Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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This invention relates to vehicle brake actuators.
Brake actuators are known which comprise a piston
working in a cylinder and engageable with a brake-actua*ing
assembly movable to apply and release the brake. The
cylinder has two chambers located one on each side of the
piston. One of the chambers contains a coil compression
spring which biases the piston towards a brake-applying
position, the spring force being transmitted through the
piston and the brake-actuating assembly during braking,
and the other chamber is normally filled with fluid, usually
air, under pressure to hold off the piston and compress the
spring.
Such brake actuators are usually used for emergency
braking or parking and may be combined with service brake
actuators.
A problem with such known actuators is that the
spring chamber is open to ambient air. The ingress of dirt
and water into the chamber, particularly salt water in winter
conditions or near the coast, causes corrosion and premature
failure of the spring.
It has been found that the use of filters and venting
pipes between the spring chamber and the atmosphere do not
solve the problem.
In accordance with one aspect of the present invention,
there is provided an actuator for a vehicle wheel brake,
comprising a cylinder divided by a movable pressure operable
member into two separate chambers, and resilient means located
in one of the chambers and arranged to bias said pressure
operable member, wherein a one-way valve is provided which
permits the flow of air from within said one chamber to the
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exterior thereof, but prevents the flow of air into the one
chamber through the valve, the one chamber being normally
sealed except for the one-way valve.
The valve may be set such that air can flow to
atmosphere through the valve only after a predetermined
minimum threshold pressure has been reached in the chamber.
Thus, the valve prevents ambient air passing
therethrough into the said one chamber. Any gas leakage from
the other chamber across the pressure operable member will be
exhausted through the valve.
An actuator in accordance with the invention for
a vehicle wheel brake and a modification thereof will now be
described, by way of example, with reference to the accompany-
ing drawings, in which: -
Figure 1 is an axial sectional view through the
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actuator,
Figure 2 is a view similar to Figure 1 of the
modified actuator, parts being removed for clarity, and
Figure 3 is a side view of a release tool for
use with the actuators of Figures 1 and 2.
Referring to Figure 1, the actuator comprises an
emergency or parking brake part 1 attached to a service
brake part 2.
The service brake part 2 comprises an aluminium
housing 3 formed in two parts 3A,3B between which is clamped
a diaphragm 4 enclosing a pressure chamber 5. Attached to
the diaphragm 4 is a brake actuating rod 6. The introduction
of pressurised fluid into chamber 5 urges the diaphragm from
its illustrated brakes-off position to an operative position
in which the rod 6 applies the brake. The service brake part
2 can operate independently of the emergency brake part 1.
The part 3B of the housing 3 is sealingly attached
to an aluminium cylinder 7 of the emergency brake part 2.
In the actuator of Figure 1, the attachment is by way of a
circlip 8 which can only be removed by machining away part
of the aluminium housing.
The emergency brake part 1 comprises a fluid pressure
operable member in the form of a piston 9 carrying sliding
seals 10 and being slidable in the cylinder 7, the piston
dividing the cylinder into two chambers 11,12. The piston ~
has an inner axially extending cylindrical extension 13 which
surrounds and retains a steel insert 14 engageable with a
push rod 15. The rod 15 extends sealingly through a wa]l 16
formed by the housing part 3B into engagement with the diaphragm --
3 4.
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One chamber 11 contains a coil compression spring
17, shown in its compressed, i.e. brake-off condition, and
the other chamber 12 is normally pressurised by fluid,
usually air, to hold the piston off against the spring bias.
The spring chamber 11 is closed by a removable plug 18 having
a one-way valve 19 which permits the flow of air therethrough
from within chamber 11 to the atmosphere.
Also contained within the chamber 11 is an expansible
annular envelope 21 which contains air. The pressure of air
in the envelope, and thus in the chamber 11, when the
envelope is in its illustrated fully compressed state is in
excess of atmospheric pressure. The excess pressure is
relatively small.
In operation of the emergency brake part 1, release
f pressure from the chamber 12 allows the spring 17 to expand
and urge the piston 9 and rod 15 in a direction to apply the
brake. Ambient air cannot pass through valve 19, but the
envelope 21 expands to prevent a substantial vacuum being
created in chamber 11. Re-application of pressure in chamber
12 returns the spring 17 and envelope 21 to their illustrated
positions.
Should air leak past the piston seal 10 it will be
exhausted through the valve 19.
The actuator of Figure 2 is similar to that of Figure
1, and operates in the same way. The main differences are
that the spring 17 is contained within the envelope 21 to
further protect the spring from corrosion and that the housing
part 3B is attached in a different way to cylinder 7. In
this case the open end 7A of cylinder 7 is rolled over an
enlarged peripheral portion 3C of housing part 3B to secure
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the housing 3 to the cylinder 7.
The retraction or release tool of Figure 3 enables
the brakes to be released in the event of release of pressure
from chamber 12 due to a failure in the pressure line. The
tool has a screw portion 22 which can fit in a cbmplementary
threaded recess 23 in the steel insert 14. The screw portion
22 carries a flanged nut 24.
To release the brake, the plug 18 is removed and
the screw portion 22 is screwed with recess 23 by means of
a square headed portion 25 and the nut 24 rotated until it
engages the end wall of the cylinder 7. Further rotation of
the nut retracts the piston 9 against the bias of the spring
17 to release the brake.
The above-described actuators have the advantages
that:
a) the one-way valve 19 prevents the ingress of
corrosive atmospheric moisture so that the life of the spring
17 is prolonged,
b) the pressurised envelope 21 reduces or obviates
the vacuum which would otherwise be created in the spring
chamber 11 during actuation of the emergency brake part 1,
c) the form of construction used provides a light-
weight actuator which makes mounting on the vehicle easier,
d) the attachment of the housing 3B to the cylinder
7 is such that it can only be readily released for service
in the factory and not in a maintenance garage so that the
risk of injury to relatively unskilled personnel by the
expansion of the spring is reduced and the possibility of the
:: replacement of actuator parts by parts of inferior quality
is also reduced, and
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e) the actuator can be designed to replace existing
actuators.
It will be appreciated that the envelope 21
may be of any suitable configuration.
