US3669571A

US3669571A - Tandem,free-piston machine

Info

Publication number: US3669571A
Application number: US887637A
Authority: US
Inventors: Henry Benaroya
Original assignee: Individual
Current assignee: Individual
Priority date: 1968-12-27
Filing date: 1969-12-23
Publication date: 1972-06-13
Anticipated expiration: 1989-06-13
Also published as: SU409436A3; DK130085B; NL6919362A; GB1296713A; CH520866A; DE1964768A1; SE352137B; JPS492206B1; LU60074A1; AT325344B; FR1601540A; DK130085C; NL167753C; CA927345A; DE6949804U; BE743552A; NL167753B; ES374935A1

Abstract

TANDEM, FREE-PISTON MACHINE INCLUDING AT LEAST ONE GROUP OF COMPRESSOR PISTONS CONNECTED TO DRIVE PISTONS, THE LATTER FORMING AT LEAST TWO GROUPS WHICH COMPRISE EACH TWO PAIRS OF DRIVE PISTONS, THESE TWO PAIRS WORKING IN TWO DRIVE CYLINDERS ARRANGED IN ALIGNMENT AND THE TWO DRIVE PISTONS OF EACH PAIR WORKING IN OPPOSITION IN THE CORRESPONDING DRIVE CYLINDER, THE OUTER DRIVE PORTIONS OF SAID TWO PAIRS BEING CONNECTED BY A CENTRAL ROD AND THE INNER DRIVE PISTONS OF SAID TWO PAIRS BEING CONNECTED BY A LINKING ELEMENT MOUNTED SLIDABLY ON SAID CENTRAL ROD, SO THAT OUTER DRIVE PISTONS AND THE INNER DRIVE PISTONS FORM, RESPECTIVELY, AN OUTER MOVABLE TRAIN AND AN INNER MOVABLE TRAIN RECIPOCATING IN OPPOSITION, MEANS OF

SYNCHRONIZATION BEING PROVIDED BETWEEN THE OUTER MOVEABLE TRAIN AND THE INNER MOVABLE TRAIN.

Description

4 Sheets-Sheet 1 Filed Dec. 23, 1969 Q N I.

'IIIII/fiII/IJIIIIIIIII INVENTOR HENRY BENAROYA Qm mm R s m wxww m. i Q E X m 1 m I l w 9 N 1 0 & J y m. mw mm mm N WN wmwmwm mm QM W A ORNEYS June 1972 H- BENAROYA ,669,571

' TANDEM, FREE-PISTON MACHINE Filed Dec. 23; 1969 4 Sheets-Sheet 2 June 13, 1972 H. BENAROYA 3,669,57fl

TANDEM, FREE-PISTON MACHINE Filed Dec. 23, 1969 4 Sheets-Sheet 5 June 13, 1972 H. BENAROYA 3,669,572

TANDEM, FREE-PISTON MACHINE Filed Dec. 23, 1969 4 Sheets-Sheet 4 Q3 m o Unitd States Patent 3,669,571 TANDEM, FREE-PISTON MACHINE Henry Benaroya, 41 Boulevard du. Commandant Charcot, Neuilly-snr-Seine, France Filed Dec. 23, 1969, Sen'No. 887,637 Claims priority, appliclation France, Dec. 27, 1968,

Int. Cl. F041) 17/00: 35/00; F02b 71/00 US. Cl. 417-341 I 16 Claims ABSTRACT OF THE DISCLOSURE Tandem, free-piston machine including at least one group of compressor pistons connected to drive pistons, the latter forming at least two groups which comprise each two pairs of drive pistons, these two pairs working in two drive cylinders arranged in alignment and the two drive pistons of each pair working in opposition in the corresponding drive cylinder, the outer drive portions of said two pairs being connected by a central rod and the inner drive pistons of said two pairs being connected by a linking element mounted slidably on said central rod, so that the outer drive pistons and the inner drive pistons form, respectively, an outer movable train and an inner movable train reciprocating in opposition, means of synchronization being provided between the outer movable train and the inner movable train.

The present invention relates to tandem, free-piston machines.

The invention relates more particularly, because it is in this case that its application seems to be most advantageous, but not exclusively, among these machines, to those intended for supplying one or several turbines.

