Background Art Compressors for high pressures with both two and more compression stages have till now been distinguished by large, heavy and unwieldy constructions.

In different fields of applications, there has for a long time been a need for small and light compressors which are capable of supplying high pressures. Such a field of application is compressors for systems using high pressure pure air for charging gas bottles, which are used when cooling and cleaning electronics in infra- red equipment, e.g. thermocameras.

US-A-2,519,580 discloses a one-stage compressor with an integrated electric motor. Parts of the gas that is to be compressed are conducted through the housing for cool- ing the motor and are thus heated, which results in an initial increase of the volume of the gas. Consequently, the possibilities of compressing the gas will be limited to a considerable extent. Although the compressor has two chambers, only the second chamber is used for compres- sion. The first chamber accomplishes a gas transport to the second chamber and communicates with the inside of the compressor housing to equalise the pressure in rela- tion to the gas passing through the compressor for cool- ing the same. The compressor is also provided with non- return valves of the wafer-valve type for the internal gas transport, which makes high pressures in the com- pressor impossible. Moreover, the compressor construction would in operation be impaired by considerable wear and much heating owing to friction between piston and rotor

as well as between piston and housing, which would render high speeds and high pressures impossible.

Even if, at first glance, US-A-2,519,580 may seem to constitute a relevant example of the prior art technique, a closer study shows, as explained above, that the com- pressor described therein is neither intended nor suited to supply high pressures. US-A-2,519,580 thus gives no guidance or indication to a person skilled in the art to solve the problem of providing a compact and light two- stage compressor for high pressures.

Summary of the Invention An object of the present invention is to provide a compressor which is capable of supplying higher pres- sures, in relation to its weight and size, than were previous constructions.

According to the invention, this and other objects that will appear from the following specification are now achieved by a two-stage compressor having the features stated in the characterising clause of claim 1.

Thus, compression is carried out in two stages using an axially reciprocating piston assembly, whose opposing piston ends cooperate with one chamber each. The chambers, a first and a second compression chamber, are intercon- nected via a duct extending through the piston assembly.

The piston assembly, which comprises at least one part non-rotatably connected to the housing, is driven via a cam assembly by a rotor arranged radially outside the piston assembly. With this construction, a compressor has now been provided, which in spite of compact dimensions and a low weight is capable of generating extremely high pressures.

By the compressor besides being arranged to be driven by an external source of power, both the dimen- sions of the compressor and the internal generation of heat can be limited.

In a preferred embodiment of the compressor, the piston assembly comprises a part which is rotatable

relative to the non-rotatable part and arranged at the first piston end. The first compression chamber is sealingly defined by the first piston end and by the rotor. This enables compact dimensions in the radial direction, which means that the performance of the compressor in relation to its outer dimensions can be further limited.

In a preferred embodiment, the cam assembly com- prises two cam surfaces, which at least partly are directed in axially opposite directions, and at least one cam follower acting against each cam surface. The one of the rotor and the piston assembly is connected to the cam surfaces and the other is connected to the cam followers.

In a thus-designed cam assembly, axial motion of the piston assembly is accomplished in both axial directions.

Motion in a first direction is effected by the one cam surface in cooperation with the associated cam follower, and motion in a second direction is effected by the second cam surface in cooperation with the associated cam follower. No spring is required to maintain the contact between the cam surface and the cam follower. Moreover, a compressor designed in this manner is independent of the rotary direction of the rotor.

Preferably, each cam follower has a rolling means for rolling engagement with the cam surface. This reduces the friction, which in turn results in low inner genera- tion of heat and permits higher speeds and, thus, also higher pressures.

Brief Description of the Drawing The invention will now be described in more detail for the purpose of exemplification, with reference to the accompanying drawing which shows a presently preferred embodiment.

The figure is a longitudinal section of a compressor according to the present invention.

Description of a Preferred Embodiment of the Invention The figure shows an example of a two-stage compres- sor according to the invention. The compressor can supply extremely high pressures, above 50 bar and even up to about 350 bar.

