EP1659295A1

EP1659295A1 - Accumulator

Info

Publication number: EP1659295A1
Application number: EP05257117A
Authority: EP
Inventors: Andrew Lindgren
Original assignee: Parker Hannifin PLC
Current assignee: Parker Hannifin Manufacturing Ltd
Priority date: 2004-11-18
Filing date: 2005-11-18
Publication date: 2006-05-24
Also published as: GB2420380A; GB0425428D0

Abstract

An accumulator 100 is described. The accumulator comprises a cylinder 110 and a port 120, 130 for the passage of hydraulic fluid into and out of the cylinder at each axial end of the cylinder. Two pistons 140, 150 are arranged to reciprocate in an axial direction within the cylinder with a peripheral edge of each of the pistons sealing against the inside surface of the cylinder as they reciprocate. In use a volume of fluid is provided between the pistons to act on both pistons. The volume of fluid, which may be a gas such as nitrogen, acts on both pistons to provide a constant spring rate to both pistons. A valve is preferably provided in one of the pistons, for fluid to be supplied between the pistons.

A hydraulic system with two hydraulic lines may incorporate the accumulator in the hydraulic system with one of the hydraulic fluid ports of the accumulator connected to each of the two hydraulic lines.

Description

The present invention relates to an accumulator, particularly a piston accumulator, and a hydraulic system including the accumulator.
An accumulator stores hydraulic fluid under pressure. An example of a piston accumulator and its operation is shown in Figures 1A to 1F. As shown in Figure 1A, piston accumulator 1 comprises an accumulator body or cylinder 10 with a piston 20 arranged to reciprocate within the cylinder 10.
A seal 30, usually in the form of one or more O-rings, is provided around the outside surface of the piston 20 to seal against the inside surface of the cylinder 10 as the piston 20 reciprocates. A gas cap 40 with a gas port 41 and valve 42 is provided at one axial end of the cylinder 10. A hydraulic fluid cap 50 with a hydraulic fluid port 51 is provided at the other axial end of the cylinder 10. The hydraulic fluid port 51 is connected to a hydraulic system (not shown).
As shown in Figure 1B, the accumulator 1 is pre-charged with gas to a desired pressure through gas port 41 and valve 42. Although referred to as gas, any suitable liquid or gas could be used. However, generally a relatively inert gas such as nitrogen is used.
Figure 1C shows the hydraulic system as it is pressurised. When the hydraulic system pressure exceeds the pre-charged gas pressure, hydraulic fluid flows through the hydraulic fluid port 51 into the accumulator 1 as shown by the arrows. This flow of hydraulic fluid moves the piston 20 away from the hydraulic fluid cap 50.
Figure 1D shows the accumulator 1 filled with hydraulic fluid at a peak in the pressure of the hydraulic system.
Figure IE shows the accumulator 1 as the pressure within the hydraulic system falls with the pre-charged gas pressure forcing hydraulic fluid out of the accumulator 1 back into the hydraulic system.
Figure 1F shows the accumulator 1 after a further fall in pressure within the hydraulic system with the pre-charged gas pressure having forced more hydraulic fluid out of the accumulator 1 back into the hydraulic system.
Such an accumulator may be used in any suitable mobile or industrial application. In many applications it is desirable to provide an accumulator in each of two or more parallel hydraulic fluid lines such as parallel hydraulic fluid lines leading to steering mechanisms for each wheel of a vehicle, mechanisms to rotate an arm of a crane in opposite directions etc. Figure 2A shows such an application with a steering mechanism 60 connected to two parallel hydraulic fluid lines 61, 62 and each hydraulic fluid line 61, 62 connected to a steering mechanism 63, 64 for a wheel. In the example of Figure 2A an accumulator 1 as described in Figures 1A to 1F is provided connected to each of the hydraulic fluid lines 61, 62. However, in order to provide the same steering feel and response in both directions, the volumes and pre-charge pressures in each accumulator need to be matched exactly which is very difficult to achieve in practice. Furthermore, each accumulator could react differently to outside influences such as temperature, especially if turned in one direction more than the other. This would then produce further uneven feel and response in both directions. Furthermore, the provision of two accumulators increases costs.
Instead of providing an accumulator 1 in each of the two hydraulic fluid lines 61, 62 a spring operated device 70 could be connected to both hydraulic fluid lines 61, 62 as schematically shown in Figure 2B. The spring operated device 70 may comprise a cylinder with a hydraulic fluid port at each axial end and with two pistons arranged to reciprocate within the cylinder. The pistons are interconnected by a spring. However, the device 70 will have a fixed spring rate set by the type of spring used. Furthermore, it would be desirable to reduce the weight and cost of the device 70 which will require connectors and washers to secure the spring to the pistons.
According to the present invention there is provided an accumulator comprising:

a cylinder;
a port for the passage of hydraulic fluid into and out of the cylinder at each axial end of the cylinder;
two pistons arranged to reciprocate in an axial direction within the cylinder with a peripheral edge of each of the pistons sealing against the inside surface of the cylinder as they reciprocate and
in use, a volume of fluid between the pistons arranged to act on both pistons.

