The unit is built up from three components:
a/ A cage which is fixed to the input shaft;
b/ pinions free to turn on the input shaft;
c/ Rack strips fixed to guides which slide in the cage.
As the input shaft is free to turn in its mountings, the cage can take up any angle, which is determined by whatever the rack strips are attached to or actuate.
The motor output is geared to the actuating pinions, which are free running on the input shaft. The unit is as compact as possible and very strongly built.
Each side of the cage is built up as follows: the assembly consists of two 1" corner brackets (part 133a), a crank (part 62) and a fish plate.
The fish plate is fixed to the same side of the crank as the boss. It is fixed by its oval hole to the middle hole of a crank, in such a way that its round hole overlies the slotted hole of the crank, but the part of the slot which is adjacent to the boss.
The retaining bolt head is also on the same side as the boss, and the bolt also holds the two corner brackets. The corner brackets overlap each other, providing three holes below, and it is by their apical holes that they are fixed to the bolt. This assembly is very firmly fastened together.
Having made the two sides of the cage, assemble the actuating pinion assembly. This is simply two 1/2" pinions fixed into each side of a socket coupling. Ensure that the unit runs freely on a rod.
The rack strips and guides can now be assembled. Use six 3 1/2" rack strips, three overlapped on each side. Each bank of three strips is very firmly fixed at each end to a 1 1/2" perforated strip. The strips are angled towards the teeth side of the rack strips by 1 degree.
Each assembly is now fixed to a 5 1/2" perforated strip by the middle holes of the 1 1/2" strips, but with three washers between the two on each bolt. The two assemblies can now be fastened to each other at one end only at this stage by a 2" screwed rod as follows: The screwed rod is passed through the long perforated strip, three washers, then the short strip, four collars and three washers, then the short strip of the second assembly, three washers, the long strip and finally the fixation nuts on each end of the rod.
Assembly of the mechanism.
A 3" rod is used for the input shaft. The pinion assembly is slid onto this rod and then the sides of the cage. The bosses of the cranks are turned inwards to lie against the pinions and are very firmly fixed to the rod, allowing the pinion assembly to rotate freely.
Three 3" rods each carrying two 1/2" pulleys are now assembled in the end holes of the cranks, and in the free holes of the corner brackets. Each rod is retained by collars on the outer side of the cage on each side.
The rack strip assembly can now be slipped in from one side. The 5 1/2" perforated strips are located in the pulleys, and the rack strips mesh with the pinions. The free ends of this assembly can now be fastened together in the same way as before.
There should be almost no play between strips and pulleys, and there should be no binding between rack strips and pinions. The reason for the angled 1 1/2" perforated strip becomes only too clear at this stage; to maintain the proper mesh distance between rack strips and gears.
This mechanism can be used to activate a linear drive, such as the opening of a drawbridge. The input shaft would be journalled in the machine, but free to rotate. The drive to the pinions would be 1/2" away, would also be a 1/2 pinion which would be activated by the drive motor.
One end of the rack strip assembly would be fastened to the part of the machine to be moved.
If a longer throw is required, simply substitute longer components in the linear part of the assembly.
Michael Adler - February 1999
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