Machines of this type are well known and have already been described in numerous patents, among which may be mentioned the French Pat. No. 1,547,421, published Oct. 24, 1968.

Up to this day, these machines had a considerable bulk, especially because they comprised in order to connect together the elements of their outer movable train, parts of large size sensitive to vibrations, to bending, and

.necessitating several guide bearings.

It is an object of the invention, particularly, to provide such above-said machines that respond better than hitherto to certain desiderata of practice, especially as regards their bulk, their simplicity, their reliability and their power to weight ratio.

It consists, principally, in making tandem, free-plston machines include at least one group of compressor pistons comprising at least three compressor pistons, namely, at least two outer compressor pistons and at least one inner compressor piston, moving in respective compressor cylinders,

at least two groups of drive pistons, each group comprising two pairs of said drive pistons, the two pistons pairs working in two drive cylinders, arranged in alignment and parallal to the compressor cylinders, and the two drive pistons of each pair working in opposition slidably on the 7 3,669,571 Patented June 13, 1972 ice means of synchronisation being provided between the outer movable train constituted by the outer compressor pistons and the outer drive pistons, and the inner movable train reciprocating in opposition to the outer movable train and constituted by the inner compressor piston and the inner drive pistons,

due to which the peripheral rods which would ensure up to the present day the connection between the outer compressor and drive pistons are eliminated, and this linkage is ensured by a mechanical element (central rod) to which sufficient size can be given to avoid any risk of vibrations and of bending, and which is possible to guide efficiently without complicating the machine in any way.

It is then seen that a machine according to the invention will have a much smaller bulk than that of a conventional machine, that it would be simpler, more reliable, and that its power to weight ratio will be increased to a considerable degree.

Moreover, by reason of the simplicity of the machin according to the invention, it will be possible to adopt stroke frequencies for the movable train much higher than those tolerable up to the present, which will enable better yields to be obtained.

The invention consists, apart from this principal arrangement, in certain other arrangements, which are used preferably at the same time and which will be more explicitly described below.

It relates more particularly, to a certain method of application (that for which tandem, free-piston machines are applied for feeding one or several turbines).

The invention will be more fully understood by the following description of several embodiments thereof, given purely by way of illustrative, but non-limiting example, with reference to the accompanying drawings, in which:

FIG. 1 is an axial section with a portion in external view of a first embodiment of a machine according to the invention;

FIG. 2 is a view in cross-section along the line II-II of FIG. 1;

FIG. 3 is an axial section, on a larger scale, illustrating a constructive detail of the embodiment shown in 'FIG. 1; 7

FIG. 4 is a view in partial axial section of another embodiment of a machine according to the invention;

FIG. 5 is a section along the line VV of FIG. 4;

FIG. 6 is an end view from VI of FIG. 4; 7

FIG. 7 is a diagramamtic axial half-section of a machine constructed according to yet another embodiment of the invention; and

FIG. 8, finally, is a diagrammatic transversal section of the machine shown in FIG. 7.

According to the invention, the tandem, free-piston machine is characterised by the fact that it comprises.

at least one group of compressor pistons -1 comprising at least three compressor pistons, namely, two outer compressor pistons 2a and Zn and at least one inner compressor piston 2b, moving in the compressor cylinders, 3a, 3n, 3b respectively,

at least two groups of

drive pistons

4a, 4b, arranged parallel and comprising each two drive cylinders, 5 and 6 respectively, arranged parallel to the

compressor cylinders

3a, 3b, 3n, and in each of which two drive pistons 7 and 8 work in opposition,

each group of

drive pistons

4a, 4b being arranged so that the two outer drive pistons 7 are rigidly fixed respectively to the outer compressor piston 2a and to the outer compressor piston 2n, and the two inner drive pistons 8 are rigidly attached to the inner compressor piston 2b through a linking element 9, the outer compressor piston 2a being connected to the outer compressor piston 2n through a central rod 10 conjugating, in both directions, the movements of these outer compressor pistons 2a and Zn and the outer drive pistons 7, the two groups of

drive pistons

4a, 4b being arranged symmetrically around the said central rod 10, the above-said linking element 9 being mounted slidably around the above-said central rod 10, synchronising means 11 being provided between the outer movable train constituted by the outer compressor pistons 2a and Zn and the outer drive pistons 7, and the inner movable train constituted by the inner compressor piston 2b and the inner drive pistons 8.

ilt is. then seen that the peripheral rods which would ensure up to the present the connection between the outer compressor pistons of the group of compressor pistons are eliminated, this connection being ensured by a mechanical element (central rod) to which may be given sufiicient dimensions to avoid any risk of vibrations and of bending, and which it is possible to guide efliciently.

