Patent Description:
Conventionally, a technique of a lift arm lifting and lowering mechanism provided to a working vehicle such as a tractor has been known. An example is disclosed in the document <CIT>.

The document <CIT> discloses a tractor including a rotary tiller lifting and lowering hydraulic cylinder structure for lifting and lowering a rotary tiller for cultivating a field. The hydraulic cylinder structure is provided with a hydraulic cylinder connected to an oil tank via an oil passage, and a piston head slidably disposed inside the hydraulic cylinder and forming a hydraulic chamber. The hydraulic cylinder structure is configured to lift and lower the rotary tiller via a lift arm provided to be rotatable upward and downward in conjunction with the sliding of the piston head.

The hydraulic cylinder structure as disclosed in the document <CIT> is configured to rotate the lift arm upward by sliding the piston head by hydraulic pressure of oil fed to the hydraulic chamber via the oil passage. The hydraulic cylinder structure is also configured to rotate the lift arm downward by discharging the oil in the hydraulic chamber via the oil passage.

When the lift arm is maintained at a position of being rotated upward in the hydraulic cylinder structure mentioned above, the discharging of the oil in the hydraulic chamber via the oil passage is restricted. In the state where the discharging of the oil in the hydraulic chamber is restricted as described above, when the temperature of the oil inside the hydraulic chamber rises, the volume of the oil may increase and thus the hydraulic pressure may rise. In this case, the hydraulic cylinder structure fails to suppress an excessive rise in the hydraulic pressure inside the hydraulic chamber. Thus, a further improvement is desired.

The document <CIT> relates to a relief valve for allowing oil to flow out due to a rise in the pressure inside an oil chamber which is attached to the piston of a lift arm driving single acting cylinder provided in a gear transmission case. The valve is structured so as to directly send the oil from the valve to the interior of the case.

The document <CIT> relates to a hydraulic device for elevating and lowering a working machine mounted on a tractor or the like. A spherical receiving portion formed by fitting a piston slidably into a liner liner and formed on the piston (in which a spherical projection of a piston rod is freely connected to and separated from the above-mentioned one). A hydraulic piston/rod connecting device comprises a protective cover made of an elastic material attached to a spherical protrusion, and a spherical bone is received through the protective cover.

Preferred embodiments of the present invention have been made in view of the above-described circumstances, and an object of preferred embodiments of the present invention is to provide a lift arm lifting and lowering mechanism that can suppress an excessive rise in hydraulic pressure inside a hydraulic chamber.

The problem to be solved by preferred embodiments of the present invention is as described above, and the following will describe a solution to this problem.

The problem is solved by the lift arm lifting and lowering mechanism according to the independent claim <NUM>. Embodiments are defined in the dependent claims of the appended set of claims.

A lift arm lifting and lowering mechanism includes: a cylinder case provided with a cylinder portion into which oil is fed; a piston slidably disposed inside the cylinder portion to form a hydraulic chamber and receiving hydraulic pressure of the hydraulic chamber on a first surface of the piston; a safety valve provided to the piston and configured to discharge the oil inside the hydraulic chamber when the hydraulic pressure of the hydraulic chamber becomes greater than or equal to a predetermined value; and a lift arm supported by the cylinder case and rotatable in conjunction with movement of the piston, in which the piston includes an oil passage allowing communication between a second surface of the piston and the first surface of the piston, and the oil passage includes: a containing portion provided to open on the first surface side and capable of containing the safety valve; and a non-containing portion provided to open on the second surface side relative to the containing portion and incapable of containing the safety valve. The piston includes a recess provided to open in the first surface and the containing portion is provided outside the recess, or a thickening portion is provided to protrude from a bottom of the recess and the containing portion is provided at the thickening portion.

In addition, the safety valve may include: a valve seat portion fixed inside the containing portion and having a through hole through which the oil is allowed to flow; a valve disc portion disposed to be movable inside the containing portion and capable of closing the through hole by contacting the valve seat portion from the second surface side; and a biasing portion biasing the valve disc portion toward the first surface side.

In addition, the containing portion may include: a valve seat containing portion containing the valve seat portion; and a valve disc containing portion formed to be continuous with the valve seat containing portion and to have a shape with a diameter smaller than a diameter of the valve seat containing portion, and containing the valve disc portion.

In addition, the through hole may include a valve disc receiving portion provided at an end portion of the valve seat portion on the second surface side and having a circular truncated cone shape that increases in diameter toward the second surface side, and the valve disc portion may include: a spherical portion having a spherical shape capable of closing the valve disc receiving portion; and a retaining portion engaging with the biasing portion and retaining the spherical portion.

In addition, the through hole may include a valve disc receiving portion provided at an end portion of the valve seat portion on the second surface side and having a circular truncated cone shape that increases in diameter toward the second surface side, and the valve disc portion may include: an engagement portion engaging with the biasing portion; and a protruding portion integrally provided to the engagement portion and having a conical shape that decreases in diameter toward the first surface side to be capable of closing the valve disc receiving portion.

In addition, the valve disc portion may include a side surface opposed to an inner surface of the containing portion, and the side surface may have a groove portion provided over an entire portion in a moving direction.

In addition, the lift arm lifting and lowering mechanism further may include a piston rod configured to transmit sliding movement of the piston to the lift arm, in which the piston includes a rod retaining portion provided to open in the second surface and receiving the piston rod, the rod retaining portion includes: a circular truncated cone portion continuous with an edge portion of the opening of the rod retaining portion and having a circular truncated cone shape extending to decrease in diameter toward the first surface side; a columnar portion continuous with the circular truncated cone portion and having a columnar shape extending in an axial direction of the piston; and a bottom portion continuous with the columnar portion and forming a bottom of the rod retaining portion.

In addition, the columnar portion and the containing portion may be provided side by side so as to at least partially overlap each other in the axial direction.

In the lift arm lifting and lowering mechanism according to the present invention, it is possible to suppress an excessive rise in hydraulic pressure inside the hydraulic chamber. Furthermore according to the present invention it is possible to suppress exposure of the safety valve from the recess. Thus, the safety valve can be easily retained with respect to the piston.

Various preferred embodiments of the present invention achieve the following effects.

In the lift arm lifting and lowering mechanism according to a preferred embodiment of the present invention, it is possible to simplify the structure of the safety valve.

In the lift arm lifting and lowering mechanism according to a preferred embodiment of the present invention, it is possible to stabilize the valve disc portion when the valve disc portion receives pressure.

In the lift arm lifting and lowering mechanism according to a preferred embodiment of the present invention, it is possible to make the structure of the safety valve relatively simple.

In the lift arm lifting and lowering mechanism according to a preferred embodiment of the present invention, it is possible to reduce the number of components of the safety valve.

In the lift arm lifting and lowering mechanism according to a preferred embodiment of the present invention, it is possible to provide a flow path for discharging oil.

In the lift arm lifting and lowering mechanism according to a preferred embodiment of the present invention, the rod retaining portion can suitably receive the piston rod.

