GB2153967A

GB2153967A - Actuator piston construction

Info

Publication number: GB2153967A
Application number: GB08503028A
Authority: GB
Inventors: Kurt Stoll; Gerhard Hihn
Original assignee: Festo SE and Co KG
Current assignee: Festo SE and Co KG
Priority date: 1984-02-07
Filing date: 1985-02-06
Publication date: 1985-08-29
Also published as: ATA18085A; ES297018U; FR2559227A1; FR2559227B1; DE3404095C2; SE457011B; ES297018Y; JPS60184768A; SE8500538L; NL8500308A; SE8500538D0; GB2153967B; GB8503028D0; AT383872B; DE3404095A1; CH666731A5

Abstract

A piston (1) for an actuator cylinder is formed of a radially inner metal core (17) and a radially outer surrounding casing body (23) moulded on the core, the latter having radially projecting anchoring structure (24) thereon embedded within the moulded body. The piston may be made bilaterally symmetrical with a plane of symmetry normal to the axis of reciprocation of the piston on a piston rod (8). Each of the two halves (15) of the piston adapted to be fitted on a piston rod is made up of a metallic core (17) with a radially projecting anchoring collar (24) thereon for making a strong join with an outer resin casing (23) molded onto the core. The anchoring part (24) has openings (25) to receive resin when the casing is produced. The casing makes direct running contact on the inner face of the cylinder. The piston (1) has plungers (37) at its two ends to run into holes in the end plates (4 and 5) of the cylinder to produce a dashpot braking effect. Such plungers are made integrally with the casing. A detector ring (32) is mounted in coaxial grooves (13) in the abutting faces of the two piston halves for detection of the position of the piston by an electromagnetic or the like system. <IMAGE>

Description

SPECIFICATION A piston The present invention relates to pistons and more particularly but not exclusively to pistons for hydraulic and pneumatic actuator cylinders.

A large variety of different forms of pistons for actuator cylinders has been proposed.

One object of the present invention is to design a piston that is low in price.

A further object of the invention is to devise a piston that is simple to manufature.

One additional object is to produce a piston that is particularly simple to assemble and mount.

In order to effect these and other objects the present invention consists in a piston designed for reciprocating motion along an axis and comprising two piston halves symmetrical to each other about a plane of symmetry therebetween generally normal to said axis, each said half being made up of a radially inner metal core and a radially surrounding casing body molded on said core, said core having a radially projecting anchoring structure thereon embedded within said molded body.

The design of the piston of the invention on the basis of two identical halves leads to simplification of the manufacture and stockholding of its parts. Although primarily the two halves are to be combined in a pair for use as a double acting actuator cylinder, they may, as a further teaching of the invention, be used separately in single acting cylinders. It is furthermore possible for the one half of the piston to be used as a module to be put together with other halves as part of a multipiston arrangement, vi. one having more than two such piston "halves". The design of the piston in the form of a composite body with a metal core and a casing of resin represents an optimum way of meeting the loads experienced in operation. The core endows the piston body with a high degree of physical strength and it is able to take up the axial forces acting thereon.The resin material of the casing body leads to low-friction running properties on the inner face of cylinder of the actuator and is able to take up the transverse forces and moments ocurring during operation elastically. Such resin is furthermore characterized by low costs as a material and it may be accurately molded at a low price, The joint between the resin and the core as produced by molding is durable and withstands high loads. Available resin molding technologies are characterized by low costs.

In accordance with a useful development of the invention, the anchoring part is designed in the form of a collar and is placed in the axially middle part of the core, viz. halfway between the axial ends of said core. Furthermore, the anchoring part may be provided with openings therethrough to receive resin during the process of molding so that the resin and the anchoring part are interlocked with pieces of the resin extending throughsuch openings from one part of the body of resin to the other.

Such a development of the present invention is responsible for aparticularly strong connection between the metal core and the casing of resin.

Furthermore the sides of the two halves that make contact with each other may be provided with locking means for connecting the two halves of the piston together and preventing relative twisting thereof. Such locking means may be in the form of projections molded on the casing body, for example in the form of an axially projecting pin fitting into a hole on the respective other half of the piston, each such half having at least one such pin and one such hole placed diametrally opposite thereto on said half in question. With such a design of the piston one may be certain that it has properties in operation fully equalling those of a single-piece piston. Sealing or piston rings mounted on the two piston halves are not loaded by an additional degree of freedom of twisting, viz. they are not twisted by the two halves twisting in relation to each other.The non-twist connection between the piston halves may be produced in a simple way by molding locking means on the resin casing producing a sort of tongue and grooves joint.

