HYDRAULIC DRIVE SYSTEM

Abstract: Hydraulic drive system for one or more hydraulic motors, comprising a pump (1), a tank (25) for hydraulic fluid, one or more control valve sections (10, 11, 12) each being provided with a shunt passage, a first shunt conduit (4) including the shunt passages of said control valve sections (10, 11, 12) connected in series for leading off to the tank (25) the temporary surplus flow from the pump (1), and a feed conduit (3) for connecting the control valve sections (10, 11, 12) in parallel to the pump (1), and a second shunt conduit (5) is connected between the pump (1) and the tank (25) and comprising a pressure responsive valve (6) which is arranged to operate in response to the pressure in said first shunt conduit (4) downstream of a first non-variable restriction (13) on one hand and in response to the pressure in said second shunt conduit (5) downstream of a second non-variable restriction (14) on the other hand to establish a direct connection through said second shunt conduit (5) from the pump (1) to the tank (5) in relation to the size of the fluid flow in said first shunt conduit (4).

Description: This invention relates to a hydraulic drive system comprising control valves of the type which on one hand are intended for determining the direction of flow to and from the pressure fluid consuming load objects, and which on the other hand are intended to adapt the size of the fluid flow to the objects by shunting off a surplus or no load flow to the hydraulic tank through a shunt passage. In case of a load which generates pressure fluid this shunting is obtained by a restriction control. The control valve type mentioned above is intended to be supplied with fluid from a pump having a constant displacement and working at a given speed level. You can see two modes of operation, namely one mode in which no fluid flow is used for effective work, and another mode in which a certain working flow is used. It is desirable to be able to control large hydraulic fluid flows (effects) by a given size of valve with moderate losses only. The mode of operation in which no work flow is delivered to the connected load objects means that the entire pump flow passes unrestricted through the shunt passage of the valves or through the so called free flow passage back to the tank. A conventional arrangement of valves comprises one or more valve sections located in parallel such that the free through passage or the shunt conduit is formed by the shunt passages of the valve sections connected in series. The restriction of the flow in the shunt conduit is obtained by cam portions on the valve slides. As the valve slides are put in their neutral positions, the area of the shunt conduit is the largest possible, but in spite of that the direction changes of the flow create considerable pressure drops which become greater the larger pump flow is forced therethrough. As one or more of the valve slides are displaced, a restriction of the surplus or no-load flow through the shunt conduit is obtained. A flow which corresponds to the decreasing shunt flow is forced into a parallel passage, the so called feed passage, which is connected to the respective load object. As long as none of the valve slides is displaced, the product of the pump flow and the pressure drop in the shunt conduit represent the no-load losses in the control valves, losses which increase in relation to the increase of the pump size. If the no-load interval is a major part of the work cycle, one realizes that it is disadvantageous to supply the valves from too big a pump or, the other way around, to choose too small valves. A previous attempt to solve this problem is disclosed in FR-A- 2 338 404. This known device includes one ore more control valve sections, all having a first shunt conduit. The device also comprises a flow dividing means which is located between the pump and the tank and comprises a pressure responsive shunt valve located in a second shunt conduit and a non-variable restriction located upstream of the control valve sections to establish a direct connection through the second shunt conduit from the pump to the tank in relation to the size of the fluid flow in the first shunt conduit. However, the flow dividing means shown in this prior device is rather complicated in structure as it requires an additional pressure limiting valve to obtain a desirable operation of the shunt valve. Through the invention there is accomplished a hydraulic drive system of the above related type having an improved yet simple flow dividing means for leading off directly to the tank temporary surplus flow from the pump. Further characteristics of the invention will appear from the following description and drawings as well as the claims. On the drawings: Fig 1 shows an operation diagram for a hydraulic drive system according to the invention including three control valve sections. Fig 2 shows diagrammatically the distribution of flow through the drive system according to the invention. Fig 3 shows a section through a flow dividing valve means comprised in the drive system in Fig 1. As appear from Fig 1, the hydraulic pump 1 delivers a flow q p to the point 2. Thereafter, the flow has three alternative ways to go, namely through the feed passage 3, the main shunt conduit 4, and the secondary shunt conduit 5. In the latter the hydraulic fluid passes the flow divider 6 and further to the tank 25. From the feed passage 3 the fluid passes through the check valves 7, 8 and 9 to the control valve sections 10, 11 and 12. The flow divider 6 comprises a two-way, two-position pressure responsive valve with spring return to closed position. See Fig 3. The activation means 16 of the valve 6 is exposed to the pressure in the main shunt conduit 4 downstream of the restriction 13, and the activation means 15 responses to the pressure in the secondary shunt conduit 5 downstream of the restriction 14. The latter pressure is related to the pump pressure as well as to the flow q D . Since this pressure initially is the same as the pressure in the point 2, (which is the reference point for the pressure balance on the valve 6), the valve 6 will open as the pressure on the activation means 16 plus the force of the spring 18 are not high enough to overcome the force generated by the activation means 15. Accordingly, the opening degree of the valve 6 is determined by the pressure drop across the restriction 14 which will increase until a balance is obtained in relation to the momentarily increasing flow q D . The relationship between the flows q F and q D is determined by the restrictions 13 and 14 as well as the spring 18. The drive system also comprises a pressure reducing valve 26 located between the pump 1 and the tank 25. In Fig 2 there is shown a flow diagram for the cases in which one or more of the control valve sections are activated. If they are not, q M = 0, whereby the entire pump flow is divided into one part-flow q F through the main shunt conduit 4 and a part-flow q D through the flow divider 6 and further to the tank. The flow q D is determined indirectly by the flow q F , i.e. by the activation condition at the control valve sections 10, 11 and 12. When these are unactivated, the flow q F passes through the main shunt conduit 4 under a minimum of resistance. The total pressure drop depends on the restriction in the main shunt conduit 4 as well as on the restriction 13 which is physicically incorporated in flow divider 6. Suitably the relationship q F/ q D is chosen as a compromise between on one hand the level of no-load losses and on the other hand how much the flow related forces can be reduced in a relative sense within the main shunt conduit at maintained acceptable control characteristics. The distribution of the different part-flows as a function of the valve slide displacement is illustrated in Fig 2. At a slide displacement S=So, the entire pump flow q p is divided into q D and q F . When S=S1, the pump flow is distributed such that q MI is directed through the feed passage 3 to the load object and the parts q DI and q FI to the tank 25. At a slide displacement S=S2, the pressure drops across the restrictions 13 and 14 are small enough to let the force balance on the valve slide of the valve 6 be dominated by the spring 18, which means than q D --> 0. The remaining displacement of the valve slide S max - S2 is then controlled by the restriction in the main shunt conduit 4 only until the entire flow goes to the activated load object. In Fig 3 there is shown how a flow dividing valve 6 may be designed practically. The right hand activation means 16 of the valve 6 is formed as a chamber which is integrated in the main shunt conduit 4 and which lodges the spring 18 for loading the valve slide 26 toward its left position, in which position the secondary shunt conduit 5 is completely blocked. The left activation means 15 is formed as a chamber 2 too and is incorporated in the secondary shunt conduit 5. The example on a flow dividing valve 6 shown in Fig 3 is constructively very simple and offers an acceptable controlability. By separating the shunt conduits 4 and 5 from the activation means 15 and 16 and instead connect the latters through separate sensing passages, the controlability may be improved to an optimum and the influence of the flow related forces may be eliminated. Such an arrangement will be somewhat more complicated but is still comprised within the scope of the invention. Suitably the flow divider is integrated in the common inlet part of the valve sections. The invention does not only include the above suggested embodiments but may be freely varied within the scope of the claims.

