In Layman's terms, A synchromesh transmission prevents gears in said transmission from grinding by spinning them at a steady, sychronized
speed. Reverse rarely has synchromesch, because it is too complex. This is why reverse tends to grind in stick shift
A synchromesh transmission is a constant mesh, collar shift transmission equipped with synchronizers, which equalize the speed of the shaft and gear before they are engaged. The action of the synchronizer eliminates gear clashing and allows for smooth changing of gears. Synchromesh transmissions are used on all current models of cars and are commonly found in other machines wherever shifting while moving is required.
Engine torque is applied to the transmission's input shaft when the clutch is engaged. The input shaft enters the transmission case, where it is supported by a large ball bearing and fitted with a gear. The output shaft (mainshaft) is inserted into, but rotates independently of the input shaft. The mainshaft is supported by the input shaft bearing and a bearing at the rear of the transmission case. The various speed gears rotate on the mainshaft. Located below or to the side of the input and mainshaft assembly is a counter shaft that is fitted with several sized gears. All of these gears, except one, are in constant mesh with the gears on the mainshaft. The remaining gear is in constant mesh with the input gear.
Gear changes occur when a gear is selected by the driver and is locked or connected to the mainshaft. This is accomplished by the movement of a collar that connects the gear to the shaft. Smooth and quiet shifting can only be possible when the gears and shaft are rotating at the same speed. This is the primary function of the synchronizers.
A synchronizer's primary purpose is to bring components that are rotating at different speeds to one synchronized speed. It also serves to lock these parts together. The forward gears of all current automotive transmissions are synchronized. A single synchronizer is placed between two different speed gears, therefore transmissions have two or three synchronizer assemblies. Reverse gear is not normally synchronized because gear rotation is required for synchronizer action and reverse is normally selected when the car is not moving.
There are four types of synchronizers used in synchromesh transmissions: block, disc and plate, plain, and pin. The most commonly used type on current transmissions is the block type. All synchronizers use friction to synchronize the speed of the gear and shaft before the connection is made.
Block synchronizers consist of a hub, sleeve, blocking ring, and inserts or spring-and-ball detent devices. The synchronizer sleeve surrounds the synchronizer assembly and meshes with the external splines of the hub. The hub is internally splined to the transmission's mainshaft. The outside of the sleeve is grooved to accept the shifting fork. Three slots are equally spaced around the outside of the hub and are fitted with the synchronizer's inserts or spring-and-ball detent assemblies.
These inserts are able to freely slide back and forth in the slots. The inserts are designed with a ridge on their outer surface and insert springs hold this ridge in contact with an internal groove in the synchronizer sleeve. When the transmission is in the "neutral" position, the inserts keep the sleeve lightly locked into position on the hub. If the synchronizer assembly uses spring- and-ball detents, the balls are held in this groove by their spring. The sleeve is machined to allow it to slide smoothly on the hub.
Bronze, brass, fiber faced blocking rings are positioned at the front and rear of each synchronizer assembly. Each blocking ring has three notches equally spaced to correspond with the three insert keys. Around the outside of the blocking ring is a set of beveled dog teeth. These teeth are used for alignment during the shift sequence. The inside of the blocking ring is shaped like a cone, the surface of which has many sharp grooves. These inner surfaces of the blocking rings match the conical shape of the shoulders of the driven gear. These cone-shaped surfaces serve as the frictional surfaces for the synchronizer. The shoulder of the gear also has a ring of beveled dog teeth designed to align with the dog teeth on the blocking ring
When the transmission is in neutral, the synchronizers are in their neutral position and are not rotating with the mainshaft. The mainshaft's gears are meshed with the counter gears and are rotating with the counter shaft. However, they turn freely, at various speeds, on the mainshaft and do not cause the shaft to rotate because they are not connected to it.
When a gear is selected, the shifting fork forces the sleeve toward the selected gear. As the sleeve moves, so do the inserts because they are locked in the sleeve's internal groove. The movement of the inserts pushes the blocking ring into contact with the shoulder of the driven gear. When this contact is made, the grooves on the blocking ring's cone cut through the film of lubrication on the gear's shoulder. If the film of lubrication is not cut by the grooves, synchronization cannot take place. Destroying the film allows for metal-to-metal contact and begins the speed synchronization of the two parts. The resultant friction between the two brings the gear's cone to the blocking ring cone's speed.
As the components reach the same speed, the synchronizer sleeve can now slide over the external dog teeth on the blocking ring and then over the dog teeth on the speed gear's shoulder. This completes the engagement of the synchronizer and the gear is now locked to the mainshaft. The blocking ring will only allow the sleeve to mesh with the gear when its teeth are lined up with the locking teeth on the gear.
Power now flows from input gear to the counter gear, back up to the speed gear locked by the synchronizer. Power then flows from the gear through the locking teeth to the sleeve, then to the hub, and finally to the mainshaft.
To disengage a gear, the shifter is moved to the neutral position, which causes the synchronizer sleeve to move away from the previous gear, thereby disconnecting it from the shaft.
In summary, synchronization occurs in three stages. In the first stage, the sleeve is moved toward the gear by the shift lever and engages the hub assembly. In the second stage, the movement of the sleeve causes the inserts to press the blocking ring onto the cone of the gear. In the third stage, the synchronizer ring completes its friction fit over the gear cone and the gear is brought up to the same speed as the synchronizer assembly. The sleeve slides onto the gear's teeth and locks the gear and its synchronizer assembly to the mainshaft.
says great wu on snynchomesh transmissions, but your statement about reverse not being synchronized is not true. my volvo 850 has synchomesh on all gears (5 forward & 1 reverse), as do all thoroughly modern manual transmissioned volvos and motorcycle transmissions don't have friction ring-type synchromesh as cars do, if they have any type of synchronization at all, btw.
Thank you Jethro