Temperature Compensated Displacement Indicating Switch

Technical Field This invention relates generally to a temperature compensated displacement indicating switch. More particularly, this invention relates to a switch useful for monitoring the pressure differential across a fluid filter, the switch having the capability of distinguishing between high pressure differentials caused by clogging of the filter and high pressure differentials to which temperature induced viscosity of the fluid is a contributing factor.

Background Art

Many fluid systems utilize one or more filters in a fluid line to remove contaminants carried by the fluid. Typical of these are forced circulation lubrication systems. As increasing amounts of contaminants are trapped by these filters, the flow of fluid through the fluid line progressively diminishes. This necessitates periodic replacement of the filter elements. ^" Failure to replace the filter once the flow through it falls below a certain level can result in damage to and/or inadequate operation of the machine incorporating the fluid system.

Systems have been developed to warn of the pressure across a filter exceeding a preselected value. Such systems typically include a sensor which monitors the fluid pressure differential from the input to the output of the filter. When this pressure differential reaches a certain magnitude, the sensor activates a device for warning of an apparent clogged filter condition.

For many fluids, especially lubricants, the fluid viscosity at low temperatures is of a magnitude sufficient to produce a differential fluid pressure exceeding the warning threshold even though the filter is in acceptable condition. To accommodate this, many displacement indicating warning systems incorporate a temperature actuated compensating element of some type to prevent activation of the warning device at these low temperatures. Thus, only when the fluid temperature exceeds the temperature threshold and the differential fluid pressure exceeds the pressure threshold, will the warning device be activated.

Existing temperature compensated differential fluid pressure warning systems may be broadly divided into two classes, those that incorporate an electrical temperature actuated compensating element and those that incorporate a mechanical temperature actuated compensating element. In the former class, the warning systems typically include a switch which goes from one to the other of an open and a closed condition in response to a pressuresensor sensing a pressure differential of at least a preselected magnitude. The warning system is normally activated in response to this change in switch condition. The electrical temperature actuated compensating element is mounted in temperature sensing relation to the fluid, often in the fluid sump. This compensating element serves as an override, preventing activation of the warning device below a threshold fluid temperature. Temperature compensated differential fluid pressure warning systems of this type are disadvantageous in that heretofore the temperature actuated compensating element has been mounted in a special port. This port establishes fluid communication between the temperature activated element

and the fluid to maintain the temperature activated element at approximately the temperature of the fluid. The need for this port necessitates additional manufacturing steps and creates a potential leakage point. Furthermore, wiring must be provided to and from this element.

Temperature compensated differential fluid pressure warning systems having mechanical temperature actuated elements are similar in many -respects to those having the electrical type temperature actuated element as detailed above. Typical of these is that disclosed in U.S. Patent 3,146,757 issued to He man et al. on September 1, 1964. This reference discloses a gimbal-like structure having a plurality of bimetallic bellow-shaped discs which are free to expand and contract with changes in the fluid temperature. This gimbal-like structure is adapted to mechanically prevent the corresponding clogged filter indicator from assuming a clogged filter state at low temperatures. Such mechanical type temperature compensated differential fluid pressure warning systems are disadvantageous in that the mechanical temperature actuated compensating components are subject to wear and failure, are difficult to calibrate, and are unduly complex and expensive of assembly.

The present invention is intended to overcome, one or more of the problems as set forth above.

Disclosure Of The Invention in one aspect of the present invention a switch has a housing, first and second terminals within said housing, and a plunger movable within said housing between first and second positions. First means is provided for establishing electrical continuity between the first and second terminals in response to the

plunger being in one of the first and second positions. Positioned within said housing is solid state second means for providing a selected one of electrical continuity and electrical discontinuity between the first and second terminals in response to a preselected portion of said second means being within a preselected range of temperatures.

