Hydraulic Actuators Equipped With Cushioning Means

1. Field of the Invention

This invention relates to a hydraulic actuator, and more specifically to a hydraulic actuator suitable for reciprocating rotation or reciprocating straight line movement of an implement, such as a back-hoe attachment, provided on an earth-moving machine or construction machine.

2. Description of the Prior Art

Usually, implements such as a back-hoe attachment provided on an earth-moving or construction machine are operated by a reciprocably straight-moving or rotating hydraulic actuator. In order to stop such an implement smoothly at its stroke end or at any desired position in its stroke, it is desired to reduce impact by properly absorbing the inertial energy caused by its motion.

With a view to realizing this desire, hydraulic actuators equipped with a cushioning means were proposed. The cushioning means of the known hydraulic actuator comprises a cushioning port provided in each of two pressure oil chambers of the actuator, a line connecting each cushioning port to a reservior, and a relief valve disposed within each line. When the implement in such a conventional actuator approaches its stroke end and therefore, an output means of the actuator, such as a piston, approaches its stroke end, a main port in a pressure oil chamber on the discharge side (that is, the pressure oil chamber whose volume has descreased as a result of the movement of the output means) is closed to increase the pressure of the chamber on the discharge side. When this pressure increases beyond a prescribed pressure of a relief valve located in the line connecting this chamber to the reservior, the relief valve is opened, whereby the pressure oil present in this pressure oil chamber passes through the cushioning port and returns to the reservior through the relief valve. Upon the return of the pressure oil to the reservior, the pressure of the pressure oil chamber on the discharge side decreases below the prescribed pressure of the relief valve, this relief valve is again closed. When a control valve is switched over to its neutral position in order to stop the implement at a desired position in its stroke, and therefore, to stop the output means at a desired position in its stroke, the main ports of the two pressure oil chambers are closed. Thus, in the same manner as in the above example, the pressure of the pressure oil chamber on the discharge side increases to return the pressure oil therein to the reservoir through the cushioning port and the relief valve.

The cushioning means described above reduces impact by absorbing the inertial energy of the implement through the flow restricting action of the relief valve during the passing of pressure oil through the relief valve. However, this conventional cushioning means cannot perform a fully satisfactory cushioning. In other words, the conventional cushioning means cannot stop the implement smoothly as desired by fully absorbing the inertial energy of the implement, that is, the inertial energy of the output means. This is because the opening pressure and the closing pressure of a relief valve which restrict the flow of the pressure oil and absorbs the inertial energy are substantially the same, and equal the prescribed pressure of the relieve valve. Accordingly, the valve is closed at substantially the same pressure as the opening pressure, and even after the closing of the relief valve, the inertial energy remains and causes impact to the implement, and thus to the output means. Inertial energy can be fully absorbed at a substantially low pressure setting, but in turn, induced pressure oil would be more likely to escape from the oil chamber, resulting in deterioration of working efficiency. It is not wise therefore to determine the pressure of the relief valve at a value lower than a reted one.

There was also proposed a hydraulic actuator in which a cushioning port of a pressure oil chamber is connected to a reservior by as first line provided with a first relief valve and also to a control valve by a second line provided with a second relief valve and an orifice placed alongside the relief valve. When the implement (therefore, its output means) approaches its stroke end, a main port of a pressure oil chamber at the discharge end of the actuator is closed, and a pressure oil in the pressure oil chamber is returned through the second line and the control valve. When the flow of the pressure oil passing through the second line is restricted by the above second relief valve and the orifice, the inertial energy of the implement is absorbed. Since the pressure oil flows through the orifice in this hydraulic actuator even after the closing of the second relief valve, the inertial energy of the implement is absorbed almost satisfactorily at its stroke end. However, when the control valve is brought to its neutral position in order to stop the implement at a desired position in its stroke, the second relief valve and the orifice are in the closed state, and therefore, the pressure oil in the pressure oil chamber on the discharge side is returned to the reservoir through the first line. The flow of the pressure oil passing through the first line is restricted by the first relief valve in the same way in the case of the above-mentioned actuator thereby to absorb the inertial energy of the implement. Since the flow of the pressure oil passing through the first line is restricted only by the first relief valve whose closing and opening pressure are substantially equal to each other, this hydraulic actuator can neither absorb the inertial energy of the implement sufficiently when stopping the implement at a desired position in its stroke. When an orifice is provided alongside the first relief in the first line, the pressure oil also escapes when being introduced into the pressure oil chamber, thereby to cause pressure loss. It is evident therefore that an orifice cannot be provided in the first line. Furthermore, this hydraulic actuator requires two relief valves for each pressure oil chamber, and thus disadvantageously has a relatively complicated structure.

