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This page provides a guided tour of the gearboxes I fabricate. My apologies for the quality of some of the photos; I will replace them with brighter and clearer ones as soon as possible.
My gearboxes comprise a phenolic worm gear, a brass worm, and brass sheet and tube. I fabricate the worm gears from 1/4" thick phenolic sheet, the worms from 3/8 dia. solid brass rod, the gearbox sides from 1/32" thick brass sheet, the worm tube from 1/32" wall brass tube, and the side extension tubes from 9/32" solid brass rod. The worm gear is 32DP 30 teeth 1" diameter, the worm is of course also 32DP and 3/8" diameter. With a single tooth worm the ratio is 30:1, with a two tooth worm, the ratio is 15:1.
The fabrication sequence is as follows:
Each gearbox consists of a pair of halves that are soldered together to create a box with narrow openings at both the top and bottom. The worm shaft assembly is fitted to the top; the bottom is covered with a plate that is stamped with "RM xxx" where xxx is the custom drive's serial number.
Note the stabilizer tubes on each side of the gearbox, which are unique
to this design. The tubes eliminate the gearbox's squirming on the
axle. Run one of your locos upside down and chances are you will see
the gearbox squirming. With my design all the energy put into the
gearbox goes into turning the axle, and no place else.
The worm's tooth (or teeth if the worm is a multiple lead worm) pushes or pulls (depending upon the worm shaft rotation) the worm gear as the worm turns. Since the worm is fixed in position longitudinally but the worm gear is free to rotate, the worm gear turns, turning the axle and the wheels. However, as the worm gear turns, its (the train's) resistance to being moved causes the worm gear to tend to push or pull the worm depending upon the locomotive's direction. This force upon the worm shaft is called end thrust. If the worm shaft is rigidly connected to (or is part of) the motor shaft, then the motor shaft is subjected to end thrust too. The reason some locos run better in one direction than the other is that end thrust on the motor shaft in one direction allows the motor to run better than it does when the end thrust is in the opposite direction. The best way to fix the worm in position so that there is no back and forth movement of the worm shaft is to use ball bearings, which efficiently absorb the end thrust caused by the resistance of the worm gear to being turned.
The distance from the center of the axle to the contact point of the worm tooth and the worm gear tooth is a lever arm. The longer the lever arm, e.g., the larger diameter the worm gear, the less work it takes to turn the worm gear.
The diameter of the worm is also a lever. Consider loosening a screw with a small diameter handled screwdriver then switching to a large diameter handled screwdriver.
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