GB2156481A

GB2156481A - Rocking piston structure for hydraulic system

Info

Publication number: GB2156481A
Application number: GB08508083A
Authority: GB
Inventors: Eckhart Rudiger; Wilhelm Heubner
Original assignee: FAG Kugelfischer Georg Schaefer KGaA; Kugelfischer Georg Schaefer and Co
Current assignee: IHO Holding GmbH and Co KG
Priority date: 1984-03-30
Filing date: 1985-03-28
Publication date: 1985-10-09
Also published as: IT1184199B; FR2562195A1; DE3411824A1; IT8520051A0; GB8508083D0

Abstract

In order to provide for hydraulic vehicle-brake and clutch systems cylinders including piston units which when pressure is applied transmit only low lateral forces to the bore wall of the cylinder (3), even when the piston rod (4) is inclined in relation to the cylinder axis (14), the piston has a part- spherical portion (15), which is rigidly joined to the piston rod (4) and which has a swivel plane (8) perpendicular to the axis of the piston rod, and is provided with a sealing plane (6) arranged parallel to or coincidental with the swivel plane. The sealing plane is formed by a sealing element (5) and is swivelable with the swivel plane. Other embodiments are shown. <IMAGE>

Description

SPECIFICATION A piston unit This invention relates to a piston unit for a transmitting and/or receiving cylinder of a hydraulic vehicle-clutch and/or brake system, the assembly comprising a sealing element, an unguided piston rod and an oscillating piston guided in a cylinder bore when in use and including a part-spherical portion, in which the piston rod is rigidly joined to the part-spherical portion, and in which the longitudinal axis of the piston rod is angularly swivable in relation to the main axis of the cylinder.

Piston units each comprising a sealing element, a piston and a piston rod are generally used in hydraulic vehicle-brake and clutch systems. The piston of each unit is cylindrical and the piston's outer surface which slides in the cylinder bore guides the piston. A pressing rod or a piston rod is used for transmitting power and for generating hydraulic power.

The pressing or piston rod swivels in the course of each piston stroke in relation to the main cylinder axis. To render this swivel motion possible the pressing or piston rod has hitherto been articulatedly connected to the piston (see, for example, published German Patent Application No. 11 97 770 or No. 29 34 218). If the stroke of the piston is accompanied by the piston rod's swivelling motion in relation to the cylinder axis radial forces are generated and transmitted across the connection to the piston and act on the cylinder wall.

These radial forces depend on the power exerted in axial direction by the piston and on the size of the angle through which the piston rod swivels. A disadvantageous result thereof is a one-sided and premature erosion of the piston's outer surface and of the cylinder wall, thus causing premature destruction of the sealing faces and failure of the sealing elements and, finally, of the entire unit. A swivable one-piece piston unit including a spherical piston and a spherically constructed sealing element is described in Swedish Patent No.

210,202. It renders it possible to dispense with the comparatively expensive articulated mounting of the piston rod and the piston unit is swivable. However, the occurrence of lateral forces remains unchanged, the reason being that the sealing plane functions in a direction normal to the cylinder axis. In addition, a piston unit of such a construction has hitherto been used only for axial-flow piston pumps but not in connection with piston units of the kind referred to in the opening paragraph hereof.

It is an aim of the present invention, therefore, to provide a piston unit which when pressure is applied does not transmit any lateral forces to the wall of the cylinder bore even if the piston rod is inclined in relation to the cylinder axis, or at least to provide a unit which applies a very much reduced lateral force relative to previously proposed units.

Accordingly, the present invention is directed to a piston unit having the construction set out in the opening paragraph of the present Specification, in which the part-spherical portion of the piston unit has a swivel plane perpendicular to the axis of the piston rod, in which, or parallel to which, plane the sealing element is arranged, and in which the sealing element forms a sealing plane swivelable with the swivel plane.

In an advantageous example of a piston unit in accordance with the present invention, the sealing element and the spherical portion, which are made of an elastomer, are vulcanized on to a metallic part of the piston.

Considering one particular embodiment of the invention in greater detail, the powertransmitting piston rod is rigidly joined to the piston carrying the sealing element. When the piston rod is swivelled through an angle of predetermined size the piston too is swivelled in the cylinder bore through an angle of the same size. To render this swivelling motion possible a piston collar has a particular construction at its contact area with the cylinder wall so as to permit such swivelling. In particular, this collar is disc-shaped with its periphery having the shape of part of a sphere.

The result is a circular contact line-for the predetermined swivel range-between the cylinder wall and the spherical portion. The plane containing this contact line is always normal to the cylinder axis. Jamming of the piston unit is inhibited. The spherical portion's end face facing the pressure chamber itself forms a support collar for the sealing element which, when the piston rod is swivelled or transmits force in an inclined direction is swivelled therewith, causing the sealing plane of the sealing element to perform the same motion in relation to the cylinder wall. Thus, the sealing plane is inclined in relation to the transverse plane of the cylinder at the same angle at which the piston rod is inclined in relation to the main cylinder axis.Since the hydraulic force acting on the inclined sealing plane comprises a component which is effective in the direction normal to the cylinder wall and the extent of which is exactly the same as that of the corresponding oppositely directed force component along the inclined piston rod, the piston unit remains free of radial forces. The invention renders it possible to manufacture the working surfaces of the pistons and cylinders from materials of reduced surface hardness which may even be elastomers; this means that the quality of the sliding fit between mated pistons and cylinders can be lowered without detrimental effect. In additon, a reduction of the over-all length and weight of the cylinder casing is possible because the guiding and supporting areas of the piston can be kept very short.

