Definition of Synchromesh Gear Box

In a synchromesh gearbox, gears can freely rotate or be locked to the shaft on which they are carried. The locking mechanism consists of a sliding collar which bridges between two circular rings with teeth on them - one travels with the gear, one with the shaft. When the rings are bridged, the gear is locked to the shaft. To correctly match the speed of the gear to that of the shaft as the gear is engaged, the collar initially applies a force to a cone-shaped clutch which is attached to the gear. This spins the gear up or down in speed to match the shaft prior to engagement of the collar. The collar is prevented from bridging the locking rings when the speeds are mismatched by baulk rings. The gear lever manipulates the collars using a set of linkages, so arranged so that only one collar may be permitted to lock a gear at any one time. In a modern gearbox, the action of all of these components is so smooth and fast it is hardly noticed.

1. I speed gear 2. II speed gear 3. main shaft 4. outer engaging unit 5. inner engaging unit 6. top gear engaging teeth 7. main drive gear 8. top gear synchronising cones 9. counter shaftSee More on this sitewww.hvtransmissions.com/prod-gear-box-rev.pdf

Fig(Synchromesh images)Lab Manual | constructional details, working principles and operation of the Automotive Transmission Systems

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?AimTo study and prepare report on the constructional details, working principles and operation of the following Automotive Transmission Systems:

a) Sliding Mesh

b) Constant Mesh

c) Synchromesh four speed range

Sliding Mesh type Gear BoxThis is the simplest type of gear box (Fig 3). The power is transferred from the engine to the clutch shaft and then to the clutch gear which is always in mesh with a gear on the layshaft. All the gears on the lay shaft are fixed and as such they are all the time rotating when the engine is running and the clutch is engaged. Three direct and one reverse speeds are attained by suitably moving the gear on the main shaft by means of selector mechanism.

Constant Mesh Gear BoxIn this type of gear box, all the gears are in constant mesh with the corresponding gears on the layshaft. The gears on the main shaft which is splined, are free. The dog clutches are provided which slide on the main shaft. The gears on the layshaft are, however, fixed.

When the left dog clutch is slided to the left by means of the selector mechanism, its teeth of the gear are engaged with the clutch gear. The same dog clutch, however, when slide to right makes contact with the second gear and second gear ratio is obtained. Similarly movement of the right dog clutch to the left results in low gear and towards right in reverse gear.

Synchromesh Gear BoxThis type of gear box is similar to the constant mesh type. In this all the gears on the mainshaft are in constant mesh with the corresponding gears on the layshaft. The gears on the layshaft are fixed to it while those on the mainshaft are free to rotate on the same shaft. Its working is also similar to the constant mesh type, but in the former there is one definite improvement over the latter. This is the provision of synchromesh device which avoids the necessity of double declutching. The parts which ultimately are required to be engaged, are first brought into frictional contact which equalizes their speed, after which these may be engaged smoothly.

Fig shows the construction and working of a synchromesh gear box. In most of the cars, however, the synchromesh devices are not fitted to all the gears. They are only on the high speed gears but on the low speed and reverse gears, ordinary dog clutches are only provided. This is done to reduce the cost.

In other words, the gear wheels which are to be positively connected are first brought into frictional contact and when the friction has equalized their speeds, the positive connections are made. Synchromesh devices can be applied to the sliding mesh gear box but they are universally used with constant mesh gear boxes used in different motor vehicles. The main features of this gear box are:

a) The output gears are free to rotate on bushes on the output shaft. They are internally located by splined thrust bearings. Single or double helical gears remain in constant mesh with the lay shaft gears.

b) The output gears are locked to their shaft by the dog clutch

c) Change of the synchronizing hub takes place when its speed equalizes or synchronizes by theirs cones.

In case this gear is not used, it is left to the skill of the driver to bring one or more gears for meshing. By the skillful use of this clutch and accelerator pedal, he can bring the gears approximately the same speed. Even for the unskilled driver it is not difficult to affect quite satisfactory the change of gears. He can do it simply by using the clutch pedal gear lever with the help of successful commercial gear synchronizers.