In keeping with a further development of the invention, on the side of each piston half opposite to the contact face there is a molded plunger sleeve running out past the core in an axial direction. This constitutes a combined guiding and damping means that is simple in construction.

For at least part of its axial length the outer face of the casing body may function as a guide face running directly on the inner face of the cylinder wall so that it is possible to do without separate sliding rings to run on the cylinder wall, this resulting in economies. Furthermore the casing body may have peripheral recesses to receive lubricant adjacent to the guide or running face so that the piston may run more freely in the cylinder.

As part of a further outgrowth of the invention, the casing body may have ring-like grooves at the contact face between the two piston halves in order to receive part of a piston detecting system in the form of a detector ring. Such a design means that the piston is easily assembled and simple in form.

Such a detector ring may be simply placed in the groove.

The ring-like groove may furthermore simultaneously serve as a means for storing lubricant, if the groove is not used for part of system for detecting its position and the groove is accordingly not occupied by a detector ring.

Further details and useful effects of the invention will be seen from the following account of one working example thereof as shown in the accompanying figures.

Figure 1 is a lengthways section through a double acting actuator cylinder, that is fitted with the piston of the present invention.

Figure 2 is a section taken through one half of the said piston on the line ll-ll of Fig. 3.

Figure 3 is an end-on view of the one piston half looking in the direction Ill as marked in Fig. 2.

Figure 4 is a plan view of the edge of the piston half seen looking in the direction IV of Fig. 2.

Figure 5 is a view of a metal core of the piston half as seen in an end-on view looking in the direction Ill of Fig. 2.

Turning firstly to Fig. 1 the reader will be able to see a working example of the invention in the form of a double acting piston 1 for use in a pneumatic or hydraulic actuator cylinder 2. The cylinder consists of a cylinder barrel 3 and two end plates 4 and 5 shutting off the ends of the cylinder and having connections 6 for pressurized driving fluid. The end plates 4 and 5 have axial holes 7 in communication with the connections 6. The axial hole 7 in the cylinder end plate 4 runs right through the end plate to accommodate a piston rod 8, whose one end protrudes from the cylinder end plate 4 and runs with a supporting and guiding action on a bush 9 in the axial hole 7 of the end plate 4; there is a piston rod seal 10 adjacent to the bush 9.

The axial hole 7 in the other cylinder end plate 5 is a blind hole. In both the cylinder end plates 4 and 5 the inner face 11 of the axial hole 7 running towards the inside 1 2 of the cylinder 2 is spaced from the piston rod 8. In the inner face 11 there is a circumferential groove 1 3 to receive a plunger seal 14.

This seal is part of a system to be described later for damping the piston 1 in its end positions.

The piston 1 is made up of two piston halves 15, that are completely identical. The piston halves 1 5 have their axial ends in contact at a contact face 1 6 forming a plane of bilateral symmetry and normal to, viz. radial in relation to, the length direction of the piston. The piston parts 1 5 take the form of modular components or elements, that as may be seen from Fig. 1 are mounted on the same piston rod 8 to form the single piston of a double acting actuator cylinder. It is however to be borne in mind that the piston halves may furthermore be used as separate pistons, more preferably in single acting actuator cylinders (not illustrated).

Each of the piston halves 1 5 has a metal core 1 7 with a generally cylindrical form. The core 1 7 has an axial through hole 1 8 allowing the piston halves to be mounted on a centering pin 1 9 on the piston rod 8. This pin 1 9 forms an axial continuation or tail of the piston rod 8 and is smaller in diameter than it is. At the transition between the piston rod 8 and the pin 1 9 a radial or ring-like shoulder 20 is formed against which the core 1 7 of one of the two piston halves 15 5 on the pin 1 9 abuts. The shoulder 20 so forms an abutment limiting the distance the piston haves 1 5 may be pushed onto the pin 19.The outer diameter of the core 1 7 at the point of abutment on the shoulder 20 is generally the same as the outer diameter of the piston rod 8.

