WO1995002750A1

WO1995002750A1 - Piston rod bearing assembly of reciprocating piston engine

Info

Publication number: WO1995002750A1
Application number: PCT/GB1994/001552
Authority: WO
Inventors: John Varga
Original assignee: Carding Specialists Canada Ltd
Current assignee: Carding Specialists Canada Ltd
Priority date: 1993-07-17
Filing date: 1994-07-18
Publication date: 1995-01-26
Anticipated expiration: 1996-01-17
Also published as: GB9314957D0

Abstract

A bearing assembly for a piston rod of a linearly reciprocating piston type engine, such as a floating piston type compressor, having a cylinder (11), a floating piston (12) movable axially in cylinder (11), at least one piston rod (13) secured to and projecting axially from one side of the piston to be received in a respective bearing assembly (17) at one end of the cylinder and which guides the movement of the piston / piston rod assembly, said bearing assembly comprising: an inner annular array of balls (18) which roll on the cylindrical outer surface of the piston rod (13) during its reciprocation, and housed in an annular space defined between the cylindrical outer surface of rod (13) and a sleeve (19); bearings (20) which rotatably mount sleeve (19); and means for rotating sleeve (19) about the axis of the piston rod (13) during linear reciprocation of the latter, to reduce wear of the balls when lubricant is provided and thereby provide more reliable operation.

Description

PISTON ROD BEARING ASSEMBLY OF RECIPROCATING PISTON ENGINE

This invention relates to a bearing assembly for a piston rod of a linearly reciprocating piston type engine.

The type of linearly reciprocating piston engine with which the invention is concerned has a cylinder, a piston movable axially in said cylinder, at least one piston rod secured to and projecting from one side of the piston, said piston rod extending axially of the piston and being movable with the piston in a fixed plane parallel to the axis of the cylinder, a cover closing one end of the cylinder and facing said one side of the piston, and an opening formed in said cover through which the piston rod extends.

Usually, the piston will have a pair of piston rods, each extending axially outwardly of a respective one of the opposed working faces of the piston.

The invention has been developed primarily in connection with a reciprocating piston type compressor having a "floating" piston, though it should be understood that the invention may be applied to other forms of reciprocating piston type engine which require mechanical power input in order to do mechanical work e.g. to pressurise a working fluid as in a pump or compressor, or which are energised by a pressurised working fluid in order to generate mechanical power e.g. an air motor.

There are many applications in industry in which a supply of compressed air is required e.g. to operate pneumatic tools in an assembly line, or in a textile mill, and in recent years rotary compressors (screw compressors, vane compressors or centrifugal compressors) have been considered to be the sole practical and reliable way of supplying compressed air.

The invention seeks to reverse this trend, and to overcome perceived drawbacks to the use of linearly reciprocating piston type compressors.

It is of course known to mount a linearly displaceable shaft in a bearing assembly, in which the shaft is taken through a circular central aperture of a ball bearing, and the balls engage the outer periphery of the shaft and each ball rotates about its own axis as the shaft moves back and forwards relative to the ball bearing during each cycle of operation.

The balls are arranged in an annular array between two coaxial annular surfaces, in which the inner annular surface is formed by the cylindrical periphery of the shaft and the outer annular surface is formed by a fixed sleeve or bush which is normally rigidly secured against the internal wall of the opening in which the bearing is mounted.

Grease or oil is normally contained within the annular space in which the balls are arranged, and lubricate the rotation of the balls. While the balls are rotating, a film of lubricant builds-up at the interfaces between the instantaneous surfaces of contact of the balls and the inner and outer annular surfaces. However, each time the shaft reverses its linear movement, there is a "dwell period" in which the rotation of the balls is arrested (prior to rotation in a reverse direction) and this tends to break down the lubricant film and allow direct metal to metal contact which is a source of wear and heat which, over a period of time, results in unacceptable wear and breakdown.

Therefore, while it would be particularly suitable for a "floating piston" type of compressor to provide rigid guidance for the movement of the piston rod in an end cover of the cylinder via a bearing assembly, (known for other uses in guiding linear movement as described above), this would not be suitable for such a use because of wear problems in the balls, and the need to provide rigid guidance by the bearing assembly to the piston / piston rod assembly.

Accordingly, in one aspect the invention seeks, by simple means, to improve the lubrication of the bearings in the bearing assembly for a piston rod of a reciprocating piston type of engine of the type defined above.

