what's a throwout bearing

https://www.niushangcn.com/resources/a-throwout-bearing-we-tell-you-here.html

Technically speaking, the correct terminology for a throwout bearing is release bearing, as in clutch release bearing. When considered in those terms, it's easier to imagine what this component is and what it does. The throwout bearing is the last piece in the series of components that composes the clutch linkage from the pedal to the clutch assembly inside the transmission bellhousing.

It's a bearing, meaning that it allows interaction between a moving surface and a stationary surface. Just like wheel bearings allow movement of the spinning wheel hub around the non-moving spindle, the throwout bearing is the component that comes between the clutch pressure plate, which is spinning when the engine is running, and the non-moving clutch fork- or the hydraulic slave cylinder typically found in newer cars. Although there are a few different ways a clutch can be actuated, mechanical or hydraulic, the function of the throwout bearing is the same. It connects moving and non-moving parts.

The throwout bearing is made up of a base, which is attached to the linkage assembly, the outer shell that pushes against the pressure plate, and the bearing assembly located between the two.

In a clutch assembly, the pressure plate is bolted to the flywheel and is rotating any time the engine is running. The friction disc slips over the splines of the transmission's input shaft and only turns when the drive wheels are turning.

The release bearing is held by the stationary clutch fork and, when the clutch pedal is pressed, pivots forward to contact the diaphragm spring of the pressure plate, which act like a lever to disengage the clutch, separating the mechanical link between the engine and the transmission's input shaft.

Clutch linkages are either mechanical or hydraulic. Hydraulic clutch systems will have either an external slave cylinder or an internal slave cylinder. Mechanical systems include actuating rods and a bellcrank, commonly referred to as a Z-bar, or via a cable between the pedal and clutch fork.

Most cars made since the late '90s have hydraulic clutch systems with an internal slave cylinder, meaning the slave cylinder is located inside the transmission bellhousing.

The slave cylinder is housed within the base of the bearing assembly. When you press the clutch pedal, fluid from the clutch master cylinder is forced through hydraulic lines into the base and to a piston below the bearing assembly. That forces the bearing surface to push the diaphragm springs of the pressure plate. In this photo, the long tube is the bleeder fitting, the fluid inlet port is located below it.

The bearing surface of many internal slave cylinders is designed to maintain contact with the pressure plate. Its outer surface is always spinning, whether the clutch is engaged or disengaged. It is self-adjusting, meaning that the spring between the bearing and base puts tension on the bearing, keeping it in contact with the pressure plate. This accommodates for clutch wear.

Some bearings of internal slave cylinders are not designed to be constantly spinning. In this case, an air gap is required between the spinning clutch assembly and the throwout bearing. The same is generally true for a system that includes a clutch fork, whether that system is fully mechanical or hydraulic. Specific air gap specs depend on application but can be found in a service manual or instructions included with the kit.

A crude but effective example of how a throwout bearing works is demonstrated on a common rolling work stool. Because the seat spins freely, we're using it as a stand-in for the engine's flywheel. The pressure plate bolts to it with the friction disc sandwiched in between. With your author's hand as the clutch fork, notice how the base of the throwout bearing remains stationary as the face spins with the pressure plate. Pushing on the pressure plate diaphragm spring disengages the clutch.

Symptoms of a Bad Throwout Bearing

The throwout bearing is a simple device that should provide years of long life. It is made with a sealed bearing assembly that doesn't require maintenance or lubrication. When a bearing goes bad, you will hear a grinding or whirring noise whenever the bearing is spinning. In a system where the bearing is in constant contact with the pressure plate, you will hear the noise all the time, though it may be louder when the clutch pedal is pushed in, putting the greatest load on the bearing surface. In systems with an air gap between the bearing and pressure plate, you will only hear the noise when the clutch is pushed in and the bearing is spinning. The sound will go away when you take your foot off the clutch pedal. If your throwout bearing is bad, you have no choice but to remove the transmission to replace it. That's why it can cost so much money to replace an inexpensive part.