The pin 1 9 projects through the through holes 1 8 in the two piston halves and has a threaded end 21 projecting axially out of the second piston half 1 5. A cylindrical nut 22 or sleeve is screwed onto the threaded end 21, such nut acting against the core 1 7 of the second piston half 1 5. The outer diameter of the nut 22 at the position of abutment is equal to the diameter of the core 1 7 viz.

about the diameter of the piston rod 8. The free end, remote from the piston 1, of the nut 22 is adapted in its shape so that it may be grasped with a suitable wrench. More specially, it may be made with a female hex socket. To assemble the piston 1 of the invention the piston halves 1 5 are placed one after the other on the pin 1 9 of the piston rod 8, whereupon the nut 22 is screwed on and screwed up tight. The piston halves 1 5 are so clamped together as not to be able to be twisted while at the same time being fixed in the axial direction on the piston rod 8.

The cores 1 7 of the piston halves 1 5 are fully covered by casing body 23, that surrounds the cores on their radially outer sides with an interlocking fit. The casing body 23 is made of resin injection molded onto the cores 1 7. In order to improve the connection between the casing body and each core 17, the latter has a radially projecting anchoring part 24, that may for example have the form of a collar. As will be seen from Fig. 1 the anchoring part 24 is located in the axially central part of the core 1 7. Between the contact face 1 6 and the anchoring part 24 the outer face of the core 1 7 is generally cylindrical, whereas in the part beyond the anchoring part 24 it tapers conically to a certain degree. The core 1 7 is embedded in the casing body 23 for its full axial length. The body 23 ends flush with the metal core 1 7 at the contact face 1 6 of the piston halves 1 5.

Referring now to Fig. 5, it will be seen that the anchoring part 24 is fashioned with openings 25, that take up resin during the molding process so that there is an interlocking effect between the anchoring part and the resin affording a particularly strong connection between the core 17 and the casing body 23.

The openings 25 have the form of slots in the example illustrated and are concentrically placed round the axis of the piston half 1 5 so that some of their limits are formed by arcs centered on the axis of the piston. It is more specially possible to have eight such slots, that subtend an angle of for example about 33 , whereas the webs between them subtend angles of about 12".

Referring now to Figs. 2 to 4 it will be seen that the piston may have mating locking means at the contact face 16, the two halves being identical in this respect, viz. having the same male and female configurations in each case, so that the two piston halves 1 5 may not be turned in relation to each other when locked together. Such locking means are best made by suitably molding the resin of the casing body 23 so that manufacture remains a simple operation. In the example of Fig. 2 it will be seen that a male member or pin 26 is molded on the outer edge of the casing body 23 so as to extend axially from the contact face 16.Diametrally opposite to the pin 26 (see Fig. 3) there is a complementary socket 27 to receive the corresponding pin 26 on the other half 1 5 of the piston so that on fitting together the two piston halves 15, that are fully identical in respect of the locking means, the pin 26 of the one half fits into the socket 27 of the other and vice versa and one may be certain that the two halves will not be able to be twisted in relation to each other. In place of having just one pin 26 and one pin socket 27 per half it would naturally be possible to have more of them. Furthermore, it would be possible to have other forms of joints with male and female elements locking into each other.

Reverting again to Fig. 1, it will be seen that the piston halves 1 5 are so designed that the casing body 23 runs on the inner face of the cylinder barrel 3. The casing body 23 is for this purpose designed in the form of a cylinder with a guide face 28 stretching along the part of its axial length. There are therefore no separate bearing rings mounted on the piston 1 of the invention to make contact with th said inner face, this greatly simplying the structure of the piston. In the present example the guide face 28 takes the form of the part of the piston half 1 5 directly adjacent to the contact face 1 6 so that the piston 1 made up of the two piston halves 1 5 has its middle part running on the inner face of the barrel.

Next to the guide face 28 in the axial direction there is a peripheral groove 29 in the respective half 1 5 to receive a piston sealing ring 30. In the case of a piston 1 for a double acting cylinder 2 it js preferred for both the piston halves 1 5 to be fitted with a piston sealing ring 30. In the present example the piston sealing rings 30 are of C-cross section so that there are two lips and the opening between the lips is turned in the axial direction away from the contact face 1 6. The longer of the two lips is in each case placed on the floor of the peripheral groove 29, whereas the sealing lip that is shorter in the axial direction runs on the inner face of the cylinder barrel 3.