Accordingly, it is an object of this invention to provide a hydraulic actuator equipped with a cushioning means that can perform a cushioning action of satisfactorily absorbing the inertial energy of an output means not only when stopping the output means at its stroke end but also when stopping it at a desired position in its stroke.

Another object of this invention is to provide a hydraulic actuator equipped with a relatively compact cushioning means having the above function.

According to this invention, there is provided a hydraulic actuator including a main body, an output means reciprocably disposed within said main body and first and second chambers defined by said output means within said main body, said first chamber having a first main port which is closed when said output means approaches one of its stroke ends and a first cushioning port provided so that it remains open after said first main port has been closed, said second chamber having a second main port which is closed when said output means approaches its other stroke end and a second cushioning port provided so that it remains open after said second main port has been closed, and said first and second main port being connected to a pressure oil source and an reservoir through a control valve; characterized in that said first cushioning port and said second cushioning port are connected to each other through a cushioning means, and said cushioning means comprises a first cracking valve which opens when the pressure on the side of said first cushioning port exceeds a first predetermined pressure and is closed when it becomes lower than said first predetermined pressure, a first restricting means which restricts the flow of a pressure oil from said first cushioning port to said second cushioning port while said first cracking valve is open, a second cracking valve which opens when the pressure on the side of said second cushioning port exceeds said first predetermined pressure and is closed when it becomes lower than said second predetermined pressure, and a second restricting means which restricts the flow of the pressure oil from said second cushioning port to said first cushioning port while said second cracking valve is open.

The basic principle of this invention can be applied both to the swing type and reciprocating type. The term "reciprocating movement", used in the present application, therefore, denotes not only a mere straight-line reciprocating movement but also a rotating reciprocating movement. Furthermore, the term "cracking valve", as used in the present application, means a valve whose closing pressure is lower than its opening pressure.

Preferably, each of the first and second restricting means is made up of a relief valve whose prescribed pressure is lower than the opening pressure of the cracking valve, or an orifice, or a combination of a relief valve whose pressure setting is lower than the opening of the cracking valve and higher than the closing pressure of the cracking valve.

Other objects and advantages of this invention will become apparent from the following description taken in conjunction with the accompanying drawings which illustrate preferred embodiments of the present invention.

FIG. 1 illustrates a simplified hydraulic circuit of a reciprocatingly rotating hydraulic actuator of this invention equipped with a cushioning means;

FIG. 2 is a sectional view showing the details of the cushioning means of the hydraulic actuator shown in FIG. 1;

FIG. 3 illustrates a simplified hydraulic circuit of a reciprocatingly straight-moving hydraulic actuator of this invention equipped with a cushioning means;

FIG. 4 is a sectional view showing a modified example of cushioning means;

FIG. 5 is a sectional view showing another modified example of cushioning means; and

FIG. 6 is a sectional view of still another example of cushioning means.

A reciprocatingly rotating hydraulic actuator constructed in accordance with this invention will be described by referring to FIG. 1.

A reciprocatingly rotating hydraulic actuator 2 includes a main body 4, a shaft (i.e., an output means) 6 disposed rottatbly within the main body 4, and two pressure oil chambers 10 and 12 defined within the main body by a vane 8 of the shaft 6. The shaft 6 is connected, for example, to an implement such as a back-hoe attachment (not shown), and drives it so that it rotates reciprocatingly. The chamber 10 includes a main port 14 and a cushioning port 16, and the chamber 12 includes a main port 18 and a cushioning port 20. The main port 14 of the chamber 10 is so positioned that it is closed by the vane 8 when the shaft 6 rotates clockwise in FIG. 1 to approach one of its stroke ends. The main port 18 of the chamber 12 is so positioned that it is closed when the shaft 6 rotates counterclockwise in FIG. 1 to approach its other stroke end. Furthermore, the cushioning port 16 and the cushioning port 20 are so positioned that they remain open after the main ports have been closed by the vane 8; in other words, they are disposed at positions in proximity to the stroke ends of the shafts 6.

