JPS597034B2 - Exhaust Pump - Google Patents

Description

[Detailed description of the invention] The present invention relates to improvements in variable discharge hydraulic pumps.

As shown in FIGS. 1 and 2, in a conventional pump having a valve plate, several pistons 3 are inserted into a cylinder block 2 fixed to a main shaft 1, and when the main shaft 1' is rotated, The piston 3 moves back and forth once every 101 revolutions of the main shaft within the cylinder 5' of the cylinder block 2' while being in pressure contact with the swash plate 4'.

Now, if the main shaft 1 rotates in the direction of arrow A in the figure, the piston 3 moves in the left half of the cylinder block 2' when viewed from the main shaft end in the direction of coming out of the cylinder 5' (suction stroke), and in the right half moves out of the cylinder. In addition, the valve plate 6' has ports 7 that open in a semicircular arc shape on the left and right sides of the cylinder block 2' where it slides into the cylinder block 2'. ' and 8', and if the main shaft 1 rotates clockwise as described above, one piston 3' moves in the direction of coming out of the cylinder 5' and sucks liquid from the left port 7', and the other piston 3'' is the cylinder 5
8', and the liquid is discharged from the right side port 8'.

Further, when changing the discharge amount, the stroke of the piston 3' is changed by changing the inclination of the swash plate 4' supported by the pump casing 9', thereby changing the discharge amount.

Furthermore, if the inclination angle of the swash plate is fixed in use, it goes without saying that a constant discharge can be achieved.

In the above-mentioned hydraulic pump, a variable stroke and variable discharge mechanism can be easily realized by changing the inclination angle of the swash plate 4', but there is a drawback that noise and vibration are generally large.

As a result of analyzing the cause of the above noise, the main cause was the piston chamber 10 partitioned by the piston 3' inside the cylinder 5'.
At the moment when the liquid is connected to the port 8 on the discharge side (high pressure side), a backflow of liquid occurs from the discharge port 8 to the piston chamber 10 side, resulting in fluctuations in the discharge flow rate, accompanying pressure fluctuations, and stress due to sudden changes in the cylinder internal pressure. The source was found to be vibrational force.

The above points will be explained in detail with reference to FIGS. 3 and 4.

Figure 3 shows the discharge stroke of the pump divided into four stages.The first step (■) is when the histone 3 is at the bottom dead center position, and the second step (■) is when the cylinder block 2 has rotated slightly and the piston is at the bottom dead center position. At the moment when the chamber 10 is connected to the discharge port 8 of the valve plate 6, the third step is the cylinder block 2' rotating further and the piston 3 moving forward, causing the internal pressure of the piston chamber 10 to become higher than the pressure of the discharge port 8. The fourth step M shows a state in which the liquid has begun to be discharged, and the piston 3 has finished its discharge stroke and is located at the top dead center.

In the above, the second step is n'lJ-problem.

In other words, the piston chamber 10', whose internal pressure is close to zero immediately after the suction stroke, suddenly becomes high pressure with an internal pressure of 100 V4/cd or more and is connected to the discharge port 8' of the valve plate 6'. At the moment of connection, a portion of the liquid suddenly flows back from the discharge port 8' toward the piston chamber 10' due to the pressure difference.

Although the absolute value of this amount of backflow liquid is not very large, since the backflow occurs within an extremely short period of time, the amount of liquid moved per unit time is extremely large. Extremely large.

Another cause of noise is the sudden drop in pressure during the suction stroke.

This kind of inhalation. In order to avoid sudden changes in pressure during the discharge stroke and to reduce noise through gradual pressure fluctuations, the fourth
As shown in Figure A, the suction port 7 and the discharge port 8 of the valve plate 6
is asymmetric about the axis, and the piston performs confining compression in the precompression angle θC section and confining expansion in the preexpansion θe section.

However, it is not clear to what extent the precompression angle θC should be set to prevent the backflow of the liquid, and even if the precompression angle θC is set, when the pressure inside the piston chamber 10 is actually measured, as shown in Figure 4. As shown in the figure, a sudden change in pressure immediately after the piston chamber 10 and the discharge port 8 communicate with each other, that is, an impact pressure P is measured.

Further, the noise on the suction side is good when the discharge pressure is constant, but when the discharge pressure changes, it becomes as shown in FIG. 1, 110.

Figure 1 shows the case when the discharge pressure is low, and Figure 11 shows the case when the discharge pressure is high.