The compressor cylinders of a same group of compressor pistons can be aligned, as will be seen in the description which follows, or displaced angularly around the central rod, especially for constructional reasons.

Preferably and as will be seen below, the two drive cylinders of a'same group of drive pistons are aligned. There will now be examined several embodiments of a machine comprising the application of the principal arrangement according to the invention.

In a first embodiment, illustrated in FIGS. 1 and 2, the machine comprises a single group of compressor pistons 1 comprising three compressor pistons, namely, two outer compressor pistons 2a and Zn and an inner compressor piston 2b, moving in aligned compressor cylinders, 3a, 3n and 3b respectively, the inner compressor piston 2b constituting the linking element 9 and having, in its central part, an orifice 12 enabling its sliding around the central rod 10, the diameters of the three above-said compressor pistons 2a, 2b and Zn being such that their respective working surfaces are equal between themselves.

j This machine can then advantageously be arranged in such manner that the work of compression of each of ,the three compressor pistons 2a, 2b and Zn in their respecone exhaust valve 26 and by at least one delivery valve 27. On this subject, itis assumed, in the embodiment illustrated in FIG. 1, that each variable volume chamber 25 was served, as regards the exhaust, by a combination of an exhaust valve 26 situated in an annular chamber 28 and exhaust ports 29 communicating the annular chamber 28 with the variable volume chamber 25 under consideration.

Preferably, this machine includes more than two groups of drive pistons; as shown in FIGS. 1 and 2, it includes three groups of

drive pistons

4a, 4b and 40 arranged regularly around the central rod 10, at angular intervals of 120.

In this machine, synchronising means 11 provided between the outer movable train (outer compressor pistons 2a and Zn and outer drive pistons 7) and the inner movable train (inner compressor piston 2b and inner drive pistons 8) are constituted by a rack 31 rigidly fixed to the

compressor piston

2a or 2n, by a rack 31 rigidly fixed to the inner compressor piston 2b, and by a gear wheel In another embodiment, illustrated in i -Iss4, s and 6,

' the machine includes several groups of compressor'pistons 1a, 1b, arranged parallel and symmetrically around compressor pistons 2a and the outer compressor pistons 2n of these groups of compressor pistons being co-axial with the aforesaid central axis 13, the connecting element 9 'having'acentral portion 14 mounted slidably around the central rod 10, and arms 15 connecting the said central portion -14,'on the one hand, to inner compressor pistons 2b of these groups of compressor pistons, and, on the other hand, to the inner drive pistons 8 of the groups ofdrive pistons.

In this case, the central rod 10 comprises, at each of its ends, a cruciform part34 provided accordingly with arms 35 according as the machine includes "groups of compressor pistons 1a, -1b and groups of

drive pistons

4a, 4b, each of these arms 35 connecting the said central rod 10 to an outer drive piston 7 or to an

outer compressor piston

2a or 2n, according to the side of the machine considered. Y

In the case illustrated in FIGS. 4, 5 and 6, this machine includes,

two groups of compressor pistons 1a and 1b regularly distributed around the central axis 13 at angular intervals of v and four groups of

drive pistons

4a, 4b, 4cand 4d arranged symmetrically two by two around the central axis 1'3. To reduce the diameter of the compressor pistons of the machine, it can advantageously be included with each group of compressor pistons 1a and 1b two outer compressor pistons 2a and Zn and two inner compressor pistons 2b and 2c, these four

compressor pistons

2a, 2b, 2c and Zn moving in the compressor cylinders, which can be aligned or displaced angularly around the central rod 10 (especially for constructional reasons), 3a, 3b, 3c and 3n respectively, the two outer compressor pistons 2a and Zn of the group being rigidly fixed to the central rod 10 through cruciform parts 34 with arms 35, the two inner compressor pistons 2b and 2c of the group being both rigidly fixed, through the connecting element 9 constituted by its central portion 14 and its arms 15, to the inner drive pistons 8 of each of the four groups of

drive pistons

4a, 4b, 4c and 4d. I

'This machine can advantageously be arranged so that the work of compression of the

compressor pistons

2a, 2b, 2c and 2n in their

respective compressor cylinders

3a, 3b, 3c and 3n is efiected in both directions, the

aforesaid compressor pistons

2a, 2b, 2c and 221 being hence doubleacting.