The main port 14 of the chamber 10 and the main port 18 of the chamber 12 are connected to a direction control valve 26 of the 4-port 3-position spring center type, for example, by lines 22 and 24 including orifices 28 and 30 for lowering the pressure acting on the actuator to proper values. The dirction control valve 26 is connected to a pressure oil source (that is, a pump and a reservoir) by the lines equipped with a safety valve (not shown). The cushioning port 16 of the chamber 10 and the cushioning port 20 of the chamber 12 are connected to the lines 22 and 24 respectively through check valves 32 and 34 respectively.

Furthermore, the cushioning port 16 and the cushioning port 18 are connected to each other by lines 36 and 38 through a cushioning means generally shown at 40 and forming the principal part of the present invention.

Now, the cushioning means 40 will be described in detail with reference to FIG. 2 taken together with FIG. 1.

The cushioning means 40 shown in FIGS. 1 and 2 is made up of a first cushioning section 42a equipped with a cracking valve 44a and a relief valve 46a and a second cushioning section 42b equipped with a cracking valve 44b and a relief valve 46b. The cracking valve 44a consists of a valve seat 48a having two concentric annular projections 50a and 52a, a valve member 56a having a shoulder 58a disposed in a bore 54a, and a compression spring 60a provided between one end wall of the bore and the shoulder 58a of the valve member 56a and urging the valve member 56a against the annular projections 50a and 52a of the valve seat 48a. The valve seat 48a has been formed therein an opening 62a which communicates with a line 38. The diameter of the opening 62a is somewhat smaller than that of the annular projection 50a. The inside of the bore 54 communicates with a relief valve 46a through a pressure oil passageway 63a. The valve member 56a of the cracking valve 44a is moved in the right direction in FIG. 1 when the pressure inside the line 38 exceeds the rated value, and the force that acts on an area within the annular projection 50a surpasses the force of the spring 60a. This results in the opening of the cracking valve 44a. The valve member 56a which has been moved in the right direction is maintained in the open state even after the pressure inside the line 38 has decreased to below the rated value. This open state lasts until the amount of the flowing oil is reduced markedly by the action of the oil that flows from the opening 62a of the seat 48a to the pressure oil passageway 63a through the bore 54a. In other words, once the cracking valve 44a has been opened by the pressure above the delivery pressure, it remains open until the pressure becomes considerably lower than the rated value, preferably until it approaches 0. The relief valve 46a which constitutes a restricting means for the flow of oil consists of a bore 64a one end of which communicates with the pressure oil passageway 63a and the other end of which communicates with a line 36, a valve member 66a having a shoulder 68a disposed with the bore 64a, and a compression spring 70a provided between one end wall of the bore 64a and the shoulder 68a of the valve member 66a and urging the valve member 66a in the left direction in FIG. 2. Preferably, an orifice 72a is provided in the valve member 66a. When the pressure inside the pressure oil passageway 63a exceeds a rated pressure, the valve member 66a is moved in the right direction in FIG. 2 in resistance to the force of the spring 70a, and opens. When the pressure inside the pressure oil passageway 63a decreases to below the rated pressure, the valve member 66a is returned to the position shown in FIG. 2 by the action of the spring 70a, and is thus closed. Since in the embodiment shown in FIG. 2, the orifice 72a is provided in the valve 66a, the pressure oil passageway 63a communicates with the line 36 through the orifice 72a even when the valve member 66a is closed at the position shown in FIG. 2 so that the pressure oil can flow from the passageway 63a to the line 36. For the reason to be given later on, the rated pressure of the relief valve 46a is preferably lower than the opening pressure of the cracking valve and higher than its closing pressure. Because the cracking valve 44a is disposed upstream of the relief valve 46a, the rated pressure of the relief valve 46a can be maintained at a very low value without causing a loss in pressure. Accordingly, the provision of the orifice 71a may be omitted.

