The invention also relates to a hydraulic system of a working machine including a hydraulic cylinder, and a damping device including a first accumulator, having a first preloading pressure Pi, and a second accumulator, having a second preloading pressure P2, wherein Pi<P2, said accumulators being arranged in hydraulic connection with the hydraulic cylinder.

The invention is particularly applicable to a working machine in the form of a load-handling vehicle, such as a loading machine, where the hydraulic cylinder for example is arranged for manoeuvring a load-handling crane or another type of lifting device of the loading machine.

One example of such a loading machine is a crane-equipped forwarder for transporting timber. In the case of a static load, i. e. a constant or time-independent load, on the lifting device, the hydraulic pressure in the hydraulic cylinder is directly proportional to said load.

However, if a swinging of the lifting device occurs, for example due to a sudden change of load or when the loading machine moves a load across uneven ground, the hydraulic pressure will fluctuate or oscillate around a pressure level corresponding to the static load. Such pressure oscillations are difficult to avoid and, since

the pressure oscillations are trying on both the loading machine and the operator of the loading machine, it is desirable to dampen them rapidly and efficiently.

Normally, a loading machine is arranged for handling loads within a large load interval and with different overhangs of the lifting device, i. e. with different working levers which are formed by different projecting working positions of the lifting device, and the pressure of the hydraulic system will fluctuate accordingly within a large pressure interval. Pressure oscillations can occur within the entire pressure interval, and the damping device should preferably be capable of damping pressure oscillations at different pressure levels within the pressure interval. Furthermore, the pressure oscillations can include two or several main frequencies which occur simultaneously. For instance, the entire loading machine can be swinging or rocking with a first frequency and amplitude, at the same time as the loading beam is rocking with a second frequency and amplitude.

US 2,638, 932 discloses a pressure equalizer for smoothing out pressure pulsations within a wide pressure interval in a line for conducting a fluid. The pressure equalizer consists of several blow accumulators being connected to the line. The accumulators have different, individual preloading pressures adapted to the pressure levels where the pressure pulsations are expected to occur. In this way, the accumulators together cover the entire pressure interval.

Accordingly, the blow accumulators according to US 2,638, 932 have a pressure-equalizing function. In a load-handling vehicle, however, it is desirable that the damping device also has a function of damping

oscillations. It is difficult, however, to achieve an efficient damping of oscillations, since the amplitude as well as the frequency of the pressure oscillations can fluctuate strongly depending on the vehicle load and the overhang of the lifting device. Thereby, the amplitude of oscillation, as a rule, is proportional to the load, i. e. a large load causes large pressure oscillations and a small load small pressure oscillations, and the frequency of the oscillation depends on the overhang, i. e. a small overhang, as a rule, causes pressure oscillations having a high frequency and a large overhang causes pressure oscillations having a low frequency. Accordingly, the damping effect should be larger with a large load and a large overhang than with a small load and a small overhang. In load-handling vehicles, one problem is to achieve a damping of oscillations which functions in an optimal way independently of load and overhang and which, in addition, functions without jeopardizing the manoeuvre accuracy of the lifting device of the vehicle.

The object of the invention is to achieve a damping device of the type mentioned by way of introduction, which at least substantially reduces the above-mentioned problems.

The invention is characterized in that the first accumulator is connected to the hydraulic cylinder via a first throttle valve, which has a flow opening having a flow area A1, and that the second accumulator is connected to the hydraulic cylinder via a second throttle valve, which has a flow opening having a flow area A2, wherein A1>A2.

In the following, the invention will be described more closely with reference to the drawings.

Figure 1 shows a circuit diagram of a hydraulic system including a damping device according to a first embodiment of the invention.

Figure 2 shows a circuit diagram of a hydraulic system including a damping device according to a second embodiment of the invention.

Figure 3 shows a damping device according to the invention including a regulating device.

Figure 4 is a sectional view of the damping device, along the line IV-IV in Figure 3.

