CA2546753C

CA2546753C - Hydraulic pump, hydraulic pump unit, and hydraulic driving unit

Info

Publication number: CA2546753C
Application number: CA2546753A
Authority: CA
Inventors: Osamu Sato; Yoshitake Sakai
Original assignee: Kayaba Industry Co Ltd
Current assignee: KYB Corp
Priority date: 2005-05-25
Filing date: 2006-05-12
Publication date: 2014-03-18
Anticipated expiration: 2026-05-12
Also published as: CN1869459A; CN1869459B; US20070243081A1; KR20060121681A; KR101241371B1; JP4897240B2; JP2006329021A; CA2546753A1

Abstract

A hydraulic pump can prevent breakage at a high pressure without providing a separate relief valve. A housing has a body side and a lid side that are divided by a surface perpendicular to a rotational axis of a hydraulic pump shaft. The lid-side housing is brought into oil-tight contact with the body- side housing by a spring. Contact between the body-side housing and the lid-side housing is maintained at a predetermined pressure corresponding to a maximum allowable pump hydraulic pressure.

Description

HYDRAULIC PUMP, HYDRAULIC PUMP UNIT, AND HYDRAULIC DRIVING UNIT
BACKGROUND OF THE INVENTION
The invention relates generally to hydraulic pumping equipment in which a hydraulic fluid is pumped in a hydraulic circuit and used as a source of motive force to drive a hydraulic actuator. The invention relates more specifically to a hydraulic pump used in a hydraulic pump unit in which at least a hydraulic pump and a hydraulic oil tank are integrated. A housing for the hydraulic pump is contained inside the hydraulic oil tank. Embodiment, of the invention may also include a hydraulic pump unit provided with a hydraulic pump according to the invention, and a hydraulic driving unit provided with such a hydraulic pump and configured to apply a driving force t) hydraulic pressure to a driven body.
Hydraulic pump units in which the hydraulic pump and the hydraulic oil tank are integrated include those in which the hydraulic pump housing is contained in the hydraulic oil tank.
Hydraulic driving units may also include an electric motor for driving the hydraulic pump, a hydraulic actuator that is driven by hydraulic oil from the hydraulic pump, and various valves for controlling the flow of the hydraulic oil between these hydraulic devices. The elements operate together to apply Zl driving force generated by hydraulic pressure to the driven body.
A hydraulic driving unit of this general type is described in 'Japanese Laid-Open Unexamined Patent Application Publication No. 02-296003, Figure 5(a) is a hydraulic circuit diagram for such a hydraulic driving unit.:

Fig. 5(b) is a view of the unit in partial section; Fig. 5(c) is a front view of a hydraulic pump; and Fig. 5(d) is a section view showing internal structures of the pump illustrated in Fig. 5(c).
Although Figs. 5(c) and 5(d) are not included in the Japanese publication, they do illustrate the structures of the embodiment described in that document. The applicant in the Japanese publication is the same as the assignee of the invention described in this application.
Figure 5(a) illustrates a hydraulic driving unit OU operable to impart a driving force to a driven body W through hydraulic pressure delivered by hydraulic oil circulating in a closed system. The hydraulic driving unit includes a hydraulic pump P for pumping hydraulic oil in both normal and reverse directions. The pump is driven by a normal/reverse rotating motor M. A
hydraulic actuator C (a hydraulic cylinder, in this case) is driven by pressure in the hydraulic oil to generate the driving force. A hydraulic oil tank T stores a quantity of the hydraulic oil inside a closed volume. A pair of operate check valves OT1 and 0T2 control the flow of the hydraulic oil in both the normal and the reverse directions between the hydraulic pump P and the hydraulic actuator C. A switching valve V controls the flow of the hydraulic oil in the normal and reverse directions between the hydraulic pump P and the hydraulic oil tank.
The hydraulic driving unit OU also includes a slow return valve SV, which prevents hunting that might otherwise be generated when an external force is applied by the driven body W while the hydraulic pump P is operating.

