GB1566577A - Vacuum pumps of the piston and cylinder type - Google Patents

Description

PATENT SPECIFICATION

( 11) 1 566 577 ( 21) Application No 8213/79 ( 22) Filed 21 Dec.

( 62) Divided out of No 1566575 ( 31) Convention Application No 20758/76 ( 32) Filed 24 Dec 1975 ( 31) Convention Application No 20758/76 ( 32) Filed 26 Feb 1976 in ( 33) Australia (AU) ( 44) Complete Specification published 8 May 1980 ( 51) INT CL 3 F 16 J 9/00 ( 52) Index at acceptance ( 72) 1976 F 2 T 37 A 10 37 A 12 37 E 1 A 1 37 E 1 K 37 E 2 A 37 E 2 B Inventors GUENTER KARL WILLI BALKAU, ECKHARD BEZ and JOHN LASCELLES FARRANT ( 54) VACUUM PUMPS OF THE PISTON AND CYLINDER TYPE ( 71) We, COMMONWEALTH SCIENTIFIC AND INDUSTRIAL RESEARCH ORGANIZATION, a Body Corporate established under the Science and Industry Research Act 1949, of Limestone Avenue, Campbell, Australian Capital Territory, Commonwealth of Australia, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement:-

This invention relates to vacuum pumps of the piston and cylinder type which can operate without use of a lubricating or sealing liquid and which can be used as backing pumps for electron microscopes.

This invention is also divided out of our copending British Patent Application No.

53342/76, (Serial No 1566575).

At present almost all electron microscopes are equipped with pumping systems based on oil diffusion pumps backed by oil-filled rotary mechanical pumps of the vane type Consequently, a considerable part of the residual gas in the columns of these microscopes is contributed by molecules of oil and fragments of oil molecules, so a contaminating layer of carbonaceous material is deposited on the specimen and on all surfaces irradiated by the electron beam The contamination of the specimen can be substantially reduced by surrounding it with a liquid nitrogen cooled trap and this practice is widely adopted.

The problem of contamination in electron microscopes could best be avoided by the use of oil-free pumps Most previous attempts to produce oil-free pumps have involved modifications of the rotary type pumps and have been unsuccessful but Australian Patent Specification No 481,072 does disclose a pump of the reciprocating piston and cylinder type which is capable of producing high vacuum conditions without the use of lubricating and sealing oil.

However, the vacuum which can be achieved with a piston and cylinder type of pump operating under oil-free conditions is limited by difficulties in sealing against gas leakage into the working spaces of the pump and, in conventional constructions, by the need to have valves which need to be subjected to gas pressure to open The vacuum that can be produced in the high vacuum stage of a multi-stage pump can then be determined by the pressure required to open an exhaust valve in that stage of the pump.

Further, in any reciprocatory piston and cylinder machine which is to operate without cylinder lubrication a major problem arises in the provision of sliding seals for the piston/cylinder interface The invention further provides novel types of seals which have proved most reliable in service and have allowed most effective sealing to be maintained over extended periods of operation.

According to the present invention there is provided a reciprocatory piston and cylinder oil free vacuum pump comprising a cylinder, a piston slidable back and forth within the cylinder and at least one piston ring of self-lubricating material extending circumferentially about the piston and providing a sliding seal between the piston and cylinder, wherein said piston ring is split so as to have free ends which fit together, said free ends are notched one to each side of the ring such that they overlap to form between them an interface extending circumferentially of the ring and the ring is held against the surface on which it slides by 1:> ut.

1,566,577 means of an endless elastomeric resilient ring extending circumferentially of the ring.

and covering a radial extremity of said interface.

The invention also relates according to another of its aspects to an oil free vacuum pump comprising:

a cylinder having a first portion closed at one end and a second portion contiguous with, but of smaller diameter than, the first portion; a piston having a head portion slidable in the first cylinder portion and a second piston portion of smaller diameter than the head portion and slidable in the second cylinder portion, said piston head portion having a front face facing the enclosed cylinder end and an annular backface; the piston being fitted with a plurality of piston rings of self-lubricating material slidable in the cylinder at least one of the piston rings being split so as to have free ends which fit together, said free ends being notched one to each side of the ring so that they overlap to form between them an interface extending circumferentially of the ring and the ring being held against the surface on which it slides by means of an endless elastomeric resilient ring extending circumferentially of the ring and covering a radial extremity of said interface; a gas inlet for inlet of gas to the interior of the first cylinder portion between the front face of the piston head portion and the closed cylinder end on reciprocation of the piston; a first exhaust duct for exhaustion of gas from the interior of the first cylinder portion ahead of the piston head portion by pumping action of the front face of the piston head portion; a one way valve in said first exhaust duct to permit exhaustion of gas from the interior of the first cylinder portion ahead of the piston head portion but closable against reverse gas flow, and an exhaust means for exhaustion of gas from the interior of the first cylinder portion behind the piston head portion by pumping action of the backface of the piston head portion.