There are many situations in which it is useful to provide an indication of when the fluid pressure across a certain element or circuit of a fluid system has exceeded a predetermined threshold. For example, it is common for earthmoving equipment to have an oil filter bypass circuit for bypassing the oil filter when the filter has become clogged. A warning device is activated by a displacement indicating switch in response to the occurrence of a bypass condition. However, when the oil is cold, the oil pressure across even a new filter can exceed this predetermined threshold. It is desirable to provide some means for preventing activation of the warning device when the oil is cold.

It has been found that for displacement indicating switches of the type in which the measure of continuity serves as an indication of a high pressure differential condition, this temperature compensation can be effected by positioning a temperature sensitive conducting element in electrical contact with the terminals of the switch. This temperature sensitive conducting element is preferably in thermal contact with the particular fluid and serves as an override, causing the switch to establish an output indicative of a low differential pressure condition within a certain range of temperatures, irrespective of the actual differential pressure condition.

Brief Description Of The Drawings

For a better understanding of the present invention, reference may be had to the accompanying drawings in which: Fig. 1 shows a diagrammatic side view, partially in cross section, of one embodiment of the present invention;

Fig. 2 shows a diagrammatic end view corresponding to Fig. 1; Fig. 3 shows a diagrammatic side view, partially in cross section, of the preferred embodiment of the present invention positioned in the access port of a fluid bypass valve with the spool biasing the plunger against the terminals; Fig. 4 shows a diagrammatic side view corresponding to Fig. 3 with the spool displaced away from the plunger;

Fig. 5 shows a schematic representation of a filter and the temperature compensated displacement indicating switch;

Fig. 6 shows a diagrammatic side view, partially in cross section, of an alternative embodiment of the present invention;

Fig. 7 shows a diagrammatic side view, partially in cross section, of another alternative embodiment of the present invention.

It is to be understood that the drawings are not intended as a definition of the invention but are provided for the purpose of illustration only.

Best Mode For Carrying Out The Invention

Referring to the drawings, a temperature compensated displacement indicating switch embodying certain of the principles of the present invention is generally indicated by the reference numeral 10. It

should be understood that the following detailed description of the temperature compensated displacement indicating switch 10 relates to the best presently known embodiment of this advance. The temperature compensated displacement indicating switch 10 can assume numerous other embodiments, as will become apparent to those skilled in the art, without departing from the basic concepts of this development.

In a preferred form of the temperature 0 compensated displacement indicating switch 10, best shown in Fig. 1, first and second terminals 12,14 are mounted within a housing 16. The terminals 12,14 each have a contact face 17 interior to the housing 16 and a connection end 18 exposed from the housing 16. 5 Preferably, the contact face 17 and connection end 18 of each terminal 12,14, are in intimate electrical contact, as by being fashioned of a continuous piece of metal. However, as will be seen subsequently, certain embodiments of the switch (10) include a thermistor or 0 some other temperature sensitive device which can be positioned in series between the connection end 18 and contact face 17 of one of the terminals 12,14. The terminal connection ends 18 are adapted to be connected to an electrical monitoring system 22 in a manner well 5 known to those skilled in the art. This electrical monitoring system 22 preferably incorporates a microprocessor, but alternatively can be hardwired.

The housing 16 does not provide an electrical path between the terminals 12,14. In the preferred _Q embodiment this is achieved by fashioning the housing

16 from a dielectric material such as VALOX 420, a PBT, 30% glass filled plastic manufactured by the General Electric Corporation. Alternatively, the housing 16 can be made of a conductive material with the terminals 5 12,14 electrically insulated from the housing 16 by nylon sleeves or the like.

