GB2136506A

GB2136506A - Improvements in and relating to linear actuators

Info

Publication number: GB2136506A
Application number: GB08307069A
Authority: GB
Inventors: Roger Durrant
Original assignee: Individual
Current assignee: Individual
Priority date: 1983-03-15
Filing date: 1983-03-15
Publication date: 1984-09-19
Also published as: GB8307069D0; GB2136506B

Abstract

A linear actuator, for example a cylinder and piston, comprises a first member 1. e.g. a piston, which is received within a hollow elongate second member 2. e.g. a cylinder, to be movable relative thereto. The members 1, 2 having corresponding generally polygonal, for example square or triangular, external and internal peripheries respectively, and means 3 are provided for sealing the gap between the members 1, 2 the sealing means contacting the corresponding surfaces of the members and rolling relative thereto during relative movement of the members. In one embodiment, the sealing means comprises at least one set of rollers 3 which are arranged with their axis in or adjacent a plane perpendicular to the direction of relative movement of the members and which are dimensioned to seal against the members. The rollers 3 may have chamfered ends 9 (Fig. 18) to engage one against another, or concave ends (Fig. 8) to engage balls 14 at the corners of the cylinder. Racks 12a, 12b on the cylinder and piston walls respectively mesh a pinion 13 formed integrally on one roller. A cage engages grooves 10 in the rollers. <IMAGE>

Description

SPECIFICATION Improvements in and relating to linear actuators The present invention relates to improvements in linear actuators and specifically devices in which one member moves relative to and within another member, the members moving relative to one another in response to linear forces, and specifically in response to or to cause changes in fluid pressure.

In converting fluid pressure forces into linear motion, traditionally a piston slidable in a cylinder or tube is used. Means for sealing the gap between the piston and the cylinder, to prevent pressure losses, are provided generally by thin annular elements of various constructions which are fitted to the piston, or by elastomeric torroidal elements fitting in grooves. These sealing means slide against the cylinder and therefore, before there relative movement, work has to be done to overcome the friction between the sealing means and the cylinder. This means that, where for example, input energy levels are low, as with measurement of changes in barometric pressure, a cylinder and piston arrangement cannot be used because it is too insensitive or because too much of the input energy is lost in overcoming the frictional forces.

Further, heat engines have traditionally converted the pressure exerted by the working fluid, for example combusted petrol-air mixture, spontaneously ignited oil-air mixture, or hot vapour, into linear motion by containing the working fluid in a cylinder and allowing a theoretically freely moving piston to respond to the pressure to travel a pre-determined distance along a cylinder, the distance being controlled by the crank shaft throw. The piston must however be effectively sealed against the cylinder walls and this is conventionally achieved by the use of piston rings which exert a uniform radial pressure on the cylinder walls of the order of 14 KN/m2, and which in use slide along the cylinder walls.

Crank case oil must also be prevented from entering the combustion space, but sufficient must be present in the cylinder to lubricate the piston rings. Oil control rings are generally used which also exert a radial pressure on the cylinder walls, typically of the order of 280 KN/m2. Because of the need for these high pressure seals and the consequently high radial forces which these seals must exert, a not insubstantial amount of the energy of the working fluid is used in overcoming the frictional forces between the sealing rings and the cylinder. Furthermore allowance has also to be made for expansion of the piston, and gaps are in the sealing rings for this purpose.

These gaps can allow blowby on starting from cold or ambient temperatures, and on reaching working temperatures a sufficient gap must still be present to prevent the rings seizing and scoring the walls of the cylinder.

The rings therefore do not always conform to the surfaces against which they are supposed to seal at all conditions and therefore allow potential leaks. Additionally, in time wear of the cylinder bore occurs producing an oval shape for the cylinder. Under these conditions piston slap can occur.

