GB2330012A

GB2330012A - Linear motor compressor

Info

Publication number: GB2330012A
Application number: GB9721090A
Authority: GB
Inventors: Wei-Min Zhang
Original assignee: Z&D Ltd
Current assignee: Z&D Ltd
Priority date: 1997-10-04
Filing date: 1997-10-04
Publication date: 1999-04-07
Anticipated expiration: 2017-10-04
Also published as: GB2330012B; GB9721090D0

Abstract

A compressor (10; 100 see figure 4A) with a built-in linear motor, comprises a cylindrical housing (20; 120) fitted with two electromagnets (40, 40'; 140, 140'), a shuttle (50; 150) supported by a central guide arrangement (30, 30'; 130) for keeping it coaxial with the other members of the compressor and protected by a suspension mechanism The central shaft can be made hollow to circulate a coolant through it to cool the compressor from inside. The shuttle carries an annular primary magnet (51; 151) and two axial magnets (55, 55'; 155,155') so as to act with the two electromagnets to form a push-and-pull driving force between them. A multi-stage compression arrangement is formed between the shuttle and the two electromagnets so that the compressor can work in a double-acting manner to compress the process fluid progressively for forming a high pressure output.

Description

Linear Motor Compressor Technical Field of Invention This invention relates to a linear motor and a compressor incorporating such a motor.

Background of Invention The present invention provides improvements to the invention disclosed in my UK patent no. GB-2299715-B, its disclosure is incorporated herewith by reference.

Summary of Invention An object of this invention is to provide an improved linear motor and/or compressor.

According to one aspect of the invention, there is provided a linear motor comprising: two opposing magnetic driving members, each with coaxial pole means; a reciprocating member disposed between said driving members; and means for energising said driving and/or reciprocating members; wherein said reciprocating member has poles matching that of the driving members to form push-and-pull driving force and wherein central guide means is arranged for keeping said reciprocating and driving members coaxiaL According to another aspect of the invention, there is provided a linear motor compressor comprising: a housing fitted with two opposing magnetic driving members, each with coaxial pole means; a reciprocating member disposed between said driving members; valve means for forming fluid passage into and out of said housing; and means for energising said driving and/or reciprocating members; wherein said reciprocating member has poles matching that of the driving members to form push-and-pull driving force and wherein central guide means is arranged to keep said reciprocating and driving members coaxial.

It is preferable that said reciprocating member has annular primary magnet means magnetised in radial direction and two secondary magnet means fitted to two axial ends of said primary magnet means and magnetised in axial direction so as to form said push-and-pull driving force with said driving members. Advantageously, the reciprocating member has pole means formed between said primary magnet means and each of said secondary magnet means for acting with the coaxial pole means of said driving members.

Advantageously, the coaxial arrangement by said central guide means ensures the reciprocating member's non-contact movements relative to said driving members, thus to allow its lubricant-free operation. The central guide means can be formed by a pair of coaxially aligned shafts, each fitted into one end of said reciprocating member. Alternatively, the central guide means can be a hollow shaft extending through said reciprocating member so that a cooling fluid can be circulated through it.

It is also preferable to have a suspension mechanism including helical springs, cushioning magnets, gas spring arrangements and/or elastomeric cushion members.

It is also preferable to form a multi-stage compression arrangement by dividing the space inside the compressor into pre-compressing and further-compressing chambers so that a process fluid can be compressed stage by stage. Advantageously, an intermediate chamber can be formed to receive compressed fluid from said pre-compressing chamber for intercooling then to supply it to said further-compressing chamber.

Brief Description of Drawings Further features, advantages and details of the invention are to be described with reference to preferred embodiments illustrated in the accompanying drawings, in which: Figs. 1A and 1B are cross-sectional views taken along the central axis of a compressor according to a first preferred embodiment of the present invention; Fig. 2 is a sectional view showing static parts of the first embodiment; Fig. 3 is a sectional view of a shuttle in the first embodiment; Figs. 4A and 4B are sectional views taken along the central axis of a compressor according to a second preferred embodiment of the present invention; Fig. 5 is a sectional view showing static parts ofthe second embodiment; and Fig. 6 is a sectional view of a shuttle in the second embodiment.

Detailed Description of Preferred Embodiments In this application, the invention is described as a compressor for the sake of easy understanding. It should be understood that the same concept can be used for gas as well as for liquid, and also for a vacuum pump. Therefore, the term compressor should be interpreted as covering all these applications, unless specifically stated otherwise. Also, the same concept can be used to build a linear motor by simply removing valves and seal members. For this reason, there is no need for a separate description on how to change each compressor embodiment into a linear motor.

