Constant Mesh Transmission

A constant mesh transmission refers to a gearbox where all or most of the mating gears are always in mesh. This is in contrast with the older style sliding gear gearbox in which gears only engage via the shifting process. Thus, gears in the constant mesh gearbox are less susceptible to wear, and the system is more driver-friendly. Today, the constant mesh transmission has been largely supplanted by the more modern synchromesh transmission.

In this article, you will learn how the constant mesh transmission works, its functionality vs a synchromesh transmission, and common applications.

How it Works

Before reviewing how a constant mesh transmission functions, one must assess its components.

Components

Components include three shafts, dog clutches, gears, and a gear lever.

Shafts

The constant mesh transmission has three shafts which include:

• Clutch/Input Shaft: This is the shaft that feeds the engine output into the gearbox.
• Lay/Counter Shaft: This is an intermediate shaft between the clutch shaft and main shaft. Moreover, the gears on this shaft are not free to rotate as they are directly mounted on the lay shaft. Also, its gears are in constant mesh with those on the main shaft.
• Main/Output Shaft: This is a splined shaft on which gears and the dog clutches are mounted. In contrast to the lay shaft, the gears here do not have a direct mounting on the shaft, so are free to rotate about it. On the other hand, the dog clutches are directly mounted on this shaft.

Dog Clutches

This is the main feature that distinguishes the constant mesh configuration from the original transmission configuration, known as the sliding mesh. These dog clutches serve to couple the lay and main shaft by interference, rather than friction. As a result, they eliminate the need for the sliding of gears to mesh with each other.

Gears

The gears of a constant mesh gearbox come in pairs of different gear ratios. Basically, two types of gear that are used are:

• Bevel Gears: The gears have a conical cross-section with angular cut teeth.
• Helical Gears: These have a cylindrical cross-section with angular cut teeth.

Gear Lever

The gear lever, also known as a selector fork, shifts the dog clutch to interfere with the driver’s desired gear ratio.

Constant Mesh vs. Sliding Mesh

The main difference between the sliding mesh and constant mess transmission is the introduction of the dog clutch. For the sliding mesh, the gear ratio that the driver wants is engaged by sliding the output shaft over until meshing occurs. To achieve this, the driver needs advanced skills to synchronize the lay and output shaft speeds and ensure smooth engagement of the gears. Nevertheless, some crashing of the gears may occur. This makes the gear teeth susceptible to wear.

A constant mesh gearbox evades this shortcoming by keeping the gears on the layshaft and output shaft constantly in mesh. Moreover, the gears do not mount directly on the output shaft. Rather, they couple to bearings. As a result, they can rotate without transmitting torque to the output shaft. So, to engage a gear ratio, the driver uses the gear lever to move the dog clutch to contact any of the gears on the output shaft. Because the dog clutch has a direct mounting to the output shaft, torque transmits to the shaft. If the dog clutch is not in contact with any other gears, the main shaft does not rotate. The advantage of this system includes:

• The gear teeth are less prone to damage in comparison to the sliding mesh gearbox.
• This design limits any damage that occurs while engaging the gears to the dog clutch. Thus, the gear wheels can last longer.
• A constant mesh gearbox is quiet because of the use of helical/bevel gears, instead of spur gears.

Constant Mesh vs Synchromesh

A synchromesh transmission provides refinement vs. the constant mesh transmission. Similarly, the gears of both transmission systems are always in mesh, and they both deploy dog clutches. Moreover, their difference lies in the structure and functionality of the dog clutch as the table below highlights.

Constant Mesh Transmission	Synchromesh Transmission
The dog clutch acts alone in engaging the gears.	A cone clutch/collar engages the gear first to synchronize the speed of lay and output shafts before the dog clutch latches the gear.
There is no cone-shaped brass clutch for synchronizing shaft speeds.	A cone-shaped brass clutch is attached to each gear to enable the collar from the dog clutch to latch on.
There is a need for the driver to double de-clutch when shifting gears.	Double de-clutching is not needed due to the synchronization of shaft speeds.
Slip condition between the dog clutch and gears.	No-slip condition between the dog clutch and gears.

Because of the synchronizing feature, the synchromesh transmission gives a smoother operation than the constant mesh gearbox. Therefore, it enjoys preference in modern manual transmission systems.

Applications of Constant Mesh Transmission

After the invention of the manual transmission in 1894, with the sliding mesh gearbox, the automobile industry enjoyed a significant boom. Since then, several developments of the manual transmission system have been rolled out. The first was the constant mesh transmission to overcome gear crashing issues of the sliding mesh system.

However, the introduction of synchromesh transmission in 1928 by General Motors, led to a gradual phasing out of the constant mesh gearbox. Although, it was still common to find vehicles with constant mesh until the 1950s. Beyond the 1950s, the constant mesh transmission has largely become obsolete, with synchronized systems becoming the standard for manual transmission. Prior to the 1950s, the constant-mesh gearbox was common in:

• Farm trucks and other heavy-duty machinery.
• Motor bikes.
• Cars such as Ford Model T.

In present times, it is unlikely to find a manual transmission with a constant mesh or sliding mesh. Modern vehicles and machinery use synchronized systems.