Safety valve

A safety valve of the type that includes a sleeve (1) with a high pressure port (11) and a low pressure port (12); a valve seat (2) and a poppet (3), both slidably inserted in the sleeve and adapted to be brought together to provide a valve interface that is capable of establishing and blocking a fluid communication between the high pressure port and the low pressure port; and a spring (7) adapted to force the poppet against the valve seat, wherein a difference in force between a fluid pressurized in the high pressure port acting on a pressure receiving surface of the poppet and the spring acting against said pressurized fluid urges the poppet to open the valve interface, thereby relieving the high pressure port by a portion of the pressurized fluid admitted out thereof into the low pressure port. The safety valve comprises a damper chamber (A) formed between the sleeve and the valve seat mentioned above, whose volume is reduced by a sliding movement of the valve seat against the action of the above mentioned spring; and a constriction adapted to bring the damper chamber into fluid communication with a low pressure side, the constriction (35) having an area of the fluid communication that is variable in such a way that it may be reduced by a sliding movement of the valve seat against the action of the spring and that it may be increased by a sliding movement in an opposite direction of the valve seat under the action of the spring.

TECHNICAL FIELD 
The present invention relates to a safety valve that is adapted for use in 
a hydraulic circuit such as those for supplying a discharge pressure fluid 
from a hydraulic pump into a hydraulic motor. 
BACKGROUND ART 
There has been known in the art a hydraulic circuit for supplying a 
discharge pressure fluid of a hydraulic pump into a hydraulic motor, in 
which the discharge pressure fluid of the hydraulic pump is supplied via 
an operating valve into one of a pair of principal circuits, of which a 
first is connected to a first port of the hydraulic motor and a second is 
connected to a second port of the hydraulic motor. 
In a hydraulic circuit of this type, it is customary to use a safety valve 
of modulation type that is designed to make a relief of a portion of fluid 
of pressure elevated in the first and second principal circuits to ensure 
that the elevated pressure may not exceed a pre-established pressure, so 
that when the hydraulic motor is either to start or to stop driving, fluid 
pressure in the first or second principal circuit which is active may 
slowly be elevated in order to diminish a shock that can then be 
disadvantageously brought about therein. Such a safety valve as known in 
the art, that is capable of performing a modulation operation in which 
fluid pressure may rapidly be elevated up to a modulation start pressure 
and after then slowly be elevated up to a preset pressure, is shown in 
FIG. 1 of the drawings attached hereto. 
More specifically, the safety valve has a valve seat 2 slidably inserted in 
a sleeve 1 in which a poppet 3 is also slidably inserted so that a concave 
conical surface 5 of the poppet 3 may be thrusted by a spring 4 against a 
convex conical surface 6 of the valve seat 2 to provide a valve interface. 
A piston 7 is slidably inserted in the poppet 3 to form a chamber 8 which 
is in fluid communication via a small opening 9 and via an inner bore 10 
of the valve seat 2 with a high pressure port 11 whereas a low pressure 
port 12 is formed in the sleeve 1. 
And, the sleeve 1 is formed at an end thereof with a large diameter section 
20 whose open end has a plug 21 securely fitted therewith. The valve seat 
2 is shaped to provide a stepped peripheral configuration having a small 
diameter one end portion 22, a large diameter mid portion 23 and an 
intermediate diameter opposite end portion 24. The small diameter one end 
portion 22 is inserted in the plug 21 to fit with an inner surface 21a 
thereof and is then sealed with a sealing material 25, whereas the large 
diameter mid portion 23 is inserted in the sleeve 1 to fit with the large 
diameter section 20 thereof and is then sealed with a sealing material 26. 
And, the intermediate diameter opposite end portion 24 is inserted in the 
sleeve 1 to fit with its inner surface 1a. Thus, the valve seat 2 is 
allowed to axially be slid and be displaceable by a distance L. Besides, 
the large diameter mid portion 23 and the sleeve 1 are shaped to provide 
an annular space 27 between the large diameter mid portion 23 and a step 
portion 1b of the sleeve 1. The annular space 27 is opening to the low 
pressure port 12 via an interstice 28 between the sleeve inner surface 1a 
and the intermediate opposite end portion 24, which is here designed to 
form a damper chamber A. 