Now follows a brief description of the construction of the compressor. The compressor comprises a housing 1 and a rotor 10 rotatably supported in the housing 1. The rotor 10 has a drive means in the form of a drive shaft 12, which extends axially outwards from an inlet end of the housing 1 to be driven by an external source of power (not shown). Radially inside the rotor 10 there is a piston assembly 20 which is connected to the housing and which is arranged for axial stroke motion and comprises two piston ends 21, 22 which are interconnected at an invariable distance from each other. The piston ends each cooperate with a compression chamber 31, 32. These are positioned axially on either side of the piston assembly 20 and, in operation, communicate with each other via spring-loaded non-return valves and a duct 33 extending through the piston assembly 20. A cam assembly 40 is connected to the rotor 10 and the piston assembly 20 to transfer the rotary motion of the rotor to the stroke motion of the piston assembly 20.

The function of the compressor will now be briefly described for compression of air. From the starting position shown in the figure, the volume of the first chamber 31 being fully compressed, the piston assembly is moved towards the second chamber 32. A vacuum forms in the first chamber 31, an inlet non-return valve 61 opens and ambient air is sucked into the first chamber 31 via an inlet duct 34 in the rotor 10. When the piston assembly is in its opposite position (not shown), where the volume of the chamber 31 is at its maximum, and when the difference in pressure between the chamber 31 and ambient air has been equalised, the valve 61 closes.

When the piston assembly 20 is again moved towards the position illustrated, the air in the first chamber 31 will be compressed. When a sufficiently high pressure has been achieved in the chamber 31, a non-return valve 63 associated with the duct 33 in the piston assembly 20 opens. The amount of pressure required for the opening of the non-return valve 63 depends on what pressure already prevails in the duct 33 and, thus, on the difference in pressure between air in the chamber 31 and the duct 33.

At the other end of the duct 33, adjacent to the second chamber 32, there is a further non-return valve 64, which is arranged to open when the difference in pressure between the duct 33 and the chamber 32 is sufficient, which occurs in connection with the piston assembly 20 moving towards the first chamber 31. When the volume of the chamber 32 is at its maximum and the pressure between the duct 33 and the chamber 32 has been equalised, the valve 64 closes, whereupon the air in the chamber 32 is compressed as the piston assembly 20 again moves towards the chamber 31. An outlet non-return valve 65 arranged in an outlet duct 35 of the chamber 32 opens when the pres- sure in the chamber 32 exceeds a predetermined initial pressure level.

Now follows a more detailed description of the pre- ferred embodiment of the compressor and its function. The housing 1 of the compressor is preferably made of metal and comprises a cylindrical body 2. A circular end piece 3 is connected at a first end (inlet end) of the body 2.

The end piece 3 has a central, circular opening through which extends the drive shaft 12 of the rotor. A seal 4 is arranged in the opening to seal against the drive shaft 12. The housing is preferably filled with oil.

At the other end (outlet end) of the housing, a high-pressure cylinder 5 is concentrically mounted in the end portion of the body 2 by means of bolts 51. The high- pressure cylinder 5 extending through the body 2 is arranged to receive the second piston end 22 (high-

pressure end) of the piston assembly 20. At the end of the high-pressure cylinder 5 facing away from the piston assembly 20, a high-pressure plug 6 is screwed in, con- centrically therewith. The outlet non-return valve 65 is positioned in the high-pressure plug 6, and a high- pressure conduit for withdrawing highly compressed gas can be connected to the plug 6.

