Piston pump

Abstract

 A pump cylinder (18) is driven in reciprocation relative to a hollow piston (22) within the cylinder. The piston and cylinder each have valved inlet ports (33, 34; 43, 44) at each end. A tubular member (14) communicates with the hollow of the piston centrally between the piston ends and extends perpendicular to the piston, serving further as a stationary pivot for oscillation of the piston while the cylinder is driven in a swinging reciprocation relative to the piston. The tubular member extends through and works relative to a slot (19) formed through the cylinder wall so that reciprocation of the cylinder pumps a fluid along a flow path through the valved inlet ports and along said tubular member. The cylinder is driven at low speed by a motor (15) through a gear box (16) rigidly secured to a tubular frame (71) of the pump. A number of piston and cylinder devices may be driven by a common drive means, and employ a common tubular member, in a modular arrangement. The pump is simple, reliable and quiet, and is effective to provide small quantities of air at low pressure, e.g. for aeration at isolated effluent. treatment plants and at fish farms.

Description

[0001] This invention relates to pumps.

[0002] Fluid pumps are available in a wide range of pressure ratios, capacity, size, weight, expense and other characteristics. A need has been apparent for some time for a low pressure, high volume, simple, inexpensive and lightweight air pump capable of running reliably on a low power consumption for extended periods without attention. Such pumps may be used for example to supply modest quantities of air at low pressure to submerged aerators at isolated sewage treatment plants and at fish farms, the pressure requirement being merely that necessary to overcome a head of a few metres of water. Pumps or compressors of the type currently used for such purposes tend to be heavy and inflexible.

[0003] An object of the present invention is to provide a pump suitable for such purposes.

[0004] According to the present invention there is provided a pump comprising a piston adapted for reciprocation within and relative to a cylinder, said cylinder having a first valved port at at least one end thereof, said piston being hollow and having a second valved port at at least the corresponding end thereof, a tubular member communicating with the interior of said hollow piston at a point between the ends of the piston and extending substantially perpendicular to the.path of said reciprocation, said tubular member extending through and working relative to a slot formed through a wall of said cylinder, whereby relative reciprocation of said piston and cylinder pumps a fluid along a path including said first and second valved ports and said tubular member.

[0005] In one embodiment of the invention the cylinder is driven in reciprocation relative to the piston. In that embodiment the cylinder may be driven by a crank rotatable about an axis extending parallel to and at a fixed distance from the axis of said tubular member. In consequence, during each stroke, the piston oscillates about the axis of the tubular member and the cylinder describes a swinging motion with opposite ends moving in circles in anti-phase. The tubular member may serve as a pivot for said oscillation of the piston, the piston oscillating relative to the tubular member. Alternatively the tubular member may be secured to the piston and oscillate about its axis with the piston.

[0006] The cylinder and the piston are preferably provided with respective first and second valved ports at each end thereof. The pump is preferably symmetrical: that is, the valve arrangements at opposite ends of the cylinder, and of the piston, are symmetrical, and the slot is symmetrically disposed with respect to the cylinder ends. The slot suitably has an axial length no more than about one third of the length of the cylinder, the tubular member is centrally disposed with respect to the length of the piston and the axial length of the piston is not less than about two thirds of the length of the cylinder. It will be appreciated that the stroke of the piston cannot be greater than the axial length of the slot less the diameter of the tubular member. Thus, with the above proportions, the piston covers the slot internally throughout its stroke in the sense of avoiding any direct communication from the working volumes in the cylinder through said slot to the exterior of said cylinder, at any point in the stroke.

[0007] The slot preferably extends parallel to the longitudinal axis of the cylinder. The cylinder may be provided with two said slots on opposite sides and the tubular member may then extend across the hollow and work in both said slots. This provides a more balanced action, particularly where the tubular member serves as a pivot for an oscillating piston as disclosed above. The piston preferably has a length significantly greater than its diameter.

[0008] The valves are preferably arranged so that the first valved port at the cylinder end is an inlet valve into the cylinder, and the second valved port at the piston end is an inlet valve into the hollow of the piston. The tubular member thereby serves as an outlet conduit for the pumped fluid. In another embodiment the tubular member may be an inlet and the valves may each be outlet valves.