The slow return valve throttles the flow of hydraulic oil from a bottom-side oil chamber C2 at the bottom side of the hydraulic cylinder C to one of the check

-2-valves 0T2. The other check valve OT1 connects to a rod-side oil chamber of the hydraulic cylinder.
Relief valves RV are provided in lines that branch back to the hydraulic tank T from the main lines between the hydraulic pump P and each of the operate check valves OT1 and 0T2. These relieve valves allow excess hydraulic oil to escape back to the tank T when an abnormal pressure is generated in thc pipe line to which the relief valve connects.
An additional line branches back to the tank OT from the lines between each of the operate check valves OT1 and 0T2 and the hydraulic cylinder C.
This additional line is provided with an emergency manual valve so that oil in the lines from the cylinder's rod-side oil chamber Cl and bottom-side oil chamber C2 can be released back to the tank when, due to a loss of power or some other problem, the pump cannot operate and is stopped. Opening the emergency manual valve MV thereby enables manual operation of the hydraulic cylinder C.
Such a hydraulic driving unit thereby ensures safety, reliability, and the ability to avoid an accident or damage to the unit by preventing breakage in the unit even if an abnormal pressure state is encountered while the unit is in operation.
A portion of the hydraulic pump P that relates particularly to the invention is described in more detail below.
As Fig. 5(b) illustrates, the hydraulic pump P in the unit OU is mounted on the side of a housing body He, inside of which various hydraulic

-3-lines are located in addition to various valves OT1, 0T2, and V. of the types described above. A lid-side housing Hb is fastened and fixed by a fastening bolt Hj over the pump. An oil-tight seal is formed between the lid-side housing Hb and the body-side housing Hc, so that the pump is contained inside the tank and oil cannot leak out of the tank.
The hydraulic pump is located so that it is close to the source of hydraulic oil, which improves pumping efficiency. If hydraulic oil leaks between the body-side housing Hc and the lid-side housing Hb, the hydraulic oil leaks not to the outside of the unit OU, but merely into the hydraulic oil tank T.
which confers on this unit a certain degree of safety against that particular potential failure.
The body-side housing Hc and the lid-side housing Hb are combined to form a combined housing structure Ha.
The means by which the body-side housing Hc and the lid-side housing Hb are fastened and fixed together is described in detail below in connection with Figs. 5(c) and 5(d).
The hydraulic pump P includes, in addition to the above-mentioned body-side housing Hc and lid-side housing Hb, a driving shaft Hd, which is driven by an electric motor (not shown). A driving gear He is fixed to the driving shaft Hd, with a driven gear Hg meshed with the driving gear He. The driven gear Hg is mounted on a rotary driven shaft Hf. A hydraulic oil port H1-allows hydraulic oil to flow in and out of the pump as these gears rotate.
Mounting holes Hi are provided for mounting the lid-side housing Hb to the body-side housing He.

-4-In this configuration the hydraulic pump P is fastened to the body-side housing He and the lid-side housing Hb by the fastening bolt Flj. These elements are assembled in an oil-tight manner so that hydraulic oil cannot leak between them. If hydraulic pressure above a predetermined maximum allowed pressure is generated inside the hydraulic pump P or the lines connected to it, the hydraulic oil is allowed to escape through the relieve valve depicted in Fig. 5(a) back into the tank so as to prevent breakage of the hydraulic pump or other such damage.
This construction, though, requires the relief valve and its related lines to be provided as separate elements, which is problematic in units such as this one in which simplicity and reductions in size and costs are very much desired.
This is true not only in the particular hydraulic drive unit described just above, but in other various hydraulic driving units, hydraulic pump units, and hydraulic pumps used in similar conditions.
The invention was developed in response to these concerns and in the context of an effort to provide a hydraulic pump in which damage due to unintended high pressure conditions can be prevented without the need to provide a separate relief valve. Pumps that embody the invention can be used in hydraulic pump units and in hydraulic driving units that include such a pump.
SUMMARY OF THE INVENTION
The invention provides a hydraulic pump that can be used in a hydraulic pump unit in which at least the hydraulic pump and a hydraulic oil tank are integrated together so that a housing of the hydraulic pump is

