TECHNICAL FIELD

The invention relates to a braking system for a motor vehicle and more particularly to a braking system for a motor vehicle comprising a friction disc brake including one or more friction discs. The invention also relates to a piston seal for use in a braking system and a vehicle comprising the braking system.

BACKGROUND

Friction disc brakes for vehicles comprises one or more friction discs rotating with the wheel. A friction disc braking systems may include a hydraulic piston being movably arranged in a stationary piston housing. When regular braking is applied, hydraulic fluid behind a piston pressurizes, as the pressure increases the piston moves and restricts the rotation of the friction disc(s). When fluid pressure drops, the brake is released and fluid begins draining from the piston housing. A retract spring pulls the piston back, releasing the pressure on the friction discs. To prevent fluid leakage from the piston housing, a sealing ring may be arranged in a circumferential direction of the hydraulic piston, either in a groove provided in the hydraulic piston or in a groove provided in the piston housing.

In a wet disc brake a plurality of friction discs are provided within a sealed brake housing containing oil. In wet disc brakes grooved friction discs rotate and other plain discs are stationary and braking effect is achieved by axially forcing the discs together. The oil is provided primarily as a coolant and for this purpose oil circulates through the grooves in the rotating discs.

Brakes on vehicles with friction discs, and in particular wet disc brakes, may suffer from what is perceived as a noise and snatch at brake application. A theoretical explanation can be given by looking at the piston travel as a function of pressure build up on the brake discs. Pressure build up is directly proportional to the torque transferred by the brake. The rapid change in torque may results in noise and snatch at brake application.

One solution is to provide damping material from rubber/cork compound in the brake disc or as a separate disc behind the brake disc. This damper smoothens the touch up between piston and brake disc and torque increase becomes slower. Damping material from rubber/cork compound are usually inexpensive but the material is often stressed to failure because of heat in combination with high surface pressure. Although inexpensive it increases the brake disc part cost and adds complexity and decreases durability.

An object of the present disclosure is to provide an improved braking system comprising a friction disc brake, the braking system providing damping during braking application. A further object of the disclosure is to provide an improved piston seal for sealing between a hydraulic piston and a stationary piston housing.

SUMMARY

An object of the present disclosure is to provide an improved braking system comprising a friction disc brake, the braking system providing damping during braking application. A further object of the disclosure is to provide an improved piston seal for sealing between a hydraulic piston and a stationary piston housing and a vehicle including the braking system.

This and other objects may be achieved by a braking system according to claim 1 , a piston seal according to claim 18 and a vehicle according to claim 21.

According to a first aspect of the invention, the present disclosure relates to a braking system for a motor vehicle. The braking system comprises a friction disc brake including one or more friction discs, a hydraulic piston being movably arranged within a stationary piston housing. The hydraulic piston is arranged to movably engage and release the friction disc brake in an axial direction upon application of a hydraulic pressure. A groove is arranged in a circumferential direction of the hydraulic piston and an elastically deformable circumferential seal is arranged in the groove providing a piston seal sealing between the hydraulic piston and the stationary piston housing. The groove comprises a first and second groove side wall and a groove bottom. The piston seal has a piston seal bottom side arranged to be in contact with and sealing against the groove bottom, and a piston seal outer side being a side opposite to the piston seal bottom side. The piston seal has a first seal side facing the one or more friction discs and an opposing second seal side facing the second side wall of the groove. The piston seal comprises one or more elastically deformable protruding portion(s) arranged on the first seal side and upon influence of the hydraulic pressure to the elastically deformable circumferential seal hydraulic fluid is allowed to enter into a volume between the second seal side and the second groove side wall.

Braking systems for motor vehicles with friction disc brake, such as wet disc brakes, sometimes suffer from what is perceived as noise and snatch at brake application. A theoretical explanation can be given by looking at the piston travel as a function of pressure build up on the brake discs. Pressure build up is directly proportional to the torque transferred by the brake. The rapid change in torque may result in noise and snatch. The piston seal according to the present disclosure is provided with one or more elastically deformable protruding portion(s) arranged on the first seal side which protruding portions upon influence of the hydraulic pressure deform and thereby allow hydraulic fluid to enter into a volume created between the second seal side and the second groove side wall. The protruding portions on the first seal side thereby acts as a pressure damping element and thereby smoothens the touch up between the piston and the friction disc and the torque increase becomes slower. A further advantage is that the protruding portion(s) may act as a spring to move the seal back to start position when the hydraulic pressure is reduced to zero. The reset of the seal in the groove is very important. Without reset, the next braking event cannot benefit from the extra displacement.

