The speed gears are in constant mesh with opposing
gears that determine the ratio for each speed the driver can select.
The speed gears freewheel on the shafts they are mounted on, and the
synchronizer assemblies are splined to the shafts on which they are
mounted. Since synchronize means to make several things happen at the
same time, the design is made to get the speed gear being selected to
move at the same speed as the shaft and synchronizer assembly. When the
shaft speeds are equal it is possible to engage the sliding sleeve of
the synchronizer assembly to the speed gear being selected without
grinding or clashing. This process must work during both upshifts and
downshifts and coordinate the different shaft speeds with a precise
timing under differing throttle openings.
Many conditions affect the operation of the
synchronizers and can create shift problems. Many of these conditions
are external to the transmission. To diagnose and repair a manual
transmission properly, the technician must understand both the theory
of operations and how the parts relate to each other to produce a good,
smooth-shifting end product for the customer.
Understanding the theory:
Although there are many different designs of
synchronizers, all of them function in the same way. The synchronizer
assembly consists of a hub with internal splines that bind it to the
shaft on which it rides. The hub has a set of external splines that
mate with splines inside the sliding sleeve, which can move back and
forth on the hub. At either end of the sliding sleeve are matched sets
of engagement teeth that engage the speed gear. Each sliding sleeve can
mate with two speed gears: 1st and 2nd gears, 3rd and 4th gears, and
5th and 6th gears, and one for reverse gear in many late-model
transmissions.
The synchronizer assembly also has slots machined
in the hub for keys (struts, dogs, pressure pieces etc.) that move in
the same direction as the sliding sleeve. These keys engage slots
milled into the synchronizer or blocking ring, which rides on a
machined cone that is part of the speed gear. Behind the cone of the
speed gear is a machined set of engagement teeth that mate with the
engagement teeth on either end of the slider. The synchronizer ring
also has a set of engagement teeth that match those on the sliding
sleeve and the speed gear.
The mechanics of a synchronized shift are
relatively simple. The driver selects a gear to be engaged using the
shift lever, which in turn moves a shift fork connected to the
synchronizer sliding sleeve. As the sleeve moves toward the speed gear,
the keys move the synchronizer ring into contact with the cone of the
speed gear. The synchronizer ring is a wet clutch that provides
friction to the cone of the speed gear to slow or accelerate the speed
gear to match the speed of the shaft on which the synchronizer assembly
is riding.
At this point the synchronizer ring will not let
the sliding sleeve engage the teeth on the speed gear until the shaft
speed and gear speed match (synchronize). When the shaft speed
equalizes with that of the speed gear, the friction on the blocking or
synchronizer ring relaxes and the ring’s external teeth line up
with the sliding sleeve and permit it to travel to fully engage the
teeth of the speed gear, completing the shift quietly. It’s
obvious that if any of these components are worn or damaged this timing
process will not work and gear grind occurs.
As part of this discussion of theory it is now time
to understand the relationship of gear ratio and shaft speeds to the
shift process. Using a 1st-to-2nd shift for this example and to make
the math easy, assume that the ratio of 1st gear is 2-1 and the ratio
for 2nd gear is 1.5-1. The driver accelerates to 4,000 rpm and begins
the shift. At this moment the main shaft is traveling at 2,000 rpm
(4,000 divided by 2 = 2,000) and the drive wheels are driving the main
shaft. At this point the 2nd speed gear is rotating at 2,667 rpm (4,000
divided by 1.5.) The speed of the clutch disc has to drop to 3,000 rpm
for 2nd gear to reach the same speed as the main shaft and synchronizer
assembly (3,000 divided by 1.5 = 2,000). At this point the shift can be
completed with no clash.
Here you can begin to realize external conditions
that can create problems with the shift, where a poor clutch release or
a driver hurrying the shift can
Synchronizer-ring design:
A synchronizer or blocking ring works as a friction
device much the same as brakes and clutches. These components, through
friction and pressure, turn motion (kinetic energy) into heat and slow
or accelerate a moving shaft.
Older designs of synchronizer rings are either
brass or bronze and have sharp threads running around the inside
diameter of the ring, with a series of grooves running across the
threads perpendicularly. The sharp threads are designed to cut through
the oil film on the cone of the speed gear, and the slots help to
exhaust the lube through centrifugal force, ensuring that the ring can
grab the speed gear instead of skidding on a film of oil.
Later designs use linings made of paper (similar to
automatic
clutches), carbon fiber and various sintered-metal
alloys. All these materials provide better holding power than brass or
bronze. An important note here is that all these designs are made to
work with a specified transmission lubricant. The 90-weight gear oil
used in the past is almost extinct and should never be used with
later-design ring linings.
In the first place, the later-design linings are
porous and absorb lubricant, and once they absorb 90-weight they will
never fully purge themselves of the high-viscosity fluid. It stands to
reason that the thicker the fluid, the higher the drag on the gear
train running through it. High-viscosity fluids create cold-shift
problems and add parasitic drag to the components, robbing fuel
mileage.
