Re: Re: Thermodynamics of rubber bands
To all...This is from the Sympo and talks F1B motors, but the physics is
the same, various batches which are VERY temperature sensitive relative to
the ratio of natural versus synthetic rubbers in the blend. Fred mentioned a
certain batch exploding in flight at an indoor contest...the temp at
flight altitude in a high temp gradient facility was above the practical temp
limit for the batch, anyway...here are some excerpts regarding the
thermodynamics:
"To begin, the energy available to a rubber motor is less than the input
of winding a rubber motor. A physics term, Hysteresis is the lag of effect
when forces acting on a rubber motor are changed. For a rubber motor,
there is a loss of input energy due to hysteresis, called the “hysteresis heat.
” This hysteresis heat is a complex matter. It consists of friction losses
between the strands and that internal to the strands. A well-lubricated
motor has relatively little friction losses. Almost all the energy lost is
due to “change-of-state” loss. An example is that of water freezing and then
melting. The phase changes are first giving up heat to freeze, and
requiring the same amount of heat to melt. The laws of physics dictate that ice,
having a more ordered crystalline molecular state than water with its random
molecules, must give up heat and regain the same amount of heat to melt.
.....
Temperature changes encountered during winding and any pre-winding stretch
to increase turns can be surprisingly large. Without a pre-winding
stretch the motor temperature will rise 25 degrees F (13.9 deg C) above ambient.
......
The pre-winding stretch, hold and relax procedure before winding is really
a mini break-in procedure. It enables the motor to take more turns because
it is temporarily longer when relaxed. The phenomenon is caused by very
short term slippage between polymer chains. The technical term is plastic
flow. In a full break-in procedure the immediate measurement of the relaxed
motor indicates an approximate 8% increase in length which overnight
typically declines to a permanent set of about 3.5%. An estimate of the
transient elongation of the motor from a pre-wind stretch is about 3%. If without
the procedure you expected 400 turns, you now get 12 more turns. This is
a major boost.
.....
Remember, the old child’s trick of stretching a rubber band or strip of
rubber and touching it to your lips? It will be distinctly warm. Pull it
away and relax it and again touch it to your lips. It will be cold. This
effect can be used during a pre-winding stretch to cool the motor in hot
weather. It might be called “auto-refrigeration.”
......
In stretch testing of rubber, the liberation of heat has a significant
effect on the results. A quick testing procedure, especially during the high
pull force period does not give the heat of hysteresis time to dissipate
to the atmosphere. Thus, you can measure a higher relaxation energy that if
you were using a slower paced technique. Since the instantaneous rise in
temperature from the heat of hysteresis might be over 20 deg F (11 deg C).
The quick test might give as much as 10% more relaxation energy than a slow
test.
When [an F1B Motor] stretched to about 40 pounds (18.2 Kg) the motors’
temperature rises about 8 degrees F (5 deg C) above the ambient. The rise
in temperature is from the hysteresis of the change of state taking place.
If held at the stretch for about a minute, all this heat will be dissipated
to the atmosphere. Then when relaxed, the motor will drop its temperature
8 degrees F ( 5 deg C) below ambient to reverse the change of state. So,
when you stretch to begin winding you start out 8 degrees F (5 deg C)
cooler. This might save blowing the motor in hot weather. But, on a cold day
you don’t want the to cool the motor. You avoid this by not relaxing after
the hold period. You just begin winding.
During the holding period whole looking for good air the addition of
hand-cranked turns does not add any appreciable hysteresis heat. This is because
all the possible crystallization of the natural rubber polymer chains has
already taken place. And, all the significant change of state from
stretching out the tangled and balled up polymer chains toward a more orderly
state has taken place."
FYI.....summary...it's the mechanical forces on the rubber that induce
phase changes in the polymers and the particular batches vary in thermodynamic
response. It's OK though because if you sacrifice a few motors, you can
get the best method of getting maximum performance out of a given
batch....test...test...test...
Rick Pangell
Editor of "The Max-Out" Newsletter of
The Magnificent Mountain Men FF Club of Colorado
In a message dated 6/23/2010 12:14:57 P.M. Mountain Daylight Time,
tapio.linkosalo_at_iki.fi writes:
Benjamin Saks wrote:
>
>
> So after all this great discussion, I am still left to wonder. Would it
> be better to "warm up" a rubber so that the change in temperature is
> less? If you have a smaller Delta T would that make the rubber less
> brittle, reducing the chances of breaking? Or would it help to cool down
> the motor significantly while winding in hopes of keeping the whole
> temperature range lower than the 100 F ceiling?
The ideal is to have your rubber at 30 to 35C. So warm it up or cool it
down to get to that. If you cannot, select a batch that can stand the
heat, or giver better torque in the cold.
I think that at indoor motor dimensions all heat produced by winding is
dissipated at once to the surrounding air.
-Tapio-
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Received on Wed Jun 23 2010 - 13:04:27 CEST
This archive was generated by Yannick on Sat Dec 14 2019 - 19:13:46 CET