The elastically deformable seal portions may be formed of the same material as the elastically deformable seal, such that the seal and the protruding portions are integrally formed into a single piece.

The groove may be arranged in the stationary brake housing or in the hydraulic piston housing.

The piston seal may be a piston seal ring. Depending on the particular brake actuation liquid being used the piston seal may for example be manufactured from oil resistant hydrogenated nitrile rubber or from fluoro rubber, or alternatively they can be manufactured from a brake fluid resistant material such as EPDM rubber.

According to one embodiment, the one or more protruding portion(s) is a first ridge extending continuously along a circumference of the piston seal on the seal first side. The piston seal may comprise two or more spaced apart protruding portions arranged along the circumference of the piston seal on the first seal side.

The piston seal may comprise a plurality of, such as four or more, spaced apart protruding portions arranged along the circumference of the piston seal on the first seal side. The piston seal may optionally comprise six or more spaced apart protruding portions arranged along the circumference of the piston seal on the first seal side.

The spaced apart protruding portions may be arranged with equal distance between each protruding portion along the circumference of the piston seal. An advantage with the protruding portions being arranged with equal distance along the circumference of the piston seal is that upon pressure build up, a controlled and balanced deformation of the seal is achieved, allowing hydraulic liquid to enter the volume between the second seal side and the second groove side wall continuously along the seal circumference.

According to one embodiment, the spaced apart protruding portions are spaced apart with a distance, as seen in the circumferential direction of the piston seal, corresponding at least to an extension of the protruding portions, as seen in the circumferential direction. If the protruding portions have a circular shape, the distance between the spaced apart protruding portions may correspond to the diameter of the protruding portions, as seen at the base of the protruding portions. This has been found by the present inventor to promote a desired balance of the deformation achieved and volume created.

According to a further embodiment, the piston seal has a height, as measured between the piston seal bottom side and the piston seal outer side, the spaced apart protrusions each has a minimum extension in the circumferential direction being from 1/10 of the height of the piston seal. Optionally the spaced apart protrusions each has a maximum extension in the circumferential direction of up to 9/10 of the height of the piston seal. The spaced apart protrusions each has a minimum extension in the circumferential direction being from 3/10 of the height of the piston seal, or from 4/10 of the height of the piston seal. The spaced apart protrusions each has a maximum extension in the circumferential direction of up to 3/4 of the height of the piston seal, or up to 7/10 pf the height of the piston seal. According to a further embodiment, the second seal side comprises one or more second side protruding portion(s) arranged along a circumference of the piston seal on the seal second side. The fact that the piston seal is provided with one or more second side protruding portion(s) improves the positioning of the piston seal in the groove, thereby promoting a predictable deformation also over time. Additionally, the provision of a piston seal having one or more second side protruding portion(s) reduces the risk of faulty assembly of the piston seal or at least reduces the complications of a faulty assembly.

The second side of the piston seal may comprise a plurality of spaced apart second side protruding portions arranged along a circumference of the piston seal on the seal second side. Optionally, the first side of the piston seal may comprise a plurality of spaced apart protruding portions and wherein the plurality of spaced apart second side protruding portions are arranged symmetrically with the spaced apart protruding portions on the first side.

According to one embodiment, the one or more second side protruding portions is a second ridge extending continuously along a circumference of the piston seal on the seal second side.

According to a further embodiment, the piston seal has one of the following cross- sectional shapes, a D-shape, a rectangular shape, a square shape, an oval shape, and an hourglass shape or a trilobal shape.

According to one embodiment, the piston seal has a width wherein a height of the one or more protruding portion(s), as measured from a surface of the first seal side, is within the range of from 5 % to 50% of the width of the piston seal. The height of the protruding portions being measured from an adjacent, non-protruding, surface. The width of the piston seal being measured at a transverse centreline, taken at equal distance from the innermost point of the seal bottom side and the outermost side of the seal outer side and not including the protruding portions.