This invention, therefore, obviates the need for articulated connections between swivable piston rods and pistons. "Clogging"-brought about by dirt and corrosion-of the cylindrical pistons experienced in connection with receiver cylinders of drum brakes is reduced.

A further advantageous application of the present invention is of particular merit in the case of a cylinder made of a synthetic material being used together with a piston also made of a synthetic material because in addition to the foregoing advantages, only one temperature independent test of the piston clearance in the cylinder bore is necessary at the time of manufacture.

Examples of piston units made in accordance with the present invention are illustrated in the accompanying drawings, in which: Figure 1 is an axial sectional view of a construction of a piston unit showing theoretically occurring forces; Figure 2 is an axial sectional view of a further piston unit in which a sealing plane extends parallel to a swivel plane thereof; Figure 3 is an axial sectional view of a piston unit in which the sealing plane is coincidental with the swivel plane; Figure 4 shows axial sectional views of two piston units each having an integral construction of sealing and guiding elements as an elastomeric-metallic union; and Figure 5 is an axial sectional view of a further piston unit in which a sealing element is integral with and made of the same material as a spherical portion of the unit.

The piston unit 1 illustrated in Fig. 1 is displaced in the bore 2 of a cylinder 3. The unit comprises a rigidly joined piston rod 4 perpendicular to a sealing plane 6 of a sealing element 5. The latter has a sealing collar 7 which provides a sealing plane 8 dividing a pressure chamber 9 from the rest of the bore 2 of the cylinder 3. The piston unit 1 is displaced in the sense that it is swivelled through an angle a relative to the cylinder axis. The vectors of the resulting force components and their directions of application are also illustrated. In particular, these force components comprise the force FD-transmitted by the piston rod 4.The force FD may be resolved into an associated axial component FDaxia =fed. cos a and-completing the triangle of forces-a radial component FD radial = FD.sin a. The lines of application of these two forces are naturally perpendicular to each other. A hydraulic force FH = p .Az from the pressure chamber 9 acts in an axial direction and is made up of the product of the pressure p exerted by the medium in the chamber 9 and the surface of projection referred to hereinafter as Az of the bore diameter D. The radial component FH,adja, = p. A proj. where A proj. is the projection of the piston head surface onto an axial plane which is perpendicular to the plane containing the piston and cylinder axis.A proj. may be called the radial projection surface.

In the case of an equilibrium of forces then FH + FHradialFD and FD = axial + FDradial' in which the oppositely directed and equal radial components FHradial =FDradial cancel each other so that there is absence of lateral forces between the piston unit and the wall of the bore.

However, this analysis is valid only in the case of the sealing plane 6 being in coincidence with the sealing plane 8 of the collar 7.

If lip rings are used, as is the general practice in hydraulic vehicle-brake systems, the sealing plane 6 is then disposed mainly in front of the swivelling plane spaced therefrom by a distance ''f'' in the direction towards the pressure chamber 9, as is shown in Fig. 2.

The result is that, the more the swivel angle a increases and depending on the extent of "f" within the operationally possible limit of about 8" swivel, there is an effective shift, without a change in direction, of the application line of FD acting on the piston rod 4. This shift produces a lateral force at the other end of the piston rod 4 of Fs, so that the moment so generated is Fs. 1 k where 1 k is the length of the piston rod.

Now

The resulting lateral force Fs, which is determined by the distance "f" between the sealing plane 6 and the swivel plane 8 and by the length 1 k of the piston rod 4 amounts-assuming a customary swivel angle and a customary length of piston rod-to less than 8% of the lateral force produced by a piston rod articulatedly joined to the piston.

Fig. 3 illustrates a construction of the part spherically shaped piston unit 1 including a primary sealing ring 51 of polytetrafluoroethylene arranged in the annular groove 1 3 and an elastomeric O-ring 21 serving as an elastic biasing element. A sealing edge 22 (equivalent to the sealing plane 6 of the primary sealing ring) is coincidental with the swivel plane 8 of the part-spherical portion 1 6 of the piston and is instrumental for the aforementioned absence of lateral forces. The primary sealing ring 5', which is hard, assumes-as a result of the sealing edge 22 being disposed in the swivel plane 8-the additional function of guiding the piston in the cylinder casing 3.

A restoring ring is present in the cylinder 3.

Otherwise, the construction is the same as the one already described with reference to Fig.

1.

