Piston for use in a fluid pump low pressure accumulator assembly

A low pressure accumulator assembly for use with a fluid pump comprises a pump housing with a longitudinally extending chamber having a base with a fluid flow opening. An accumulator is secured in the pump housing chamber. The accumulator includes an accumulator housing with a longitudinally extending bore. A piston is biased outwardly of the bore to selectively engage the base of the pump housing chamber. A protrusion extends outwardly from the piston to limit an area of engagement between the piston and the base so that fluid flowing from said opening into the chamber may engage a greater area of the piston.

BACKGROUND OF THE INVENTION 
The present invention relates generally to a fluid pump having an 
accumulator assembly comprising an accumulator residing in a cavity of a 
pump housing with a transverse base, and more particularly to an 
accumulator piston having a protrusion to limit the area of engagement 
between the piston and a base of the pump housing cavity. 
A low pressure fluid accumulator assembly comprising an accumulator 
temporarily receives and stores fluid flowing through a fluid flow opening 
in a base of a pump housing cavity. Accumulators generally include an 
accumulator housing with a longitudinally extending bore to receive a 
piston. A biasing means is disposed between a floor of the accumulator 
housing and the piston to bias the piston longitudinally outwardly from 
the accumulator housing. The piston selectively engages the base of the 
pump housing cavity. 
A gap is required between the piston and the base of the pump housing 
cavity in order for pressurized fluid to properly flow into the 
accumulator assembly. If the area of contact between the piston and the 
fluid flow is reduced to that portion of the piston immediately adjacent 
the fluid flow opening, insufficient force will be applied to the 
outwardly biased piston. 
A snap ring has typically been inserted into an upper groove of the 
accumulator housing to limit the longitudinal travel of the piston, 
providing the necessary gap between the piston and the base of the pump 
housing cavity. The high spring rate of the snap ring destroys the tooling 
used to guide the snap ring or directly damages the accumulator housing. 
Resulting contamination interferes with proper operation of the 
accumulator assembly. When used in a system critical application such as 
an antilock brake, in which proper operation of the accumulator assembly 
is essential during an antilock braking cycle, such contamination is 
unacceptable. 
SUMMARY OF THE INVENTION 
An improved low pressure accumulator assembly for a fluid pump eliminates a 
need for a snap ring to limit longitudinal travel of an accumulator piston 
with a piston head biased outwardly from the longitudinal bore of an 
accumulator housing. Instead, a protrusion extends outwardly from an outer 
surface of the piston head to selectively engage a transverse base of a 
pump housing cavity that receives the accumulator. The protrusion provides 
a gap between the outer surface of the piston head and the base. Thus, 
pressurized fluid flowing from a fluid flow opening in the base of the 
pump housing cavity contacts virtually the entire outer surface of the 
piston head. Sufficient force may be applied by the fluid to overcome the 
biased piston, resulting in proper operation of the accumulator assembly. 
Preferably, a single protrusion is centered on the piston head to prevent 
induced radial loading between the piston and accumulator housing. In 
addition, the fluid flow opening is preferably offset from the protrusion 
to prevent interference between the protrusion and the fluid flow opening. 
In a preferred embodiment, the protrusion comprises a nipple integrally 
formed with the piston head. Alternatively, the protrusion comprises a 
thrust button disposed in a longitudinal bore of the piston head.

