Making a Rubber Motor

Start by collecting everything you will need onto your work table.
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We are going to make a 25″ loop of 1/16″ rubber strip.  This requires sufficient 1/16″ rubber strip, a tape measure to measure it, scissors to cut it, a clean diaper to dry it after washing, a 1/8″ O-ring, a vial of 1/2 strength lube, a bottle of motor lube, a Ziploc baggie, a vial to store the motor, masking tape and a pen to make a label and, if you want, a penny balance and centimeter scale or some other means to weigh the finished motor.
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Start by measuring off the required amount of strip.  A 25″ long motor will require 50″ of strip, plus 1/4″ extra on each end for the knots, so a total length of 50 1/2″ is required.  Usually I lay it out on the kitchen floor in a straight line, but that wouldn’t fit on the table for the picture, so I doubled it over and measured off 25 1/4″.
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Cut with scissors, sharp razor blade, wire cutters or finger nail clips.  Whatever works!  You star troopers can use your laser.  But don’t burn a hole in your building board!
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Wash the strip with liquid kitchen detergent and rinse it thoroughly.  You don’t know where this rubber has been and you don’t want any hard, sharp grit that can start a tear in tightly wound rubber.  This also gets your hands clean.
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This isn’t really necessary, but I like to blot it dry to keep drops of water falling all over the place and diluting the lube.  Just be sure your diapers are clean!
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An O-ring makes it so much easier to get the motor onto the prop hook without losing turns or having the slippery motor slip out of your fingers and jam way down inside the fuselage.  This one weighs only 0.043 grams and is worth its weight in trouble avoided.  Slip the end through the ring and slip the ring to the middle of the strip.  This is very easy to forget.  Every time you start to tie a knot in rubber, ask yourself “Did I put the O-ring on?”
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Some people like to “braid” the motor.  This is not actually a braid, it is done by twisting the ends of the strip so the two halves twine around each other.  This makes the motor a little shorter and stiffer so it doesn’t slop around as much in the fuselage when it runs down.  With a motor this long, that might be a good idea, although I’m not expecting this motor to ever run all the way down, since the gym ceiling limits how high the plane can fly and keeps us from using all the turns on the motor.  You actually turn each end in the “same” direction, here clockwise.  If you turn them in opposite directions, no twist will result.  (Because then they actually are turning the same direction.)  Try it.  You twist just enough so the entire length is twined.
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Leave about two inches free on each end and dip them in some diluted lube.  If you tie knots on full strength lube, you will be able to watch the knots slowly untie themselves.  That’s right, lube makes the rubber slippery!  This is even more fun to do than watching glue dry. The straight lube is made of one volume of glycerin and one volume of green soap, both available at the pharmacy.  (There are other lubes available, you can buy proprietary mixes in little bottles or you can use silicon lubes, but this stuff works just fine.  If green soap is hard to find, try baby shampoo or liquid dish detergent.  The glycerin is slippery, but it is in the formula mainly as a moisturizer.)  Shake and mix it well. The knot tying lube is one part straight lube mixed with one parts of water.  It doesn’t take much, one teaspoon of lube and two teaspoons of water is plenty.  You can put it in a pill bottle or a plastic film can.  Some people put the ends in their mouth and wet the ends with saliva.  OK with me if you want to do it that way, but I find that my saliva isn’t slippery enough to prevent little tears in the rubber during the knot tying.
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Smear a little knot lube on both index fingers and thumbs.
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There are many, many kinds of knots.  Some are so complicated that it takes an entire crew of sailors to keep track of all the twists and turns.  This one is very simple and does the job well.  It is just two overhand knots, tied in opposite directions.  Start by putting the running end under the standing length and bringing it up over, around and down into the loop.  This is about the simplest knot in the world to tie and you already know how to tie it.
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Pull the knot up snug, but not tight, a little distance from the ends.
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Then pull the two lengths apart to pull the knot up to about 1/8″ from the shortest end.
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Grab the two short ends and the long pieces and pull the knot as tight as you can get it.  You will feel the rubber hardening as you reach its maximum extension.
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Dip the knotted end into the knot lube again.
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Tie the second knot in the opposite direction from the first one.  This time the running end went over the standing length, down under and up through the loop.  This is the most difficult knot in the world to tie and will take weeks of intense practice to master.  😉
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Pull the knot up snug.
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Pull this knot up against the first knot.  Pull it pretty tight.  You can grab the ends and pull tight again, like you did with the first knot, but finish by pulling the second knot tight up against the first knot.
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At this point you may want to weigh the motor.  This one weighed 2.13 grams, dry.
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You will need a clean, dark place to store your motor.  Keeping them inside a Ziploc bag inside a dark can or box will work.  The main thing is to protect them from sunlight.  Some people insist on storing their rubber in a refrigerator, but that does not seem to make much difference.  You can use a black plastic film can.  I am using the vial my blood glucose test strips come in.  After peeling off the label and washing the vial, I put on a piece of masking tape so I can relabel it.
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I wrote the length, width, weight and estimated maximum turns on the label.  You can also write the rubber batch, airplane name and make a check every time you use the motor.
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Pour about a teaspoonful of lube into the Ziploc bag.
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Put the motor into the bag and massage it until the entire surface is coated with lube.  The bag makes it unnecessary for you to take a bath after lubing your motor.  You will get only a little on your fingertips that you can wipe off on the diaper.
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Grab one end and pull the motor out between two fingers to squeeze off excess lube.  You may want to weight it again.  Mine weighed 2.30 grams wet, meaning that the lube weighed 0.17 grams.  Hmm, maybe I will wipe it again.
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Loop and gently twist the skein…
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…twisting and folding it until it will fit into the can.  Put it in your flight kit and go flying.
Gary Hinze