Each of the variable volume chambers 36 defined by a surface of a compressor piston and the corresponding side of the compressor cylinder is then served by at least one exhaust valve 37 and at least one delivery valve 38.

In this machine, synchronising means 11 provided between the outer movable train (outer compressor pistons 2a and Zn of the two groups 1a and "1b, and outer drive pistons 7 of the four

groups

4a, 4b, 4c'and 4d) and the inner movable train (inner compressor pistons 2b and 2c of the two groups 1a and lb'and inner drive pistons 8 of the four

groups

4a, 4b, 4c and 4d) are constituted by a rack 39 rigidly fixed to one of the outer compressor pistons (for example through one of the arms 35), by a rack 40 rigidly fixed to one of the inner pistons (forexample through one of the arms 15), and by a gear wheel 41 mounted on a fixed axis 42 and constantlyin mesh with the two above-said racks 39 and 40 (FIG. 5). I

.At this point of-the description on the invention, it would be appropriate to indicate that,'with respect to the embodiment illustrated in FIG. 1, the embodiment illustrated in FIG. 4 has the advantage of separating the mechanical functions (linking functions) and the thermodynamic functions (compressor functions) of the compressor pistons.

In fact, in the embodiment shown in FIG. 1, the inner compressor piston 2b ensures the mechanical linkage between the inner drive pistons 8, the

outer compressor pistons

2a and 2n ensuring the mechanical linkage between the outer drive pistons 7. Thesecompressor pistons must hence be calculated, on the one hand, to resist the forces generated by these mechanical linkages (which forces can be disturbed by possible irregularities in the operation of the groups of drive pistons), and, on the other hand, to ensure their work of compression (which work can be disturbed by possible jammings of the exhaust and/or delivery valves of the groups of compressor pistons).

n the other hand, in the embodiment shown in FIG. 4, the outer compressor pistons 2a and Zn, and the one or more inner compressor pistons 2b and 2c ensure only their work of compression, the mechanical linkages being ensured by the cruciform parts '34 with arms 35 or the linking element 9 with central portion 14 and arm 15, to which it is possible to give dimensions appropriate to their role without being obliged to take other factors into consideration.

In another embodiment shown in FIGS. 7 and '8, the machine includes several groups of compressor pistons which are distributed in groups of compressor pistons in which the delivery pressures are at different levels, the groups of compressor pistons at equal levels of pressure being preferably arranged symmetrically around the central axis 13.

In the case illustrated, this machine includes,

two groups of low pressure compressor pistons 1a and 1b, regularly spaced around the central axis .13 at angular intervals of 180,

two groups of high pressure compressor pistons 1c and 1d, regularly spaced around the central axis 13 at angu- It is advantageous to include in this machine heat exchangers 43 ensuring the cooling of the compressed gas between the two groups of low pressure compressor pistons 1a and 1b and the two groups of high pressure compressor pistons 1c and 1d.

As shown in FIG. 7, each group of

compressor pistons

1a, 1b, 1c and 1d of this machine may include two

outer compressor pistons

2a and 2n and two inner compressor pistons 21) and 2c, these four

compressor pistons

2a, 2b, 2c and 21: moving in aligned

compressor cylinders

3a, 3b, 3c and 311 respectively, the two outer compressor pistons 2a and Zn of the group being rigidly connected to the central rod through the cruciform parts 34 with arms 35, the two inner compressor pistons 2b and 2c of the group being both rigidly fixed, through the linking element constituted by its central portion 14 and its arms 15, to the inner drive pistons of each of the four groups of drive pistons 4a, '4b, 4c and 4d.

