BRAKE VALVE WITH PREFILL CHAMBER UNLOADING

DISTRIBUTEUR DE FREINAGE AVEC DELESTAGE DE LA PRECHAMBRE

English Abstract

Left and right two stage brake valves communicate fluid
pressure to left and right brake mechanism. Each brake valve
has an inlet for receiving pressurized brake fluid from a
pump, a reservoir port, a stepped bore and a stepped piston
member comprising a larger diameter prefill piston and a
smaller diameter high pressure piston. The stepped piston is
movable away from a rest position to transfer fluid to the
brake mechanism. The stepped bore and the stepped piston form
a high volume prefill chamber and a high pressure chamber.
The high pressure piston has a groove communicated with the
brake mechanism, and the groove communicates with the inlet
when the high pressure piston has moved a certain distance to
transfer fluid from the high pressure chamber to the brake
mechanism. The prefill piston has a sealing land and a second
land on either side of a sealing ring which is slidable in the
stepped bore. The sealing land sealingly engages the sealing
ring when the prefill piston moves to transfer fluid from the
prefill chamber to the brake mechanism. The sealing land is
spaced apart from the sealing ring by a distance which is
greater than the distance between the groove and the inlet
when the stepped piston is in its rest position.

French Abstract

Cette invention concerne des robinets de freinage bi-étagés, un gauche et un droit, qui alimentent en liquide sous pression les mécanismes de freinage correspondants. Chaque robinet comporte une admission de liquide de frein sous pression provenant d'une pompe, un orifice de retour vers un réservoir, un alésage à gradin et un piston à gradin dont la partie à grand diamètre a une fonction de préremplissage et la partie à petit diamètre une fonction d'alimentation sous haute pression. Ce piston est mobile entre une position de repos et une position de refoulement de liquide vers le mécanisme de freinage. L'alésage à gradin et ce piston définissent ensemble une chambre de préremplissage à grand volume et une chambre haute pression. La partie du piston assurant l'alimentation sous haute pression comporte une gorge mettant en communication le mécanisme de freinage et l'admission précitée lorsqu'elle s'est déplacée d'une certaine distance pour refouler le liquide de la chambre haute pression vers le mécanisme de freinage. La partie du piston assurant le préremplissage comporte, de part et d'autre d'une bague d'étanchéité glissée dans l'alésage à gradin, une portée d'étanchéité et une seconde portée. La première portée prend appui sur la bague d'étanchéité lorsque la partie du piston assurant le préremplissage se déplace pour refouler le liquide de la chambre de préremplissage vers le mécanisme de freinage. La portée d'étanchéité est espacée de la bague d'étanchéité d'une distance supérieure à la distance entre la gorge et l'admission précitées lorsque le piston à gradin est au repos.

Claims

THE EMBODIMENTS OF THE INVENTION IN WHICH AN
EXCLUSIVE PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS
FOLLOWS:
1. A two stage brake valve for communicating fluid
pressure to a brake mechanism, the brake valve having an inlet
for receiving pressurized brake fluid from a pump, a reservoir
port, a stepped bore and a stepped piston member comprising a
larger diameter prefill piston and a smaller diameter high
pressure piston, the stepped piston being movable away from a
rest position to transfer fluid to the brake mechanism, the
stepped bore and the stepped piston forming a high volume
prefill chamber and a high pressure chamber, the high pressure
piston having a groove communicated with the brake mechanism,
the groove being communicated with the inlet when the high
pressure piston has moved a certain distance to transfer fluid
from the high pressure chamber to the brake mechanism, the
prefill piston having a sealing land and a second land on
either side of a sealing ring which is slidable in the stepped
bore, the sealing land being sealingly engagable with the
sealing ring when the prefill piston moves to transfer fluid
from the prefill chamber to the brake mechanism, characterized
by:
the sealing land is spaced apart from the sealing ring by
a distance which is greater than the distance between the
groove and the inlet when the stepped piston is in its rest
position.

