Patent Application
20070253849
This application relates to liquid pumps.
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.
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.
The apparatus 1 shown in
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
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
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
The intake stroke starts with the piston 50 fully extended as shown in
During the delivery and intake strokes, the bias spring 80 functions as follows: At the start of the delivery stroke, portrayed in
If the output pressure Pout remains below a threshold level sufficient to overcome the spring preload, ΔD will be zero. Above that threshold, over a range of output pressures Pout for which the pump 10 is designed, ΔD is a smooth positive function of output pressure Pout. The function is “positive” in that ΔD increases with increasing Pout throughout the pressure range, and “smooth” in that the second derivative of ΔD verses Pout 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 DP of the piston 50, which equals displacement DD of the driver 72 minus ΔD. Therefore, when Pout is above the threshold pressure, the delivery stroke volume is smoothly and inversely related to Pout. When Pout is below the threshold pressure, the delivery stroke volume is unaffected by varying Pout.
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 Pout. Since the delivery stroke volume of this pump 10 decreases with increasing Pout, the required power will tend to vary less with Pout 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 Pout, i.e., proportional to 1/Pout. That renders the input power approximately invariant with Pout, 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
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.
