Note: Descriptions are shown in the official language in which they were submitted.
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HYDRAULIC ACTUATOR
FIELD OF THE INVENTION
This invention relates to cyclic hydraulic actuators and more particularly to
cyclic
actuators such as those used in rock drills and other mining machinery.
SUMMARY OF THE INVENTION
A hydraulic actuator according to the invention includes a housing, a drive
chamber in the housing, a hydraulic supply fluid inlet into the drive chamber,
an
elongated piston which extends from and is reciprocal in the housing, with the
piston including a first diametrical step to provide a piston drive area on
the
piston in the drive chamber, a reciprocal sleeve within the drive chamber
clear of
the drive chamber inner wall and which surrounds, is spaced from and extends
over a length of the piston to define between its inner surface, the outer
surface
of the piston and first and second opposed surfaces in the housing, a piston
return chamber, a second diametrical step in the piston in the return chamber
to
provide a piston return area, the first surface of the opposed surfaces in the
housing defining with a first end surface of the sleeve a fluid exhaust valve
which
on reciprocation of the sleeve opens and closes the return chamber to an
exhaust port from the housing, a blind bore in a wall of the drive chamber in
which a portion of the first end portion of the sleeve is sealingly,
reciprocal to seal
the exhaust valve in the bore from the drive chamber, the second of the
opposed
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surfaces in the housing defining with the second end surface of the sleeve a
fluid
inlet valve to the return chamber which on reciprocation of the sleeve opens
and
closes the return chamber to and from the drive chamber, formations on the
piston and sleeve which interact during the reciprocation of the piston to
generate
hydraulic fluid pressure changes in the return chamber which will, during
travel of
the piston in a first direction towards the exhaust valve, cause the sleeve to
move
to open the return chamber inlet valve and close the exhaust valve to result
in
deceleration and then acceleration of the piston in its opposite second
direction
towards the inlet valve, formations on the piston and the housing downstream
of
the return chamber inlet valve which interact to generate hydraulic fluid
pressure
changes in the return chamber which, during travel of the piston in its second
direction, cause the sleeve to be moved to close the inlet valve open the
exhaust
valve to result in the deceleration and then acceleration of the piston in its
first
direction.
The return chamber sleeve preferably is cylindrical with a portion of its
length
from its first end downwardly stepped from a first diameter in the drive
chamber
to a second smaller diameter which is partially located in the blind bore in
the
housing wall with the step in the sleeve being located in the drive chamber to
provide a hydraulically exposed return area for the sleeve in the drive
chamber
which is permanently exposed to fluid at supply pressure in use.
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Preferably, the actuator includes a sleeve guide formation which surrounds the
piston and projects from the second of the opposed housing surfaces into the
return chamber and on which a second end portion of the sleeve is sealingly
reciprocal to open and close the inlet valve to the return chamber,
The reduced diameter portion of the piston on the return area side of the
piston
may again be stepped, at a position spaced from the second diametrical step,
from a first diameter to a further reduced second diameter portion which is
sealingly reciprocal through the actuator housing wall with the reduced
diameter
second end portion of the sleeve including an inner rib which is spaced from
the
first end of the sleeve with the radially inner surface of the rib having a
diameter
just greater than the outer diameter of the piston first diameter.
The inner surface of the larger diameter portion of the wall of the sleeve may
be
recessed radially outwardly from a position spaced from the step in the
sleeve, to
a larger diameter which extends over the remainder of its length, with an
intermediate zone of the piston between its drive and return areas in the
return
chamber including an outwardly stepped portion having a diameter just less
than
the non-recessed portion of the sleeve wall inner surface.
In a preferred form of the invention the drive chamber in the actuator body is
divided into first and second chamber sections by a partition wall which
includes
at least one aperture through which the chamber sections are continually open
to
each other with the first drive chamber section housing the portion of the
piston
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which includes its drive area; the second chamber section housing the larger
diameter portion of the sleeve which projects from the blind bore in the drive
chamber wall and, on the partition wall, the second of the two opposed housing
surfaces and the sleeve guide formation.
