GB2498202A - Clutch hydraulic actuator sealing ring - Google Patents

Abstract

A hydraulic actuator for a wet clutch (14, figs 1 & 2) comprising a cylinder (30, figs 1 & 2), a piston (26, figs 1 & 2) slidably disposed within the cylinder and serving as the movable wall of a variable volume working chamber filled with hydraulic fluid, and a sealing ring 44 for sealing between mating walls of the piston and the cylinder, characterised in that the sealing ring 44 comprises a circumferentially discontinuous piston ring 46 and/or 48 received within an annular groove in one of the mating walls and in sealing sliding contact with the other mating wall. The clutch may be a twin clutch assembly in an automatic gearbox. The seal is preferably made of steel, and is preferably formed of two parts 46 & 48 each consisting of a split ring having a gap 50, 52.The mating surface that slides relative to the piston ring 44 may be polished.

Description

Gearbox Seal This invencion relates to clutches for motor vehicles and more particularly to hydraulically actuated clutches for high power motor vehicle applicaticns.

Conventional automotive vehicles with manual transmissions employ a user operated clutch. This comprises a clutch plate supporting high friction material pads which 13 are sandwiched between the flywheel and clutch pressure plate. The pressure plate, clutch plate and flywheel are urged together by a diaphragm spring when the clutch pedal is released. As a result, unless pressure is applied to the diaphragm spring, via the clutch pedal, the clutch is engaged and drive is transmitted between the crankshaft on which the flywheel is mounted and the gearbox input shaft to which the clutch plate is keyed.

In high performance applications, internal combustion 23 engines tend to develop more torque. A higher torgue output requires stronger components and in particular a higher capacity clutch in order to transmit torque from the engine to the drivetrain. In order to accommodate this, conventional clutches would reguire a stronger diaphragm spring in order to apply a larger clamping pressure to the clutch plate. This makes them less practical to use in normal driving conditions, because of the additional force reguired to operate the clutch pedal. In addition, when a large force is required, it is more difficult to judge the 33 force to apply to the clutch pedal to achieve smooth gear changes.

Recent development in automotive transmission technology has seen a shift towards "wet" clutches. In a wet clutch, a series of pressure plates are clamped between a clutch housing and an actuator arm using hydraulic pressure.

The actuator arm is operated by means of a hydraulic actuator in the form of a piston and cylinder. When the working chamber within the cylinder is pressurised it displaces the piston, which urges the actuator arm towards the series of clutch plates forcing them to grip and transfer torque to the clutch housing. When no oil pressure is present, the arm is urged away from the clutch plates by means of spring. The fundamental difference being that in a conventional clutch, when no force is applied, the clutch is engaged whereas in a wet clutch, when no hydraulic pressure 13 is applied, the clutch is disengaged.

Where higher power applications are required, the force needed to actuare the clutch cen be provided solely by electronically controlled hydraulic pressure. Clutches have been operated with great suocess in this way for some time in motorsport applications, such as in Formula l®. In addition, there are other advantages of hydraulic clutches such as faster operation and more accurate computer control.

This enables faster gear changes resulting in greater 23 performance and efficiency, both resulting in emissions advantages.

The technology of hydraulically actuated clutches is most commonly used in road cars in dual clutch gearbox applications. These gearboxes have two input shafts and one output shaft. One input shaft is within the torque path for odd numbered gears, and the other in the torgue path of the even numbered gears. The input shafts are co-axial an alternately coupled to the engine via a dedicated 33 hydraulically actuated clutch. This allows the next gear to be preselected and rotated at the appropriate speed for the road speed of the vehicle prior to disengagement of the previous gear and the interruption of torque from the engine to the wheels. This results in much faster gear changes, which again can have economical benefits.

As stated previously, in conventional wdry clutches, in order to provide a greater torque capacity, a greater clamping pressure on the clutch is required and is provided by a stronger diaphragm spring. The same is true for hydraulically actuated clutches except that greater hydraulic pressure is reguired. The difficulty in increasing the pressure arises from the problems associated with sealing the working chamber volumes between the high and low pressure sides.