According to one aspect of the present invention there is provided a linear actuator comprising a first member which is received within a hollow elongate second member to be movable longitudinally relative thereto, wherein the members have corresponding generally polygonal external and internal peripheries respectively, and means are provided for sealing the gap between the members, the sealing means contacting corresponding surfaces of the members and rolling relative thereto during relative movement of the members.

The sealing means advantageously comprises a set of rollers which are arranged with their axes in or adjacent to a plane perpendicular to the direction of relative movement of the members. The sealing means may comprise one set of rollers or a plurality which are spaced apart in the direction of relative movement of the members. The rollers are advantageously mounted in a frame or cage surrounding the first member, the cage or frame and rollers moving relative to both the members during relative movement of the members.

At the corners between adjacent planar surfaces, sealing may be achieved by appropriate shaping of the ends of the adjacent rollers of a set so that, at the ends, the rollers make rolling contact with each other and with the surfaces of the second member.

Alternatively, the corners between adjacent planar surfaces of the second member may be curved to receive spherical sealing members which are retained by and seal against correspondingly shaped ends of adjacent rollers.

The sealing means may be positively located relative to the first and second members by for example a rack and pinion system, racks being provided correspondingly on corresponding surfaces of the members and a pinion being provided on one of the rollers or in association therewith.

For use under high temperature conditions, for example in a heat engine, the sealing means may be protected from the high temperatures by reducing the gap between the members at the appropriate end, or at both ends, of one or both members. For example, the first member may be provided with an enlarged end or the second member may have a reduced internal periphery at one or both ends. Additional relatively low pressure seals, or hydrocarbon or ferromagnetic fluids may be used to increase the protection and seal this reduced gap.

It will be appreciated that the forces required to overcome the frictional forces presented by the above described sealing means having rolling contact is much smaller than those for overcoming sliding friction and therefore the energy losses are much reduced, increasing the sensitivity and efficiency of the actuator. Additionally, wear of the sealing means and of the members will be substantially reduced not only because rolling contact rather than sliding contact is involved but also because, by virtue of the fact that the sealing means roll relative to both members, the sealing means will move only half the distance moved by the members.

The diameters of the rollers of the sealing means are advantageously selected so that, for the reciprocating system, in each stroke they rotate through a complete revolution to prevent uneven wear of the rollers.

The above described arrangement not only provides a linear actuator with an efficient seal but, where a plurality of sets of rollers are provided, the sealing means also serves to guide the members in their relative movement and, when used as a cylinder and piston in a heat engine can reduce piston slap.

Embodiments according to the present invention will now be described, by way of example only, with reference to the accompanying drawings, in which: Figure 1 is a diagrammatic vertical section on the line ll-ll of Fig. 2 through an embodiment of linear actuator according to the are sent invention; Figure 2 is a plan view of the embodiment of Fig. 1; Figure 3 is a section on the line Ill-Ill of Fig. 1; Figure 4 is a section on the line IV-IV of Fig. 3; Figures 5 and 6 are plan views of components of the actuator of Figs. 3 and 4 Figure 7 is a detailed view of part of the actuator of Figs. 3 and 4; Figure 8 is a section through another embodiment of linear actuator according to the present invention; Figure 9 is a section on the line IX-IX of Fig. 8;; Figures 10 and 11 are plan views of components of the linear actuator shown in Figs.

8 and 9; Figure 12 is a detailed view of part of the linear actuator of Figs. 8 and 9; Figure 13 is a section through a further embodiment of linear actuator according to the present invention; Figure 14 is a section on the line XIV-XIV of Fig. 13; Figures 15 and 16 are plan views of components of the linear actuator of Figs. 1 3 and 14; Figure 1 7 is a detailed view of part of the linear actuator of Figs. 1 3 and 14; Figure 18 is a section through a further embodiment of linear actuator according to the present invention; and Figure 19 is a section on the line XIX-XIX of Fig. 18.

