Hydraulic clutch release systems
On vehicles with foot-actuated single disc clutch, a mechanism is required to transfer power from the pedal to the clutch. Developing such a mechanism has brought forth a number of different solutions. Originally, a cable was used to transmit the pedal forces to a lever mechanism in the clutch bell; the clutch then was actuated by the lever and a release bearing. Today, these systems have only little share of the market, as it is more and more diffi cult to install the clutch cable in a straight line between the pedal and the lever due to less and less space available in the engine compartment. Installing a cable in narrow radii is impossible as friction and wear rise to impermissible levels and driving comfort and clutch actuation smoothness are impaired. Therefore, modern foot-actuated clutch systems use hydraulic clutch actuation mechanisms. There are two basic systems: in semi-hydraulic systems the clutch cable is replaced by a hydraulic system consisting of a master cylinder at the pedal, a hydraulic line and a slave cylinder on the gearbox outside. On release systems with concentric slave cylinder (CSC) the lever in the transmission housing and the conventional release bearing are replaced by a hydraulic cylinder with integrated bearing which is located inside the clutch housing and positioned centrally on the clutch disc hub and clutch diaphragm spring.
Clutch release system with CSC
1 Transmission input shaft
2 Concentric slave cylinder CSC
3 Anti-vibration unit
4 Peak torque limiter
5 Reservoir with hydraulic fl uid
6 Hose
7 Master cylinder
8 Pedal
Fully hydraulic release systems are more economic for vehicle manufacturers as they are easier to install owing to the reduced number of individual parts, and the installation of the hydraulic line offers a lot of fl exibility in designing engine compartment layouts.
Benefits of hydraulic release systems:
• Flexible installation of the hydraulic line
• Good actuation comfort through less friction
• Vibration and noise optimized
• Ease of installation and maintenance
• Integrated wear adjustment
Master cylinder
The master cylinder consists of a housing, a piston with piston rod and a pair of seals (primary and
secondary seal). It has a hydraulic port, normally a quick connector, for the pressure line through which it is connected to the slave cylinder. On some models the screw connector commonly used on brake systems can still be found. The master cylinder is connected to the hydraulic fluid circuit. In many cases, it is connected to the brake fluid reservoir via a hose; but there are also solutions providing a separate reservoir for the clutch cylinder. The primary seal separates the reservoir from the hydraulic chamber, thus generating the pressure required to actuate the clutch. The secondary seal seals up the low-pressure chamber of the reservoir against its environment. With the clutch pedal released, a spring at the pedal or in the master cylinder pushes the piston back entirely. In this position, the connection between the reservoir and the pressure chamber is open which allows the air trapped in the system to escape and liquid to enter. The selfadjusting mechanism of the hydraulic system is used.
1 Connection to the reservoir
2 Primary seal
3 Secondary seal
4 Connection to the pressure line
5 Housing
6 Piston
7 Piston rod
Vibration damper
The internal combustion process of the engine causes vibration which is passed from the clutch components via the release bearing to the clutch pedal. This is noticeable to the driver in the form of a tingling in the foot and noise. In order to prevent this, fi lter elements can be integrated in the tubes or hoses. This could be either membrane dampers or anti-vibration units with two non-return valves acting in the opposite direction.
Peak torque limiter
Peak torque limiters are moveable orifices within the hydraulic line which reduce the volume flow during clutch engagement. They protect the drive train from overload caused by a sudden clutch engagement, e.g. if
the driver’s foot slips off the clutch pedal. During maintenance, peak torque limiters must never be removed from the hydraulic system, as this can damage the transmission, the drive shafts or the Dual-Mass Flywheel.
Peak torque limiter. Moveable orifices inside the hydraulic line control the flow volume when the clutch is engaged at high speeds.
1 Moveable orifice
2 Housing
Slave cylinder
Concentric slave cylinder
In designs with CSC the release bearing is directly connected to the piston. The release movement of the clutch is initiated by hydraulic pressure: when engaging the clutch the diaphragm spring pushes the central piston toward its orig inal position and allows the fluid to flow back into the master cylinder.
