U.S. patent number 3,731,021 [Application Number 05/176,568] was granted by the patent office on 1973-05-01 for time delay switch with improved dashpot means.
This patent grant is currently assigned to Smiths Industries Limited. Invention is credited to Dominoc Paul Edmund Barnard.
United States Patent |
3,731,021 |
Barnard |
May 1, 1973 |
TIME DELAY SWITCH WITH IMPROVED DASHPOT MEANS
Abstract
An electrical time delay switch has its two electrical contacts
housed within a cylinder in which a piston is slidable for
operation of the contacts. The first contact is rigidly attached to
an end wall of the cylinder but longitudinally spaced therefrom,
and the second contact is disposed between the end wall and the
first contact and is resiliently urged into engagement of the first
contact by a leaf spring. The piston at the end of its stroke
engages the leaf spring to break interengagement of the contacts.
As the piston travels towards the leaf spring it expels air out of
the cylinder at a predetermined rate through porous ceramic or
sintered bronze constituents of the end wall, this predetermined
rate defining the switch's time delay. Towards the end of its
stroke the piston reaches a wider section of the cylinder which
provides a large air leakage path round the piston to invoke sudden
operation of the contacts by the piston.
Inventors: |
Barnard; Dominoc Paul Edmund
(Witney, EN) |
Assignee: |
Smiths Industries Limited
(London, EN)
|
Family
ID: |
10428924 |
Appl.
No.: |
05/176,568 |
Filed: |
August 31, 1971 |
Foreign Application Priority Data
|
|
|
|
|
Sep 11, 1970 [GB] |
|
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43,478/70 |
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Current U.S.
Class: |
200/34 |
Current CPC
Class: |
H01H
7/03 (20130101) |
Current International
Class: |
H01H
7/00 (20060101); H01H 7/03 (20060101); H01h
007/03 () |
Field of
Search: |
;200/34 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Scott; J. R.
Claims
I claim:
1. A time delay switch comprising a casing having a cylindrical
portion joined by a transition section to one end of enlarged
diameter, end closure means of predetermined porosity secured to
said one end to provide a predetermined rate of air leakage
therethrough relative to the interior of said casing, a pair of
electrical contacts within said cylinder, first means mounting one
of said contacts to the end closure means in substantially fixed
spaced relation to the end closure means, second means mounting the
other of said contacts resiliently to said end closure means
between said end closure means and said one of the contacts for
engagement of said one of the contacts, a piston slidably mounted
within the cylindrical portion of said casing, biassing means to
urge said piston longitudinally of the casing towards the end
closure means for sudden engagement of said second mounting means
and consequential disengagement of said contacts upon its reaching
said transition section, and means for selectively moving said
piston away from said end closure means against the restraint of
said biassing means to permit reengagement of said contacts.
2. A time delay switch according to claim 1, wherein said piston
comprises a first component for sealing engagement with said
cylindrical portion, and a second component for engagement with
said second mounting means, spring means urging said first and
second components into sealing interengagement with one another for
movement in unison, and means for counteracting said spring means
to displace said first and second components relative to one
another in a direction breaking said sealing interengagement.
3. An electrical time delay switch comprising a cylinder having a
first portion of first cross-section communicating with a second
portion of second cross-section different from that of said first
portion; closure means for said cylinder comprising a body of
porous material having a predetermined rate of air leakage
therethrough relative to the interior of said cylinder; a piston
slidably mounted within said cylinder, said cylinder having sealing
means shaped in conformity with said first cross-section for
resiliently engaging said first portion of the cylinder when said
piston is adjacent said first portion to provide an air-tight seal
therebetween and operative to disengage from said second
cross-section when said piston is adjacent said second portion to
break said air-tight seal; biassing means for urging said piston
from said first portion of the cylinder to said second portion
thereof whereby the breaking of said seal causes a sudden increase
in the rate of movement of the piston when it enters said second
portion; a pair of electrical contacts; means mounting said
contacts for operation by said piston to change the state of said
switch when the piston attains a predetermined position within said
second portion of the cylinder; and means coupled to said piston
for enabling the piston to be moved into said first portion of the
cylinder against the action of said biassing means to permit the
piston thereafter to be urged by the biassing means toward said
second portion to effect said change in the state of the switch
after a predetermined time delay.
