Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
2129290
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TITLE OF THE INVENTION:
Pressure Switch
NAME OF INVENTOR:
James Richard Ellett
FIELD OF THE INVENTION
This invention relates to pressure switches.
R~K5ROUND AND SUMMARY OF THE INVENTION
In the design of pressure switches, a rod
slidable within a housing and movable by changes in
external fluid pressure operates a plunger of a
mechanically operated electrical switch. Resistance of
the rod to movement, and hence the pressure at which
the switch trips, is adjusted by a spring within the
pressure switch that is biased against movement of the
rod due to external fluid pressure. Adjustment of the
compression on the spring changes the tripping
pressure.
In one prior art pressure switch, model no.
RS73 of Camrose Electric Controls Ltd. of Edmonton,
Alberta, Canada, the spring is adjusted directly by a
sleeve threaded onto the outside of the housing. As
the sleeve moves along the outside of the housing, it
compresses the spring by direct connection with the
spring. This design, while it protects the inside of
the housing from the environment, is prone to
accidental movement of the sleeve, hence unintended
changes in the tripping pressure.
In another prior art pressure switch, model
DHTE of Barber Industries Ltd. of Edmonton, Alberta,
Canada, the compression of the spring is adjusted by
several wedges opposed to one another transverse to
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the direction of compression of the spring. As the
wedges are moved towards each other by a screw
abutting the end of one of the wedges, the
longitudinal thickness of the combined wedges
increases, thus compressing the spring. Again, with
this device the adjustment mechanism is not enclosed
and is susceptible to tampering.
The model DHTE of Barber also includes a
mechanically operated pressure switch that is secured
in place by a single screw, with the tripping point of
the switch adjusted using a threaded piston rod inside
a threaded trip plate. This trip point setting system
is difficult to adjust.
The invention described and claimed here is
intended to overcome the limitations of the known
prior art. In one aspect of the invention, there is
provided a pressure switch comprising a housing with
an interior bore extending from one end to the other.
A rod slidable within the bore and operated by
external fluid pressure extends between a piston at
one end of the housing and a mechanically operated
electrical switch at the other end of the housing. A
spring is disposed about the rod between a first stop
on the rod and a second stop on the housing. The
spring provides resistance against movement of the rod
under external fluid pressure. The resistance of the
spring to movement is adjusted by means disposed
within the housing. A port in the housing allows
access to the adjusting means and a cover is provided
for the port. The cover preferably includes a ring
disposed around the housing and threaded onto the
housing over the port. The adjustment means preferably
includes a sleeve engaging the spring, with the sleeve
threaded into the bore of the housing for movement
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longitudinally within the housing. The sleeve includes
plural radially extending slots disposed around the
sleeve. Rotation of the sleeve changes the compression
of the spring.
In a further aspect of the invention, the
mechanically operated electrical switch is mounted on
a switch mounting block by three cooperating screws.
A central hold down screw passes through the switch
mounting block and is threaded longitudinally into the
housing. A pair of jacking screws flank the central
screw and are threaded into the housing. Each of the
jacking screws is received in a screw receiving
counterbore in the switch mounting block to provide
limit points for movement of the switch mounting block
about the central hold down screw. Each of the screws
is accessible from the second end of the housing,
preferably through holes extending from the screw
receiving counterbores.
The pressure switch may include a stop
limiting movement of the cover away from the port, and
may include a lock to prevent movement of the
adjustment sleeve due for example to vibration of the
pressure switch.
These and other aspects of the invention are
described in the detailed description and claims that
follow.
BRIEF DESCRIPTION OF THE DRAWINGS
There will now be described preferred
embodiments of the invention, with reference to the
drawings, by way of illustration, in which like
numerals denote like elements and in which:
Fig. 1 is a longitudinal section through a
pressure switch according to the invention;
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Fig. 2A is a cross-section through an
adjustment sleeve for a pressure switch according to
the invention showing an anti-vibration adjustment
lock;
5Fig. 2B is a side view of the anti-vibration
adjustment lock of Fig. 2A;
Fig. 2C is an exploded view of the anti-
vibration adjustment lock of Fig. 2A;
Fig. 3 is a cross-section through a switch
10mounting block for use with the pressure switch of
Fig. 1;
Fig. 4 is a top end view of the pressure
switch of Fig. 1;
Fig. 5 is a side section through an
15adjustment access sealing assembly for use with the
pressure switch of Fig. 1;
Fig. 6 is a side view of the adjustment
access sealing assembly of Fig. 5; and
Fig. 7 is an exploded view of a holding
20block and screw for use with the adjustment access
sealing assembly of Fig. 5.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
Referring to Figs. 1 and 2A, there is shown
25a pressure switch 10 according to the invention. The
pressure switch 10 is formed of a housing 12 having a
first end 14 and a second end 16 threaded together and
sealed with seals 13. Set screw 17 locks the first end
14 and second end 16 of the housing together. A bore
3018 of variable inner diameter passes through the
housing 12 from the first end 14 to the second end 16.