On the inside of the high-pressure cylinder 5 ex- tends a guide means 7 comprising an annular base member 8 which fits round the inwardly directed end of the high- pressure cylinder 5 and is fixedly connected to the housing 1 by means of said bolts 51. From the base member extend two guide bars 9 axially inward from diametrically opposite positions along the circumference of the base member 8. The two guide bars 9 are intended to hold the piston assembly 20 in a non-rotatable manner. On each side of each of the two guide bars 9, an axial roller bearing 52 in the form of a ball 53 in a race 54 is arranged. The guide bars 9 extend into corresponding axially directed recesses 29 in the piston assembly 20, such that the balls 53 engage and roll against an axial recess wall in the recesses 29 when the piston assembly 20 is in operation. By the relative axial motion being received by means of roller bearings, the friction de- creases, and therefore higher speeds are permitted while at the same time the generation of heat will be limited.

The rotor 10 is rotatably supported in the housing 1 by means of two ball bearings 55, 56 which are arranged at either end of the housing 1. The outer rings of the two ball bearings 55, 56 are held axially apart by means of a spacing sleeve 57 arranged inside the body 2.

The rotor 10 has an inlet part 11 extending through the end piece 3 of the housing 1 and comprising the above-mentioned drive shaft 12, which is tubular and defines an inlet duct 34. Ambient air is supplied to the compressor. The air is compressed without any preceding heating, which makes it possible to achieve high pres-

sures. The inlet part 11 is directly supported in the ball bearing 55 and has an internal flat surface 13 of radial extent. This surface 13 constitutes the bottom of the first compression chamber 31. In this bottom, there is arranged a mushroom-shaped and spring-loaded inlet non-return valve 61 with an associated valve seat 62. The surface of the valve 61 facing the compression chamber 31 is smooth and extends in the same plane as the surface 13, thereby achieving a maximum active volume in the compression chamber 31.

Inwards from the inlet part 11 extends a tubular low-pressure cylinder 14 which engages an annular recess in the periphery of the surface 13. On the inside in the radial direction, the low-pressure cylinder 14 forms the chamber wall of the compression chamber 31 and receives the first piston end 21 of the piston assembly 20. An annular flange 44 associated with a cam ring 41, which will be described in more detail below, engages the side of the low-pressure cylinder 14 facing away from the inlet part 11. On the axially opposite side of the cam ring 41 extends a tubular end sleeve 15 to the second ball bearing 56, on the outlet side. The inlet part 11, the low-pressure cylinder 14, the cam ring 41 and the end sleeve 15 are all fixed to each other by means of axial bolts extending through all these piston parts. Moreover, the parts of the rotor 10 are axially clamped between the ball bearings 55, 56.

As mentioned above, the piston assembly 20 has a first piston end 21, which is concentrically arranged in the low-pressure cylinder 14 about a through centre axis A. The first piston end 21 belongs to a low-pressure piston 23 and forms a flat surface of radial extent. The low-pressure piston 23 has a cylindrical piston wall 24 extending axially from the surface 21 and having a com- pression ring 71 and an oil wiping ring 72, which seal between the low-pressure piston 23 and the low-pressure cylinder 13. The flat surface of the piston end 21 is

directed towards the surface 13 and surrounds a centrally arranged non-return valve 63 which is ball-shaped and spring-loaded. The non-return valve 63 is arranged in the central duct 33 of the piston assembly 20 and, more specifically, in an annular part 25 belonging to the low- pressure piston 23 and extending axially from the first piston end 21. The low-pressure piston 23 is rotatably arranged relative to the rest of the piston assembly 20, and therefore the low-pressure piston 23 will in opera- tion rotate together with the rotor 10. More precisely, the low-pressure piston is via a ball bearing 58 con- nected to a shuttle 27, which is in turn non-rotatably connected both to the housing 1 and to the rest of the piston assembly. The annular part 25 extends into a cylindrical recess in the shuttle 27. A shaft seal 73 is arranged in the cylindrical recess of the shuttle to seal against the annular part 25. By the low-pressure piston 23 received in the rotor 10 being allowed to rotate to- gether with the rotor 10, a reliable and strong pressure sealing of the first compression chamber is ensured. It is in fact easier to obtain a satisfactory and strong seal between rotating and non-rotating parts over a shorter distance with a smaller difference in speed, such as round the annular part 25, compared with a longer distance with a greater difference in speed, such as between the low-pressure piston 23 and the low-pressure cylinder 14.

