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Comptometer - The "Controlled Key" mechanism


The purpose of the "Controlled Key" mechanism is to ensure the accuracy of Comptometer calculations by preventing two of the most common operator errors - failing to fully depress a key, and commencing a repeated keystroke before the key has fully risen.

The "incomplete downstroke" problem is solved with an elaborate "trigger" mechanism. The trigger is "armed" at the commencement of every keystroke, and disarmed at the very bottom of the downstroke. If the key reverses while the trigger is armed, the mechanism trips a "touch-off bar" which drops a set of locks onto the segment levers in every column. The entire keyboard immediately locks, except for the column causing the error. The operator is immediately aware of the problem, completes the offending keystroke, releases the lock, and the calculation continues without error.

The "incomplete upstroke" problem is solved with a "pinion ratchet reverse lock", which is engaged during the upstroke to prevent reversal of the segment lever.


Trigger Parts The trigger components.

This view shows the components of the trigger mechanism in each column.

The trigger itself is the black multi-armed lever, just over 3" long, at the top right-hand side. A pivot near its centre connects to a vertical mounting link and a long horizontal connecting link.

The four parts below the trigger assembly are (left to right): the trigger retainer horizontal (TRetH), the trigger retainer vertical (TRetV), the pinion ratchet reverse lock (PRRL), and the accumulator locking hook (ALH).


Trigger Link Rear) The trigger connecting link.

The tail of the trigger connecting link attaches to the segment lever bell crank via a pin-and-slot and a short horizontal spring. The short downward arm on the brake lever acts as a retainer to keep the link in position on the bell crank pin.

This view also shows the two nested key stop levers pivoted above the segment and lock levers at the rear of the machine, and passing between the two vertical arms of the bell crank.


Trigger retainers The trigger retainers.

The two trigger retainers (perhaps better called "latches") are mounted on the same 1/16" wire near the top centre of the frame.

The lower (right-hand) arm of the trigger retainer horizontal (red) passes behind the frame plate and rests on a vertical post on the top of the segment lever. It is held in contact with the post by a light spring (not shown). The two small downward projections on the frame plate prevent the arm from rubbing on the plate, and limit its vertical movement.

The forward arm of the trigger retainer vertical (blue) is held against the central stop wire by a light spring.

Several other points (unrelated to the triggers) can be seen in this view:


Trigger Installed The trigger installed.

The trigger hangs from a wide U-shaped link which occupies the full width of the column. The trigger can move horizontally fore-and-aft, and can rotate about its central pivot. The trigger is spring-loaded anti-clockwise and rearwards by the two coil springs attached to its forward arm.

The rearward arm of the trigger has a horizontal cross-arm (just forward of the stop wire), which extends across to the right-hand side of the column and rests on the top of the segment lever, thus setting the initial rotation.

With the segment lever and cross-arm correctly adjusted, the upwards vertical arm of the trigger will stand just under (but not touching) the tip of the upper arm of the trigger retainer horizontal.

The downward vertical arm at the rear of the trigger also has a short cross-ways section, which sits just below (but not touching) the rear-facing point on the trigger retainer vertical.


Accumulator interlocks The accumulator interlocks.

The accumulator locking hook (red) and the pinion ratchet reverse lock (blue) are mounted on the same 1/16" wire, immediately below the eccentric frame spacers at the front of the machine. A second wire (the pivot for the subtraction cutoff levers) passes through a cut-out in the vertical arm of the reverse lock and serves to limit its rearward travel. In early machines (as shown) the two levers are positioned laterally by a tag on the top of the locking hook which fits into a slot in the under side of the frame spacer. In later versions the ALH has "ears" which position it centrally between the frame plates.

The fork on the rear of the accumulator locking hook fits around the pin on the front of the trigger.

The two interlock levers are interconnected by a spring-and-stop arrangement near their hubs, such that one or the other is always engaged with the accumulator.

With the trigger in its rest position, the front of the accumulator locking hook is raised clear of the lantern wheel, and the pinion ratchet reverse lock is spring-loaded lightly against the outside shell of the pinion ratchet. (In this view the pinion ratchet is hidden behind the lantern wheel). When the trigger pin is raised, the accumulator lock will drop onto the lantern wheel, and the reverse lock will be released.


Trigger and touch-off bar The touch-off bar.

The touch-off bar (red) is a 1/16" square bar suspended on fixed arms under a 1/8" x 1/4" mounting bar. The mounting bar extends the full width of the machine, and pivots between the two outside frame plates. The touch-off bar is held forward against a stop by gravity and a light spring.

With the machine at rest, the rear-facing projection on the tail of the trigger is positioned just above the touch-off bar.

As the trigger is armed, its tail moves forward and then down, bringing it directly in front of the vertical face of the touch-off bar.

If the keystroke is completed normally, the tail of the trigger will reverse its travel and move up and back to its original position. However, if the trigger is tripped by a short keystroke, it will move directly rearwards, pushing the touch-off bar rearwards and activating the locking mechanism.


Segment lever locking hook The column lock mechanism.

Just rearward of the touch-off bar is a second 1/8" x 1/4" bar pivoted between the outside frame plates. The cross-arms on this "column lock retainer bar" support two 0.088" wires, which are retained by the bronze leaf spring at the left-hand end.

The "column lock" or "segment lever locking hook" is the small black U-shaped hook which is suspended from another 0.088" wire. The right-hand arm of the U passes behind the frame plate and is positioned by a tab which curls over the top of the plate.