The volume of fluid, which may be a gas such as nitrogen, acts on both pistons to provide a constant spring rate providing the same feel and response in both directions in appropriate applications and provides a simple, lightweight and lower cost accumulator.
A valve is preferably provided in one of the pistons, through which the volume of gas is supplied. The spring rate between the pistons can be adjusted by changing the pressure of the volume of gas. As the valve is provided in a piston rather than, say, in a side wall of the cylinder, a seal on the peripheral edge of the pistons is not worn by contact with the valve, there is no leakage over the edge of the piston whilst passing the valve and precise positioning of the pistons in the cylinder when supplying gas is not required.
An example of the present invention will now be described with reference to the accompanying drawings, in which:

Figures 1A to 1F show examples of operation of a conventional accumulator;
Figures 2A and 2B show examples of providing an accumulator in two parallel hydraulic fluid lines;
Figure 3 shows an example of an accumulator illustrating the present invention;
Figures 4A to 4D show the operation of the accumulator of Figure 3;
Figure 5 shows an example of an accumulator illustrating the present invention connected between two parallel hydraulic fluid lines;
Figure 6 shows a crimping die to mechanically deform a cylinder into a groove in the circumferential periphery of an end cap and
Figure 7 shows a cross-section of an edge of an end cap.

The accumulator 100 shown in Figure 3 has a cylinder 110 with a hydraulic port 120 at one axial end and another hydraulic port 130 at the other axial end. Each of the hydraulic ports 120, 130 is arranged to be connected to a line of a hydraulic system (not shown) when in use. The cylinder 110 has two pistons 140, 150 with seals around their periphery (not shown) to seal against the inside surface of the cylinder 110. The pistons 140, 150 are arranged to reciprocate within the cylinder 110. Fluid, in this example an inert gas such as nitrogen, is provided between the pistons 140, 150. The single gas volume acts on both pistons providing a constant spring rate for both of the pistons. When the hydraulic ports 120, 130 are connected to parallel hydraulic lines such as those of the steering mechanism 60 as shown in Figures 2A and 2B, the same feel and response is provided in both directions.
One of the pistons, in this example piston 150, is provided with a valve 160 for charging the volume between the pistons 140, 150 with fluid. As explained above, in this example the fluid provided between the pistons 140, 150 is an inert gas such as nitrogen. The valve 160 is provided in or on a conduit 170 shown schematically by dotted lines passing axially through the piston 150. The valve 160 and conduit 170 are provided internally within the piston 150 so that in use they do not interfere with the seals around the circumferential periphery of the piston 150 so that the integrity of the seal is not degraded.
Each of the hydraulic ports 120, 130 is provided on a respective end cap 121, 131. The end caps 121, 131 may be joined to the cylinder 110 in any desired manner such as by using corresponding threads on the circumferential periphery of the end caps 121, 131 and the inner surface of the end portions of the cylinder 110 or by mechanically deforming the cylinder 110 into one or more grooves in the circumferential periphery of one or both of the end caps 121, 131 (crimping) for example.
In the example shown in Figure 3, end cap 131 is provided with an extension portion 132 to prevent contact between valve 160 and end cap 131 when piston 150 bottoms out. The extension portion 132 is arranged to be connected between an end of the cylinder 110 and the end cap 131 in any desired manner such as by corresponding threaded portions.
Figures 4A to 4D schematically show an example of an accumulator of the present invention at various stages of operation.
Figure 4A shows the assembled accumulator 100 prior to use without a pre-charge of gas. The assembled accumulator comprises cylinder 110 with ports 120, 130 for the passage of hydraulic fluid at each axial end of the cylinder 110. Each port 120, 130 is connected to a respective hydraulic system (not shown). Two pistons 140, 150 are arranged to reciprocate in an axial direction within the cylinder 110 with a peripheral edge of each of the pistons sealing against the inside surface of the cylinder 110 as they reciprocate. The hydraulic fluid ports 120, 130 are provided in respective end caps 121, 131.
The accumulator 100 is provided with a pre-charge of gas between pistons 140, 150 through valve 160 which passes internally through one piston 150. The spring rate for the two pistons 140, 150 may be selected by providing any desired pressure of gas between the pistons 140, 150. The spring rate may be adjusted subsequently by changing the pre-charge pressure of gas between the pistons 140, 150.
Figure 4B shows the accumulator 100 with the pre-charge of gas between the pistons 140, 150. As indicated by the double ended arrow, the pre-charge of gas exerts the same spring rate on both pistons 140, 150.
Figure 4C shows the accumulator 100 working from the left hand end as shown with hydraulic fluid such as oil entering through port 120 from a hydraulic system (not shown).
Figure 4D shows the accumulator 100 working from the right hand end as shown with hydraulic fluid such as oil entering through port 130 from a hydraulic system (not shown).
The accumulator 100 of Figures 3 and 4 may be connected between two hydraulic lines of a hydraulic system, such as the hydraulic lines 61, 62 as shown in Figure 5.
If the cylinder 110 is mechanically deformed (crimped) into one or more grooves in the circumferential periphery of one or both of the end caps 121, 131 as described above, this mechanical deformation is preferably performed using a crimping die as shown in Figure 6. The crimping die has a plurality of teeth 200, in this example six, which surround one end of the cylinder 110 with an end cap 121 positioned therein. The mechanical deformation of the cylinder 110 onto the circumferential periphery of the end cap 121 is performed by each tooth 200 simultaneously moving radially inwardly, contacting the outer surface of the cylinder 110 and mechanically deforming it onto the circumferential periphery of the end cap 121. This mechanical deformation is easily performed by a single compressive action of the teeth 200 of the crimping die and enables relatively thick walled cylinders to be mechanically deformed onto the end caps.
Figure 7 shows a cross-section of the edge of one of the end caps 121, 131. As can be seen an annular groove 300 is formed in the circumferential periphery of the end cap 121, 131 into which the cylinder 110 is mechanically deformed. The groove 300 preferably includes a sharp entry radius 310 and a smooth, curved sidewall 320 to the bottom of the groove. A wider portion 330 of the end cap 121, 131 provides guidance for the positioning of the cylinder 110 onto the end cap prior to mechanical deformation. The sharp entry radius 310 provides enhanced fatigue resistance and the curved sidewall 320 reduces tensile stress. The groove may have a depth of at least 1mm or at least 2mm.
Many variations may be made to the example shown without departing from the inventive concept. For example, the hydraulic fluid ports 120, 130 may be provided in end caps 121, 131 which could be connected to the cylinder 110 in any desired manner such as by threads or by crimping. Furthermore, one end cap may or may not be provided with an extension 132 to prevent contact between the valve in the piston and the end cap. The accumulator may, if desired, be connected between any two hydraulic fluid lines.
The invention could apply to any size accumulator such as 1 litre, 2 litre for example.