In other words, as shown in Figure 4, a constant discharge pressure is maintained at Pd.
If the discharge pressure is low, the internal pressure of the piston chamber 10' in figure i drops to atmospheric pressure at point a in the middle of the pre-expansion section θe, and vacuum is generated in the section from point a to point b due to overexpansion. This leaves an adverse effect such as bubble generation in the piston chamber 10.

In addition, when the discharge pressure is high, the drop in the internal pressure of the piston chamber 1σ is not completed at point C' at the end of the pre-expansion period in figure ii, so the internal pressure of the piston chamber 10' drops rapidly from point C' to point d'. It has little effect in reducing noise.

As mentioned above, the hydraulic pump with the valve plate shown in Figure 4 is only effective at a certain discharge pressure, and furthermore, it is difficult to secure a sufficient pre-expansion section due to the danger of vacuum, etc. The expansion angle θe could only be designed to be about 10 to 20 degrees.

Furthermore, in the pump shown in FIG. 1, the suction hole 11 and discharge hole 12 of the cylinder are bent, so that thrust is applied to the cylinder shaft 2' by the hydraulic pressure of the suction hole 11 and discharge hole 12. This causes vibration of rotating members such as the cylinder block, and since the valve plate 6 is in direct pressure contact with the cylinder block 2, when the pressure in the piston chamber 10 increases, the valve plate 6 and the cylinder block However, since the hydraulic pressure in the piston chamber 10 is fluctuating while being compressed and pressurized, it is extremely difficult to balance the static pressure with the valve disk 6, and this causes vibrations. was induced, causing noise.

Due to the combination of the above-mentioned causes of noise, conventional variable discharge hydraulic pumps generate high noise of nearly 100 phons, which seriously impedes the working environment.

The present invention eliminates the noise causes of conventional pumps as described above,
The purpose is to provide a pump with low noise.

The features of the present invention include a valve plate fixed to the casing, a main shaft rotatably supported by the casing,
A cylinder block is fixed to the main shaft and rotates in sliding contact with the valve plate, and each cylinder block is inserted and built into a number of cylinders bored in the cylinder block to perform the discharge stroke and suction stroke alternately. I was forced to move back and forth between the
In a variable discharge hydraulic pump of the type having a piston that completes its discharge stroke at top dead center and a swash plate on which one end of each piston slides, the suction side semi-circular shape provided on the valve plate a slipper pad inserted into the piston chamber, pressed against the valve plate by the piston chamber pressure, and communicating with the suction side semicircular port during the suction stroke;
A discharge-only port opened in the direction of the rotation center of the cylinder block from each cylinder drilled in the cylinder block, and a discharge hole provided in the main shaft so as to communicate the discharge-only port with a discharge port provided in the casing. Then, when the piston chamber pressure is higher than the discharge pressure during the discharge stroke, a check valve built into the discharge-only port opens and the piston moves forward against the valve plate or the swash plate on which one end of the piston slides. and a depressurizing orifice provided at a position before reaching the top dead center, and the slichahad inserted into the piston chamber is pressed against the valve plate by the piston chamber pressure,
Since it communicates with the suction-side semicircular port during the suction stroke, the balance can be achieved automatically, and vibrations are not induced.

The check valve built into the discharge-only port prevents fluid from flowing into the piston chamber from the discharge side and backflow, eliminating noise caused by sudden changes in pressure within the piston chamber and preventing valve opening/closing sounds from leaking to the outside. .

In addition, the discharge hole provided in the main shaft allows the closing direction of the check valve to be placed in the same direction as the centrifugal force caused by the rotation of the cylinder block, simplifying the structure of the check valve, improving durability, and reducing costs. reduce

Furthermore, since the pressure is reduced by the pressure reduction orifice at a position that does not reach the top dead center, a gradual pressure drop is achieved and the occurrence of cavitation at low pressures is also eliminated.

In this way, noise reduction is achieved.

Hereinafter, one embodiment of the present invention will be described with reference to FIG. 6. 1 is a main shaft, 2 is a cylinder hook fixed to the main shaft 1, 3 is a plurality of cylinders bored in the cylinder block 2, and 4 is each cylinder. 3 is a piston inserted, 5 is a swash plate 7 to which one end of the piston 4 is attached via a retainer 6, a valve plate is fixed to a casing 8 by a pin 9, 10 is a piston chamber, and 11 is a valve plate. A slipper pad is inserted into each piston chamber 10 and is pressed and slid against the valve plate 7 by a retainer ring 12 and a spring 13, 15 is a discharge groove provided around the main shaft 1 of the cylinder block 2, and 14 is a discharge groove. 15 and each piston chamber 10, a discharge-only port is bored in the direction of the rotation center of the cylinder block 20,
16 is a check valve provided in the discharge-only port 14, 17 is a discharge hole provided in the main shaft 1, and 18 is a sealing device provided at one end of the main shaft 1, to which a seal ring 19 is fixed.