Reference Books:1 Automotive Mechanics Crouse/Anglin

2 Automobile Engineering by Dr Kirpal Singh

3 Automobile Engineering by Pitman

Viva Questions

1 Why gear box is used in a vehicle

2 What is a synchromesh device?

3 Explain clearly the necessity of a transmission in a vehicle

4 Discuss the advantages of a constant mesh gear box over the sliding mesh type gear box

5 Describe the working of a synchromesh gear box with the help of a neat sketch

6 What is transfer box ? where it is used ?

7 What are the merits and demerits of synchromesh gear box compared to a sliding mesh on constant mesh type

8 Explain the common troubles encountered in gear boxes and suggest suitable remedies

9 What is a synchronizer

10 Why is synchromesh device usually not employed for the reverse gear

11 What may be the possible reason when the gear slips out of engagement

12 What should be the cause when the vehicle is running with excessive noise in the gear box

Figure 13-8.Dissembled main shaft assembly. (fig. 13-8, A) toward the third-speed gear (fig. 13-8, D). This action engages the external teeth of the clutch gear with the internal teeth of the third-speed gear. Since the third-speed gear is rotating with the rotating counter-shaft gear, the clutch gear also must rotate. The clutch gear is splined to the main shaft, and therefore, the main shaft rotates with the clutch gear. This principle is carried out when the shift lever moves from one speed to the next. Constant-mesh gears are seldom used for all speeds. Common practice is to use such gears for the higher gears, with sliding gears for first and reverse speeds, or for reverse only. When the shift is made to first or reverse, the first and reverse sliding gear is moved to the left on the main shaft. The inner teeth of the sliding gear mesh with the main shaft first gear. SYNCHROMESH TRANSMISSION The synchromesh transmission is a type of constant-mesh transmission. It synchronizes the speeds of mating parts before they engage to allow the selection of gears without their clashing. It employs a combination metal-to-metal friction cone clutch and a dog or gear positive clutch. These clutches allow the main drive gear and second-speed main shaft gear to engage with the transmission main shaft. The friction cone clutch engages first, bringing the driving and driven members to the same speed, after which the dog clutch engages easily without clashing. This process is accomplished in one continuous operation when the driver declutches and moves the control lever in the usual manner. The construction of synchromesh transmissions varies somewhat with different manufacturers, but the principle is the same in all. The construction of a popular synchromesh clutch is shown in figure 13-9. The driving member consists of a sliding gear splined to the transmission main shaft with bronze internal cones on each side. It is surrounded by a sliding sleeve having internal teeth that are meshed with the external teeth of the sliding gear. The sliding sleeve has grooves around the outside to receive the shift fork. Six spring-loaded balls in radially drilled holes in the gear fit into an internal groove in the sliding sleeve. That prevents the sliding sleeve from moving endwise relative to the gear until the latter has reached the end of its travel. The driven members are the main drive gear and second-speed main shaft gear. Each has external cones and external teeth machined on its sides to engage the internal cones of the sliding gear and the internal teeth of the sliding sleeve. The synchromesh clutch operates as follows: when the driver moves the transmission control lever to the third-speed, or direct-drive, position the shift fork moves the sliding gear and sliding sleeve forward as a unit until the internal cone on the sliding gear engages the external cone on the main drive gear. This action brings the two gears to the same speed and stops endwise travel of the sliding gear. The sliding sleeve slides over the balls and silently engages the external teeth on the main drive gear. This action locks the main drive gear and transmission main shaft together as shown in

GEARS: Synchromesh Gear Box

This type of gear box is similar to the constant mesh type in that all the gears on the main shaft are in constant mesh with the corresponding gears on the lay shaft. The gears on the lay shaft are fixed to it while those on the main shaft are free to rotate on the same. Its working is also similar to the constant mesh type, but in the former there is one definite improvement over the latter. This is the provision of synchromesh device which avoids the necessity of double declutching. The parts which ultimately are to be engaged are first brought into frictional contact which equalizes their speed, after which these may be engaged smoothly.

Figure shows the construction and working of a synchromesh gear box. In most of the cars, however the synchromesh devices are mot fitted to all the gears as is shown in this figure. They are fitted only on the high gears and on the low and reverse gears ordinary dog clutches are only provided. This is done to reduce the cost.

< Previousn the above figure is the engine shaft, Gears B, C, D, E are free on the main shaft and are always in mesh with corresponding gears in the lay shaft. Thus all the gears on main shaft as well as on lay shaft continue to rotate so long as shaft A is rotating. Menders F1 and F2 are free to slide on splines on the main shaft. G1 and G2 are ring shaped members having internal teeth fit onto the external teeth members F1 and F2 respectively. K1 and K2 are dog teeth on B and D respectively and these also fit onto the teeth of G1 and G2. S1and S2 are the forks. T1and T2 are the ball supported by springs. These tend to prevent the sliding of members G1 (G2) on F1 (F2).however, when the force applied in G1 (G2) through fork S1 (S2) exceeds a certain value, the balls are overcome and member G1 (G2) slides over F1 (F2). There are usually six of these balls symmetrically placed circumferentially in one synchromesh device. M1, M2, N1, N2, P1, P2, R1, R2 are the frictional surfaces.

The working of the gear box is as follows .for direct gear, member G1 and hence member F1 (through spring loaded balls) is slid towards left till comes M1 and M2 rub and friction makes their speed equal. Further pushing the member G1 to left causes it to override the balls and get engaged with dogs K1. Now the drive to the main shaft is direct from B via F1 and the splines. We have to give sufficient time for synchronization of speeds, otherwise clash may result.

For the second gear the members F1 and G1 are slid to the right so that finally the internal teeth on G1 are engaged with L1.then the drive to main shaft will be from B via U1, U2, C, F1 and splines.For first gear, G2 and F2 are moved towards right. In this case the drive will be from B via U1, U3, D, F2 and splines to the main shaft.For reverse, G2 and F2 are slid towards right. In this case the drive will be from B via, U1, U4, U5, D, F2 are splines to the main shaft.