-5-contained inside the hydraulic oil tank. The housing of the pump includes a body-side and a lid-side housing that come together at a surface perpendicular to a rotary pump shaft of the hydraulic pump. The lid-side housing is brought into oil-tight contact with the body housing at a predetermined pressure that corresponds to a maximum allowable pump hydraulic pressure.
The invention may be further embodied in a hydraulic pump unit or a hydraulic driving unit that includes such a hydraulic pump as one of its working elements.
Because the lid-side housing is held in oil-tight contact with the body -side housing with a predetermined pressure that corresponds to the maximum allowable pump hydraulic pressure, the lid-side housing serves the role of a relief valve, and breakage of the hydraulic pump by high pressure inside the pump can thereby be avoided without the presence of a relief valve as a separate element.
Since the outside of the pump is covered by the hydraulic oil tank, moreover, hydraulic oil that leaks from the pump will return to the tank without leaking outside of the hydraulic pump unit assembly.
BRIEF DESCRIPTION OF THE DRAWINGS
Preferred embodiments of the invention are described in more detail below in connection with the appended drawings, in which:
Fig. 1(a) is a hydraulic circuit diagram illustrating a hydraulic driving unit as one embodiment of the invention;
Fig. 1(b) is a section view of a hydraulic pump used in the hydraulic driving unit of Fig. 1(a);

-6-Fig. 1(c) is a front view of the pump shown in Fig. 1(b);
Figs. 2(a) and 2(b) are section views illustrating one aspect of the operation of the pump depicted in Figs. 1(b) and 1(c);
Fig. 2(c) is an enlarged view of a part of the pump depicted in Figs.
2(a) and 2(b);
Fig. 2(d) is an enlarged view of a part used in an alternative embodiment;
Figs. 3(a) and 3(b) are section views illustrating one aspect of the operation of an alternative embodiment;
Figs. 4(a) and 4(b) are section views illustrating one aspect of the operation of another alternative embodiment;
Fig. 4(c) is a front view of a pump depicted in Figs 4(a) and 4(b);
Fig. 4(d) is a section view on section line A¨A in Fig. 4(c);
Fig. 5(a) is a hydraulic circuit diagram of a conventional hydraulic driving unit;
Fig. 5(b) is a view in partial section of the hydraulic driving unit of Fig. 5(a);
Fig. 5(c) is a front view of a pump used in the hydraulic driving unit of Figs 5(a) and 5(b); and Fig. 5(d) is a section view of the pump illustrated in Fig. 5(c).
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
Figures 1(a)-1(c) show an example of a hydraulic driving unit that includes a hydraulic pump that embodies the present invention. Figure (a) is a hydraulic circuit diagram of the driving unit. Figure 1(b) is a section view through a part of the hydraulic pump shown in Fig. 1(a). Figure 1(c) is a front view of the pump shown in Figs. 1(a) and 1(b).

-7-Figures 2(a) and 2(b) illustrate the operation of the hydraulic pump shown in Fig. 1(b). Figure 2(c) is an enlarged view of a part of the pump shown in Fig. 2(a), and Fig. 2(d) is an enlarged view of a part of another exemplary embodiment.
Hydraulic driving unit 10 is used, for example. to lift a working element in a special agricultural vehicle over cultivated ground. A hydraulic pump unit 9 uses hydraulic pressure to drive a hydraulic actuator. which is illustrated in this embodiment as a hydraulic cylinder 2. Other types of hydraulic actuators may also be used with hydraulic pump units of this general type.
A hydraulic pump 1 of the type depicted here in connection with this hydraulic driving unit 10 may also be used in other hydraulic circuits inside of a hydraulic oil tank to avoid leakage of oil from the pump outside of the oil tank or the unit.
The drive unit 10 includes a hydraulic pump 1 with an electric motor 11.
The motor drives the pump to feed hydraulic oil under pressure in both normal and reverse directions. A hydraulic cylinder 2 delivers a driving force generated through the hydraulic oil pressure to a driven body W. A hydraulic oil tank 3 stores hydraulic oil in a closed space, and operate check valves 4 control the flow of hydraulic oil between the hydraulic pump 1 and the hydraulic cylinder in the normal and reverse directions. A switching valve 5 controls the flow of hydraulic oil in the normal and reverse directions between the hydraulic pump 1 and the tank 3. This embodiment differs from the conventional one described above in that this one does not require a separate relief valve.