[0009] In yet another embodiment the piston may be driven in reciprocation relative to the cylinder by a reciprocating driving force supplied to the tubular member.

[0010] Embodiments of the invention will now be described, by way of example, with reference to the accompanying drawings, in which:-

Figure 1 is a side view of a pump according to the invention;

Figure 2 is a longitudinal section through the piston and cylinder;

Figure 3 is a cross-section on line A-A of Figure 2;

Figure 4 is an end view of the cylinder;

Figure 5 is an end view of the piston;

Figure 6 is a longitudinal section through a second embodiment of piston and cylinder; and

Figure 7 is a side view of another pump according to the invention.

[0011] Referring to Figure 1 there is shown a low pressure fluid pump or compressor comprising a structural frame 10 with a carrying handle 11. The frame includes a base 1.2 and a central post 13 having a transverse tubular member 14 secured thereto towards the lower end thereof. An electric motor 15 and a gear box 16 are mounted at one side of the post 13, and switch gear 17 for the motor is mounted to the upper end of the post 13. A pair of similar right circular cylinders 18 extend parallel to one another within the frame 10, one extending in front of the post 13 and the other behind (and therefore not seen in Figure 1). The gear box 16 is rigidly secured directly to the base 12, and the motor 1.5 is mounted to the gear box. This arrangement tends to isolate the motor 15 and the cylinders 18 from one another as regards mechanical vibration and torque loadings.

[0012] Each cylinder 18 has slots 19 through its opposite walls, and the common transverse tubular member 14 extends through and works in both slots of both cylinders. The cylinders are driven in swinging motion about the tubular member by means of individual cranks 20 disposed at the front and rear of the gear box 16 and pivotally connected to trunnions 21 secured to the heads of the cylinders. The arrangement of the rear cylinder is in all respects similar to the front cylinder shown in Figure 1, except that the respective cranks 20 preferably operate in anti-phase.

[0013] Referring to Figure 2, each cylinder 18 receives a hollow right circular cylindrical piston 22 dimensioned for a close tolerance longitudinal sliding fit within the cylinder. The tubular member 14 extends through the slots 19 in the opposite walls of the cylinder and through a circular cross-section bore 23 through the opposite walls of the piston, as shown in Figure 3. A spacer collar 24 is disposed on the tubular member between the cylinder and the post 13, and a retainer bush 25 is disposed on the tubular member at the opposite side of the cylinder. The piston may thereby pivot, i.e. oscillate, freely about the tubular member while the cylinder reciprocates in its swinging motion. The tubular member communicates with the hollow of the piston through two slot-shaped ports 26, shown in Figures 2 and 3.

[0014] Certain relative dimensions are selected as follows:-The piston has a length greater than its diameter, preferably at least twice as great. The greater the ratio of length to diameter the more stable is the piston, but the greater the sliding contact area, in this embodiment, between the piston and cylinder for a given capacity. The axial dimension of the piston from the tubular member axis to each working face of the piston is greater than the maximum axial stroke of the piston. The slots 19 are symmetrically disposed with respect to the cylinder ends and have an axial length very slightly greater than the stroke plus the tubular member diameter. Thus in effect the axial length of the piston is slightly greater than twice, the stroke length and, the tubular member being halfway between the piston faces, the slots are covered internally by the piston throughout its stroke.

[0015] It is desirable for each piston face closely to approach its associated cylinder end face at the appropriate end of each stroke. Consequently, in view of the above relative dimensions, the slots have an axial length of about one third of the cylinder length and the piston has an axial length of slightly more than two thirds of the cylinder length. The piston walls are of substantial thickness, as shown in Figure 2, to preserve dimensional stability during operation, bearing in mind that the piston serves as a manifold for the pumped fluid.

[0016] The ends of the the cylinder 18 are each provided with similar first valved ports as shown in Figures 2 and 4. Each end comprises a circular plate 30 bolted to the cylinder wall 31 by bolts 32. One end has the trunnion 21 secured thereto or formed integrally therewith. Two inlet ports 33 extend through the plate. A single flap valve 34 is screwed to the interior face of plate 30 by screws 35 and covers both ports 33, the valve opening and closing automatically in response to pressure differential across the two faces thereof. The inlet ports are open externally to the atmosphere where air is the working fluid.

