in the opposite direction, wheel a is freed, and wheel b held fast, and if the clutch be held stationary at a point midway between the two, both wheels are free and neither is driven. In another form of clutch, connection is made by expanding friction rings on the inner faces of drums, fixed to the parts to be driven. Positive clutches are used temporarily to connect various portions of the gear from time to time, as may be required, and these take the form of notches or projections upon a sliding ring or collar or other part of the machinery which, when moved along a shaft, can engage with or slip into
_0227.jpg)
Fig. 10
corresponding notches or projections on the part with which it is desired to make connection. In this case the connection is sudden, and from a position of absolute rest the part put into gear by the clutch is instantly moved forward at the same velocity as the rest of the machinery the moment the clutch teeth slip into their places. Figs, 11, 12, and 13 show a common form of positive clutch. This form of clutch, it may be noted, is—or should be—always used in conjunction with a clutch of the friction order—i.e. whilst individual portions of gear may be connected with and disconnected