The peripheral grooves 29 for the piston sealing rings 30 are placed axially halfway between the ends of each casing body 23 so that they are radially alined with the anchoring part 24 of the core 1 7. As noted earlier, the outer face of the casing body 23 has the part 28 running on the inner face of the barrel between the peripheral groove 29 and the contact face 1 6. On the other hand the part 31 of the piston on the side of the groove 29 opposite to the contact face 1 6 is smaller in diameter, the casing body here not running on the inner barrel face and only serving to keep the piston sealing ring 30 in place.

As may be seen from Fig. 1, the piston 1 is fitted with a detector ring 32 for use in a systm for electromagnetically ascertaining the position, speed and/or acceleration of the piston. That is to say, the motion of the piston may be magnetically or inductively detected.

The detector ring 32 is held between the two piston halves 1 5 within the contact face 1 6.

To make this possible each piston half has a concentric groove 33 opening in the axial direction and designed to take up one respective half of the said detector ring 32. In the illustrated example of the invention. each of the annular grooves 33 has a rectangular cross section. The two groves have the same cross section and diameter so that a continuous annular space is produced to accommodate the detector ring 32.

In order to reduce friction of the piston 1 in the cylinder it is possible to have grease pockets 34 or recesses in the running face 28 to receive a suitable lubricant for permanent lubrication of the piston. With reference to Figs. 2 to 4 it will be seen that a particularly simple arrangement is possible in this respect in which the recesses 34 are open towards the contact face 1 6 of the two piston halves 15. The recesses 34 are in the form of cutouts with the form of a sector between the annular groove 33 for the detector ring 32 and the outer face of the casing body 23. The axial depth of the cutouts is in this respect less than the depth of the annular grooves 33.The cutouts are so spaced out about the axis of the piston halves 1 5 that in the assembled condition they are opposite to each other and in cross section form double lubrication openings. It is for example possible to have eight recesses 34 each subtending an angle of about 15 for example and being separated by humps each subtending an angle of the order of 30 .

If electromagnetic detection of the piston position is preferred, the annular groove 33 will be fitted with a detector ring 32 and lubricant will only be charged into the peripheral recesses 34. If on the other hand no detector ring is needed, the annular groove 33 will be filled with lubricant which will escape through the recesses as it is needed.

This is a substantial advantage of the lubricant recesses 34 running into the annular groove 33.

Reverting again to Fig. 1, it will be seen that the piston halves 15, that are held together, are sealed at their contact face 1 6.

This sealing function is preferably performed adjacent to the metallic core 17, that is radially inside the piston and may readily be machined to produce a high-quality sealing face. As a sealing means use is made of a gasket ring 35 placed around the pin 1 9 of the piston rod 8. The through hole 18, receiving the pin 19, of each piston half is made wider adjacent to the contact face 1 6 so tha two halves of a groove 36 will be formed to accommodate the said gasket ring 35. When the piston halves 1 5 are pressed together the gasket ring 35 will be compressed somewhat to sealingly engage the pin 1 9 on the piston rod 8.

The piston halves 1 5 are formed with plungers 37 for damping the piston 1 at the end of a stroke. The plungers 37 are integral parts of the casing body 23 made of resin and project axially from the main parts of the piston halves on the sides thereof opposite to the contact face 1 6 past the cores 1 7. The inner diameters of the plungers are the same as the outer diameter of the piston rod 8 so that they may be fitted on a part of the piston rod 8 or of the nut 22. The plungers 37 of the piston 1 move in and out of the holes 7 in the end plates 4 and 5 and make sealing contact with the seals 14. The ends 38 of the plungers 7 are rounded to make it easier for them to enter the seals 14.