The second cushioning section 42b is the same as the first cushioning section 42a except that its constituent elements are arranged in the reverse direction with respect to the lines 36 and 38. Therefore, it will not be described in detail in this specification.

Now, the operation of the hydraulic actuator illustrated in FIGS. 1 and 2 will be described below. When in the state shown in FIG. 1, the control valve 26 is moved in the direction shown by a solid-line arrow to connect the chamber 10 to a pump and the chamber 12 to a reservoir, the pressure oil is fed from the main port 14 and the cushioning port 16 to the chamber 10, and the pressure oil in the chamber 12 is returned to the reservoir from the main port 18. This causes the rotation of the shaft 6 in a direction shown by a solid-line arrow. When the shaft 6 rotates and approaches one end of its strokes, the vane 8 of the shaft 6 closes the main port 18 of the chamber 12 on the discharge side. Even after the main port 18 of the chamber 12 has been closed by the vane 8, the shaft 6 remains rotating in the direction shown by a solid-line arrow by the inertial energy accompanying the shaft 6. As a result, the pressure inside the chamber 12, and therefore, the pressure inside the line 38 which communicates with the chamber 12 through the cushioning port 20, increase. When these pressures increase to an extext that surpasses the force of the spring 60a of the cracking valve 44a, the valve member 56a of the cracking valve is moved in the right direction in FIG. 2 to open the cracking valve 44a. When the cracking valve 44a is opened, the pressure oil within the chamber 12 flows from the line 38 to the pressure oil passageway 63a through the bore 54a of the cracking valve 44a, and passes through the orifice 72a. At the same time, the valve member 66a of the relief valve 46a is moved in the right direction in FIG. 2 to open the relief valve 46a (note that the rated pressure of the relief valve 46a is lower than the opening pressure of the cracking valve 44a). Thus, the pressure oil within the chamber 12 reaches the line 36 through the relief valve 46a and an orifice formed in a main body of the valve, and flows into the chamber 10 from the cushioning port 16. When the pressure oil flows through the relief valve 46a and the orifice 72a, the flow of the oil is restricted by the relief valve 46a and the orifice 72a thereby to absorb inertial energy accompanying the shaft 6. The cracking valve 44a remains open even when the pressure inside the chamber 12 becomes lower than the opening pressure of the cracking valve 44a by the flowing of the pressure oil from the chamber 12 to the chamber 10. When the pressure inside the chamber 12 is reduced to the rated pressure of the relief valve 46a (which is lower than the opening pressure of the cracking valve 44a and higher than its closing pressure), the valve member 66a is returned to the position shown in FIG. 2 by the action of the spring 70g to close the relief valve 46a. But the pressure oil within the chamber 12 flows into the chamber 10 through the orifice 72a formed in the valve member 66a, and therefore by the flow restricting action of the orifice 72a, the inertial energy accompanying the shaft 6 is continuously absorbed. When the inertial energy of the shaft 6 has been fully absorbed, and the amount of the pressure oil flowing from the chamber 12 to the chamber 10 through the cracking valve 44a becomes very small, the valve member 56a of the cracking valve 44a is returned to the position shown in FIG. 2 by the action of the spring 60a, and the cracking valve 44a is closed. Since at this time, almost all of the inertial energy accompanying the shaft 6 has been absorbed, the shaft 6 is stopped very smoothly at its stroke end.