This machine can then be advantageously arranged so that the work of compression of the

compressor pistons

2a, 2b, 2c and 2n in their

respective compressor cylinders

3a, 3b, 3c and 3n is eifected in both directions, the

abovesaid compressor pistons

2a, 2b, 2c and 211 being hence double-acting and, preferably, mounted so that one outer compressor piston works in opposition with one inner compressor piston.

Each of the variable volume chambers 44 bounded by one surface of a compressor piston and the corresponding side of the compressor cylinder is then served by at least one exhaust valve 45 and at least one delivery valve 46.

As to the means of synchronisation provided, in this machine, between the outer movable train and the inner movable train, they can be arranged as indicated with respect to the preceding embodiments and, for reasons of simplification of the drawings, they have not been shown in FIGS. 7 and 8.

However, synchronising means are well-known in this field and are described, for example, in US. Pat. NO. 2,587,167, and in the aforementioned French patent.

There will now be described in more detail certain arrangements of the invention applied to the machine illustrated in FIGS. 1, 2 and 3.

According to one of these arrangements, the central rod 10 is guided in sliding bearings arranged in the bottoms of the

compressor cylinders

3a, 3b and 311 traversed by'the said central rod 10.

According to another of these arrangements, the central rod 10 is surrounded by three tubular cross-pieces, namely,

an inner tubular cross-piece 51 interposed between the two bottoms of the inner compressor cylinder 3b and on which slides the inner compressor piston 2b of which the orifice 12 has a diameter greater than the outer diameter of this inner tubular cross-piece 51,

and two outer tubular cross-pieces 52 and 53 interposed each between the bottoms appearing opposite the inner compressor cylinder 3b and respectively the bottoms of the two

outer compressor cylinders

3a and 3n,

passages 54 being provided to connect each of the two variable volume chambers 25 limited by the inner compressor piston 2b and the inner compressor cylinder 3b with the two annular spaces 55 and 56 bounded by the central rod 10 and the two outer tubular crosspieces 52 and 53.

A sliding sealing joint 57, constituted by segments, is then interposed, at the right of the inner compressor piston 2b, between the central rod 10 and the inner tubular cross-piece 51.

A pressure balance is thus maintained between the variable -volume chambers 25 where there must exist, 0perationally, the same pressure.

According to yet another of these arrangements, the guidance of each of the two outer compressor pistons 2a and Zn, already ensured by the central rod 10 and the rods of the outer drive pistons 7, is completed by three additional rods 58 each extending the rod of the corresponding outer drive pistons 7, these additional rods 58 sliding in slide bearings 59 arranged in the outer bottom of the

outer compressor cylinder

3a or 3n considered.

There will now be considered in more detail certain arrangements of the invention which relate more particularly to the embodiments illustrated in FIGS. 4 and 7.

According to one of these arrangements, the guidance of the central rod 10 with respect to the chassis 60 of the machine is ensured principally by two slide bearings 61 arranged in vicinity of the ends of the said central rod 10.

On account of possible irregularities of the drive forces (risks of irregularities of operation of the groups of drive pistons) and/or compressor forces (risks of jamming of the exhaust and/ or delivery valves of the groups of compressor pistons), it is advantageous to give a certain degree of radial freedom to the guidance of the central rod 10 by the aforesaid sliding bearings 61.

To this end, these slide bearings 61 may be given a v slight conicity towards the exterior.

These slide bearings 61 can also be mounted on balland-socket joints.

According to another of these arrangements, there is interposed, between the

outer compressor pistons

2a, 2n and the arms 35 of the cruciform part 34 connecting the

outer compressor pistons

2a, 2n to the central rod 10, elastic elements 62 capable of transmitting additional forces due to possible jamming of an exhaust and/ or delivery. valve in the variable volume chambers swept by the aforesaid

outer compressor pistons

2a, 2n.

According to yet another of these arrangements, there isinterposed, between the outer drive pistons 7 and the arms 35 of the cruciform part 34 connecting these outer drive pistons 7 to the central rod 10, elastic elements 63 capable of transmitting supplementary forces due to possible irregularities of operation of the groups of drive pistons. But there could also, instead of elastic elements, be interposed between the outer drive pistons and the arms of. the cruciform part, spherical ball-joint devices.