4. The time delay switch of claim 3 wherein said piston comprises
two separate components, further biassing means urging said two
components into sealing engagement with one another for movement in
unison towards said predetermined position under the action of the
first-mentioned biassing means, and means for selectively effecting
movement of said two components relative to one another against the
action of said further biassing means to break the seal between
said two components.
5. The electrical time delay switch of claim 3 wherein said closure
means is disposed at an end of the cylinder remote from said first
portion thereof.
6. The electrical time delay switch of claim 3 wherein each of said
first and second cross-sections is circular, said second
cross-section being of greater diameter than said first
cross-section.
7. An electrical time delay switch comprising a cylinder having
first and second portions, the second portion being of greater
cross-section than said first portion, a piston slidably mounted
within said cylinder between said first and second portions,
biassing means coupled to said piston for urging said piston from
said first portion of the cylinder to said second portion thereof,
means having a predetermined rate of air leakage therethrough for
controlling the rate of movement of the piston effected by said
biassing means when said piston is in said first portion of the
cylinder, the greater cross-section of said second portion of the
cylinder permitting a sudden increase in the rate of movement of
said piston by said biassing means when said piston enters said
second portion, a pair of electrical contacts, means mounting said
contacts for operation by said piston to change the state of said
switch when the piston attains a predetermined position within said
second portion of the cylinder, and means coupled to said piston
for moving said piston out of said second portion and into said
first portion of the cylinder against the action of said biassing
means to enable the piston thereafter to be urged by said biassing
means toward said second portion to effect said change in state of
the switch after a predetermined time delay.
8. The electrical time delay switch of claim 7 wherein said means
having a predetermined rate of air leakage therethrough comprises a
cylinder closure member.
9. The electrical time delay switch of claim 7 wherein said means
having a predetermined rate of air leakage therethrough comprises a
porous member.
10. The electrical time delay switch of claim 9 wherein said porous
member comprises a ceramic material.
11. The electrical time delay switch of claim 9 wherein said porous
member comprises sintered metal.
12. The electrical time delay switch of claim 9 wherein said piston
includes means sealingly engaging said first portion of the
cylinder.
13. The electrical time delay switch of claim 9 wherein said
contacts are disposed within said second portion of the cylinder.
Description
SUMMARY OF THE INVENTION
This invention relates to switches and in particular to electrical
switches whose contacts are operated after a period of time has
elapsed from actuation or setting of the switch. Such an electrical
switch is herein referred to as a "time delay switch."
According to one aspect of this invention there is provided a time
delay switch wherein time delay is achieved by means providing a
predetermined rate of air leakage into or out of a chamber.
According to another aspect of this invention there is provided a
time delay switch comprising a pair of relatively movable contacts;
a piston slidable within a cylinder, biassed in one direction
longitudinally of the cylinder and arranged to operate the contacts
when it is at a predetermined longitudinal position relative to the
cylinder; and a variable volume chamber defined at least in part by
the piston and cylinder, the chamber being closed by means
providing a predetermined rate of air leakage into or out of the
chamber.
In use a pressure differential provided across the piston decreases
as air leakage occurs until the piston causes operation of the
contacts.
Preferably the means providing a predetermined rate of air leakage
is a porous member. The porous member, which preferably provides an
end "closure" for the chamber in line with the cylinder, may be of
ceramic material, or of sintered metal, e.g., bronze.
Conveniently there is provided means mounting one contact of the
pair for movement relative to the other contact of the pair, the
mounting means lying in the path of movement of the piston.
Preferably the mounting means comprises a resilient metal strip,
and in this case one end of the strip may constitute the one
contact.