In the lift arm lifting and lowering mechanism according to a preferred embodiment of the present invention, it is possible to easily obtain a certain thickness between the containing portion and the rod retaining portion in the piston.

The embodiments shown in <FIG> and described with reference to such Figures below are not encompassed by the wording of the claims but are considered as useful for understanding the invention.

First, the overall configuration of a tractor <NUM> provided with a lift arm lifting and lowering mechanism <NUM> according to a first embodiment will be described with reference to <FIG>.

The tractor <NUM> mainly includes a body frame <NUM>, front wheels <NUM>, rear wheels <NUM>, an engine <NUM>, a hood <NUM>, a transmission case <NUM>, a cabin <NUM>, a lift arm lifting and lowering mechanism <NUM>, a link mechanism <NUM>, and a rotary tilling device <NUM>.

The body frame <NUM> is disposed such that its longitudinal direction is in the front-rear direction. A front portion of the body frame <NUM> is supported by a pair of left and right front wheels <NUM> via a front axle mechanism (not shown). The transmission case <NUM> is provided at a rear portion of the body frame <NUM>. The transmission case <NUM> contains a transmission device (not shown) and the like. The transmission case <NUM> is supported by a pair of left and right rear wheels <NUM> via a rear axle mechanism (not shown).

The engine <NUM> is provided at a front portion of the body frame <NUM>. The engine <NUM> is covered by the hood <NUM>. Power of the engine <NUM> is subjected to a speed conversion at the transmission device, and is then made transmittable to the front wheels <NUM> via the front axle mechanism and made transmittable to the rear wheels <NUM> via the rear axle mechanism. The front wheels <NUM> and the rear wheels <NUM> are rotationally driven by the power of the engine <NUM>, so that the tractor <NUM> can travel.

The power of the engine <NUM> is also subjected to a speed conversion at the transmission device and is then made transmittable to a PTO shaft. In the present embodiment, the rotary tilling device <NUM> is connected to the PTO shaft. The rotary tilling device <NUM> is disposed at a rear portion of the tractor <NUM> and includes a power transmission case <NUM>, a rotary shaft <NUM>, and a plurality of blades <NUM>. The power transmission case <NUM> transmits the power from the PTO shaft to the rotary shaft <NUM> to rotate the rotary shaft <NUM>. In this manner, the rotary tilling device <NUM> can cultivate a field or the like by rotating the plurality of blades <NUM>.

The rotary tilling device <NUM> is connected to the lift arm lifting and lowering mechanism <NUM>, which will be described later, via the link mechanism <NUM>. In the present embodiment, the link mechanism <NUM> forms a three-point linkage. Note that the link mechanism <NUM> is not limited to the three-point linkage and various manners may be used.

The cabin <NUM> is provided at a middle portion of the tractor <NUM> in the front-rear direction (behind the engine <NUM>). An occupant space 8a in which an operator rides is formed inside the cabin <NUM>. A steering wheel 8b for adjusting the steering angle of the front wheels <NUM>, a seat 8c for the operator to be seated, and the like are disposed in the occupant space 8a. Appropriate operation tools are also disposed in the occupant space 8a.

Next, the configuration of the lift arm lifting and lowering mechanism <NUM> will be described with reference to the principle examples not covered by the present invention of <FIG>.

The lift arm lifting and lowering mechanism <NUM> supports the rotary tilling device <NUM> such that it can be lifted and lowered. The lift arm lifting and lowering mechanism <NUM> is provided at a rear portion of the body frame <NUM>. The lift arm lifting and lowering mechanism <NUM> includes a cylinder case <NUM>, a piston <NUM>, a safety valve <NUM>, a piston rod <NUM>, an interlocked arm <NUM>, and a lift arm <NUM>.

The cylinder case <NUM> shown in the principle example not covered by the present invention of <FIG> and <FIG>, contains the piston <NUM>, the piston rod <NUM>, and the interlocked arm <NUM>, which will be described later. The cylinder case <NUM> is disposed above the transmission case <NUM>. The cylinder case <NUM> has a box shape that opens downward. The interior of the cylinder case <NUM> communicates with the interior of the transmission case <NUM> via the opening. The cylinder case <NUM> includes a cylinder portion <NUM> and an oil feed passage <NUM>.

The cylinder portion <NUM> is a portion into which oil (hydraulic oil) is fed. The cylinder portion <NUM> forms a front-side portion of the cylinder case <NUM>. The cylinder portion <NUM> has a bottomed cylindrical shape. The cylinder portion <NUM> has a bottom portion and a side portion. The cylinder portion <NUM> is disposed such that the bottom surface faces (is inclined) in a diagonally rear-lower direction.

The oil feed passage <NUM> is an oil passage through which oil flows. The oil feed passage <NUM> has an end penetrating the bottom portion of the cylinder portion <NUM> and communicates with the interior of the cylinder portion <NUM>. The other end of the oil feed passage <NUM> communicates with the transmission case <NUM> via a predetermined oil passage. A predetermined oil pump (not shown) is provided to the oil passage. Oil reserved inside the transmission case <NUM> can be fed through the oil feed passage <NUM> via the oil pump. A predetermined switching valve (not shown) for switching the flowing direction of the oil is provided to the oil feed passage <NUM>. The switching valve can switch between a state where the oil can be fed to the cylinder portion <NUM> via the oil pump, a state where the oil can be discharged from the cylinder portion <NUM>, and a state where the oil feed passage <NUM> is closed.

The piston <NUM> shown in the principle examples not covered by the present invention of <FIG>, is slidably disposed inside the cylinder portion <NUM>. The piston <NUM> has a columnar shape corresponding to the interior of the cylinder portion <NUM>. The side surface of the piston <NUM> slides relative to the inner peripheral surface of the cylinder portion <NUM>. The front surface (surface facing in a diagonally front-up direction) of the piston <NUM> is opposed to the bottom surface of the cylinder portion <NUM>. The rear surface (surface facing in a diagonally rear-lower direction) of the piston <NUM> is opposed to the piston rod <NUM>, which will be described later. A hydraulic chamber R enclosing oil fed to the cylinder portion <NUM> is formed by the front surface of the piston <NUM>, the bottom surface of the cylinder portion <NUM>, and the inner peripheral surface of the cylinder portion <NUM>. The piston <NUM> includes a recess <NUM>, a rod retaining portion <NUM>, a recessed groove <NUM>, an O-ring <NUM>, and an oil passage <NUM>.

The recess <NUM> shown in the principle example of a piston not covered by the present invention of <FIG> and <FIG>, is a portion that opens in the front surface of the piston <NUM> and is recessed toward the rear surface side of the piston <NUM> in its axial direction (hereinafter referred to as the "axially rear surface side"). The recess <NUM> has a circular shape as seen in the axial direction. The recess <NUM> is formed over a large portion of the front surface of the piston <NUM>. The recess <NUM> has a shape that is curvedly recessed in its entirety and curvedly protrudes from the bottom at its central portion. Thus, the recess <NUM> is formed such that surrounding portions of the central portion are deeper than the central portion.

