The wheels that engage each other in which turning of one turns the other is called gearing. A wheel having teeth made especially to engage similar teeth or other wheels is known as a Gear. If the two gears in a gear set are considerably different size the smaller one is called the pinion. The size of the teeth on any gear is determined by the pitch of the gear. The purpose of gearing are to provide a means for mechanical advantage in transmitting power which is in the form of torque or to change the direction of torque. Know about the purpose of gear box?
A gear is a form of lever, but without the limitation of movement of an ordinary lever. When two gear wheels with different numbers of teeth are meshed together and one is driven, the other will obviously revolve at a different speed. Consider the gears in the illustration in which gear “A” and “B” round for a certain distance until they move apart. Before one pair of teeth goes out of contact, however, another pair come into contact or “mesh” as it is called. In one complete turn of gear “A” each of its teeth will have meshed with a tooth of gear “B”. As “B” has twice the number of teeth of gear “A” it will make half a rotation than ‘A’.
The relationship between the numbers of teeth on two gears, which mesh with one another, is known as “gear ratio”.
Various types of Gears
(a) Gears Cut on Cylinders. Several types of gears have teeth cut across the periphery (outside) of cylindrical shapes. The axis of the gears are parallel when the gears are running together.
- Spur Gear. In this type teeth are cut straight across the face of the gear and parallel to the axis of rotation.
- Helical Gears. Helical gears have teeth, which are cut according to a geometric figure called a helix. The thread on an ordinary bolt is an example of the helix, although on a bolt the helix angle is much greater than that on most gears.
(b) Gears Cut on Cones. Where it is desired to change the direction of torque, gears cut on parts of cones are used. The cones fit together so that the axis of shaft make an angle. If the axis of the cones intersects, the gears are known as bevel gears.
- Bevel Gears. If the teeth on a conical gear are straight and would if continued meet at the tip of the cone, the gear is called a spur bevel gear or sometimes a straight bevel gear.
- Spiral Gears. Conical gears having teeth curved in this manner are spiral gears. Because of the general similarity to bevel gears, spiral gears frequently are called spiral bevel gears.
(c) Gears for Special Applications. If the flow of torque through a gear train is such that the shafts or the axis of the gears are not parallel and do not intersect, special forms of gears must be used.
- Worms and Worm Gears. A worm resembles an ordinary bolt. The teeth wind around the gear just as the screw thread winds around the bolt. The sliding surface of the teeth however is cut according to the same contour used on all gears.
- Hypoid Gears. Hypoid gears are similar to spiral bevel gears, but they differ in that the axis of the shafts do not intersect. Since one shaft is lower than the other, the teeth cannot be cut in the direction of a true spiral and till mesh properly. Instead, the teeth on meshing bevel gears, the axis
of which do not intersect are cut in the direction of a complicated geometric figure known as the hyperbatic parabolied. The word “hypoid” is the popular contraction of the true mathematical term.
Various types of Gearbox
Sliding Mesh or Crash Type Gear Box
The gears in a crash type gearbox are of spur type (sliding mesh). Although spur gears have high efficiency they are now mainly used on commercial vehicles, because besides being noisy in operation, before two rotating gears can be slide into mesh with each other they must be running at near the same speed, which calls for considerable skill by the driver to prevent clashing of the gears and this skill termed as ‘double declutching’ and necessitates allowing the engine to decelerate or to be accelerate dependent on whether the change is up or down and thus matching the engine speed to the road speed.
Silent or Constant Mesh type Gearbox
This is development of the crash type gear box to give a quieter operation and make gear changing easier by employing helical gears for the constant mesh train and in addition an intermediate gear on the main shaft and lay shaft is made with helical teeth. The intermediate gear on the main shaft remains in constant mesh with its opposite member on the lay shaft irrespective of which gear wheel transmit the drive, but it floats on plain bush-on the main shaft. Thus, the intermediate gear on the main shaft is always driven by the lay shaft, but it does not transmit the drive until it is selected by the selector mechanism which moves a dog mounted on splines on the main shaft to mesh with dogs cut on a flange of the gear wheel thus locking the shaft and wheel together. All the pinions are in constant mesh and cannot slide along the shaft. The pinions on the main shaft is prevented from moving along the shaft by locking rings, and are mounted on bushes so that no drive is transmitted direct.
Construction of Gearbox
The main components of gearbox.
(a) The Box. It is a casing, usually made of aluminium alloy or malleable cast iron, that houses the gears. It is bolted to the clutch bell housing and also supported by chassis cross member. It contains lubricating oil, maintained at a predetermined level. A top cover or side cover usually incorporates for a housing for the gear selector mechanism. Rotation of the lay shaft, gears and bearings are lubricated by splash lubrication.
(b) Primary Shaft. The driving shaft to the gear box is called the primary shaft. The front end of this is supported in a spigot bearing fitted in the engine flywheel or camshaft rear flange. The shaft is splined at the front end, which fits in the hub of clutch plate. The rear end of the shaft is spigotted to support the main shaft and carries a pinion, which is in permanent mesh with the lay shaft driving pinion and this arrangement, is known as the constant mesh gears. The dog teeth formed on the rear end of the shaft facilitates to transmit the drive to the main or sliding shaft when top gear is selected.
(c) Lay Shaft. A shaft driven by and in constant mesh with the primary shaft carries different sized gears for meshing with main shaft gears. It is supported in bushes or bearings in the gearbox.
(d) Main Shaft. Carries different sizes gears for meshing as required with the lay shaft gears. It is supported at the front end in a spigot bearing in the primary shaft and at the rear in a bearing in the gearbox casing.
(e) Reverse Shaft . It is a short fixed shaft carrying a spur gear permanently in mesh with the lay shaft, having the largest gear wheel on the main shaft is brought into mesh to reverse the direction of drive. In a four-speed gearbox, the reverse gear wheel is moved into mesh with the lay shaft gear wheel and the reverse drive is obtained when the largest gear wheel on the main shaft is meshed with the pinion on the reverse shaft.