Advice: Clutch brakes explained

One of the most common questions we get asked is, what is a clutch brake? Closely followed by, how does it work?

In this tech topic, we'll explain the function a clutch brake plays, how and when to operate it and how to check it is properly adjusted.

We are not suggesting that after reading this article you'll be able to slip under the truck and adjust your clutch brake. We are, however, attempting to provide an in-depth understanding of its operation so you'll be able to better utilise it and maximise its life, and also know when it requires adjustment or replacement.

The main purpose of the transmission cultch brake is to stop the rotation of the transmission gears and input shaft, allowing quicker engagement of a selected gear, either first or reverse, when moving off from an initial start. Clutch brakes are typically used with pull-type clutches such as those used with a manual (non-synchro) Roadranger transmission.

Related reading:
Advice: Emissions systems explained
Advice: Buying used
Advice: Compact tipper versus ute

A standard clutch brake has a simple construction comprising a steel washer splined to the transmission input shaft that is sandwiched between two fibre discs.

A torque-limiting clutch brake is typically installed in new trucks by the OEM and is recommended for all severe-duty applications. It features a hub with self-contained Belleville washers that are designed to slip after approximately 12ftlb (16Nm) of torque. This provides smoother engagement, and protects the clutch brake from overloading or tang breakage resulting from driver abuse. It is a one-piece design that can only be installed with the transmission removed.

Two-piece clutch brakes work in the same manner as standard clutch brakes, but their main benefit is the ease with which they can be installed and replaced – a process that doesn't necessitate the removal of the transmission.

Tools like the Kiene Diesel K-1360 one-piece clutch brake remover is designed for removing a one-piece clutch brake should it fail, without requiring the removal of the transmission.

To use the K-1360, place the drill guide on the clutch brake, clamp it down tightly and drill through the brake up to the collar. Remove the bushing and finish the cut with the chisel, then rotate the clutch brake to repeat the process on the other side.

The clutch brake cutter kit comes with a chisel, drill collar, drill guide and drill bushing. Now a two-piece clutch brake can be easily installed.

To check the clutch brake squeeze, first ensure the clutch has the correct release travel and free travel settings. The pedal should be between 12-25mm from the end of the stroke at the beginning of the clutch brake squeeze. Make sure the vehicle is stationary, park brakes applied, wheels chocked and that the engine is stopped and the vehicle tagged out, so it cannot start accidently.

To check this, you will need an assistant to push the clutch while you insert a 0.25mm feeler gauge between the release bearing and the clutch brake.

Get your assistant to depress the clutch pedal to squeeze and hold the feeler gauge. Then get them to slowly let the pedal up. As soon as the feeler gauge can be removed, get them to stop raising the pedal. The pedal should be between 12-25mm from the end of the pedal stroke for proper clutch brake squeeze.

When driving and changing between gears on the move it is important to remember not to push the clutch pedal all the way to the floor, as this will cause serious damage to the clutch brake. The clutch brake should only be used when the vehicle is stationary and a gear selection from neutral is required.

It is vitally important to make sure the vehicle is not rolling when the clutch brake is being engaged, as this too can cause serious damage to the clutch brake.