Patent Grant
7007465
The present invention relates to a mobile handling device with a hydraulic circuit, which hydraulic circuit comprises a lifting cylinder arranged in a lifting device intended for the handling of a variable load and an accumulator for recovering or recycling the lowering load energy, the hydraulic circuit also comprising a variable hydraulic machine with two ports, said hydraulic machine being able to give full system pressure by a driving device in two flow directions to said ports, wherein one of the ports is connected to said accumulator and the other port is connected to said lifting cylinder.
Excavators, trucks, container handlers etc. and a large number of other mobile handling machines which are intended to handle a variable load have one or more lifting cylinders for lifting the load for which the unit is designed. The great majority of mobile handling devices used today have no energy recovery facility whatever for the lowering load, meaning that the lowering load energy, most often in connection with passage via a control valve which determines the lifting and lowering motion, is converted to heat which then has to be cooled away. The heating of the hydraulic oil to undesirable temperatures is a long familiar problem for machinery manufacturers and end customers.
For several years, a plurality of inventors have been working on recovering the energy losses which arise in a lifting system without any ballast weight balancing away the weight of the arm system. For different reasons they have not managed to obtain a commercially useful solution, as all the time there exist weaknesses implying unacceptable results. Below, different reasons for said problems are mentioned.
A device with an auxiliary cylinder, which is more or less directly connected to one or more accumulators, creates difficulties, as the arm system to a large extent influences the hydraulic pressure in the lifting cylinder depending on the working radius used. The system must thus balance towards the lowest pressure which may exist at a short working radius, which is a problem.
In those solutions where attempts have been made with the lifting cylinder in a closed circuit together with a hydraulic machine, wherein oil is pumped to and from an accumulator the problem is to compensate the leakage losses, which are unavoidable in all rotating hydraulic machines. When the hydraulic oil has run short in the accumulator, which happens simultaneously, the need of a power peak will immediately arise, which results in difficult problems, which to a great extent lessens the value of such a solution.
An object of the invention is to eliminate or at least minimize the above mentioned drawbacks, which object is achieved by a mobile handling device according to the characteristic part of patent claim 1.
The invention provides many advantages and i.a. a considerable reduction of the engine power. The invention has so far been tested in an excavator of the size 20 tons but is also applicable on practically all lifting devices.
At least the following important advantages are achieved with the invention:
The invention will be described below more in detail in connection with the enclosed drawings, in which:
The hydraulic machine 3 is a variable reciprocating pump which can both receive and emit oil at the ports 10, 11. The pump is of a known type which permits full system pressure at both outlet ports and in which the flow can be adjusted from zero to maximum by means of the variable setting, which is normally achieved by means of a so-called swash plate. Using a pump of this kind eliminates the need to regulate the circuit via a control valve, whereby a considerable simplification is achieved at the same time as control losses are reduced.
Furthermore, a sequential valve 7 is included in the hydraulic circuit. The sequential valve 7 is disposed in a line 1-6, which connects the lifting cylinder 1 to the accumulator 6, by means of which it is possible to relieve any excess pressure in the line 1-2 between the lifting cylinder and the logic element 2 via the sequential valve 7 to the accumulator 6, so that the energy is retained in the system.
A safety valve 8 is provided in the system between the accumulator 6 and a tank 42, which ensures that a certain maximum pressure for the circuit is not exceeded. A pressure-reducing valve 23 is disposed between the accumulator 6 and the logic element 5. The pressure-reducing valve ensures that the accumulator pressure does not exceed the maximum value permitted for the accumulator type, meaning that the accumulator does not necessarily need to be of the same pressure class as the rest of the system.
Furthermore, it is shown that the hydraulic circuit is connected to the handling device's conventional hydraulic pump 12, the flow of which is regulated in a conventional manner via a control valve 13. Due to this oil can be routed via one of the ports 14 on the control valve 13 to the opposite side 1 A of the double-acting cylinder 1. Furthermore, oil can be supplied via the control valve 13 via a second port 15 to the piston side 1 B of the lifting cylinder 1. In the line 15-1, disposed between the control valve 13 and the piston side 1 B of the lifting cylinder 1 is a non-return valve 16 which prevents oil being routed from the piston side 1 B of the lifting cylinder to the control valve 13. The hydraulic pump 12 collects its oil in the normal manner from the tank 42. The control valve 13 is normally connected by one end 13-42 to the tank 42, while its other end 13-12 is connected to the hydraulic pump 12. Furthermore, the system has a sequential valve 19 which can return surplus oil from the lifting circuit 1. 3, 6 to the control valve 13, where it can be used for example to manoeuvre the stick on an excavator. Finally, it is shown that the system can include an additional accumulator 21, which can either be disposed to be connected or not connected to the circuit via a valve 22. This extra accumulator 21 can be used either to ensure that sufficient hydraulic oil is to be found in connection with certain working operations and/or to provide the circuit with a different pressure level in connection with certain working operations.
