IBM Selectric Typewriter - Selectric Mechanism

Selectric Mechanism

Slow-motion video of whiffletree linkage in Selectric mechanism

Mechanically, the Selectric borrowed some design elements from a toy typewriter produced earlier by Marx Toys. IBM bought the rights to the design. The typeball and carriage mechanism was similar to the design of the Teletype Model 26 and later, which used a rotating cylinder that moved along a fixed platen.

The mechanism that positions the typing element ("ball") is partly binary, and includes two mechanical digital-to-analog converters, which are "whiffletree" linkages of the type used for adding and subtracting in linkage-type mechanical analog computers. (The nomenclature used by IBM Office Product Customer Engineers and in IBM maintenance publications for the machine's "whiffletrees" is "Rotate and Tilt Differentials.") Every character position on the element has a two-part binary code, one for tilt and one for rotate.

When the typist presses a key, it unlatches a metal bar for that key. The bar is parallel to the side of the mechanism. This bar has several short projections ("fingers"). Only some of the fingers are present on any given code bar, those present corresponding to the binary code for the desired character.

When the key's bar moves, its projections push against a second set of bars that extend all the way across the keyboard mechanism; each bar corresponds to one bit. All bars for the keys contact some of these crosswise bars. Those bars that move define the binary code.

The bars that have been moved cause cams on the drive shaft (which is rotating) to move the ends of the links in the whiffletree linkage, which sums (adds together) the amounts ("weights") of movement corresponding to the selected bits. The sum of the weighted inputs is the required movement of the typing element. There are two sets of similar mechanisms, one for tilt, one for rotate. The type element has four rows of 22 characters. By tilting and rotating the element to the location of a character, the element can be thrust against the ribbon and platen, leaving an imprint of the chosen character. The rotate whiffletree subtracts when the type element needs to rotate clockwise to access the characters on that part of the ball.

The motor at the back of the machine drove a belt connected to a two-part shaft located roughly halfway through the machine. The Cycle Shaft on the left side powered the tilt and rotate mechanism. The Operational Shaft on the right side powered functions such as spacing, backspacing and case shifting. Additionally, the Op Shaft was used as a governor, limiting the left-to-right speed with which the carrier moved. A series of spring clutches were used to power the cams which provided the motion needed to perform functions such as backspacing. The Cycle Shaft was rotated when a spring clutch was released, driving a set of cams whose rotational motion was then converted into left-and-right motion by the whiffletree.

The system was highly dependent upon lubrication and adjustment and much of IBM's revenue stream came from the sale of Service Contracts on the machines. Repair was fairly expensive, so maintenance contracts were an easy sell.

The arrangement of characters on the element was not random. Punctuation marks and the underscore were deliberately placed so the maximum amount of energy was used to position the element, thus reducing the impact made by them and lessening the chance that the underscore would cut through the paper. However on the 88 character element used on the Selectric I and II machines, the character "W" is on the same rotate band as the underscore and on the adjacent tilt band. To ameliorate this there is a "Dual Velocity" cam in the carrier that drives the element toward the paper. It has 2 separate profiles, one for low velocity that serves characters such as periods, commas, and underline and one for high velocity that serves the larger characters.

Tilt and rotate movements are transferred to the ball carrier, which moves across the page, by two taut metal tapes, one for tilt and one for rotate. The tilt and rotate tapes are both anchored to the right side of the carrier (the mechanism that supports the type element). They both wrap around separate pulleys at the right side of the frame. They then extend across the machine behind the carrier, and then wrap around two separate pulleys at the left side of the frame. The tilt tape is then anchored to a small, quarter-circle pulley which, through a link, tips the tilt ring to one of four possible locations (The tilt ring is the device to which the type element is connected). The rotate tape is wrapped around a spring-loaded pulley located in the middle of the carrier. The rotate pulley under the tilt ring is connected through a universal joint (called a "dog bone"; it looked like a small bone) to the center of the tilt ring. The type element is spring-latched onto that central post. The type element rotates counter-clockwise when the rotate tape is tightened. The spiral "clock" spring underneath the rotate pulley rotates the element in the clockwise direction. As the carrier moves across the page (such as when it returns), the tapes travel over their pulleys, but the spring-loaded pulleys on the ball carrier do not pivot or rotate.

To position the ball, both of the pulleys on the left side of the frame are moved by the whiffletree linkage. When the rotate pulley is moved to the right or left, the rotate tape spins the type element to the appropriate location. When the tilt pulley is moved, it tips the tilt ring to the appropriate location. When it moves, the tape rotates the spring-loaded pulley on the ball carrier independent of the carrier's location on the page.

Case was shifted between caps and lower case by rotating the element by exactly half a turn. This was accomplished by moving the right-hand rotate pulley using a cam mounted on the end of the operation shaft.

Both Selectric and the later Selectric II were available in standard, medium, and wide-carriage models and in various colors, including red and blue as well as traditional neutral colors.