Meccano Torque Amplifier

Torque (or Torsional Moment) is an action on a shaft tending to cause rotation. It is defined as the force acting at a tangent to the circumference of the shaft multiplied by the radius of the shaft, i.e. a force times a distance, and is expressed in lb-ft or N-m. The torque will thus increase with a larger force for the same radius. A torque amplifier acts to increase the torque of a shaft without a change of speed. This can only be achieved by the introduction of an external form of energy such as an electric motor.

Why should one want to amplify torque? The best example in Meccano is its use in the differential analyser. This is a mechanical analogue computer used for solving differential equations. It can handle the sort of problem where for example one needed to know how far a car had travelled in unit time while accelerating steadily. To solve this problem, one would have to take frequent samples of the speed and distance travelled. The more samples, the greater the accuracy and adding them together is the integrating process. The integrator is merely a large horizontal disc, on which a wheel at right angle is driven by friction. The wheel can be moved along the surface of the disc by a lead screw, and even across the centre of rotation, and so the speed and direction of the wheel can be changed. Slippage of the wheel has to be avoided, but it is required to drive a system of gears, and needs torque amplification in order to do this.

The action of a torque amplifier is not at all clear from an inspection of a photograph or diagram. Even an explanation can be quite hard to understand. The principle on which it works is not unlike that of a deckhand who works with a power capstan. Pulling on the rope wrapped around the capstan results in a huge increase in force at the other end because of friction between the two, the difference being supplied by the capstan. Controlled slippage of the rope on the capstan varies the amount of force obtained. There is no gain in speed, the fastest speed being that of the capstan output from the motor.

The Meccano model utilises the capstan principle, and contra-rotating drums driven by an electric motor at constant speed running free on the output shaft in opposite directions provide the power and reversing. Cord is wrapped around each drum, and the input tightens the cord on one or other drum depending on the direction of input rotation. The other end of each cord is attached to an output crank fixed on an output shaft.

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