Figures 5-7 show a further embodiment of a damping device according to the invention, in section and in three different positions.

Figure 1 is a schematic representation of a circuit diagram of a hydraulic system of a working machine in the form of a loading machine (not shown). The hydraulic system includes a hydraulic cylinder 1, being arranged for manoeuvring a lifting device in order to hoist and lower a load. The hydraulic system further includes a directional valve 2, being arranged for manoeuvring the hydraulic cylinder 1 via hydraulic lines 3,4. The hydraulic system also includes a damping device 5, being arranged for damping pressure oscillations in the hydraulic system, which pressure oscillations can arise, for example, when the load is hoisted or lowered in connection with loading work, or when the loading machine moves a load across uneven ground.

The damping device 5 includes a first accumulator 6, having a predetermined, first preloading pressure P1, and a second accumulator 7, having a second predetermined, preloading pressure P2 being higher than the first preloading pressure P1, i. e. Pi<P2. The damping device 5 further includes a first throttle valve 8, which has a first flow opening having a flow area A1. The damping device 5 also includes a second throttle valve 9, which has a second flow opening having a flow area A2. The first throttle valve 8 is arranged for connecting the first accumulator 6 to the hydraulic cylinder 1 via the hydraulic line 4, and a second throttle valve 9 is arranged for connecting the second accumulator 7 to the hydraulic cylinder 1 in the same way.

The damping of the pressure oscillations takes place by means of kinetic energy of the swinging lifting device being transformed into heat, when hydraulic oil passes through the throttle valves 8,9. Thereby, the damping effect is a function of the quantity of hydraulic oil passing through each throttle valve per unit of time, on one hand, i. e. the flow of hydraulic oil through the throttle valve, and the fall of pressure though the throttle valve, on the other hand.

The first accumulator 6 is activated at a first pressure level, corresponding to P1, and the second accumulator 7 is activated at a second pressure level, corresponding to P2. Accordingly, since P1<P2, the second accumulator 7 is activated at a higher pressure level than the first accumulator 6. High pressure levels normally arise in the hydraulic system with large loads and/or large overhangs of the lifting device and, consequently, the second accumulator 7 will be active at such large loads and/or large overhangs. According to the same reasoning, the

first accumulator 6 will be active with small loads and/or small overhangs. According to the invention, the flow area A1 is larger than the flow area A2, i. e. A1>A2, in order to obtain a uniform damping at the different pressure levels. Thereby, a larger damping effect is obtained in the second accumulator 7, i. e. with relatively large loads and/or large overhangs, than in the first accumulator 6, i. e. with relatively small loads and/or small overhangs.

It will be appreciated that the damping range of the hydraulic system can be widened by means of connecting more than two accumulators to the hydraulic system.

Figure 2 is a schematic representation of a circuit diagram of such a hydraulic system of a loading machine, said hydraulic system including a damping device 10 having four accumulators. As the system described previously in connection with Figure 1, the hydraulic system includes a hydraulic cylinder 1 being arranged for manoeuvring a lifting device of the loading machine in order to hoist and lower a load, a directional valve 2 being arranged for manoeuvring the hydraulic cylinder 1 via hydraulic lines 3,4, and said damping device 10. The damping device 10 includes first, second, third and fourth accumulators 6,7, 11,12, being connected to the hydraulic cylinder 1 through first, second, third and fourth throttle valves 8,9, 13,14, respectively, via the hydraulic line 4. The accumulators 6,7, 11,12 have preloading pressures Pi, P2, P3, P4, wherein P1<P2<P3<P4, i. e. the first accumulator 6 has a first, lowest preloading pressure P1, the second accumulator 7 has a second, second lowest preloading pressure P2, the third accumulator 11 has a third, second highest preloading pressure P3 and the fourth accumulator 12 has a fourth, highest preloading pressure P4. According to the

