The movement along the main shaft of the sliding gears, driving dogs or Synchromesh unit is affected by the gear change mechanism and gear selector mechanism. The former is operated manually by the driver; the selector mechanism is incorporated in the gearbox itself and in response to movement of the gear lever, the required gear or driving dog is selected and moved into its correct position on the main shaft.
Gear change Mechanisms
In its simplest form the gear change mechanism consists of a gear lever, the lower end of which engages directly with the mechanism. A ball formed on the bottom of gear lever, is mounted in spherical seating in the gear box top permitting the gear lever to be moved fore and aft and laterally as required. A later development is the steering column mounted gear change lever, with this type of control, movement for the gear lever is transmitted to the selector mechanism by system of connecting rods and levers. Another type of gear change mechanism is the remote control fitted to load carriers of the forward control type, where the change mechanism is housed in a box, bolted to the side of the engine crank case and movements of gear lever is transmitted to the gearbox by control shaft.
Gear Selector Mechanisms
Gear selector mechanism for gearboxes with four forward speeds are similar to the type shown in image above. The selectors are mounted in top of the gearbox casing and consist of three selector shafts that can be moved separately backwards and forwards. Each selector shaft has a selector fork attached to it; one for first and second one for third and top, and one for reverse gear. Each selector fork has a slot into which the need of the gear lever fits. When the gears are in neutral the slots are in line and the gear lever can be moved from side without selecting a gear. To select a gear lever is first move into the appropriate selector fork slot, and then moved forward or backward to move the selector fork, and therefore the gear required into engagement. Spring loaded balls engaging in recesses in the selector shaft lock the shaft in neutral or in the selected gear position as required.
To prevent an accidental engagement of two gears at once, various methods are employed.
(a) Plungers and Sliding Pin. The plungers are located between the selected shafts in the gearbox cover, are separated by the pin, which is located in a drilling in the centre selector shaft. The two outer shafts are notched to receive the rounded ends of the plungers. The combined length of the plungers and pins greater than the distance between the two outer selector shafts by an amount equal to the depth of the notch and the centre shat is locked by the plunger are displaced outwards into the notches of two side shafts to the gear box cover.
(b) Swinging Fork. The swinging fork shaft interlock is so designed that when the slot and the gear lever are in line with one of the selector fork grooves, the other two selector forks are positively locked by bosses which form part of swinging fork, engaging in their grooves.
(c) Interlock Plate. The interlock plate mechanism consists of an interlock in a guide plate, which is riveted to the gearbox cover. The guide allows the interlock to slide across the gearbox but prevents movement in a fore and aft direction. A projection on the underside of the interlock engages the slots in the top of the selector fork. The end of the gear lever projects through a slot in the interlock to move the fork of the gear selected while the other forks are locked in the neutral position.
(d) Reverse Gear Stop. To prevent the accidental selection of reverse gear in a four forward speed gearbox a spring loaded catch or plunger mechanism is usually incorporated. It requires either extra effort at the gear lever to overcome the spring, or a special movement such as lifting up or pressing down the gear lever as a whole lifting a catch or trigger, before reverse gear can be selected.
Synchro Mesh Gearbox
It is a heavy duty gear box and attached to the rear face of the flywheel bell housing, supported on rubber insulators on the cross member. Gear changing, is smooth and positive through a control lever mounted in selector housing at the top of the assembly. Poppet and springs retain the gears in mesh and an interlock prevents slipping out of mesh. Synchromesh units provide an automatic means of synchronising the speeds of the driving and the driven shafts before the gears engaged and thus minimising the difficulties of gear changing. The working principle of the synchromesh engagement of the constant mesh gear consists in essence of interposing small cone clutches between the dogs that are to be engaged. The cone makes contact before the dog teeth themselves come together and friction between the cones bring the parts to identical speeds.
Operation Of Three Speed Synchromesh Gear Box
(a) 1st Speed Synchromesh and Reverse drive. The lay shaft is driven by the primary shaft. Lay shaft, main shaft and constant mesh gears revolves as one unit except 1st gear. The gear speed gear has straight cut teeth. When the gear lever is moved into engagement, the sliding 1st gear on the main shaft is coming into contact with the lay shaft 1st speed gear and the power is transmitted to the main shaft. The reverse drive idler gear is also of the normal type and is moved into mesh with the gear on the main shaft and lay shaft and thus the direction of rotation is reversed.
(b) 2nd and Top Synchromesh Gear Operation . When the selector shaft is moved the synchromesh unit and the mating cones of the synchromesh hub and 2nd gear wheel will come into contact first, the sped is synchronised, further movement of the sleeve will move the entire, hub till the dog come into engagement whereby locking the 2nd speed gear wheel with the main shaft. On top gear the synchromesh unit is moved forward to the extreme position on the main shaft. In the position the dogs on the synchromesh sleeve are engaged with those on the primary shaft gear and the primary shaft is thus rigidly connected to the main shaft. Drives, therefore, transmitted direct and all other gears revolve idly.