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Shifting gears, well sort of.........
Just to add a little to the discussion about shifting gears.....actually
it should be called "engaging" gears.
For many years the synchronized transmissions used in passenger cars
have forward gears that are constantly meshed together which means
that the gear position is never "shifted" or moved. For many years
problems with changing gears and the idea of "grinding
gears" during a shift gives the impression that the gears are grinding
together when they try and mesh with each other
and come together during a "shift". The only gear that is
sometimes shifted or moved is reverse gear. This can cause
actual gear "grinding" when shifting into reverse. This grinding is usually
caused by the clutch disc not disengaging completely during a reverse
Here is a description of an actual gear "shift". For example,
the mainshaft (input shaft) of the FWD Audi transmissions has 3rd and
4th gears that spin freely on the mainshaft on needle bearings. Inbetween
3rd and 4th gears is a hub that is splined (connected) to the mainshaft. This
hub has an outer "sleeve" which is a round ring that can slide towards either
3rd or 4th gear on either side. Additionally there are brass synchronizer rings
inbetween the sleeve and the gear. The edge of the gears, synchroizer ring
and the "sleeve" all have small slotted tapered "teeth" on them. These teeth
on the 3rd/4th gears are separate from the larger meshed gear teeth.
In addition, the synchronizer rings and the edge of 3rd and 4th gears
have a tapered contact area to allow the synchronizer ring
to contact the tapered area on the gear and match the gear speed to the
rotating speed of the mainshaft.
When you "shift" gears into 3rd for instance this "sleeve" with tapered teeth
is moved towards 3rd gear where it contacts the synchronizer ring and as
the teeth line up on the synchro the round sleeve continues to slide over
the synchronizer ring and continues toward the tapered edge of the
gear. The slinding sleeve which now has hold of the synchroizer ring
pushes the synchroizer ring against the tapered edge of the gear which
makes the gear spin at the speed as the sleeve/synchronizer attached
to the mainshaft.
Finally the sleeve/synchronizer/gear small teeth all line up and the gear
gets locked to the mainshaft so 3rd gear can now transfer power from the
mainshaft (input shaft connected to the engine flywheel/clutch disc)
down through the constantly meshed 3rd gear on the pinion shaft.
Shifting problems are usually caused by worn teeth on the sleeve,
synchonizer and gears which prevent the teeth from lining up. The
sharp edge of the teeth get rounded off after many miles. The
synchronizer tapered section can also get worn to the point where
it does not speed up or slow down the gear effectively which does
not allow the small teeth on the gear to line up with the sleeve.
Popping out of gear on deceleration can be caused by a worn out
or loose sleeve/hub assembly. Too much play in the sleeve/hub
allows the sleeve to jump off the gear teeth and no longer lock
the gear to the mainshaft.
Happy gear shifting.......I mean gear engaging.........