Ruberband for measuring torque

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Padu posted on
January 14, 2006, 11:04 pm

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I have to specify a motor/gearhead combination for automating the steering
column of an ATV. The first step I believe is to measure the torque
necessary to move the wheels with the vehicle stopped on asphalt. I figure
that if I choose a motor that has that torque and a little margin, I won't
have problems when the ATV is in movement.

Here's the method I'm using. I got 20 rubberbands together, hooked at the
end of the ATV's handlebar and pulled (tangent to the steering column) as
fast as I want the motor to pull it. I chose 20 rubberbands because if I
used less, it would stretch to the limit, with 20 I still have plenty of
room to stretch. In this experiment, the rubberbands went from 3 inches (no
torque) to 6 inches (max. torque), so 3 inches of displacement.
Then I used the same 20 rubberbands to lift a weight of 30 pounds, it
stretched 9 inches (6 inches displacement).
Can I assume that the elasticity of rubber is linear in this case? If so,
then 3 inches of displacement would lift 15 pounds right?

The next thing I did was to calculate the force necessary to cause that 3
inches displacement multiplying 15 inches by g, and then multiplying by the
handlebar distance from the center of the axle.

I ended up with a torque close to 25Nm, so I'm thinking about choosing a
motor/gearhead with a 50Nm available torque. RPM should be close to 20rpm.

Does that make sense? I saw some nice motors from Harmonic Drive that would
fit my application perfectly (small, very torquey, hollow shaft), for
instance the FHA17C, but they are too damn expensive (around $2800)
http://www.hdsi.net/products/rotaryactuator/fha.cfm

I need something around $100-$200 (motor+gearhead), am I being realistic?

Cheers

Padu

Gordon McComb posted on
January 15, 2006, 3:36 pm
Permalink

Re: Ruberband for measuring torque

The force needed to stretch rubber isn't linear, especially over
repeated tests. Think of a rubber balloon; they're easier to blow up if
they're stretched first. I think after the first couple of tests your
results will start to go way off. You're better off with fish weights or
something with a metal spring. You'll find some articles and example
test results of doing this type of thing with a good Google search.

To get what you want for the price you need, you may have to get a more
routine motor and add some sub-gearing either on the steering yoke or
the steering column. Look for a motor with an initial output speed of
around 500-1000 RPM unloaded, so that it can be further geared down to
around 50-80 RPM. That will achieve a 90 degree arc in a few hundred
milliseconds; anything less is like a drunk behind the wheel. I think
you want to be able to steer right/left in under 250ms. (Assuming a 90
degree arc for the steering, that's 1 RPS, or 60 RPM -- but remember,
under load.) Go to Industrial Liquidators on Convoy for a selection of
nice large metal gears perfect for this job.

Another thing to look at is pneumatics or even hydraulics. You can find
pneumatic cylinders with throws of 10-12 inches surplus for under $20;
C&H in Pasadena is a good local source. Add an air ballast and a pump
for maybe $100-150. The trick is in making this proportional, which can
be done using feedback circuits and PWM to the valves. Might be a nice
project for one of your team mates... <g> In any case, good speed and
nice torque with air/liquid.

-- Gordon

Padu wrote:

Padu posted on
January 16, 2006, 7:52 pm
Permalink

Re: Ruberband for measuring torque

"Gordon McComb"

I'll repeat my tests with a more formal method. The rubber band was a very
informal test just to get a ballpark torque number. Turns out that it may be
way off, either because of my measurement method or wrong calculations as I
found very difficult to fina a 50Nm (double of what I measured) motor.

Thanks for the hint, I'll take a look.

Well, I have very little knowledge on motors. Seems like I'll have to go
through an intensive course this coming days. Luckily (or unluckily), there
are lots of information on the internet mainly on the motor manufacturers. I
even downloaded a software from Parker's that help you specify and choose
your motor, but most of the parameters it requires are still way over my
head.

Thanks

Padu