It is no novelty to hear a grinding or clashing within the car when a careless chauffeur starts, or when he changes from one speed to another. If the owner knew what was going on inside to make all that noise, a new chauffeur would have a job quickly and there would be laid down starting, speeding, slowing down, and stopping rules as stringent as those of the traffic policeman.
The illustration gives the mechanism of a simple transmission gear case. The engine shaft J has on the end a gear wheel A, and on the face of the gear are four engaging teeth I. The end of the shaft J is hollow and in this revolves one end of the transmission shaft K, which is square. On it are two gear wheels of varying size, D and E, one having, say, thirty teeth and the other forty. The smaller is yoked to the larger and both slide along the square shaft when moved by a lever. The gear D has on its face engaging teeth I, corresponding to those on the engine shaft gear A, and when the two are engaged the transmission shaft revolves at the same speed as the engine shaft, giving the highest speed of which the car is capable.
To provide for varying speeds, another shaft is suspended in the transmission case, on which are other gears. If gear A has twenty teeth, B will have, say, forty. This reduces the motion of the gear shaft to one-half that of the engine shaft. Farther along the gear shaft, gear F, with twenty teeth, engages gear E with forty, further reducing the speed, so that the transmission shaft revolves one-fourth as fast as the engine shaft, making the low gear, or slowest speed.
When the car is standing, of course, gears E and F are not engaged, but the engine is running in neutral—that is, no gear on the transmission shaft is engaged, gear E being shifted just far enough to miss gear F. To start, it is customary to disconnect the engine and move the lever so that gears E and F engage. If the speed of the engine shaft be 600 r.p.m. that of gear F would be 300 r.p.m., or 6000 teeth pass a given point per minute (300 × 20)—something of a buzz-saw motion. Into this revolving mass of teeth the forty teeth of gear E, which is at rest, must penetrate and mesh. It does not require much of a mechanic to see that the meshing must be quite perfect or there would be a clash and grind that does no good to delicate machinery. So it is good practice to allow enough time after the clutch is released for the moving shaft to come to rest.
Once the car is under way and it is desired to increase the speed, the lever is shoved forward, moving the transmission gears forward until gear D engages gear C. These are the same size and have, say, thirty teeth each. But they are not moving at the same speed. Gear C, revolving at 300 r.p.m. puts 9000 teeth per minute past the engaging point, while gear D, moving at 150 r.p.m. puts just 4500 around per minute. The difference of 4500 represents the possibilities of clashing and breaking or stripping the gear. The wise chauffeur just at the instant of shifting the gear, would throttle down his engine one-half and bring the number of revolutions of the gear shaft to approximately that of the transmission shaft, which is kept in motion by the momentum of the car. He also will hesitate in the shift—that is, stop for an instant in neutral before completing the shift, to allow for adjustment. It is possible in this way to lessen the difference in teeth speed. If it were possible to make both gears revolve at exactly the same speed the shift would be noiseless and frictionless. This is practically impossible in actual running, though in theory it can be done. But they may be brought near enough to minimize the clash.
In shifting to high speed from medium, the engine should be throttled more closely and the shift lever should hesitate again, if one would avoid the thump and jerk commonly felt when the high gear is thrown in. The engaging teeth of gears A and D will stand a sledge-hammer blow, but “constant tapping wears away the hardest rock,” you know, and the best gears made wear and break. Besides there is the jar to engine and car to consider. Constant jerking and jumping rack the mechanism, chassis, and body and shorten the life of each, so that economy, if not comfort, would seem to dictate care by the driver.
In reversing the operation—that is, going from high to medium and medium to low, one needs to reverse the directions given for increasing speed. Still assuming the engine shaft to be running 600 r.p.m., gear D would have that speed and would throw 18,000 teeth per minute (600 × 30), while gear C, as before, would be going at 9000 teeth per minute (300 × 30). It would therefore be wise to stop in neutral, engaging the clutch to speed the engine up, and then release the clutch before engaging the lower gear, bringing gear C to somewhere near the speed of gear D. In practice it is approximated by not releasing the clutch fully when changing to lower gear, thus preventing the clutch from reducing its speed. To accomplish this speedily, however, the car speed must be reduced considerably before attempting to make the shift.
Going on to lowest speed, gear E would now be moving at 300 r.p.m. and gear F at 300 r.p.m., but gear E’s forty teeth move at 12,000 per minute and gear F’s twenty teeth at 6000 p.m., to correct which one should speed up the engine, or check the car, in the same manner as just described.
In reversing, to back the car, the gear operation intensifies the problem. In the illustration, gear G operates gear H constantly, the action being to reverse the motion in the latter, and when gear E engages gear H to reverse the motion of the transmission shaft and thus back the car. In addition to the difference in speed and variance of teeth revolutions, there is added the contrary direction of the two gears which are to engage. To throw back on reverse even at moderate speed menaces the gears and shakes things up uncomfortably. Fortunately it is almost invariably necessary to fully stop the car before reversing, and necessity of caution in backing prompts very low speed throughout the operation.