Selectric Resources

 

HALF BACKSPACE OPERATIONAL THEORY

 

The half backspace mechanism allows the typist to move the carrier one half space to the left. As long as the half backspace lever is held forward, the carrier remains one half space to the left and the typewriter will type between the normal escapement positions. This is done by moving the escapement rack one half space to the left (Figure 1).

 

The movement of the escapement rack is provided by the half backspace lever, operating link, cam lever and half backspace cam. When the half backspace lever is operated, it pulls up on the operating link, which rotates the cam lever, forcing it to the left, out of its bottom position with the half backspace cam. The cam lever then contacts the escapement rack gear, which is attached to the escapement rack. This cam action moves the escapement rack to the left, moving the escapement pawl and carrier, to get a half backspace operation (Figure 1).

Figure 1 — Half Backspace Mechanism

 

HALF BACKSPACE LEVEL 1

On level 1 machines, the movement of the escapement rack is provided by the half backspace lever, intermediate lever and half backspace bellcrank. When the half backspace lever is operated, it operates the intermediate bellcrank, which operates the intermediate lever and pulls down on the half backspace bellcrank (Figure 2).

Figure 2 — Half Backspace Mechanism (Level 1)

 

ESCAPEMENT RACK MOVEMENT

A lug on the half backspace bellcrank is in a notch in the escapement rack. As the bellcrank is operated, the lug moves to the left, moving the escapement rack to the left. A compression spring restores the bellcrank and the escapement rack to their normal position. The working edge of the escapement rack notch has a step in it to allow for the different motion requirements for the two pitches (Figure 3).

Figure 3 — Escapement Rack Movement (Rear View)