The sleeve guide formation is conveniently a guide sleeve having an internal
diameter just greater than the outwardly stepped portion of the piston and
which
projects from the partition wall to be in sliding engagement with the recessed
inner surface of the second end portion of the reciprocal sleeve with the
guide
sleeve including a fluid inlet port which passes through its wall at or
adjacent the
second opposed housing surface on the partition wall for hydraulically
connecting
the return chamber to the second drive chamber section as the reciprocal
sleeve
opens the return chamber inlet valve, in use.
The actuator may include a start/stop valve arrangement which is located in
the
hydraulic fluid supply inlet to the first drive chamber section.
In this specification the term "hydraulic fluid" is intended to include water,
oil and
emulsions of water and oil.
BRIEF DESCRIPTION OF THE DRAWINGS
The invention is now described by way of example only with reference to the
drawings in which:
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FIGURE 1 is a diagrammatic sectioned side elevation of an embodiment of the
actuator of the invention as used in a rock drill, and
FIGURE 2 is an enlarged fragmentary view of the return chamber inlet valve
arrangement of the Figure 1 actuator.
5 DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
The drill actuator of the invention is shown in Figure 1 to include a housing
10, a
piston 12, a sleeve 14 and a start/stop valve 16.
The actuator housing includes a composite piston drive chamber which is
composed of two drive chamber sections 18 and 20 which are separated from
each other by a divider wall 22 through which at least one open fluid passage
23
passes. The start/stop valve 16 is connected to a hydraulic fluid inlet which
is
open into the drive chamber section 20. The forward end wall of the drive
chamber 18 is recessed to provide a blind bore 25 from which a plurality of
exhaust ports 24 lead to the outside of the housing. The body additionally
includes three seal bearings 26,28 and 30 which support the piston 12 and in
which the piston is sealingly reciprocal. A further seal bearing 34 is located
in the
bore 25 and sealingly supports the sleeve 14 for reciprocal movement.
The piston 12 is downwardly stepped on either side of a central zone 36 to
provide a hydraulically exposed piston drive area 38 in the drive chamber
section
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20 and a piston return area which is a combination of a portion of piston area
40
and a further stepped area 44. The piston further includes an outwardly
stepped
portion 42. The net return area of the piston is larger than the piston drive
area.
The sleeve 14 is in the form of a stepped cylindrical sleeve with its forward
smaller diameter portion located in the bore 25 for reciprocatory movement on
the seal bearing 34. The step in the sleeve extends beyond the inner surface
of
the small diameter portion of the sleeve to provide an inner rib 46 in the
sleeve
cavity. The inner surface of the rib is just larger in diameter than the
diameter of
the piston portion between the piston steps 42 and 44. (The term "just" in
this
specification is to be taken to mean a separation distance between components
which is optimally less than 0,15mm).
The rear portion of the inner surface 47 of the large diameter portion of the
sleeve is recessed from a position rearward of the sleeve step, as shown in
the
drawing, to provide a fluid flow restrictor surface 48 in the forward portion
of the
sleeve cavity. The diameter of the restrictor surface 48 is just greater than
the
outer diameter of the piston step 42. The step in the sleeve wall provides, on
the
outside of the sleeve, a hydraulically exposed return area 52 which is, during
operation of the actuator, continually exposed to fluid under pressure in the
drive
chamber section 18. The net return area of the sleeve is the diametrical
difference between the inner sleeve surface 47 and the outer diameter of the
reduced diameter forward portion of the sleeve. The annular sutface at the
front
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end of the sleeve seats on a valve seat 56, which is a transverse surface
portion
of the end wall of the bore 25, with the two surfaces defining an exhaust
valve for
the actuator.
The space between the inner surface of the larger diameter portion of the
sleeve
14 and the outer surface of the piston defines a piston return chamber 58. The
rear annular surface of the sleeve defines with a seat 59 on the partition
wall 22
an inlet valve to the return chamber 58.
The divider wall 22 carries a guide formation which, in this embodiment of the
invention, is a short circular sleeve 60 which is made integral with the wall
22.
The inner surface of the guide sleeve 60 is spaced from the piston zone 36 by
a
distance to be just clear of the piston step 42 on the return stroke of the
piston.
The inner surface 47 of the sleeve 14, adjacent its free end, carries a seal
bearing 62 which is slidably engaged with the outer surface of the guide
sleeve
60 to hold the sleeve centralised about the piston. The sleeve 60 carries a
plurality of circumferentially spaced fluid inlet ports 64 which pass through
it at
the inlet valve seat 59, as is more clearly seen in Figure 2.
