The carriage is shifted sideways by a toothed rack attached to the rear of the upper frame. The rack engages with a driving disc at the right-hand rear of the machine. The disc is driven from the number 3 shaft in the drive train via a dog clutch and reversing gear.
The carriage shift can be initiated manually via the control keys, or automatically during multiplication and division. There is extensive interlocking to prevent mis-operation.
The carriage positioning rack.
This view shows the carriage positioning rack at the right-hand rear of the machine. The driving disc is just visible behind the vertical frame plate. The small sloping tab on the outer edge of the first (and last) tooth operates the shift terminating mechanism at the end of the travel.
The rack is not rigidly attached to the carriage, but is hinged from the upper frame spacer shaft. It is normally held flat against the driving disc by the stiff spring visible at the near end. The rack is profiled to give a constant linear velocity, so that the carriage will move in a smooth and continuous manner as it returns to its starting position. Buffer springs at each end of the rack absorb the impact of rapid movement.
The driving disc.
The shifter disc has three drive rollers, and rotates one-third of a turn to move the carriage by one position (5/8") in either direction. When the carriage reaches the end of its travel, the chamfered edge of the active roller comes to rest against the sloping tab at the end of the rack. If a further shift is initiated, the roller will lift the rack and pass harmlessly underneath.
The carriage is not locked in position when at rest, but has a certain amount of movement due to the buffer springs and the free play in the drive train. Final positioning is not done until the start of the setting cyle, when the carriage centraliser (the broad arrowhead at the left-hand end of the actuator unit) rises between the frame plates of the main register to align the carriage and hold it in position.
The two "ears" above the shifter disc are part of the shift terminating mechanism (described below).
The drive mechanism.
The shifter disc is driven via a 3:2 bevel gearset from a half-turn shaft extending horizontally from the lower rear of the control unit.
The far end of the horizontal shaft is driven from the number 3 shaft in the drive train via a dog clutch and a reversing mechanism, partly visible through the cut-out in the frame plates. The cam and detent roller hold the drive shaft in the two rest positions when the clutch is disengaged. The supporting bracket attaches to the motor base plate.
During automatic division, a two-lobe cam on the horizontal shaft (hidden) signals the completion of the carriage shift and re-starts the subtraction process in the new column.
The shift clutch linkage.
The shift clutch linkage is located along the left-hand side of the control unit.
The lever arm at the right (front) is attached to a shaft operated by the manual shift keys. The lower horizontal link has a notch at its rear end, which engages with a shelf on the vertical shift control arm (centre). Several interlocks can press down on the horizontal link to disconnect it from the central lever and disable the manual control.
At the left of this view are the shift clutch operating arm, the clutch assembly with its driving gear (hidden) and reversing gears, and the clutch release dog.
The upper horizontal link swings the shift clutch operating arm fore and aft. The two short arms at its lower left operate a sliding collar which selects one or other of the reversing gears. The longer slotted arm operates the clutch release dog, which engages the clutch and starts the motor when the arm moves in either direction. A spring at the bottom of the release dog normally holds the operating pin in the centre of the "V" slot, with the dog preventing the clutch from turning. A non-return detent (not shown) drops into a small notch at the top of the clutch disc to hold it in position against the dog at the end of the cycle.
A mechanism driven from the carriage dip arm also operates on the central shift control lever to initiate automatic left shifts when the carriage rises during multiplication and division. This mechanism is disabled during normal addition cycles, or by the "Non-Shift" key in multiplication.
The shift terminating mechanism.
The shift terminating mechanism has two functions: to stop a manual shift when the carriage reaches the end of its travel, and to terminate the automatic right shifts initiated by the "Clear/Return" key.
When the carriage reaches the end of its travel, the small tab on the end of the positioning rack (first picture above) will sit just under the ear on the terminating lever above the shifter disc (second picture). If a further manual shift is requested (or the key is held down), the disc will rotate, the rollers will raise the rack and pass underneath, and the tab on the rack will raise the terminating lever. The terminating lever disconnects the lower horizontal link from the vertical shift control lever, disabling the manual request and allowing the clutch dog to spring back to its neutral or off position when the shift cycle is completed. The linkage resets when the manual shift key is released.
When the "Clear/Return" key is pressed, a mechanism at the rear of the control unit initiates and latches a manual right shift, as if the operator held down the key. The shift terminating lever is extended via a shaft to the left-hand side of the machine, and when this is raised by the tabulator finger it terminates the shift and releases the latch.