When the control valve 26 is moved in the direction shown by a dotted-line arrow in order to rotate clockwise the shaft 6 which has been stopped in its stroke end in the counterclockwise direction, the pressure oil from the pump flows into the chamber 12 from the cushioning port through the line 24 and a check valve 34 (at the stroke end, the main port 18 is closed by the vane 8). At the same time, the pressure oil in the chamber 10 is returned to the reservoir from the main port 14 through the line 22. This results in the rotation of the shaft 6 in the direction shown by a dotted-line arrow. When the shaft 6 has rotated to some extent in the direction shown by a dotted-line arrow, the main port 18 is opened, and the pressure oil from the pump flows into the chamber 12 chiefly from the main port 18. When the control valve 26 is set at its neutral position (the position illustrated in FIG. 1) in order to stop the shaft 6 during its stroke, for example, at the time of the shaft 6 being situated at the position shown in FIGS. 1 and 2, the main port 14 of the chamber 10 and the main port 18 of the chamber 12 are closed. Even then, the shaft 6 remains rotating in the direction shown by a dotted-line arrow by the inertial energy accompanying the shaft 6. As a result, the pressure inside the chamber 10, and therefore, the pressure of the line 36 increase. When these pressures increase to the extent of surpassing the force of the spring 60b of the cracking valve 44b, the valve member 56b of the cracking valve 44b is moved in the left direction in FIG. 2 to open the cracking valve 44b. When the cracking valve 44b is opened, the pressure oil within the chamber 10 reaches the pressure oil passageway 63b from the line 38 through the bore 54b of the valve 44b, and flows through the orifice 72b. At the same time, the valve member 66b of the relief valve 46b is moved in the right direction in FIG. 2 to open the relief valve 46b (note that the rated pressure of the relief valve 46b is lower than the opening pressure of the cracking valve 44b). Thus, the pressure oil in the chamber 10 passes through the relief valve 46b and the orifice formed in the valve member and reaches the line 36. Then, the pressure oil flows from the cushioning port 20 into the chamber 12. When the pressure oil flows through the relief valve 46b and the orifice 72b, the flow of the pressure oil is restricted by the relief valve 46b and the orifice 72b, whereby the inertial energy accompanying the shaft 6 is absorbed. Even when the pressure inside the chamber 10 is reduced below the opening pressure of the cracking valve 44b as a result of the flowing of the pressure oil from the chamber 10 to the chamber 12, the cracking valve 44b remains open. When the pressure inside the chamber 10 decreases to the predetermined pressure of the relief valve 46b (which is lower than the opening pressure of the cracking valve 44b and higher than its closing pressure), the valve member 66b of the relief valve 46b is returned to the position shown in FIG. 2 by the action of the spring 70b to close the relief valve 46b. However, the pressure oil within the chamber 10 flows into the chamber 12 through the orifice 72b formed in the valve member 66b, and therefore, by the flow restricting action of the orifice 72b, the inertial energy accompanying the shaft 6 continues to be absorbed. When the inertial energy of the shaft 6 has been fully absorbed and the amount of the pressure oil flowing from the chamber 10 into the chamber 12 through the cracking valve 44b is very much reduced, the valve member 56b of the cracking valve 44b is returned to the position shown in FIG. 2 by the action of the spring 60b to close the cracking valve 44b. Since at this time, the inertial energy of the shaft 6 has been absorbed almost completely, the shaft 6 is stopped at the desired position in its stroke very smoothly.

Even when orifice 72b is not provided in the valve members 66a and 66b of the relief valves 46a and 46b, the pressures of the relief valves 46a and 46b can be predetermined at a very low level without causing a loss in pressure. Therefore, the inertial energy accompanying the output means can be absorbed to a considerable extent, and the stopping of the output means can be effected more smoothly than in the conventional device.

A reciprocatingly straight-moving hydraulic actuator constructed in accordance with the present invention will be described below by referrence to FIG. 3.