Sensors
Master and slave cylinders are increasingly equipped with sensors which measure actuation travel and transmit the reading to the engine and transmission control unit. Whether or not a system is sensor-equipped can be seen from a little case attached to either the master or slave cylinder which houses plugs and cable connectors. Sensors are precisely tuned to the master or slave cylinder to which they are attached and must therefore never be removed from and attached to another cylinder. Failure of one component mandates the replacement of the entire cylinder/sensor unit.
Hydraulic fluid
If not otherwise indicated by the vehicle manufacturer, hydraulic systems use brake fl uid. When leaving production, the system is prefi lled with fluid. When in operation, water accumulates in the brake fluid and causes the boiling point to decrease. In the worst case when ambient temperatures are high, this can lead to vapor bubbles in the slave cylinder, which in turn can cause clutch decoupling problems. In order to prevent this from happening, it is recommended to change the brake fluid two to three times a year. It is mandatory to observe the manufacturer’s recommendations to make sure the correct fluid is used. Failure to do so can damage the seals or cause noise emissions at the master cylinder.
Maintaining a hydraulic release system normally requires no more than replacing the brake fl uid at regular
intervals. Professional workshops use a special filling device for a quick and clean procedure. If no special tool is available, the re-filling of the system with fluid can be performed by repeatedly depressing and releasing the pedal while opening and closing the bleed screw simultaneously. To make sure the system is flushed entirely and to avoid air bubbles trapped in the system, here too, the manufacturer’s specifi cations should be observed.
Cleanliness when working on the hydraulic system is a crucial factor. Even the smallest contamination can cause leakages and malfunctions. Systems designed for brake fluid must be protected from mineral oil ingress. Relubrication of the cylinders or connectors is forbidden for the same reason. Even the smallest amount of mineral oil can destroy the seals. For clutch systems using the same reservoir as the brake system, there is the risk of carrying over contaminations from the brake system.
Important
When replacing the clutch, performing a visual inspection of the CSC is recommend ed. Replace the CSC
if there are any signs indicating leakage, excessive thermal load, stiff operation of the bearing or hydraulic
system, or advanced wear of the bearing ring at the diaphragm spring.
Clutch release system with CSC
1 Transmission input shaft
2 Concentric slave cylinder CSC
3 Anti-vibration unit
4 Peak torque limiter
5 Reservoir with hydraulic fl uid
6 Hose
7 Master cylinder
8 Pedal
Fully hydraulic release systems are more economic for vehicle manufacturers as they are easier to install owing to the reduced number of individual parts, and the installation of the hydraulic line offers a lot of fl exibility in designing engine compartment layouts.
Benefits of hydraulic release systems:
• Flexible installation of the hydraulic line
• Good actuation comfort through less friction
• Vibration and noise optimized
• Ease of installation and maintenance
• Integrated wear adjustment
Master cylinder
The master cylinder consists of a housing, a piston with piston rod and a pair of seals (primary and
secondary seal). It has a hydraulic port, normally a quick connector, for the pressure line through which it is connected to the slave cylinder. On some models the screw connector commonly used on brake systems can still be found. The master cylinder is connected to the hydraulic fluid circuit. In many cases, it is connected to the brake fluid reservoir via a hose; but there are also solutions providing a separate reservoir for the clutch cylinder. The primary seal separates the reservoir from the hydraulic chamber, thus generating the pressure required to actuate the clutch. The secondary seal seals up the low-pressure chamber of the reservoir against its environment. With the clutch pedal released, a spring at the pedal or in the master cylinder pushes the piston back entirely. In this position, the connection between the reservoir and the pressure chamber is open which allows the air trapped in the system to escape and liquid to enter. The selfadjusting mechanism of the hydraulic system is used.
1 Connection to the reservoir
2 Primary seal
3 Secondary seal
4 Connection to the pressure line
5 Housing
6 Piston
7 Piston rod
Vibration damper
The internal combustion process of the engine causes vibration which is passed from the clutch components via the release bearing to the clutch pedal. This is noticeable to the driver in the form of a tingling in the foot and noise. In order to prevent this, fi lter elements can be integrated in the tubes or hoses. This could be either membrane dampers or anti-vibration units with two non-return valves acting in the opposite direction.