General Structure of the First Embodiment Figs. 1A to 3 show a first embodiment of the present invention. In Fig. 1A, the compressor 10 has a housing 20, two annular electromagnets 40 and 40' secured to the two ends of the housing, two short shafts 30 and 30' (not in section), each fitted in the centre of one electromagnet, a shuttle member 50 carried by the short shafts, and a suspension mechanism including helical springs 61 and 61' and cushioning magnets 62 and 62'. The two electromagnets 40 and 40' are arranged in a similar way as in my UK patent no. GB2299715-B for generating a push-and-pull force onto the shuttle 50.

The compressor 10 is a double-acting unit with an internal multi-stage compression arrangement, which includes three pairs of separated chambers formed between the shuttle 50 and the electromagnets 40 and 40'. Specifically, two first-stage compressing chambers I and I' are the annular spaces between each axial end of the shuttle 50 and a cushioning magnet 62 or 62', into which the process fluid is supplied from outside the compressor via channels 70 and 70'. Two second-stage compressing chambers are formed by the cylindrical spaces 11 and II' inside the shuttle 50, into which the process fluid is supplied from the chambers I and I' via channels 80 and 80'. Two fiuther chambers G and G' are the annular spaces around the shuttle pole pieces 54 and 54', forming gas springs as part of the shuttle's suspension mechanism In Fig. 2, the housing 20 has a non-magnetic cylindrical wall 21 with a number of outer fins 22 for better mechanical strength and thermal dissipation. The two electromagnets are identical, therefore only the right one 40 is described here. The corresponding parts of the left electromagnet 40' is indicated by the same reference numbers with an apostrophe '.

The electromagnet 40 has a magnetic base member 41 secured by fastening means 23 to the housing 20. Its magnetic circuit is formed by an annular outer pole piece 42, which is connected to the base 41 via a magnetic member 43, and a coaxial inner pole piece 44, directly connected to the base at its rear end. A toroidal coil 45 is fitted in the annular space defined by the members 42, 43, 41 and 44. A magnetic gap is formed between the pole pieces 42 and 44 for acting with the shuttle's pole piece, as to be described later.

Clamped between the two electromagnets is a separation ring 24 made of a nonmagnetic and thermally conductive material, such as brass or alummrum, to improve the compressor's heat dissipation. A non-magnetic lining member 25 is fitted inside the ring 24, extending into the magnetic gaps of the two electromagnets. That is to say, the lining member 25 provides a complete and smooth inner surface for closely matching the outer surface of the shuttle 50, so as to form a non-contact sliding seal between them. The two end portions ofthe lining member 25 also define the outer wall ofthe gas spring chambers G and G'. The inner pole piece 44 has on its inner surface a lining member 46 forming the outer wall ofthe pre-compressing chamber I.

The two shafts 30 and 30' are axially aligned to guide the shuttle's movements. They are identical so only one, ie. the right one 30, is described here. The shaft 30 has a tubular portion 31, a piston head 32 fitted to one end ofthe portion 31, a one-way valve 34 in the piston head and a seal 33 on the outer surface of the piston head. The seal 33 is made of a (RTM) self-lubricating material, such as Teflon, for lubricant-free operation.

Shuttle Structure In Fig. 3, the shuttle 50 has an annular primary permanent magnet 51 magnetised in radial direction, e.g. its outer periphery is the North (the outer pole) and the inner periphery the South (the inner pole). A magnetic tubular member 52 is fitted inside the annular magnet 51, serving as its inner pole piece.

Two intermediate pole pieces 54 and 54' are each fitted to one end of the tube 52, and at their axial ends are the secondary permanent magnets 55 and 55', which are magnetised in axial direction. The polarity ofthe magnets are arranged in a way that the pole piece 54 or 54' is magnetically connected to the south pole of the primary magnet 51 and that of the secondary magnet 55 or 55', therefore it provides highly concentrated magnetic flux for acting with the magnetic gap of the electromagnet 40 or 40'. The annular space between the pole piece 54 or 54' and the magnet 51 is filled with a non-magnetic and lightweight filling stuff 58 or 58', such as foamed plastics or resin, and covered by a nonmagnetic low-friction sleeve 59 to form a smooth bearing surface. The two axial magnets 55 and 55' are also covered by non-magnetic low-friction sleeve members 56 and 56'.