Let it be assumed that the section in which the valve surface 5 and the 
seating surface 6 are to be in contact with each other has a diameter d1, 
the piston 7 has a diameter d2, and the small diameter one end portion 22 
of the valve seat 2 has a diameter d3. 
An explanation is now given below of an operation of the conventional 
safety valve whose construction has been described above. 
Assuming that the high pressure port 11 has a pressure that ranges between 
P1 and 0, the spring 4 forces the poppet 3 which in turn pushes the valve 
seat 2 to cause it to move by the distance L1 leftwards in the Figure to 
contact with the plug 21. Here, it should be noted that the spring load F1 
of the spring 4 is set at a value that is lower than a conventional value 
by L1.times.K where K is a spring constant. 
Under the state described, if the pressure P1 in the high pressure port 11 
is suddenly elevated, the poppet 3 starts sliding rightwards when a thrust 
force due to the pressure P1 acting on the pressure receiving surface of 
the poppet 3 with an area A1=.pi./4.multidot.(d.sub.1.sup.2 
-d.sub.2.sup.2) is balanced with the spring load F0 and then causes a 
section between the valve surface 5 and the seating surface 6, that is, 
the valve interface to open, thereby permitting a fluid of the elevated 
pressure to commence being admitted into the low pressure port 12, thus 
relieving the high pressure port 11. The pressure at this instant is a 
modulation start pressure. 
Then, with the elevated fluid pressure P1 acting on the end surface 2a of 
an area A2=.pi./4.multidot.(d.sub.3.sup.2 -d.sub.1.sup.2) of the valve 
seat 2, the valve seat 2 is placed under a thrust force=A2.times.P1 to 
tend to move rightwards. However, also under the action of the damper 
chamber A, that is, the action in which pressure fluid in the annular 
space 27 is restricted in its flow by the interstice 28, flowing out of it 
and gradually into the low pressure port 12, the valve seat 2 must be more 
slow to move rightwards than the poppet 3. As a consequence, the pressure 
rises with a reduced slope towards arriving at a preset pressure when the 
seating surface 6 is urged to contact the valve surface 5. 
The pressure of fluid in the high pressure port 11 under such a relief 
action will thus be to assume a waveform as represented by the solid curve 
shown in FIG. 2, rising in two stages with a long time elapsed until the 
preset pressure is reached. 
With such a safety valve, it is therefore seen that a drop of pressure in 
the high pressure port 11 while it is under a relief action causes the 
poppet 3 and the valve seat 2 to move leftwards with the spring 4, 
returning to their initial positions. As the valve seat 2 is moved 
leftwards, fluid in the low pressure port 12 is forced to flow via the 
interstice 28, sucked into the damper chamber A. 
However, since the interstice 28 that must be small and minimum in order to 
retard the rightward movement of the valve seat 2 impedes the fluid 
flowing that is sucking into the damper chamber A, the fluid fails to be 
sucked into the damper chamber A to an extent that is proportionate to the 
rate at which the valve seat 2 is moving leftwards via the poppet 3 with 
the spring 4. A negative pressure then is caused in the damper chamber A 
and gives rise to the formation of air bubbles therein. 
The rightward movement of the valve seat 2 produced by rising of pressure 
again in the high pressure port 11 while air bubbles are formed in the 
damper chamber A causes the air bubbles to tend to be broken in the damper 
chamber A, allowing the valve seat 2 to be displaced rightwards quicker 
than at a rate that is solely governed by the flow restrictive action of 
the interstice 28. It follows then that a desired length of the time of 
modulation (i. e., the time elapsed from the instant at which the pressure 
is at a modulation start pressure until an instant at which the pressure 
reaches a preset pressure) cannot be achieved. 