According to and whatever the embodiment adopted, there is provided a free-piston; tandem machine of small bulk, of great simplicity, of high reliability and of large power to weight ratio.

In addition, due to the arrangements adopted in the embodiments illustrated in FIGS. 4 and 7, there can be adopted groups of compressor pistons having a relatively small bore and, for this reason, does not impose the problems relative to bores of high magnitude.

In conclusion, it can be atfirmed that the machine according to the invention has a reduced bulk, a smaller weight of the movable train, an increased stroke frequency and a greater power to weight ratio.

Themachine according to the invention can work as a motor-compressor supplying compressed air or as an antigenerator supplying a gaseous mixture of compressed. air

.having passed the drive cylinders and/or the combustion gases of said cylinders or as a machine part of which works :as motor-compressor and part of which works as antigenerator.

As goes without saying and as emerges besides already from the preceding, the invention is in no way limited to those of its embodiments, nor to those of its method of realisation of its various parts which have been more particularly described; it embraces, on the contrary, all variations thereof.

What I claim is: I

V 1. A tandem, free-piston machine having inner and outer movable trains arranged respectively to reciprocate in synchronism in opposite directions, said machine comprising:

at least two parallel groups of compressor pistons comprising each at least three compressor pistons, namely, at least two outer compressor pistons and at least one inner compressor piston moving in respective compressor cylinders, the compressor cylinders of each group being arranged in alignment to each other,

at least two groups of drive pistons, each group comprising two pairs of said drive pistons, each pair of the two drive piston pairs working in opposition in drive cylinders, the drive cylinders being arranged in alignment to each other and parallel to the compressor cylinders,

said groups of compressor and drive pistons being arranged parallel and symmetrically around a central axis, I each group of drive pistons being arranged so that the outer drive pistons of the two drive piston pairs .are

- rigidly attached respectively to the outer compressor pistons, and the inner drive pistons of the two drive ,pistonpairs are rigidly attached to the inner compressor pistons,

the outer compressor and drive pistons being connected by-acentral rod coaxial with said central axis, said central rod sliding in fixed bearings, .and the inner compressor and drive pistons being connected by a linking element having a central part coaxially positioned around and spaced from the central rod over the entire length of said central part and arms connecting said central part, on the one hand, to the inner compressor pistons of said groups of compressor pistons, and, on the other hand, to the drive pistons of the groups of drive pistons.

2. A machine according to claim 1, wherein are provided,

two groups of compressor pistons regularly spaced around the central axis at angular intervals of 180", and four groups of drive pistons arranged symmetrically two by two around the central axis. 3. A machine according to claim 1, wherein the groups of compressor pistons are distributed in groups of com- .pressor pistons in which the delivery pressures are at different levels, the groups of compressor pistons at equal pressure levels being arranged symmetrically around the central axis.

4, A machine according to claim 3, wherein there ar provided,

two groups of low pressure compressor pistons, dis.-

tributed regularly around the central axis at angular intervals of 180,

two groups of high pressure compressor pistons, distributed regularly around the central axis at angular intervals of 180 and arranged at with respect to the groups of low pressure compressor pistons, and four groups of drive pistons arranged symmetrically two by two around the central axis.

5. A machine according to claim 3, including at least one heat exchanger ensuring the cooling of the compressed gas between the groups of compressor pistons in which the delivery pressures are at different levels.

6. A machine according to claim 4, wherein the outer compressor pistons and the inner compressorpistons work in opposition.

7. A machine according to claim 1, wherein there is provided,

an inner tubular cross-piece inserted between the two bottoms of the inner compressor cylinder and on which the inner compressor piston slides, whose orifice has a diameter greater than the outer diameter of this inner tubular cross-piece,

and two outer tubular cross-pieces interposed each between the facing bottoms of the inner compressor cylinder and repectively the bottoms of the two outer compressor cylinders,

passages being provided to connect each of the two variable volume chambers, defined by the inner compressor piston and the inner compressor cylinder, with the two annular spaces, defined by the central rod and the two outer tubular cross-pieces,

a sliding sealing joint, constituted by segments, being then interposed at the right of the inner compressor piston, between the central rod and the inner tubular cross-piece.

