Rubber Motor Secrets. Drawing, description

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Rubber Motor Secrets. Tips for a modeler

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For many years, the rubber motor, due to its ease of operation and low cost, was the main engine for modellers. However, even now, with an abundance of micromotors, ship models and car models with a rubber engine find their ardent supporters, as evidenced, in particular, by editorial mail. Among your questions, dear readers, two are the most common. So, T. Zavgorodny from Volgograd, E. Shcherbina from Semipalatinsk, A. Zhdanov from Moscow and others are interested in how to calculate the number of revolutions of a rubber motor in order to make the most of its energy, and what is the reason for its maximum energy efficiency.

The rubber motor works due to the internal energy of the stretched rubber threads. They can increase their length quite strongly: 6-7 or more times, and distinguishable varieties are stretched in different ways. There are several formulas that modellers can use when calculating a rubber motor. One of them can be used to determine the relative elongation λ (lambda) - the ratio of the increment in length to the original. This is written as a formula:

where 1 is the free length of the rubber and Δ1 is the length increment.

Stretching a rubber thread, we spend a certain amount of energy, but we get back only 65-67%. The loss is caused by the cost of internal friction in the rubber itself. Therefore, the coefficient of performance (COP) of the rubber motor cannot be more than 0,65-0,67.

In order to accumulate more energy in the rubber motor, it is necessary, while maintaining the mass of rubber, to increase its twist. Moreover, it will be the smaller, the thicker the rubber motor. This contradiction is solved by composing a rubber motor from several separate threads. Since during the spin-up of the rubber motor, the loss of power is caused both by internal friction and by friction between the individual threads, the practical number of revolutions is always less than that which should be obtained according to the calculations. We propose an approximate formula for calculating the number of revolutions:

here: N is the maximum rubber motor winding in revolutions, E is the initial length of the rubber motor in centimeters, λ is the maximum relative elongation according to the characteristics of the rubber or obtained empirically, S is the total cross section of the rubber motor threads in cm2, K is some experimental coefficient

It should be said that the accuracy of the calculation according to this formula will increase if the rubber motor is pre-stretched before the factory. Experiments have shown that with an increase in preliminary drawing, the number of revolutions increases, which is taken into account by introducing the experimental coefficient K in formula 2. This effect on the plant is apparently due to the fact that the distribution of friction between the fibers, and especially between the threads of the rubber motor, changes.

The values of the experimental coefficient K, depending on the preliminary drawing, are given in the table.

To reduce friction between the strips, the rubber motor is usually lubricated with castor oil or glycerin (this also increases its durability). If we take the useful energy of the rubber motor as 100%, then when twisting without preliminary drawing, we will lose 30-35% for friction, and by lubricating it and applying preliminary drawing from 1 to 3 times, we can reduce the loss to 5%.

Author: A.Schulmeister

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