-8-The basic functions, relationships, and operations of the hydraulic pump 1, hydraulic cylinder 2, hydraulic oil tank 3, operate check valve -1 and switching valve 5 are the same as those of the hydraulic pump P. hydraulic cylinder C, tank T, operate check valves OT1 and 0T2, and switching valve V
that make up the hydraulic driving unit OU of the conventional example described above and depicted in Figs. 5(a)-5(d), and duplicative description is therefore omitted from this discussion. Reference symbol 2a refers to a bottom-side oil chamber of the hydraulic cylinder 2, and reference symbol 2b to a rod-side oil chamber.
A slow return valve 7 and an emergency manual valve 6 are also provided in this hydraulic driving unit 10. These elements are generally the same as the slow return valve SV and the emergency manual valve MV
provided in the hydraulic driving unit OU in the example described above.
The pump 1 in this embodiment is such that the hydraulic driving unit 10 does not need a separate relief valve and as a result the relief valves RV that are required and depicted in Fig. 5(a) are no longer required in the hydraulic circuit shown in Fig. 1(a).
Exemplary characteristics of this hydraulic pump are described below in further detail in connection with Figs. 1(b), 1(c), 2(a), 2(b), and 2(c).
The location of this hydraulic pump 1 in the overall unit is, though not specifically illustrated, similar to the position of the hydraulic pump P in the conventional example shown in Fig. 5(b), and accordingly, the unit's construction is such that hydraulic oil that leaks out of the hydraulic pump 1 returns to the hydraulic oil tank 3.

-9-The hydraulic pump 1 is provided with a lid-side housing lb. a body-side housing 1c, a driving shaft id driven by an electric motor 1 1 , a dri viii gear le fixed to this rotary driving shaft id. a driven gear lg meshed with this driving gear le and driven, a rotary shaft if of this driven gear lg, a hydraulic oil port lh through which the hydraulic oil supplied with rotation of the driving gear le and the driven gear lg goes in and out, a mounting hole 1i for mounting the lid-side housing lb, and a fastening bolt lj screwed with this mounting hole li for mounting the lid-side housing lb to the body-side housing lc in the oil tight manner.
Certain elements are generally the same as corresponding elements of the conventional hydraulic pump P depicted in Figs. 5(a)-5(d). These include lid-side housing Hb, body-side housing Hc, driving shaft Hd, driving gear He, driven gear Hg, rotary shaft Hf, hydraulic oil port Hh, mounting holes Hi and fastening bolt Hj, except for portions mentioned specifically below.
The lid-side housing lb and the body-side housing lc together form an overall housing la, in generally the same way as the lid-side housing Hb, the body-side housing Hc, and the housing Ha in the conventional example described above in connection with Figs. 5(a)-5(d).
The hydraulic pump 1 of the present invention includes structure in addition to the conventional pump of the prior example. In particular. the housing la includes a body-side housing member lc and a lid-side housing member lb, which come together at a surface perpendicular to the rotary shafts 1d and if of the hydraulic pump 1. The lid-side housing lb is brought into contact with the body-side housing lc in an oil-tight manner with a predetermined pressure that corresponds to a maximum allowable pump

