Hi Roger
It is becoming harder to find interesting articles which we have not covered
here or which are not on our Blind Handy Man Files Area.
But the Research Team is constantly looking
Regards
Ray
________________________________
From: blindhandyman@yahoogroups.com [mailto:[EMAIL PROTECTED] On Behalf
Of Roger Bachelder
Sent: Tuesday, July 10, 2007 8:23 AM
To: blindhandyman@yahoogroups.com
Subject: RE: [BlindHandyMan] How a Clutch Works
Great post Ray, with out a doubt, a keeper!
Roger C Bachelder 3rd
[EMAIL PROTECTED] <mailto:Bachelder3%40verizon.net>
_____
From: blindhandyman@yahoogroups.com
<mailto:blindhandyman%40yahoogroups.com> [mailto:blindhandyman@yahoogroups.com
<mailto:blindhandyman%40yahoogroups.com> ]
On Behalf Of Ray Boyce
Sent: Wednesday, July 11, 2007 5:44 PM
To: blindhandyman@yahoogroups.com
<mailto:blindhandyman%40yahoogroups.com>
Subject: [BlindHandyMan] How a Clutch Works
Hi Everyone
If you've ever driven a vehicle with a manual transmission, you know
how to
depress the clutch, select a gear, and release the clutch while
applying
power
to get the car to move. But what really happens when you depress and
release
the clutch? Let's get to the bottom of that question.
In its simplest form, the clutch allows engine power to be applied
gradually
when a vehicle is starting out and interrupts power to avoid gear
crunching
when shifting. Engaging the clutch allows power to transfer from the
engine
to the transmission and drive wheels. Disengaging the clutch stops the
power
transfer and allows the engine to continue turning without force to the
drive wheels. To understand how a clutch works, we first need to
understand
who
the players are and how the whole shebang works. So let's look at the
basic
components: the flywheel, clutch disk, pressure plate, throw-out
bearing and
linkage.
A large steel or aluminum "disc," the flywheel is bolted to the
crankshaft
of the engine. The flywheel does many things - acts as balancer for the
engine,
dampens engine vibrations caused by the firing of each cylinder, and
provides a smooth-machined "friction" surface that the clutch can
contact.
But its
main function is to transfer engine torque from the engine to the
transmission. The flywheel also has teeth along the circumference,
allowing
the starter
motor to contact when turning the engine over.
The clutch disc is basically a steel plate, covered with a frictional
material that goes between the flywheel and the pressure plate. In the
center of the
disc is the hub, which is designed to fit over the spines of the input
shaft
of the transmission. When the clutch is engaged, the disc is "squeezed"
between
the flywheel and pressure plate, and power from the engine is
transmitted by
the disc's hub to the input shaft of the transmission.
In layman's terms, a pressure plate is a spring-loaded "clamp," which
is
bolted to the flywheel. It includes a sheetmetal cover, heavy release
springs,
a metal pressure ring that provides a friction surface for the clutch
disc,
a thrust ring or fingers for the release bearing, and release levers.
The
release
levers lighten the holding force of the springs when the clutch is
disengaged. The springs used in most pressure plates are of a
diaphragm-type, however
a few use multiple coil springs. Some high-performance pressure plates
are
"semi-centrifugal," meaning they use small weights on the tips of the
diaphragm
springs to increase the clamping force as engine revolutions increase.
The "throw-out bearing" is the heart of clutch operation. When the
clutch
pedal is depressed, the throw-out bearing moves toward the flywheel,
pushing
in
the pressure plate's release fingers and moving the pressure plate
fingers
or levers against pressure plate spring force. This action moves the
pressure
plate away from the clutch disc, thus interrupting power flow.
Mounted on an iron casting called a hub, the throw-out bearing slides
on a
hollow shaft at the front of the transmission housing. The clutch fork
and
connecting
linkage convert the movement of the clutch pedal to the back and forth
movement of the clutch throw-out bearing. To disengage the clutch, the
release bearing
is moved toward the flywheel by the clutch fork. As the bearing
contacts the
pressure plate's release fingers, it begins to rotate with the pressure
plate
assembly. The release bearing continues to move forward and pressure on
the
release levers or fingers causes the force of the pressure plate's
spring to
move away from the clutch disc. To engage the clutch, the clutch pedal
is
released and the release bearing moves away from the pressure plate.
