Selectric Resources
EXPRESS BACKSPACE OPERATIONAL THEORY
The purpose of the express backspace mechanism is to allow the operator to move the carrier quickly to the left, from the keyboard. When the operator depresses the express backspace keybutton, the carrier will move to the left at the same speed as carrier return, with the escapement pawl dragging. The carrier movement stops when the keybutton is released or the left margin is reached.
The express backspace bellcrank pivots on an extension of the operational cam checkpawl shaft. The express backspace shoe is mounted on the bellcrank, which is positioned under the carrier return clutch spring. When the EXP keybutton is depressed, the keylever pawl moves down and pushes on the right extension of the bellcrank. This causes the bellcrank to pivot up and press the express backspace shoe against the carrier return clutch spring. The carrier return clutch spring drives the carrier return pinion, moving the carrier to the left (Figure 1).
Figure 1 — Express Backspace
EXPRESS BACKSPAEE LATCH (SINGLE PITCH ONLY)
On the NRB/S “Selectric” II Typewriter, there would be a possibility of the carrier holding on the backspace pawl when the express backspace keybutton is released and the carrier stops. An additional part, called the express backspace latch, prevents this from happening by causing a backspace operation when the EXP keybutton is released. The express backspace latch is mounted on the operational keylever pawl guide. When the EXP key is depressed, the express backspace keylever pawl rotates the express backspace latch, which releases the backspace interposer. At the same time, another extension of the latch moves behind a latching surface on the interposer and prevents it from moving far enough to the rear to release the operational cam, When the EXP key is released, it allows the express backspace latch to restore up, allowing the interposer to continue to the rear, causing a backspace operation (Figure 2).
Figure 2 — Express Backspace{Single Pitch)