In Fig. 1 the piston 1 will be seen at the left hand end of its stroke next to and abutting against the left end plate 5 with the radial face 39 of the casing body 23. To move back the piston in the opposite direction fluid under pressure will be admitted to the connection 6 of the left hand cylinder end plate 5 whereas the connection 6 of the right hand end plate 4 will be opened for the discharge of fluid therefrom. During the backward stroke of the piston 1 the fluid within the interior 1 2 of the actuator cylinder 2 will firstly discharge through the full cross section of the axial hole 7. However this outlet then will be shut off when the plunger 37 enters the axial hole 7 and engages the seal 14. As a result fluid is trapped in the cylinder between the piston and the right hand end plate to damp the motion of the piston shortly before the end of its stroke. The seal 14 is in the form of a special-purpose 3-lip seal and allows fluid to flow to a limited degree past the groove 1 3 so that the said trapped fluid is slowly discharged. The possible back pressure of the trapped fluid is therefore limited and the piston moves till it abuts the cylinder end plate 4.

The invention is not limited to the piston made up of two piston halves 1 5 and there is the possibility of using one such half 1 5 in a cylinder alone, more specially in a singleacting actuator.

Claims

1. A piston designed for reciprocating motion along an axis, the piston selectively comprising one or more piston parts, each being formed of a radially inner metal core and a radially surrounding casing body molded on the core, the latter having a radially projecting anchoring structure thereon embedded within the molded body, the piston parts, when the piston is selected to comprise two piston parts, being symmetrical to each other about a plane of symmetry therebetween substantially normal to said axis.

2. A piston as claimed in claim 1, wherein the anchoring structure is in the form of a collar located substantially halfway along the length of the respective piston part in the direction of the axis.

3. A piston as claimed in claim 1 or claim 2, wherein the anchoring structure has openings therein containing parts of said molded casing body to form interlocking arrangements securing the body in place.

4. A piston as claimed in any of claims 1 to 3, wherein the piston comprises two of the piston parts.

5. A piston as claimed in claim 4, wherein the piston parts are in contact with each other at a common contact face that is substantially normal to the axis.

6. A piston as claimed in claim 5, comprising interlocking means at the contact face joining the parts together with a detent effect and locking the parts together against relative rotation about the axis.

7. A piston as claimed in claim 6, wherein the interlocking means comprise a male and a female means on each piston part, the female means on each part being diametrally opposite to the male means thereon.

8. A piston as claimed in any of claims 4 to 7, comprising plungers projecting from the ends thereof and adapted to fit into holes in the end plates of a cylinder to perform a damping function.

9. A piston as claimed in any of claims 4 to 8, comprising a piston sealing ring mounted on each piston part in a peripheral groove.

1 0. A piston as claimed in claim 9, wherein the peripheral grooves for the piston sealing rings are located axially in the middle between the ends of each piston part, and wherein the portion of an outer face of the piston part, between the groove and a plane of contact between the piston parts normal to the axis, is adapted to run on the inner face of a cylinder to receive the piston, whereas a further portion of each piston part facing away from the axis and extending between the end of each piston part and the groove, is of lesser diameter than the portion adapted to run on the cylinder inner face.

11. A piston as claimed in any of claims 4 to 10, having an outer cylindrical support face on each piston part, the outer face extending along at least part of the axial length of the piston and being adapted to run on the inner face of a cylinder for use with the piston.

1 2. A piston as claimed in claim 11, wherein, adjacent to the support face, the casing body is formed with peripheral recesses adapted to receive a lubricant.

1 3. A piston as claimed in claim 12, when dependent on claim 5, wherein the recesses are open towards the common contact face between said piston parts.

14. A piston as claimed in any of claims 4 to 11, wherein the casing bodies have grooves in faces thereof at the plane of symmetry, the grooves being complementary and being adapted to receive a detector ring which may be detected by electromagnetic means outside a cylinder in which said piston is to be fitted.

15. A piston as claimed in claim 14, wherein each casing body is formed with peripheral recesses adapted to receive a lubricant, the recesses being in the form of sectorlike cutouts between the groove and the radially outer limit of each piston part.

1 6. A piston as claimed in any of claims 4 to 15, wherein the piston parts make sealing contact with each other.

1 7. A piston as claimed in any of claims 4 to 16, wherein the piston parts make sealing contact with each other at adjacent ends of the cores.

1 8. A piston as claimed in any of claims 4 to 16, wherein the cores have axial holes extending therethrough, the holes being wider at adjacent ends of the parts and having a sealing ring mounted partly in the wider section in one part and partly in the wider section in the other part.

1 9. A piston substantially as hereinbefore described with reference to the accompanying drawings.