The rod retaining portion <NUM> shown in the principle example of a piston not covered by the present invention of <FIG> and <FIG>, is a portion that opens in the rear surface of the piston <NUM> and is recessed toward the front surface side of the piston <NUM> in its axial direction (hereinafter referred to as the "axially front surface side"). The rod retaining portion <NUM> has a circular shape as seen in the axial direction. The rod retaining portion <NUM> is formed over a large portion of the rear surface of the piston <NUM>. The rod retaining portion <NUM> includes a circular truncated cone portion <NUM>, a columnar portion <NUM>, and a bottom portion <NUM>.

The circular truncated cone portion <NUM> is a portion that is continuous with the edge portion of the opening of the rod retaining portion <NUM>. The circular truncated cone portion <NUM> has a circular truncated cone shape that extends to decrease in diameter toward the axially front surface side. That is, the inner peripheral surface of the circular truncated cone portion <NUM> has a shape of an inclined surface (tapered surface) that decreases in diameter toward the axially front surface side. The illustrated example shows an example where the inclination angle of the side surface of the circular truncated cone portion <NUM> with respect to the axial line as seen in a cross-sectional view is substantially <NUM> degrees. In addition, in the illustrated example, the inclination angle of a portion of the side surface close to the edge portion of the opening the circular truncated cone portion <NUM> is smaller than that of other portions (an acute angle). A portion connecting the circular truncated cone portion <NUM> and the edge portion of the opening has a curved-surface shape.

The columnar portion <NUM> is a portion connecting the circular truncated cone portion <NUM> and the bottom portion <NUM>, which will be described later. The columnar portion <NUM> is continuous with the circular truncated cone portion <NUM> on the axially front surface side of the circular truncated cone portion <NUM>. The columnar portion <NUM> has a substantially columnar shape extending in the axial direction. A portion connecting the columnar portion <NUM> and the circular truncated cone portion <NUM> has a curved-surface shape.

The bottom portion <NUM> is a portion that forms the bottom of the rod retaining portion <NUM>. The bottom portion <NUM> is continuous with the columnar portion <NUM> on the axially front surface side of the columnar portion <NUM>. The inner surface of the bottom portion <NUM> has a curved-surface shape. A portion connecting the bottom portion <NUM> and the columnar portion <NUM> has a stepped shape with a diameter smaller than a diameter of the columnar portion <NUM>.

The recessed groove <NUM> shown in the principle examples not covered by the present invention of <FIG>, is a groove provided in the side surface of the piston <NUM>. The recessed groove <NUM> is provided over the entire periphery of an axially central portion of the side surface of the piston <NUM>. The recessed groove <NUM> has a rectangular shape as seen in a cross-sectional view.

The O-ring <NUM> shown in the principle example not covered by the present invention of <FIG>, is an annular member for sealing oil in the hydraulic chamber R. The O-ring <NUM> is fitted into the recessed groove <NUM> and contacts the bottom surface of the recessed groove <NUM> and the inner peripheral surface of the cylinder portion <NUM>.

The oil passage <NUM> shown in the principle examples of a piston and a safety valve not covered by the present invention of <FIG>, allows communication between the front surface and the rear surface of the piston <NUM>. The oil passage <NUM> penetrates the piston <NUM> in the axial direction. The oil passage <NUM> is provided at a radially outer portion of the piston <NUM>. The oil passage <NUM> is provided to at least partially overlap the recess <NUM> and the rod retaining portion <NUM> as seen in the axial direction. The oil passage <NUM> has a circular shape as seen in the axial direction. The oil passage <NUM> includes a containing portion <NUM> and a non-containing portion <NUM>.

The containing portion <NUM> shown in in the principle example of a safety valve not covered by the present invention of <FIG> and <FIG>, is a portion capable of containing the safety valve <NUM>, which will be described later. The containing portion <NUM> forms a portion of the oil passage <NUM> on the axially front surface side. The containing portion <NUM> includes a valve seat containing portion <NUM> and a valve disc containing portion <NUM>.

The valve seat containing portion <NUM> is a portion containing a valve seat portion <NUM> of the safety valve <NUM>, which will be described later. The valve seat containing portion <NUM> includes an increased diameter portion 24a and a decreased diameter portion 24b. The increased diameter portion 24a is a portion of the valve seat containing portion <NUM> on the axially front surface side. The increased diameter portion 24a opens in the front surface of the piston <NUM> to communicate with the recess <NUM>. The decreased diameter portion 24b is a portion of the valve seat containing portion <NUM> on the axially rear surface side. The decreased diameter portion 24b has a shape with a diameter smaller than a diameter of the increased diameter portion 24a. A female screw portion is formed on the inner surface of the decreased diameter portion 24b.

The valve disc containing portion <NUM> is a portion containing a valve disc portion <NUM> and a biasing portion <NUM> of the safety valve <NUM>, which will be described later. The valve disc containing portion <NUM> is continuous with the decreased diameter portion 24b of the valve seat containing portion <NUM> on the axially rear surface side relative to the decreased diameter portion 24b. The valve disc containing portion <NUM> has a shape with a diameter smaller than a diameter of the decreased diameter portion 24b.

The non-containing portion <NUM> is a portion incapable of containing the safety valve <NUM>, which will be described later. The non-containing portion <NUM> includes a decreased diameter portion 26a and an increased diameter portion 26b. The decreased diameter portion 26a is a portion of the non-containing portion <NUM> on the axially front surface side. The decreased diameter portion 26a is continuous with the valve disc containing portion <NUM> on the axially rear surface side relative to the valve disc containing portion <NUM>. The decreased diameter portion 26a has a shape with a diameter smaller than a diameter of the valve disc containing portion <NUM>. The increased diameter portion 26b is a portion of the non-containing portion <NUM> on the axially rear surface side. The increased diameter portion 26b has a shape with a diameter larger than a diameter of the decreased diameter portion 26a. The increased diameter portion 26b opens in the circular truncated cone portion <NUM> of the rod retaining portion <NUM>.

As shown in the principle example of a piston not covered by the present invention of <FIG>, the above-mentioned containing portion <NUM> and the columnar portion <NUM> of the rod retaining portion <NUM> are provided side by side so as to at least partially overlap each other in the axial direction of the piston <NUM>. In the present principle example of a piston not covered by the present invention, a portion of the valve disc containing portion <NUM> on the axially rear surface side and substantially the entire portion of the columnar portion <NUM> are provided side by side so as to overlap each other in the axial direction. In this manner, a certain thickness is obtained between the portion of the valve disc containing portion <NUM> on the axially rear surface side and substantially the entire portion of the columnar portion <NUM>.

The safety valve <NUM> shown in the principle example of a safety valve not covered by the present invention of <FIG>, is capable of switching between opening and closing the oil passage <NUM>. The safety valve <NUM> opens the oil passage <NUM> when the hydraulic pressure of the hydraulic chamber R becomes greater than or equal to a predetermined value. The safety valve <NUM> is contained in the containing portion <NUM>. The safety valve <NUM> includes the valve seat portion <NUM>, the valve disc portion <NUM>, and the biasing portion <NUM>.

