Selectric Resources

 

NONROTARY BACKSPACE OPERATIONAL THEORY (NRB/S)

 

The purpose of the backspace mechanism is to move the carrier to the left one space at a time. When the backspace mechanism is operated, cam motion is transferred through a cam follower, latch assembly, intermediate lever, backspace rack and backspace pawl to move the carrier to the left (Figure 1).

 

The complete operational theory of the operational control mechanism can be found in the operational control section of this manual.

 

BACKSPACE OPERATION

When the backspace cam is rotated, motion is transferred through the cam follower to operate the backspace latch.

 

As the backspace latch is pulled down by the cam follower, the latch assembly pivots around its mounting stud, rotating the intermediate backspace lever clockwise on its mounting stud. The front of the intermediate backspace lever moves the backspace rack to the left.

 

PAWL MOUNTING

The backspace pawl is mounted to the escapement bracket directly above the escapement pawl. Movement of the backspace pawl to the left also moves the carrier and the escapement pawl to the left.

Figure 1 — Backspace Mechanism

 

PAWL OPERATION

The escapement pawl has a small stud on its upper surface which extends through an elongated slot in the backspace pawl (Figure 2). This stud ensures that the pawls move laterally together. The slot in the backspace pawl allows either pawl to move front to rear. When the backspace rack pushes to the left against the backspace pawl, the carrier moves to the left. The escapement pawl moves with the backspace pawl and will move out of the rack tooth and into the previous rack tooth.

Figure 2 — Pawl Movement — Operating (Top View)

 

The backspace rack is spring loaded to the right. When the backspace operation is completed, the backspace rack returns to its rest position. The carrier is held by the escapement pawl; the stud maintains the position of the backspace pawl allowing it to drop into the next tooth of the backspace rack as the rack restores (Figure 3).

Figure 3 — Pawl Movement — Restoring (Top View)