INTRODUCTION TO GEARING AND GEAR TRAINS
The wheels that engage each other so that one is turned and the other turns are called Gearing. A wheel having teeth made especially to engage similar teeth or other wheels is known as a Gear. If the wheel is used to drive a chain it is called a sprocket. Gear wheels, before the gear are cut in them, are called gear blank. If the two gears in a gear set are of 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. At least two teeth on any gear should be in engagement at all times. 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. It enables a given power output to be applied to the road wheels, either in a short time, i.e., for the moving of a given load under easy conditions at a high speed, or over a longer period, i.e. movement of the same load under hard conditions, e.g., starting or climbing a hill at a lower speed. It is thus a means of changing the rate of power application by altering the leverage that the engine has at the driving wheels.
OBJECT AND TYPES OF GEAR BOX
The gear box is a mechanism which is used to transfer the drive received from the clutch, to the propeller shaft through a fixed reduction or increased ratio mechanism under varying engine and vehicle speed condition. Types of gear boxes are as follows.
(a) Sliding Mesh or Crash Type Gear Box
(b) Silent or Constant Mesh type Gear Box
(c) Synchromesh Gear Box
SLIDING MESH OR CRASH TYPE GEAR BOX
The gears in a crash type gear box are of spur type (sliding mesh) and the engagements of the various ratios are done by sliding the gears. Although spur gears have high efficiency they are now mainly used on commercial vehicles, besides being noisy in operation, before two rotating gears can be slide into mesh with each other they must be running at nearly the same speed, which calls for considerable skill by the driver to prevent clashing of the gears. This skill is termed ‘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 GEAR BOX
This is a development of the crash type gear box to give a quiter 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 transmits 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 spines 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 from the pinions to the shaft till sliding dogs locks it to main shaft.
SYNCHROMESH GEAR BOX
On modern vehicles the dogs are replaced by synchromesh units. This arrangement makes a gear change from top to intermediate or form low to intermediate much easier. All the other gear wheels other than constant mesh gears have straight cut teeth. 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.
GEAR CHANGE LEVER, SELECTOR AND SHIFTER MECHANISM
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.
(a) Gear Change Lever 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 a 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.
(b) Gear Shifter and Selector Mechanisms. 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, first move into the appropriate selector fork slot, and then move 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.
Purpose is to lower the whole range of gear ratios under more difficult road and load conditions. It is operated by a separate gear lever having three positions, high, low and neutral.. It is important that the gear change must be made, only when the vehicle is stationary.
Note. Transfer gear boxes are sometime known as Auxiliary gear boxes.
INTRODUCTION AND PURPOSE
On some vehicles an auxiliary gearbox is fitted immediately behind the main gearbox. Its purpose is to lower the whole range of gear ratios under more difficult road and load conditions. It is operated by a separate gear lever having three positions, high, low and neutral. It is important that the gear change must be made, only when the vehicle is stationary.
CONSTRUCTION AND OPERATION OF AUXILIARY GEAR BOX
(a) Construction: An Auxiliary Gearbox has three shafts, namely:
(i) The Main or Driving Shafts. Driven from the main Gear box but with its rear end supported in a spigot formed in the tail shaft. The main shaft is splined and carries a single gear wheel which, in addition to straight cut teeth, has dogs machined on one face.
(ii) A Subsidiary Shaft. With one large gear wheel, and another gear wheel, this is in constant mesh with the tail shaft gear wheel. Both the gear wheels are solid with the shaft.
(iii) A Tail Shaft. This is connected by a universal joint to the propeller shaft. It carries one gear wheel, which, in addition to straight cut teeth, has dogs at one face.
(b) Operation. A selector mechanism similar to that of a main gearbox can be manipulated to move the sliding gear on the splined main shaft to obtain either high or low gear. It is operated in the following manner.
(i) High Gear. The sliding gear wheel is moved along the splines of the main shaft, so that the gear wheel dogs engage those of the gear wheel on the tail shaft. The drive is, therefore, straight through with no reduced gear ratio.
(ii) Low Gear. The sliding, gear wheel is moved in the opposite direction along the splines of the main shaft, so that it meshes with the larger gear wheel mounted on the subsidiary shaft. The drive is through the main shaft to the subsidiary shaft and then the tail shaft through the constant mesh gears. The reduction is effected by the ratio between the number of teeth on the sliding gear wheel and the number of teeth on the subsidiary shaft gearwheel.
(iii) Neutral. Neutral position is obtained when the gear lever is in a central position, so that the sliding gear wheel is midway between the gear wheel on the subsidiary shaft and that of the tail shaft.
On vehicles where four wheel drive is employed; i.e. where the driving power can be applied to the front axle as well as the rear, it is necessary to transfer the drive from the main gear box to another gearbox known as the transfer gearbox.
Purpose. The driving power can be applied to the front axle as well as the rear, or driving power can be used for any other purpose like water pump of DFT or to operate any machine by engine power.
CONSTRUCTION AND OPERATION OF TRANSFER GEAR BOX
(a) Construction. The transfer gearbox has four shafts, namely:
(i) The main shaft driven by the propeller shaft from the main gearbox is splined and carries a toothed gear wheel which has dogs machined on its rear face. The gear-wheel can be moved along the splined shaft by selector mechanism to engage similar dogs machined on the front face of another gear wheel also mounted on the main shaft but not keyed to it. The sliding gear wheel cam also be moved in the opposite direction, so that the teeth engage those of a large gear wheel fixed on the lay shaft (Idler shaft).
(ii) The lay shaft carries two fixed gear wheels. A large one, which when engaged by the sliding gear wheel of the main shaft transmits the drive and gives a gear reduction. A smaller gear wheel at the rear end of the lay shaft is in constant mesh with a wheel on the main shaft and with one on the output shaft connected to the rear axle propeller shaft. The wheel on the main shaft is freely mounted, but the wheel on the output shaft is fixed on that shaft.
(iii) There are two output shafts, the front one being connected to the front axle propeller shaft and the rear one to the rear axle propeller shaft. The front output shaft is splined and carries a sliding dog which can be moved along the splines by selector mechanism to engage dogs cut on the front face of the gear wheel fixed to the rear axle output. The rear end of the front output shaft is supported in spigot formed in the rear output shaft.
(b) Operation . When the high gear and rear axle drive only is selected, the sliding gear wheel on the main shaft is moved along the splines and its dog engage those of the constant mesh gear that is freely mounted on the main shaft. The drive is then transmitted through the main shaft and constant mesh gears to the rear output shaft and there is no gear reduction. When the low gear and four wheel drive is selected then the sliding gear on the main shaft moves along the splines in a forward direction and its teeth engage those of the large gear wheel fixed to the lay shaft. At the same time the sliding dog on the output shaft connected to the front axle propeller shaft, is moved rear-wards to engage the dogs cut on the front face of the constant mesh wheel fixed to the output shaft which is in turn connected to the rear axle propeller shaft. Therefore, the drive line is from the main shaft to the lay shaft (with gear reduction) the constant mesh gears to both output shafts, so that the drive is conveyed to the front and rear axles.