invention, the throttle valves 8,9, 13,14 have flow openings having flow areas Ai, A2, A3, A4, wherein A1>A2>A3>A4, i. e. the first throttle valve 8 has a flow opening having a largest flow area A1, the second throttle valve 9 has a flow opening having a second largest flow area A2, the third throttle valve 13 has a flow opening having a second smallest flow area A3 and the fourth throttle valve 14 has a flow opening having a smallest flow area A4. By means of adapting the flow areas to the preloading pressures, a uniform damping is achieved throughout the entire damping range, since the flow of hydraulic oil out of and into the accumulators per unit of pressure change becomes relatively constant at the different pressure levels, at the same time as the fall of pressure through the flow valves become larger at high pressure levels than at low pressure levels.

Furthermore, as a result of the uniform damping, a very good manoeuvre accuracy of the hydraulic system is achieved.

It will be appreciated that the damping range of the damping device can be widened further by means of connecting more than four accumulators to the hydraulic system. In the general case, in which N accumulators having preloading pressures P1, P2,... PN-i, PN are connected to the hydraulic system via throttle valves having flow areas A1, A2,... AN_1, AN, wherein the preloading pressures of the accumulators are P1<P2<... <PN-1<PN, according to the invention, the flow openings are arranged so that their flow areas are A1>A2>... >AN-i>AN.

Preferably, the damping device is adjustable in order to enable the damping characteristic of the damping device to be varied when necessary. Preferably, the regulation

is such that the flow openings of the throttle valves are adjusted simultaneously, while maintaining the ratio between the flow areas of the flow openings. Figures 3 and 4 show the principle of function of an embodiment of such an adjustable damping device, said damping device intended to be connected to a hydraulic system of a working machine in order to damp pressure oscillations in the hydraulic system. The damping device includes a regulating device in the form of a circular-cylindrical pipe 15 and an adjusting spindle 16 being rotatably arranged in the pipe. The adjusting spindle 16 includes an axial, central channel 17 arranged for being brought into hydraulic connection with a hydraulic cylinder in the hydraulic system. Furthermore, the damping device includes four accumulators 18, having different preloading pressures P1, P2, P3 and P4, respectively, wherein P1<P2<P3<P4, said accumulators 18 being fitted to the pipe 15. The central channel 17 communicates with each accumulator 18 via a radial connecting channel 19 and a groove 20, tapering in a tangential direction and having a progressive depth, of the adjusting spindle 16, and a radial opening 21 in the pipe 15. Accordingly, for each accumulator 18, the connecting channel 19, the groove 20 and the opening 21 constitute a throttle valve which has a flow opening having a flow area A1, A2, A3 and A4, respectively. Due to the progressive depth of the grooves 20, the flow areas can be caused to increase or decrease simultaneously by means of rotating the adjusting spindle 16 in relation to the pipe 15.

Accordingly, the adjusting spindle 16 can be rotated between a first end position where the throttle valves have their smallest flow areas, and a second end position where the throttle valves have their maximum flow areas.

The angle of rotation of the adjusting spindle 16 can be, for example, a function of the frequency of the pressure

oscillations, wherein a low frequency preferably results in smaller flow areas than a high frequency. Thereby, a further improved manoeuvring ability of the hydraulic system is achieved. However, the connecting channels 19, the grooves 20 and the openings 21 should be designed so that the basic relationship between the flow openings, i. e. Al>A2>A3>A4, is maintained in the different regulating positions.

Alternatively, the damping device exhibits a predetermined, non-adjustable damping characteristic, wherein the flow openings have fixed flow areas. When after-mounting a non-adjustable damping device in a working machine, the damping characteristic of the damping device preferably is adapted to the working machine in question. In order to clarify the need of damping, the pressure oscillations in the hydraulic system of the working machine are recorded when it is operated within a normal speed interval and with loads within a normal load interval. The recorded pressure oscillations are then analysed in order to find the frequencies and amplitudes at which the need of damping is largest. Thereafter, the volumes of the accumulators and the flow areas of the throttle valves are dimensioned, while taking the found frequencies and amplitudes and the weight of the working machine into consideration. Preferably, this dimensioning is done by means of a computer program in a computer unit.