Description

21~4246
BRAKE VALVE WITH PREFI~ CHAMBER UNLOADING
Backqrou~d of the Invention
The invention relates to a brake control valve assembly
which includes a pair of dual stage brake control valves.
A known dual stage brake control valve is described in
U.S. Patent No. 4,738,109, issued 19 April 1988 and assigned
to the assignee of this application. Such a brake valve
provides the cZ~r~h; l; ty for manual braking when pump pressure
is not available at the brake valve pump pressure inlet. This
capability is achieved with a dual stage valve arrangement
that includes a prefill piston and a high pressure piston.
The prefill piston provides the large volume of oil and
associated low hydraulic pressure required for e~gagement of
the rear axle brake ~ n~nt~. Once this volume is
satisfied, the brake valve transitions from the larger prefill
piston diameter to the smaller diameter high pressure piston
in order to generate higher pressures required for braking.
With pump pressure available at the brake valve pump
inlet, application of the brake valve (prefill piston/high
pressure piston) requires only a small amount of travel from
the neutral position to the pressure metering position.
However, during this travel, fluid is subst~nt;~lly trapped
and pressure is generated in the prefill chamber, resulting in
a heavy pedal feel. The heavy pedal feel and associated
higher pedal force required to apply the brake results in
reduced control of the brake pressure due to the eventual
pressure decay of the prefill chamber. This heavy pedal feel
is more noticeable in cold weather condition3 due to the
increased pressure decay time.
C rv of the Inv~n~ i on
Accordingly, an object of this invention is to provide a
brake valve which does not have a heavy pedal feel.
This and other objects are achieved by the present
invention, wherein left and right two stage brake valves
communicate fluid pressure to left and right brake ~e~h~n; ~
Each brake valve has an inlet for receiving pressurized brake
fluid from a pump, a reservoir port, a stepped bore and a
stepped piston member compri8ing a larger rl;~r-t~r prefill
_ _ . .... . . .

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piston and a smaller diameter high pres~ure piston. The
stepped piston is movable away from a rest position to
transfer fluid to the brake mechanism. The stepped bore and
the stepped piston form a high volume prefill chamber and a
5 high pressure chamber. The high pressure piston has a groove
communicated with the brake ^hAn; ~m, and the groove
communicates with the inlet when the high pres~ure piston has
moved a certain distance to transfer fluid from the high
prebsure chamber to the brake mechanism. The prefill piston
10 has a sealing land and a second land on either side of a
sealing ring which is slidable in the stepped bore. The
sealing land sealingly engages the sealing ring when the
prefill piston moves to transfer fluid from the prefill
chamber to the brake ~h:ln; r~. The sealing land is spaced
15 apart from the ~ealing ring by a distance which is greater
than the distance between the groove and the inlet when the
stepped piston is in its rest position.
I~rief Descri~tion of thP ~rawinqs
Fig. 1 is a cross-sectional view of a prior art two-stage
20 brake valve; and
Fig. 2 is a cross-sectional view of a two-stage brake
valve according to the present invention.
Detailed De~cri~tion
Referring to Figs. 1, there 18 shown one side of a prior
25 art dual two stage brake valve 10 a housing 11 which encloses
a left two stage brake valve (not shown) and a right two stage
brake valve 12 for communicating fluid pressure to a left
brake ~_-hiln; rT (not shown) and right brake mechanism 14 .
Each brake valve has an inlet 16 for receiving pre~surized
30 brake ~luid from a pump 18, an inlet groove 19 communicating
with the inlet 16, a reservoir port 20 for communicating with
a reservoir 22, a ~tepped bore 24 with large diameter section
23 and small diameter section 25. A stepped piston member 26
includes a larger diameter prefill piston 28 and a smaller
35 diameter high pressure piston 30. The stepped bore 24 and the
stepped piston member 26 form a high volume prefill chamber 32
and a high pressure chamber 34. The high pressure pisto~ 30

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has a groove 36 communicated with the brake --~hi~n; r~ 14 via
outlet 38. The groove 36 communicates with the inlet 16 when
the high pressure piston 30 has moved a certain distance D to
transfer fluid from the high pressure chamber 34 to the brake
r--~hi~n;~m 14~ The prefill piston 28 ha~ a sealing land 40 and
a second land 42 on either side of a bealing ~ing 44 which is
slidable in the bore 23. The sealing land 40 sealingly
engages the sealing ring 44 when the prefill piston 28 moves
through distance d, whereupon further v - ' of the prefill
piston 28 to the right viewing FIG. 1 will transfer fluid from
the prefill chamber 32 to the brake mechanism 14. A spring 46
is biased to urge the piston member 26 to the left to a rest
pos ition .
Ref erring now to FIG . 2, according to the present
invention, the sealing land 40' is spaced apart from sealing
ring 44 by a distance d' which is greater than the distance D
between the edges of grooves 19 and 36 when the piston member
26 is in its rest position. Thus, land 40 sealingly engages
the sealing ring 44 when the prefill piston 28 moves through
distance d, whereupon further ,v~ t of the prefill piston
28 to the right viewing FIG. 1 will transfer fluid from the
prefill chamber 32 to the brake mechanism 14. This prevents
land 40' from engaging ring 44 during the time the piston
member 26 travels from the rest position to the pressure
metering position where grooves l9 and 36 start to overlap
each other. As a result, during this initial piston travel,
fluid will not be trapped in prefill chamber 32, thus avoiding
a heavy pedal f eel .
While the present invention has been described in
conjunction with a specific embodiment, it is understood that
many alternatives, modifications and variations will be
apparent to those skilled in the art in light of the foregoing
description. Accordingly, this invention is intended to
embrace all such alternatives, modif ications and variations
which fall within the spirit and scope of the appended claims.

Representative Drawing