Manual Transmission Clutches

[Counterman]

The terms manual, standard or stick shift all refer to the same exact thing: a type of transmission that requires the driver to select and change the gears. Even though there are few new cars today that require this input from the driver, it is preferred by many, and there are still many of them on the road.

Automatic transmissions are in the majority of all new vehicles, however a manual transmission has some advantages in efficiency and performance, and during the last 20 years, even if a car outwardly appears as if it’s an “automatic,” it might actually be a dual clutch transmission, or DCT for short.

A DCT is basically an automated manual transmission. As the name suggests, it has two clutches. The traditional manual transmission that we’ve known for years has one clutch that you engage or disengage by using the clutch pedal to the left of the brake pedal, and you also use a manual shifter to select the gears. They also have one input shaft, which is splined to a clutch disc, that transfers power from the engine into the transmission.

A DCT has two clutches, but also two input shafts, each one splined to its own clutch, and that’s the key in how they work. The clutches and the shifter in a DCT are controlled by a combination of electronics and hydraulics, so no clutch pedal or input from the driver is needed. It’s all done by a computer.

What does this mean to you as a counter-professional? DCT clutches are often clutch packs, and while theoretically share the same functional aspects, they generally differ from the clutches of a traditional manual transmission. These traditional manual transmissions are popular among enthusiasts and their clutches are still a common service item, so let’s take a look at how they work.

There are three main components involved, the flywheel, friction disc and pressure plate. The flywheel is bolted to the crankshaft and has a machined surface for contact with the friction disc.

The pressure plate, which is an assembly made up of the clutch cover, pressure plate and diaphragm spring, bolts to the flywheel, so the flywheel and pressure plate are always moving at engine speed. The friction disc is sandwiched in between the two and it’s the friction disc that is splined to the input shaft of the transmission.

When the clutch is engaged, the diaphragm spring applies force to the pressure plate to tightly grip the friction disc between it and the flywheel, so the power of the engine flows into the transmission. When the clutch is disengaged, a throw-out bearing pushes on the center of the diaphragm spring, causing it to pull the pressure plate away from the friction disc, letting it slip freely so no power flows into the transmission.

The throw-out bearing is located on the end of the clutch fork, a lever that transfers the motion from the control side of the clutch system, which can be linkage, cable or hydraulically operated.

Due to the advantage of smooth operation and low maintenance, hydraulic clutch control systems are the most popular today, utilizing a master cylinder at the clutch pedal and slave cylinder at the clutch fork. Some systems eliminate the clutch fork, integrating the throw-out bearing onto the end of the slave cylinder.

When a customer is replacing a clutch, the most important aspect of the service is that they get a complete kit with a new friction disc, pressure plate and throwout bearing. Flywheels can often be resurfaced, and they should be resurfaced or replaced. Reusing a flywheel can cause immediate damage to a new disc and at minimum shorten the life of the clutch.

Depending on the design of the transmission, there may be a pilot bearing or bushing located in the end of the crankshaft which supports the input shaft of the transmission. Be sure this is replaced at the same time, and it’s also a good time to replace the flywheel and pressure plate bolts, as well as inspect and replace any worn clutch control components. 

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