It can also be seen that since the damper chamber A is in fluid 
communication with the low pressure port 12 via the interstice 28 between 
the inner surface 1a of the sleeve 1 and the intermediate diameter 
opposite end portion 24 of the valve seat 2, a change in the diameter of 
either of the sleeve 1 and the valve seat 2 as caused by a change in 
temperature alters the size of the interstice 28. The size of the 
interstice 28 also fluctuates when the sleeve 1 or the valve seat 2 is 
eccentric. Such a change or fluctuation in the size of the interstice 28 
bars a stabilized time span of modulation from being obtained. 
As observed in the foregoing description, it has hitherto been recognized 
in the art to be difficult to adjust the time of modulation to be 
sufficiently long while the interstice 28 is kept minimum, which in turn 
makes it difficult to significantly diminish a shock that is entailed in a 
hydraulic motor when it is to start or end driving. 
It is accordingly an object of the present invention to provide a safety 
valve which can overcome the problems mentioned above. 
SUMMARY OF THE INVENTION 
In order to achieve the above mentioned object, there is provided in 
accordance with the present invention in a certain aspect thereof a safety 
valve that includes: 
a sleeve having a high pressure port and a low pressure port; 
a valve seat and a poppet, both slidably inserted in the sleeve and adapted 
to be brought together to provide a valve interface that is capable of 
establishing and blocking a fluid communication between the high pressure 
port and the low pressure port; and 
a spring adapted to force the poppet against the valve seat, 
wherein a difference in force between a fluid pressurized in the high 
pressure port acting on a pressure receiving surface of the poppet and the 
spring acting against the pressurized fluid urges the poppet to open the 
valve interface, thereby relieving the high pressure port by a portion of 
the pressurized fluid admitted out thereof into the low pressure port, 
and is characterized in that 
the safety valve comprises: 
a damper chamber formed between the said sleeve and the said valve seat and 
having a volume thereof, wherein a sliding movement of the said valve seat 
against the action of the said spring reduces the volume of the said 
damper chamber; and 
a constriction adapted to bring the said damper chamber into fluid 
communication with a reduced pressure side, the said constriction having 
an area of the fluid communication that is variable in such a way that it 
may be reduced by a sliding movement of the said valve seat against the 
action of the said spring and that it may be increased by a sliding 
movement in an opposite direction of the said valve seat under the action 
of the said spring. 
According to the construction mentioned above, it can be noted and should 
be understood that the constriction which permits fluid in the damper 
chamber to flow out of it and to a reduced pressure side and of which the 
area of fluid communication is reduced when the valve seat is slid under a 
pressure elevated in the high pressure port against the spring action is 
allowed to slide so slowly, thus causing it to take longer time for the 
pressure to rise from a relief start pressure up to a preset pressure, 
hence a prolonged time of the modulation that ensues. 
Also, it can be seen and should be appreciated that the constriction 
through which fluid in the reduced pressure side is sucked into the damper 
chamber and whose area of communication is increased when the valve seat 
is slid under the spring force with the pressure reduced in the high 
pressure port allows the fluid in the reduced pressure side to flow into 
the damper chamber smoothly, thereby preventing the damper chamber from 
becoming negative in pressure inside and from producing air bubbles 
therein. 
This provides a pre-established time of modulation that results if the 
safety valve is brought iteratively into a relief action. Consequently, 
this safety valve, used in a hydraulic circuit for supplying a pressurized 
fluid into a hydraulic motor, significantly diminishes a shock which 
occurs when the motor starts and stops driving. 
It is yet possible to further prolong, without any inconvenience suffered, 
the time of modulation in which the valve seat is even more slowed in its 
sliding movement with the constriction having the area of fluid 
communication further reduced when the valve seat is slid against the 
spring action. But, even so the constriction can have its area of fluid 
communication enlarged when the valve seat is to be slid under the spring 
force, without regard to how far the area of fluid communication was then 
reduced. 
By virtue of the fact that a further longer time of modulation does not 
allow air bubbles to be generated in the damper chamber, this safety valve 
if operated with a repeated relief action does not fail to effectively 
operate in any prolonged time of modulation that is necessary and 
sufficient. Therefore, the safety valve according to the present invention 
is adapted for use in a hydraulic circuit for supplying a pressure fluid 
into a hydraulic motor while attaining significant reduction of a shock 
occurring when the hydraulic motor is starting or stopping a drive. 
