EP2365226B1

EP2365226B1 - Hydraulic system

Info

Publication number: EP2365226B1
Application number: EP11157419.0A
Authority: EP
Inventors: Ricccardo Morselli
Original assignee: CNH Industrial Italia SpA
Current assignee: CNH Industrial Italia SpA
Priority date: 2010-03-12
Filing date: 2011-03-09
Publication date: 2013-05-22
Anticipated expiration: 2031-03-09
Also published as: ITTO20100190A1; EP2365226A1

Description

Field of the invention

The present invention relates to a hydraulic system for controlling the supply of hydraulic power to a load, such as a jack.

Background of the invention

Tractors and other agricultural vehicles often have hydraulic output lines, sometimes termed an electro-hydraulic remote, which are used to supply hydraulic power to ancillary equipment. Two output lines are used, one to supply hydraulic fluid under pressure and the other acts a return line for the fluid discharged by the load. Each of these two output lines is connectible by a leak proof coupling to a hose leading to a respective side of the load.
Assuming that the load is a hydraulic jack, it may be required for example to raise the jack, lower it, lock it in a fixed position and allow it to float freely. To achieve this, it is well known in the art to use a five port, four position spool valve, as shown in Figures 1A to 1D which schematic show a prior art valve with the spool in four different positions. The two output ports A and B of the spool valve 10 lead to two output lines connectable to the load 12. Of the three input ports, a first port R is connected to a return line leading to a fluid reservoir, a second port PC is connected to a source of pressurised fluid and the third port PLS is connected to a sensing line for allowing the back pressure to be monitored.
In the spool position represented by Figure 1A, the output ports A and B are isolated from one another and from the supply and return port PC and R ports. The jack 12 is locked in its existing position as fluid can neither enter nor escape from either of its working chambers.
In Figure 1B the spool is shown in its position to extend the jack 12. Fluid under pressure is supplied from the supply port PC to the working chamber on the right of the piston of the jack 12 while fluid from the other working chamber is allowed to return to the reservoir through the return port R, causing the piston to move from right to left as viewed.
The connections of Figure 1B are reversed when the spool is moved to the position represented by Figure 1C. In this case, fluid is supplied under pressure to the working chamber on the left of the piston as viewed and allowed to return to the reservoir from the working chamber to the right. In this way, the piston is caused to retract, i.e. the piston rod moves back into the cylinder from left to right.
Lastly, in the position shown in Figure 1D, the two output ports A and B are connected to one another. This allows the piston to float freely within the cylinder. As the working chambers do not have the same area, both chambers are connected to the return port R so that surplus fluid can be discharged to the reservoir or additional fluid can be drawn from the reservoir.
The load sensing port PLS is connected to the return port R when the cylinder is locked or floating. When the jack is being extended or retracted, the load sensing port PLS is connected to the output port A or B that is connected to the supply port PC. Within the spool, throttles are provided in the connections leading from the supply port PC to the respective output port A or B. The purpose of each throttle in the spool is not to damp the movement of the load but merely to allow a load sensing pressure difference to be developed across it indicative of the resistance offered by the load. If the load is low and the piston moves freely, there will be a high flow rate in the throttle and the pressure measured at the port PLS will be lower than the supply pressure PC. However, when the load offers high resistance, the flow rate through the throttle will be low and the sensed pressure PLS will be nearly equal to supply pressure PC.
When driving a load that is offering resistance, there should ideally be no resistance to flow in the return line R leading to the reservoir. Any resistance to flow offered by the return line will cause a pressure drop and reduce the efficiency of the hydraulic system. In particular, the throttling effect of the return line will result in energy losses equal to the product of the fluid flow rate and the pressure at the port R connected to the return line.
In practice, there needs to be present resistance in the return line to allow for the fact that the load does not always offer high resistance and can operate in a draft mode. Supposing for example that in Figure 1B, the jack 12 is being used to raise a heavy weight. The force to extend the jack is resisted by the weight being raised and the jack can only extend relatively slowly. However, when the spool valve is moved to the position shown in Figure 1C to lower the jack, instead of opposing the movement of the piston of the jack, the weight will assist it. In the absence of some form of hydraulic damping, the weight may drop too rapidly. A throttle is therefore included in the spool to provided resistance in the return line in order to damp the movement of the piston when it is operating in a draft mode, that is to say when the gravitational or other forces acting on the piston assist the applied hydraulic pressure instead of resisting it.
DE 10 2005 022 275 A1 discloses a hydraulic control arrangement with a pilot operated pressure relief valve provided with a pressure switching stage by which means the pressure regulated by the pressure relief valve can be lowered according to the pressure in the another pressure chamber.

