Building the World's First Computer: Babbage's Analytical Engine

One of the subtleties of locking that Babbage doesn't seem to have detailed anywhere is what happens at the top and bottom of the digit stacks when shifts (which multiply or divide by 10) are done. If all digits are unlocked in the normal fashion, then the least significant digit (for left shifts) or the most significant digit (for right shifts) are undriven and are subject to accidental movement.  The solution is to devise a mechanism that will lock only that one digit. Here's an illustration for number stacks that are four digits high, showing which digits need to be locked. In my continuing quest to minimize the number of vertical axes, I came up with a way to do that with the existing long pinion locks: a single extra locking lever that is engaged by a "backwards" rotation of the axis.  When fully rotated counterclockwise, all locks are engaged with the inner gears of all digits. When fully rotated clockwise for shifts, only the one "backward" lock is en...

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: locks for Plan 27, from BAB/A/097 where he needs four supporting vertical axes: the lock pi...