[0017] The ends of the piston 22 are also each provided with similar second valved ports as shown in Figures 2 and 5. Each end comprises a circular plate 40 bolted to the piston wall 41 by bolts 42. Four inlet ports 43 extend through the plate. A single clover leaf flap valve 44 is screwed to the centre of the interior face of plate 40 by screw 45 and covers all four ports 43. The open and closed positions of the valves 44 are shown in full line at the left hand and right hand ends of the piston in Figure 2. The flap valves are conveniently formed of plastics material.

[0018] In use the cylinder is reciprocated by the motor 15. As the cylinder moves generally to the left from the extreme right hand end of its stroke shown in Figure 2, air is drawn in through the left hand ports 33 to occupy the increasing internal volume between the left hand end of the cylinder and the left hand of the piston. At the same time the left hand end piston valves 44 close because the pressure within the piston hollow, the manifold 50,is greater than in said increasing internal volume. The air previously drawn into the right hand internal volume 51 is now placed under pressure as the volume 51 reduces. Consequently the right hand cylinder valves 34 promptly close and the right hand piston valves 44 subsequently open to pump the air from the volume 51 into the manifold 50 when the pressure in volume 51 exceeds that existing in the manifold 50. When the cylinder has attained and passes the extreme left hand end of its stroke, all four sets of valves each change position in response to the appropriate pressure changes and a second pumping action commences.

[0019] The air in the manifold 50 enters the tubular member through the ports 26 and flows along the tubular member to a point of use. A suitable hose or other conduit may be connected to the outlet end of the tubular member. The opposite end of the tubular member is closed. The amount of leakage flow in or out through the slots 19 is found in practice to be insignificant at the low pressures with which the pump is primarily intended to function, typically no more than about ten psi (70,000 N/m² ). It will be noted that when the cylinder is at one extreme stroke position the internal volume at that end is at maximum pressure but is simultaneously most distant from the slots 19. Any leakage path is thus of maximum length. At the same time the other internal volume is at minimum pressure, typically ambient atmospheric pressure, while it is closest to the slot, e.g. the right hand chamber in Figure 2.

[0020] The piston and cylinder of the embodiments of Figures 1 to 5 may be made substantially wholly of engineering plastics material, suitably by injection moulding. The plastics material chosen preferably does not weaken at temperatures up to about 100°C and several such materials are available. The inherent self-sealing self-lubricating properties of plastics material reduce the frictional generation of heat between the piston and cylinder to a minimum. Plastics material is also not adversely affected by water and thus, 'for uses involving the pump working at the high end of its pressure range and at high capacity for an extended period, it is possible to spray cooling water in through the inlet manifold without adversely affecting pump operation. For the same reason the pump can be used to pump or compress air/ water mixture and other fluids. In a typical prior art motor/pump set the pump is about 50% of the total weight. The present embodiment permits that 50% to be very substantially reduced.

[0021] A second embodiment of the piston and cylinder of a pump according to the invention is shown in Figure 6. A cylinder 60 is formed of aluminium tubing and has slots 61 through its opposite side walls. The cylinder 60 receives a hollow right circular cylindrical piston 62 formed of aluminium tubing of substantially smaller external diameter than the internal diameter of the cylinder. The opposite ends of the piston are closed by polypropylene bushes 63 provided with peripheral ring seals 64 of polytetrafluoroethylene and suitably of square cross-section. The bushes and seals are dimensioned for a closed tolerance sliding fit within the cylinder 60. The bushes are located and secured in position on the piston tubing by a pair of aluminium tie rods 65 which extend between the two bushes and receive screws 66 through each bush.

[0022] The opposite ends of the cylinder 60 are closed by square metal plates 67 located and secured in position by four aluminium tie rods 68 which extend between the two plates 67 at the corners thereof and receive screws 69 through each plate.

[0023] i A circular cross-section bore 70 extends through the opposite walls of the piston and a tubular metal bush 71 is located and secured, e.g. by adhesive, in position spanning the bores 70. The bush 71 is apertured to communicate freely with the hollow of the piston in a similar manner to apertures 26 shown in Figure 2. In use, a tubular member extends through the cylinder slots 61 and the bush 71 in a similar manner to the tubular member 14 described in detail in relation to Figures 1 to 5. In this case the tubular member may have a nylon bush secured thereto to oscillate within and relative to the bush 71.

