Motor-car principles; the gasoline automobile

by Roger B. Whitman

Fig. 32.—Multiple Disk Clutch.

An internal-expanding clutch consists of a broad ring, or drum, against the inner surface of which bear two pieces of metal shaped to fit. The pieces of metal, or shoes, are pivoted together at one end so that they may be moved in or out, after the manner of the handles of a pair of scissors; when open they bear against the inside surface of the drum, and when closed they are free from it. The drum is attached to the engine shaft and the shoes to the transmission shaft, the friction between them being so great that the transmission shaft is carried around as the drum revolves. The shoes are kept in contact with the drum by a coil spring, the depression of a pedal releasing them from its pressure.

CHANGE-SPEED MECHANISM

The change-speed mechanism, to which the clutch transmits the power of the engine, is to the engine what a block and tackle is to a man who lifts a heavy weight, and is necessary because of the varying resistance to the movement of the car in traversing steep, rough hills and smooth avenues. A change-speed mechanism may be defined as an arrangement by which the relative number of revolutions of the crank shaft and driving wheels may be altered to suit conditions. If the driving wheels revolve but once while the crank shaft makes twelve revolutions, the car will move at one sixth the speed that it would have if the wheels revolved once to every two revolutions of the crank shaft, but it will have six times the ability to overcome the resistance presented by a hill or sandy road.

If a gasoline engine were so connected that the relative number of revolutions of the crank shaft and wheels could not be changed, a slowing down of the car through the resistance presented by a rough hill would slow the engine to correspond, and as speed is an important factor in the power that the engine delivers, it would be prevented from doing the work of which it is capable at the time when it was most necessary. By means of the change-speed mechanisms in most general use, the relative number of revolutions of the crank shaft to one of the wheels may be varied to from two to eighteen, the former giving the car high speed over a smooth road, and the latter slow speed, but greater ability to overcome hills and heavy roads.

To attain this result, gears are used. If two gears having the same number of teeth are in mesh, they will make the same number of revolutions, and the force with which the driven gear will revolve will be the same as that of the driving gear, less the friction of the teeth. If the driven gear has twice the number of teeth of the driving gear, it will revolve at half the speed, but with twice the force.

The forms of change-speed mechanism most largely used are based on this principle, which is so applied that the gear driven by the engine may be in mesh with a gear that has many more teeth and revolves much slower in consequence, or a gear that has possibly one and a half times the number of teeth, or a gear that has the same number of teeth, and therefore revolves at the same speed. The sliding-gear mechanism takes its name from the arrangement by which the changes in the combination of gears is effected by sliding them along a shaft, to mesh with other gears on a shaft driven by the engine.

Fig. 33.—Sliding Gear—Progressive Type. A, Sleeve driven by engine; B, gear on sleeve; C, gear on countershaft; D, low-speed gears; E, second-speed gears; F, idler for reverse; G, clutch for high speed; H, connected to rear wheels; J, gears sliding on square shaft.

The driver changes the gears by moving a lever that in the progressive type moves forward by degrees to move the car on the slow speed, the intermediate speeds, and the high. A typical arrangement of the progressive type of sliding change-speed mechanism is shown in Fig. 33. The power of the engine is transmitted to a short, hollow shaft, called a sleeve (A), which carries a gear (B) that is in permanent mesh with a gear (C) on the end of the countershaft. Parallel to the countershaft is another shaft, one end of which is held in a bearing in the hollow sleeve; while the sleeve supports this shaft, the two may revolve independently of each other. The second shaft is square, or of such a construction that while the two gears that it carries may slide along, they revolve with it. The gears on the square shaft are of different sizes, and in sliding on it come successively into mesh with gears carried on the countershaft. Because of the gears between them, the countershaft revolves when the engine revolves the sleeve; but the speed of the square shaft depends on the combination of gears in mesh between it and the countershaft. When the sliding gears are in such a position that they are not in mesh with the countershaft gears, the square shaft is independent of the countershaft, and may revolve or be stationary, the gears then being in the neutral position. When the sliding part is moved so that its largest gear is in mesh with the smallest of the countershaft gears (D), the square shaft will revolve at a slower speed than the countershaft, because its gear is larger than the one driving it. Again sliding the moving part will separate these gears, and bring the next pair (E) into mesh, the square shaft then moving at a higher speed, but still slower than the countershaft because of the difference in the size of the gears. Sliding the moving part still farther along the shaft will disengage the second-speed gears and engage the high speed, in which the square shaft revolves at the speed of the sleeve and crank shaft, this being effected by locking the moving part to the sleeve by means of a clutch (G). This clutch consists of several fingers projecting from the moving part, corresponding to the spaces between similar fingers on the end of the sleeve. The locking together of the square shaft and sleeve gives what is known as the direct drive, which is of comparatively recent development; many designs of sliding gears still use a third pair of gears which, being of the same size, give the square shaft the speed of the crank shaft. By the use of the direct drive, the power of the engine is directly applied to the square shaft, avoiding the loss that occurs through the friction of the teeth of the gears.