Object of the invention

The present invention seeks therefore to provide a hydraulic system which operates efficiently when the controlled load is operating in a resistive mode yet provides hydraulic damping when the load is operating in a draft mode.

Summary of the invention

According to the present invention, there is provided a hydraulic system having a spool valve with two output ports connectible to opposite sides of a hydraulic load and three input ports which include a supply port connectable to a pressure supply line, a return port connectable to a return line, and a pressure sensing part connectable to a load sensing line wherein a first throttle is provided in the flow path between the pressure supply line and each output port, the downstream side of the first throttle being connected to the pressure sensing port, and a second throttle is provided in the flow path from each output port to the return line, characterised in that a low resistance discharge path bypassing the valve spool is provided between at least one of the output ports and the return port, the low resistance discharge path including a normally closed valve that opens when the pressure difference between the sensing port and the output port exceeds a predetermined level.
In the invention, when the sensing port is at a lower pressure than the return side of the load, indicating that the load is a draft load, the low resistance discharge path remains closed and the fluid from the return side of the load has to pass the throttled flow path through the spool valve before reaching the reservoir, resulting in damping of the movement of the load. When however the sensing port is at a higher pressure than the output port, indicating a resistive load, the discharge path is opened so that less resistance to flow is offered by the fluid returning to the reservoir and an improved efficiency is achieved.
The normally closed valve is preferably a hydraulically controlled spool valve urged by a spring towards a closed position, the spool being moved towards the open position by the pressure at the sensing port and towards the closed position by the pressure at the output port.
To provide improved operation in both directions of movement of the load, each of the output ports may have a respective discharge path incorporating a respective normally closed valve.

Brief description of the drawings

The invention will now be described further, by way of example, with reference to the accompanying drawings, wherein:

Fig. 1, as earlier described in the background art section, shows representations of a spool valve used in controlling a load connected to the electro-hydraulic remote lines of a tractor, the spool valve being shown in four different positions; and
Figures 2 and 3 show hydraulic systems embodying the present invention and incorporating a spool valve as shown in Figure 1.

Detailed description of the preferred embodiment(s)