This
action
allows the pressure plate's springs to force against the clutch disc,
engaging the clutch to the flywheel. Once the clutch is fully engaged,
the
release
bearing is normally stationary and does not rotate with the pressure
plate.
Now that we have the parts, how do they all work together? Thankfully,
it's
not rocket science.
A mechanical or hydraulic linkage usually operates the clutch in a
manual
transmission. If your vehicle has a mechanical linkage, it is usually
either
a
cable or shaft and lever style. The shaft and lever linkage has many
parts
and various pivot points, including a release lever and rod, an
equalizer or
cross shaft, a pedal to equalizer rod, an "over-center" spring (to
return
the clutch pedal to the rest position), and the pedal assembly that
transfers
the movement of the clutch pedal to the throw-out bearing. In older
vehicles, these pivot points need to be lubricated properly on a
regular
basis to keep
the movement buttery smooth and prevent wear.
If you have a newer vehicle, you're lucky, as pivot points are now
fitted
with low-friction plastic grommets or bushings. As the older
"lube-it-yourself"
pivot points wear, the extra play in the linkage makes clutch pedal
"free-play" adjustments difficult. When the pedal is released, the
assist
spring returns
the linkage to its normal "up" position and removes the pressure on the
release rod. This action causes the release bearing to move away from
the
pressure
plate.
A cable-type clutch linkage is simple, lightweight and is the most
common
linkage on newer cars today. Normally, a cable connects the pivot of
the
clutch
pedal directly to the release fork. This simple design is flexible,
compact,
and eliminates nearly all of the wearing pivot points found in a shaft
and
lever linkage. There is one downside to this type of setup: cables will
gradually stretch and can break due to excessive wear and electrolysis.
On a typical installation, one end of the cable is connected to the
clutch
pedal and a spring is attached to the pedal assembly to keep the pedal
in
the
"up" position. The other end of the cable is connected to the clutch
release
fork with a fitting that allows for free-play adjustments. When the
clutch
pedal is depressed, the cable pulls the clutch fork, causing the
release
bearing to move forward against the pressure plate.
Commonly found in mid- and rear-engine vehicles, a hydraulic clutch
linkage
is basically a mini hydraulic brake system. A master cylinder is
attached to
the clutch pedal by an actuator rod, and the slave cylinder is
connected to
the master cylinder by high-pressure tubing. The slave cylinder is
normally
attached to a bracket next to the bell housing, so that it can move the
clutch release fork directly.
Just like depressing the brake pedal on your car, depressing the clutch
pedal pushes a plunger into the bore of the master cylinder. A valve at
the
end
of the master cylinder bore closes the port to the fluid reservoir, and
the
movement of the plunger forces fluid from the master cylinder through
the
tubing
to the slave cylinder. Since the fluid is under pressure, it causes the
piston of the slave cylinder to move its pushrod against the release
fork
and bearing,
thus disengaging the clutch. When the clutch pedal is released, the
springs
of the pressure plate push the slave cylinder's pushrod back, which
forces
the hydraulic fluid back into the master cylinder. The biggest plus to
a
hydraulic linkage is the physics: a small amount of pedal force can be
used
to
manipulate what would normally be a heavy clutch with a shaft and lever
linkage.
Now that you know what happens when you depress the clutch pedal, what
are
the warning signs that a clutch needs adjustment or replacement? While
most
new
car clutch linkages are self-adjusting, there are some telltale signs
that
will tell you if adjustment is needed. For instance, if the clutch
engages
and
disengages close to the floorboard or the transmission "grinds" when
shifting, your clutch may need attention. Does the clutch pedal move
easily,
but the
transmission will not go into gear? More than likely, the clutch
linkage has
become disconnected or a clutch cable has snapped. If the clutch slips
(doesn't
fully engage), the linkage could be grossly out of adjustment, or the
clutch
disk could be worn to the point of replacement. Clutch "chatter" is
often
caused by an overheated clutch (normally from "slipping" the clutch
when
starting on an incline) or from oil on the clutch disk. In either case,
the
clutch
must be replaced. No matter what symptoms your vehicle may have, always
consult with a certified ASE mechanic to diagnose the problem properly.
Although it may seem like there's not much to getting your car in and
out of
gear, a lot is going on behind the scenes each time you depress the
clutch
pedal. Now you have something to think about each time you're faced
with
rush-hour traffic.
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