Meccano Torque Amplifier
by Michael Adler
after a design by Tim Robinson
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 traveled in unit time while accelerating steadily. To solve this problem, one would have to take frequent samples of the speed and distance traveled. 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. This is shown in Fig. 1.
Construction of the frame is shown in the Isomec diagram (Fig. 2). Each side is built up from two longitudinal 7 1/2" angle girders joined at each end by 1 1/2" girders. The side frames are joined to each other by transverse 5 1/2" girders. Two transverse 5 1/2" strips are fastened to the long girders top and bottom. Add vertical 1 1/2" strips between the side girders as shown. Bolt a 1 1/2" strip to a 1 1/2" flat girder to overlap its oval holes at exactly 1/2" spacing from the round holes of the flat girder. Now bolt the flat girder to 1 1/8" bolts inside the left side girders to form bearings for short rods. . On the right side, fix a vertical 1 1/2" strip to 1 1/8" bolts inside the side girders to form a bearing for a short rod.
Assemble the drums and their 2 1/2" gears according to the photo and diagram (4,5) (Fig. 3). Each drum consists of two wheel flanges and a boiler end between them. The boiler end drum surface should be highly polished to obtain smooth operation. To do this, run the drums at speed and use 1500 grade abrasive paper followed by a liquid metal polish. A 1 1/8" bolt is just long enough to hold the drum and gear together, but the components must be located very carefully on the four bolts so that there is no eccentricity. Use rods through all the holes to locate before progressively substituting the screws.
A 1" angle bracket (3) is fixed to the input 57 tooth gear but separated from it by a washer. A second 1" angle bracket is fixed to the boss side of the gear, but separated from it by a collar and washer. The left output crank is a 1" angle bracket (7) fixed to a crank. The right output crank (1) is a similar structure.
Assemble the drums, input cranks and output cranks on a long output shaft (8). Note that there are two washers between output crank (7) and input crank gear. The shaft is located by collars inside the frame. Do not at this stage tighten any component on this shaft
Use thick woven thread of about 1.5mm. The material of the bands is important. Most cord for some reason contracts when heated. If the mechanism is run under real load for an extended time the bands become warm or even hot. If
they shorten, they tighten and that results in positive feedback as the tightening results in more friction. Dracon or similar material can be used. Meccano cord is not suitable.
The motor must turn only one way and the left-hand drum must rotate clockwise as shown in the diagram. In order for the input crank mounted on the 57 tooth gear to tighten the thread on this drum, the input gear on the output shaft must be rotated clockwise as well. Fasten one end of the thread to the input crank, and then run the thread in an anti-clockwise direction for two and three quarter turns around the drum, being careful to lay the turns next to each other. A crossed thread must be avoided. Note that there is a slight difference in alignment of the cranks which assists in maintaining the thread turns. End by fastening the thread on the left hand output crank with the two cranks at about 90 degrees to each other. Test by rotating the input gear clockwise. The thread should tighten on the drum.
Now fix the thread for the second drum in place. This must be done in a direction opposite to the first thread. Start by fixing the thread to the input crank of this drum. Now run the thread clockwise round the drum for two and three quarter turns, and fix it to the output crank, again with the two cranks about 90 degrees to each other. When the input crank is rotated anti-clockwise, it should tighten the thread on this drum, which should always be turning anti-clockwise.
Now loosely tighten the set screws on the output cranks on the output shaft and rotate the cranks on the shaft until all slack is taken out of both threads but do not over-tighten. Now firmly tighten the crank set screws. Slippage must occur between threads and drums when the there is no input.
Fix a second 57 tooth gear (6) on the input shaft to mesh with the a similar gear and locate the shaft with collars.
All that remains is to provide a drive from the motor to the drums in such a way that they rotate equally in opposite directions. This is done by using two 1" gears on one side, and three 1/2" gears on the other from the motor drive. A 3/4" pinion on the motor output shaft meshes with a 50 tooth gear. This gear is fixed on a shaft on which a 1/2" pinion is fixed at one end, and a 1" gear at the other. The motor used here is a Faulhaber 6volt with built in reduction epicyclic gearbox, but if a faster speed is required, a different motor and reduction gearing can be substituted.
Action. The motor should be started and run at full speed and the drums should be rotated in the desired direction. Once the motor is started, operation of the device depends only on movement of the input shaft. The threads should slip on both drums. When the left thread is tightened, power is transferred to its output crank, and the output shaft starts to rotate, powered of course by the motor. When the right thread is tightened, the output is in the opposite direction. The increased torque given to the output shaft can easily be felt.
1. Constructive use of friction in torque amplifiers and constant torque devices by Dr. Ir. I.H.Boedijk - CQ No. 4 June 1989
2. Torque Amplifiers by Alan Partridge - CQ No. 19 March 1993.
3. Differential Analyser by Ron Fail - Midlands Meccano Guild Gazette No. 16 April 1993
4. Differential Analyser GMM Supermodel No 4. December 1967
5. ISM Meccano Online Website www.actcom.co.il/~anthias/
Michael Adler - May 2001
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