Preferably the chamber's dimension in the vicinity of the
predetermined longitudinal position is larger than the cylinder's
dimension so that the last part of the piston's movement towards
the position in the one direction is independent of air leakage. In
this way, the operation of the contacts is rapidly achieved whereby
the risk of arcing between the contacts is minimized. Preferably
the position is at or near the limit of permissible movement of the
piston in the one direction.
In a preferred arrangement the piston comprises two relatively
movable components that move in unison under the bias but are
separable to provide a large air leakage path between them when the
time delay is to be curtailed.
By way of example, time delay switches according to this invention
will now be described, reference being had to the accompanying
drawings of which:
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a longitudinal cross-section through one exemplary time
delay switch;
FIG. 2 is a transverse cross-section taken along the line II--II of
FIG. 1;
FIG. 3 is a longitudinal cross-section through another exemplary
time delay switch; and
FIG. 4 is a transverse cross-section taken along line IV--IV of
FIG. 3 with the switch primed.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
A cup-shaped housing 1 (FIG. 1) is formed by impact extrusion of an
aluminum blank with a centrally apertured base 2 and a side wall
composed of two coaxial cylindrical portions 3, 4 interconnected by
a sloping or part-conical portion 5. The inner surface of
cylindrical portion 3 provides a cylinder within which a piston 6
is slidably mounted. The piston 6 comprises a rubber peripherally
lipped ring seal 7 that is trapped between flanges 8, 9 of the
piston. The flange 8 is integrally moulded of plastics material
(e.g., polypropylene) with cylindrical hollow bosses 10, 11 one
each side of the flange. The external surface of boss 10 is
provided circumferentially with a radially-directed projection 12
that co-operates with a mating depression provided in the internal
surface of a ring 13 integrally moulded of plastics material (e.g.,
polypropylene) with the flange 9. The hollow boss 11 frictionally
retains the serrated end of a metal actuating rod 14. The actuating
rod 14 extends through an externally threaded bearing bush 15
staked in the central aperture of the housing base 2 and has an
actuating knob 16 frictionally retained on the serrated opposite
end of the actuating rod 14. A low rate coiled compression spring
17 acts between the housing base 2 and the flange 8 of the piston
to bias the piston 6 in a longitudinal direction away from the base
2 towards an end "closure" disc 18 of the cup-shaped housing 1.
The disc 18 is of ceramic or other similar material of known
porosity. It is retained in position against a rubber annular seal
19 that abuts an annular shoulder of the housing side wall by
swaging over the lip of the housing 1. A chamber 20 is thereby
defined by the end disc 18, the piston 6 and the housing side wall
(which last includes the portions 4 and 5 and also the portion 3
when the piston 6 is remote from the disc 18). On the outer surface
of disc 18, two L-shaped electrical blade terminals 21, 22 are
provided that are rivetted through rubber sealing washers 26 to two
metal strips 23, 24 disposed inwardly of the chamber 20. As will be
seen from FIG. 2 the metal strip 23, which is resilient, is at an
angle to the imaginary line joining the rivets. The metal strip 24,
which is substantially rigid, is suitably bent so that the free end
of resilient metal strip 23, which constitutes one contact, is
disposed between the ceramic disc 18 and the free end of metal
strip 24. A contact point 25 rivetted to the free end of metal
strip 24 is engaged (see FIG. 1) by the free end of resilient metal
strip 23 when the latter is not acted upon by the piston 6 (see
below).
