Section 31.—DIFFERENTIAL GEAR.
Devices to utilise the difference of velocity, or power, between two distinct moving parts.
[547]. Equational box. Two drivers A, A′, equally speeded in opposite directions, will drive the bevil gear at same velocity without revolving the spur wheel C′ which is loose on the shaft; but any alteration in the relative speeds of A and A′, causes the bevil pinion to travel round, carrying the spur wheel C′ at a speed equal to half the difference of the two velocities. This gear is used on traction engines to drive the swivelling wheels round curves, where the proportionate velocities of the wheels will vary with the radius of the curve. In this application of the gear B is the driving shaft, and A and A′ the swivelling wheels.
[548]. Is a modification of [547]. The pinion A may be controlled in speed by any hand or automatic device, to vary the speed of the driven pinion B. The belt pulley C carries round the bevil wheel D, driving B at a speed varying with the motion given to A.
[549]. Two wheels (one of which has a different number of teeth to the other) gearing into one pinion; used for counters and slow motions of all kinds.
[550]. Is an application of [No. 549] by internal or epicycloidal gear to pulley blocks. Moore’s patent ([No. 1545]) and Pickering’s patent are examples. The arm shown is not required where two internal loose wheels are used with different numbers of teeth, and one pinion as in [No. 1545], but if used is fixed to the pinion so that it is prevented from revolving, but retains its circular swaying motion; in this case, one internal wheel is movable and the other fixed, the speed being equal to the difference in number of teeth of the loose wheel and pinion at each revolution of the eccentric shaft.
[551]. Weston’s differential pulley block, consisting of a two-grooved pitched chain-sheave having different numbers of teeth, in combination with a return block and endless chain.
[552]. Differential screws. These may be both of the same hand, or one right and one left-handed, and any fractional speed secured by proportioning the pitches.
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[553]. Two-speed gear, operated by a double clutch, which throws either pair into gear as required.
[554]. Stewarts’ differential gear. Two cranks, one fixed to a sleeve and the other to a centre shaft, are driven round at varying velocities by a slotted crosshead revolving with the driving shaft. The two shafts are not in the same line.
[555]. Differential hydraulic accumulator. The effective area of the ram is the annular shoulder, or the difference between the areas of the top and the bottom rams.
[556]. Differential governing device. The motive power drives A which winds up the large weight; the small weight tending to run down, drives the fan regulator, and the two weights are so adjusted that when the proper speed is attained, both weights are stationary; any change of speed causes them to run up or down, so actuating the regulation by the bell crank lever and rod.
[557]. Varying differential regulator. The upper rod A is connected to the regulator valve or other device, and it is capable of receiving motion from either the piston, which acts against a spring, or from the rod B attached to some positive reciprocating part, so that the nett movement of A is due to the difference of motion of B and the piston C.
Differential Worm Gear, [No. 1559].