WHY GEARS STRIP
Some of the most unnecessary and expensive repairs to an automobile are
those connected with its transmission. It is not only exasperating, but
unnecessary to be told that the gears of your car are stripped, or that
the teeth are broken so that smooth running is impossible, if indeed
the car can be run at all.
It is not in the permanent mesh gears, where shaft motion is turned
into axle motion, that the trouble comes. Properly set and packed the
rear axle gears never should make trouble. But it is in the shifting
gears of the transmission, where gears come to mesh at varying rates
of speed, and with the number of revolutions, load, and several other
things to complicate the situation, that there is wear and tear—largely
tear.
If an automobile could run at an unvarying rate of speed, if there
were no hills which require the translation of speed into power, or if
the engine controls alone could give sufficient speed regulation to
cover the emergencies constantly arising, the transmission might be
simplified into a mere coupling and reverse. But simplify as you will,
there must be provision for varying speeds and these must be attained
while the car is in motion, and this means the meshing together of
finely built gears revolving so fast the teeth cannot be seen.
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.
[Illustration]
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.
Care in operating the gears will add much to the life of the car and to
the pleasure of the owner, and chauffeurs should be instructed in the
proper handling of the speed and reverse levers. It racks one’s nerves
when riding to hear gears clash and grind on a passing car, and many of
the taxis seem to have this trouble.
The chauffeurs run the cars on a slap-dash method and disregard of the
gears is a part of it all, but it costs the owner much in upkeep and
renewals.