According to a further embodiment, hydraulic fluid is allowed to enter into a volume between the seal second side and the second vertical side wall, at least upon influence of the hydraulic pressure of 2 bars or more against the piston seal second side. Deformation of the protruding portion(s) should this start at around 2 bars, as this commonly is the pressure at which brake play adjusters are compressed. The protruding portions may be completely deformed at a pressure of about 20 to 30 bar.

The braking system may be a wet disc braking system comprising a single or multiple friction discs. The wet disc braking system may comprise one or more grooved friction discs rotating and one or more plain stationary discs arranged alternatingly.

According to one embodiment, the groove is an inner groove and the piston seal is an inner piston seal. The friction disc brake may include a second groove, being an outer groove, and a second outer piston seal. The outer groove and the outer seal may include any one of the features disclosed for the inner groove and for the inner piston seal.

According to a second aspect of the present invention a piston seal for use in a braking system is provided. The braking system including one or more friction discs, a hydraulic piston being movably arranged within a stationary piston housing, the hydraulic piston being arranged to movably engage and release the friction disc brake. The piston seal being adapted to be arranged in a circumferential groove being arranged in a circumferential direction of the hydraulic piston, the piston seal being an elastically deformable circumferential seal, the piston seal having a piston seal bottom side, a piston seal outer side being a side opposite to the piston seal bottom side, the piston seal having a first seal side and a second seal side. The piston seal comprises one or more protruding portion(s) arranged on the first seal side and extending in a direction perpendicular to the first seal side.

The piston seal is typically a piston seal ring. Depending on the particular brake actuation liquid being used the piston seal may for example be manufactured from oil resistant hydrogenated nitrile rubber or from fluoro rubber, or alternatively they can be manufactured from a brake fluid resistant material such as EPDM rubber.

According to one embodiment, the one or more protruding portion(s) is a first ridge extending continuously along a circumference of the piston seal on the seal first side.

The piston seal may comprise two or more spaced apart protruding portions arranged along the circumference of the piston seal on the first seal side. The piston seal may comprise a plurality of, such as four or more, spaced apart protruding portions arranged along the circumference of the piston seal on the first seal side. The piston seal may optionally comprise six or more spaced apart protruding portions arranged along the circumference of the piston seal on the first seal side.

The spaced apart protruding portions may be arranged with equal distance between each protruding portion along the circumference of the piston seal. An advantage with the protruding portions being arranged with equal distance along the circumference of the piston seal is that upon pressure build up, a controlled and balanced deformation of the seal is achieved, allowing hydraulic liquid to enter the volume between the second seal side and the second groove side wall continuously along the seal circumference.

According to one embodiment, the spaced apart protruding portions are spaced apart with a distance, as seen in the circumferential direction of the piston seal, corresponding at least to an extension of the protruding portions, as seen in the circumferential direction. If the protruding portions have a circular shape, the distance between the spaced apart protruding portions may correspond to the diameter of the protruding portions, as seen at the base of the protruding portions. This has been found by the present inventor to promote an advantageous balance of the deformation.

The spaced apart protruding portions may be arranged with equal distance between each protruding portion along the circumference of the piston seal. An advantage with that the protruding portions are arranged with equal distance along the circumference of the piston seal is that upon pressure build up, a controlled and balanced deformation is achieved.

According to one embodiment, the spaced apart protruding portions are spaced apart with a distance, as seen in the circumferential direction of the piston seal, corresponding at least to an extension of the protruding portions, as seen in the circumferential direction. If the protruding portions have a circular shape, the distance between the spaced apart protruding portions may correspond to the diameter of the protruding portions, as seen at the base of the protruding portions. This has been found by the present inventor to promote a balance between deformation achieved and volume created.

According to a further embodiment, the piston seal has a height, as measured between the piston seal bottom side and the piston seal outer side, the spaced apart protrusions each has a minimum extension in the circumferential direction being from 1/10 of the height of the piston seal. Optionally the spaced apart protrusions each has a maximum extension in the circumferential direction of up to 9/10 of the height of the piston seal. The spaced apart protrusions each has a minimum extension in the circumferential direction being from 3/10 of the height of the piston seal, or from 4/10 of the height of the piston seal. The spaced apart protrusions each has a maximum extension in the circumferential direction of up to 3/4 of the height of the piston seal, or up to 7/10 of the height of the piston seal.