A reciprocatingly straight-moving hydraulic actuator 74 shown in FIG. 3 includes a main body 76, a piston (output means) 78 disposed reicprocably within the main body 76, and two pressure oil chambers 80 and 82 defined by the piston within the main body 76. The piston 78 is connected to an implement (not shown) to be operated, and reciprocatingly moves it. When it is necessary to rotate the implement reciprocatingly, the piston is connected to the implement through a suitable connecting mechanism to move the implement reciprocatingly. The chamber 80 includes a main port 84 and a cushioning port 86, and the chamber 82 includes a main port 88 and a cushioning port 90. To the ends of the piston 78 are secured members 92 and 90 which serve to close the main port of the chamber on the discharge side when the piston 78 approaches its stroke end. The main port 84 of the chamber 80 and the main port 88 of the chamber 82 are connected to a pressure oil source (that is, a pump) and a reservoir through a control valve 100, for example, a control valve of the 4-port 3-position spring center type, by means of lines 96 and 98, respectively. Orifices 102 and 104 are provided respectively in the lines 96 and 98 so as to reduce the pressure acting on the actuator to a suitable level. The cushioning port 86 of the chamber 80 is connected to the cushioning port 86 of the chamber 82 by means of lines 106 and 108 through a cushioning means generally shown at 40'. The cushioning means 40' is of the same structure as the cushioning means 40 illustrated in FIGS. 1 and 2, and its details will not be made in this specification. The constituent elements of the cushioning means 40' are shown by the same numerals as used to indicate the elements of the cushioning means 40 shown in FIGS. 1 and 3 but with primes attached thereto.

The operation of the reciprocatingly straight-moving hydraulic actuator illustrated in FIG. 3 is substantially the same as that of the reciprocatingly rotating hydraulic actuator shown in FIGS. 1 and 2.

A modified embodiment of the cushioning means is illustrated in FIG. 4. In the cushioning means 240 shown in FIG. 4 the first cushioning section and the second cushioning section are combined into a unitary structure to make it compact. The cushioning means 240 includes a cylindrical bore 254 having valve seats 248a and 248 b provided at its ends. The valve seats 248a and 248 b have two concentric annular projections 250a and 252a, and 250b and 252b, respectively at their inner surfaces, respectively, in the same way as shown in FIG. 2. Furthermore, the valve seats 248a and 248b have formed at their central parts openings 262a and 262b respectively which communicate with the lines 36 and 38. In the bore 254, a pair of opposing cracking valve members 256a and 256b are disposed. These cracking valve members 256a and 256b are urged by a compression spring 260 in the mutually departing directions, and are pushed against the valve seats 248a and 248b. The cracking valve member 256a has two bores 264a and 265a therein, which are connected to each other by an opening 267a (a smaller-diameter portion). The bore 264a is connected to an opening 262a of the valve seat 248a and a line 38 by an opening 269a provided at the bottom of the cracking valve member 256a. The bore 265a is connected to the bore 254 by means of an opening 271a provided at the top of the cracking valve member 256a. Within the bore 264a, a relief valve 266a urged in the right direction in FIG. 4 by means of a compression spring 270 is disposed. Preferably, the relief valve member 266a has an orifice 272a extending therethrough. A check ball 275a urged in the right direction in FIG. 4 by means of a compression spring 273a and closing the opening 271a is disposed within the bore 265a. In the same way as the cracking valve member 56a illustrated in FIG. 2, the cracking valve member 256a is moved in the right direction of FIG. 2 in resistance to the action of the spring 260 when the pressure inside the line 38 exceeds a rated value, and is thus opened. Once it has been moved in the right direction, it remains open until the pressure inside the line 38 decreases to a value considerably lower than the above rated pressure, preferably to a value near zero. The rated pressure of the relief valve member 266a is adjusted to a value lower than the opening pressure of the cracking valve member and higher than its closing pressure. The check ball 275a is moved in the left direction in FIG. 4 by the application of the slight pressure. Since the cracking valve member 256b is the same as the cracking valve member 256a except that it is reversed, the description of the details of this cracking valve member 256a will be omitted.

The operation of the cushioning means 240 will be apparent from the above description, but will be briefly described below.

When the pressure of the line 38 increases beyond a predetermined value, the cracking valve member 256a is moved in the right direction against the spring 260, and the pressure oil in the line 38 flows into the bore 254. The pressure oil that has flowed into the bore 254 moves the check ball 275b in the right direction to open the opening 271b, and moves the relief valve 266b in the right direction to open it. Thus, it flows into the line 36 through the bore 264b and the orifice 272b. Even when the pressure in the line 38 decreases to below the above rated pressure, that is the opening pressure of the cracking valve, the cracking valve member 256a remains open. When the pressure inside the line 38 decreases to below the rated pressure of the relief valve member 266b, the relief valve member 266b is returned to the position shown in FIG. 4. However, the pressure oil from the line 38 continues to flow into the line 36 through the orifice 272b. When the pressure inside the line 38 further decreases to the closing pressure of the cracking valve, the cracking valve member 256a is returned to the position shown in FIG. 4, and so is the check ball 275b. When the pressure of the line 36 exceeds the predetermined value, the flow of the pressure oil is reversed. It it obvious that the inertial energy accompanying the output means is absorbed by the oil flow restricting action of the relief valve and the orifice.