The interior ofthe tube 52 is divided by a separation member 53 into the two secondstage compressing chambers II and II'. The inner surface of the tube 52 forms gas-tight fit with the piston seals 33 and 33' of the two short shafts 30 and 30' shown in Fig. 2. A number of fluid communication channels 80 and 80' are formed between the outer surface of the tube 52 and the inner surfaces of the pole pieces 54, 54' and the primary magnet 51, so that the fluid in the first-stage compressing chamber I or I' can flow into an axial magnet 55 or 55' then into the inlet hole 81 or 81' ofthe channel 80 or 80' and come out of the outlet hole 82 or 82' in the second-stage compressing chamber II' or II. Since there are a number of channels 80 and 80' evenly cut around the periphery of the tube 52 (only one of each is shown), the outlet holes 82 or 82' have gas bearing effects on the piston seal 33 or 33'.

Within each axial magnet 55 or 55', there is a non-magnetic bushing 57 or 57' sitting against each end ofthe tube 52 for supporting the suspension spring 61 or 61' shown in Fig.

1A, and also magnetically separating the tube from the spring. When assembled, a small gap is formed between the bushing 57 and the shaft 30, allowing the fluid in the chamber I to enter the inlet holes 81 ofthe channels 80.

Reciprocating Operation Now return to Figs. 1A and 1B. In Fig. 1A, the shuttle 50 is at a neutral position, e.g. when there is no current in either of the two electromagnets so the shuttle position is decided by the biasing forces ofthe two suspension springs 61 and 61'. At this position each of the shuttle's pole pieces 54 and 54' is positioned between the inner and outer pole pieces ofthe corresponding electromagnet, i.e. in its magnetic gap. In Fig. 1B, the electromagnets are energised and the polarity of their pole pieces is marked by low case letters n and s showing the interaction between the electromagnets and the shuttle. The shuttle is driven by the magnetic forces to the left. More particularly, the electromagnet 40 is at a status that its outer pole piece 42 is n and the inner one 44 is s so its effect is to push the shuttle's main magnet 51 away and to pull the axial magnet 55 into the gap. On the other hand, the electromagnet 40' is at a status to push the other axial magnet 55' away and to pull the main magnet 51 towards the gap. That is to say, the axial magnet 55 forms a closed magnetic circuit with the electromagnet 40 and the main magnet 51 with the electromagnet 40'. When the current passing through the two electromagnets is reversed, the shuttle will be driven to the other end in a similar way. Because each electromagnet would always act with at least one of the shuttle magnets, its energy efficiency is high.

In Fig. 1B, as the axial magnet 55' approaches the cushioning magnet 62', a strong repelling force is generated between them This strong cushioning force together with the effects of the suspension spring 61', which is compressed close to its solid length, and the gas spring formed by the chamber G', which is reduced to a small "dead" space, provide adequate protection to prevent the shuttle from hitting the pole piece 44'. The three elements ofthe suspension mechanism, ie. the cushion magnets, suspension springs and gas springs, are selected to make the shuttle's natural frequency close to that of the power supply, e.g. 50Hz or 60Hz, so that the shuttle can operate at its resonant frequency.

During operation, the shuttle is precisely aligned by the piston heads and kept coaxial with the other parts of the compressor. Because the total contacting area between the shuttle and the piston heads is very small, friction and wear are also smalL On the other hand, since the electromagnets' pole faces are generally annular or cylindrical in shape, matching that of the shuttle's, they produce no sideways driving forces during the shuttle's reciprocating movements. The only possible source of side force is the helical springs 61 and 61', but their sideways effects are restricted by the shafts 30 and 30'. All these elements ensure that the total fiction resistance to the shuttle's movements is very small and the shuttle can operate lubricant-free.

Multi-stage Compression The compressor 10 is a double-acting machine for its shuttle movements in two opposite directions produce the same compressing effects. When considering the path travelled by a process fluid, there are two gas communication/compression routes, which are mutually separated. One starts from the right end at the gas inlet holes 71 on the base 41, goes into the pre-compressing chamber I via the channels 70, then as the shuttle moves back, the gas flows into the axial magnet 55 and then via the channel 80 into the second-stage compressing chamber II', and finally at the next shuttle stroke the gas leaves the compressor through the short shaft 30' at the left end. The other route is formed in the same way in opposite direction. In each gas flow route, a first one-way valve is formed by the shuttle's sealing surface 56 which opens and closes the outlet holes 72 of the channels 70 and a second one-way valve is formed by the piston's sealing surface 33' which opens and closes the outlet holes 82' of the channels 80. That is to say, whether the channels 70 and 80 are opened or closed depend on the axial position ofthe shuttle 50.