-10-hydraulic pressure. A spacer lk and a spring lm are illustrated in Figs. 2(a)-2(d) as an example of structure that performs this function in this embodiment The spacer 11 is located between the mounting hole of the lid-side housing lb and the fastening bolt 1j, so as to fasten and fix the fastening bolt 1 j to the body-side housing lc.
The spring lm is interposed between the underside of the head of I he fastening bolt lj and the lid-side housing lb so as to bring the lid-side housing lb into oil-tight contact with the body-side housing lc with a predeterin lied pressure that corresponds to the maximum allowable pump hydraulic pressure.
The mounting hole of the lid-side housing lb is configured so that the spacer lk can be interposed between it and the fastening bolt 1j. The hole diameter is thus larger than that of the lid-side housing Hb of the conventional example by the amount necessary to receive the spacer.
When the hydraulic pump 1 is operating at a normal working pressure as shown in Fig. 2(a), the lid-side housing lb is held in oil-tight contact with the body-side housing lc. The pump thus operates normally without leaking oil.
If, though, the hydraulic pressure inside the hydraulic pump I
increases for some reason above the allowable maximum pressure. the lid-side housing lb lifts away from the body-side housing 1c against a biasing force of the spring lm, and as shown in FIG. 2(b), a gap is thereby created between the lid-side housing 1b and the body-side housing lc. The hydraulic oil then begins to leak under pressure from this gap and is thereby returned into the hydraulic oil tank 3.

-11-When enough of the hydraulic oil has leaked out of the hydraulic pump 1, the hydraulic pressure inside the pump 1 will once again become less than the maximum allowable pump pressure, and the pump will return once again to the normal state illustrated in Fig. 2(a).
In this way, breakage of the pump 1 or the like due to unintended high pressure can be avoided without providing a separate relief valve. Such a system eliminates the need for a separate relief valve and its associated hydraulic lines, and so the size, complexity, number of assembly processes.
and cost of the system can all be reduced.
In other respects, the hydraulic driving unit 10 and the hydraulic pump unit 9 that include this hydraulic pump perform the same functions as in prior such systems.
The spacer 1k enables the fastening bolt lj to be installed with force sufficient to ensure that the bolt will stay in place and not be loosened inadvertently. So long as this potential for loosening is guarded against, means other than the spacer 1k might also be employed. A spring washer. for example.

might be installed under the head of the fastening bolt lj to provide the predetermined resistance against the hydraulic pressure in the pump through the elastic force exerted by the spring washer itself.
A stepped fastening bolt in shown in Fig. 2(d) effectively integrates the spacer 1k with the fastening bolt 1j in Fig. 2(c). A bolt of this type is sometimes referred to as a reamer bolt, in which an outer diameter of the bolt under the bolt head (a stepped portion) lo is equal to an outer diameter of the spacer lk

-12-and larger than a diameter of the lower bolt end and the corresponding mounting hole ii in the body-side housing 1c.
This combination of the stepped fastening bolt in and the spring- ho performs the same function as that of the combination of the straight fastening bolt 1j, the spacer 1k, and the spring lm in Fig. 2(c).
Figures 3(a) and 3(b) depict a hydraulic pump as another embodiment of the invention. The same reference characters are assigned to corresponding parts as those described previously so that duplicated explanation is omitted.
This hydraulic pump lA does not include a spacer and a spring as was the case with the pump shown in Fig. 1(b). This pump lA instead combines a frame body lp fixed to the body-side housing I c. The frame body lp contains a lid-side housing lr as illustrated in Figs. 3(a) and 3(b). The lid-side housing can slide perpendicularly to the interface between the body-side housing 1c and the frame body lp. A spring is is located between the frame body tp and the lid-side housing tr.
The frame body lp is fastened and fixed to the body-side housing lc with fastening bolts lj. The frame body's outer shape is otherwise generally similar to that of the lid-side housing lb of the gear pump 1 of FIG. 1(b).
The lid-side housing iris contained inside the frame body I p and normally held in oil-tight contact with the body-side housing 1.c by the force of the spring is, which applies a force that corresponds to a predetermined maximum allowable pressure appropriate for the system's pump. The lid-side

