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  
       [0001]     This application relates to liquid pumps.  
       BACKGROUND  
       [0002]     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  
       [0003]     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.  
         [0004]     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  
       [0005]      FIG. 1  is a schematic view of a pressure washer that includes a pump;  
         [0006]      FIGS. 2-4  are schematic sectional views of the pump at different stages during its operation; and  
         [0007]      FIG. 5  is a schematic view of a spring of the pump.  
     
    
     DESCRIPTION  
       [0008]     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.  
         [0009]     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.  
         [0010]     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 .  
         [0011]     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 .  
         [0012]     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 .  
         [0013]     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 .  
         [0014]     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 .  
         [0015]     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 .  
         [0016]     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.  
         [0017]     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 .  
         [0018]     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 .  
         [0019]     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 .  
         [0020]     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.  
         [0021]     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.  
         [0022]     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.  
         [0023]     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&#39;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 .  
         [0024]     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.