In Fig. 1A, the shuttle is positioned to block all the channels 70, 70' 80 and 80' so there is no gas flow into the compressor or between the chambers inside the compressor. In Fig. 1B, the shuttle 50 moves to the left, the space in the right chamber I is expanded and the channels 70 is opened so gas is sucked in; at the same time the channels 80 is blocked and the space in the chamber II' is reduced to zero so the gas in it is forced out through the outlet valve 34' in the piston 32'. On the other hand, the same shuttle movement blocks the left channels 70' and the space in the chamber I' is reduced to minimum so the gas in it can only go via the channels 80' into the next stage chamber II which is expanded to suck the gas in. In next stroke when the shuttle moves to right the process reverses direction.

In addition to the advantage of achieving two-stage compression by a single moving part, the arrangement also has the advantage that each compressing operation has minimum leakage. For example, when the shuttle moves to the left, the left chambers I' and II' are both at their compressing phase so their internal pressure increases at the same time, and the leakage from the chamber II' into the chamber I' is reduced. Similarly, the pressure in the gas spring chamber G' is also high, which helps to reduce the leakage from the chamber I'.

Further, although there is a very small gap between the shuttle's outer bearing surface 59 and the inner bearing surface ofthe lining member 25 for keeping them virtually non-contact, this gap forms a long leakage route compared with the shuttle's outer diameter so when the shuttle reciprocates at high speed any leakage through this long gap would be ignorable. All these features make the lubricant-free operation practical because the shuttle does not rely on lubricant for sealing. Finally, it is worth mentioning that because the gas inlet holes into each chamber are arranged peripherally around the corresponding bearing surfaces, they also produce gas bearing effects which help to reduce fiction and wear.

General Structure of the Second Embodiment Now the second embodiment of this invention is described with reference to Figs. 4A to 6. In Figs. 4A and 4B, the general structure of a compressor 100 and its operation are similar to that of the first embodiment. The main differences are in its central guide structure, suspension mechanism, multi-stage compression and intercooling arrangement.

Only these new features are described below.

In Fig. 5, a central guide 130 has two end sections 131 and 131', each secured to a base of an electromagnet, and a middle section 132 fitted between the end sections. Each end section is covered by a low-fnction sleeve 133 or 133' which provides a sliding seal surface. Two bushings 134 and 134' are fitted at the two ends ofthe middle section 132 for supporting suspension springs. A through hole is formed along the axis of the central guide 130, which can be used to circulate a fluid to cool the compressor from within.

In Fig. 6, the shuttle 150 has an inner pole piece 152 with an internal ridge 153 which bears two seal rings 157 and 157'. The seals also serve as bushings for suspension springs.

The shuttle's other parts, ie. its main magnet, two axial magnets and two intermediate pole pieces 154 and 154' are similar to that ofthe first embodiment.

As shown in Fig. 4A, when the shuttle 150 is assembled in the compressor 100, its two axial magnets have sliding fit on the two end sections of the central guide and the two seals 157 and 157' have sliding fit on the middle section ofthe central guide, which ensure the shuttle is precisely coaxial with the other parts during operation. The seals also divide the shuttle's internal space into two gas-tight chambers, in each of which is fitted a suspension spring 161 or 161' so that in operation the shuttle is suspended by the effects of both the helical springs and the gas springs.

The annular space between each ofthe shuttle's pole piece 154 or 154' and the inner pole piece of a corresponding electromagnet forms a first-stage compressing chamber I or I'.

Two second-stage compressing chambers are formed by the annular spaces H and H' between the shuttle's axial magnets and their corresponding cushioning magnets. In the firststage compressing chambers I and I' there are elastomeric rings 147 fitted on the pole face of the inner pole piece, which ring is to cushion the shuttle from hitting the pole piece directly and at the same time avoid any dead space in the compressing chamber. There are similar rings 167 and 167' in the second-stage chambers II and II'. An intercooling chamber C is formed between the two electromagnets, which chamber has gas communication passages for receiving compressed gas from the two first-stage compressing chambers I and I' and for supplying the gas to the two second-stage compressing chambers II and II'. All these passages are formed around the periphery ofthe lining member 125.

