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Counter Pro Course -
Clutches

What's a "Clutch?"



There are about 40,000,000 passenger cars and light trucks on the road which don't have automatic transmissions. They have "stick shifts" or "manual transmissions". And each and every one of these vehicles has a device called a "clutch" sitting between its engine and transmission.

In spite of this huge number of manual transmission vehicles, there is a good possibility that you may have never driven a car with a clutch in it. If you are a performance car enthusiast, drag racer or a sports car fan, or, in fact, enjoy almost any kind of serious automotive performance sport, you probably have driven a manual transmission.

Nevertheless, even if you've driven one, there is still the possibility you might not know much about the clutch you've used.

Indeed, clutches are a "mystery" to many professional technicians, as well. If this elite group of automotive pros is occasionally in the dark about the clutch you can't afford to be, because you, as a counterman, are responsible for handling the parts requirements of the professional shop. You're the expert with all the answers!

Therefore, this training course is designed to help you understand the following:

1. What a clutch is.
2. How it operates.
3. How it fails during normal operation.
4. How it fails when it is removed or installed incorrectly.
5. How to identify various faults, visually, so you can tell your professional (and DIY) customers when to look for evidence of problems which can cause premature failure of newly replaced clutch units.
6. How to handle some of the more difficult problems associated with clutches when working at the jobber counter.

Ready to learn? Read on!

Who needs a "Clutch?"

About 85% of all passenger cars (and nearly as large a segment of the light truck market) have automatic transmissions. Automatic transmissions "think for the driver" and do all of the power management which sends the rotary motion of the engine's crankshaft to the drive wheels of the car.
The other 15% of passenger cars and light trucks use what are called manual transmissions. In this type of system, the driver of the car selects various gears in the transmission or gear box to do the job of moving the drive wheels of the car. The transmission does no "thinking" the driver does it, all by himself.

Why a car needs a transmission
Engine/Vehicle speed differentials.



Gears are used as a method of "stepping down" the speed of the engine to meet the speed of the road wheels, at any time the vehicle isn't going at highway cruising speeds.

Engines, as you know, spin at RPMs from about 900 RPM (idling) to 5,000 RPM (top cruising speed or passing speeds). And while engine speed can rise and fall very quickly, the vehicle in which the engine is used has plenty of inertia and moves (relatively) quite slowly.

Large differences between engine and road wheel RPM's require a transmission or "gearbox" to allow the difference to exist.

The car's road wheels and tires typically rotate much more slowly than most people imagine. For instance, at 30 mph, a typical 13" tire rotates at about 450 RPM. The engine, on the other hand, may be rotating at several thousand RPM while turning the tires that slow speed.

In order to provide this difference in rotational speeds between the engine and road wheels, a gearbox or transmission is certainly required.

Also, because of the vehicle's inertia, it takes a multiplication of the engine's power to get the vehicle accelerating more or less quickly when moving up to speed from a dead stop. Gears can be used to provide this multiplication of power. (A fact you should have learned in grade school science or high school physics.)

Here is a fairly typical road speed range and RPM range for an average sort of passenger car or light truck. We are giving this hypothetical vehicle a four-speed transmission, although there are plenty of vehicles on the road which have three-speed, five-speed and even six-speed transmissions, as well.

At each of the gear changes noted, the transmission lever is moved from one position to another within the gearbox. In the case of this vehicle accelerating from rest to highway cruising speed, the driver "upshifts" from 1st, to 2nd, to 3rd and finally to 4th gear. On the other hand, when the driver goes from high speed to a stop, he typically "downshifts" from 4th, to 3rd, to 2nd, and into neutral or lst gear matching the engine speed to the road speeds. He does so to keep everything running smoothly and under maximum control.

The purpose of a clutch

At each of these "shift points", the engine must be disconnected from the transmission in order to allow the gears in the transmission to slow down and mesh. If the engine is not disconnected from the transmission during this shifting interval (which normally takes about 1/2 to 1 second), or crash, etc. Any of these words are fairly descriptive of the nasty sound resulting from failure of the gears to mesh at reasonably matching speeds. And of course, when the car comes to a complete stop (such as coming to a stop sign or light), the driver may want to keep the car in a drive gear. The engine would stop at the same time the vehicle stopped its forward motion, unless the clutch was used to allow the driver to keep the engine disconnected from the transmission. He does so by keeping the clutch pedal pressed (and therefore, the engine disengaged) until he is ready to get the car moving again. As we say, the device used to disconnect the power of the engine from the drive system within the gearbox is the clutch.
Clutch pedal is located on left of brake pedal, so driver can operate the clutch while the right foot can handle the chores of braking and matching engine speed to road speed during gear changes using the accelerator.	Clutch pedal uses linkages to connect with mechanism in bell housing.
The driver operates the clutch at will, when needed. He does this by pressing a pedal which is to the left of the brake pedal, underneath the dashboard of the car. (This clutch pedal mechanism is connected, using either a mechanical linkage of levers, bellcranks, rods, cables or other relatively direct hookup devices or a hydraulic master cylinder/slave cylinder system to the clutch mechanism closer to the bell housing on the engine.)
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