In order that the invention may be more fully explained, one particular vacuum pump and a piston ring construction will be described with reference to the accompanying drawings, in which:

Figure 1 is a partly sectioned elevation of a multi-stage vacuum pump constructed in accordance with the invention; Figure 2 is a perspective view of a piston ring of the vacuum pump; and Figure 3 is a scrap cross-section through -art of a cylinder and piston of the pump.

The vacuum pump illustrated in Figures 1 to 3 comprises a pair of pump units 11, 12 mounted one at either side of a central crankcase 13 supported on a pedestal 14.

Pumping units 11, 12 are of similar construction, unit 12 providing two high vacuum stages and unit 11 providing two backing stages Each unit comprises a piston, denoted as 16 in pumping unit 12, which reciprocates within a cylinder 17 and, in each case, cylinder 17 has a peripheral wall 18, a cylinder end 19 and external cooling fins 21.

The two pistons of pumping units 11, 12 are connected by means of crank arms 22 to a common crankshaft 23 which extends through crankcase 13 and is connected to an electric drive motor (not shown).

Crankshaft 23 has two cranks of mutually opposite eccentricities so that the pistons of the two pump units are reciprocated in opposition to one another.

Details of the construction of pumping unit 12 can be seen in the right-hand side of Figure 1 As shown, piston 16 and cylinder 17 are both of stepped configuration More particularly, piston 16, which is hollow, has a relatively large diameter head portion 24 and a smaller diameter rear skirt portion 26 so that an annular piston face 27 is defined at the rear of the head portion directed oppositely to the main piston face 28.

Cylinder 17 has a relatively large diameter portion 29 within which the head portion of the piston slides and a smaller diameter portion 31 to receive the skirt portion 26 of the piston An annular shoulder 32 is defined in the cylinder between cylinder portions 29, 31 in opposition to the annular piston face 27 Thus, a differential piston arrangement is provided whereby the cylinder has a front cylindrical working space 33 and a rear annular working space 34.

Each cylinder 17 has a tubular inlet 36 which provides communication with the interior of the cylinder through a set of inlet ports 37 extending through the peripheral wall of the cylinder at a location such that they are exposed only when the piston is near bottom dead centre but are covered by the piston during the greater part of the movement of the piston.

Each pumping unit 11, 12 has a gas outlet fitted to its cylinder end closure 19.

However, as will be explained below, this outlet acts only as a pressure relieving bypass when the pump is required to handle large volumes of gas, as during initial pumping down toward high vacuum conditions or in the event of a surge of excess gas thereafter The pump is capable of reaching high vacuum conditions in which gas outlets 30 become inoperative and gas is instead expelled from working space 33 of each unit via a gas transfer 3 1,566,577 3 passage which extends longitudinally within the cylinder wall of that unit.

The gas transfer passage of unit 12 is seen in the right-hand side of Figure 1 and is indicated by the numeral 81 One end of this passage 81 communicates with working space 33 via a port 82 and it extends at its other end to a set of ports 83 which open into the interior of the cylinder at such a position that they are uncovered to communicate with working space 34 as piston 16 approaches the top dead centre position In the formation of port 83 a hole 84 must be drilled in cylinder portion 29 and subsequently closed off with a plug 85 and sealing "O"-ring 86 Port 82 may penetrate the inner face 52 of cylinder closure 19 to ensure that it is not blocked by the piston when the piston approaches the top dead centre position.

Differential piston face 27 acts to exhaust air from working space 34 via an exhaust duct 67 extended through cylinder wall portion 31 to the working space 34 Exhaust duct 67 may be fitted with a one-way valve 66 comprised of a valve plug 68 at the inner end of duct 67 and a valve biasing spring 69.

