double helix looks like).

It is able to translate motion through the gears, the gears of the first tug at the gears of the other pulling the other gears along as the first moves.   

Obviously it has some limitations.  If you don't want the pinion to roll off of the rack, there is only so much that you can rotate the pinion.  And if you want the rack to move forward and backward, then you'll have to find another way of changing the pinion's direction.  

I can see this mechanism being use in situations where something is moved forward and backwards a certain limited amount.  Places where I could see something moving back and forth like that would be something like an automatic stamper, or some device that automatically puts pizza in an oven and takes it out again after it's cooked, or something that lifts and takes down pop up targets at a shooting range, because all of those have simple back and forth motions, and especially for setting up moving targets, you definitely don't want a person there setting up the movements.  In the other direction from linear motion to spinning motion, I remember a plastic toy that required a plastic line with perforations like zip ties to spin a little plastic helicopter that few off.  I could imagine that being implemented with a plastic rack and a pinion on the shaft of the flying thing such that when the plastic rack was pulled, the flying thing spun really quickly.  In that situation, the rack definitely flew away from the pinion. Actually the pinion was on the flying helicopter thing so it would be more correct to say the pinion flew away from the rack.  

Actually Wikipedia mentions a perfectly logical application of this to raise and lower the gates of a lock (as in a canal).  The gates of a lock only go up so far and only go down so far, and so this system does very well in that situation.  My only worry is corrosion corroding the gears of the rack, but I assume that must not be a big problem if they are using this method.  By a similar logic, a Pinion and Racket is also used in stair lifts for the mobility impaired

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Another fascinating usage of this is for locomotives to have a third wheel that is actually a pinion, that contacts a length of rack and then pull themselves up, or lower themselves down a very steep incline that it could not climb on two smooth rails and friction alone.   In this case, it is actually desired that the pinion rolls on and off the rack, because when the locomotive is not traveling a steep incline it does not need the strong grip that the pinion and rack provides