10 thoughts on “Making a Rubber Motor

  1. I found the Glycerin and Green Soap. The Green soap has 30 percent alcohol, do you simmer the green soap first? I mixed 50/50 Glycerin/Green soap straight, simmered until no alcohol and let cool. This made for a pretty thick lube, much thicker than the old Peck Lube.

    1. Green soap is soap made from plant oils rather than animal fat. Green soap comes in several forms. There are solid bars, soap dissolved with water and a tincture in alcohol. I buy the water solution. Heating the tincture to drive off the flammable alcohol over a gas flame could be a fire hazard. I would add water to the heated soap until it is about the same consistency as the glycerin. The goal is to get an even, thin coating all over the rubber. You don’t want it to splatter all over the inside of your airplane. You don’t want it to dry out too fast. The glycerin helps keep it moist, but you can tell when the lube has dried out in the air, it gets harder to put maximum turns into the motor. Time to refresh the lube. Keep your lube in a bottle with a secure cap to keep it from drying out in the bottle.

      1. Gary, I looked on the internet and cannot find the Green Soap in Water, can you recommend a soap to use? thank you. David

        1. I have a bottle of lube dated 18 May ’78 and another dated 11 June 2002. I don’t have the empty bottles of green soap, so can’t quote the labels. In 2002 I had to special order the green soap at Longs Drugs. For the Klutz book project, I used Johnson’s baby shampoo. I have used bubble soap without glycerin. It works, but dries out quickly. I think it would work well mixed with glycerin. I got it to show wind direction on the field. A mix of Joy dish soap and glycerin is used for bubble soap. Maybe that would work for lube, too.

  2. Hi Gary

    I’ve recently picked up my old hobby and I’ve got a knack for vintage stick & tissue models (found a whole bunch of plans from the 50ies & 60ies in my dad’s cabinet).
    Where can you buy the O-rings you mention in your article? I take it that the O-ring in the the above article is for smaller (indoor) planes? What type should I take for larger outdoor planes (wingspan 30″ and above)?

    regards

    K-J

    1. Hi Karel-Jan,

      O-rings may be bought in bulk from industrial suppliers, but most modelers buy them in smaller quantities from one of the model suppliers. These sites list many other things that may interest you, too.

      http://www.faimodelsupply.com/

      http://www.indoormodelsupply.com/

      https://www.a2zcorp.us/store/

      http://www.campbellscustomkits.com/

      Larger models use metal rings or hooks. You will find those described in the above catalogs.

      You may prefer to buy from a European source.

      http://www.samsmodels.com/index.html

      http://www.freeflightsupplies.co.uk/

      Gary

  3. On a related matter, I can find no article discussing when you use a motor of 1/8 or of 3/16. For example, for the Gollywock, I know a number of flyers who swear by using a 3/16 rubber size for their model. I also know a similar group of Gollywock fliers who swear by using 1/8. How does one know what size motor to use on a flying model?? Thanks for any help you can give me…………………Rick Isaacson

    1. Rick,

      The size of a motor is determined by its length and its cross section. The cross section is the area of a single strand times the number of strands. Your question relates to the width of a single strand. The width of the individual strand is not important, as long as you can get the cross section required to develop the torque to fly.

      There are those of the opinion that a few wide strands will provide more turns, torque and energy than many narrow strands, made up into motors of equal total cross section. There are those of the opposite opinion. That tells me the difference is not very strong. My analysis of 20 motors with 2 to 12 strands found a very weak negative correlation between number of strands and turns and torque over the whole range. The weak relationship also was not statistically significant. Given the closeness of 1/8″ and 3/16″ width, the numbers of strands would not differ by much, making any difference in turns and torque proportionally small. In other words, the width and number of strands is not important. Use whatever width you have to make the required cross section and weight.

      A strand of 3/16″ has 1.5 times the cross section of a strand of 1/8″. Two strands of 3/16″ is equivalent to three strands of 1/8″ in cross section. However, we make motors using loops containing even numbers of strands, because the ends must come together to be tied. If you needed to make a motor with this cross section, you would chose the 3/16″ strip.

      In competition, motor weight is often limited in the rules. Weight is the product of motor volume and rubber density. (Our Tan rubber has a density of around 0.93 grams per cubic centimeter. Add a little for lube.) Volume is the product of length and cross section. A weight specification thus defines a relationship between length and cross section. Typically a modeler will decide on the length, work out the cross section, and figure out the number of strands to make that cross section. (Modelers often assume a weight of 1 gram per foot of 1/8″ lubed rubber. He can multiply that by the length they want and the number of strands to see whether it comes in under the minimum weight.) If the numbers don’t work out, he can adjust the length or he can use a different strand width.

      Gary

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