In the construction described above, it is desirable that the said 
constriction comprise: 
a stepped bore having a small diameter section and a large diameter 
section; 
a ball adapted to be fitted in the said stepped bore; and 
a spring, which is separate from the first mentioned spring, for thrusting 
the said ball against the said small diameter section, 
wherein the said small diameter section is in fluid communication with the 
said reduced pressure side and the said large diameter section is in fluid 
communication with the said damper chamber. 
According to the construction mentioned above, it can be noted and should 
be understood that an area of fluid communication as referred to is 
established by the diameter of the stepped bore and the diameter of the 
ball. Thus, a constriction with such an area of fluid communication as so 
established becomes analogous to a sharp edge orifice, presenting no 
change in constriction property (i. e. area of fluid communication) 
against a change in temperature. Hence, a stabilized time span of 
modulation is provided. 
In addition to the constructive elements mentioned above, the safety valve 
according to the present invention may include a piston that is inserted 
in the said poppet to provide a chamber, a narrow opening that is formed 
in the said poppet and is in fluid communication with the said high 
pressure port, and a throughgoing bore that is formed in the said valve 
seat.

BEST MODES FOR CARRYING OUT THE INVENTION 
Hereinafter, suitable embodiments of the present invention with respect to 
a safety valve are set forth with reference to the accompanying drawings 
hereof. 
An explanation is given of an embodiment of the present invention based on 
FIG. 3. It should be noted that the same components as those in the prior 
art are designated by the same reference characters and a further 
explanation of details thereof are here omitted. 
A plug 21 is interiorly formed with a bore having a small diameter section 
30 and a large diameter section 31. The plug 21 is fitted with an end of 
the sleeve 1 to fit over an outer peripheral surface 1c thereof in the 
large diameter bore section 31. The small diameter one end portion 22 of 
the valve seat 2 formed at one end thereof as mentioned previously is 
fitted with the small diameter section 30 of the plug 21 interiorly 
thereof and is sealed thereto with a sealing material 32. The large 
diameter mid portion 23 of the valve seat 2 is fitted with the large 
diameter section 31 of the plug 21 interiorly thereof and is sealed 
thereto with a sealing material 33. The intermediate diameter opposite end 
portion 24 formed at the other end thereof is fitted with the inner 
peripheral surface 1a of the sleeve 1 and is sealed thereto with a sealing 
material 34. 
A damper chamber A which is constituted by an annular space 27 is here 
formed by the large diameter section 31 of the plug 21, an end surface 1d 
of the sleeve 1 and the intermediate diameter opposite end portion 24 of 
the valve seat 2 together so as to be in fluid communication via a 
constriction 35 with an auxiliary low pressure port 36 that is formed in 
the small diameter bore section 30 of the plug 21. 
Referring to FIG. 4, an explanation is given of a detailed structure of the 
constriction 35. 
The large diameter mid portion 23 of the valve seat 2 is formed with an 
annular groove 40, and also includes a round portion 41 lying closer to 
the damper chamber A than the annular groove 40, which is smaller in 
diameter than the large diameter section 31 of the plug 21. An annular 
slit 42 is formed between the round portion 41 of the valve seat 2 and the 
large diameter section 31 of the plug 2 to provide a fluid communication 
between the damper chamber and the annular groove 40. The annular slit 42 
is so much of size that it may not affect the time of modulation. 
The annular groove 40 is formed with a stepped bore 45 having a small 
diameter section 43 and a large diameter section 44 and has a ball 46 
fitted therein. In the stepped bore 45, the small diameter section 43 has 
a diameter D1, the large diameter section 44 has a diameter D2, and the 
ball 46 has a diameter D3 which is a bit smaller than the diameter D1 of 
the small diameter section 43. Further, the small diameter section 43 has 
a depth L2 which is a bit greater than one half (1/2) of the diameter D3 
of the ball 46. 