Claims

An accumulator comprising:
a cylinder;

a port for the passage of hydraulic fluid into and out of the cylinder at each axial end of the cylinder;

two pistons arranged to reciprocate in an axial direction within the cylinder with a peripheral edge of each of the pistons sealing against the inside surface of the cylinder as they reciprocate and

in use, a volume of fluid between the pistons arranged to act on both pistons.
An accumulator according to claim 1, wherein a valve is provided in one of the pistons for fluid to be supplied between the pistons.
An accumulator according to claim 2, wherein the valve is provided in or on a conduit provided internally within a piston such that it does not form part of the circumferential periphery of the piston arranged to engage the inside surface of the cylinder.
An accumulator according to claim 3, wherein the conduit passes in an axial direction through the piston.
An accumulator according to any one of the preceding claims wherein the hydraulic fluid ports are each provided in an end cap.
An accumulator according to claim 5, wherein the cylinder is sealed to at least one of the end caps using corresponding threads on the inside surface of an end portion of the cylinder and the circumferential periphery of an end cap.
An accumulator according to claim 5 or claim 6, wherein the cylinder is sealed to at least one of the end caps by mechanically deforming the cylinder into one or more grooves in the circumferential periphery of the at least one end cap.
An accumulator according to claim 7, wherein the cylinder is mechanically deformed using a crimping die.
An accumulator according to claim 7 or claim 8, wherein the one or more grooves have a cross-section including a sharp entry radius and a curved sidewall.
An accumulator according to any one of claims 5 to 9 when dependent upon any one of claims 2 to 4, wherein the end cap to be sealed to the end of the cylinder with the piston having a valve has an extension portion between the cylinder and that end cap.
A hydraulic system with at least two hydraulic lines and the hydraulic ports of an accumulator according to any one of the previous claims connected between the two hydraulic lines.