By the time Babbage had gotten to Plan 15 in 1837, he had elaborated the wheel locks to have a third state in addition to locked and unlocked: weakly locked. In that state the wheel was indirectly retained in position by a spring, but if the wheel were driven to move, it could. That prevents a free wheel from accidentally moving, perhaps because of vibration, while still allowing it to be forcibly changed.
A prime example is a figure wheel during carriage, It may or may not be receiving a carry or a borrow, so it has to be loose to allow that to happen or not. I've found weak locks to be helpful to improve reliability, not so much because of vibration, but because my concentric-axis arrangement without horizontal framing plates for the carriage digit wheels can transmit a very slight turning torque to undriven wheels.
Babbage's typical mechanism for weak locks looks something like this:
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| locks for Plan 27, from BAB/A/097 |
where he needs four supporting vertical axes: the lock pivot L3, the locking tab F, the detent pivot L2, and the spring holder a.
In my quest for simplification, I've implemented a related but simpler scheme that only requires two vertical axes: one for the lock pivot which also holds the tab (posts, in my case) that move for locking and unlocking, and one to hold the detent which has an embedded spring. (See the previous post about detents.) The "weak lock" position is where neither of the two posts are in contact with the locking arm, which is then free to move to either the lock or unlock position, but is weakly held in those positions by the detents on the other arm.
This is what it looks like when fabricated and installed. The white Teflon washers help isolate the movement of the carry arms from that of the locking posts that are driven by rotation of the slotted shaft. The carry arm hubs have oil-impregnated brass bushings, which you can't see, that further reduce friction to the shaft.
Once the detent pressure is adjusted, it seems to work well in the case where the digit wheels are being directly driven by carry sectors during the operation of the anticipating carriage for addition or subtraction.
It does not work when the digit wheels are being driven a long chain of gears. The locks are in the gear teeth, and given the geometry -- which perhaps could be improved -- it takes a reasonable torque to force the lock out of the gear tooth valley while working against resistance of the arm to move the small distance that depresses the detent and has it move to the next groove. When the driving force is many gears away, the backlash accumulates, increasing the force to the point where the lock finally breaks free, but with so much energy stored in the gear train that the wheel travels too far.
Perhaps this is a symptom of excessive backlash caused by sloppy 3D printed gears that needs to be addressed generally. But in the meantime weak links will be used only for wheels driven directly by a prime mover.
More and more of Babbage's design elements seem to be making their way into my design...
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