According to a further embodiment, the second seal side comprises one or more second side protruding portion(s) arranged along a circumference of the piston seal on the seal second side. The fact that the piston seal is provided with one or more second side protruding portion(s) improves the positioning of the piston seal in the groove, thereby promoting a predictable deformation also over time.

The second side of the piston seal may comprise a plurality of spaced apart second side protruding portions arranged along a circumference of the piston seal on the seal second side. Optionally, the first side of the piston seal may comprise a plurality of spaced apart protruding portions and wherein the plurality of spaced apart second side protruding portions are arranged symmetrically with the spaced apart protruding portions on the first side.

According to one embodiment, the one or more second side protruding portions is a second ridge extending continuously along a circumference of the piston seal on the seal second side.

According to a further embodiment, the piston seal has one of the following cross- sectional shapes, a D-shape, a rectangular shape, a square shape, an oval shape, and an hourglass shape or a trilobal shape.

According to one embodiment, the piston seal has a width, wherein a height of the one or more protruding portion(s), as measured from a surface of the first seal side, is within the range of from 5 % to 50% of the width of the piston seal. The height of the protruding portions being measured from an adjacent, non-protruding, surface. The width of the piston seal being measured at a transverse centreline, taken at equal distance from the innermost point of the seal bottom side and the outermost side of the seal outer side and not including the protruding portions.

According to a third aspect of the present invention, a vehicle, such as a heavy-duty vehicle is provided. The vehicle being a motor vehicle and comprises a braking system according to the first aspect of the invention.

The heavy-duty vehicle may for example be a truck.

Further advantages and advantageous features of the invention are disclosed in the following description and in the dependent claims.

BRIEF DESCRIPTION OF THE DRAWINGS

With reference to the appended drawings, below follows a more detailed description of embodiments of the invention cited as examples.

In the drawings:

Fig. 1 is a cross-sectional view of a braking system according to the present disclosure;

Fig. 2 is a perspective view of a piston seal according to the present disclosure;

Figs. 3a-3c are cross-sectional views of a piston seal in a groove during braking and deformation of the piston seal; and

Fig. 4 shows a diagram with the hydraulic pressure applied upon a braking application on the x axis and the piston travel at the hydraulic pressure applied.

DETAILED DESCRIPTION OF EXAMPLE EMBODIMENTS OF THE INVENTION

It is to be understood that the present invention is not limited to the embodiments described above and illustrated in the drawings; rather, the skilled person will recognize that many changes and modifications may be made within the scope of the appended claims. Figure 1 is a cross-sectional view of a braking system 1 for a motor vehicle according to the present disclosure. The braking system 1 in this figure is a wet braking system and comprises a friction disc brake 2 being hydraulically actuated. The friction disc brake 2 includes one or more friction discs 2a and one or more steel discs 2b, the friction discs 2a and the steel discs 2b being arranged alternatingly in the braking system 1. The friction discs 2a are rotating discs while the steel discs 2b are stationary. A hydraulic brake piston 3 is movably arranged within a stationary brake housing 4. The hydraulic piston 3 is arranged to movably engage and release the friction disc brake 2 in an axial direction A, upon application of a hydraulic pressure. The braking system 1 is coupled to a wheel hub 9.

A groove 5 is arranged in a circumferential direction C of the hydraulic piston 3 and an elastically deformable circumferential seal 6 is arranged in the groove 5 providing a sealing between the hydraulic piston 3 and the stationary brake housing 4. The groove 5 comprises a first groove side wall 5a, a second groove side wall 5b and a groove bottom 5c. The piston seal 6 has a piston seal bottom side 6c being arranged to be in contact with and sealing against the groove bottom 5c, and a piston seal outer side 6d is a side opposite to the piston seal bottom side 6c. The piston seal 6 has a first seal side 6a facing the one or more friction discs and an opposing second seal side 6b. The second seal side 6b faces the second side wall 5b. As may be seen in the enlarged view, the piston seal 6 comprises one or more elastically deformable protruding portion(s) 7 arranged on the first seal side 6a. Upon actuation of the disc brake 2, fluid is supplied to the brake housing 4 building up a pressure and causing the hydraulic brake piston 3 to move relative its starting position in the axial direction A. Upon influence of the hydraulic pressure to the elastically deformable circumferential seal 6, hydraulic fluid is allowed to enter into a volume 8 formed between the second seal side 6b and the second groove side wall 5b due to deformation of the elastically deformable protruding portion(s) 7. The elastically deformable circumferential seal 6 thus functions as a damper smoothening the touch up between the hydraulic brake piston 3 and the friction disc brake 2 such that the torque increase becomes slower thereby reducing or eliminating noise and snatch at brake application.