The linear actuator shown diagrammatically in Figs. 1 and 2 is intended for use as a cylinder and piston in a reciprocating heat engine. The cylinder and piston assembly is shown in Fig. 1 at mid stroke and comprises a generally square section piston 1 which moves within a generally square section bore in a cylinder 2. The gap between the cylinder and piston is sealed, and the piston is supported and guided in its movement relative to the cylinder, by sealing means which comprise, as shown, two sets of rollers 3a, 3b, 3c and 3d, and 3e, 3f, 39 and 3h (of which rollers 3c, 3dare not shown in Fig. 1).

These rollers 3 roll against both the cylinder and piston and are dimensioned to seal against the walls of both the cylinder and the piston. The rollers of each set are arranged with their axes in a plane perpendicular to the direction of relative movement of the cylinder and piston to create a continuous seal between the cylinder and piston in that plane.

For additional sealing and/or to protect the rollers 3 from external conditions, the dimensions of the bore on either side of the sealing means is reduced to conform more closely to that of the piston 1, to thereby create an annular recess 4 within which the sealing means are located. It will be appreciated that the axial length of the recess 4 is calculated in dependence on the stroke of the piston 1 within the cylinders 2 and the corresponding movement of the roller 3. The piston 1 has an axial length such that the ends are always received within the portions of the bore in the cylinder of reduced dimensions.

More positive sealing may be achieved in the region of the portions of the cylinder of reduced dimension by the use of hydrocarbon or ferromagnetic fluids.

As shown in Fig. 2, in a heat engine, the cylinder and piston shown in Fig. 1 is advantageously arranged with the crank shaft 7 extending diagonally of the cylinder 2 to allow small end and connecting rod 8 forces to be absorbed by the rollers 3 on all four surfaces of the cylinder and piston, instead of only two were the cylinder placed squarely over the crankshaft.

The sealing means of Figs. 1 and 2 is shown in greater detail in Figs. 3 to 7. As shown each roller 3 comprises a cylindrical portion corresponding in length to a face of the piston 1 and conical end portions 9.

These end portions have a cone angle of 45 so that the end portions 9 of adjacent rollers of a set contact and seal against each other to fully seal the periphery of the piston. The corners of the cylinder are then correspondin gly shaped, in other words they are bevelled at 135 to the adjacent surfaces of the cylinder, so that the end portion 9 of each roller also makes sealing contact with the bevelled surface to fully seal the periphery of the bore of the cylinder.

The sets of rollers 3 are retained by a cage which comprises rectangular side cages 11 holding corresponding rollers of each set in relative position, the side cages 11 being connected by braces (not shown) which encircle the piston. As shown, the rollers 3 are retained in the side cages 11 by providing the rollers with grooves 10 which engage in slots 11 a in the side cages 11.

To keep the sealing means, comprising the sets of rollers, in position relative to the cylinder and piston, a rack and pinion system may be provided comprising corresponding racks 1 2a, 1 2b on opposed faces of the cylinder and piston and a pinion 1 3 which meshes with both racks and is provided on one of the rollers 3.

The embodiment shown in Figs. 8 to 1 2 is similar to that shown in Figs. 3 to 7 and corresponding parts have been designated by the same reference numerals. The main difference between the two embodiments is in the manner of sealing at the corners of the cylinder and piston. As shown, in this embodiment, the rollers 3 have a length equal to the length of the faces of the piston and are provided with inwardly concave or, as shown conical, ends. The corners of the cylinder are provided with hemispherical recesses in which spheres 14 are located, the spheres being retained in place by engagement with the ends of the adjacent rollers.

In the embodiment shown in Figs. 1 3 to 17, sealing at the corners of the cylinder is achieyed by square section elements 1 5 which are slidably received in the corners of the cylinder and seal there against. The rollers 3 are conveniently mounted in these elements by shafts 1 6 received in the elements. To accommodate the shafts 16, adjacent rollers of each set may be longitudinally staggered.

Alternatively reduced length shafts may be used so that the axes of rollers in each set can be coplanar. The elements 1 5 are advantageously interconnected by braces 1 7 which form with the elements 1 5 a cage for the rollers 3.