'8. A machine according to claim 1, wherein the guidance of each-of two outer compressor pistons, already ensured by the central rod and the rods of the outer drive pistons, is completed by supplementary rods each extending the rod of the corresponding outer drive pistons, said supplementary rods sliding in slide bearings arranged in the outer bottom of the outer compressor cylinder concerned.

9. A machine according to claim 1, wherein the guidance of the central rod is ensured by two sliding bearings arranged in the vicinity of the ends of said central rod, said two sliding bearings being arranged so as to give a certain degree of radial freedom to the guidance of the central rod.

' 10. A machine according to claim 9, wherein the slidmg bearrings have a slight conicity towards the exterior.

11. A machine according to claim 9, wherein the slid transmitting additional forces due to possible wedging of an exhaust and/or delivery valve into chambers of variable volume swept by said outer compressors.

13. A machine according to claim 1, wherein there is interposed, between the outer drive pistons and the arms of a cruciform part connecting said outer drive pistons to the central rod, elastic elements capable of transmitting additional forces due to possible irregularities of operation of the groups of drive pistons.

14. A machine according to claim 1, wherein is interposed, between the outer drive pistons and the arms of a cruciform part connecting said outer drive pistons to the central rod, spherical ball-and-socket joint devices capable of transmitting additional forces due to possible irregularities of operation of the groups of drive pistons.

15. A tandem, free-piston machine having inner and outer movable trains arranged respectively to reciprocate in synchroism in opposite directions, said machine comprising:

at least two parallel groups of compressor pistons comprising each at least four compressor pistons, namely, at least two outer compressor pistons and at least two inner compressor pistons moving in respective compressor cylinders, the compressor cylinders of each group being arranged in alignment to each other,

said groups of compressor and drive pistons being arranged parallel and symmetrically around a central axis,

each group of drive pistons being arranged so that the outer drive pistons of the two drive piston pairs are rigidly attached respectively to the outer compressor pistons, and the inner drive pistons of the two drive piston pairs are rigidly attached to the inner compressor pistons,

the outer compressor and drive pistons being connected by a central rod coaxial with said central axis, said central rod sliding in fixed bearings, and the inner compressor and drive pistons being connected by a linking element having a central part, a support coaxially positioned around and spaced from said central rod, said central part being mounted slidingly on said support, and arms connecting said central part, on the one hand, to the inner compressor pistons of said groups of compressor pistons, and, on the other hand, to the inner drive pistons of the groups of drive pistons.

16. A tandem, free-piston machine having inner and outer movable trains arranged respectively to reciprocate in synchronism in opposite directions, said machine comprising:

10 at least two parallel groups of compressor pistons comprising each of at least four compressor pistons, namely, at least two outer compressor pistons and at least two inner compressor pistons moving in respective compressor cylinders, the compressor cylinders of each group being arranged in alignment to each other, at least two groups of drive pistons, each group comprising two pairs of said drive pistons, each pair of the two drive piston pairs working in opposition in drive cylinders, the drive cylinders being arranged in alignment to each other and parallel to the compressor cylinders, said groups of compressor and drive pistons being arranged parallel and symmetrically around a central axis, each group of drive pistons being arranged so that the outer drive pistons of the two drive piston pairs are rigidly attached respectively to the outer compressor pistons, and the inner drive pistons of the two drive piston pairs are rigidly attached to the inner compressor pistons, the outer compressor and drive pistons being connected by a central rod coaxial with said central axis, said central rod sliding in fixed bearings, and the inner compressor and drive pistons being connected by a linking element having a central part coaxially positioned around and spaced from said central rod over the entire length of said central part and arms connecting said central part, on the one hand, to the inner compressor pistons of said groups of compressor pistons, and, on the other hand, to the inner drive pistons of the groups of drive pistons, the work of compression of the compressor pistons in their respective compressor cylinders being efiected in both directions, the said compressor pistons hence being double-acting.

References Cited UNITED STATES PATENTS 2,313,284 3/1943 Valentine 4l7536 2,491,679 12/ 1949 Mercier 123-46 2,510,127 6/1950 Mercier 417341 2,578,162 12/1951 Wallace l2346 B 2,736,488 2/1956 Dros 417341 ROBERT M. WALKER, Primary Examiner US. Cl. X.R. 123-46R