Clutch Brake

What is a clutch brake on manual heavy duty truck transmissions?
In: Cars and Vehicles, Clutches and Flywheels, Mechanical Engineering [Edit categories]


A clutch brake is a circular disc with a friction surface that is splined to the input shaft of the transmission between the release bearing and transmission. Its purpose is to slow and/or stop the input shaft from turning to allow initial forward or reverse gear engagement in non-synchronized transmissions. It helps prevent premature wear of the internal transmission parts during those initial shifts.
To understand this in more detail, let's consider what is going on when a heavy truck transmission is shifted from neutral to first or reverse. When a truck is in neutral and the clutch pedal is not depressed, the master clutch is engaged. This causes power from the engine to drive the transmission input shaft, resulting in rotation of some shafts and gears in the transmission. However, the gears aren't engaged or meshed in a way that transmits power to the drive shaft. In other words, parts of the transmission are just idling but spinning.

When the clutch pedal is depressed, it moves the release or throw-out bearing toward the transmission and away from the flywheel. This unloads the springs in the master clutch, releasing the master clutch and decoupling the flywheel from the transmission output shaft.

Without the engine driving the transmission input shaft, the spinning shafts and gears in the transmission start to coast down. However, it would take a long time for these rotating masses to slow down, because they have a lot of mass and the only thing that is slowing them is gear and bearing drag.
If everything is properly adjusted, the master clutch is opened or released when the pedal is depressed about half way or so.
If there's no clutch-brake, and you then shift into first or reverse, the rotation of gears in the transmission won't all be the same on a non-synchronized transmission, and then the gears can clash or make a "thunk" as gears that are spinning are meshed with gears that are not spinning. Over time, this can wear or even damage the gears.
When a clutch-brake is installed, pushing the clutch pedal all the way to the floor pulls the release bearing even further away from the master clutch toward the transmission, which eventually squeezes the clutch-brake disc between the release bearing and the transmission case. Friction between these parts then slows and soon stops the transmission input shaft from rotating.
Now, gears can be moved into mesh in the transmission and there might be a bit of noise because the teeth aren't perfectly aligned, but they won't be rotating. If you time everything just right, you'll make the shift from neutral to first or reverse just as the gears are almost stopped, but not quite. A little rotation helps ease the meshing and can almost eliminate noise or "thunk" sounds.

The clutch-brake is unusual because it always picks up grease from the release bearing. Modern clutch-brakes are designed to operate with grease, and they even require greasing or oiling before installation.

Clutch-brakes fail for two reasons. First, over time, the friction material will wear out. At some point, there will be metal-to-metal contact in the "sandwich" consisting of the bearing, clutch-brake plate, and the transmission cover. This might still slow the gears, but it can cause damage to the bearing or the transmission cover. It's also possible that the release bearing won't be able to move far enough toward the transmission to squeeze the disc, and the clutch-brake won't slow down the transmission shaft because there won't be friction in the "sandwich."

If you keep the pedal linkage properly adjusted and periodically grease the release bearing and clutch-brake disc, the disc can last for a long time.

However, the most common cause of clutch-brake failure is depressing the clutch pedal all the way to the floor when shifting during a range change--when the truck is moving. When you do this, you are forcing the little clutch-brake to try and stall the engine and stop the truck! Doing that only one time will often destroy the clutch-brake completely. This can also occur if the pedal linkage isn't properly adjusted and the release bearing squeezes the clutch-brake into the transmission case when making a range gear change.