Figure 2 shows the spool valve 10 in the same position as in Figure 1B. The passage in the valve spool connecting the output port B to return line R is shown as incorporating a throttle but this throttle may alternatively be disposed with the return port. This throttle offers resistance to flow to damp the movement of the load when it is operating in a draft mode.
When operating in draft mode, if the piston of the jack 12 in Figure 2 were allowed to float, it would move to the left on account of the gravitational forces acting on it. The fluid supplied under pressure through the port A would encounter little or no resistance and fluid will also be returned under pressure through port B to the reservoir. Were it not for the effect of the throttle in the return flow path, the piston could move dangerously quickly. Therefore, the throttling effect within the return line is therefore required when operating in draft more.
When operating in a resistive mode on the other hand, the piston in Figure 2 would attempt to move to the right under the gravitational forces and would offer resistance to fluid supplied through port A. There will therefore be a higher back pressure and there would be no danger of the piston moving too quickly. However, all the fluid returning to the reservoir would encounter unwanted resistance when flowing through the return line, the work done in forcing the fluid through the restriction in the return line unnecessarily reducing the overall efficiency of the hydraulic system.
The system of the invention shown in Figure 2 overcomes this energy waste by providing two flow paths for the return fluid. One flow path has a higher resistance to provide damping when operating in draft mode and the other having lower resistance when operating in resistive mode. Furthermore, the switching between flow paths is automatic and requires no intervention from the operator.
A hydraulically controlled on/off spool valve 24 is provided in a low resistance discharge line 22 leading from the port B to the reservoir. Under normal condition, this valve 24 is closed as represented by the spring at the left of the valve in Figure 2.
The spool of the valve 24 is urged in a direction to close by the pressure in the port B and is urged in a direction to open by the pressure PLS in the sensing port.
If operating a resistive load, the sensed back pressure will be greater than the pressure in the return port and when the net force on the spool of the valve 24 exceeds the spring force, the spool 24 moves to the left as shown and opens the valve 24. This opening immediately reduces the pressure in port B to nearly ambient pressure and as long as the sensed back pressure remains sufficient to overcome the return spring acting on the spool, the valve 24 will remain open. Thus, when operating in a resistive mode, there is little or no pressure drop in the return line from port B to the reservoir.
On the other hand, when the load is operating in a draft mode, the pressure PLS in the sensing line remains low and cannot overcome the spring load on the spool of the valve 24 and the pressure drop across the throttle in the return flow path further assists in keeping the valve 24 closed. The return fluid must therefore pass through the return port R of the spool valve 10 and encounter the throttling effect of the return line to provide the necessary damping of the piston movement.
The valve 34 serves the same function as the valve 24 but is only active when the spool of the valve 10 is moved to the position shown in Figure 1C. For as long as the spool valve is in the position shown in Figure 2, port A is under the same pressure as the back pressure PLS in the sensing port and the valve 34 is therefore kept in its closed position by the action of its own spring.
Though the invention has been described with reference to a load constructed as a jack, it should be clear that it applies equally to other forms of load. For example, a hydraulic motor can have a draft mode when attempting to reduce the speed of a flywheel having a high moment of inertia.
The embodiment of the invention shown in Figure 3 is essentially the same as that of Figure 2 save for the addition of an electronically controlled discharge valve 44 which allows the discharge flow through the valves 24 and 34 to be enabled and disabled. When the discharge valve 44 is closed, the hydraulic system behaves in the same way as a conventional system, as described above by reference to Figures 1A to 1D. On the other hand, when the valve 44 is opened, the hydraulic system behaves in the same manner as the embodiment of the invention shown in Figure 2. The advantages offered by this additional control are that it results in smooth transitions when one of the valves 24 and 34 opens and it allows disablement of the discharge flows which bypass the spool valve 10, when necessary for safety or specific working conditions.
It will also be appreciated that the invention is not restricted to the particular form of spool valve described above. For example, in some applications, there may be no requirement for a floating position of the valve spool. Furthermore, a throttle may be provided in series with the supply line instead of two throttles being integrated into the valve spool.

Claims

A hydraulic system having a spool valve (10) with two output ports connectible to opposite sides of a hydraulic load (12) and three input ports which include a supply port connectable to a pressure supply line, a return port connectable to a return line, and a pressure sensing port connectable to a load sensing line, wherein a first throttle is provided in the flow path between the pressure supply line and each output port, the downstream side of the first throttle being connected to the pressure sensing port, and a second throttle is provided in the flow path from each output port to the return line;
hydraulic system characterised in that a low resistance discharge path (22,32) bypassing the valve spool (10) is provided between at least one of the output ports and the return port, the low resistance discharge path (22,32) including a normally closed valve (24,34) that opens when the pressure difference between the sensing port and the output port exceeds a predetermined level.
A hydraulic system as claimed in Claim 1, wherein the normally closed valve (24,34) is a hydraulically controlled spool valve urged by a spring towards a closed position, the spool being moved towards the open position by the pressure (PLS) at the sensing port and towards the closed position by the pressure at the output port (B).
A hydraulic system as claimed in claim 1 or 2, wherein both the output ports (B,A) have respective discharge paths (22,32) and respective normally closed valves (24,34).
A hydraulic system as in any of the preceding claims wherein said system further comprises an electronically controlled discharge valve (44) allowing to disable said low resistance discharge path (22,32).