# pic: Inside the torque multiplier

Partially disassembled torque multiplier. Note the inner gear has 32 teeth and the outer ring has 33. Output inner gear (not visible here) also has 32 teeth and is coupled to the input gear via 6 pins seen in this view. Both input and output gears are coupled to standard 1/2" square (socket wrench type) drives.

Interesting…that has the same principle as an oil pump…but i wonder how it ‘multiplies’ torque

looks messey though

It multiplies the torque, by dividing the revolutions per minute. Since the power output cannot be greater than the power input.

Basically, its a gear reduction.

This is useful

Thanks for that link, Conor. The handtorque multiplier example shown there is an industrial grade unit with an epicyclic gear set.

The low-cost (read: Harbor Freight) unit does not have a conventional epicyclic gear set. It appears to operate differently. The input gear center hole pilots on an eccentric section of the input shaft, causing its tooth engagement point on the outer ring to revolve. The input gear revolves 33 times for every single revolution of the output gear, and both gears react against the teeth of the stationary ring. And as mentioned earlier, the gears are loosely pinned together in six places, allowing relative freedom of movement between the gears of about one tooth pitch. Also, the output gear rotation is opposite that of the input gear. So to loosen a lug nut you turn the crank clockwise. Maybe this really is a kind of epicyclic gear set, with the sun gear replaced by an eccentric – sort of like a gear with one tooth?

I’m hoping someone with more experience in gear set design will jump in here an enlighten us to the mystery of this gear system. I am actually an electrical engineer, with just enough knowledge of mechanisms to be dangerous.

This is a good question: anybody have a good explanation for an epicyclic gear drive?

These are great drives because you get tremendous turndown (torque multiplication) in a very compact package. There is a company called Harmonic (at least that is what they used to be called) that makes a similar drive but very elegant. The inner “gear” is actually flexible, such that it is deformed into an ellipse that meshes the ring gear in two places. The teeth are very fine pitch. Therefore, the one tooth difference between inner and outer gears makes for a huge turndown (something like 75:1 or better in a transmission not much larger than a coupling), and the drive is incredibly smooth. The biggest advantage in our application was the lack of any perceptible gear noise, as the drive was for cooling rolls on a polycarbonate sheet line. Any ripple would have produced an optically perceptible imperfection in the lexan. Very cool, but expensive.

Thanks, John. For many years I’ve been using a Harmonic Drive Model PCR-5C 100:1 reducer as part of low-speed motor tester. [Note: this reducer is now obsolete, replaced by the CSG series, I think.] We use this tester to measure cogging, ripple, and frictional torques developed by brushless permanent magnet motors at 2 RPM

. The Harmonic Drive unit operates with negligible backlash, is very quiet, and as you pointed out it provides very high reduction ratio in a compact package. It is also not cheap.

I did not recognize the similarity of the harmonic drive to the low-cost lugnut remover gear set.

Substitute the word planetary for epicyclic. It’s another word for the same thing.

The wiki comes through with a decent explanation, Epicyclic gearing - Wikipedia but it won’t touch a good machine design book like Shigley or Norton. The Lindeburg PE exam reference book is the best explanation I have but I can’t post it.
http://ppi2pass.com/ppi/PPIShop

Pete

edit: When you say it operates like an oil pump I’m a little confused. Do you mean it’s like a gearotor hydraulic pump?

The way the harmonic drives operate is explained here in detail: http://www.harmonicdrive.net/reference/operatingprinciples/

This is an interesting explanation of how a multi-speed planetary transmission operates: http://www.railcar.co.uk/mechanical/gears/work.htm This was actually for a locomotive: http://www.railcar.co.uk/mechanical/gears/intro.htm

And this is a great animation. You can click the buttons and see the different modes of operation: http://www.mekanizmalar.com/transmission.html

I usually don’t like to revive dead threads. However, when I saw a description of how hypocycloidal gearing works, it immediately reminded me of the lugnut remover. The animation in the link above seems to capture the operation of that gadget better than any of the previous explanations.