1 Housing
2 Hose element
3 Hose support
4 Swell seal ring
Peak torque limiter
Peak torque limiters are moveable orifices within the hydraulic line which reduce the volume flow during clutch engagement. They protect the drive train from overload caused by a sudden clutch engagement, e.g. if
the driver’s foot slips off the clutch pedal. During maintenance, peak torque limiters must never be removed from the hydraulic system, as this can damage the transmission, the drive shafts or the Dual-Mass Flywheel.
Peak torque limiter. Moveable orifices inside the hydraulic line control the flow volume when the clutch is engaged at high speeds.
1 Moveable orifice
2 Housing
Slave cylinder
 |
| Slave cylinder attached to the gearbox exterior 1 Clutch pressure line 2 Slave cylinder 3 Release bearing 4 Release fork |
In semi-hydraulic systems the slave cylinder is typically located at the outside of the transmission housing or
serves as an actuation device for the clutch lever. In this case, the slave cylinder comprises the housing, the piston with sealing, a precharge spring and a bleed screw. The precharge spring applies constant preload pressure on the release bearing which therefore rotates even if the release system is free of load. In this manner, no undesirable noise occurs. The bleed screw facilitates the fl ushing of the system during maintenance. In systems with CSC, the release bearing is directly connected to the piston and tensioned against the diaphragm spring tips of the clutch by the integrated precharge spring. The release movement of the clutch is initiated by hydraulic pressure: when engaging the clutch, the diaphragm spring pushes the central piston into its original position and the fluid fl ows back into the master cylinder. Designed with a large travel, the slave cylinder is able to adjust to tolerances occurring during installation or as a result of clutch wear and tear.
 |
1 Preload spring
2 Piston
3 Seal
4 Housing
5 Dust bellows
6 Release bearing
|
In designs with CSC the release bearing is directly connected to the piston. The release movement of the clutch is initiated by hydraulic pressure: when engaging the clutch the diaphragm spring pushes the central piston toward its orig inal position and allows the fluid to flow back into the master cylinder.
Sensors
Master and slave cylinders are increasingly equipped with sensors which measure actuation travel and transmit the reading to the engine and transmission control unit. Whether or not a system is sensor-equipped can be seen from a little case attached to either the master or slave cylinder which houses plugs and cable connectors. Sensors are precisely tuned to the master or slave cylinder to which they are attached and must therefore never be removed from and attached to another cylinder. Failure of one component mandates the replacement of the entire cylinder/sensor unit.
Hydraulic fluid
If not otherwise indicated by the vehicle manufacturer, hydraulic systems use brake fl uid. When leaving production, the system is prefi lled with fluid. When in operation, water accumulates in the brake fluid and causes the boiling point to decrease. In the worst case when ambient temperatures are high, this can lead to vapor bubbles in the slave cylinder, which in turn can cause clutch decoupling problems. In order to prevent this from happening, it is recommended to change the brake fluid two to three times a year. It is mandatory to observe the manufacturer’s recommendations to make sure the correct fluid is used. Failure to do so can damage the seals or cause noise emissions at the master cylinder.
Maintaining a hydraulic release system normally requires no more than replacing the brake fl uid at regular
intervals. Professional workshops use a special filling device for a quick and clean procedure. If no special tool is available, the re-filling of the system with fluid can be performed by repeatedly depressing and releasing the pedal while opening and closing the bleed screw simultaneously. To make sure the system is flushed entirely and to avoid air bubbles trapped in the system, here too, the manufacturer’s specifi cations should be observed.
Cleanliness when working on the hydraulic system is a crucial factor. Even the smallest contamination can cause leakages and malfunctions. Systems designed for brake fluid must be protected from mineral oil ingress. Relubrication of the cylinders or connectors is forbidden for the same reason. Even the smallest amount of mineral oil can destroy the seals. For clutch systems using the same reservoir as the brake system, there is the risk of carrying over contaminations from the brake system.
Important
When replacing the clutch, performing a visual inspection of the CSC is recommend ed. Replace the CSC
if there are any signs indicating leakage, excessive thermal load, stiff operation of the bearing or hydraulic
system, or advanced wear of the bearing ring at the diaphragm spring.
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