Re: Re: Thermodynamics of rubber bands

From: Bill Gowen <wdgowen_at_gmail.com>
Date: Wed, 23 Jun 2010 16:49:01 -0400

This info is going into my permanent collection. Thanks Rick!

  ----- Original Message -----
  From: themaxout_at_aol.com
  To: Indoor_Construction_at_yahoogroups.com
  Sent: Wednesday, June 23, 2010 4:03 PM
  Subject: Re: [Indoor_Construction] 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 - 13:49:10 CEST