The housing 16 has first and second end portions 19,20. Access to the terminals 12,14 is provided at the first end portion 19 for connection to the electrical monitoring system, and the second end portion 20 is adapted to be attached in activating relationship to an activating element 24. It is the displacement of some portion of the activating element 24 that is monitored by the present temperature • compensated displacement indicating switch 10. In the preferred embodiment this activating element 24 is a displacement indicating filter bypass valve 27 having a spool 26. More generally, the activating element 24 is a device having a movable element 26 which translates in response to the occurrence of a certain condition. The spool 26 is movable from a first spool position to a second spool position in response to the pressure across a filter 28.of a fluid system 31 exceeding a preselected value. In the preferred embodiment, mo.tion of the spool 26 in a direction away from the temperature compensated displacement indicating switch 10 is indicative of the filter 28 being clogged. The hydraulic relationship of the filter 28, bypass valve 27 and temperature compensated displacement indicating switch 10 are diagrammatically represented in Fig. 5. Together the bypass valve 27, the filter 28, the switch 10, and the electrical monitoring system 21 make up a filter monitoring device 29.

In the preferred embodiment, the second end portion 20 of the housing 16 is adapted to be received into a threaded aperture 30 of the filter bypass valve 27. As is best shown in Figs. 3 and 4, the spool 26 is movable toward and away from the switch 10.

The compensated displacement indicating switch ιo includes means 41 for providing electrical

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continuity intermediate the first and second terminals 12,14 in response to the activating element 24 being in one of said first and second positions. More specifically the housing 16 defines a first aperture 32 extending from the second end portion 20 to the contact face 17 of the terminals 12,14. Positioned within this first aperture 32 is a plunger 36 which, in the preferred embodiment, has a contact face 38 and a stop face 40. The plunger 36 is adapted to be moved within the aperture 32 between first and second positions. In the first position, the plunger contact face 38 is in abutment with both of the terminals 12,14. In the second position, the plunger contact face 38 is displaced from abutment with the terminals 12,14. A stop 42 is attached to the housing 16 at the second end portion 20 thereof. This stop 42 is preferably an annular disc defining a second aperture 44 of smaller diameter than the first aperature 32. The first and second apertures 32,44 are preferably coaxial. In the second position, the plunger stop face 40 is in contact with the stop 42. This is best shown in Fig 4. In the preferred embodiment, the first aperture 32 and the plunger 36 are generally cylindrical and have a common axis which defines the direction of movement of the plunger 36.

Means 52 is provided for biasing the plunger 36 toward the stop 42. Preferably, this biasing means 52 includes a coil spring 54 positioned intermediate the housing 16 and the plunger 36. As best shown in Fig. 1, the first aperture 32 is bounded furthest from the housing second end portion 20 by an end wall 56. A first spring seat aperture 58 extends from this end wall 56 in a direction toward the housing first end portion 18. Similarly, the plunger 36 defines a second spring seat aperture 60 extending from the plunger

contact face 38 in a direction toward the plunger stop face 40. The first spring seat aperture 58 receives a first end 62 of the spring 54 and the second spring seat aperture 60 receives a second end 64 of the spring 54. As best shown. in Fig. 3, it is advantageous to size the spring 54 such that it is not in full compression when the plunger contact face 38 abuts the terminals 12,14.

The plunger 36 preferably includes a main body element 66 and a conducting element 68. The main body element 66 has a guiding portion 70 which is sized relative to the first aperture 32 to substantially prevent axial misalignment between the plunger 36 and the first aperture 32. More specifically, the guiding portion 70 includes a plurality of fins 72 each of which is in actual or virtual abutment with the side wall of the first aperture 32. Spaces intermediate the fins 72 allow fluid communication intermediate the second aperture 44 and the terminals 12,14. The guiding portion 70 and fins 72 are best shown in hidden lines in Fig. 2.

