Editing Clutch (section)

=== Single-revolution clutch ===
[[File:TTY-ASR-33-Clutch.jpg|thumb|Cascaded-pawl single-revolution clutch in a [[teleprinter]] ]]

The ''single-revolution clutch'' was developed in the 19th century to power machinery such as [[Shear (sheet metal)|shears]] or [[Machine press|presses]] where a single pull of the operating lever or (later) press of a button would trip the mechanism, engaging the clutch between the power source and the machine's [[crankshaft]] for exactly one revolution before disengaging the clutch. When the clutch is disengaged, the driven member is stationary. Early designs were typically [[dog clutch]]es with a [[Cam (mechanism)|cam]] on the driven member used to disengage the dogs at the appropriate point.<ref>Frank Wheeler, Clutch and stop mechanism for presses, {{patent|US|470797}}, granted Dec. 14, 1891.</ref><ref>Samuel Trethewey, Clutch, {{patent|US|495686}}, granted Apr. 18, 1893.</ref>

Greatly simplified single-revolution clutches were developed in the 20th century, requiring much smaller operating forces and in some variations, allowing for a fixed fraction of a revolution per operation.<ref>Fred. R. Allen, Clutch, {{patent|US|1025043}}, granted Apr. 30, 1912.</ref> Fast action friction clutches replaced dog clutches in some applications, eliminating the problem of impact loading on the dogs every time the clutch engaged.<ref>John J. Zeitz, Friction-clutch, {{patent|US|906181}}, granted Dec. 8, 1908.</ref><ref>William Lautenschlager, Friction Clutch, {{patent|US|1439314}}, granted Dec. 19, 1922.</ref>

In addition to their use in heavy manufacturing equipment, single-revolution clutches were applied to numerous small machines. In [[tabulating machine]]s, for example, pressing the operate key would trip a single revolution clutch to process the most recently entered number.<ref>Fred. M. Carroll, Key adding device for tabulating machines, {{patent|US|1848106}}, granted Mar. 8, 1932.</ref> In [[Hot metal typesetting|typesetting machines]], pressing any key selected a particular character and also engaged a single rotation clutch to cycle the mechanism to typeset that character.<ref>Clifton Chisholm, Typesetting machine, {{patent|US|1889914}}, granted Dec. 6, 1932.</ref> Similarly, in [[teleprinter]]s, the receipt of each character tripped a single-revolution clutch to operate one cycle of the print mechanism.<ref>Arthur H, Adams, Selecting and typing means for printing telegraphs, {{patent|US|2161840}}, issued Jun. 13, 1928.</ref>

In 1928, [[Frederick G. Creed]] developed a single-turn [[#Other designs|wrap spring clutch]] that was particularly well suited to the repetitive start-stop action required in [[teleprinter]]s.<ref>[[Frederick G. Creed]], Clutch Mechanism, {{patent|US|1659724}}, granted Feb. 21, 1928</ref> In 1942, two employees of [[Pitney Bowes|Pitney Bowes Postage Meter Company]] developed an improved single turn spring clutch.<ref>Alva G. Russell, Alfred Burkhardt, and Samuel E. Calhoun, Spring Clutch, {{patent|US|2298970}}, granted Oct. 13, 1942.</ref> In these clutches, a coil spring is wrapped around the driven shaft and held in an expanded configuration by the trip lever. When tripped, the spring rapidly contracts around the power shaft engaging the clutch. At the end of one revolution, if the trip lever has been reset, it catches the end of the spring (or a pawl attached to it), and the [[angular momentum]] of the driven member releases the tension on the spring. These clutches have long operating lives—many have performed tens and perhaps hundreds of millions of cycles without the need of maintenance other than occasional lubrication.

''Cascaded-pawl single-revolution clutches'' superseded wrap-spring single-revolution clutches in page printers, such as [[teleprinter]]s, including the [[Teletype Model 28]] and its successors, using the same design principles. [[IBM Selectric typewriter]]s also used them. These are typically disc-shaped assemblies mounted on the driven shaft. Inside the hollow disc-shaped drive drum are two or three freely floating pawls arranged so that when the clutch is tripped, the pawls spring outward much like the shoes in a [[drum brake]]. When engaged, the load torque on each pawl transfers to the others to keep them engaged.  These clutches do not slip once locked up, and they engage very quickly, on the order of milliseconds. A trip projection extends out from the assembly. If the trip lever engaged this projection, the clutch was disengaged. When the trip lever releases this projection, internal springs and friction engage the clutch. The clutch then rotates one or more turns, stopping when the trip lever again engages the trip projection.