Figure 2 illustrates a perspective view of the circumferential seal 6 as seen from the first seal side 6a. The seal 6 shown in this figure is a D-shaped seal 6. The first seal side 6a is provided with spaced apart protruding portions 7 arranged along the circumference of the piston seal 6 on the first seal side 6a. The spaced apart protrusions 7 each has a minimum extension in a circumferential direction C being from 1/10 of the height h of the piston seal 6. Optionally the spaced apart protrusions 7 each has a maximum extension in the circumferential direction C of up to 3/4 of the height h of the piston seal 6. In this figure where the protrusions 7 has a circular shape, as seen from the first seal side 6a, the maximum extension corresponds to the diameter of the protrusions 7. The piston seal 6 has a width w, the width being measured at a centreline arranged at equal distance from the seal bottom side and the seal outer side, but not including the protrusions, wherein a height of the one or more protruding portion(s) 7, as measured from a surface of the first seal side 6a, is within the range of from 5 % to 50% of the width w of the piston seal 6.

Figures 3a-3c are schematic cross-sectional views of a D-shaped piston seal 6 arranged in a groove 5. The groove 5 comprises a first groove side wall 5a and second groove side wall 5b and a groove bottom 5c. The piston seal 6 has a piston seal bottom side 6c being arranged to be in contact with and sealing against the groove bottom 5c, and a piston seal outer side 6d is a side opposite to the piston seal bottom side 6c. The piston seal 6 has a first seal side 6a and an opposing second seal side 6b, the second seal side 6b facing the second side wall 5b. The figures 3a-3c illustrate the piston seal 6 during different stages of a braking application.

Figure 3a shows the piston seal 6 in an initial non-pressurized position. Figure 3b shows the piston seal 6 upon influence of hydraulic pressure of about 5 bars against the piston seal second side 6b. When hydraulic fluid is supplied to the brake housing 4 pressure is built up causing the hydraulic brake piston 3 to move relative it’s starting position in the axial direction A. Upon influence of the hydraulic pressure to the elastically deformable circumferential seal 6, the elastically deformable protruding portions 7 arranged on the first seal side 6a deforms and thereby creates an increased volume 8 between the second seal side 6b and the second groove side wall 5b. The piston seal 6 is provided with a plurality of spaced apart second side protruding portions 11, the second side protruding portions 11 being arranged symmetrically with the spaced apart protruding portions 7 on the first side 6a. The fact that the piston seal 6 is provided with the plurality of spaced apart protruding portions 11 improves the positioning of the piston seal 6 in the groove 5, thereby promoting a predictable deformation also over time and reduces the complication at a possible faulty assembly of the seal. Figure 3c shows the piston seal 6 upon influence of a hydraulic pressure of 20 bars and in a fully compressed and deformed condition. The protruding portions 7 on the first seal side 6a thus act as pressure damping elements allowing hydraulic fluid to enter the volume 8 between the second seal side 6b and the second groove side wall 5b resulting in a smoother contact between the piston and the friction disc and a slower torque increase.

Figure 4 shows a diagram with the hydraulic pressure applied upon a braking application on the x axis and the piston travel, given in mm, at the hydraulic pressure applied. As illustrated in the figure, the piston travel occurs rapidly at about 2 bars and the behavior illustrated with the sharp knee on the curve may be the cause of the perceived noise and snatch at the brake application. The dotted line illustrates the effect when using a piston seal according to the present disclosure, which results in a dampening effect illustrated by the smoothening of the bend on the curve.