Extending from the guiding portion 70 in a direction toward the end wall 56 is a conducting element support portion 74. This support portion 74 is generally cylindrical and is of smaller diameter than the guiding portion 70. In the preferred embodiment, the conducting element 68 is a generally annular brass member which is press fit onto the support portion 74. As best shown in Fig. 3, the conducting element 68 is configured and positioned such that in response to the plunger 36 being biased toward the housing first end portion 19, the conducting element 68 comes into conductive contact with both of the terminals 12,14. As previously detailed, the preferred embodiment of the temperature compensated displacement

indicating switch 10 incorporates a mechanical switch in which a conducting element 68 establishes electrical continuity intermediate the terminal contact faces 17 upon being forced into contact therewith. Those skilled in the art will appreciate that other mechanical switches could also be utilized in conjunction with the principles of the present development. For example, a type of mechanical switch could have been utilized in which one of the terminals has a movable portion which, upon inward displacement of a plunger, is brought into physical and electrical contact with the other of the terminals.

A projecting portion 76 of the plunger 36 extends from the guiding portion 70 in a direction away from the end wall 56. The projecting portion 76 is preferably frustoconical, having a major diameter smaller than the diameter of the second aperture 44 so as to be capable of projecting therethrough. The projecting portion 76 preferably extends from the guiding portion 70 a distance greater than the sum of the thickness of the stop 42 and the travel distance of the plunger 36 within the housing 16. As best shown in Fig. 3, this ensures that the conducting element 68 contacts the terminals 12,14 in response to the spool 26 acting against the projecting portion 76. Were the length of the projecting portion 76 insufficient, a sufficiently broad spool 26 could come to rest against the stop 42 without displacing the plunger 36 sufficiently to bring the conducting element 68 into contact with the terminals 12,14.

Further included is second means 78 for establishing electrical continuity between the first and second terminals 12,14 in response to said second means 78 being within a preselected range of temperatures. Preferably, this second means 78

includes a thermistor having a positive temperature coefficient of resistivity and is placed across the two terminals 12,14. Preferably this is an RL3006-50-50-25-PTO PTC thermistor manufactured by the Thermistor Division of the Keystone Carbon Co. ,

St. Mary's, Pennsylvania, U.S.A. This thermistor has a relatively high resistance at temperatures above 50 C and a relatively low resistance at temperatures below 50°C. . It will be appreciated by those skilled in the _o art that thermistors of other values can be substituted to obtain a displacement indicating switch 10 having other desired temperature sensitivities. It is preferable that the thermistor utilized have a sharp transition between its low and high impedance 5 conditions. That is, the thermistor preferably shifts from a low to high impedance state over a relatively short temperature span.

The thermistor is preferably positioned in a third recess 80 intermediate the two terminals 12,14. o First and second leads 81,83 of the thermistor connect the thermistor to a corresponding one of the first and second terminals 12,14. The third recess 80 is preferably in fluid contact with the second aperture 44. This fluid contact is established by a fluid 5 passage 99 made up of the first aperture 32, the second aperture 44, the first spring seat aperture 58 and the third recess 80. Hence, the thermistor is in fluid contact with the activating element 24.

Fig. 6 sets forth an alternative embodiment of the present invention, in which corresponding elements are denoted by primed reference numerals. In this alternative embodiment, the housing 16' has a first aperture 79 from which there extends an axially directed central aperture 82. This central aperture 82

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does not extend completely through the housing 16' but. terminates at an end wall 56' proximate the first end portion 19' .

The plunger 36' is positioned within this central aperture 82 and is axially displaceable therein. A stop 42' is provided for limiting the movement of the plunger 36' away from the end wall 56'. Preferably, this stop 42', as shown in Fig. 6, is a disc defining a second aperture 44'. The plunger 36' has a main body element 66' and a projecting portion 76' of relatively smaller cross section. The projecting portion 76' is adapted to project from the main body element 66' through the second aperture 44'. The main body element 66' includes a magnet 88 preferably extending to a position distal from the projecting portion 76'.