-13-housing iris sized to cover only the area around the gears le and lg and the hydraulic oil port lh where oil tightness is required of the hydraulic pump 1,\.
The frame body lp has a non oil-tight structure that allows communication of the hydraulic oil to an appropriate location ¨a hydraulic oil hole it shown in Figs. 3(a) and 3(b), for example. Instead of providing this hydraulic oil hole it, the structure may be made non oil-tight with a simple frame structure in which the frame body does not cover the whole circumference of the lid-side housing In the hydraulic pump lA with this construction, in the normal-pressure operating state shown in Fig. 3(a) the body-side housing 1c and the lid-side housing 1r are brought into oil-tight contact with one another so that leakage of the hydraulic oil does not occur.
When the hydraulic pressure in the pump IA exceeds the allowable hydraulic pressure as shown in Fig. 3(b), on the other hand. the lid-side housing lr slides outward against the biasing force of the spring is so that a gap is generated between the body-side housing 1c and the lid-side housing 1r.

Hydraulic oil can then leak from the pump and return into the hydraulic tank 3 through the gap between the lid-side housing 1r, the frame body 1p, and the hydraulic oil hole it.
This hydraulic pump thus performs the same function as the pump shown in Fig. 1(b), alone and in combinations comprising hydraulic driving units and hydraulic pump units.

-14-Figures 4(a) and 4(b) illustrate another hydraulic pump as an embodiment of the invention. Figure 4(c) is a front view of the pump shown in Fig. 4(b). Figure 4(d) is a section view on section line A¨A in Fig. 4(c).
Hydraulic pump 1B includes a lid-side housing lu that is fixed to the body-side housing lc with fastening bolts 1w. The lid-side housing as a whole is an elastic body with a special flexing portion lv configured to deform elastically when a maximum allowable hydraulic pump pressure is applied to it.
As can best be seen in Fig. 4(c), the lid-side housing lu has a shape that covers only the periphery of a rotary driving gear le and a rotary gear 1g.
with an overlap sufficient to establish an oil-tight seal. Projecting portions lv are formed with a shape such that the fastening bolts do not bear directly on them.
The pump's normal operating state is illustrated in Fig. 4(a), but when a hydraulic pressure greater than the predetermined maximum allowable pressure is generated inside the pump, the pump operates in the condition illustrated in Figs. 4(b)-4(d). In particular, the flexing portion lv is elastically deformed so that the hydraulic oil in the hydraulic pump 1B is allowed to escape out of the pump and back into the hydraulic oil tank 3.
This hydraulic pump 1B thus functions in the same way as the pump I.
shown in Fig. 1(b), which provides corresponding advantages to hydraulic driving units and hydraulic pump units that use this pump.
Hydraulic pumps in the form of gear pumps have been described in t he examples discussed above. Hydraulic oil escape structures like those described above may also be used in other types of pumps and in other applications,

-15-though, and the present invention is not restricted to use with gear pumps.
The invention may find use, for example, in other hydraulic pumps such as vane pumps.
Hydraulic actuators other than hydraulic cylinders may also be used as parts of overall hydraulic assemblies. A rotary hydraulic actuator might be used, for example, in which a driving force may be generated as a torque derived from hydraulic pressure in the actuator.
The hydraulic pump, hydraulic pump unit, and hydraulic driving unit of the present invention can be used in any industrial field in which size and cost reduction are desired.

-16-

Claims

WHAT IS CLAIMED IS:

1. A hydraulic pump, having a self-relief function, used in a hydraulic pump unit with a construction that at least a hydraulic pump and a hydraulic oil tank are integrated and a housing of said hydraulic pump is contained in the hydraulic oil tank, wherein said housing is provided with a body side housing and a lid side housing divided by a surface perpendicular to a rotary shaft of the hydraulic pump, this lid-side housing is brought into contact with the body-side housing in the oil tight manner with a predetermined pressure corresponding to a pump allowable hydraulic pressure, and hydraulic oil is made to flow between said body-side housing and said lid-side housing and into the hydraulic oil tank if said allowable hydraulic pressure is exceeded.

