Pump with variable stroke piston

Abstract

A pump apparatus includes a housing located on an axis. The housing has a chamber, an inlet valve and an outlet valve. A piston driver is configured to axially reciprocate. A piston is a piston configured to reciprocate in the chamber to draw liquid into the chamber through the inlet valve during an intake stroke and to discharge the liquid out of the chamber through the outlet valve during a delivery stroke. A spring axially biases the piston to a base position relative to the driver, so that the driver, when reciprocating, will drive the piston to reciprocate. A preload structure preloads the spring to enable pressure of the liquid in the chamber to displace the piston away from the base position against the spring bias after the pressure exceeds a threshold level.

Description

TECHNICAL FIELD

This application relates to liquid pumps.

BACKGROUND

A liquid pump includes a piston that reciprocates in a cylindrical chamber. The piston draws liquid through an inlet valve into the chamber during an intake stroke and forces the liquid out of the chamber through an outlet valve during a delivery stroke.

SUMMARY

A pump apparatus includes a housing located on an axis. The housing has a chamber, an inlet valve and an outlet valve. A piston driver is configured to axially reciprocate. A piston is a piston configured to reciprocate in the chamber to draw liquid into the chamber through the inlet valve during an intake stroke and to discharge the liquid out of the chamber through the outlet valve during a delivery stroke. A spring axially biases the piston to a base position relative to the driver, so that the driver, when reciprocating, will drive the piston to reciprocate. A preload structure preloads the spring to enable pressure of the liquid in the chamber to displace the piston away from the base position against the spring bias after the pressure exceeds a threshold level.

Preferably, the spring has a spring constant that increases with increasing compression of the spring. The spring is configured to render the volume of liquid delivered during each delivery stroke inversely related to output pressure of the pump. The preload is manually adjustable. The preload structure includes a protrusion on the piston within the driver, and further includes a stop surface in the driver that blocks the protrusion from exiting the driver and against which the protrusion is biased by the spring. The spring is configured to absorb the entire reciprocation of the driver in a situation where liquid is blocked from exiting the outlet valve piston while the driver continues to reciprocate.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view of a pressure washer that includes a pump;

FIGS. 2-4 are schematic sectional views of the pump at different stages during its operation; and

FIG. 5 is a schematic view of a spring of the pump.

DESCRIPTION

The apparatus 1 shown in FIG. 1 has parts that are examples of the elements recited in the claims. The apparatus thus includes examples of how a person of ordinary skill in the art can make and use the claimed invention. It is described here to meet the requirements of enablement and best mode without imposing limitations that are not recited in the claims.

The apparatus 1 is a pressure washer. It includes a pump 10 for pumping a liquid from a supply line 12 to an outlet line 14. The supply line 12 has an inlet hose 20 with a threaded end 22 configured to be screwed onto a water faucet. The outlet line 14 has an outlet hose 24 connected to a spray nozzle 26. The pump 10 draws water from the inlet line 12 and forces it out the nozzle 26 in the form of a pressurized spray.

As shown in FIG. 2, the pump 10 includes a housing 30 located on a central axis A. The housing 30 has axially front and rear ends 32 and 34 and a cylindrical piston-bearing surface 36 defining a cylindrical chamber 38. The chamber 38 is centered on the axis A and extends forward from a rear opening 40 of the housing 30. Liquid enters the chamber 38 from the supply line 12 through an inlet check valve 42. The liquid exits the chamber 38 into the outlet line 14 through an outlet check valve 44.

A piston 50 includes piston head 52 rigidly fixed to a piston rod 54. A threaded front end 56 of the rod 54 is screwed into a threaded bore 57 of the head 52. The length L of the piston 50 depends on the depth to which the rod 54 is screwed into the head 52. The head 52 extends from the rod 54 into the chamber 38. It forms an annular liquid-tight seal with, and is axially slidable against, the piston-bearing surface 36. The head 52 and the housing 30 together enclose a compression cavity 58, which is a closed section of the chamber 38 that has a volume that varies as the head 52 reciprocates. A nut 60 is screwed onto the rear end 62 of the rod 54 and protrudes radially outward from the rod 54.