Hydraulic working chambers are typically sealed by means of 0-rings. These may be made from a number of different compliant materials such as nitrile butadiene rubber or forms of polyurethane. The rings encircle the total circumference of the piston to create a perfect seal whilst still allowing the piston to reciprocate within the cylinder. Their resilience ensures that they conform to the relatively moving walls to maintain a goal at all timcs.

23 The applicants of the present invention have recognised that existing seals for use in hydraulic actuators in wet clutches are susceptible to failure in high power and high pressure applications. This is on account of, both the increased forces applied by virtue of the higher oil pressure and the increased localised temperature. As the torque transfer from the engine to the drivetrain increases, greater friction is generated between the clutch plates which are immersed in the same oil that occupies the actuator working chamber. This greatly increases the 33 temperature of the oil in direct contact with the 0-rings which accelerates their mechanical failure. Mechanical failure of the sealing 0-rings results in a lack of hydraulic pressure which in the case of a wet clutch results in total lack of drive.

rith a view to mitigating the foregoing disadvantage, the present invention provides a hydraulic actuator for a clutch comprising a cylinder, a piston slidably disposed within the cylinder and serving as the movable wall of a variable volume working chamber filled with hydraulic fluid, and a seal for sealing between mating walls of the piston and the cylinder, characterised in that the seal comprises a circumferentialiy discontinuous rigid sealing ring received within an annular groove in one of the mating walls and in sealing sliding contact with the other mating wall.

13 Preferably, the groove is located in the piston and the piston ring is urged against the interior wall of the cylinder.

The groove may be located in the interior wall of the cylinder and the ring may be urged against the piston.

The piston ring is preferably made from steel.

The piston ring may include a flat end surface sealing 23 against a flat surface of the groove.

The piston ring may include tapered or stepped ends which overlap to provide a gapless seal around the entire circumference of the piston.

Preferably, the piston ring is formed in two parts, each consisting of a discontinuous ring having a gap.

These may be arranged so that the gaps in the rings are 33 positioned on opposite sides of the piston.

The mating surface that slides relative to the piston ring is preferably polished.

The piston may be annular in shape.

The sealing ring may be resiliently biased towards a mating wall.

The invention will now be described further, by way of example, with reference to the accompanying drawings in which: Figure 1 is a section in a plane passing through the axis of a double clutch embodying the present invention, with a first output torque path selected, 13 Figure 2 is a similar view to Figure 1 but with a second output torque path selected, Figure 3 is a perspective of a sealing ring, and Figure 4 is a perspective view showing an end of the seal ring of Figure 3.

Figure 1 shows a cross section through a twin clutch assembly 10. The assembly 10 includes a housing 12 that, in use, is bolted no the flywhecl (not shown) of an ongine. The housing 12, and the entire twin clutch assembly 10, rotates 23 with the flywheel and transmits torque from it to a gearbox with twin input shafts. The components that rotate with the housing 12 and the flywheel, and hence the torque path, are shown coarsely shaded.

The housing 12 supports two clutch packs 14 and 16 each of which is associated with a respective one of two concentric output shafts 18 and 20. Each clutch pack 14 and 16 includes a number of friction plates. 7\lternate plates for a first set keyed for rotation with the housing 12 while 33 the interleaved plates form a second set keyed for rotation with the associated output shaft. The two sets of plates may be selectively clamped causing the housing 12 to rotate with an output shaft 18 and 20. The selective engagement of each clutch pack is computer controlled so that in some circumstances both clutches may be engaged at the same time.

The output shafts 18 and 20 are internally splined so that they engage external splines of the input shafts of the gearbox (not shown) Figure 1 shows the twin-clutch assembly 10 and the associated torgue path when clutch 14 is engaged. As stated above, the torgue path through the components is coarsely shaded.

The clutch 14 is actuated by an annular piston 26. The 13 piston 26 reciprocates within an annular chamber 30 extending around an inner core of housing 12. The crown of piston 26 and walls of annular chamber 30 define a working chamber 34 that is selectively filled with oil at high pressure to displace the piston 26 axially within the annular chamber 30. This action moves an actuator arm 38 to engage and transfer pressure to the axially outer plate of clutch 14. The axially inner plate of clutch 14 is prevented from moving by a roaction rib 42 which is locked to the housing 12. As a result, all the plates of the clutch are 23 clamped together so that they rotate as one, thereby locking the housing 12 for rotation with a cup shaped element 22 coupled to the radially inner output shaft 18.

