Variable Speed Gear
Built by Dick Watson in October 2014
This mechanism is shown, with no instructions, on the last page of the post-war Mechanisms Set manual. It could not have been built from the contents of the outfit, in fact, none of the gears used are contained in it. It is taken from a suggestion by R. Fordham from Crewe published in the March 1933 Meccano Magazine.
The 4” circular plate is driven by the motor. Turning the handle on the right-hand side of the mechanism moves two 1½” pulleys across the face of the circular plate. These pulleys drive the bevel gears on the two sides of the differential in opposite directions. The rod carrying the other two bevels rotates at the average speed of the driven bevels, and is connected to the output shaft.
If the 1½” pulleys are the same distance from the centre of the Circular Plate, the output shaft will be stationary, but moving them one way or the other will cause the output shaft to turn in different directions. The speed increases the further they are moved outwards.
The same could be achieved with a single 1½” pulley, but it would have to move right across the face of the 4” pulley, catching on the nuts in the centre.
The mechanism is like a motor car differential working in reverse, and demonstrates how it works.