However, as will be explained below exhaust duct 67 is connected to the air inlet of pumping unit 11 and by appropriate phasing of the two pistons of the pumping units 11, 12 it is possible to eliminate valve 66.

As mentioned above, cylinder end outlet 30 serves as a pressure relief by-pass only It is connected together with exhaust duct 67 from working space 34 to the air inlet of pumping unit 11 and it comprises a member 38 fitted across the mouth of an opening 39 which extends through cylinder end closure 19 Opening 39 forms the inlet end of a gas transfer duct 40 extending out through member 38 and the interface between closure 19 and member 38 is sealed by an "O"-ring 41 A one-way valve 42 is disposed within the inlet end of the duct 40 This valve is comprised of an elastomeric valve plate or disc 48 biased by a helical valve spring 49 against a thin annular flange 51 formed in cylinder end closure 19 to project inwardly of passage 40 at the inner face 52 of cylinder closure 19 Spring 49 acts directly between member 38 and valve disc 48 The face of disc 48 which is presented to flange 51 has a central projecting boss portion to project within and to fill the space within the rim of flange 51 when valve 42 is closed.

Each pumping unit 11, 12 is provided with two sets of sealing rings which provide sliding seals between the respective piston and cylinder The first set is constituted by five piston rings 53 fitted to the head portion 24 of the piston to slide in cylinder portion 29 and the second set is constituted by three further piston rings 54 fitted to the skirt portion 26 of the piston to slide within the cylinder portion 31.

The construction of piston rings 53 and the manner in which they are fitted to the piston is illustrated in Figures 2 and 3 Each ring 53 is of simple rectangular cross-section and is split so as to have two free end portions 56, 57 which fit together Ring end portions 56, 57 are notched one to each side of the ring, i e end portion 56 has a notch 58 at one side of the ring and end portion 57 has a notch 59 at the other side of the ring, such that they overlap to form between them a radial interface 61 which extends circumferentially of the ring Notches 58, 59 each extend across half the width of the ring so that radial interface 61 is disposed midway between the opposite sides of the ring The ring ends come together at the radial interface 61 and they can slide relative to one another at this radial interface back and forth in the circumferential direction of the ring to permit limited expansion and contraction of the ring without destroying the circumferential continuity of the ring.

The head portion 24 of piston 16 is provided with five axially spaced circumferential grooves 62 within which the sealing rings 53 are located The sealing rings are disposed around endless resilient expander rings 63 which are seated within channels 64 formed in the roots of grooves 62 Expander rings 63 are moulded in one piece and are of circular cross-section They are a neat fit within channels 64, which are of rectangular cross-section Channels 64 are narrower than grooves 62 and are disposed centrally of those grooves Thus each expander ring 63 is positively located centrally, of the respective sealing ring 53 and it covers the radial interface 61 of that sealing ring at the inner periphery of the sealing ring.

Figure 3 is, in effect, a composite of three cross-sections indicated by the lines A-A, B-B, C-C in Figure 2 showing the manner in which the expander rings 63 engage the respective sealing rings around their entire circumference including across the radial interface 61 It will be seen that the ring 63 presents a barrier to leakage across sealing ring 53 around its entire circumference, including across the split at the ends of the ring Ring 63 thus serves the dual function of expanding sealing ring 53 outwardly against the cylinder wall and ensuring maximum sealing around the whole periphery of the sealing ring It is found that because of their circular crosssection rings 63 can expand to take up wear of rings 53 without substantial alteration of the pressure on rings 53 throughout their service life.

1,566,577 1,566,577 To further decrease the possibility of leakage across the set of piston rings 53 the overlapped ends of the rings are successively staggered in the circumferential direction through 1800 so as greatly to increase the potential leakage path across the rings via the split ends.

Sealing rings 53 may be formed of a polyimide resin filled with polytetrafluorethylene or other suitable material or polytetrafluoroethylene rings filled with graphite powder, other carbonaceous material, molybdenum disulphide, bronze powder, glass fibre, polyimide resin or mixtures of these or similar materials These materials can provide the correct balance of a low coefficient of friction, a not unduly high coefficient of expansion, low wear and reasonable thermal conductivity The polyimide resin is preferred since it can withstand higher temperatures Expander rings 63 may be formed as simple "O"-rings' of neoprene or other suitable elastomer.

Sealing rings 54 fitted to the skirt portion 26 of piston 16 are similar to sealing rings 53.