A spring 47 is also provided that is ring shaped and wound fittedly in the 
annular groove 40 to force the ball 46 against the bottom wall of the 
stepped bore 45. A fluid bore 48 is further formed which provides a fluid 
communication of the small diameter section 43 in the stepped bore 45 with 
the auxiliary low pressure port 36 via a space 49. The space 49 lies 
between one end surface of the large diameter mid portion 23 of the valve 
seat 2 and the large diameter bore section 31 of the plug 21 and always 
communicates with the auxiliary low pressure port 36 via an inclined 
surface 23a of the large diameter mid portion 23. 
An explanation is next given of the operation of the embodiment described. 
When the pressure in the high pressure port 11 rises to a modulation 
commencing pressure, in the same manner as with the conventional safety 
valve previously described, the poppet 3 is displaced rightwards against 
the spring action of the spring 4 to commence relieving the high pressure 
port 11 by pressure fluid admitted out thereof into the low pressure port 
12. Thereafter, the valve seat 2 is displaced rightwards under the 
pressure in the high pressure port 11 to allow fluid in the damper chamber 
A to flow through the annular slit 42, the annular groove 40, an 
interstice between the large diameter section 44 and the ball 46 and an 
interstice between the small diameter section 43 and the ball 46 into the 
fluid bore 48 and then through the latter and the space 49 into the 
auxiliary low pressure port 36. 
The pressure fluid that flows out of the damper chamber A towards the 
auxiliary low pressure port 36, in the course of passing through a small 
interstice provided by the small diameter section 43 and the ball 46 that 
serves as a restrictor or constrictor, is restricted or constricted in 
flow with less fluid flow admitted therethrough and out thereof. In other 
words, a small difference in diameter between the small diameter section 
43 and the ball 46 provides a reduced area of fluid communication between 
the damper chamber A and the auxiliary low pressure port 36, hence a 
reduced flow of fluid that is admitted into the auxiliary low pressure 
port 36 from the damper chamber A. This causes the valve seat 2 to be 
displaced rightwards at a reduced speed, which in turn causes the pressure 
to rise at a reduced rate up to a preset pressure. Hence, a prolonged time 
of modulation ensues. 
Specifically, a flow constriction as described above is provided by an 
interstice formed between a circular rim defined by the ball 46 cut by a 
horizontal plane at a height of its radius as shown and the wall of the 
small diameter orifice 43. Thus, such a constriction is considered to be 
essentially equivalent to one in a sharp edge orifice in general, there 
being no significant change in constrictive characteristics with respect 
to a change in temperature. It should also be noted that the pressurized 
fluid passing over the ball 46 to allow it to be rotated serves 
effectively to expel a foreign matter that may possibly have been 
entrapped in the interstice. 
It can also be seen that if the pressure in the high pressure port 11 is 
reduced, the valve seat 2 is displaced leftwards under the action of the 
spring 4. Fluid is then caused to flow through the low pressure port 36, 
the space 49 and the fluid bore 48 into the small diameter section 43 and 
then, pushing up the ball 46, to pass through a large interstice created 
between the large diameter section 44 and the ball 46, flowing into the 
annular groove 40 and through the latter and the annular slit 42 into the 
damper chamber A. 
The enlarged interstice in this case in which fluid flows in the opposite 
direction provides a larger area of fluid communication between the low 
pressure port 36 and the damper chamber A and allows the fluid much more 
to flow into the damper chamber A without restriction, permitting the seat 
valve 2 to quickly return to its initial state. Thus, the inside of the 
damper chamber A is effectively prevented from becoming negative in 
pressure and hence is freed from a formation of bubbles therein. 
There is thus provided a safety valve which may be used in a hydraulic 
circuit for supplying a hydraulic motor with a pressure fluid whereby a 
shock occurring when the motor is to start or stop driving is markedly 
diminished. 
While the present invention has hereinbefore been set forth with respect to 
certain illustrative embodiments thereof, it will readily be appreciated 
by a person skilled in the art to be obvious that many alterations 
thereof, omissions therefrom and additions thereto can be made without 
departing from the essence and the scope of the present invention. 
Accordingly, it should be understood that the present invention is not 
intended to be limited to the specific embodiments thereof set out above, 
but to include all possible embodiments thereof that can be made within 
the scope with respect to the features specifically set forth in the 
appended claims and encompasses all the equivalents thereof.