FIG. 5 illustrates a cushioning means 340 obtained by somewhat modifying the cushioning means 40 illustrated in FIG. 2. Same as in the case of the cushioning means 40 shown in FIG. 2, the cushioning means 340 shown in FIG. 5 consists of a first cushioning section 342a and a second cushioning section 342b. The first cushioning section 342a has a cylindrical bore 354a having a valve seat 348a disposed at its one end. The valve seat 348a, same as the valve seat 48a shown in FIG. 2, includes two concentric annular projections 350a and 352a and an opening 362a. At the central part of the bore 354a, an annular member 355a having an outside diameter substantially corresponding to the inside diameter of the bore 354a is provided. This annular member 355a has formed therein a bore 359a of a stepwise structure caused by a shoulder 357a. Between the valve seat 348a and the annular member 355a, a cracking valve member 356 a urged against the valve seat 348a by the compression spring 360a is provided. Within the bore 359a of the annular member 355a, a member 361a whose one end is urged against the shoulder 357a by the compression spring 370a is disposed. The member 361a includes a plurality of orifices 363a which are closed when its one end is urged against the shoulder 357a by the compression spring 370a, but are open when the member 361a is moved in the right direction in FIG. 5, and a plurality of normally open orifices 365a. The other end of the bore 354a opposite to the end at which the valve east 348a is disposed communicates with the line 36.

The member 361a is moved in the right direction in FIG. 5 when the pressure acting on the left side in FIG. 5 becomes lower than the opening pressure of the cracking valve member 356a and higher than the closing pressure of the cracking valve member 356a. When this pressure becomes lower than the above rated pressure, the member 361a is returned to the state shown in FIG. 5. The second cushioning section 342b is the same as the first cushioning section 342a except that it is reversed.

The operation of the cushioning means 340 shown in FIG. 5 will be briefly described below.

When the pressure inside the line 38 becomes higher than the opening pressure of the cracking valve, the cracking valve member 356a moves in the right direction against the spring 360a, and opens. Thus, the pressure oil within the line 38 flows into the bore 354a, and causes the member 361a to move in the right direction. Thus, the pressure oil that has flowed into the bore 354a flows into the line 36 through the orifices 363a and 365a. When the pressure on the side of the line 38 becomes below the rated pressure of the member 361a, the member 361a is returned to the position shown in FIG. 5. Since the orifices 365a are open even when the member 361a has returned to the position shown in FIG. 5, the pressure oil flows into the line 36 from the line 38 through the orifices 365a. When the pressure on the side of the line 38 further decreases and becomes lower than the closing pressure of the cracking valve, the cracking valve member 356a is returned to the position shown in FIG. 5. When the pressure on the side of the line 36 increases, the second cushioning section 342 acts in the same way. Needless to say, the inertial energy accompanying the output means is abosrbed by the oil flow restricting action of the orifices 363a and the orifices 365a.

A cushioning means 440 shown in FIG. 6 results from the omission of the relief valve members 266a and 266b from the cushioning means 240 shown in FIG. 4. The cushioning means 440 differs from the cushioning means 240 shown in FIG. 4 in that the diameters of the openings at the bottom of the cracking valve members 456a and 456b are reduced to form orifices 466a and 466b in addition to the omission of the relief valve members 456a and 456b.

In the cushioning means 440, the inertial energy accompanying the output means is absorbed by the oil flow restricting action of the orifices 466a and 466b.

While the present invention has been described with reference to some preferred embodiments, the invention is not limited to these specific structures, but any changes and modifications are possible so long as they do not depart from the spirit and scope of the present invention.