They are fitted within grooves in the piston and provided with internal neoprene expander rings in the same manner.

Pumping unit 11 is identical to unit 12 and it provides two backing stages for the unit 12 which provides two high vacuum stages To achieve this the outlet 30 and exhaust duct 67 of unit 12 are connected, as indicated by broken lines in Figure 1, to the intake 36 ' of unit 11 and the cylinder end outlet 30 ' and exhaust duct 67 ' of unit 12 are opened to atmosphere.

It will be appreciated that a certain gas pressure must be generated within working space 33 in order to open valve 42, this pressure being determined by the biasing force provided by the valve spring 49 and the vacuum condition in exhaust duct 40 created by the backing stage unit 11 The valve therefore constitutes a restriction to exhaustion of gas through exhaust passage On the other hand, transfer passage 81, although relatively small compared with exhaust duct 40 is permanently unrestricted and, during the short part of the piston stroke during which port 83 is opened, it provides a preferential path for displacement of gas from working space 33 to working space 34.

When the pump is required to handle a large gas flow, as when initially pumping down to high vacuum conditions, the gas which enters the cylindrical working space 33 through the intake ports 37 is compressed to a pressure sufficient to cause valve 42 to open before piston 16 approaches the end of its forward stroke to open port 83 During such operation, the bulk of the gas is expelled via valve 42 and exhaust duct 30 into the fore-vacuum created by the backing stage pump unit 11 When the piston approaches the end of its forward stroke to open port 83 some of the residual gas will be displaced through passage 81 into working space 34 This can occur at a pressure lower than that required to open valve 41 and the latter valve may then close As pumping proceeds, a stage is reached at which the amount of gas taken into working space 33 through intake ports 37 is so small that it cannot be compressed to a pressure sufficient to open valve 42 before the piston has reached the position at which port 83 is opened At this stage of operation valve 42 remains closed and the only transfer of gas from working space 33 is via transfer duct 81 at pressures lower than would be necessary for exhaustion through outlet duct 30 In this way it is possible to generate a higher vacuum condition than could be achieved by exhaustion of gas through the valve In arrangements where gas can be exhausted only through a valve, the vacuum which can be achieved will be limited by the extent to which the valve can operate reliably at extremely light spring pressures whereas the arrangement described herein eliminates this limitation and valve spring 49 can be strong enough to provide reliable valve operation With this arrangement, the limitations on the vacuum which can be achieved are determined mainly by the extent to which gas can be caused to pass rapidly from transfer duct 81 into working space 34 at the end of the forward stroke of the piston and without flowing backwardly before port 83 is re-closed at the start of the back stroke of the piston This demands that the total volume of transfer passage 81 be very small compared with the piston swept volume of working space 33 in order to achieve a high compression ratio, since transfer passage 81 constitutes an unswept "dead space" in permanent communication with working space 33 In order to achieve vacua significantly lower than could be achieved with exhaustion through spring loaded valves, the volume of transfer passage 81 should be not more than 5 % of the piston swept volume of working space 33.

In order to avoid significant back flow of gas into transfer passage 81 port 83 must remain open only during a very short part of the stroke of the piston More specifically it should remain closed except for back and forth movements of the piston through less than 5 % of the stroke of the piston at the end of the piston forward stroke Moreover, port 83 is not at any stage completely uncovered It provides the required degree of communication with working space 34 when the trailing edge of the rearmost piston ring 53 passes across it at which stage 1,566,577 compressed gas in transfer passage 81 escapes into working space 34 via the small clearance space between the piston and the cylinder immediately behind that ring The rear part of the piston behind the rearmost ring continues to cover port 83 even when the piston reaches the top dead centre position This rearpart of the piston may be slightly relieved to allow adequate gas flow.

The operation of the backing stage unit 11 is similar to that of unit 12 Outlet 30 ' of the backing stage is operative only when that stage is required to handle large gas flows.

At other times gas is expelled from the cylindrical working space of unit 11 solely via the cylinder wall transfer passage to the annular working space whence it is pumped by the differential piston face to atmosphere via the respective exhaust duct 67 ' Because the pistons of the two pumping units are reciprocated in opposite phase the air intake ports of unit 11 will be open only during the pumping stroke of the differential piston face of unit 12 and the one-way valve 66 in exhaust duct 67 of unit 12 may be omitted but the exhaust duct of unit 11 must, of course, be fitted with such a valve.