The valve seat portion <NUM> shown in in the principle example of a safety valve not covered by the present invention of <FIG> and <FIG>, is a member fixed in the valve seat containing portion <NUM>. The valve seat portion <NUM> includes a collar portion 31a, a body portion 31b, and a through hole <NUM>. The collar portion 31a is a portion of the valve seat portion <NUM> on the axially front surface side. The collar portion 31a is contained in the increased diameter portion 24a of the valve seat containing portion <NUM>. The collar portion 31a has a columnar shape corresponding to the increased diameter portion 24a of the valve seat containing portion <NUM>. The collar portion 31a contacts the bottom surface (front surface) of the increased diameter portion 24a so that the movement of the valve seat portion <NUM> toward the axially rear surface side is restricted.

The body portion 31b is a portion of the valve seat portion <NUM> on the axially rear surface side.

The body portion 31b is contained in the decreased diameter portion 24b of the valve seat containing portion <NUM>. The body portion 31b has a shape with a diameter smaller than a diameter of the collar portion 31a. The body portion 31b has a columnar shape corresponding to the decreased diameter portion 24b of the valve seat containing portion <NUM>. The body portion 31b has a columnar shape corresponding to the decreased diameter portion 24b of the valve seat containing portion <NUM>. A male screw portion corresponding to the female screw portion of the decreased diameter portion 24b of the valve seat containing portion <NUM> is formed on the side surface of the body portion 31b. By fastening the body portion 31b to the decreased diameter portion 24b, the valve seat portion <NUM> is fixed relative to the valve seat containing portion <NUM>.

The through hole <NUM> is a hole penetrating the valve seat portion <NUM> in the axial direction. The through hole <NUM> forms an oil passage through which oil is allowed to flow. The through hole <NUM> has a circular shape as seen in the axial direction. The through hole <NUM> has a shape such that the portion formed in the collar portion 31a has a diameter larger than a diameter of the portion formed in the body portion 31b. The through hole <NUM> includes a valve disc receiving portion 32a.

The valve disc receiving portion 32a is a portion for receiving the valve disc portion <NUM>, which will be described later. The valve disc receiving portion 32a forms an end portion of the through hole <NUM> on the axially rear surface side and opens in the rear surface of the body portion 31b. The valve disc receiving portion 32a has a circular truncated cone shape that increases in diameter toward the axially rear surface side. That is, the inner peripheral surface of the valve disc receiving portion 32a has a shape of an inclined surface (tapered surface) that increases in diameter toward the axially rear surface side. The illustrated example shows an example where the inclination angle of the inner peripheral surface of the valve disc receiving portion 32a with respect to the axial line of the piston <NUM> is substantially <NUM> degrees.

The valve disc portion <NUM> is capable of closing the valve disc receiving portion 32a. The valve disc portion <NUM> is disposed to be slidable inside the valve disc containing portion <NUM>. The valve disc portion <NUM> includes a spherical portion <NUM> and a retaining portion <NUM>.

The spherical portion <NUM> is a member having a spherical shape. A general steel sphere (ball) can be used as the spherical portion <NUM>. The outer diameter dimension of the spherical portion <NUM> is formed to be substantially the same as the inner diameter dimension of the valve disc containing portion <NUM> (slightly smaller than the inner diameter dimension of the valve disc containing portion <NUM>). The spherical portion <NUM> can close the valve disc receiving portion 32a by contacting the inclined surface of the valve disc receiving portion 32a.

The retaining portion <NUM> retains the spherical portion <NUM>. The retaining portion <NUM> is disposed on the axially rear surface side relative to the spherical portion <NUM>. The retaining portion <NUM> includes a body portion <NUM> and an engagement portion <NUM>.

The body portion <NUM> is a portion of the retaining portion <NUM> on the axially front surface side. The body portion <NUM> has a columnar shape. The outer diameter dimension of the body portion <NUM> is formed to be substantially the same as the inner diameter dimension of the valve disc containing portion <NUM> (slightly smaller than the inner diameter dimension of the valve disc containing portion <NUM>). The body portion <NUM> includes a recess 36a.

The recess 36a is a portion that opens in the front surface of the body portion <NUM> and is recessed toward the axially rear surface side. The recess 36a has a conical shape that increases in diameter toward the axially front surface side. That is, the inner peripheral surface of the recess 36a has a shape of an inclined surface (tapered surface) that increases in diameter toward the axially front surface side. The illustrated example shows an example where the inclination angle of the inner peripheral surface of the recess 36a with respect to the axial line of the piston <NUM> is substantially <NUM> degrees. The spherical portion <NUM> contacts the inclined surface of the recess 36a, and thus the spherical portion <NUM> is retained such that its movement in the radial direction is suppressed.

The engagement portion <NUM> is a portion of the retaining portion <NUM> on the axially rear surface side. The engagement portion <NUM> engages with the biasing portion <NUM>, which will be described later. The engagement portion <NUM> is formed to protrude from the rear surface of the body portion <NUM> toward the axially rear surface side.

The biasing portion <NUM> biases the valve disc portion <NUM> toward the axially front surface side.

The biasing portion <NUM> is disposed on the axially rear surface side relative to the valve disc portion <NUM> inside the valve disc containing portion <NUM>. The biasing portion <NUM> is a coil spring capable of extending and contracting in the axial direction. The outer diameter dimension of the biasing portion <NUM> is formed to be substantially the same as the inner diameter dimension of the valve disc containing portion <NUM> (slightly smaller than the inner diameter dimension of the valve disc containing portion <NUM>). An end portion of the biasing portion <NUM> on the axially front surface side is fitted to the engagement portion <NUM> of the retaining portion <NUM>. An end portion of the biasing portion <NUM> on the axially rear surface side contacts the bottom surface (surface facing frontward) of the valve disc containing portion <NUM>.

As shown in in the principle example of a safety valve not covered by the present invention of <FIG>, when the biasing portion <NUM> is extended, the spherical portion <NUM> biased via the retaining portion <NUM> closes the valve disc receiving portion 32a. The biasing portion <NUM> is capable of contracting by receiving the hydraulic pressure inside the hydraulic chamber R via the spherical portion <NUM> and the retaining portion <NUM>. The biasing force of the biasing portion <NUM> is set such that it contracts when the hydraulic pressure inside the hydraulic chamber R becomes greater than or equal to a predetermined value.

The principle example of a safety valve not covered by the present invention of <FIG>, shows a case where the hydraulic pressure inside the hydraulic chamber R is greater than or equal to the predetermined value. In this state, the hydraulic pressure applied to the valve disc portion <NUM> (the spherical portion <NUM>) via the through hole <NUM> is larger than the biasing force of the biasing portion <NUM>. In this manner, the valve disc portion <NUM> moves toward the axially rear surface side against the biasing force of the biasing portion <NUM>, so that the valve disc receiving portion 32a is opened.

