What is a Sliding mesh gearbox?
Sliding mesh gearbox is a transmission system that consists of various sets of gears and shafts that are arranged together in an organized fashion and the shifting or meshing of different gear ratios is done by the sliding of gears towards the right and left over the splined shaft with the help of a gear lever operated by the driver.
An obsolete type of transmission or gearbox in which the gears on the layshaft are fixed to the shaft rigidly, whereas the gears on the main shaft can slide on it by means of splines but are otherwise permanent rotational mesh with the shaft. The figure below shows the arrangement of the sliding mesh gearbox.
Sliding Mesh Gearbox was the first gearbox or transmission system invented in the automobile. The first transmission system was given by French Inventors Louis-Rene and Emile Levassor who invented the world’s largest-ever transmission system with 3-speed sliding mesh transmission in 1894. The gear ratio is achieved by sliding the required gears to bring it with appropriate mating gears.
Parts of Sliding Mesh Gearbox:
The main parts of the sliding mesh gearbox are:
- Gear Lever
The shaft is generally used to transmit the power from one end to another end. There are three types of shafts in the case of Sliding Mesh Gearbox.
- Clutch Shaft: The clutch shaft is used to transmit the power from the engine when the clutch is in the engaged position. When the clutch shaft is rotating, the clutch gear which is attached at the end of it also rotates along with the clutch shaft.
- Lay Shaft: The layshaft is an intermediate shaft between the clutch shaft and the Main shaft which provides meshing of fixed gears to the movable gears to provide the output appropriately. The fixed gears are attached to the layshaft and they are in mesh with the gears of the main shaft and the gear of the clutch shaft.
- Main Shaft: This shaft acts as an output shaft for the transmission of power from the engine shaft via layshaft. The gears with internally splined grooves are arranged on the main shaft so that they can mesh easily with the gears of the layshaft.
Usually, two types of gears were used in the sliding mesh gearbox. They are: –
- Spur gear: Spur gears have straight teeth that are produced parallel to the axis of the gear. These gears are the most economical types of gear but tend to vibrate and become noisy at high speed.
- Helical gear: The teeth of helical gears are not parallel to the gear axis. The teeth of this gear type are at an angle to the gear axis. These gears are less noisy and have a smoother operation than a spur gear. Also, these gears have higher tooth strength and a higher load-carrying capacity.
It is used to slide the gears in the main shaft to obtain the appropriate gear ratio. It is operated by the driver.
Construction and Working of sliding mesh gear box
The gear connected to the clutch shaft also rotates. As the gear of the clutch shaft rotates, the layshaft gear which is connected to the clutch shaft gear also rotates but in opposite direction. So, the layshaft rotates due to the rotation of layshaft gear that is rigidly fixed in the layshaft.
Sliding mesh gearboxes are made with spur gear. The gears in the layshaft are rigidly fixed shafts. The gears in the main shaft alone can move along the spline. The selector fork moves the gear and moves the perfect mesh with the required gear in layshaft when the gear lever is shifted.
An idler gear is provided for reverse gearing. Thus, the power that forms the clutch shaft is transmitted to the main shaft through the layshaft.
- First gear: In the first gear, the gearbox provides maximum torque at low speed. The smallest gear on the layshaft engages with the biggest gear in the main shaft, thus transmitting high torque. First Gear is used at the time when the vehicle starts its movement in a forwarding direction.
- Second gear: In the second gear, the gearbox provides low torque and high speed when compared to 1st gear. Second Gear is obtained when second largest gear of second smallest gear of layshaft meshes with middle size gear of the main shaft.
- Third gear: In the third gear, the gearbox provides low torque and high speed when compared to 2nd gear. The third gear is the last gear or top gear of the Sliding Mesh Gearbox. This gear is obtained when the biggest gear of layshaft meshes with the smallest gear of the main shaft.
- Fourth gear: For the fourth gear, the clutch shaft and the main shaft are engaged by a dog clutch, thereby the main shaft rotates at maximum speed as that of the clutch shaft.
- Reverse gear: When selecting reverse, the direction of rotation of the output shaft is changed This is achieved by using an idler gear. The idler gear meshes between a countershaft gear (layshaft or 2nd motion shaft) and an output shaft gear (main shaft or 3rd motion shaft).
Advantages of Sliding Mesh Gearbox:
- Since only one gear is in mess in the sliding mesh gearbox so less fluctuating loads on shafts causing less vibration and noise
- unlike the contact mesh gearbox in which all gears are in constant mesh.
- Its efficiency is more than a constant gearbox as only one gear is in mess, unlike the contact mesh gearbox in which all gears are in constant mesh.
- Its manufacturing is easy as compared to a constant mesh gearbox.
- Its mechanism is simple.
Disadvantages of Sliding Mesh Gearbox:
- Only spur gears can be used as gears are not in a constant-mesh gearbox in which helical or herringbone gears can be used.
- More effort is required to engage the gear as the gear has to be slide in sliding mesh gearbox, unlike constant mesh gearbox where only dog clutch has to be a slide for engagement of different gears.
- Less life of gear as more wear and tear of gear is caused in sliding mesh gearbox due to friction.
- It takes more time and money to replace the gears if the gearbox fails but in constant mesh gearbox only dog clutches are to be replaced at failure which takes less time and money.
- Sliding Mesh Gearbox is used in Alfa 12HP with 4-speed manual transmission.
- It is also used in Fiat 6HP with 3-speed manual transmission.
- It is also used in Mercedes 35 HP and Renault Voiturette.