The gas communication routes in the compressor 100 are described in detail with reference to Fig. 5, in which the hatching patterns of different parts are deleted and the gas passages to be described are shaded so as to make the illustration easy to read. A gas inlet line passes through the housing wall to enter the chamber C and form a coil 200. The coil helps heat exchange between the gas in the inlet tube and that outside it to produce cooling effects in the chamber C.

In operation, the gas in the tube 200 is supplied via channels 170', which have outlet holes 172' in the first-stage compressing chamber I', into that chamber. Then the same gas is forced by the shuttle movement out of the chamber through channels 180' and into the chamber C via the one-way valves 182', so the pressure in the chamber C builds up. At the same time, the high pressure gas in the chamber C enters channels 190 which have outlet holes in the second-stage chamber II at the other end. When the shuttle reverses its movement, the gas in the chamber II is forced out via the outlet valves 193 in channels 195.

Another parallel gas flow route is arranged in the same way in opposite direction.

It should be noted that the intercooling chamber C receives compressed gas from both first-stage chambers I and I', and supplies the same gas to both second-stage chambers II and II', so the operating conditions at both sides are the same. This helps to stabilise the shuttle's double acting operation in opposite directions. In such an arrangement, although there is only one moving part inside the compressor, it achieves high output pressure and flow rate in a very efficient manner.

Industrial Applicability It is not difficult to understand from the above description that the linear motor and/or compressor according to the present invention has at least the following advantages. a) High energy efficiency, compact structure and low manufacturing costs. b) High pressure and now rate output. c) Lubricant-free, leak-free and maintenance-free operation. d) Low noise and vibration operation.

Finally, there is no need to mention that the embodiments in this application are only exemplary, which can be easily adjusted or altered by those skilled in the art once the basic concepts ofthe invention are understood. For example, by combining the two embodiments, it is easy to achieve three-stage compression within one unit.

Claims

Claims 1. A linear motor comprising: two opposing magnetic driving members, each with coaxial pole means; a reciprocating member disposed between said driving members; and means for energising said driving and/or reciprocating members; wherein said reciprocating member has pole means matching that of said driving members to form push-and-pull driving force and wherein central guide means is arranged for keeping said reciprocating member coaxial with said driving members.
2. A linear motor compressor comprising: a housing fitted with two opposing magnetic driving members, each with coaxial pole means; a reciprocating member disposed between said two driving members; valve means for forming at least one fluid passage into and out of said housing; and means for energising said driving and/or reciprocating members; wherein said reciprocating member has pole means matching that of said driving members to form push-and-pull driving force and wherein central guide means is arranged for keeping said reciprocating member coaxial with said driving members.
3. An apparatus of claim 1 or claim 2, wherein said reciprocating member has a primary magnet means magnetised in radial direction and two secondary magnet means fitted to two axial ends thereof and magnetised in axial direction so as to form said push-and-pull force with said driving members.
4. An apparatus of claim 3, wherein said reciprocating member's pole means are formed between said primary magnet means and each of said secondary magnet means for acting with the coaxial pole means of said driving members.
5. An apparatus of claim 3 or claim 4, wherein said primary and secondary magnet means are formed by permanent magnet.
6. An apparatus of claim 3 or claim 4, wherein said primary and secondary magnet means are formed by electromagnets.
7. An apparatus of any of the preceding claims, wherein said central guide means includes a pair of coaxially aligned shafts, each fitted into one end of said reciprocating member.
8. An apparatus of claim 7 in combination with claim 2, wherein each said shaft is fitted with a piston head and valve means for forming a compressing chamber with said reciprocating member.
9. An apparatus of any of the preceding claims 1 to 6, wherein said central guide means includes a shaft extending through said reciprocating member.
10. An apparatus of claim 9, wherein said shaft has a through passage for circulating a cooling fluid.
11. An apparatus of any of the preceding claims, further comprising a suspension mechanism for said reciprocating member which includes helical springs.
12. An apparatus of any of the preceding claims, further comprising a suspension mechanism for said reciprocating member which includes cushioning magnets.
13. An apparatus of any of the preceding claims, fiuther comprising a suspension mechanism for said reciprocating member which includes gas spring arrangements.
14. An apparatus of any ofthe preceding claims, further comprising elastomeric members for providing cushioning effects to said reciprocating member.
15. An apparatus of claim 2 or any of claims 3 to 14 in combination with claim 2, wherein said reciprocating member and the driving members are arranged to divide the space inside the housing into at least one pre-compressing space and one further compressing space, and a one-way fluid passage is formed therebetween so that a process fluid can be compressed progressively therein.
16. An apparatus of claim 15, wherein said one-way fluid passage is formed in said reciprocating member.
17. An apparatus of claim 15, wherein said one-way fluid passage is formed in said housing between said driving members.
18. An apparatus of claim 17, further comprising an intermediate space having a one-way fluid inlet for receiving compressed fluid from said pre-compressing space and a one-way fluid outlet for supplying the same fluid to said further-compressing space.
19. An apparatus of any of the preceding claims, wherein gas bearing means is formed between said reciprocating member and said driving members or central guide means.
20. A linear motor constructed substantially as described herein with reference to any of Figs. 1A to 3 or Figs. 4A to 6 ofthe accompanying drawings.
21. A compressor constructed substantially as described herein with reference to any of Figs. 1A to 3 or Figs 4A to 6 of the accompanying drawings.