[0024] The relative dimensional criteria for the cylinder, piston and slot are also similar to those described in relation to Figures 1 to 5. The ends of the cylinder and the ends of the piston are each provided with valved ports, again suitably similar to those described in relation to Figures 1 to 5. For simplicity it is possible to employ one flap valve and one port in each position rather than the two ports 33 and four ports 43 shown in Figures 2, 4 and 5. A cylinder valve 72 and port 73, and a piston valve 74 and port 75 are shown in Figure 6.

[0025] The plastics material construction of Figures 1 to 5 and the aluminium construction of Figure 6 have different characteristics suiting them for particular applications. An advantage of the aluminium construction is that heat generated by the pumping action is readily dissipated by conduction and then convection cooling by an air current. Nevertheless a plastics material pump has run in practice for a substantial period of months without overheating and with maintenance other than a little silicone grease between the piston and the cylinder. The aluminium construction permits harder working and thus a higher speed. In one embodiment a 2000 r.p.m. motor speed is geared down to a 200 r.p.m. crank shaft.

[0026] Pumps according to the invention are intended to run at a relatively low speed as compared to the prior art in which generally a compressor runs at the same speed as a motor. A speed of 200 r.p.m. or less gives a notably quiet performance. An a.c. or d.c. drive motor may be used. One embodiment uses a 25 watt motor to provide 0.8 c.f.m.(1.5 m ³/ hour) of air at 1.5 p.s.i. (10500 N/m²), the internal diameter of the cylinder being 50 mm.

[0027] A number of pumps may be arranged in modular fashion and driven by a common drive means, e.g. an electric motor. The tubular member may be common to a number of pumps. In a particularly convenient arrangement a single stationary tubular member is secured to a frame and two, or more, piston and cylinder devices are mounted thereon, the cylinders being driven in reciprocation by a common electric motor through a crank and the tubular member serving as a common pivot for the pistons and as a common outlet manifold for the pumped fluid. Other modular arrangements may utilise the pumps in one or more axial rows or in a radial arrangement. The pumps in a modular array may be connected in series or in parallel with respect to the supply of pumped working fluid.

[0028] It .is also possible to have a pair of piston and cylinder devices of different working capacity driven by a common drive motor and mounted on a common tubular member as a pivot, the tubular member being closed between the cylinders and thus having separate outlets for the fluid pumped by the two devices. For example an isolated small sewage or effluent treatment plant may have a need for low pressure air at a delivery rate of 10 m³/hour, e.g. for aeration, and sludge return, and a simultaneous need for air at a delivery rate of 2 m³/hour, e.g. for air lifts and scum return.

[0029] Further flexibility of output can be achieved by providing a crank arm, e.g. crank 20, with two or more bolts apertures at different distances from the drive motor output axis. The cylinder trunnion may then be selectively pivot bolted to a particular crank aperture to determine the stroke .of the piston and cylinder device.

[0030] Figure 7 is a side view of another pump according to the invention. Two piston and cylinder devices 80 incorporated therein may each take the form either of that shown in Figures 2 to 5 or that shown in Figure 6. This pump comprises a structural frame 81 in the form of a single vertical metal tube which may be provided with flanges 82 to enable the frame to be bolted to a wall. Alternatively frame 81 may have feet for free standing in a vertical position. A short transverse metal tube 83 extends from tube 81 and communicates internally therewith. The tube 83 carries a tubular member 84 which is similar in structure and purpose to the tubular member 14 described in relation to Figure 1 to 5.

[0031] The tubular member 84 serves as common pivot for the pistons of the two devices 80. Moreoever the tubular member 84 communicates internally with tube 83 and thus also with the tube 81 forming the frame. The frame thereby also serves the purpose of an outlet manifold for the pumped fluid. The pumped fluid may be taken by hose or other conduit from the top or bottom end or both of the tube 81.

