The over-run mechanism prevents the racks being thrown ahead of the multiplier body by operators who disregard the instruction: "Do not turn the crank too quickly".
The mechanism consists of a toothed star wheel on the cross-shaft and a rotating gate mounted below. The gate moves in synchronism with the rack frame.
If the rack remains in contact with the multiplier body, the star wheel teeth will pass through the gate. If it is thrown ahead, the movement will be blocked until the gate opens.
Even this was insufficient for the motor-driven machine, which was operated at 95 cycles per minute. Additional frictional rubbing strips were added to retard the movement of the racks, as previously described.
The star wheels.
The star wheels are fixed in position towards the rear of the cross-shafts. When the shafts are installed, the star wheels lie between the zero rack and the rear drive frame rail.
The teeth are narrowed and shaped at the tips in order to pass through the gates.
The overrrun gate shaft.
The overrun shaft is 7mm diameter and 180mm long, with 9 toothed gate wheels of 22mm diameter (8 for the accumulator shafts, plus one for the counter).
The shaft is mounted below the racks at the centre rear of the machine.
Overrrun shaft drive.
The shaft is driven via a pair of bevel gears from a short fine-toothed rack attached to the rear drive frame rail.
The shaft moves in synchronism with the rack frame, and makes a full turn for every stroke.
There is no centre bearing, but the shaft is restrained from flexing at the drive point by a close-fitting cut-out in the cast-iron bracket behind.
The long rack frame rail is prevented from flexing upwards by a roller mounted on a bracket on the rear cross-shaft support.
Overrrun shaft timing.
The timing of the gate wheels relative to the movement of the rack frame is critical to the successful operation of the machine.
The short driving rack is dowelled onto the frame rail, then screwed, then lock-screwed. Timing marks are provided on the rack, the drive gear, and the adjacent gate wheel.
Alternate gate wheels are slightly offset because of the overlapping of the star wheels. The wheels are secured to the shaft with tapered pins.
Overrrun shaft engagement.
This view (looking down through the racks at the right-hand end) shows the intersection of the two rotating wheels.
In normal operation the star wheel teeth will pass cleanly through the gate and will never come into contact.