20 is a discharge port provided in the casing 8 so as to communicate with the discharge hole 11, 21 is a suction port provided in the casing 8, and 22 is a semicircular-arc shaped suction provided in the valve plate 7. The dedicated port 23 is a pressure reducing (oil release) orifice installed in the valve plate 7, and the orifice 23 is installed at a position before the top dead center of the piston, and extends to a position slightly beyond the top dead center. It's growing.

Reference numeral 24 is an oil release hole that communicates the orifice 23 with the inside of the casing 25.

In such a device, when the main shaft 1 is rotated, the piston 4 descends during the suction stroke, and the liquid flows from the suction port 21 to the suction side semicircular port 22 provided in the valve plate 7, as shown by arrow a. It is sucked into the piston chamber 10 via the slip pad 11.

On the other hand, when the discharge stroke begins, the suction port 22 is closed during the discharge stroke, the piston 4 rises, and when the pressure inside the piston chamber 10 becomes greater than the discharge pressure, the check valve 16 opens and the liquid is discharged from the discharge-only port 14. It is discharged from the discharge port 20 through the groove 15 and the discharge hole 17 as shown by the arrow b.

According to such a device, rapid backflow of liquid from the discharge port to the piston chamber can be prevented.

FIG. 7 shows a developed view of the cylinder during one revolution, and shows the delivery process divided into four steps.

The first step ■ is a state in which the piston 4 is at the bottom dead center position,
In the second step ■, the cylinder block 2 is slightly rotated, and in the third step ■, the cylinder block 2 further rotates, and as the piston 4 moves forward, the internal pressure of the piston chamber 10 becomes higher than the internal pressure of the discharge groove 15, and the cylinder block 2 is rotated slightly. A state in which liquid has begun to be discharged through the port 14, and the fourth step (3) indicates a state in which the piston 4 has further advanced.

Although there is no difference between the pump of the present invention and the conventional pump in the first step (3), there is a difference in the second step (H).

That is, the discharge port of the piston chamber is the high pressure side port of the valve plate,
That is, at the moment of connection to the discharge side semicircular port, in the conventional pump structure shown in FIGS. 1 and 2, a backflow of liquid from the discharge side semicircular port 8' to the piston chamber 101 occurs. This causes noise, but in this embodiment, since the check valve 16 is provided in the discharge-only port 14, the check valve 16 closes due to the pressure difference, and the liquid piston chamber 10 It is designed to completely prevent backflow.

While the check valve 16 is closed, the outlet of the liquid in the piston chamber 10 is blocked, so when the cylinder block 2 further rotates in this state, the piston 4 moves forward. Since the liquid in the piston chamber 10 is pressurized by
When the pressure of the liquid gradually increases and reaches the position of the third step (3), it finally becomes higher than the internal pressure of the discharge groove 15.

At that moment, the check valve 16 is pushed open and the pump starts discharging liquid, and as a result, the pressure within the piston chamber 10 no longer changes rapidly as indicated by P in FIG.

(Note that θcv is the precompression angle.

) When the piston 4 further advances and comes to point D shown in FIG. 71, the piston chamber 10 communicates with the pressure reducing orifice 23,
If the amount of oil escaping from the orifice 23 to the inside of the casing 25 through the oil relief hole 24 is greater than the amount of oil discharged due to the forward movement of the piston 4, the pressure inside the piston chamber 10 becomes less than the discharge pressure, so the check valve 16 immediately closes and the piston chamber closes. 10 internal pressure starts to reduce.

However, the piston 4 moves forward from point D to top dead center, and the pressure drop is gradual to compensate for the amount of oil escaping from the orifice 23, and when it reaches point F, the piston chamber 10 closes to the valve. It communicates with the suction side port 22 of the panel 7.

Therefore, the pressure reduction section is from point D to point F as shown in FIG. 7, and the pressure reduction angle .theta.d can be maintained at 40 to 50 degrees, which enables a gradual pressure drop and contributes to a reduction in noise.

Further, a decompression pump employing a depressurization method using a decompression orifice 23 as in the present invention is effective even when the discharge pressure changes as shown in FIG.