In use, the above-described time delay switch is actuated or primed
by pulling knob 16 outwardly to bring the piston 6 towards the
housing base 2 against the biassing action of spring 17 and cause
the contacts to close and/or to remain closed. During this priming
action air from the space between piston 6 and housing base 2 is
forced past the piston to enter the increased volume chamber 20. As
the spring 17 attempts to urge the piston 6 back towards disc 18 it
compresses the air in chamber 20, the consequential pressure
difference across the piston 6 resisting the return motion of the
piston. However since the end disc 18 of ceramic material has a
predetermined porosity, it provides a controlled rate of air
leakage out of the chamber 20 so that the pressure difference
decreases as does the volume of chamber 20. The rate of air leakage
and the length of cylinder 3 are arranged to provide the desired
time elapse for the piston to travel from near the housing base 2
to the sloping or part-conical portion 5 of the housing's side
wall. As soon as the piston reaches the sloping or part-conical
portion 5 of the housing's side wall, air from the decreased volume
chamber 20 can enter the space between the piston 6 and the
housing's base 2 so that the air pressure difference across the
piston is immediately brought to zero. The compression spring 17,
having now no resistance to further expansion, causes the piston 6
to move rapidly towards the disc 18 until the leading edge of the
piston's ring 13 engages the resilient metal strip 23 (constituting
the aforesaid mounting means for one of the contacts) and deflects
it towards disc 18 to thereby open the contacts and thereafter
maintain them open.
In the second embodiment of FIGS. 3 and 4, the cup-shaped housing
31 is formed by impact extrusion of an aluminum blank with a
centrally apertured base 32 and a side wall composed of two
coaxially cylindrical portions 33, 34 interconnected by a sloping
or part-conical portion 35. The inner surface of cylindrical
portion 33 provides a cylinder within which a piston 36 is slidably
mounted. The piston 36 comprises a sliding cup-shaped component 38,
a peripherally lipped rubber ring seal 37 abutting the rear surface
of the cup-shaped sliding component 38, and an operating component
39 having an annular bead 39a of V-shaped cross-section sealingly
abutting the radial face of the rubber ring seal 37. The components
38 and 39 are each moulded of plastics material (e.g.,
polypropylene), the cup-shaped component 38 being provided with
axially extending grooves 38a in its external cylindrical wall
surface and an apertured central boss 40, and the operating
component 39 being provided centrally with an integral hollow boss
41. The hollow boss 41 frictionally retains the serrated end of a
metal actuating rod 44 that passes through the apertured boss 40
and is provided with a shoulder 44a intermediate its ends. An
apertured dished plate 43 is urged against the shoulder 44a by a
coiled compression spring 42 acting between the plate 43 and the
cup-shaped component 38, and this spring 42 serves to urge together
the two components 38 and 39 such that no air leakage can occur
across the annular bead 39a in contact with the seal 37. The
actuating rod 44 extends through an externally threaded bearing
bush 45 staked in the central aperture of the housing base 32 and
is provided with a radially directed, spring loaded nipple 46 for
the coupling thereto of an actuating knob (not shown). A low rate
coiled compression spring 47 acts between the housing base 32 and
the component 38 to bias the piston 36 in a longitudinal direction
away from the base 32 towards an end "closure" unit 48 of the
cup-shaped housing 31.
The unit 48 comprises a disc 50 moulded of plastics material with
an apertured boss 51. A resilient rubber sleeve 52 is disposed
within the apertured boss 51 and houses four axially aligned,
"solid," cylindrical, slug-like elements 53 of sintered bronze.