Through the shuttle 27 extends a tubular centre bolt 26. The centre bolt defines a longitudinal portion of the duct 33 and extends from the central annular part 25 of the low-pressure piston 23 and is at a second end con- nected to high-pressure piston 28. The shuttle 27 extends round the centre bolt 26. The shuttle 27 has two im- portant functions, viz. on the one hand to hold four cam followers 45, 46, which will be described in more detail below, to transfer the rotary motion of the rotor 6 to the stroke motion of the piston assembly 20 and, on the

other hand, to cooperate with the above-mentioned guide means 7 to hold the piston assembly 20 in a non-rotatable manner. The cam followers 45, 46 extend radially away from the shuttle 27 and are rotatably connected thereto in fixed and diametrically opposite positions. The cam followers 45, 46 are arranged in a constant spaced-apart relationship in the axial direction. The cam surfaces are correspondingly arranged at an intermediate distance in the axial direction, which is constant along each circum- ferential position. In positions which are offset from the cam followers 45, 46 by 90°, the shuttle 27 has two diametrically opposite, axially directed recesses 29, which are arranged to engage the axial roller bearing 52 of the guide bars 9, thereby holding the piston assembly 20 in a non-rotatable manner.

The high-pressure piston 28 extends from the centre bolt to the second piston end 22 of the high-pressure cylinder 5 and is supported in the high-pressure cylinder 5 by means of linear roller bearings 59. The linear roller bearings result in reduced friction and more reli- able guiding in the radial direction, which improves the performance of the compressor. A mushroom-shaped, spring- loaded non-return valve 64 is arranged in the duct 33 at the piston end 22. The end of the valve 64 facing the compression chamber 32 is flat and aligned with the end surface 22 (piston end) of the high-pressure piston, thereby achieving a maximum active volume in the com- pression chamber 32. The valve 64 is, like the above- described valves, adapted to open when the difference in pressure over the valve exceeds a predetermined value.

Seals 74 are arranged in the periphery of the high- pressure piston 28 to seal against the high-pressure piston 5.

In a manner corresponding to that of the above- described valve 63, a non-return valve 65 is placed in the outlet plug 6, said valve 65 opening as the pressure

in the second compression chamber 32 exceeds a predeter- mined pressure.

The cam assembly 40 comprises, as mentioned above, a cam ring 41, which has an annular flange 44 and an inner cam portion, which has two axially directed opposed cam surfaces 42, 43. The cam surfaces 42, 43 describe paral- lel sine curves with two diametrically opposed troughs and two corresponding ridges positioned therebetween. The distance between the cam surfaces in the axial direction is constant.

The shuttle 27 has, on each side of the cam surfaces 42, 43 and in diametrically opposed positions, cam fol- lowers 45, 46, which thus are four in number. In the embodiment shown, these cam followers consist of cam rollers 45, 46 which are arranged to roll against the cam surfaces 42, 43. The cam rollers are, via pins 47, mounted on the shuttle 27 to transfer motion to the piston as- sembly 20. By double cam followers being symmetrically arranged at each cam surface, a symmetric distribution of power is achieved, which enables higher speeds. The com- pressor according to the embodiment can be made to rotate by up to 900 revolutions per min.

In a further embodiment of the invention, the cam ring of the cam assembly can be U-shaped in cross- section, with two inwardly directed cam surfaces, facing each other, in the form of ball races. In this embodi- ment, the shuttle has two ball races, which are directed outwards away from each other and extend circularly round the shuttle. In positions corresponding to the embodiment described above, four balls are confined between the ball races, thereby effecting the stroke motion of the shuttle as the cam ring rotates.

The non-return valves that have been described are adapted to control, together with associated springs, the high pressures stated. The ratio of the mass of the valve members to the spring force is also adapted to the accele- rations that arise in the compressor.