That is, as shown in FIG. 9A, when the discharge pressure is low, the amount of escape from the orifice 23 is small, so the pressure reduction effect is small, and the rate of decrease in the internal pressure of the piston chamber 10 is slow (as shown by curve P1).

When the discharge pressure is high, the opposite is true, as shown by curve P2 in diagram B (the descending speed is fast).

In any case, it is gentler than the curve P" in FIG. 5, which shows the conventional case without pressure reducing means.

In addition, problems such as cavitation at low pressure due to normal confinement expansion and sudden drop in pressure at high pressure are greatly alleviated.

In the above embodiment, the pressure reducing orifice 23 is provided on the valve plate 7, but it may be provided on the swash plate 5 on which the piston 4 slides.

As described above, in the present invention, the pressure fluctuations during discharge and suction are reduced by the check valve and the pressure reducing orifice, so the noise can be significantly reduced. All you have to do is improve it.

In other words, when the check valve 16 is used as a discharge valve of a hydraulic pump, it opens and closes several tens of times per second, so when the check valve 16 opens and closes, it causes abnormal vibrations in the poppet and generates abnormal noise, which increases noise and damages the equipment. Adversely affects durability.

To alleviate this, an oil reservoir 164 is provided in the spring receiver 162 or valve seat 163 around the poppet 161 of the check valve 16, as shown in FIGS. 10 and 11, and an orifice 165 is provided in the oil reservoir 164. All you have to do is set it up.

Note that 166 is a spring.

That is, when the poppet 161 tries to vibrate at a high frequency, it compresses the oil in the oil reservoir 164, and the oil reservoir 164 is a closed room except for the orifice 165, thereby preventing rapid movement.

Therefore, by appropriately selecting the diameter of the orifice 165, normal opening and closing will not be hindered at all, and abnormal vibrations can be prevented and noise can be reduced.

Note that 166 is a spring, and various shapes of this check valve can be considered.

As specifically explained above, the present invention includes a valve plate fixed to a casing, a main shaft rotatably supported by the casing, and a cylinder fixed to the main shaft and rotated while sliding on the valve plate. The block and the cylinder block are each inserted into a number of cylinders bored into the same block, and are made to reciprocate between top dead center and bottom dead center in order to perform the discharge stroke and suction stroke alternately. In a variable discharge hydraulic pump of the type having a piston that completes its discharge stroke at a time, and a swash plate on which one end of each piston slides, a suction side semicircular port provided in the valve plate; A slipper pad is inserted into the piston chamber, is pressed against the valve plate by the piston chamber pressure, and communicates with the suction side semicircular port during the suction stroke, and rotation of the cylinder block from each cylinder bored in the cylinder block. A discharge-only port opened toward the center, a discharge hole provided in the main shaft to communicate the discharge-only port with a discharge port provided in the casing, and a discharge hole provided in the main shaft to communicate with the discharge port provided in the casing. a check valve built into the discharge-only port so as to be opened; and a decompression orifice provided at a position before the piston advances and reaches at least the top dead center on the valve plate or the swash plate on which one end of the piston slides. A variable discharge, low-noise hydraulic pump is constructed with the It is possible to suppress the excitation force that causes noise due to sudden changes in the pressure inside the piston chamber.

(2) In addition, by installing the check valve that does not open until a certain pressure, the pressure inside the piston chamber is confined and compressed until it reaches the pressure on the discharge side (discharge pressure), and the pressure change is made gradual and mechanical The excitation force can be greatly improved.

Furthermore, by consciously increasing the piston chamber volume, it is possible to proactively lengthen the time required for the confinement compression stroke, thereby making the pressure change (discharge pressure) more gradual.

(3) Conventionally, there are hydraulic pumps in which the cylinder block is fixed and the swash plate is rotated to cause the piston to reciprocate, and this type of pump cannot have a valve plate due to its structure. It is necessary to have a discharge valve (check valve), and this type of pump generally has excellent characteristics with less noise and vibration than hydraulic pumps with a valve plate, but because the swash plate rotates, It is difficult to manipulate the oblique angle and it is difficult to make the discharge amount variable.

In this regard, the present invention provides a low-noise pump which is capable of variable stroke and variable discharge since the swash plate does not rotate in a pump having a valve plate, and which is almost equivalent to or even superior to the type of pump without a valve plate. I can do it.