The piston rod <NUM> shown in the the principle example not covered by the present invention of <FIG> and <FIG>, transmits the sliding movement of the piston <NUM> to the lift arm <NUM>, which will be described later. The piston rod <NUM> has a columnar shape that is longer in the front-rear direction. The piston rod <NUM> has a distal end portion (front end portion) received by the rod retaining portion <NUM> of the piston <NUM>. The piston rod <NUM> has a front end surface having a curved-surface shape, and the front end surface contacts the bottom portion <NUM> of the rod retaining portion <NUM>. The piston rod <NUM> has a base end portion (rear end portion) rotatably connected to the interlocked arm <NUM>, which will be described later. In the present embodiment, since the circular truncated cone portion <NUM> is provided to the rod retaining portion <NUM> as described above, the distal end portion of the piston rod <NUM> can be easily guided toward the bottom portion <NUM>. In addition, since the columnar portion <NUM> is provided to the rod retaining portion <NUM>, a sufficient depth dimension of the rod retaining portion <NUM> can be obtained, and the distal end portion of the piston rod <NUM> can be easily retained.

The interlocked arm <NUM> connects the piston rod <NUM> and the lift arm <NUM>, which will be described later. A first rotation shaft <NUM> is provided at the distal end portion (front end portion) of the interlocked arm <NUM> and disposed such that its axial direction is in the left-right direction. The interlocked arm <NUM> is rotatably connected to the base end portion of the piston rod <NUM> via the first rotation shaft <NUM>. The interlocked arm <NUM> has a base end portion (rear end portion) rotatably and integrally connected to the lift arm <NUM>, which will be described later.

The lift arm <NUM> is supported to be rotatable relative to the cylinder case <NUM>. The lift arm <NUM> is provided such that its distal end portion (rear end portion) protrudes rearward. A second rotation shaft <NUM> is provided at the base end portion (front end portion) of the lift arm <NUM> and disposed such that its axial direction is in the left-right direction. The lift arm <NUM> is supported to be rotatable upward and downward relative to the cylinder case <NUM> via the second rotation shaft <NUM>. The lift arm <NUM> is fixed unrotatably to the base end portion of the interlocked arm <NUM> via the second rotation shaft <NUM>. The lift arm <NUM> is displaceable to a predetermined position between a position at which it is lifted most as shown in the principle example of a lift arm lifting and lowering mechanism not covered by the present invention of <FIG> (hereinafter referred to as a "lifted position") and a position at which it is lowered most as shown in the principle example of a lift arm lifting and lowering mechanism not covered by the present invention of <FIG> (hereinafter referred to as a "lowered position") by rotating about the second rotation shaft <NUM>.

The link mechanism <NUM> shown in <FIG> is rotatably connected to the distal end portion of the lift arm <NUM>. When the lift arm <NUM> is lifted, the link mechanism <NUM> lifts the rotary tilling device <NUM>. When the lift arm <NUM> is lowered, the link mechanism <NUM> lowers the rotary tilling device <NUM>. The following will describe rotating motion of the lift arm <NUM> in the lift arm lifting and lowering mechanism <NUM> configured as described above.

First, force applied to the lift arm lifting and lowering mechanism <NUM> will be described. Due to the self-weight of the lift arm <NUM> and the weight of the rotary tilling device <NUM> connected to the lift arm <NUM> (hereinafter referred to as "the self-weight of the lift arm <NUM> and the like"), the lift arm <NUM> undergoes a force to rotate it about the second rotation shaft <NUM> in the clockwise direction as seen in a side view (moment of force in the clockwise direction as seen in a side view). The force presses the piston <NUM> toward the axially front surface side via the interlocked arm <NUM> and the piston rod <NUM>.

When the lift arm <NUM> is at the lowered position as shown in the principle example of a lift arm lifting and lowering mechanism not covered by the present invention of <FIG>, the front surface of the piston <NUM> pressed toward the axially front surface side as described above contacts the bottom surface of the cylinder portion <NUM>.

The following will describe, as an example of rotating motion to lift the lift arm <NUM>, rotating motion in which the lift arm <NUM> at the lowered position as shown the principle example of a lift arm lifting and lowering mechanism not covered by the present invention of <FIG>, is moved to the lifted position as shown in the principle example of a lift arm lifting and lowering mechanism not covered by the present invention of <FIG>.

First, a switching valve provided to the oil feed passage <NUM> is switched to the state where oil can be fed to the cylinder portion <NUM>. In this manner, oil from the transmission case <NUM> is fed to the hydraulic chamber R inside the cylinder portion <NUM> via the oil pump.

The oil fed to the hydraulic chamber R first flows into the space defined by the bottom surface of the cylinder portion <NUM> and the recess <NUM> of the piston <NUM>. In this state, when further oil is fed, the hydraulic pressure of the oil presses the recess <NUM> of the piston <NUM>. In this manner, the piston <NUM> slides toward the axially rear surface side inside the cylinder portion <NUM> against the force due to the self-weight of the lift arm <NUM> and the like.

In association with the sliding movement of the piston <NUM>, the piston rod <NUM> is pressed toward the axially rear surface side. In this manner, the interlocked arm <NUM> rotates about the second rotation shaft <NUM> in the counterclockwise direction as seen in a side view. Accordingly, the lift arm <NUM> unrotatably fixed to the interlocked arm <NUM> rotates in the counterclockwise direction as seen in a side view. The lift arm <NUM> rotates to a predetermined position in the counterclockwise direction as seen in a side view to reach the lifted position as shown in the principle example of a lift arm lifting and lowering mechanism not covered by the present invention of <FIG>.

When the lift arm <NUM> is at the lifted position, the switching valve is switched to the state where the oil feed passage <NUM> is closed. In this manner, the state where the lift arm <NUM> is at the lifted position and the rotary tilling device <NUM> is lifted can be retained.

Next, as an example of rotating motion to lower the lift arm <NUM>, rotating motion in which the lift arm <NUM> at the lifted position as shown in the principle example of a lift arm lifting and lowering mechanism not covered by the present invention of <FIG>, is moved to the lowered position as shown in the principle example of a lift arm lifting and lowering mechanism not covered by the present invention of <FIG> will be described.

First, the switching valve is switched to the state where oil can be discharged from the cylinder portion <NUM>. In this state, the oil inside the hydraulic chamber R is discharged via the oil feed passage <NUM>, and the piston <NUM> is pressed by the piston rod <NUM> by the force due to the self-weight of the lift arm <NUM> and the like and slides toward the axially front surface side. In this manner, the lift arm <NUM> rotates in the clockwise direction as seen in a side view. The lift arm <NUM> rotates to a predetermined position to reach the lowered position as shown in <FIG>.

When the lift arm <NUM> is at the lowered position, the switching valve is switched to the state where the oil feed passage <NUM> is closed. In this manner, the state where the lift arm <NUM> is at the lowered position and the rotary tilling device <NUM> is lowered can be retained.

The following will describe operation of the safety valve <NUM> when the lift arm <NUM> is at the lifted position.

