Title:
TIME DELAY SWITCH WITH IMPROVED DASHPOT MEANS
United States Patent 3731021


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 D
Application Number:
05/176568
Publication Date:
05/01/1973
Filing Date:
08/31/1971
Assignee:
SMITHS IND LTD,GB
Primary Class:
International Classes:
H01H7/03; (IPC1-7): H01H7/03
Field of Search:
200/34
View Patent Images:
US Patent References:



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/m2) 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.