(4) In conventional hydraulic pumps with valve discs, this problem occurs because the piston chamber is suddenly connected to the low-pressure suction-side semicircular port when the pump finishes the discharge stroke and moves on to the suction stroke. In order to avoid shock caused by a sudden drop in pressure in the piston chamber, the starting end of the semicircular port on the suction side is sometimes shortened and the liquid in the piston chamber is trapped and expanded in this short section. If gas is mixed in, the above-mentioned backflow phenomenon will be further promoted during the discharge stroke, making it difficult to ensure a sufficient confinement and expansion section.

That is, it could only be applied to a constant working pressure and constant discharge amount type.

In contrast, according to the present invention, even if gas is mixed into the piston chamber due to cavitation, during discharge, the piston chamber is confined and compressed until the pressure in the piston chamber becomes equal to the pressure on the discharge side, so the occurrence of abnormal noise due to cavitation is avoided. Since there is almost no effect, and the amount of discharged liquid is slightly affected, it is possible to provide a confinement expansion section at the top dead center to avoid shock caused by sudden pressure reduction.

:5) The closing direction of the check valve is in the same direction as the centrifugal force, which not only simplifies the design of the spring for the check valve, but also enables the structure of the check valve without using a spring, reducing cost and durability. There are great advantages in this respect.

:6) At the time of startup, the air in the cylinder gathers toward the center of the cylinder block 2 due to centrifugal force due to the difference in specific gravity. can be easily discharged, which is advantageous in terms of noise.

:7) Since both the discharge pressure and the centrifugal force act in a direction to prevent the check valve from loosening, the reliability of mounting and fixing the check valve becomes high.

(8) Also, since the check valve 16 is located inside the pump in a dedicated discharge port, the sound of opening and closing the valve will not leak to the outside.
《It is advantageous in terms of noise.

(9) In addition, in the present invention, a pressure reducing orifice 23 is provided on the valve plate or swash plate to release oil at a position just before the top dead center of the piston, so that while the piston is moving forward, this reduces the pressure at a position that is not suitable for the top dead center. Therefore, it is possible to bring about a gradual pressure drop of 400 or more in the effective depressurization section, eliminate the occurrence of cavitation at low pressure, and make it possible to reduce noise.

Note that, as shown in the embodiment, the slip pad 11 provided between the valve plate 7 and the cylinder 30 is directly pressed against the valve plate 7 by the cylinder internal pressure, so the force balance can be ideally achieved and vibrations are not induced. Noise is reduced because there is no risk of vibration or thrust being generated against the cylinder, and there is no risk of vibrational leakage.

The reason why the provision of the slip pad 11 between the valve disk I and the cylinder 3 makes the force balance ideal will be explained below.

Even if each piston chamber 10 has an independent internal pressure,
The slip pad 11 of each piston chamber is as shown in Fig. 12.
All you have to do is balance, so you can achieve balance.

For example, in a certain piston chamber 10, the piston diameter dp, slipper pad outer diameter d2, slipper pad inner diameter dl, and the repelling force and pressing force of the slipper pad are respectively F r + F
If a, then the inner diameter d1 of the slipper pad is
By appropriately determining the outer diameter d2, the hydraulic equilibrium ratio ε becomes ε=1 regardless of the internal pressure of the piston chamber, that is, the force balance becomes ideal.

Further, in this embodiment, each piston chamber has a slichhad, and since it is sufficient to balance each piston chamber, it is possible to achieve balance with a simple structure.

In addition, the pressure inside the piston in Fig. 1 is uniformly distributed perpendicular to the wall surface as shown in Fig. 13, and when this distributed load is offset, a thrust force remains.

In other words, it is very difficult to make both the thrust force and the bending moment zero.

In Fig. 13, the thrust force and bending moment in one piston chamber are described, but in reality, the number of pistons is n.
Although it was not possible to achieve perfect balance as it was necessary to dynamically balance each component, in the embodiment, the discharge-only port 14 was provided in the radial direction of the cylinder block 20, and the hole in the cylinder 3 was designed to pass straight through the cylinder block 2. Because of this, no thrust is applied to the cylinder block 2 even if the pressure in the piston chamber 10 rises, and therefore the rotating shaft portion rotates extremely smoothly, eliminating vibration factors.

Furthermore, by providing the check valve with an oil reservoir and an orifice to function as an oil damper, it is possible to prevent abnormal vibrations of the check valve, thereby reducing noise and improving the durability of the hydraulic pump.

By combining the above-mentioned low-noise elements, the noise level can be significantly lowered as shown in FIG.