Another external source of shift problems is the
use of an incorrect lubricant. The high content of sulfur compounds in
gear oils attacks the ring linings, and the viscosity prevents the
rings from exhausting the fluid, creating gear clash. Most late-model
transmissions are specified to run on ATF or specially formulated 5W-30
motor oils with special additive packages to provide the proper
coefficient of friction for smooth synchronizer function. It is
surprising how many shops, in their need to get a job out the door,
will put any kind of lube in the transmission without regard to the
shift consequences. The correct lube is as important a part as any
bearing, seal or gear you will install. An incorrect lube may force you
to disassemble the transmission you just rebuilt and replace the new
synchro rings you just installed.
Design issue with synchronizer rings:
With the older-design brass or bronze rings, the
service manuals would list a specification for measuring
synchronizer-ring reserve, which was measuring by placing the ring on
the cone of the speed gear and using a feeler gauge to measure the ride
height of the ring from the synchronizer teeth on the speed gear. The
problem with this is that there was no way to measure the integrity of
the cone on the speed gear. The cone is tapered and impossible for the
average shop to measure even if it had the proper specs from a print.
The correct and simple way to measure the quality
of the taper on the speed gear is with machinist layout bluing or a
good permanent felt marker. Clean and degrease all the cones on the
speed gears and coat the cones with layout dye or felt marker. Then
place a new ring on the cone and twist it on with force. Remove the
ring and look at the pattern it made on the cone. It should be a
constant mark around the cone from the top to the bottom of the ring.
If there are skips or just a thin band around the gear cone at the top
or bottom, the speed gear needs to be replaced. This is something that
needs to be done during the teardown and rebuild process before you
quote a price. Anything you do not set a price on will become your
problem once the customer has agreed to a “final” price.
Newer ring designs have double- and triple-cone
synchronizer rings. Having friction surfaces on both the inside and
outside of the ring doubles or triples the surface area available for
working the ring without increasing the diameters of the gears and
synchronizers. This type of setup is common today, and measuring the
synchronizer reserve with a feeler gauge is almost impossible. The only
way to ensure that you have complete working coverage of the cones is
to use the dye or felt-marker method. Many of the sintered-alloy type
of linings on late-model rings have an aggressive coefficient of
friction and will wear out the speed-gear cones.
Diagnosis and separating synchronizer-ring problems
from other causes:
This is where the most confusion begins in
diagnosing shift problems. Synchronizer rings can cause grinding and
gear clash. They also can cause shift “block-out,” where a
shift cannot be completed and is literally blocked from further
movement of the shift lever. The ring will never create gear jump-out
and in many instances will not be the cause of notchy, dragging shifts.
To quickly eliminate external causes with shift problems, you need to
make a series of checks of the driveline components. Is the unit filled
with the correct oil? Is the lube in good condition or burnt beyond
recognition? Is the clutch properly adjusted for a full release, and
are the hydraulics or release linkage functioning properly?
A quick, easy test of the clutch setup is to jack
the vehicle to get the drive wheels off the floor or put the vehicle on
a lift with all four wheels off the ground. Now start the engine and go
through all the gears with the drive wheels spinning freely. If the
transmission now shifts better with little or no gear clash, the
problem is related to clutch release. This, however, does not mean that
the customer has not done internal damage to the unit by driving with a
bad release for an unknown amount of time. Also important to note is
that a poor clutch release will not always show up in all gears. Many
3-4 shift problems are clutch related, but the technician falsely
believes a bad clutch will show up in all gears.
Another clue about clutch problems is sheared
synchronizer keys, usually on the 3-4 synchronizer. The keys shear into
pieces because the clutch did not disconnect the input shaft from the
engine. This can be caused by the clutch not releasing as desired or by
a driver who is either rushing a shift or “power shifting”
without the clutch or not getting off the throttle when making a shift.
Gear jump-out is defined as the shift lever placing
the transmission in the desired gear and then having the unit jump out
of gear immediately when the clutch is engaged or when a change in
throttle position occurs while driving. The synchronizer ring has
NOTHING to do with gear jump-out. The causes are excess end play within
the unit allowing the shafts to move the slider off the speed gear,
worn or damaged shift forks or shift linkage, worn or damaged back
taper on the speed gear or sliding sleeve, worn or damaged powertrain
mounts, bent or out-of-balance driveshafts, or misadjusted shifters or
cables.
Notchy, grinding shifts or blocked-out shifts are
the result of worn synchro rings, damaged engagement teeth on the
slider or speed gear, excess end play on a gear or shaft, worn or
misadjusted shift linkage or forks, bad clutch release, driver shifting
at too high an engine speed for the transmission components, or trying
to rush the shifts or failing to use the clutch.
Understanding the components and their relationship
to synchronized shifts should help you to make faster, more-accurate
diagnostic decisions. Paying attention to details during teardown and
rebuild will help reduce comebacks and make the work more profitable.
Using the right lube for the unit will ensure your customer’s
satisfaction and, it’s hoped, repeat business. Word-of-mouth
recommendations are the best advertising. 