In the bore 18 at the first end 14 is a piston 22 that
is free to move longitudinally a limited amount within
the bore 18. The piston 22 is shown here as including
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a piston head 24 snugly fitted in piston guide 26 at
the first end 14 of the housing 12. The piston guide
26 is secured within the housing by a locking nut 28
threaded in the first end of the housing 12. End 20 of
the bore 18 is sealed by diaphragm 30 that is held
firmly within the end 14 by the piston guide 26 and
sealed with O-ring seal 32 or a suitable alternative
such as a gasket. Alternatively, the piston
configuration may omit the diaphragm and use a sealed
piston within a piston guide, but this configuration
is more complicated in design and not preferred. The
piston 22 abuts against the diaphragm 30 and assists
in supporting it against collapse from external fluid
pressure. Range of movement of the piston is limited
by shoulder 35a on the piston guide 26 and by shoulder
35b on the piston 22. A bleed valve 33 releases
pressure from within the bore 18 of the housing 12.
A mechanically operated electrical switch 34
(micro-switch) is disposed at the second end 16 of the
housing 12. A rod 36 is mounted slidably within the
housing 12 and extends between the piston 22 and the
mechanically operated electrical switch 34. By direct
contact with the piston 22 and plunger 38 of the
switch 34, the rod 36 is operatively connected to both
the piston 22 and the switch 34, but this operative
connection may be accomplished using intervening
devices, with added complexity. The diaphragm 30,
piston 22 and rod 36 function as a mechanism to
transfer external fluid pressure along rod 36 to the
mechanically operated electrical switch 34. The
mechanically operated electrical switch ~common in the
art in itself) is operated by a plunger 38, which
abuts against hub 39 threaded onto the end 37 of rod
36. Movement of the rod 36 and hub 39 in the direction
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from the first end of the housing 12 to the second end
depresses the plunger 38 and activates the switch 34.
The end 37 of the rod 36 is sealed within the bore 18
by elastomer seals 40.
S A spring 42 is disposed about the rod 36
between a first stop 44 on the rod 36 and a second
stop 46 forming part of the housing 12. The spring 42
provides resistance against movement of the rod 36
from the first end 14 of the housing 12 to the second
end 16 of the housing 12. The spring 42 is preferably
a compression spring, but may conceivably be in
tension. The degree of resistance of the spring 42 to
external fluid pressure on piston 22, hence movement
of rod 36, is adjustable by adjustment means 48
encapsulated within the housing 12.
The adjustment means 48 includes a sleeve 52
surrounding and thus engaging one end of the spring
42. The sleeve 52 includes a threaded portion 54
threaded into the bore 18 of the housing 12 for
movement longitudinally within the housing 12 by
rotation of the sleeve 52. Referring in particular to
Fig. 2A, plural radially extending slots 56 are
disposed around the sleeve 52. The slots 56 may be of
various shapes, but should be shaped to receive an
implement, such as a screw driver, used to rotate the
sleeve 52. A port 58 or opening in the housing 12 is
provided and makes the adjustment means 48 accessible,
for example by a screwdriver or other means for
operating the adjustment means 48. A cover 60 for the
port 58 is provided by a ring 62 disposed around the
housing 12 and threaded onto the housing 12 over the
port 58. The opposed ends of ring 62 are sealed
against the housing 12 when the cover is in the closed
position by O-ring seals 64 placed in grooves 66
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formed in the housing 12. By unthreading the ring 62
towards the second end 16 of the housing 12, the port
58 is uncovered, allowing access to the adjustment
means 48. Tightening of the cover 60 against the seals
64 ensures isolation of the adjustment means from the
external environment.