[0032] The devices 80 are provided with a common drive means comprising an electric motor 85 and a gearbox 86. The gear box is directly secured to the frame tube 81 and the motor is mounted on top of the gear box 86. Drive from the gear box to the devices 80 is by way of cranks 87 operating in an anti-phase as illustrated in the drawing.

[0033] The vertical arrangement shown in Figure 7, with the motor 85 at the top, is helpful in generating and sustaining a convective flow of cooling air up past the pistons and cylinders. If desired a generally tubular housing or cowl may be fitted over the apparatus of Figure 7, for example as shown by dashed lines 88. The cowl would be provided with substantial apertures in its side-wall near the top and the bottom for flow of working and cooling air.

Claims

1. A pump comprising a piston (22) adapted for reciprocation within and relative to a cylinder (18), said cylinder having a first valved port (33,34) at at least one end thereof, characterised in that said piston (22) is hollow and has a second valved port (43,44) at at least the end thereof nearer a said valved end of the cylinder, a tubular member (14) communicating with the interior of said hollow piston at a point between the ends of the piston and extending substantially perpendicular to the path of said reciprocation, said tubular member (14) extending through and working relative to a slot (19) formed through a wall of said cylinder (18), whereby relative reciprocation of said piston and cylinder pumps a fluid along a flow path including said first and second valved ports and said tubular member.

2. A pump according to claim 1 characterised by drive means (15,16) connected to drive said cylinder (18) in reciprocation relative to the piston (22).

3. A pump according to claim 2 characterised by crank means (20) connecting said drive means (15,16) to said cylinder, said crank means (20) being rotatable about an axis parallel to the axis of said tubular member (14) whereby the cylinder is driven in a swinging motion with opposite ends moving in circles in anti-phase and the piston oscillates about the axis of the tubular member.

4. A pump according to claim 3 characterised in that said tubular member (14) serves as a pivot for said oscillation of said piston (22), the piston oscillating relative to the tubular member.

5. A pump according to any one of claims 1 to 4 characterised in that the cylinder (18) and the piston (22) are each provided with respective first and second said valved ports (33,34; 43,44) at each end thereof.

6. A pump according to any one of claims 1 to 5 characterised in that said slot (19) is centrally and symmetrically disposed between said cylinder ends, the tubular member (14) is centrally disposed with respect to the length of the piston (22), and the relative lengths of the piston (22), cylinder (18) and slot (19) are such that there is no direct communication from the working volumes within the cylinder through said slot to the exterior of said cylinder at any point in the stroke.

7. A pump according to claim 6 characterised in that said stroke and said slot each have an axial length no more than about one third of the length of the cylinder (18), and the axial length of the piston (22).,is not less than about two thirds of the length of the cylinder.

8. A pump according to any one of claims 1 to 7 characterised in that the cylinder (18) is provided with two said slots (19) on opposite sides, each said slot extending parallel to the longitudinal axis of the cylinder, and in that the tubular member (14) extends across the hollow and works in both said slots.

9. A pump according to any one of claims 1 to 8 characterised in that said first valved port (33,34) is an inlet valve into the cylinder (18), the second valved port (43,44) is an inlet valve into the hollow of the piston (22), and the tubular member (14) serves as an outlet conduit for pumped fluid.

10. A pump according to any one of claims 1 to 9 characterised in that said valves (34,44) are flap- valves of plastics material.

11. A pump according to any one of claims 1 to 10 including drive means (15,16) connected to drive said cylinder (18) in reciprocation relative to the piston (22), characterised by a frame (10,81) supporting said pump, and wherein said drive means (15,16) comprises a motor (15,86) and a gear box (16,85), said gear box and said tubular member (14,84) being rigidly secured to said frame, and said drive motor being mounted to said gear box.

12. A pump according to any one of claims 1 to 11 including a frame (81) supporting said pump, characterised in that said frame (81) consists of a metal tube (81) in fluid flow connection with said tubular member (84) to form a portion of said fluid flow path.

5 13. A pump according to any one of claims 1 to 12 comprising at least two said pistons (22,80) and cylinders (18, 89) driven by a common drive means (15,16;85,86), a single said tubular member (14,84) serving as a common pivot for oscillation of said pistons.

14. A pump according to claim 13 wherein said two piston and cylinder devices are of different pumping capacities.

Drawing