The rear end 62 of the rod 54 is captured in a bore 70 of a piston driver 72. A threaded ring 74 surrounding the rod 54 is screwed into a threaded front end 76 of the bore 54. A rearward-facing stop surface 78 of the ring 74 blocks the nut 60 from exiting the bore 70.

A bias spring 80 is wrapped about the rod 54 and compressed between respective spring bearing surfaces 82 and 84 of the head 52 and the driver 72. The spring 80 biases the rod 54 into a base position relative to the driver 72, as shown in FIG. 2, in which the nut 60 abuts the stop surface 78. The nut 60 and the stop surface 78 thus together preload the spring 80. The stop surface 78 is axially between the nut 60 and the bias spring 80.

A return spring 90 is wrapped about the piston head 54 and compressed between respective spring bearing surfaces 92 and 94 of the housing 30 and the head 52. The return spring 90 keeps the driver 72 in contact with a front wobble surface 96 of a wobble plate 98. The plate 98 is attached to an axially-extending output shaft 100 of a motor 102. The wobble surface 96 is inclined with respect to the axis A so that it reciprocatingly pushes the driver 72 forward against the bias of the return spring 90 as the plate 98 rotates. The piston 50 is driven by the driver 72 to reciprocate, with a series of intake and delivery strokes in phase with forward and rearward strokes of the driver 72.

The delivery stroke starts with the piston 50 fully retracted as shown in FIG. 2, and pressure P_cav in the cavity 58 equaling supply line pressure P_in plus crack pressure P_crack of the inlet valve 42. Thereafter during the delivery stroke, the piston 50 advances, causing the pressure in the cavity 58 to increase. At some point, as in FIG. 3, when the cavity pressure P_cav starts to exceed P_out+P_crack, the outlet valve 44 starts to open to let the liquid into the outlet line 14. From then on, further advancement of the piston 50 delivers liquid into the outlet line 14 while P_cav remains constant at P_out+P_crack. This continues until the piston 50 reaches a fully forward position shown in FIG. 4, the outlet valve 44 closes, and cavity pressure P_cav remains at P_out+_crack.

The intake stroke starts with the piston 50 fully extended as shown in FIG. 4. As the return spring 90 pushes the piston 50 rearward, cavity pressure P_cav gradually decreases. When P_cav recedes below P_in−P_crack, the inlet valve 42 opens to let liquid from the supply line 12 into the cavity 58. Further retraction of the piston 50 draws liquid through the inlet valve 42 into the cavity 58, while P_cav remains constant at P_in−P_crack. The intake stroke ends as shown in FIG. 2 with the piston 50 fully retracted.

During the delivery and intake strokes, the bias spring 80 functions as follows: At the start of the delivery stroke, portrayed in FIG. 2, the cavity pressure P_cav is too weak to overcome the preload of the bias spring 80 urging the nut 60 against the stop surface 78. At some point during the delivery stroke, if and when the cavity pressure P_cav increases sufficiently to overcome the preload, the nut 60 will start to separate from the stop surface 78. For comparison purposes, FIG. 4 shows the positions of the driver 72 and the piston 50 at the start of the delivery stroke in dashed lines and their positions at the end of the delivery stroke in solid lines. By the end of the delivery stroke, the displacement distance D_p of the piston 50 is shorter than the displacement distance D_D of the driver 72 by the separation distance AD of the nut 60 from the stop surface 78.

If the output pressure P_out remains below a threshold level sufficient to overcome the spring preload, ΔD will be zero. Above that threshold, over a range of output pressures P_out for which the pump 10 is designed, ΔD is a smooth positive function of output pressure P_out. The function is “positive” in that ΔD increases with increasing P_out throughout the pressure range, and “smooth” in that the second derivative of ΔD verses P_out is finite over the operating range. Due to the density and incompressibility of the liquid filling the cavity 58, ΔD is substantially unaffected by inertia of the piston head 52.

