Since the whole subject of transmission concerns itself with gears, it will not be out of place to discuss the gears themselves and describe the many different kinds in use. Speaking broadly, the gears used may be classified according to the position of their axes, relative to one another. Thus we have axes parallel and in the same plane; parallel but not in the same plane; at right angles and in the same plane; at right angles and not in the same plane; at some other angle than a straight or a right angle and in the same plane; and the same, but not in one plane. These classes give us the forms of gear in common use, viz, spur gears, bevel gears, helical gears, herringbone gears, spiral gears, and worm gears.
Fig. 81. Fellows Shaper for Cutting Gear Tooth.
A spur gear is not only by far the most common kind of gear, but is also the easiest to describe, consisting as it does of a round flat disk with teeth cut in its circumference, i.e., around the periphery of the disk. The cutting of these teeth has had much to do with their universal use, since the very low cost of cutting the teeth, due to special machinery developed for that purpose, just about explains the matter. Formerly, the teeth were cut, one gear at a time, in the milling machine, this being practically a hand operation, since all movements of the gear or cutter had to be made by hand. Later, improvements made it possible to cut more than one gear at a time, which resulted in lowering the cost, but did not eliminate the hand work.
Step by step special machinery was developed for this work, until finally a perfected machine was brought out which does all the work. With this machine, the workman places the cutter on the machine spindle, sets the gear blanks into position, and starts the machine, after which it goes on automatically cutting tooth after tooth to a correct shape, until the gear is finished, when the workman is again necessary to shut it off, and, after taking out the finished gears, put in a fresh supply of gear blanks.
This machine, known as the Fellows gear shaper, Fig. 81, has reduced the manual labor of gear-cutting to such a point that it is possible for one man to operate, unassisted, from three to six machines at one time. Usually, these are placed together and located near the automatic machinery, a group of them being called a battery. By having a battery of five machines cared for by a single man, the cost of spur-gear cutting has been brought down to the absolute limit.
Bevel gears, in which the shafts are at right angles and in the same plane, or in the same plane but not at right angles, are more difficult to cut and therefore less used. Their cutting is now done, like the spurs, in an automatic, or nearly automatic, machine, which requires little attention, but it does require ' more care than the spur-gear machine. Both spurs and bevels sometimes require a chamfered tooth-edge, spur gears as used in the Panhard or clash-gear transmission always being in need of it. This work was formerly done by hand, but now a special machine has been manufactured for this purpose.
There are no real restrictions against the use of the spur and bevel, either or both being used interchangeably. Very often they are used in combinations, which appear, peculiar, as the one shown in Fig. 82. This is the final drive and reduction gear of the Autocar commercial cars, made by the Autocar Company, Ardmore, Pennsylvania. In this it will be noticed the drive from the engine is to an intermediate shaft through bevels and final drive by spur gears.
Fig. 82. Combination of Gears in the Autocar Final Drive.
In situations where quiet running is deemed necessary, the use of a helical gear frequently finds favor, since it accomplishes the desired result, although the cost of cutting is high. Of late, these have come into general use for camshaft drives and similar places. A pair of helical gears set so that the helices run in opposite directions, forms a herringbone gear. This is even more quiet in its action than the single helix, and. pos:
Besses other virtues as well. One well-known firm has adopted it for camshaft driving gear, and makes it, as described, to save cutting-cost, as the cost of cutting a true herringbone would be prohibitive. So, a pair of helical gears of opposite direction are set back to back and riveted or otherwise fastened together, forming a herringbone gear at a low cost. Both of these may be used when the two shafts are parallel and in the same plane, but for cases where the shafts are neither in the same plane nor parallel, some form of spiral gear must be used.
Fig. 83. Hindley Worm Steering Gear for Heavy Trucks.
Spiral gears, as such, not being generally understood, and that variety of the spiral known as the worm gear being very simple and easily understood, the latter has attained much popularity within the past few years. This has been due in part to superior facilities for cutting correct worms and gears, but, in the main, to a superior knowledge of the principles upon which the worm works, and the things which spelled failure or success. Thus, one of the earliest experimenters in this line laid down the law that the rubbing velocity should not exceed 300 feet per minute if success was desired, or in rotary speed about 80 to 100 revolutions. For automobile use, this was out of the question; but later experimenters found that these results only attached to the forms of gear used by the early workers, and did not apply to a strictly modern gear laid down on scientific principles.