According to one aspect of the invention there is provided a linearly reciprocating piston engine comprising: a cylinder; a piston movable axially in said cylinder; a piston rod secured to and projecting from one side of the piston, said piston rod extending parallel to the axis of the piston and being movable with the piston in a fixed plane parallel to the axis of the cylinder; a cover closing one end of the cylinder and facing said one side of the piston; an opening formed in said cover through which the piston rod extends; and, a bearing assembly mounted in said opening and arranged to guide the linear reciprocating movement of the piston rod, said bearing assembly comprising an annular array of balls surrounding and making contact with the cylindrical periphery of the piston rod, and housing in which the balls are received; and, means for applying rotation to the balls about respective axes extending generally parallel to the axis of the piston rod.

Thus, in use, the balls rotate about respective axes extending perpendicular to the axis of the piston rod, by reason of linear reciprocation of the piston rod, but also carry out further rotation about axes extending parallel to the axis of the piston rod. Therefore, even when the rod is reversing its reciprocation (which in known constructions would arrest the rotation of the balls), the balls continue to rotate about their axes which extend parallel to the rod axis (under action of said rotation applying means) so that lubrication of the balls can be maintained at all stages of reciprocation of the piston rod.

Any suitable means may be used to apply rotation to the balls, and which may comprise means to apply intermittent, or continuous rotation to the piston, or the piston rod. However, in a preferred embodiment, the bearing assembly comprises an annular array of balls surrounding and making contact with the cylindrical periphery of the piston rod; a sleeve radially spaced from the axis of the piston rod to define an annular space in which the array of balls is housed; means rotatably mounting the sleeve in the opening in the cover; and means for applying rotation to the sleeve about the axis of the piston rod during linear reciprocation of the latter.

Therefore, in the preferred embodiment of linearly reciprocating piston engine according to the invention, during operation the sleeve is rotated while the piston rod carries out backwards and forwards movement during each cycle of operation, and the rotation of the sleeve applies rotation to the balls, even when the piston rod is reversing its direction of reciprocation, (which in known constructions of reciprocating shafts normally arrests the rotation of the balls and gives rise to metal to metal contact).

By rotating the balls during operation, grease or oil provided as lubricant can maintain a protective lubricant film between the interfaces of both the inner and outer annular surfaces on which the balls rotate, thereby reducing wear, and providing more reliable operation.

The means by which the sleeve is rotatably mounted in the cover opening through which the piston rod extends may take any convenient form, which may be, for example, a further annular array of bearing balls. However, other arrangements may be provided, which allows the sleeve to rotate about the axis of the piston rod during operation.

Preferably, the invention is applied to a reciprocating piston type compressor having a "floating piston", and which will have a pair of oppositely projecting piston rods, each secured to a respective one of the opposed working faces of the piston, and extending through a mounting opening in a corresponding cover end plate of the housing.

The piston rods guide the movement of the piston so that the piston does not normally engage the side wall of the cylinder, which avoids necessity for piston seals, and provides substantially friction-free movement of the piston. A small proportion of gas can leak from one side of the piston to the other side (via the radial clearance between piston and cylinder wall), but this can be tolerated for many applications in view of the considerably enhanced service life and reduced maintenance costs of a friction-free movement of the piston and absence of piston seals. However, according to a second aspect of the invention, which is concerned exclusively with a floating piston type of reciprocating piston type of compressor, there is provided: a reciprocating piston type compressor having a cylinder, a piston mounted for linear reciprocation in said cylinder substantially without guidance of sliding contact with the inner wall of the cylinder, at least one piston rod rigidly secured to the piston, and a bearing assembly provided in one end of said cylinder and which slidably receives said piston rod so as to guide the reciprocating movement of the piston.

Preferably, a pair of piston rods is provided, each rigidly secured to a respective end face of the piston, and mounted slidably in a respective bearing assembly.

A preferred embodiment of the invention will now be described in detail, by way of example only, with reference to the accompanying drawings, in which:

Figure 1 is a schematic illustration of a linearly reciprocating piston type engine according to the invention, but in which bearing housings through which the piston rods extend are omitted for clarity; and

Figure 2 is a schematic illustration of one of the bearing housings.