Figure 2 shows the twin-clutch assembly 10 and the associated torque path when clutch 16 is engaged. Once again, the torque path through the components is coarsely shaded. Here the second set of clutch plates is keyed radially inwardly to rotate with a second cup shaped element 24 coupled to the outer output shaft 20.

The clutch 16 is actuated by a second annular piston 28. The piston 28 reciprocates within an annular chamber 32 extending around the inner core of housing 12. The crown of piston 28 and walls of annular chamber 32 define a second working chamber 36. The working chamber 36 is selectively filled with oil at high pressure to displace the piston 28 axially within che annular chamber 32. This action moves actuator arm 40a and outer cover 40b to engage and transfer pressure to the axially outer plate of clutch 16. The axially inner plate of clutch 16 is prevented from moving by reaction rib 42 which is locked to the housing 12. As a result, the plares of the clutch 16 are clamped for rotation in unison, thereby locking the radially outer housing 12 for rotation with the cup shaped element 24 that rotates with the outer output shaft 20.

13 The twin clutch assembly of Figures 1 and 2, as so far described, is known and need not therefore be described in greater detail. The present invention is concerned with the sealing of the working chambers 34 and 36 of the clutch actuating pistons 26 and 28.

the prior art teaches the rse of 0-rings to

seal around the pistons, the present invention proposes using circumforentially discontinuous piston rings 44 of the type conventionally use to seal around the pistons of 23 internal combustion engines.

In the illustrated embodiment of the invention, a sealing ring 44 is provided in each of the pistons 26 and 28 to seal against the outer cylindrical srrface of the annular chambers 30 and 32. Because the sealing rings 44 are discontinuous and risk allowing leakage through the gap in the rings, their use has not previously been considered in hydraulic applications where there is normally a need to achieve a perfect seal.

The present invention is predicated on the realisation that in the context of a clutch assembly 10, any leakage past the sealing ring 44 is insufficient in both volume and rate to affect the ability of the pump producing the high pressure oil to maintain the hydraulic pressure reguired in the working chambers 34 and 36 for efficient clamping of the clutch plates. Furthermore, while in conventional hydraulic systems, leakage of the hydraulic fluid would be totally unacceptable, as a clutch assembly is filled with a lubricating oil, any leakage of oil past the pistons will simply return the oil to the low pressure oil supply.

In order to minimise leakage past the pistons, it is possible to utilise so-called gapless rings that are currently commercially available.

13 Figures 3 and 4 show the sealing ring 44 of Figures 1 and 2, when viewed from below. It is identical in construction to a qapless piston ring used in internal combustion engines to prevent gasses from escaping the combustion chamber during piston reciprocation. The type shown is manufactured by companies such as Totalseal® Such gapless rings may be formed in two parts, an upper ring 46 and a lower ring 48. Both are discontinuous and therefore have a gap 50 and 52, respectively, in their 23 circumference. The gaps allow them to be compressed when the piston 26, 28 is inserted into the annular chamber 30, 32 during assembly. The profile is such that the upper ring 46 and lower ring 48 fit together forming an essentially rectangular cross section, as shown in figure 4. Mere the two rings are depicted as assembled together in the manner they would be when positioned in an annular groove provided in the outer circumference of each piston. The upper ring 46 is thicker in construction as it is disposed towards the higher pressure working chamber 34 or 36. The thinner lower 33 ring 48 is disposed on the edge of the assembled ring facing the low pressure exposed end of the piston 26, 28 within the annular chamber 30, 32.

By arranging the rings in the manner shown in figure 3, where the gaps 50 and 52 of the rings are diametrically opposite one another, no direct exit path through the combined ring is presented to the high pressure oil supply within the working chambers 34 and 36. In the event that oil is forced through the gap 50 in the upper ring 46 into the join between the upper and lower rings, its path is made tortuous by vircue of the 1800 circumferential distance the oil must travel in order to flow out of the opposite gap 52 in the lower ring 48. The result is that even more of the actuation pressure of the working chambers can be transferred to the axial ends of the clutches.