The principle example of a safety valve not covered by the present invention of <FIG>, exemplary shows a case where the hydraulic pressure inside the hydraulic chamber R is less than the predetermined value. In this state, the biasing force of the biasing portion <NUM> is larger than the hydraulic pressure applied to the valve disc portion <NUM> (the spherical portion <NUM> and the retaining portion <NUM>) via the through hole <NUM>. In this manner, the valve disc portion <NUM> (the spherical portion <NUM>) biased by the biasing portion <NUM> closes the valve disc receiving portion 32a.

The principle example of a safety valve not covered by the present invention of <FIG>, exemplary shows a case where the hydraulic pressure inside the hydraulic chamber R is greater than or equal to the predetermined value. In this state, the hydraulic pressure applied to the valve disc portion <NUM> (the spherical portion <NUM>) via the through hole <NUM> is larger than the biasing force of the biasing portion <NUM>. In this manner, the valve disc portion <NUM> moves toward the axially rear surface side by the hydraulic pressure inside the hydraulic chamber R against the biasing force of the biasing portion <NUM>, so that the valve disc receiving portion 32a is opened.

When the valve disc receiving portion 32a is opened, the oil inside the hydraulic chamber R can flow through the oil passage <NUM>. That is, when the valve disc receiving portion 32a is opened, the oil flowing through the through hole <NUM> is fed into the valve disc containing portion <NUM> of the containing portion <NUM>. The oil fed into the valve disc containing portion <NUM> flows through the gap between the inner surface of the valve disc containing portion <NUM> and the spherical portion <NUM> and the side surface of the body portion <NUM> of the retaining portion <NUM> toward the axially rear surface side, as shown in principle example of a safety valve not covered by the present invention of <FIG>. In this manner, the oil inside the hydraulic chamber R can be discharged from the rear surface of the piston <NUM>.

Since the safety valve <NUM> as described above is provided, it is possible to suppress an excessive rise in the hydraulic pressure inside the hydraulic chamber R. In particular, the hydraulic pressure inside the hydraulic chamber R may excessively rise under certain conditions. Specifically, when a load is applied to the lifted rotary tilling device <NUM>, a force is applied to rotate the lift arm <NUM> at the lifted position in the clockwise direction as seen in a side view, and the piston <NUM> is pressed toward the axially front surface side, so that the hydraulic pressure inside of the hydraulic chamber R may excessively rise. In addition, when the temperature of the oil inside the hydraulic chamber R rises, the volume of the oil may increase and thus the hydraulic pressure may excessively rise. Since the safety valve <NUM> as described above is provided, the hydraulic pressure inside the hydraulic chamber R is decreased when the hydraulic pressure inside the hydraulic chamber R becomes greater than or equal to the predetermined value, and thus it is possible to suppress an excessive rise in the hydraulic pressure.

In addition, since the safety valve <NUM> is contained in the piston <NUM>, it is possible to suppress increase in the size of the cylinder case <NUM>, unlike the case where the safety valve <NUM> is provided to the cylinder case <NUM>.

In addition, although the safety valve <NUM> is pushed by the hydraulic pressure of the hydraulic chamber R toward the axially rear surface side, the non-containing portion <NUM> is provided on the axially rear surface side relative to the containing portion <NUM>, and thus the movement of the safety valve <NUM> beyond the non-containing portion <NUM> toward the axially rear surface is restricted. In this manner, it is possible to prevent the safety valve <NUM> pressed by the hydraulic pressure from coming off piston <NUM> from the axially rear surface.

In addition, in the explained principle examples not covered by the present invention, oil discharged from the rear surface of the piston <NUM> is fed into the transmission case <NUM> via an opening at a lower portion of the cylinder case <NUM>. Thus, it is possible to eliminate the necessity to provide an oil passage for returning the discharged oil to the transmission case <NUM> and simplify the configuration.

In addition, in the explained principle examples not covered by the present invention, since the valve disc containing portion <NUM> has a shape with a diameter smaller than a diameter of the valve seat containing portion <NUM> and the valve disc portion <NUM> and the biasing portion <NUM> are contained in the valve disc containing portion <NUM>, it is possible to stabilize the valve disc portion <NUM> and the biasing portion <NUM> (suppress the wobbling thereof) when the valve disc portion <NUM> and the biasing portion <NUM> receive pressure. In this manner, it is possible to suppress the event that the safety valve <NUM> fails in operating at a predetermined pressure and variation in the pressure for operating the safety valve <NUM>.

As described above, the lift arm lifting and lowering mechanism <NUM> according to the principle examples not covered by the present invention, includes:.

In this configuration, it is possible to suppress an excessive rise in the hydraulic pressure inside the hydraulic chamber R. That is, when the hydraulic pressure inside the hydraulic chamber R becomes greater than or equal to the predetermined value, the oil inside the hydraulic chamber R is discharged to the outside by the safety valve <NUM> provided to the piston <NUM>, so that the pressure inside the hydraulic chamber R can be decreased. In addition, since the piston <NUM> is provided to the safety valve <NUM>, the attachment of the safety valve <NUM> can be made relatively easy. In addition, since the containing portion <NUM> is provided to open on one end side (axially front surface side) of the oil passage <NUM> and the non-containing portion <NUM> is provided on the other end side (axially rear surface side), it is possible to prevent the safety valve <NUM> pressed by hydraulic pressure from coming off the piston <NUM> from the axially rear surface side.

In addition, the safety valve <NUM> includes:.

In this configuration, the structure of the safety valve <NUM> can be simplified. That is, the structure of the safety valve <NUM> can be simplified as compared to a control valve using electric power.

In addition, the containing portion <NUM> includes:.

In this configuration, it is possible to stabilize the valve disc portion <NUM> when the valve disc portion <NUM> receives pressure.

In addition, the through hole <NUM> includes the valve disc receiving portion 32a provided at the end portion of the valve seat portion <NUM> on the axially rear surface side and having a circular truncated cone shape that increases in diameter toward the axially rear surface side, and
the valve disc portion <NUM> includes:.

In this configuration, the structure of the safety valve <NUM> can be made relatively simple.

That is, the general spherical portion <NUM> can be used to close the valve disc receiving portion 32a regardless of the inclination angle (taper angle) of the inner peripheral surface of the valve disc receiving portion 32a, and thus the structure of the safety valve <NUM> can be made relatively simple.

In addition, the lift arm lifting and lowering mechanism <NUM> further includes the piston rod <NUM> configured to transmit sliding movement of the piston <NUM> to the lift arm <NUM>, in which.

In this configuration, the rod retaining portion <NUM> can suitably receive the piston rod <NUM>. That is, since the circular truncated cone portion <NUM> is provided, the distal end portion of the piston rod <NUM> can be guided toward the axially front surface side (bottom portion <NUM> side). In addition, since the columnar portion <NUM> is provided, it is possible to obtain a sufficient depth dimension of the rod retaining portion <NUM> and easily retain the distal end portion of the piston rod <NUM>.