In the above described embodiments, the cylinder and piston of the actuator are described as having square sections. It will be appreciated that they may have other polygonal sections. For example, as shown in Figs.

1 8 and 19, the cylinder and piston may be triangular in section. The embodiment of Figs.

1 8 and 1 9 in fact differs from that shown in Figs. 3 to 7 only in that the end portions of the roller 3 have a cone angle of 30 and the corners of the cylinder are not bevelled to create a single flat surface but two inclined surfaces, relatively inclined at 120 to each other and at a 150 to the adjacent surfaces of the cylinder.

While specific reference has been made to the use of the above described linear actuators in relation to the heat engines, it will be appreciated that they may be used in many other contexts for translating linear forces into fluid pressure changes and vice versa.

Claims

1. A linear actuator comprising a first member which is received within a hollow elongate second member to be movable longitudinally relative thereto, wherein the members have corresponding generally polygonal external and internal peripheries respectively, and means are provided for sealing the gap between the members, the sealing means contacting corresponding surfaces of the members and rolling relative thereto during relative movement of the members.

2. A linear actuator as claimed in Claim 1, wherein the sealing means comprises a set of rollers arranged with their axis in or adjacent a plane perpendicular to the direction of relative movement of the members.

3. A linear actuator as claimed in Claim 2, wherein the sealing means comprise a plurality of sets of rollers which sets are spaced apart in the direction of relative movement of the members.

4. A linear actuator as claimed in any one of Claims 3 to 9, where the sets of roller also serve to guide the members in their relative movement.

5. A linear actuator as claimed in either Claim 2 or Claim 4, wherein the rollers are mounted in a frame or cage surrounding the first member, the frame or cage and rollers moving relative to both members during relative movement of the members.

6. A linear actuator as claimed in any of of Claims 2 to 5, wherein each roller extends the full width of the corresponding surface of the first member and is shaped at its ends so as to make rolling contact with the end of an adjacent roller and with a surface of the second member.

7. A linear actuator as claimed in Claim 6, wherein the ends of the rollers are conical.

8. A linear actuator as claimed in any one of Claims 2 to 5, wherein each roller extends the full width of the corresponding surface of the first member and, at each end, engages a spherical sealing member which is received in a correspondingly shaped recess in the respective corner of the second member between adjacent planar surfaces.

9. A linear actuator as claimed in any one of Claims 2 to 5, wherein each member extends the full width of the corresponding surface of the first member and, at each end, engages an element which is slidably received in the respective corner of the second member to seal there against.

1 0. A linear actuator as claimed in any of Claims 2 to 9, wherein the members reciprocate and the diameters of the rollers are selected so that, in each stroke, they rotate through a complete revolution.

11. A linear actuator as claimed in any one of the preceding claims, wherein the sealing means are positively located relative to the first and second members.

1 2. A linear actuator as claimed in any one of Claims 2 to 10, wherein the or each set of rollers is positively located relative to the first and second members by pinion means associated with one of the rollers which engages with rack means associated with the members.

1 3. A linear actuator as claimed in any one of the preceding claims, wherein at one or both ends of the members, the gap between the members is reduced in width.

14. A linear actuator as claimed in Claim 13, wherein the external periphery of the first member is enlarged to close the gap.

1 5. A linear actuator as claimed in Claim 13, wherein the internal periphery of the second member is reduced to reduce the gap.

1 6. A linear actuator as claimed in any one of Claims 1 3 to 15, including sealing means between the members in the region of the reduced gap.

1 7. A linear actuator as claimed in any one of the preceding claims, wherein the first and second members have generally square external and internal peripheries respectively.

1 8. A linear actuator as claimed in any one of Claims 1 to 16, wherein the members have generally triangular external and internal peripheries respectively.

1 9. A linear actuator substantially as herein described with reference to the accom paning drawings.