DESCRIPTION OF A PREFERRED EMBODIMENT 
A pump 20 having a low pressure accumulator assembly 22 is illustrated in 
FIG. 1. In the disclosed embodiment, pump 20 is used with an antilock 
braking system. Low pressure accumulator assembly 22 is activated during 
an antilock braking cycle to receive pressurized fluid flowing through a 
fluid flow opening 24 of a pump housing 26. While low pressure accumulator 
assembly 22 comprises the use of two, differently sized fluid accumulators 
28 to react with fluid from openings 24, the assembly and operation of 
each of accumulators 28 is identical. 
Fluid flow opening 24 extends through a transverse base 30 of a 
longitudinally extending chamber 32 within pump housing 26, chamber 32 
having sidewalls 34. Accumulator 28 includes an accumulator housing 36 
with a laterally spaced outer surface 38 that engages sidewalls 34 of 
chamber 32. Accumulator housing 36 has a longitudinally extending bore 40 
that receives an accumulator piston 42 with an upper piston body 44 and a 
lower upper piston head 46. Piston body 44 has a longitudinally extending 
cavity 48 with a transverse spring thrust surface 50. An outwardly biased 
spring 52 is disposed within bore 38 between a transverse floor 54 of 
accumulator housing 36 and spring thrust surface 50. Spring 52 acts as a 
biasing means to bias piston head 46 longitudinally outwardly from 
accumulator housing 36 to selectively engage transverse base 30 of pump 
housing 32. At least one protrusion 56 extends away from an outer surface 
58 of piston head 46 to limit an area of engagement between outer surface 
58 and base 30 of pump housing 26. 
A gap 60 between outer surface 58 of piston head 46 and base 30 of pump 
housing 26 is required for the proper operation of low pressure 
accumulator assembly 22. During an antilock braking cycle, pressurized 
brake fluid flows through fluid flow opening 24 into accumulator 28 
against the biasing effect of spring 52. If there is no gap 60, then the 
pressure applied by the fluid is limited to the area of outer surface 58 
in contact with opening 24. Insufficient force results to counter the 
biasing effect of spring 52, resulting in the failure of accumulator 
assembly 22. 
Gap 60 also prevents sticking between outer surface 58 and base 30. 
Sticking may result if outer surface 58 and base 30 are particularly 
smooth or flat. 
As shown in prior art FIG. 2, before the use of protrusion 56, a snap ring 
62 has typically been inserted into an upper groove 64 of accumulator 
housing 36 to limit the longitudinal travel of piston 42. Further, snap 
ring 62 locks spring 52 and piston 42 within accumulator housing 36, 
ostensibly to prevent component separation of accumulator 28 during 
assembly. Piston 42 includes a chamfered edge 66 that engages snap ring 
62. To insert snap ring 62 into groove 64, a hollow insertion tool 68 with 
a tapered sleeve 70 is positioned over the open end 72 of accumulator 
housing 36 to act as a guide. Extending a short longitudinal distance into 
bore 40 of accumulator housing 36, sleeve 70 locates snap ring 62 at the 
proper depth. An assembly piston 74 pushes the snap ring 62 through tool 
68 and into groove 64. However, the high spring rate of snap ring 62 
destroys sleeve 70 of insertion tool 68, causing unwanted contamination 
and unacceptable failure of accumulator assembly 22 during antilock 
braking cycles. 
A hollow insertion tool 76, shown in prior art FIG. 3, does not have a 
tapered sleeve. Assembly piston 74 pushes the snap ring 62 through the 
insertion tool and into groove 64. However, by removing tapered sleeve 70, 
the snap ring prematurely expands before entering groove 64, creating a 
burr on an upper radially inner surface 78 of accumulator housing 36. 
Thus, unwanted contamination still results in unacceptable failure of 
accumulator assembly 22. 
Low pressure accumulator assembly 22 is shown in greater detail in FIG. 4. 
Accumulator housing 36 is press fit within chamber 32 of pump housing 26. 
Alternatively, sidewalls 34 of chamber 32 and outer surface 38 of 
accumulator housing 36 may have mating threads to secure the two housings 
together. A fluid seal 80 is disposed in a groove 82 formed in accumulator 
housing 36 and engages a ledge 84 of cavity 48. Fluid seal 80 prevents 
leakage from accumulator assembly 22. 
Oil ring lip seal 86, received in a groove 88 of piston 42 between piston 
body 44 and piston head 46, engages bore 40 to seal piston 42 within 
accumulator housing 36. Thus, fluid does not leak around piston head 46 to 
interfere with the operation of accumulator 28. In practice, the 
engagement of lip seal 86 with side walls 90 of bore 40 is sufficient to 
retain piston 42 within accumulator housing 36 during assembly. Thus, a 
snap ring such as snap ring 62, shown in FIGS. 2 and 3, is not required to 
prevent component separation. Alternatively, once spring 52 and piston 42 
are received within accumulator housing 36, open end 72 of the accumulator 
housing may be rolled radially inwardly to lock spring 52 and piston 42 in 
place. As a result, the present invention uses fewer components and 
requires less assembly time. While cost is reduced, however, accumulator 
assembly 22 also works better than prior art assemblies. 
In the illustrated embodiment, a single protrusion 56 is centered on outer 
surface 58 of piston head 46. Otherwise, when protrusion 56 engages base 
30, some longitudinal biasing of spring 52 may be converted to a radial 
loading between piston 42 and walls 88 of bore 40. Radial loading 
increases resistance to longitudinal travel by piston 42 and decreased 
efficiency of accumulator 28 during operation. 
Fluid flow opening 24 is offset from protrusion 56 to prevent interference 
between the protrusion and the fluid opening. If the opening is blocked by 
the protrusion, the accumulator assembly will not work properly. 
A plurality of protrusions may be used. Again, however, the elimination of 
induced radial loading is important. Typically, such protrusions will be 
equally spaced from the center of outer surface 58, and share a common 
longitudinal length. 
In the illustrated embodiment, protrusion 52 is a machined nipple that is 
integrally formed with the rest of piston head 46. An alternative 
embodiment of protrusion 52 is shown in FIG. 5. A longitudinal bore 92 is 
adapted to receive a thrust button 94 that includes protrusion 56. This 
alternative embodiment allows retrofitting of existing accumulator 
pistons. 
Preferred embodiments of the present invention have been described. It is 
to be understood that variations and modifications may be employed without 
departing from the scope of the present invention. Accordingly, the 
following claims should be studied to learn the true scope of the present 
invention.