In addition, the columnar portion <NUM> and the containing portion <NUM> are provided to at least partially overlap each other in the axial direction.

In this configuration, it is possible to easily obtain a certain thickness between the containing portion <NUM> and the rod retaining portion <NUM> in the piston <NUM>. Thus, it is possible suppress the concentration of stress at a partial area between the containing portion <NUM> and the rod retaining portion <NUM>.

Note that the first surface according to the present embodiment is a form of embodying the front surface.

The second surface according to the present embodiment is a form of embodying the rear surface.

Furthemore, exemplary shown in the further principle example of a safety valve not covered by the present invention of <FIG> and <FIG>, a modification may be made to the structure of a valve disc portion 33A of a safety valve <NUM>. Note that the further principle example of the safety valve not covered by the present invention of <FIG>,<FIG> is generally similar to the lift arm lifting and lowering mechanism <NUM> according to the above-described first embodiment except for the structure of the valve disc portion 33A.

The valve disc portion 33A is different from that in the above-described first embodiment in that a body portion 36A can close the valve disc receiving portion 32a without the intervention of the spherical portion <NUM>. The body portion 36A is provided with an engagement portion <NUM> protruding from the rear surface toward the axially rear surface side in the same way as in the above-described first embodiment. The body portion 36A includes a protruding portion 36Aa.

The protruding portion 36Aa is a portion capable of closing the valve disc receiving portion 32a. The protruding portion 36Aa is integrally provided to the body portion 36A so as to protrude toward the axially front surface side on the front surface of the body portion 36A. The protruding portion 36Aa has a circular truncated cone shape that decreases in diameter toward the axially front surface side. That is, the outer peripheral surface of the protruding portion 36Aa has a shape of an inclined surface (tapered surface) that decreases in diameter toward the axially front surface side. The inclination angle of the outer peripheral surface of the protruding portion 36Aa with respect to the axial line is set to be smaller than the inclination angle of the inner peripheral surface of the valve disc receiving portion 32a with respect to the axial line. Thus, as shown in the further principle example of a safety valve not covered by the present invention of <FIG>, the distal end portion of the protruding portion 36Aa can close the valve disc receiving portion 32a by being inserted into the valve disc receiving portion 32a. The illustrated example shows an example where the inclination angle of the outer peripheral surface of the protruding portion 36Aa with respect to the axial line in a cross-sectional view is substantially <NUM> degrees.

As shown in the further principle example of a safety valve not covered by the present invention of <FIG>, when the hydraulic pressure in the hydraulic chamber R becomes greater than or equal to a predetermined value, the valve disc portion 33A receives the hydraulic pressure in the hydraulic chamber R at the distal end portion of the protruding portion 36Aa and moves toward the axially rear surface side, so that the valve disc receiving portion 32a is opened. In this case, the oil fed into the valve disc containing portion <NUM> flows through the gap between the inner surface of the valve disc containing portion <NUM> and the side surface of the body portion 36A toward the axially rear surface side.

As described above, the through hole <NUM> according to the further principle example of a safety valve not covered by the present invention of <FIG>,<FIG> includes the valve disc receiving portion 32a provided at the end portion of the valve seat portion <NUM> on the axially rear surface side and having a circular truncated cone shape that increases in diameter toward the axially rear surface side, and
the valve disc portion <NUM> includes:.

In this configuration, the number of components of the safety valve <NUM> can be reduced.

That is, the number of components can be reduced as compared to the case where a component closing the valve disc receiving portion 32a and a component engaging with the biasing portion <NUM> are separately prepared.

The structure of the safety valve <NUM> is not limited to those in the above-described principle examples not covered by the present invention.

For example, as in a lift arm lifting and lowering mechanism <NUM> according to a second embodiment, a modification may be made to the structure of a valve disc portion 33B shown in <FIG> and <FIG>. Note that the lift arm lifting and lowering mechanism <NUM> according to the second embodiment is generally similar to the above-described further principle example of a safety valve not covered by the present invention of <FIG>,<FIG>, except for the structure of the valve disc portion 33B.

The valve disc portion 33B is different from that in the above-described further principle example of a safety valve not covered by the present invention of <FIG>,<FIG>, in that groove portions 36Bb are provided in the side surface of a body portion 36B. Note that the body portion 36B is provided with a protruding portion 36Ba and an engagement portion <NUM> in the same way as in the above-described first embodiment.

The groove portions 36Bb are grooves provided over the entire side surface of the body portion 36B in the axial direction (moving direction). A pair of groove portions 36Bb are provided at upper and lower positions of the body portion 36B as shown in <FIG> and <FIG>. The portion connecting the groove portion 36Bb and the side surface of the body portion 36B is chamfered.

In the present embodiment, the outer diameter dimension of the body portion 36B is larger than the outer diameter dimension of the body portion 36A in the further principle example of a safety valve not covered by the present invention of <FIG>,<FIG>.

As a result, the gap between the inner surface of the valve disc containing portion <NUM> and the side surface of the body portion 36B is made small. In the above configuration, the wobbling of the valve disc portion 33B relative to the valve disc containing portion <NUM> can be suppressed, and the valve disc portion 33B can be stably moved.

<FIG> shows a state where the hydraulic pressure inside the hydraulic chamber R is less than a predetermined value. In this state, the distal end portion of the protruding portion 36Ba can close the valve disc receiving portion 32a by being inserted into the valve disc receiving portion 32a.

<FIG> shows a state where the hydraulic pressure inside the hydraulic chamber R is greater than or equal to the predetermined value. In this state, the valve disc portion 33B receives the hydraulic pressure in the hydraulic chamber R at the distal end portion of the protruding portion 36Ba and moves toward the axially rear surface side, so that the valve disc receiving portion 32a is opened. In this case, although the gap between the inner surface of the valve disc containing portion <NUM> and the side surface of the body portion 36B is relatively small, the groove portions 36Bb are provided in the side surface, so that a flow path for discharging oil can be provided.

As described above, the valve disc portion 33B according to the second embodiment includes the side surface opposed to the inner surface of the containing portion <NUM>, and the side surface has the groove portion 36Bb provided over the entire portion in the moving direction.

In this configuration, a flow path for discharging oil can be provided. That is, even if the gap between the side surface of the valve disc portion 33B and the inner surface of the valve disc containing portion <NUM> is small, a flow path for discharging oil can be provided.

Note that, although the protruding portion 36Aa and the protruding portion 36Ba have a circular truncated cone shape in the above-described second embodiment and the further principle example of a safety valve not covered by the present invention of <FIG>,<FIG>, the protruding portion 36Aa and the protruding portion 36Ba are not limited to the above-mentioned shape as long as they have a shape that is circular as seen in the axial direction and decreases in diameter toward the axially front surface side (conical shape). For example, the protruding portion 36Aa and the protruding portion 36Ba may have a conical shape. In addition, the protruding portion 36Aa and the protruding portion 36Ba are not limited to a shape in which its generatrix is a straight line as seen in a side view, and may have a shape in which its generatrix is a curved line, for example.