Fig. 4 illustrates a further construction of piston unit 1 and its location in the cylinder 3 In view of the geometry of the sealing element used in this embodiment the swivel and sealing planes are spaced from each other by the distance "f". The unit substantially consists of axially arranged symmetrical elements and is illustrated as an embodiment which may or may not include a restoring spring 10 in the cylinder 3. In the event of the restoring spring 10 being used the unit is provided at its end facing the pressure chamber 9 with a tapered spigot 11 which secures the abutting restoring spring 10 and which by means of a diametrically larger collar 1 2 secures the sealing element 5 against axial displacement in a groove 1 3 within which the sealing element is seated.The collar 7, which has parallel end faces, serves as a stop for the sealing element's bottom face which is directed away from the pressure chamber 9. It has, in side view, a circular generatrix 1 5 and forms a part-spherical portion 1 6 so that when in operation and the piston unit 1 is swivelled through the angle a in relation to the axis 14 of the cylinder bore, it can, without jamming, be tilted or swivelled all-round against the bore wall of the cylinder 3. The part-spherical portion 1 6 with its parallel end faces is part of a notional sphere 1 7 the diameter "D" of which corresponds to the nominal size of the cylinder bore 2 of the cylinder 3.The centre 18 of the notional sphere 1 7 must be located either on the plane face 1 9 of or within the parallel-faced part-spherical portion 1 6 so that, when the latter is swivelled, line contact with the cylinder wall is always ensured. The minimum width "2e" of the part-spherical portion 1 6 required for this purpose depends, in this embodiment, on the angle a through which the piston unit 1 is swivelled in relation to the axis 1 4 of the cylinder bore and on the nominal diameter "D" of the cylinder bore 2.

The width is symmetrically positioned in relation to the centre of the notional sphere 1 7.

In the event of the part-spherical portion 1 6 being constructed with the minimum width "e", the centre 1 8 of the notional sphere 1 7 is located on the plane face 1 9 which, as a consequence, forms the stop for the bottom face of the sealing element 5. The piston unit 1 having the part-spherical portion width "e" and being symmetrical about the axis is compared, as an alternative, with one not including the restoring spring 10 and being attached to an actuating pedal. In this instance, the tapered spigot 11, the restoring spring 10 and also one dimension "e" of the partspherical portion 1 6 are rendered superfluous so that the overall length of the piston unit 1 can be reduced.

The application of the two forms of piston units 1 illustrated in Fig. 4, which during swivelling also show-in view of the elliptical configuration of the cross-section to be sealed and in dependence on the swivel angle-a different increase in the width of a gap "s", shown in Fig. 2, will always be related to the intended ultimate use of the piston unit 1, the reason being that the gap "s" in the case of the width "e" of the part-spherical portionwithout a continuous line contact-uniformly increases on both sides but is doubled on one side if the width of the part-spherical portion is "2e". However, in the latter case the continuous line contact or guidance is maintained. Extrusion defects on the sealing element 5 can be avoided by inserting a support disc 20 for the lip ring illustrated in Fig. 4 and generally used in hydraulic systems for vehicles.

Fig. 5 illustrates an embodiment in which the sealing element 5 is integral with and made of the same material as the part-spherical portion 16. Advantageously, a metallic piston part 23 is a deep-drawn component combined with a separate piston rod 4, the integration being effected by rigidly joining the two components by virtue of their matching shapes or by interlocking them. The metallic piston part 23 is a blind-hole cup 24 from the open end of which the piston bottom 25 extends in radial direction perpendicularly to the longitudinal axis; this extension terminates via an interposed curve in a flange 26 running parallel to the axis. The piston bottom 25 and the flange 26 of the metallic piston part 23 form the fixing surfaces for a onepiece elastomeric sealing element 5" including the elastomeric part-spherical portion 1 6 vulcanized on to said surfaces. The effect of swivelling the sealing plane 6 together with the part-spherical portion 1 6 and the resulting substantial absence of radial forces render it possible, therefore, to manufacture the centering and guiding component, the part-spherical portion 1 6 together with the sealing element 5". as a one-piece elastomer of relatively low compression strength and to vulcanize it on to the piston bottom and flange in one operation-which is the best possible solution as far as damage and wear are concerned and which reduces manufacturing costs.

Claims

1. A piston unit for a transmitting and/or receiving cylinder of a hydraulic vehicle-clutch and/or brake system, the assembly comprising a sealing element, an unguided piston rod and an oscillating piston guided in a cylinder bore when in use and including a part-spherical portion, in which the piston rod is rigidly joined to the part-spherical portion, in which the longitudinal axis of the piston rod is angularly swivelable in relation to the main axis of the cylinder, in which the part-spherical portion of the piston unit has a swivel plane perpendicular to the axis of the piston rod, in which, or parallel to which, plane the sealing element is arranged, and in which the sealing element forms a sealing plane swivelable with the swivel plane.

2. A piston unit according to claim 1, in which the sealing element is arranged in an annular groove of the piston unit, the element having a shape matching that of the groove and/or being lockingly held therein.

3. A piston unit according to claim 1, in which the sealing element and the spherical portion, which are made of an elastomer, are vulcanized on to a metallic part of the piston.

4. A piston unit substantially as described herein with reference to any one of Figs. 1 to 5 of the accompanying drawings.