These porous elements 53 compress the sleeve 52 radially so that it
is firmly gripped in position within the boss 51 and itself firmly
grips the elements 53. Axial insertion of the elements 53 into the
sleeve 52 is facilitated by the slightly tapered shape of the
sleeve's outer surface. The unit 48 is retained in position by
swaging over the lip of housing 31 to trap the disc 50 between that
lip and an annular shoulder of the housing side wall. An annular
rubber seal 49 is interposed between the disc 50 and said housing
shoulder. A chamber 55 is thereby defined by the unit 48, the
piston 36 and the housing side wall (which last includes the
portions 34 and 35 and also the portion 33 when the piston 36 is
remote from disc 50). Two similar L-shaped electrical blade
terminals 56, 57 are provided on the outer surface of disc 50 one
each side of the cross-sectional plane of FIG. 3. These terminals
56, 57 are each located against rotation by axially directed pips
54 moulded integrally with disc 50, and are rivetted through rubber
sealing washers 61 to two metal strips 58, 59 disposed inwardly of
the chamber 55. As shown in FIG. 4, both metal strips 58, 59 are
located adjacent the rivets by axially directed pips 62 moulded
integrally with the disc 50, and each strip has an electrical
contact point 60 securely rivetted to its opposite end. The shorter
metal strip 59 is substantially rigid and is stepped in the axial
direction so that its contact point 60 is at a fixed distance from
the disc 50. The longer metal strip 58 is resilient and is inclined
to strip 59 in the plane of FIG. 4 so that its contact point 60 is
in alignment with that of strip 59 and lies between it and the disc
50. The longer metal strip 58 is also inclined towards the strip 59
in the plane of FIG. 3 so that in the absence of restraint the two
contact points 60 engage one another. The relative disposition of
the two contact points 60 is shown in FIG. 3 in the presence of
restraint due to the disposition of the piston 36, that is to say
the normal, contacts open state of the time delay switch.
Prior to fitting the assembled end "closure" unit 48 to the housing
31 it is tested for air leakage across it. The preferred test is
such that, at 0.408 atmospheres (41.3kN/m.sup.2) pressure
differential, the air leakage rate should not exceed 5.3 cubic
centimeters of air in 1 minute.
The manner of operation of the time delay switch of FIGS. 3 and 4
will be apparent from the corresponding description of the
operation of the time delay switch of FIGS. 1 and 2, air leakage
occuring at a controlled rate through the elements 53 which have a
predetermined porosity. However, the embodiment shown in FIGS. 3
and 4 facilitates manual over-riding of the operation prior to full
time elapse. That is to say once the switch has been primed (by
pulling the actuating rod 44 to withdraw the piston 36 away from
the contact points 60 and permit them to close together), the
predetermined time delay prior to the contacts again opening--when
the flat radial end face of the piston's operating component 39
suddenly contacts the resilient strip 58 -- can be curtailed by
suddenly relieving the pressure within the chamber 55. This is
achieved by manually depressing the knob attached to the actuating
rod 44 so that the piston's operating component 39 is separated
from the piston's sliding component 38 against the action of spring
42. As soon as this occurs, the seal between these two components
38, 39 (provided by the annular bead 39a) is broken so that air
from the chamber 55 can escape rapidly through the aperture in
control boss 40. Once the pressure differential has been removed
from across the piston 36, the spring 47 causes the piston 36 to
move rapidly towards the disc 50 until the leading radial end face
of operating component 39 engages the strip 58 to deflect it
towards the disc 50 and thereby open the contacts.
It will be appreciated that in normal operation, i.e., when full
time elapse is obtained, the arrival of the lipped ring seal 37 at
the sloping or part-conical portion 35 of the housing suddenly
relieves the pressure within chamber 55 whose air rapidly escapes
through the axially extending grooves 38a at the periphery of the
sliding component 38.
Where it is desired to regulate the time during which a vehicle
rear screen is to be heated by a suitable electrically operated
heating means (e.g., the heating element described in copending
U.S. Pat. application No. 121,819 filed Mar. 8, 1971, for "Heating
Elements"), the provision of a time delay switch is considered
desirable in view of the high current, of the order of 10 to 15
amps, taken by such heating means. If such a current is maintained
for long periods of time with the vehicle engine switched off, the
drain on the vehicle battery may be sufficient to hinder subsequent
starting of the engine. With either of the above-described
exemplary time delay switches connected in series with the heating
means across the vehicle battery's terminals, the rear screen
heating means can not be maintained in operation for longer than
the time elapse period of the switch (unless of course, the switch
is again primed). Conveniently this period is within the range of
12 to 15 minutes. If necessary, the switch of FIGS. 1 and 2 may be
suitably calibrated after assembly to accord with this (or any
other) required time elapse period by painting the exterior surface
of the ceramic disc 18 to reduce its effective porosity.
* * * * *