A pressure-sensing element 17 is provided to register the pressure in the line between the lifting cylinder 1 and the logic element 2. In the event of a lowering motion which requires power, the pressure-sensing element 17 will register that the pressure is below that required for the function and ensure that the control valve 13 emits oil to the rod side of the lifting cylinder via the port 14.
The system functions such that in the event of a lifting motion, the operator will send a control signal to the control servo (not shown), which will activate the valve 9 which in turn ensures that the valves 2 and 5 open. The connection between the accumulator 6, hydraulic machine 3 and lifting cylinder 1 is thus completely open. The pressurized oil in the accumulator 6 flows then to the variable hydraulic machine 3, which conveys the oil onwards to the lifting cylinder 1. If the pressure in the accumulator in this case is higher than that required to carry out the work using the lifting cylinder 1, the surplus energy will be supplied by the hydraulic machine 3 to the drive system, best achieved via the transmission T. If the accumulator pressure should not be quite sufficient, the variable hydraulic machine 3 provides a pressure increase to reach the requisite pressure level, which is achieved by means of power which is supplied via the handling machine's motor D. Thus in such a situation only as much energy is supplied as is required to overcome the pressure difference between the accumulator and the lifting cylinder's requirement. In the event of a lowering movement, the direction of flow in the pump is changed and oil is supplied at port 10 and emitted at port 11 to be supplied to the accumulator 6. If the pressure in the accumulator 6 is then lower than at the lifting cylinder 1, the variable hydraulic machine 3 will be able to supply energy to the transmission T. If on the other hand the pressure in the accumulator is higher than in the lifting cylinder, additional energy from the motor D will need to be supplied to the variable hydraulic machine 3 to obtain a lowering movement. However, this energy supplied is stored in the accumulator 6 and is therefore accessible in connection with the next lifting movement. It is evident from the above that the system is energy-saving and eliminates heat-generating throttling of the oil flow which normally occurs when the lowering energy is handled in conventional systems.
The task of the pressure-sensing element 17 is to ensure that the hydraulic machine 3 adjusts the flow down to zero when the hydraulic cylinder no longer has any pressure, for example when the bucket has reached ground level.
In the case of a lifting motion which it is desired to be performed quickly, a normal requirement for example in deep cut digging, both the variable hydraulic machine 3 and the hydraulic pump 12 can be activated, in which case the oil obtained from the accumulator does not fully correspond to the amount of oil of the lifting cylinder. During a lowering movement, the non-return valve 16 will prevent the oil from flowing to port 15. On the next lowering movement, therefore, an amount corresponding to that obtained from the pump 12 must be evacuated from the circuit via the safety valve 8. Alternatively, the sequential valve 19 can be used to return the surplus oil to the inlet side of the control valve 13, to be used for example for the slewing motion on an excavator. Oil for the rod side of the double-acting lifting cylinder 1 can be obtained via a so-called refill valve 18, in the form of a non-return valve, which is disposed between the outlet side of the control valve and the line 14-1 which leads to the rod side of the lifting cylinder 1.
When lowering the lifting cylinder, the major portion of the oil will be pumped to the accumulator system 6, but when the arm system suddenly is relieved, when the bucket for instance hits the ground a pressure sensor 73 in the lifting circuit must emit a signal to the computer to justify the pumping capacity downwards. During the transient time of the hydraulic machine, it must be supplied with oil in order not to be destroyed (not to seize), and this amount is obtained from the refilling circuit, which consists of the accumulator 20, the non-return valve 31 and the pressure reducer 59, which receives its oil from the open circuit of the machine.
The hydraulic machine chosen in the system has like all rotating pumps a volumetric loss, which at full flow and pressure may be expected to amount to 5% but at low flows it may be close on 100%, and said loss of liquid must inevitably be replaced. It is important to realize that said loss is practically independent of the deflection of the hydraulic machines or its flow. At a lowering motion, the amount of oil which is delivered by the lifting cylinder will thus not be found in the accumulator but a portion thereof will run to the tank 42 via the leakage line of the hydraulic machine. Except said leakage, consideration must also be taken to the amount which is drained via the valve 62. It must be possible to control the lowering motion of a machine with great accuracy, and the hydraulic machine 3 does then not give sufficient control. For this reason, there is a valve 62 in the lowering circuit, which allows complete control. A lowering motion will take place only via the valve 62 if small motions or great accuracy are required.
