Miscellaneous notes

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Though it was not shown in any of the animations, there are several auxiliary parts that ensure the correct functioning of the Engine. 

One of such devices are the locks.  The wheels are seldom left loose.  When not engaged by gears, levers or arms, they are held in place by locks that come in and out at different times to make sure that the wheels cannot move.  At full speed, the wheels might overshoot their expected positions or they might be carried along by friction against their neighbors.  With all those gears coming up and down, engaging and disengaging, if the wheels were not where they are expected, the Engine would jam.

Moreover, these locks are wedge shaped and usually come in in between the teeth of the wheel so that they ensure the wheels are left in proper alignment even if slightly offset.

Other useful devices are the zero stops.  Some of them are static, like the ones for the sector wheels.  When the restore arm turns them to zero, they end up against a stationary vertical bar that is the zero stop.  This bar is shaped so that when the sector wheels rise to their upmost position and are thus disengaged from any other wheel, an extra tooth, not shown in the model, gets lodged in a grove in the stop bar, so that it can't move.

The figure wheels also have a zero stop, which is not stationary but comes in and out on every cycle.  It comes in when the restore arm engages, to provide a stop when the figure wheel is returned to zero, preventing it from overshooting.  This zero stop does move away, unlike the one for the sector wheel, since the figure wheels do have to move full circles, while the sector wheels just come and go.

Though the locks are small and their movement is minimal, they have to do it quite fast.   Though they are indeed quite important in ensuring the integrity of the Engine itself and the calculation, they do not participate in the calculation itself and Babbage didn't give them more than gaps in the whole cycle to do their thing.  Thus, while doing the first tests of the Engine at the Science Museum, it was found that when it came the time in the cycle to disengage the locks, the person at the hand crank just couldn't move the handle.  Thus, a set of springs were added to the original design to help with the weight of the locking mechanism and making the whole cycle much smoother.

I have used the word  'fast' several times in this page so, you might be wondering how fast is fast.  The Engine can be operated so that it makes 10 calculations a minute, that is, a result every 6 seconds.  If it goes any faster, the momentum of some of the parts pushes too hard against all the transmission mechanism and bends it out of tolerance, so that the gears are not where expected and the teeth start hitting each other either jamming the Engine or breaking the teeth.

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