Figures 5-7 show an embodiment of a damping device having a predetermined damping characteristic, said damping device further including function-checking members. Like the damping device described previously in connection with Figure 3 and 4, the damping device includes a circular-cylindrical pipe 22 and an adjusting spindle 23

being rotatably arranged in the pipe. The adjusting spindle 23 includes an axial, central channel 24 arranged for being brought into hydraulic connection with a hydraulic cylinder in the hydraulic system. The damping device further includes two accumulators 25 having different preloading pressures P1 and P2, respectively, wherein P1<P2, said accumulators 25 being fitted to the pipe 22. Alternatively, the damping device can include more than two accumulators, for example four of them.

In the following, the function of the damping device will be described in greater detail with reference to the left-hand accumulator 25 in Figures 5-7. The damping device can be adjusted into an active position, as shown in Figure 5. In the active position, where the damping device is during normal operation, the central channel 24 communicates with the accumulator 25 via a radial connecting channel 26 of the adjusting spindle 23 and a radial opening 27 in the pipe 22. Accordingly, for the left-hand accumulator 25, the connecting channel 26 and the opening 27 constitute a throttle valve having a fixed flow opening with a predetermined, constant flow area. In the same way, a connecting channel and an opening constitute a throttle valve having a fixed flow opening and a predetermined and constant flow area for the right-hand accumulator, where the flow area, Ai and A2, respectively, of each throttle valve stands in an inverse proportion to the preloading pressure, P1 and P2, respectively, i. e. A, >A2 for preloading pressures P1<P2.

Figure 5 illustrates this by means of the left-hand accumulator having a larger flow area than the right-hand accumulator and, consequently, the left-hand accumulator has a lower preloading pressure than the right-hand accumulator.

As mentioned above, the damping device includes function-checking members. These members are used for checking whether the accumulators are unbroken, something which is important when the damping device is used for increasing the breakdown safety and if this increased safety already has been taken into consideration in the strength calculation of the working machine. The function-checking members have the form of a control channel, corresponding to each accumulator and extending beside the connecting channel of the accumulator in a radial direction from the central channel 24 through the adjusting spindle 23.

In the following, a control sequence where the left-hand accumulator 25 is checked by means of its control channel 28 will be described. In the active position, the connecting channel 26 is open and the control channel 28 is blocked, as is evident from Figure 5. Before a control sequence, the hydraulic cylinder connected to the damping device is unloaded, whereupon the damping device, by means of a clockwise rotary motion of the adjusting spindle 23, is brought from its active position into an inactive or closed position, shown in Figure 6, where the connecting channel 26 as well as the control channel 28 are blocked. Accordingly, in the closed position, no exchange of hydraulic oil between the accumulators and the remaining hydraulic system is allowed. When the damping device is in the closed position, the hydraulic cylinder is loaded again, whereupon the damping device, by means of a counter-clockwise rotary motion of the adjusting spindle 23, is brought from the closed position into a control position, shown in Figure 7, where only the control channel 28 is open. If the left-hand accumulator 25 is operative, an exchange of hydraulic oil between the left-hand accumulator 25 and the remaining

hydraulic system will be allowed, in the control position, via the control channel 28 of the left-hand accumulator, at the same time as the hydraulic cylinder is compressed under its load. Accordingly, a compression of the hydraulic cylinder in this position indicates an accumulator which is in working order. The right-hand accumulator of the damping device is checked in the same way.

In the foregoing, the invention has been described in connection with a working machine in the form of a loading machine. As mentioned previously, such a loading machine can be, for example, a crane-equipped forwarder for transporting timber. It will be appreciated, however, that the principle of the invention is equally applicable to other types of working machines including hydraulic systems in which pressure oscillations can occur.