The hydraulic machine 3 has a size which allows full lifting speed, but it will be considerably more expensive to give the hydraulic machine a size which also manages full lowering speed, which is approximately 50% higher, i.e. which should require a flow which is approximately 50% higher. Further, this would imply a considerably extension of the line areas etc. The valve 62 thus has two functions, partly to allow complete control at low lowering speeds, partly to increase the maximal lowering speed at high lowering speeds. Or in other words, the valve 62 allows that a hydraulic machine having considerably lower capacity than what is shown in
In order to solve the problem which arises in connection with the filling of the accumulator 6 with oil to ensure next lifting motion the following details have been added. The lifting piston 1 has been provided with a position sensor 90 giving a signal to the computer, which also receives a signal from the accumulator system 6 by a sensor 91. Then the computer 94 calculates the need and emits a signal to the pump 71, which attends to desired/sufficient pressure being established, which in turn determines the amount in the accumulator. Said refilling of the accumulator is thus performed independent of a lowering motion or lifting motion being made or other functions being utilized. If the maximal pumping capacity for the lifting motion is, say 100, the capacity of the pump 71 only has to be a fraction thereof.
The reason is that said refilling of the accumulator takes place during the entire operation period of the machine. Let's assume that the lifting cylinder needs 35 l. To perform a full stroke, there must be a sufficient amount plus an amount for the volumetric loss in the accumulator, now assuming that this amount is 5 l. At the preceding lowering motion an amount of 35 l less the volumetric loss less the amount which was drained by the valve 2, was obtained, which can be assumed to be 10 l. The pumping capacity is calculated to perform a lifting motion of 6 sec, which implies a need of 350 l/min. A complete digging operation can be assumed to take minimum 20 sec. and the capacity of the pump 71 must then be 15 l/20 sec or 45 l/min.
In order to be able to perform a complete lifting cycle at full speed, a power of 350×250/600=145.8 kW×the efficiency is needed. The pressure is according to experience the mean value which is used in this assumption. If the mean pressure in the accumulator is assumed to be 175 bars, the following energy is required according to the invention, 350×75/600=43.7 kW plus 45×175/600=13.1 kW, thus totally 56.8 kW×the efficiency, and the power need has thus been reduced by approximately 60% in the lifting motion. In order to improve the efficiency of the system further, the capacity of the pump 71 is increased, so that the loading of the accumulator can be performed during the 14 sec when no lifting motion is going on.
In order to determine the usefulness of the invention the following should be considered.
A great advantage according to the invention thus depends on the system with a separate valve 62 for the control of the lowering speed, which implies complete control and that the same valve can be used to obtain full lowering speed. Through the inevitable volumetric losses which are the case in a pressurized hydraulic system, the lowering motion will require that the hydraulic machine gets an increase signal when low lowering speeds are required. In addition, when the hydraulic machine is not pressurized, the lowering speed will be load dependent, which is not acceptable from an operating point of view. When low lowering speeds are desired, the computer does not emit any signal to the hydraulic machine 3 or to the valves 2 and 6 but only to the valves 7 and 62. In this way, an exactly controlled motion with immediate response is obtained. In this connection it should be pointed out that the adjusting times of such a hydraulic machine 3 normally are felt too long. When a higher lowering speed is desired, the computer emits a signal to the valves 2 and 5 to open while the hydraulic machine 3 is moved outwards. When a complete movement outwards of the hydraulic machine 3 has been achieved, the computer emits a signal to the valve 62 to increase the flow to a desired level. The maximal flow via the valve is 50% of the pumping capacity. The overflow valve 63 is provided in order to pressurize the hydraulic machine 3 before the valves 2 and 6 open. This implies that a “dip” in the lowering operation is avoided. The non-return valve 51 is provided so that no “dip” may occur at the lifting operation. The non-return valves 65 and 31 do not prevent desirable flows.