Typically, a pump as illustrated in Figures 1 to 3 may be constructed in accordance with the following:

Diameters of cylindrical 10 cm Piston stroke Swept volumes of ' cylindrical working spaces Diameter of transfer passages 81 Volume of transfer passages 81 Part of piston stroke during which port 83 is opened 2.7 cm 212 cc 148 cc 0.2 cm 0.14 cc 4 % It has been found that a pump of the above dimensions run at a speed of about 400 strokes per minute can be operated quite satisfactorily without the need for any piston lubrication and will maintain a pressure of less than 0 030 Torr of permanent gases.

The illustrated pump provides four successive pump stages in a most compact and effective arrangement Specifically, each cylinder provides two stages and pumping through these stages is achieved by a single moving piston This permits the gas transfer passage to be formed directly in the cylinder wall and accurate phasing of the opening and closing of the gas transfer port to the lower vacuum stage relative to the movement of the piston face in the higher vacuum stage Moreover, the differential piston arrangement, in creating a vacuum behind the piston rings, minimizes leakage of gas into the higher vacuum stage and with the provision of piston rings of the illustrated type such leakage can be successfully reduced to insignificant levels while maintaining oil-free operation of the pump.

Claims (6)

WHAT WE CLAIM IS:-

1 A reciprocatory piston and cylinder oil free vacuum pump comprising a cylinder, a piston slidable back and forth within the cylinder and at least one piston ring of selflubricating material extending circumferentially about the piston and providing a sliding seal between the piston and cylinder, wherein said piston ring is split so as to have free ends which fit together, said free ends are notched one to each side of the ring such that they overlap to form between them an interface extending circumferentially of the ring and the ring is held against the surface on which it slides by means of an endless elastomeric resilient ring extending circumferentially of the ring and covering a radial extremity of said interface.

2 An oil free vacuum pump comprising:

a cylinder having a first portion closed at one end and a second portion contiguous with, but of smaller diameter than, the first portion; a piston having a head portion slidable in the first cylinder portion and a second piston portion of smaller diameter than the head portion and slidable in the second cylinder portion, said piston head portion having a front face facing the enclosed cylinder end and an annular backface; the piston being fitted with a plurality of piston rings of self-lubricating material slidable in the cylinder at least one of the piston rings being split so as to have free ends which fit together, said free ends being notched one to each side of the ring so that they overlap to form between them an interface extending circumferentially of the ring and the ring being held against the surface on which it slides by means of an endless elastomeric resilient ring extending circumferentially of the ring and covering a radial extremity of said interface; a gas inlet for inlet of gas to the interior of the first cylinder portion between the front face of the piston head portion and the closed cylinder end on reciprocation of the piston; a first exhaust duct for exhaustion of gas from the interior of the first cylinder portion ahead of the piston head portion by pumping action of the front face of the piston head portion; a one way valve in said first exhaust duct 1,566,577 to permit exhaustion of gas from the interior of the first cylinder portion ahead of the piston head portion but closable against reverse gas flow; and an exhaust means for exhaustion of gas from the interior of the first cylinder portion behind the piston head portion by pumping action of the backface of the piston head portion.

3 A vacuum pump as claimed in either claim I or claim 2 wherein said interface is a radial interface disposed midway between opposite sides of the piston ring.

4 A vacuum pump as claimed in any one of claims 1 to 3 wherein the piston ring is carried by the piston and is expanded outwardly by the endless ring against the cylinder wall.

A vacuum pump as claimed in claim 4 wherein the piston ring is seated in a circumferential groove in the piston and said endless ring is located in a circumferential channel formed in the root of the groove.

6 A vacuum pump according to any one of the preceding claims wherein the said piston ring is formed of any one of the following materials:(a) polyimide resin filled with polytetrafluoroethylene, or (b) polytetrafluoroethylene filled with:

(i) graphite powder or (ii) molybdenum disulphide, or (iii) bronze powder, or (iv) glass fibre, or (v) polyimide resin, or (vi) mixture of these materials.

Agents for the Applicants, GILL, JENNINGS & EVERY, Chartered Patent Agents, 53 to 64 Chancery Lane, London WC 2 A IHN.

Printed for Her Majesty's Stationery Office, by the Courier Press, Leamington Spa 1980 Published by The Patent Office, 25 Southampton Buildings, London, WC 2 A l AY, from which copies may be obtained.