Referring to Figs. 2A, 2B and 2C, there is
also provided an anti-vibration adjustment lock 51 for
the pressure switch. The lock 51 resists rotational
movement of the sleeve 52 within the first end 14 of
the housing 12 that may occur due to vibration of the
pressure switch in use. The lock 51 is formed by a
retaining screw 53 threaded into a spring spool 55
with spring washer 57 and tension spring 59 secured on
the shank of spring spool 55. The shank of screw 53
terminates in an unthreaded portion dimensioned to fit
snugly within any of the slots 56. The tension spring
59 has laterally extending arms 59a that press against
the sides of port 58, thus resisting rotational
movement of the sleeve 52 and preventing rotation of
the sleeve 52 more than one slot spacing.
At the second end 16 of the housing 12, the
bore 18 is enlarged to receive the electrical switch
34. A switch mounting block 72 secures the electrical
switch 34 in a fixed position. This fixed position of
the electrical switch must be set carefully upon
manufacture of the pressure switch so that travel of
the rod 36 within its limited range of motion is
sufficient to trip the plunger 38 of the electrical
switch 34. The precise setting of the position of the
mechanically operated electrical switch that is
permitted by this design of the mounting block avoids
the need for precise tolerances during manufacturing,
which would be impractical to achieve.
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Referring in particular to Figs. 3 and 4,
the switch mounting block 72 includes a pair of
transverse mounting screws 74 to secure the
mechanically operated electrical switch 34 on the
switch mounting block 72. A ground screw 77 is
provided within the enlarged end 16 of the housing to
provide a termination point for a grounding wire.
A central hold down screw 76 passes through
the switch mounting block 72 and is threaded
longitudinally into the second end 16 of the housing
12 towards the first end of the housing 14. A pair of
jacking screws 78 flank the central screw 76 and are
screwed into the second end of the housing 12. Each of
the jacking screws 78 is received in a screw receiving
counterbore 79 in the switch mounting block 72 whose
shoulders 81 provide limit points for movement of the
switch mounting block about the central hold down
screw 76. Each of the screws 76, 78 is accessible for
rotation from the second end 16 of the housing 12.
Reduced diameter portions 83 of the bores 79 extend
through the switch mounting block 72 thereby provided
access to the jacking screws 78. The jacking screws 78
support the switch mounting block and relative
adjustment of the jacking screws to each other permits
accurate alignment of the switch mounting block. The
height of the jacking screws 78 above the switch
mounting block 72 establishes the tripping point of
the mechanically operated electrical switch 34. The
hold down screw 76 is not threaded into the switch
mounting block and thus holds down and fixes the
switch mounting block in relation to the jacking
screws and therefore the housing. An end cap 80
threads onto the second end 16 of the housing 12 with
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O-ring seal 82 to enclose and secure within it the
mechanically operated electrical switch 34.
Referring to Figs. 5, 6 and 7, there is
shown an adjustment access sealing assembly 84 that
may be used as a stop to prevent the adjustment cover
ring 62 from moving axially along the pressure switch.
The assembly 84 is formed from a holding block 86 and
holding screw 88. The holding block 86 is secured to
the housing 16 by threading of the holding screw 88
through the holding block 86 into the housing 16
adjacent the cover ring 62. A sealing wire or cable
(not shown) forming a seal can be inserted in cross
bore 89 to prevent unauthorized removal of the holding
screw 88 and the holding block 86. Any intrusion into
the pressure switch will then be made apparent from
the breaking of the seal.
Adjustment of the tripping pressure is as
follows. The cover 60 is unthreaded, revealing the
port 58. A screwdriver or like instrument is used to
rotate the sleeve 52 and move it up or down in
accordance with the direction of rotation, thus
altering the compression of the spring. The pressure
setting can be calibrated if desired with marks on the
sleeve 52 or on the sides of the port 58 or on both
the sleeve and the sides of the port 58. The
rotational and hence axial location of the sleeve 52
may be secured by insertion of the anti-vibration
adjustment lock 51 into one of the slots 56 with
spring 59 biased against the sides of port 58. After
adjustment of the sleeve 52, the cover may be threaded
down over the port 58, thus securing the adjustment
means against tampering. To further secure the
adjustment means, the sealing assembly 84 may be
threaded into the housing 16 to prevent axial movement
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of the cover ring 62 thus limiting movement of the
cover away from the port and ensuring complete
enclosure and sealing of the adjustment means 60.
A person skilled in the art could make
immaterial modifications to the invention described
and claimed in this patent without departing from the
essence of the invention.