21. A compressor constructed substantially as described herein with reference to any of Figs. 1A to 3 or Figs 4A to 6 of the accompanying drawings.

Amendments to the claims have been filed as follows 1. A linear motor comprising: two opposing magnetic driving members, each with coaxial inner and outer pole means; a reciprocating member disposed between said driving members; and means for energising said driving and/or reciprocating members; wherein said reciprocating member has poles corresponding to said inner and outer pole means of each said driving member to form push-and-pull driving force and wherein central guide means is arranged for keeping said reciprocating member coaxial with said driving members.

2. A linear motor compressor comprising: a housing fitted with two opposing magnetic driving members defining an inner space therebetween, each member having coaxial inner and outer pole means; a reciprocating member disposed in said inner space; valve means for forming at least one fluid passage into and out of said inner space; and means for energising said driving and/or reciprocating members; wherein said reciprocating member has poles corresponding to said inner and outer pole means of each said driving member to form push-and-pull driving force and wherein central guide means is arranged for keeping said reciprocating member coaxial with said driving members.

3. An apparatus of claim 1 or claim 2, wherein said reciprocating member has a primary magnet means magnetised in radial direction and two secondary magnet means fitted to two axial ends thereof and magnetised in axial direction so as to form said push-and-pull force with said driving members.

4. An apparatus of claim 3, wherein said reciprocating member's pole means are formed between said primary magnet means and each of said secondary magnet means for acting with the coaxial pole means of said driving members.

5. An apparatus of claim 3 or claim 4, wherein said primary and secondary magnet means are formed by permanent magnet.

6. An apparatus of claim 3 or claim 4, wherein said primary and secondary magnet means are formed by electromagnets.

7. An apparatus of any of the preceding claims, wherein said central guide means includes a pair of coaxially aligned shafts, each fitted into one end of said reciprocating member.

8. An apparatus of claim 7 in combination with claim 2, wherein each said shaft is fitted with a piston head and valve means for forming a compressing chamber with said reciprocating member.

9. An apparatus of any of the preceding claims 1 to 6, wherein said central guide means includes a shaft extending through said reciprocating member.

10. An apparatus of claim 9, wherein said shaft has a through passage for circulating a cooling fluid.

Il. An apparatus of any of the preceding claims, further comprising a suspension mechanism for said reciprocating member which includes helical springs.

12. An apparatus of any of the preceding claims, further comprising a suspension mechanism for said reciprocating member which includes cushioning magnets.

13. An apparatus of any of the preceding claims, further comprising a suspension mechanism for said reciprocating member which includes gas spring arrangements.

14. An apparatus of any ofthe preceding claims, further comprising elastomeric members for providing cushioning effects to said reciprocating member.

15. An apparatus of claim 2 or any of claims 3 to 14 in combination with claim 2, wherein said reciprocating member and the driving members are arranged to divide the space inside the housing into at least one pre-compressing space and one further compressing space, and a one-way fluid passage is formed therebetween so that a process fluid can be compressed progressively therein.

16. An apparatus of claim 15, wherein said one-way fluid passage is formed in said reciprocating member.

17. An apparatus of claim 15, wherein said one-way fluid passage is formed in said housing between said driving members.

18. An apparatus of claim 17, further comprising an intermediate space having a one-way fluid inlet for receiving compressed fluid from said pre-compressing space and a one-way fluid outlet for supplying the same fluid to said further-compressing space.

19. An apparatus of any of the preceding claims, wherein gas bearing means is formed between said reciprocating member and said driving members or central guide means.

20. A linear motor constructed substantially as described herein with reference to any of Figs. 1A to 3 or Figs. 4A to 6 of the accompanying drawings.