Means 52' is provided for biasing the plunger 36' toward the stop 42'. Preferably, this biasing means 52' includes a coil spring positioned intermediate the end wall 56' and the plunger 36' The biasing means 52' and plunger 36' are preferably configured to permit the plunger 36' to be displaced a sufficient amount to allow an extreme outer end 89 of the projecting portion 76' to be brought substantially flush with an outer face 90 of the stop 42' This is shown in Fig. 6. As the spool 26' is withdrawn from the stop 42', the plunger 36' moves under the influence of the biasing means 52' to a position in which the main body element 66' abuts the stop 42'. Means 92 for changing the impedance of an electrical circuit in response to the presence of a predetermined portion of said plunger 36' is positioned adjacent the central aperture 82. Together the impedance changing means 92 and the plunger 36' make up means 41' for providing electrical continuity

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intermediate the first and second terminals 12,14 in response to the activating element 24 being in one of said first and second positions. This impedance changing means 92 is preferably fixed within a fourth aperture 94 which is displaced a small lateral distance from the central aperture 82. In the preferred embodiment this impedance changing means 92 includes a normally open magnetic reed switch adapted to be closed when the magnet 88 is positioned proximate thereto, and adapted to be open when the magnet 88 is spaced a significant axial distance from the reed switch. In the embodiment shown in Fig. 6, the reed switch is closed when the projecting element outer end 89 is brought adjacent to the stop outer face 90, and is opened when the projecting element outer end 89 is allowed to project a significant distance beyond the stop outer face 90. It will be apparent to those skilled in the art that the means 92 could also be a Hall effect type device, an optical proximity device, a capacitative device or another variety of proximity type device.

Second means 78' for establishing electrical continuity in response to said second means being within a preselected range of temperatures is located within the housing 16', preferably being positioned proximate the first aperture 32'. Preferably, this electrical continuity establishing means 78' is similar to that detailed previously, and is connected in parallel with the impedance changing means 92. The second means 78' is preferably located within a fifth aperture 96 at a position proximate the first aperture 32'. In the embodiment shown in Fig. 6, the fourth and fifth apertures 94,96 are fabricated to communicate with the first end portion 19'. Once the corresponding

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elements are positioned within these apertures 94,96, the apertures 94,96 are potted, in a manner well known to those skilled in the art.

In applications for which a rapid response to temperature changes is required it is advantageous to maintain a portion of the fifth aperture 96, intermediate the impedance changing means 78' and the first aperture 32', free from potting material. This area is maintained in fluid communication with the first aperture 32' In the preferred embodiment this permits the fluid which activates the bypass valve to be in contact with the second means 78'.

The embodiments of the temperature compensated displacement indicating switch 10 described heretofore incorporate a displacement detecting contact switch 37 and positive temperature coefficient thermistor in parallel. The displacement detecting contact switch 37 of these embodiments is adapted to be closed in response to a "no fault" condition and the thermistor is selected and arranged to disable a "fault" condition in response to the temperature being below a predetermined level. It should be pointed out, however, that using many or all of the principles of the present development, temperature compensated displacement indicating switches of other logic schemes can be developed.

For example, it may be desirable that a temperature compensated displacement indicating switch 10 be used to monitor an activating element 24 in a situation in which movement of the activating element 24 or a portion of the activating element toward the switch 10 is indicative of a potentially deleterious situation. Such a switch could resemble that of the preferred or alternative embodiment with the exception that the thermistor be of the negative temperature

coefficient type and be in series rather than in parallel with the circuit formed by the plunger 36 across the terminals 12,14. That is, the negative coefficient thermistor would, in this embodiment, be connected in series with the connection end 18 and contact face 17 of one of the two terminals 12,14. The warning system 22 would monitor such a switch for low impedance and activate the warning device in response to detecting a low impedance condition. As in the previous embodiments, the thermistor utilized preferably has a relatively sharp transition from high to low impedance.