The start/stop valve 16 is, in this embodiment of the invention, a ball valve
which
is operable between a first position, which is shown in the drawing, to open
the
composite drive chamber to hydraulic fluid under pressure and a second
transverse position in which the drive chamber sections are' closed to fluid
at
supply pressure.
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The actuator additionally includes a vent valve 66 for venting fluid at supply
pressure from the drive chamber section 20 for starting as will be explained
below.
The actuator, in a typical application might include a conventional
pressurised
gas accumulator, not shown in the drawing, which ensures that the hydraulic
fluid
supply pressure in the machine remains constant within acceptable limits.
Prior to use, the valve 16 is closed, and the vent valve 66 is opened to drop
any
fluid pressure in both the drive chamber sections 18 and 20 and the return
chamber 58. In use the valve 66 is closed and the valve 16 is connected to a
hydraulic hose through which the drive chamber sections 18 and 20 are, on
opening of the valve 16, charged with hydraulic fluid at supply pressure.
Pressure
fluid in the drive chamber 20 acting on the piston drive area 38 drives the
piston
to the left in the drawing with substantially the return area 40 of the piston
driving
hydraulic fluid in the return chamber 58 through the open exhaust ports 24
until
the return area 40 on the piston engages the step area 54 on the inner surface
of
the sleeve to entrain the sleeve to the left in the drawing until its leading
edge
abuts the seat 56 to close the exhaust valve leading to the exhaust port 24.
The fluid enters the return chamber 58 through the inlet valve and the open
inlet
ports 64 to the return chamber. The fluid pressure acting on the sleeve drive
area
54 hydraulically locks the sleeve to the exhaust seat 56 and the hydraulic
pressures acting on the piston drive and return areas 38, 40 and 44 returns
the
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piston to the right in the drawing. As the piston moves to the right and its
outwardly stepped portion 42 enters the guide sleeve 60 the fluid pressure in
the
return chamber 58 drops and the force acting on the net return area of the
sleeve
14 breaks the hydraulic lock of the sleeve 14 on the exhaust valve seat 56 to
force the sleeve rearwardly until the rear edge of the sleeve closes the inlet
valve
59 on the divider wall 22. The return chamber 58 is vented to atmosphere
through the exhaust ports 24 to cause the pressure acting on the sleeve 14
return area hydraulically to lock the sleeve onto its inlet seat 59 to prevent
fluid
flow from the drive chamber section 18 into the return chamber 58. With the
fluid
pressure in the return chamber dropped the high fluid pressure acting on the
piston drive area 38 in the drive chamber section 20, stops and reverses the
direction of piston travel into its drive stroke. It is to be noted that the
outwardly
directed step 42 on the piston 12 enters and leaves the underside of the guide
sleeve 60 downstream of the return chamber inlet valve. This causes the drive
chamber fluid at supply pressure to be isolated from the interaction space
between the underside of the guide sleeve 60 and the piston step 42 while the
piston is driven into its drive stroke thereby minimising cavitation.
As the piston commences its drive stroke the rapid acceleration of the piston
to
the left in the drawing causes the piston return areas 40 and 44 to expel
hydraulic fluid in the return chamber 58 from the open exhaust ports 24 until
the
outward step 44 on the piston reaches the restriction surface on the rib 46 in
the
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sleeve to restrict exhaust fluid flow from the machine and to cause a fluid
pressure build up in the return chamber 58 ahead of the piston return area 40.
The pressure build up acting on the sleeve drive area 54 breaks the hydraulic
lock between the sleeve and its inlet valve seat 59 and the net fluid force
now
acting on the sleeve drives the sleeve onto its exhaust valve seat 56. The
high
speed piston is now decelerated by fluid force acting on its return areas 40
and
44 until the free end 66 of the piston strikes the drill steel or until the
hydraulic
cushion formed between the drive area 54 of the sleeve and the return area 40
of
the piston, by the hydraulic interaction of the flow restrictor surface 48
with the
step 42 on the piston, rapidly decelerates the piston.
While the invention has been described with reference to a specific embodiment
it is to be appreciated that modifications and variations of the invention are
possible without departing from the scope of the invention which is defined in
the
following claims.
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