WO1996028659A1

WO1996028659A1 - Piston pump

Info

Publication number: WO1996028659A1
Application number: PCT/GB1996/000548
Authority: WO
Inventors: Alec Thornilow; Simon John Baseley
Original assignee: Hobourn Automotive Limited
Priority date: 1995-03-10
Filing date: 1996-03-08
Publication date: 1996-09-19
Also published as: GB9504892D0

Abstract

A piston pump in which one or more pistons are slidably mounted in respective open-ended cylinders (3). The head (15) of each piston is located towards the open end of each cylinder (3) and acts as a cam follower resting on a cam surface (5') defined by an eccentric lobe formed on a central rotating shaft (5). As the shaft (5) rotates, each piston reciprocates within its respective cynlinder (3). Each piston is formed in two parts (6, 7) which are capable of limited relative motion with respect to one another. A dampening mechanism is located between the two parts (6, 7) of each cylinder by which noise, caused by shock waves travelling from the head (15) of each cylinder to its other end, is reduced. The dampening mechanism comprises a small chamber (16), formed between the two parts (6, 7) of each cylinder, which chamber (16) is in fluid communication with an oil reservoir located in the respective cylinder (3) via a restrictive passage (14).

Description

PISTON PUMP

The present invention relates to a piston pump and particularly to a piston pump capable of withstanding large loads.

In conventional piston pumps a number of pistons are spaced radially around a central rotating shaft which is mounted on bearings in the pump housing. The shaft has one or more eccentric lobes which act on each of the pistons in turn as the shaft rotates. Each of the pistons are tubular in construction and are mounted in a respective cylinder having a valved inlet port and a valved outlet port. Each of the pistons are biased against the shaft by means of a spring which is located in the hollow end of the piston and is attached at its opposite end to an end cap of the cylinder. The difficulty with the piston pump described above is that the rapid changes in pressure in the pump cylinders causes high shock loads to be generated between the piston and the eccentric lobe on the shaft. This in turn can result in high noise levels during operation.

With the present invention a piston is provided which is capable of withstanding the high loads associated with piston pumps and enables operation without the high noise levels formerly associated with such pumps.

The present invention provides a piston pump comprising a cylinder, a piston body slidably movable within the cylinder and means for causing movement of the piston body within the cylinder, the piston body having first and second separable parts capable of movement relative to each other and a damping member therebetween for damping the transmission of a load through the piston body.

With the invention described the damping member and the two parts of the piston body reduce the transmission of shock loads through the piston to the shaft during use thereby also reducing noise.

Preferably, the first part of the piston body is a substantially cylindrical member having an inwardly projecting ledge and the second part is an insert for location within the cylindrical member and having a shoulder for defining with the ledge a chamber which functions as the damping member and is in fluid communication with the interior of the piston cylinder by means of a narrow passage.

An embodiment of the present invention will now be described by way of example with reference to the accompanying drawings, in which:

Figure 1 is a cross-sectional view through a piston pump in accordance with the present invention; and

Figure 2 is an enlarged sectional view of the piston of Figure 1. A piston pump is shown in Figure 1 which consists of six pistons 2 and their associated cylinders 3 mounted in the pump housing 4 in equal spaced radial relationship about a central rotating shaft 5 having a cam surface 5' defined by an eccentric lobe on the shaft. It will of course be understood that alternative arrangements of the pistons and different numbers of pistons may be employed as appropriate.

One of the pistons 2 is shown in greater detail in Figure 2. The piston 2 is formed in two parts 6,7 which are capable of limited relative motion with respect to each other. The piston body 6 is a substantially cylindrical member the outer surface of which is in sliding engagement with the wall of the cylinder 3 and which moves to expose and close the inlet port 8 of the cylinder. Projecting inwardly from the piston body 6 is a shelf 9 which provides an abutment surface for engagement with an end of a spring 10. The spring extends radially outwardly from the shelf 9 to the cap or end closure 11 of the cylinder. The second part of the piston 2 is an insert 7 which includes a spigot 12 which projects through the centre of the piston body 6. The piston body 6 and the spigot 12 define a recess 13 in which the spring 10 is located. As may be seen from Figure 2 the clearance between the spigot 12 and the free edge of the shelf 9 is small and defines a narrow passage 14 therebetween. The head 15 of the spigot 7, which is adjacent the shaft 5, has a diameter greater than the diameter of the body of the spigot so that the head 15 overlaps with the shelf 9 and forms a sealing fit with the piston body 6.

The shelf 9 of the piston body 6 and the head 15 of the insert 7 define a chamber 16 which is in fluid communication with the interior of the piston cylinder 3 via the passage 14. The chamber 16, which act as a damping device, contains a resilient member in the forms of a backing ring 17 and an energising ring 18, for example an O ring, which also act to seal the chamber 16 and the piston bore.

During operation, the cam 5' engages with the head 15 of the insert 7 to cause the insert and the piston body 6 to move within the cylinder 3. As the piston 2 moves to the lowest position in the cylinder 3, during its downward stroke, the pressure in the cylinder 3 above the piston 2 is low and the energising ring 18 expands to cause the chamber 16 to assume its greatest volume. The piston 2 moves to uncover the inlet port of the cylinder 8 thereby introducing fluid , for example oil, into the interior of the cylinder. Oil is also drawn into the chamber 16. As the piston 2 then moves on the return upward stroke the inlet port 8 is covered and the pressure in the cylinder rises sharply as does the pressure in the chamber 16. The pressure acting on the piston body is opposed by the pressure in the chamber 16 and the resilient member 17, 18 and causes the volume of the chamber 16 to be reduced slightly thereby increasing the pressure on the resilient member. Also, as the volume of the chamber 16 is compressed, the oil in the chamber 16 is only able to escape through the narrow passage 14 at a controlled rate thereby providing a damping or cushioning effect. The oil is then fed out through the one-way valve of the outlet port 19 of the cylinder until the piston returns to its downward stroke.

The cushioning or damping effect of the oil in the chamber 16 and the sealing member reduces the transmission of shock loads through the piston to the shaft. This in turn reduces the amount of noise generated by the piston pump when in use.

It will be apparent that alternative arrangements of the parts of the piston may be employed without departing from the spirit and scope of the present invention. For example, the attachment of the spring to the piston need not be by means of the shelf 9. Also, the piston body may be attached to a surface which engages the cam surface of the shaft and the insert may be in engagement with the spring which still enables a damping chamber to be provided and relative movement between the piston body and the insert of the piston.

Claims

1. A piston pump comprising a cylinder, a piston body slidably movable within the cylinder and means for causing movement of the piston body within the cylinder, the piston body having first and second separable parts capable of movement relative to each other and a damping member therebetween for damping the transmission of a load through the piston body.

2. The piston pump as claimed in claim 1 , wherein the damping member is a chamber defined by the first and second parts of the piston body which is in fluid communication with a supply of fluid, the chamber including limiting means for restricting the flow of fluid from the chamber.

3. A piston pump as claimed in claim 2, wherein the supply of fluid is the interior of the cylinder.

4. A piston pump as claimed in claim 3, wherein the limiting means is a passage having a cross-sectional area smaller than the area of the interior of the chamber.

5. A piston pump as claimed in any one of claims 2 to 4, wherein the chamber further includes a resilient sealing member.

6. A piston pump as claimed in any one of claims 2 to 5, wherein the first part of the piston body is a substantially cylindrical member having an inwardly projecting ledge and the second part is an insert for location within the cylindrical member and having a shoulder for defining with the ledge the chamber.

7. A piston pump substantially as hereinbefore described with reference to and as shown in the accompanying drawings.