Similarly, it may be desirable that a temperature compensated displacement indicating switch be used to monitor a movable element 26 in a situation in which movement of the activating element 24 away from the switch 10 is characteristic of a deleterious situation and is indicated by a low rather than high impedance condition. Such a switch could be configured such that the displacement detecting contact switch 37 established continuity intermediate the terminals 12,14 in response to the movable element 26 being displaced away from the switch 10. This could be effected, for the embodiment of Fig. 6, by positioning the reed switch 92 proximate the stop 42' such that the reed switch 92 is closed only when the plunger 36' is extended. Alternatively, the configuration shown in Fig. 7 could be utilized. A negative temperature coefficient thermistor is placed in series with the circuit formed by the plunger 36" across the terminals 12",14". The warning system would monitor such a switch for low impedance and activate the warning device in response to the detection of such a low impedance condition.

Industrial Applicability

The preferred embodiment of the switch 10 is especially applicable to fluid systems incorporating a filter bypass circuit and is useful in activating a warning system in response to the clogging of the filter. This switch 10 is temperature compensated such that at low temperatures high viscosity of the fluid utilized in the fluid circuit will not result in activation of the warning system. The operation of the temperature compensated displacement indicating switch 10 is best understood by initially considering only those features which are not related to temperature compensation. The displacement detecting contact portion 37 of the switch 10 establishes one of a relatively high and a relatively low impedance condition intermediate its terminals 12,14 in response to the plunger 36 assuming a first position, and establishes the other of the relatively high and the relatively low impedance condition in response to the plunger 36 being in a second position. More generally, this displacement detecting contact portion 37 is termed means 41 for establishing electrical continuity intermediate the terminals 12,14 in response to said movable element 26 being in one of a first and second position. This change between impedance conditions resulting from movement of the plunger 36 is effected in the preferred embodiment by including a conducting element 68 on the plunger 36 which in only one of the first and second positions, causes electrical contact between the two terminals 12,14 establishing low impedance therebetween.

In an alternative embodiment, this change in impedance conditions is brought about by positioning a magnet 88 on the plunger 36 ^" and placing a reed switch 92 in series with the terminals 12,14. Changes in the

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magnetic flux at the reed switch 92 due to movement of the magnet 88 result in a change between the open and closed conditions of the switch 92 as the plunger 36 moves between the first and second positions. Those skilled in the art will recognize other suitable apparatus for opening and closing a circuit intermediate the terminals 12,14 in response to movement of the plunger 36.

The warning system constantly monitors the impedance of the switch 10. In response to the switch 10 assuming an impedance indicative of, for example, an apparent blocked filter condition, the warning system activates an alarm or other warning indicator.

As previously stated, an important feature of the present switch 10 is that within a certain temperature range, one of the impedance conditions is disallowed. That is, except within a preselected range of temperatures the switch 10 establishes an indication, by impedance change, of the occurrence or non-occurrence of a certain condition. Within this preselected range of temperatures the condition of the switch is independent of all other variables.

In the preferred embodiment of the present invention, in which a high impedance condition is indicative of a blocked filter condition, the high impedance condition is disallowed for that range of temperatures at which the fluid is relatively viscous. The positive temperature coefficient thermistor is connected intermediate the terminals 12,14 and provides a low impedance path therebetween at temperatures below the transition temperature of the thermistor, which in the preferred embodiment in 50 C. This switching temperature can be established at other points by selecting the proper thermistor valve.

In summary, the switch 10 can distinguish between bypass conditions occurring due to a plugged filter 28 at warm fluid operating conditions and viscous fluid at cold operating temperatures. Thus, the switch 10 serves to activate the warning system only when a bypass condition is in existance at fluid temperatures above a preselected level. It should be apparent from the foregoing, however, that the switch 10 is broadly useful for providing an output dependent upon the position of a movable element in a first range of temperatures while providing an invariant output in a second range of temperatures.

Other aspects, objects, advantages and uses of this invention can be obtained from a study of the drawings, the disclosure and the appended claims. It should be understood that the temperature compensated displacement indicating switch 10 can assume many configurations other than that detailed above as the preferred embodiment, without departing from the claims.