The structure of the safety valve <NUM> is not limited to those in the above-described second embodiment and the above-described principle examples not covered by the present invention.

In the present invention as defined by the appended claims, a lift arm lifting and lowering mechanism <NUM> according to a first alternative of an embodiment covered by the present invention. a modification is made to the structures of a recess <NUM> and a valve seat portion 31A of a piston <NUM> shown in <FIG> and <FIG>. Note that the lift arm lifting and lowering mechanism <NUM> according to the first alternative of the embodiment covered by the present invention is generally similar to the principle examples not covered by the present invention of <FIG>, except for the structures of the recess <NUM> and the valve seat portion 31A.

The piston <NUM> according to the first alternative of the embodiment covered by the present invention, is different from that in the above-described principle examples not covered by the present invention of <FIG>, in that a thickening portion 15a is provided at a part of the recess <NUM>. The thickening portion 15a is a portion formed to fill (thicken) a part of the recess <NUM> recessed toward the axially rear surface side on the front surface of the piston <NUM>. The thickening portion 15a is provided at the lower end portion of the recess <NUM> to protrude from the bottom of the recess <NUM>. The thickening portion 15a has a substantially circular shape as seen in the axial direction. The front surface of the thickening portion 15a is coplanar with the portion of the front surface of the piston <NUM> where the recess <NUM> is not provided.

In the present embodiment, the containing portion <NUM> is provided at the thickening portion 15a.

Thus, the entire side surface of the collar portion 31a of the valve seat portion 31A is covered by the increased diameter portion 24a of the valve seat containing portion <NUM>.

A recessed groove 31c is provided in the side surface of the collar portion 31a of the valve seat portion 31A. The recessed groove 31c is provided over the entire periphery of the side surface of the collar portion 31a. The recessed groove 31c has a rectangular shape as seen in a cross-sectional view.

In the present first alternative of the embodiment covered by the present invention, an O-ring <NUM> is fitted into the recessed groove 31c. The O-ring <NUM> is an annular member capable of sealing oil. The O-ring <NUM> contacts the bottom surface of the recessed groove 31c and the inner peripheral surface of the increased diameter portion 24a of the valve seat containing portion <NUM>. Thus, the sealing property of the valve seat portion 31A against the valve seat containing portion <NUM> can be improved.

As described above, the piston <NUM> according to the first alternative of the embodiment covered by the present invention, includes:.

In this configuration, the exposure of the safety valve <NUM> from the recess <NUM> can be suppressed.

Thus, the safety valve <NUM> can be easily retained with respect to the piston <NUM>. In addition, the portion where the containing portion <NUM> is provided and thus reduced in weight can be thickened, so that an imbalance in weight of the piston <NUM> can be suppressed.

Alternatively, in the present invention as defined by the appended claims, in a lift arm lifting and lowering mechanism <NUM> according to a second alternative of the embodiment covered by the present invention, a modification is made to the structure of a recess 15A a piston <NUM> shown in <FIG> and <FIG> in the structure of the safety valve <NUM>. Note that the lift arm lifting and lowering mechanism <NUM> according to the second alternative of the embodiment covered by the present invention, is generally similar to the lift arm lifting and lowering mechanism <NUM> according to the above-described principle examples not covered by the present invention of <FIG>, except for the structure of the recess 15A.

The recess 15A according to the second alternative of the embodiment covered by the present invention is different from that in the above-described principle examples of a piston and a safety valve not covered by the present invention of <FIG>, in that it is formed to be smaller than the recess <NUM> according to the principle examples not covered by the present invention of <FIG>, Specifically, the recess 15A is formed in a circular shape having a size that does not overlap the containing portion <NUM> of the oil passage <NUM> as seen in the axial direction (see <FIG>). In other words, the containing portion <NUM> is formed to be located outside the recess 15A (in the radial direction) as seen in the axial direction. Thus, the recess 15A is provided so as not to communicate with the containing portion <NUM>.

As described above, the piston <NUM> according to the second alternative of the embodiment covered by the present invention, includes the recess 15A provided to open in the front surface, and the containing portion <NUM> is provided outside the recess 15A.

Thus, the safety valve <NUM> can be easily retained with respect to the piston <NUM>. In addition, the formation of the recess 15A can be simplified as compared to the case where the thickening portion 15a is provided as in the first alternative of the embodiment covered by the present invention.

Within the scope of the present invention as defined by the appended claims, in addition, the structure of the rod retaining portion <NUM> of the piston <NUM> is not limited to the manners in the above-described embodiments and various manners may be used. That is, the disposal and shapes of the circular truncated cone portion <NUM>, the columnar portion <NUM>, and the bottom portion <NUM> of the rod retaining portion <NUM> may be appropriately set from the viewpoint of suitably retaining the piston rod <NUM> and the viewpoint of obtaining a sufficient thickness between it and the oil passage <NUM>.

Within the scope of the present invention as defined by the appended claims, in addition, the structure of safety valve <NUM> is not limited to the manners in the above-described embodiments and various manners may be used. For example, the safety valve <NUM> is not limited to the manner that biases the valve disc portion <NUM> against the valve seat portion <NUM>, and a control valve using electric power may be used.

Within the scope of the present invention as defined by the appended claims, in addition, although the tractor <NUM> is shown as an example of the working vehicle provided with the lift arm lifting and lowering mechanism <NUM> in the above-described embodiments, there is no limitation thereto. For example, the working vehicle may be another agricultural vehicle, construction vehicle, industrial vehicle, or the like.

Claim 1:
A lift arm lifting and lowering mechanism (<NUM>) comprising:
- a cylinder case (<NUM>) provided with a cylinder portion (<NUM>) into which oil can be fed;
- a piston (<NUM>) slideably disposed inside the cylinder portion (<NUM>) to form a hydraulic chamber and receiving hydraulic pressure of the hydraulic chamber on a first surface of the piston when oil is fed into the cylinder case;
- a safety valve (<NUM>) provided to the piston (<NUM>) and configured to discharge the oil from inside the hydraulic chamber when the hydraulic pressure of the hydraulic chamber becomes greater than or equal to a predetermined value; and
- a lift arm (<NUM>) supported by the cylinder case (<NUM>) and rotatable in conjunction with movement of the piston (<NUM>), wherein
- the piston (<NUM>) includes an oil passage (<NUM>) allowing communication between a second surface of the piston (<NUM>) and the first surface of the piston (<NUM>), and
- the oil passage (<NUM>) includes:
- a containing portion (<NUM>) provided to open on the first surface side and capable of containing the safety valve (<NUM>); and
- a non-containing portion (<NUM>) provided to open on the second surface side relative to the containing portion (<NUM>) and incapable of containing the safety valve (<NUM>),
characterized in that the piston (<NUM>) includes a recess (<NUM>) provided to open in the first surface and in that
- the containing portion (<NUM>) is provided outside the recess (<NUM>); or
- a thickening portion (15a) is provided to protrude from a bottom of the recess (<NUM>) and the containing portion (<NUM>) is provided at the thickening portion (15a).