In other words, a conventional hydraulic pump with a discharge rate of about 400 t/min has a discharge rate of 98 kg/cd at a normal pressure of 210 kg/cd.
The pump produced an extremely high-pitched sound of up to 99 phons, but when it exceeded 90 phons, the level of noise that could be perceived by the human ear became extremely loud, making the work environment so bad that it was impossible to talk near the pump.

However, in the example, the noise level is as shown in curve a in Fig. 14, and even at a normal pressure of 210K9/crA, the noise level is 86~86.
At around 87 phons, it can be lowered by more than 10 phons than conventional pumps.

Furthermore, in the present invention, if an oil reservoir is provided in the check valve as shown in FIGS. 10 and 11, the noise level will be further lowered by 2 to 3 phons as shown in the b curve, and the sound level will be extremely low below 85 phons. This results in a low-temperature pump, which is extremely useful for noise prevention.

[Brief explanation of drawings]

Figure 1 is a vertical cross-sectional view of a conventional variable discharge type hydraulic pump;
The figure is a view from arrow 1 in Figure 1, Figure 3 is an explanatory diagram showing the state of the discharge stroke of the conventional hydraulic pump shown in Figures 1 and 2, and Figure 4 A is a view of the conventional hydraulic pump. The end view of the valve disk is shown in Figure 5, and the piston chamber pressure diagram when the valve disk is used is shown in Figure 5. Figure 5 shows the piston chamber pressure state when the discharge pressure changes in a conventional pump. is when the discharge pressure is low,
11 is a curve diagram when the height is high, FIG. 6 is a vertical cross-sectional view of a variable discharge type low-noise hydraulic pump showing an embodiment of the present invention, and FIG. 7 is a diagram when the cylinder of the pump shown in FIG. 6 is making one revolution. Developed view showing the state, A in FIG. 8 is an end view of the valve plate of the pump shown in FIG. 6, B is a curve diagram showing the state of the piston chamber pressure during pre-expansion, and FIG. Figure 10 shows the state of the pressure inside the piston when the discharge pressure of the pump changes, A is a curve diagram when the discharge pressure is low, B is a curve diagram when it is high, and Figure 10 is a non-return diagram with an oil damper installed on the spring support. Longitudinal cross-sectional view of the valve,
Fig. 11 is a vertical cross-sectional view of a check valve with an oil damper installed on the valve seat, Fig. 12 is an explanatory view showing the equilibrium state of force in each piston chamber when a slipper pad is provided, and Fig. 13 is An explanatory diagram of the thrust force bending moment that occurs in a conventional hydraulic pump, Fig. 14 is a pump capacity 4
It is a graph comparing the noise level of a conventional hydraulic pump and a pump according to the present invention in the case of 0 0 t/min. 1...Main shaft, 2...Cylinder block, 3...Cylinder, 4...Piston,
5... Swash plate, I... Valve plate, 11...
...Slipper pad, 14...Discharge-only port, 16...Check valve, 17...Discharge hole, 22...Suction side semicircular arc Type port, 23...
...Decompression orifice, 161...Poppet,
162... Spring holder, 163... Valve seat, 164... Oil sump, 165...
・Orifice for oil sump.

Claims (1)

[Claims] 1. A valve plate fixed to a casing, a main shaft rotatably supported by the casing, a cylinder block fixed to the main shaft and rotating in sliding contact with the valve plate, and a number of cylinders bored in the cylinder block. a piston, which is inserted and built into the interior, and which is made to reciprocate between top dead center and bottom dead center in order to alternately perform a discharge stroke and a suction stroke, and completes its discharge stroke at top dead center; In a variable discharge hydraulic pump having a swash plate with one end in sliding contact, the valve plate has a suction side semi-circular port provided in the valve plate, and is inserted into the piston chamber, and the valve plate is a slipper pad that is in pressure contact with the suction side semicircular port and communicates with the suction side semicircular port during the suction stroke; a discharge exclusive port that opens toward the rotation center of the cylinder block from each cylinder bored in the cylinder block; A discharge hole is provided in the main shaft so as to communicate between the port and the discharge port provided in the casing, and a reverse hole is built into the discharge port so that it opens when the piston chamber pressure is lower than the discharge pressure during the discharge stroke. A variable discharge type low noise device characterized by comprising a stop valve and a pressure reducing orifice provided on a valve plate or a swash plate on which one end of the piston slides, at a position where the piston moves forward and reaches at least the top dead center. hydraulic pump.