efficiency of simple plastic gears
June 2, 2008 8:36 PM   Subscribe

mechanical gears question - what is the efficiency of simple plastic set of small mechanical gears

in the manually operated control module for an a/c control in my car, there are two sets of gears. a straight gear at the front, off the know onto a through shaft. and a segment gear at the back in rack and pinion form, at the other end of the shaft. the dial torque is transferred through the gears to a steel bowden cable, which then operates the HVAC mecahnism.
My question is, what are some good sources to DEMONSTRATE the efficiency of these gears. i need to do a presentation, and want reputable sources to show what efficiency could be expected, and the effect of lubrication etc. any sources would be appreciated, thanks. sorry for the boring question
posted by edtut to Science & Nature (6 answers total) 1 user marked this as a favorite
The efficiency of small, unlubricated plastic gears is comparatively low (as low as 88% for simple tooth profiles of nylon 6 gears running at moderate loads and speeds); this paper tries to quantify efficiency for some simple types of unlubricated light duty plastic gears. To calculate the compound efficiency for a train of gears, you'd have to take the efficiency of successive stages as a product.
posted by paulsc at 9:20 PM on June 2, 2008

A simple way to measure efficiency would be to rig up a system where you have a measurable input and a measurable output. For instance, set up your gear set to be powered by an electric motor which is connected to a constant-voltage power supply and an ammeter. Have the output of the gear train lift a known weight over a known distance (simple winch). Output is given by mgh, input by (integral) VIt. You can actually see measurable results with LEGO gear trains and various forms of lubrication, it's really neat.

Of course, the real question is why do you need to know the efficiency of a plastic gear train that is operated by hand, much less one that doesn't see continuous revolution?
posted by merkuron at 10:19 PM on June 2, 2008

Response by poster: thanks. because the torque is directly related to the customer perception of quality.
posted by edtut at 11:11 PM on June 2, 2008

I'm with merkuron here:

The easiest way to "demonstrate" these effects is going to be setting up a test fixture to show the effect of various differences (backlash, lubrication, load).

On a typical gear train, this would not be very hard, just a matter of suspending effective torque loads on cable or string. In this assembly, it will be very hard for anything like this to reach a "steady state" of sorts.

Merk's question is valid though, I am not sure I understand what this efficiency calculation is buying you. Perhaps a more in-depth explanation might help us help you?
posted by milqman at 11:18 PM on June 2, 2008

I don't have my gearing texts with me at the moment, so I can't give you a full answer. However, reasonable approximations are discussed here, and someone's copied Boston Gear's introductory materials (PDF), which include a variety of relevant calculations for different gearing types.

Since you're looking at an HVAC control, which presumably would be rotated in both directions, you'll need to worry about backlash, with neither of my links above really deal with in any depth. For presentation purposes, that may not matter.

...however, for presentation purposes, building the little test rig described above might be pretty cool and effective.
posted by aramaic at 5:58 AM on June 3, 2008

Not sure what kind of equipment you have access to or how accurately you need to do this, but this pdf from the Faulhaber motor company shows a pretty accurate method for measuring the efficiency of a gearbox.

This is probably a bit extreme for your case but it doesn't hurt to know how it would be done with professional equipment.

In case you get awkward questions at your presentaiton, you should be aware that the efficiency of a gear train is usually measured in terms of the mechanical power in and out. The method merkuron described doesn't measure this directly so a bunch of other stuff will affect it: you're measuring the total efficiency of motor+gear train and not the mechanical efficiency of the gear train alone. But it is ceratinly pretty easy to do and makes a neat demonstration. If you only need to compare different forms of lubrication, then you might be just fine measuring the electrical power in, since you'll compare them to the same base system. Good luck.
posted by theyexpectresults at 2:53 AM on June 9, 2008

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