The synchronizer performs a
number of jobs vital to transmission/transaxle operation. Its main job
is to bring components that are rotating at different speeds to one
synchronized speed. A synchronizer ensures that the pinion shaft and the
speed gear are rotating at the same speed. The second major job of the
synchronizer is to actually lock these components together. The end
result of these two functions is a clash-free shift. In transaxles, a
synchronizer can have another important job. When spur teeth are cut
into the outer sleeve of the synchronizer, the sleeve can act as a
reverse idler gear and assist in producing the correct direction of
rotation for reverse operation.
In modern transmission and
transaxles, all forward gears are synchronized. One synchronizer is
placed between the first and second gears on the pinion shaft. Another
is placed between the third and fourth gears on the mainshaft. If the
transmission has a fifth gear, it is also equipped with a synchronizer.
Reverse gear is not normally fitted with a synchronizer. A synchronizer
requires gear reduction to do its job and reverse is selected with the
vehicle at a stop. In some older transmissions, or truck transmission,
both reverse and first gears may be unsynchronized.
The synchronizer sleeve
surrounds the synchronizer assembly and meshes with the external splines
of the clutch hub. The clutch hub is splined to the transmission pinion
shaft and is held in position by a snap ring. A few transmissions use
The synchronizer sleeve has
a small internal groove and a large external groove in which the shift
fork rests. Three slots are equally spaced around the outside of the
clutch hub. Inserts fit into these slots and are able to slide freely
back and forth. These inserts, sometimes referred to as shifter plates,
are designed with a ridge in their outer surface. Insert springs hold
the ridge in contact with the synchronizer sleeve internal groove.
The synchronizer sleeve is
precisely machined to slide onto the clutch hub smoothly. The sleeve and
hub sometimes have alignment marks to ensure proper indexing of their
splines when assembling to maintain smooth operation.
Brass or bronze
synchronizing blcoker rings are positioned at the front and rear of each
synchronizer assembly. Each blocker ring has three notches equally
spaced to correspond with the three insert keys of the hub. Around the
outside of each blocker ring is a set of beveled clutching teeth, which
is used for alignment during the shift sequence. The inside of the
blocker ring is shaped like a cone. This coned surface is lined with
many sharp grooves.
The cone of the blocker
ring makes up only one half of the total cone clutch. The second or
matching half of the cone clutch is part of the gear to be synchronized.
The shoulder of the speed gear is cone shaped to match the blocker ring.
The shoulder also contains a ring of beveled clutching teeth designed to
align with the clutching teeth on the blocker ring.
When the transmission is in
neutral or reverse, the first/second and third/fourth synchronizers are
in their neutral position and are not rotating with the pinion shaft.
Gears on the mainshaft are meshed with their countershaft partners and
are freewheeling around the pinion shaft at various speeds.
To shift the transmission
into first gear, the clutch is disengaged and the gearshift lever is
placed in first gear position. This forces the shift fork on the
synchronizer sleeve toward the first speed gear on the pinion shaft. As
the sleeve moves, the inserts also move because the insert ridges lock
the inserts to the internal groove of the sleeve.
The movement of the inserts
forces the blocking ring's coned friction surface against the coned
surface of the first speed gear shoulder. When the blocking ring and
gear shoulder come into contact, the grooves on the blocker ring cone
cut through the lubricant film on the first speed gear shoulder and a
metal-to-metal contact is made. The contact generates substantial
friction and heat. This is one reason bronze or brass blocker rings are
used. A nonferrous metal such as bronze or brass minimizes wear on the
hardened steel gear shoulder. This frictional coupling is not strong
enough to transmit loads for long periods. As the components reach the
same speed, the synchronizer sleeve can now slide over the external
clutching teeth on the blocking ring and then over the clutching teeth
on the first speed gear shoulder. This completes the engagement. Power
flow is now from the first speed gear, to the synchronizer sleeve, to
the synchronizer clutch hub, to the main output shaft, and out to the
To disengage the first
speed gear from the pinion shaft and shift into second speed gear, the
clutch must be disengaged as the shift fork is moved to pull the
synchronizer sleeve and disengage it from the first gear. As the
transmission is shifted into second gear, the inserts again lock into
the integral groove of the sleeve. As the sleeve moves forward, the
forward blocking ring is forced by the inserts against the coned
friction surface on the second speed gear shoulder. Once again, the
grooves on the blocker ring cut through the lubricant on the gear
shoulder to generate a frictional coupling that synchronizers the gear
and shaft speeds. The shift fork can then continue to move the sleeve
forward until it slides over the blocker ring and gear shoulder
clutching teeth, locking them together. Power flow is now from the
second speed gear, to the synchronizer sleeve, to the clutch hub, and
out through the pinion shaft.