Referring now to the drawings, there will be described an embodiment of linearly reciprocating piston engine (having a "floating piston") which is intended primarily for use as a high pressure or low pressure air compressor. The engine is designated generally by reference 10 and has a cylinder 11, a floating piston 12 movable axially in the cylinder 11, and at least one piston rod secured to and projecting axially from one side of the piston. However, in practice, a pair of oppositely extending piston rods 13 will usually be provided, each being secured to and projecting from a respective one of the two opposed working sides or faces 14 of the piston.

Each piston rod 13 extends axially of the piston and is movable with the piston in a fixed plane parallel to the axis of the cylinder, and when the rods 13 are centrally mounted, as shown, this plane of movement will contain the longitudinal axis of the cylinder 11. Piston rods 13 are rigidly secured to the piston 12, unlike the connection of a piston rod in an internal combustion engine, so as to provide guidance for the movement of the piston 12 without need to engage the wall of cylinder 11.

A cover 15 closes each end of the cylinder, and each faces a respective one of the opposed sides 14 of the piston. Further, as shown in Figure 1, a mounting opening 16 is formed in each cover 15 through which a respective piston rod 13 extends .

A respective bearing assembly, shown by reference 17 in Figure 2, is mounted in each mounting opening 16 and guide the movement of the piston / piston rod assembly, and comprises an inner annular array of balls 18 (of one or more rows) which roll on the cylindrical outer surface of piston rod 13 during its reciprocation, and which are housed in an annular space defined between the cylindrical outer surface of each rod 13 and a bearing housing formed by a sleeve or bush 19. Bush 19 is rotatably mounted in opening 16 in cover 15 by any suitable rotary support, which is shown schematically by two outer annular arrays of bearings 20. The bearings 20 also prevent the bush 19 from rocking during reciprocation of rod 13. Also, means (not shown) locates the bush 19 against axial displacement. A rigid assembly is obtained by interference fit in the opening 16 to provide radial loading of all engaging parts .

Balls 18 may be "caged", as shown schematically by reference 22, to be retained against relative axial displacement. Stops 23 limit the axial movement of balls 18, and also form a means of aligning the balls at the end of each reciprocation of the piston 12 (if cage 22 is omitted).

Grease or oil will be contained in the annular space in which the bearings 18 are housed, and this provides lubrication between the instantaneous contact interfaces between the rotating balls 18 and the cylindrical surface of rod 13 (or a wear sleeve mounted on rod 13) and the cylindrical inner wall of bush 19. Linear reciprocation of the piston rod 13 causes the balls 18 to rotate about their axes, and thereby guide in a smooth and lov; friction manner the axial reciprocation of the piston rod 13.

However, to minimise risk of metal to metal contact taking place between the surfaces of the balls 18 and adjoining contact surfaces whenever the rod 13 reaches the end of its linear movement in one direction, and then carries out reverse linear movement, an improved means is provided to maintain lubrication of the contact surfaces at this critical point in each linear reciprocating cycle. As is well known, a hydrodynamic lubricating effect is maintained for as long as the balls 18 are rotating, and the invention provides a means whereby the balls 18 can be caused to continue to rotate, even at the critical point of reversal of linear movement of the piston rod 13. To this end, bush 19 is rotatably mounted in the mounting opening 16, and in addition means is provided (not shown) to apply continuous rotation to the bush 19 about the axis 21 of the piston rod i3. The balls therefore continue to rotate, at all stages during the linear reciprocation of the piston rods 13, and this provides improved retention of lubricant film which builds-up at the contact interfaces.

The combined rotations applied to each ball (by reciprocation of rod 13 and rotation of sleeve 19) means that the risk of localised wear on any parts of surfaces of rod 13 and sleeve 19 is much reduced.

In a typical arrangement, with a lubricant having a viscosity of about 80 centistokes, 100 lb loading, and linear speed of piston rod of about 5 feet per second, a speed of rotation of sleeve 19 of about 100 rpm is suitable.

Theoretically, it may be sufficient for the bush 19 to be rotated whenever the piston rod 13 is approaching the end of its linear movement in one direction and / or is commencing its return path of movement, but in practice it will be generally more convenient to provide a more continuous rotary drive to bush 19 during operation of the engine.

When the engine 10 is used as an air compressor, suitable seals will be provided to provide some form of sealing to the mounting assembly of each piston rod 13, though in the case of a compressor a limited amount of air leakage through opening 16 can be tolerated. However, axially outwardly of the bearing assembly, a stepped arrangement (not shown) of cover 15 may approach the surface of rod 13 more closely to provide improved sealing. Suitable inlet and outlet valves will be provided (not shown) to control induction of air to be compressed during the induction stroke, and exhaust of compressed air during the compression stroke. Preferably, the engine operates as a double acting cylinder.