The delivery stroke volume, i.e., the volume of liquid delivered during each delivery stroke, is proportional to the displacement D_P of the piston 50, which equals displacement D_D of the driver 72 minus ΔD. Therefore, when P_out is above the threshold pressure, the delivery stroke volume is smoothly and inversely related to P_out. When P_out is below the threshold pressure, the delivery stroke volume is unaffected by varying P_out.

The threshold can be manually increased by increasing the preload on the bias spring 80. This can be done by screwing the rod 54 deeper into the head 52 or screwing the ring 74 deeper into the driver 72. Either of these steps decreases the depth of the head 52 in the chamber 38. The resulting increase in initial volume of the cavity 58 does not affect the achievable output pressure Pout, because the liquid is incompressible.

Power input by the pump 10 from the motor 102 is typically proportional to motor speed, delivery stroke volume and outlet pressure P_out. Since the delivery stroke volume of this pump 10 decreases with increasing P_out, the required power will tend to vary less with P_out than without the reduction ΔD in stroke displacement.

Preferably, the bias spring 80 is selected to yield a delivery stroke volume that is approximately inversely proportional to P_out, i.e., proportional to 1/P_out. That renders the input power approximately invariant with P_out, so that a motor 102 optimized for one power level at one outlet pressure would be optimal for other pressures too. This can be achieved by the bias spring 80 having a spring constant that increases with increasing spring compression. A step-wise increasing spring constant can be achieved by the bias spring 80 comprising coil springs 111 and 112 differing in spring constant. In the example shown in FIG. 5, the springs 111 and 112 are arranged in parallel, more specifically concentric, with successively shorter springs having successively higher spring constants. Alternatively, a smoothly increasing spring constant can be achieved by the bias spring 80 comprising a single coil spring of smoothly varying wire thickness.

The spring constant and the preload for the bias spring 80 are preferably higher than for the return spring 90. This ensures that most of the driver reciprocation will be passed to the piston 50 and absorbed by the return spring 90 and not absorbed by the bias spring 80. On the other hand, the bias spring's spring constant and preload are preferably sufficiently low, and its initial length sufficiently high, to enable the bias spring 80 to absorb the entire reciprocation stroke of the driver 72 in a situation where the piston 50 is jammed in its fully retracted position. Such a situation can occur if a clog in the outlet line 14 totally prevents the liquid from exiting the outlet valve 44.

This written description uses examples to disclose the invention, including the best mode, and also to enable any person skilled in the art to make and use the invention. The patentable scope of the invention is defined by the claims, and may include other examples that occur to those skilled in the art. Such other examples are intended to be within the scope of the claims if they have elements that do not differ from the literal language of the claims, or if they include equivalent structural elements with insubstantial differences from the literal language of the claims.

Claims

1. A pump apparatus comprising:

a housing located on an axis and having a chamber, an inlet valve and an outlet valve;

a piston driver configured to axially reciprocate;

a piston configured to reciprocate in the chamber to draw liquid into the chamber through the inlet valve during an intake stroke and to discharge the liquid out of the chamber through the outlet valve during a delivery stroke;

a spring that axially biases the piston to a base position relative to the driver, so that the driver, when reciprocating, will drive the piston to axially reciprocate; and

a preload structure that preloads the spring to enable pressure of the liquid in the chamber to displace the piston away from the base position against the spring bias after the pressure exceeds a threshold level.

2. The apparatus of claim 1 wherein the spring has a spring constant that increases with increasing compression of the spring.

3. The apparatus of claim 1 wherein the spring is configured to render the volume of liquid delivered during each delivery stroke inversely related to output pressure of the pump.