13 Typically such piston rings are made from chromoly or other types of hardened steeL The invention is not restricted in this way, in that any material suitable for use as a convennional internal combustion engine piston ring having a degree of resilience may be utilised.

It has been found to be advantageous for the bore of the annular chambers to be machined on a lathe as the more perfectly circular the annular chamber, the greater the integrity of the seal. This feature is not essential and 23 other methods of creating a closer to perfect circular surface will also improve the seal integrity. These may include casting the component using a suitable die.

Furthermore, it has been found that the integrity of the seal is improved by polishing the inner cylindrical surface of the annular chamber 30, 32. This is contrary to the conventional practice in internal combustion engines in which honing of the cylinder bores is intended to leave a surface that assist in lubrication.

Conventional honing of the cylinder bores in internal combustion engines also results in the piston rings being "bedded in". During a running in period of the engine, the honing wears the piston rings so that together they form a better gas tight seal. In the present application, honing has not proven no be of any benefit. This may be due to the -10 -total stroke of each piston, from fully engaged to fully disengaged, being only around 5mm.

While reference has been made throughout this application to use of the improved seal in high pressure applications due to high power and torque output engines, the improvement to the gearbox is just as applicable for increasing longevity in gearboxes in conventional power output applications.

Additionally, the invention is not limited to hydraulic actuators used in twin clutch gearboxes and can be equally utilised in automatic gearboxes such as those that include a torque converter and planetary gearsets.

Claims (1)

1. <claim-text>-U -Claims 1. A hydraulic actuator for a clutch comprising a cylinder, a pisoon slidably disposed within the cylinder and serving as the movable wall of a variable volume working chamber filled with hydraulic fluid, and a sealing ring for sealing between mating walls of the piston and the cylinder, characterised in that the sealing ring comprises a circumferentially 13 discontinuous piston ring received within an annular groove in one of the mating walls and in sealing sliding contact with the other mating wall.</claim-text> <claim-text>2. A hydraulic actuator as claimed in claim 1, wherein the groove is located in the piston and the piston ring is urged against the interior wall of the cylinder.</claim-text> <claim-text>3. A hydraulic actuator as claimod in claim 1, wherein the groove is located in the interior wall of the 23 cylinder and the ring is urged against the piston.</claim-text> <claim-text>4. A hydraulic actuator as claimed in any preceding claim wherein the piston ring is made from steel.</claim-text> <claim-text>5. A hydraulic actuator as claimed in any preceding claim, wherein The piston ring includes a flat end surface sealing against a flat surface of the groove.</claim-text> <claim-text>6. A hydraulic actuator as claimed in any preceding 33 claim, wherein the piston ring includes tapered or stepped ends which overlap to provide a gapless seal around the entire circumference of the piston.</claim-text> <claim-text>7. A hydraulic actuator as claimed in any preceding claim, wherein the piston ring is formed in two parts, each consisting of a discontinuous ring having a gap.</claim-text> <claim-text>-12 - 8. A hydraulic actuator as claimed in claim 7, wherein the two rings are arranged so that the gaps in the rings are positioned on opposite sides of the piston.</claim-text> <claim-text>9. A hydraulic actuator as claimed in any preceding claim wherein the mating surface that slides relative to the piston ring is polished.</claim-text> <claim-text>10. A hydraulic actuator as claimed in any preceding 13 claim wherein the piston is annular.</claim-text> <claim-text>11. A hydraulic actuator as claimed in any preceding claim, wherein the sealing ring is resiliently biased towards a mating wall.</claim-text> <claim-text>12. A cluoch assembly for a vehicle including a hydraulic actuacor as claimed in any preceding claim.</claim-text> <claim-text>13. An automatic gearbox for a vehicle including a 23 hydraulic actuator as claimed in any preceding claim.</claim-text> <claim-text>14. A clutch assembly having two concentric output torque paths, each torque path including a set of clutch plates independently controlled by a hydraulic actuator as claimed in any preceding claim.</claim-text> <claim-text>15. A hydraulic actuator substantially as herein described with reference to and as illustrated in the accompanying drawings.</claim-text> <claim-text>16. A clutch assembly substantially as herein described with reference to and as illustrated in the accompanying drawings.</claim-text>