The locking hook is pulled forward by a light spring (not shown) connected to the arm on its top left-hand side, but is held back by the short rearward arm at its top right which rests on the front wire of the column lock retainer bar. Allowing this wire to drop will let the locks swing forward under the corresponding hooks on the tops of the segment levers.


Touch-off detail Touch-off operation.

This view from the left-hand rear shows how the column lock retainer wire is held in the raised position by an extended arm (blue) which rests on the top surface of the touch-off bar, within about 0.010" of the edge. (The outermost touch-off bar support has been removed for clarity).

If the trigger pushes the touch-off bar rearwards, the arm will drop and the springs will pull the locking hooks forward. The hooks will immediately lock any columns that are at rest, and will catch the others as soon as their segment levers rise.

The lock release button on the keyplate presses down on the rear wire on the column lock retainer bar, raising the front wire and the extended arm. This releases the locking hooks, and allows the touch-off bar to spring back into position for the next operation.




Trigger operation.

Having described the components of the "Controlled Key" mechanism, the remainder of this section describes the sequence of operations that occur during every keystroke.

1. Column at rest.

With the column idle before a keystroke the rear horizontal cross-arm of the trigger is resting on the top of the segment lever. The tail of the trigger is sitting just above the touch-off bar, while the pin and fork at the front are holding the accumulator locking hook clear of the lantern wheel.

2. Trigger moves forward.

As the keystroke begins, the right-hand shoulder on the keystem presses down on the lock lever. The lock lever rotates the segment lever bell crank clockwise, pushing the trigger connecting link forward. The trigger moves horizontally forward, rotating its support link anti-clockwise. The trigger movement is less than 1/8", and ceases when the forward arm on its supporting link strikes the underside of the spring attachment wire. Any excess movement of the bell crank is absorbed by the slot and spring at the rear of the trigger connecting link. The tail of the trigger is now positioned above and just forward of the touch-off bar.

As the trigger moves forward, the downward vertical arm at its rear end strikes the hook on the trigger retainer vertical and pushes it forward.

3. Trigger rotates to armed position.

As the key continues downward, its right-hand shoulder starts to move the segment lever. The trigger follows, rotating anti-clockwise until it reaches the large stop wire under the centre of its rearward arm. The movement at this point is less than 1/16". The extreme ends of the trigger move almost 1/8", bringing the tail downwards to the "armed" position directly in front of the touch-off bar.

As the trigger rotates backwards, the trigger retainer horizontal (which also rests on the segment lever) rotates forward, until the tip of its upper arm comes to rest on the tip of the vertical trigger arm.

At the same time, the pin on the forward arm of the trigger raises the fork on the rear of the accumulator locking hook, which lowers the hook into engagement with the lantern wheel. The locking hook is linked to the pinion ratchet reverse lock, which is pushed rearward and clear of the ratchet.

The mechanism is now ready to complete the downstroke - the trigger is armed, the accumulator is locked, and the pinion ratchet reverse (ie, downwards) lock is disengaged.

4. Full keystroke.

Towards the bottom of the keystroke, the left-hand shoulder on the keystem reaches one or other of the key stop levers and begins to activate the key stops.

At the very end of the stroke, a small tab on the horizontal arm of the keystop presses down on the forward arm of the trigger, just forward of the springs. The trigger is rotated to a position slightly more clockwise than its rest position, allowing the trigger retainer horizontal to drop behind the vertical arm, and the trigger retainer vertical to swing under the tab on the rear of the trigger. The trigger is now disarmed, with its tail raised clear of the touch-off bar and held fast by the two retainers.

As the trigger is pushed downwards by the key stop, the accumulator locking hook is raised clear of the lantern wheel, and the pinion ratchet reverse lock moves forward into engagement with the ratchet. As the segment lever rises, it drives the accumulator clockwise, with the reverse lock clicking over on each step on the ratchet. If the operator attempts to start a new downstroke before the key has fully risen, it will be immediately caught by the pinion ratchet reverse lock and the key will block. The reverse lock is not released until the trigger has been reset and re-armed at the beginning of the next keystroke.

As the segment lever reaches its home position it lifts the trigger retainer horizontal back above the vertical arm of the trigger. Finally, when all key pressure is released, the lock lever rises and the segment lever bell crank spring pulls the trigger rearward. When clear of the trigger retainer vertical the trigger drops slightly until its cross-arm again rests on the segment lever. The mechanism is now back to its starting position. This sequence of operations occurs on every keystroke in every column.

5. Short keystroke.

If the operator releases the key before completing the downstroke, the trigger remains armed. The segment lever attempts to drive the accumulator forward, but the lantern wheel is held fast by the accumulator locking hook and the lever is unable to rise.

Although the segment lever can not return, the key continues upwards under pressure from the keypiece spring, and the lock lever follows under pressure from the segment lever bell crank spring. The relative motion between the lock lever and the segment lever allows the spring to rotate the bell crank anti-clockwise. The connecting link pulls the trigger rearward, operating the touch-off bar and dropping the segment lever locking hooks.

The segment lever in the offending column is still held depressed and will not be caught by the column locking hook. When the operator returns to complete the keystroke, the lock lever and bell crank will again push the trigger forward, restoring the trigger to the armed position and releasing the accumulator locking hook (but without resetting the touch-off bar). The cycle will then proceed normally to completion, except that the column locking hook will catch the segment lever when it returns to the top of its stroke. The operator then presses the red release button and proceeds as if nothing had happened.



Original text and images Copyright © John Wolff 2004-2022.
Page created: 2004. Last Updated: 8 February 2022.

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