Claims: 1. Hydraulic drive system for one or more hydraulic motors, comprising a pump (1), a tank (25) for hydraulic fluid, one or more control valve sections (10, 11, 12) each provided with a shunt passage, a first shunt conduit (4) comprising the shunt passages of said control valve sections (10, 11, 12) connected in series for leading off to the tank (25) the temporary surplus flow from the pump (1), a feed conduit (3) for connecting in parallel the control valve sections (10, 11, 12) to the pump (1), flow dividing means (5,6) is located between the pump (1) and the tank (25) and comprising a second shunt conduit (5), a pressure responsive valve (6) in said second shunt conduit (5), a first non-variable restriction (13) located in said first shunt conduit (4) upstream of said control valve sections (10, 11, 12), to establish a direct connection through said second shunt conduit (5) from the pump (1) to the tank (25) in relation to the size of the fluid flow in said first shunt conduit (4), characterized By a) a second non-variable restriction (14) located in said second shunt conduit (5) upstreams of said pressure responsive valve (6). b) said pressure responsive valve (6) being arranged to operate in response to on one hand the pressure in said second shunt conduit (5) downstream of said second restriction (14) and on the other hand in response to the pressure in said first shunt conduit (4) downstream of said first restriction (13). 2. Drive system according to claim 1, wherein said valve (6) is acted upon in its closing direction by the pressure in said first shunt conduit (4) downstream of said first restriction (13) as well as by a spring (18).

Inventors: PERSSON, BENGT-GOERAN, WENNERBO, OERJAN EGON VALDEMAR, Persson, Bengt-Göran, Wennerbo, Örjan Egon Valdemar
Applicants: ATLAS COPCO AKTIEBOLAG
Application year: 1988
Main classification: F15B11
IPC codes: F15B11