Each column of the selector unit takes a single input from the keyboard, and positions a set of five selector forks which control the pick-up arms in the actuator unit.
During the "manual setting" phase, the operator input from the keyboard rotates a small "selection cam" at the bottom of the unit and a numeric "check dial" at the top. During the "powered setting", the setting line presses a set of feelers against the cam to position the selector forks forward or back. The selector forks then position the pick-up gears in the actuator unit.
The selector unit also houses the division trip mechanism, which uses the keyboard setting as one input to the magnitude comparator. The trip mechanism is described separately in the "Division" section.
This overall view shows the selector unit fully assembled and ready for installation. There are ten identical sections on 5/8" centres. The assembly is about 5-1/2" high.
The selector unit installed.
This view shows the selector unit installed between the main frame side plates in the centre of the machine. The selector unit is the first item to be installed in the frame, and the last to be removed.
The selector frame.
The selector mechanism is supported on a light frame built from plates of 0.032" steel. The plates are held in alignment by six notched metal strips about 0.150" wide, located at most of the corners. The strips are inserted through slots and locked in position by a 1/16" rod or wire.
The frames for the actuator and the carriage are built on a similar principle.
The selector mechanism.
This view from the right-hand side shows the selector mechanism for one column, assembled for illustration on the outside of the frame.
The central vertical lever is pivoted on a 1/16" shaft just above its centre, and carries a more-or-less horizontal gear sector on either end. The lower sector drives the selection cam, while the upper drives the keyboard check dial. A torsion spring adjacent to the check dial provides a restoring force.
The sawtooth bar under the keyboard module hooks over a pin on the vertical arm, just below the pivot. The keystems push the bar rearward, rotating the arm according to the digit pressed. The total movement at the connection point is only about 3/16" - the movement from one key to the next is less than 1/32".
The selector linkage consists of three interconnected links which are pivoted on the three shafts at the bottom centre. The linkage is operated by a double cam on the setting shaft, via a follower which pivots on the shaft at the far right. (There are actually three cams spaced across the width of the machine). Marchant used several of these double or opposing cams to both advance and retract a mechanism, although in other places they used conventional single cams and spring-loaded followers.
The selector feelers.
The front selector arms pivot on the smaller shaft to the left, which is fixed to the frame. The rear arms pivot on the larger shaft to the right, which itself swings on short links from the large-diameter fixed shaft at the bottom. The moving pivot shaft is pushed forward and back horizontally by the links from the setting shaft cam followers.
The selection cam assembly consists of a driving gear and five pressed-metal discs, each of which handles two digit positions. Each position has a raised tooth on one side and a notch on the opposite side. The cam is shown in the position for digit 1, which requires the rightmost (ie, closest) selector fork to be pushed rearward.
At the start of the manual setting cycle, the upper (yoke) ends of the selector arms are still locked in place by the bail raised under the actuator pick-up arms. The moving pivot shaft is pulled well to the right, so that the feelers are wide apart and the selection cam is free to rotate in response to the keys.
When a control key is pressed, the setting line will begin to rotate. During the forward stroke (the first 90°) the selector locking bail will be released and the moving pivot shaft will be pushed to the left. The right-hand feeler will be blocked by the projection on the cam, causing the selector fork to rotate clockwise. As the moving pivot continues forward, the cross-link at the bottom causes the left-hand link to also rotate clockwise, forcing the left-hand feeler into the conveniently-located notch.
Had the cam been in the zero position, the situation would have been opposite and the selector fork would have rotated anti-clockwise. The other four sections of the cam present neither teeth nor notches at this time, so their selector arms are forced into the central position.
The selection cam detent.
This view from the left-hand side shows the spring-loaded detent which engages with the drive gear on the selection cam.
The detent is engaged as the moving pivot shaft moves forward, holding the selection cam firmly in position as the setting is transferred to the feelers and the actuator pick-up arms. The selection is then locked by the bail which rises into the slots in the lower edges of the pick-up arms.
With the selection now firmly locked into the actuator, the moving pivot shaft commences its return stroke. The selector feelers are pulled back clear of the cams, the detent is released, and the keyboard is cleared. Finally, the main clutch is engaged as the setting line comes to a stop at 180°.
The actuator pickup arms remain locked in position until the start of the next powered setting cycle. However, the keyboard and selection cams are free to move, allowing the operator to start entering the next number into the selector while the add and restore cycles are still proceeding.