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702 HANDBOOK OF PROJECTION FOR
HOW IT WORKS.— When speed control knob H, Fig. 231, is in position to stop the projector mechanism, it has moved fork B away from thrust ball bearing F, and has thus relieved spring C of all compression. This eliminates all friction between discs L and H. When knob H is in position to start the projector mechanism, it has caused fork B to shove bearing F endwise, which has the effect of compressing spring C and bringing discs L and H together under pressure, which will, of course, cause part L and pulley W to revolve, and thus drive the projector mechanism, pulley W being a part of disc H.
Understanding that pulley A and disc L are one piece, that pulley W and disc H are rigidly joined, and that both revolve freely on shaft M, pulley A being belted direct to the motor and pulley W direct to the projector mechanism, it will be seen that when discs L and H are pressed together by coil spring C, the projector mechanism will be driven by the friction set up between the two discs. That much is quite plain and simple.
THE GOVERNOR.— And now let us examine governor T, Fig. 251, and see what it is for. At any given speed of projection the "load" which must be pulled by the friction between discs L and H will require a certain, fixed amount of pressure by spring C to carry it. It will also be seen that exactly in proportion as projection speed is increased, the amount of the load to be pulled is increased, because increase in speed always requires an increase in expenditure of power.
When the projector is started, the entire force of spring C will be exerted to press friction discs L and H together, and the amount of driving force available will depend upon how much the spring is compressed, which in turn depends upon the position of speed control knob H, Fig. 231. As soon as disc H starts to revolve, however, the governor also starts to revolve, and governor arms J, Fig. 251, are thrown outward by centrifugal force. If you examine these arms you will see their ends are hooked, and that the hooks bear upon collar K, against the other end of which spring C presses. It therefore follows that any force exerted by arms J will act to compress spring C, hence to lessen the friction between discs L and H.
As speed is increased, the power exerted by arms J becomes greater, which, of course, means that they are carrying an increased amount of the pressure exerted by spring C, and in this way friction betweer the discs is decreased with in