4. The apparatus of claim 3 wherein the spring is configured to render the volume of liquid delivered during each delivery stroke inversely proportional to the output pressure.

5. The apparatus of claim 1 wherein the preload is manually adjustable.

6. The apparatus of claim 5 wherein manual adjustment of the preload adjusts the depth of the piston in the chamber.

7. The apparatus of claim 1 wherein the spring is a bias spring, and the apparatus further comprises a return spring having a lower spring constant than the bias spring urging the piston out of the chamber.

8. The apparatus of claim 1 wherein the preload structure includes a protrusion on the piston that is located in the driver and is axially urged by the spring against a stop surface of the driver.

9. The apparatus of claim 8 wherein the stop surface is between the rod protrusion and the spring.

10. The apparatus of claim 1 wherein the spring is configured to absorb the entire reciprocation stroke of the driver in a case where liquid is blocked from exiting the outlet valve piston while the driver continues to reciprocate.

11. The apparatus of claim 1 further comprising an inlet hose connected to a water source.

12. A pump apparatus comprising:

a housing located on an axis and having a chamber, an inlet valve and an outlet valve;

a piston driver configured to axially reciprocate;

a piston configured to reciprocate in the chamber to draw liquid into the chamber through the inlet valve during an intake stroke and to discharge the liquid out of the chamber through the outlet valve during a delivery stroke; and

a spring that axially biases the piston to a base position relative to the driver, so that the driver, when reciprocating, will drive the piston to axially reciprocate while enabling pressure of the liquid in the chamber to displace the piston away from the base position against the spring bias, the spring having a spring constant that increases with increasing compression of the spring.

13. The apparatus of claim 12 wherein the spring includes coil springs with different spring constants.

14. A pump apparatus comprising:

a housing located on an axis and having a chamber, an inlet valve and an outlet valve;

a piston driver configured to axially reciprocate;

a piston configured to reciprocate in the chamber to draw liquid into the chamber through the inlet valve during an intake stroke and to discharge the liquid out of the chamber through the outlet valve during a delivery stroke;

a spring that axially biases the piston to a base position relative to the driver, so that the driver, when reciprocating, will drive the piston to axially reciprocate while enabling pressure of the liquid in the chamber to displace the piston away from the base position against the spring bias; and

an inlet hose connected to the inlet valve and configured to be connected to a liquid source.

15. The apparatus of claim 14 wherein the inlet hose has a threaded end configured to be screwed onto a water faucet.

16. A pump apparatus comprising:

a housing located on an axis and having a chamber, an inlet valve and an outlet valve;

a piston driver configured to axially reciprocate;

a piston configured to reciprocate in the chamber to draw liquid into the chamber through the inlet valve during an intake stroke and to discharge the liquid out of the chamber through the outlet valve during a delivery stroke; and

a spring that axially biases the piston to a base position relative to the driver, so that the driver, when reciprocating, will drive the piston to axially reciprocate while enabling pressure of the liquid in the chamber to displace the piston away from the base position against the spring bias, the spring being configured render the volume of liquid delivered during each delivery stroke inversely related to output pressure of the pump.

17. The apparatus of claim 16 wherein the delivery stroke volume is approximately inversely proportional to output pressure of the pump.

18. A pump apparatus comprising:

a housing located on an axis and having a chamber, an inlet valve and an outlet valve;

a piston driver configured to axially reciprocate;

a piston configured to reciprocate in the chamber to draw liquid into the chamber through the inlet valve during an intake stroke and to discharge the liquid out of the chamber through the outlet valve during a delivery stroke; and

a spring that axially biases the piston to a base position relative to the driver, so that the driver, when reciprocating, will drive the piston to axially reciprocate while enabling pressure of the liquid in the chamber to displace the piston away from the base position against the spring bias, the spring being configured to absorb the entire reciprocation stroke of the driver in a situation where the liquid is blocked from exiting the outlet valve while the driver continues to reciprocate.