In the computerized control system 94 an optimal power output function is included, which is based on the fact that when no power is taken out, the engine speed will lie at overspeed for the given output position. According to experience, the engine is completely loaded when the motor speed has fallen by x %. When the engine has a loading degree which is less than a given value, for instance 80%, a signal is emitted by the computer to the pump 71 to increase the pressure level by a suitable percentage in the accumulator system 6 towards the minimal level which is required to ensure the lifting requirement. Said superposed power will additionally make a power reduction possible at the subsequent lifting operation. In the computerized program for the pressure increase in the accumulator circuit 6 an adaptive function has also been included, which will imply that the system is adapted to the pressure with which the accumulator is loaded to the position which the lifting cylinder has taken at an optional number of previous lowering operations. The accumulator system is designed and calculated to allow accommodation within the system of the amount of oil available in the lifting cylinder. The operation field of an excavator is calculated and designed to cover a considerably larger field than which the machine normally is used for. Normally, no more than 60–70% of the stroke length of the lifting cylinder is utilized but in the calculation of the size of the accumulator, the maximal amount of oil which can be received by the accumulators must be taken into consideration. In order not to obtain extremely large and expensive accumulators, the gas pressure must be lowered towards the ideal level so that the end pressure will not be too high when the lifting cylinder stands in its bottom position. The adapted function sees to it that an increase of pressure occurs, when the system has received information that only a limited portion of the stroke length of the lifting cylinder has been utilized. The overflow valve 22 ensures that no higher pressure than the permitted one occurs, when a way of driving arises, which was not present previously.
According to further aspects of the invention, the following features are also valid:
The invention is not restricted to the above description but can be varied within the scope of the following patent claims. It is perceived for example that the servo pressure can be obtained from a source in the system other than the pump 4, e.g. from the accumulator 20. It is furthermore perceived that one is not limited in any way to using just one lifting cylinder but that also two or more lifting cylinders can be used in a circuit according to the invention. The same is naturally true also of the number of accumulators, which can be varied as desired or needed. It is also perceived that a number of modifications can be made with regard to the valve arrangements without it affecting the principles of the invention. Furthermore, it is perceived that multiples of the constituent elements can be used, for example a plurality of lifting cylinders. Furthermore, it is perceived that the invention can also be used in similar handling machines other than those previously named, for example forestry machines, so-called croppers etc.
The invention can also be utilized in connection with the use of a control valve via which the hydraulic oil is routed to and from the accumulator or lifting cylinder. Here it holds good that the potential energy which is in the lifting piston will in the event of a lowering movement be returned to the accumulator via the control valve, which accumulator in turn is connected to the variable reciprocating pump. A precondition however is that the accumulator pressure is below the lifting cylinder pressure and that before a state of equilibrium arises a separate return line to the tank is opened. In a lifting movement the pressurized oil in the accumulator will provide the pressure increase or pressure drop in the reciprocating pump necessary for the requirement to execute the desired work. If for example the lifting work calls for 200 bar and the accumulator pressure is 100 bar, the stored energy has executed half the lifting work. It is preferably the case that the control valve is supplied with hydraulic medium from the lifting pistons via the regular pump inlet and that the control valve is provided with pressure compensation which on activation of the valve emits a pressure-compensated flow to the engine port.
To modify the invention for fork lift trucks, which are characterized by a form of working in which it was not possible using the previous technology to recover the lowering load energy, the following applies. The normal cycle for a fork lift truck is to lift or lower a load, it not being possible to determine the sequence for these operations, but rather the task controlling the course of events. Due to the design of the lifting cylinder, as much oil is used to lift the forks empty as with a full load, only the pressure varies. The hydraulic system for a fork lift truck with energy recovery should therefore be completed by a valve which in the event of a low lowering load automatically opens a valve which is connected to the tank when Δp between the cylinder pressure and accumulator falls below a predetermined value. In this regard a valve actuated by the operator is naturally conceivable.
| Number | Date | Country | Kind |
|---|---|---|---|
| 9904796 | Dec 1999 | SE | national |
This application is a division of U.S. patent application Ser. No. 10/168,302, filed Jun. 20, 2002 Now U.S. Pat. No. 6,804,957 which is a 371 of PCT/SE00/02360, filed Nov. 29, 2000.
| Number | Name | Date | Kind |
|---|---|---|---|
| 4188787 | Bromell et al. | Feb 1980 | A |
| 4928487 | Nikolaus | May 1990 | A |
| Number | Date | Country |
|---|---|---|
| 197 21 759 | Nov 1998 | DE |
| 455 864 | Aug 1988 | SE |
| WO 9613669 | May 1996 | WO |
| WO 0000748 | Jan 2000 | WO |
| Number | Date | Country | |
|---|---|---|---|
| 20050016169 A1 | Jan 2005 | US |
| Number | Date | Country | |
|---|---|---|---|
| Parent | 10168302 | US | |
| Child | 10926292 | US |