Evidently, the novel aspects of the bearing assembly disclosed herein (which are particularly suitable in providing substantially wear-free guidance of a floating area-sealed piston of a compressor) may be applied in other types of linearly reciprocating piston engines, including air motors.

The preferred embodiment has a bearing assembly in which the sleeve 19 forms a housing in which the balls 20 are received, and which applies rotation to the balls about axes extending parallel to the axis of the piston rod 13 (whereas linear reciprocation of the rod rotates the balls about axes which extend perpendicular to the axis of the rod).

However, other means (not shown) may be employed to apply the same rotation to the balls as provided by the sleeve, including means to apply rotation to: (a) the piston; (b) the piston rod; or (c) a unitary assembly of piston and piston rod.

The preferred embodiment of the invention provides an oil-free, floating piston type of reciprocating compressor working at pressures for industrial usage (up to 9 atmospheres). It has the advantage of high efficiency, both when compressing and when running unloaded. There is no wear between piston and cylinder, and low maintenance by reason of absence of piston seals. Further, the bearing assemblies provide much reduced friction and wear. It can be operated with a supercharger if required. The simplicity of design, reduced wear, and reduction of friction enable the compressor to be competitively priced relative to rotary compressors.

Claims

1. A linearly reciprocating piston engine comprising: a cylinder; a piston movable axially in said cylinder; a piston rod secured to and projecting from one side of the piston, said piston rod extending parallel to the axis of the piston and being movable with the piston in a fixed plane parallel to the axis of the cylinder; a cover closing one end of the cylinder and facing said one side of the piston; an opening formed in said cover through which the piston rod extends; and, a bearing assembly mounted in said opening and arranged to guide the linear reciprocating movement of the piston rod, said bearing assembly comprising an annular array of balls surrounding and making contact with the cylindrical periphery of the piston rod, and housing in which the balls are received; and, means for applying rotation to the balls about respective axes extending generally parallel to the axis of the piston rod.

2. An engine according to Claim 1, including means for applying rotation to said piston, or to said piston rod, in order to rotate the balls about said axes extending parallel to the axis of the piston rod.

3. A linearly reciprocating piston engine comprising: a cylinder; a piston movable axially in said cylinder; a piston rod secured to and projecting from one side of the piston, said piston rod extending parallel to the axis of the piston and being movable with the piston in a fixed plane parallel to the axis of the cylinder; a cover closing one end of the cylinder and facing said one side of the piston; an opening formed in said cover through which the piston rod extends; and, a bearing assembly mounted in said opening and arranged to guide the linear reciprocating movement of the piston rod: in which the bearing assembly comprises an annular array of balls surrounding and making contact with the cylindrical periphery of the piston rod; a sleeve radially spaced from the axis of the piston rod to define an annular space in which the array of balls is housed; means rotatably mounting the sleeve in the opening in the cover; and means for applying rotation to the sleeve about the axis of the piston rod during linear reciprocation of the latter.

4. An engine according to Claim 3, in which said means by which the sleeve is rotatably mounted in the cover opening comprises a further annular array of bearing balls.

5. An engine according to Claim 3 or 4, in which the piston is a "floating piston" which is linearly reciprocatable in said cylinder substantially without guidance by sliding contact with the inner wall of the cylinder, and a pair of oppositely projecting piston rods is secured one each to a respective one of the opposed working faces of the piston, and extending through a mounting opening in a corresponding cover end plate of the housing and mounted therein via a respective bearing assembly.

6. An engine according to any one of Claims 3 to 5, in which said annular array of balls surrounding and making contact with the cylindrical periphery of the piston rod is retained against axial relative displacement via a cage, or have relative axial movement limited between a pair of axially displaced end stops.

7. A floating piston type of reciprocating piston type of compressor which comprises: a cylinder, a piston mounted for linear reciprocation in said cylinder substantially without guidance by sliding contact with the inner wall of the cylinder, at least one piston rod rigidly secured to the piston, and a bearing assembly provided in one end of said cylinder and which slidably receives said piston rod so as to guide the reciprocating movement of the piston.

8. An engine according to Claim 7, including a pair of oppositely projecting piston rods, each secured to a respective one of the opposed working faces of the piston, and mounted slidably in a respective bearing assembly provided in a corresponding end of said cylinder.