The mistake made was in the pitch angle of the worm, which was formerly made small, nothing over 15 degrees being attempted. This was the item that was at fault and caused this very useful and efficient mode of driving to fall into disuse. As soon as this fact was ascertained and larger pitch angles utilized, better results were attained, until with 20-degree angles, 700 feet per minute pitch-line velocity was attained, followed shortly by the use of even higher angles, resulting even more successfully. As the efficiency depends directly upon the pitch angle, these changes brought the efficiency of this form of gearing from the former despised 30, 40, and sometimes 50 per cent up to 87,88, and even 90 per cent, thus putting it on a par with any but the very best of spur gears, and above bevel gearing.' In fact, in the light of modern knowledge of worm gears, it could easily be said without departing from the truth that it is possible to obtain from this form an efficiency of 93 per cent. In automobile work it has been used mostly for steering gears and final drives. For the former its irreversible quality is brought out, while for the latter this must be made subordinate to a great reduction, which may be attained in a very small, compact space. Many modern machines make use of worm gears: as Jeffery, and the Baker, Detroit, Hupp-Yeats, and Woods electrics; Pierce, Packard, Locomobile, Mack, Atterbury, Blair, Chase, Gramm, G. M. C, Hulburti Moreland, Standard, Sterling, and other trucks; Dennis (English) busses and trucks, and Greenwood and Batley (English) trucks. Among those using the spiral bevel may be noted Packard, Cadillac, Reo, Steams-Knight, Velie, Kline, Apperson, Buick, Chalmers, Chandler, Cole, Haynes, Hupmobile, Jackson, King, Locomobile, and many others. Figs. 83, 84, 85, and 87 show applications of the worm, and Fig. 86 shows a separate detail of a worm as used on a prominent truck.
Fig. 84. Rear View of Timken Worm-Driven Rear Axle Courtesy of Ttmken-Dttrait Axle Company, Detroit, Michigan.
Fig. 85. Worm Gear Applied to Rear Axle Drive of Touring Car.
The spiral bevel is a new development, having been brought out in 1914 as a compromise between the worm and the straight bevel. As such, it is supposed to have practically all the advantages of both, except that it does not afford the great speed reduction that can be accomplished with a worm in the same apace, being more like the bevel in this respect.
Fig. 86. Worm Used on Locomobile Trucks.
Courtesy of Locomobile Company of America, Bridgeport, Connecticut.
Progress in the application of worm gears for rear-axle use has been considerable in the last few years. In one respect, at least, designers have found it an advantage. The top position for the worm was not much used at first, as it was thought impossible for it to receive sufficient lubricant there. Consequently, it was always placed in the bottom position, which cut down the clearance considerably; in fact, in this position the clearance was less than with the ordinary bevel. With the proof that the worm could be lubricated in a satisfactory manner in the top position, the majority of them are so placed, thus converting what was formerly a disadvantage into an advantage, for in the upper position the clearance is greater than with bevel gears. This is shown quite clearly in Fig. 84, where it will be noted that the worm-gear housing in the center is actually higher than are the brake drums at either end of the axle. This too, despite the fact that a truss rod passes beneath the center of the axle. For heavy trucks especially, and for pleasure electric cars, the worm has proved an ideal drive. In these situations there is the condition of high engine or electric-motor speed, coupled with low vehicle speed requirements, which necessitate a considerable reduction. As pointed out, the worm gives this in a small space.
For 1916, the very apparent tendency in final drives is toward spiral bevels for pleasure cars and worms for electrics and trucks. The tendency toward spirals is very great, amounting practically to a landslide, 57 per cent using it against 10 for 1915. The development of special machinery for cutting these gears and the understanding of their use has brought this about. In the truck field there has been a similar movement toward the worm, due to similar causes.