# Ruberband for measuring torque

• posted
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)

I need something around \$100-\$200 (motor+gearhead), am I being realistic?
Cheers
• posted
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... In any case, good speed and nice torque with air/liquid.
-- Gordon
Padu wrote: > > 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) >