2. The hydraulic pump according to claim 1, wherein said oil-tight contact with said predetermined pressure is realized by a spacer fitted between a mounting hole of said lid-side housing and a fastening bolt so as to achieve fastening and fixing of the fastening bolt to said body-side housing and a spring interposed between an underneck of the fastening bolt which has been fastened and fixed and said lid-side housing for generating said predetermined pressure.

3. The hydraulic pump according to claim 1, wherein said oil-tight contact with said predetermined pressure is realized by a frame body fixed to said body-side housing containing said lid-side housing, wherein said lid-side housing is capable of sliding in a direction perpendicular to said divided surface to allow oil leakage, and a spring interposed between the frame body and said lid-side housing for generating said predetermined pressure.

4. The hydraulic pump according to claim 1, wherein said oil-tight contact with said predetermined pressure is realized by a lid-side housing constituted so that it is fixed to said body-side housing with a fastening bolt and comprised by an elastic body as a whole and a part of which is elastically deformed when the part receives said pump allowable hydraulic pressure.

5. The hydraulic pump unit provided with the hydraulic pump in any one of clams 1 to 4.

6. A hydraulic driving unit in which at least an electric motor for driving said hydraulic pump and a hydraulic actuator driven by a hydraulic oil from said hydraulic pump are further integrated to the hydraulic pump unit in claim 4.

7. A hydraulic pump, having a self-relief function, used in a hydraulic pump unit with a construction that at least a hydraulic pump and a hydraulic oil tank are integrated and a housing of said hydraulic pump is contained in the hydraulic oil tank, wherein said housing is provided with a body side housing and a lid side housing divided by a surface perpendicular to a rotary shaft of the hydraulic pump, this lid-side housing is brought into contact with the body-side housing in the oil tight manner with a predetermined pressure corresponding to a pump allowable hydraulic pressure, wherein said oil-tight contact with said predetermined pressure is realized by a spacer fitted between a mounting hole of said lid-side housing and a fastening bolt so as to achieve fastening and fixing of the fastening bolt to said body-side housing and a spring interposed between an underneck of the fastening bolt which has been fastened and fixed and said lid-side housing for generating said predetermined pressure.

8. A hydraulic pump, having a self-relief function, used in a hydraulic pump unit with a construction that at least a hydraulic pump and a hydraulic oil tank are integrated and a housing of said hydraulic pump is contained in the hydraulic oil tank, wherein said housing is provided with a body side housing and a lid side housing divided by a surface perpendicular to a rotary shaft of the hydraulic pump, this lid-side housing is brought into contact with the body-side housing in the oil tight manner with a predetermined pressure corresponding to a pump allowable hydraulic pressure, wherein said oil-tight contact with said predetermined pressure is realized by a frame body fixed to said body-side housing containing said lid-side housing, wherein said lid-side housing is capable of sliding in a direction perpendicular to said divided surface to allow oil leakage, and a spring interposed between the frame body and said lid-side housing for generating said predetermined pressure.

9. A hydraulic pump, having a self-relief function, used in a hydraulic pump unit with a construction that at least a hydraulic pump and a hydraulic oil tank are integrated and a housing of said hydraulic pump is contained in the hydraulic oil tank, wherein said housing is provided with a body side housing and a lid side housing divided by a surface perpendicular to a rotary shaft of the hydraulic pump, this lid-side housing is brought into contact with the body-side housing in the oil tight manner with a predetermined pressure corresponding to a pump allowable hydraulic pressure, wherein said oil-tight contact with said predetermined pressure is realized by a lid-side housing constituted so that it is fixed to said body-side housing with a fastening bolt and comprised by an elastic body as a whole and a part of which is elastically deformed when the part receives said pump allowable hydraulic pressure.