A Temperature Compensated Switch For Monitoring A Filter Bypass Valve

Technical Field This invention relates generally to a switch and, more particularly, to a switch for monitoring a filter bypass valve.

Background Art

Fluidic systems such as lubricating oil or fuel systems use filters in a fluid line to remove contaminants carried by the fluid. A filter which becomes clogged or plugged by contaminants will result in a decrease in the fluid flow. A reduction in fluid flow through the filter also can occur during cold fluid operating conditions when the viscosity of the fluid is high. ~

A loss of lubricant due to reduced oil flow or a loss of power due to reduced fuel flow will have obvious adverse consequences. Consequently, fluidic systems also have bypass vlaves which ensure fluid flow in the event of a plugged filter or highly viscous fluid during cold fluid operating conditions. Typically, a bypass valve can be a spool valve having a spool which is moved to open the valve and to bypass the fluid around the filter in response to a predetermined fluid pressure differential across the filter.

A switch for monitoring the open and closed states of the bypass valve has been employed to produce a signal when the valve is open or in the bypass mode. The switch may have electrical contacts and a pin or piston which is operatively coupled to

- L> __Λ _* ~-

and moves with the spool. When the spool is moved to the open or bypass position, the piston is moved to open the contacts to generate the signal. ^" When the spool is returned to the closed position, the piston follows the spool to close the contacts and, thereby, turn off the signal.

A problem with such a prior switch is that it cannot distinguish between a bypass mode due to a plugged filter or a bypass mode due to the high viscosity of the fluid at cold fluid temperatures. In either mode, the bypass valve will be open, resulting in the switch generating the signal. This is disadvantageous since it usually is of primary importance to know that the bypass valve is open due to a plugged filter which should be cleaned or replaced, rather than being open due to a viscous fluid.

Another problem with the prior switch is that its piston will follow or move with the spool at all times that the bypass valve changes between open and closed states. Consequently, the piston is continually subjected to wear, which can reduce its lifetime.

The present invention is directed to overcoming one or more of the problems as set forth above.

Disclosure of the Invention

In one aspect of the present invention, a switch for monitoring a fluid bypass valve has a movable member adapted to be operatively coupled to the fluid bypass valve, means for moving the movable member towards the fluid bypass valve, and means for generating a signal in response to a predetermined

movement of the movable member, the improvement comprising means for sensing the temperature of the fluid and inhibiting movement of the movable member in response to a preselected temperature of the fluid.

A prior filter bypass monitoring switch, in producing such a signal, cannot distinguish between bypass modes due to a plugged filter or viscous fluid, and has a piston which continually follows the bypass valve, causing undue piston wear. The switch of the present invention has a fluid temperature sensor which inhibits movement of its movable member only during cold fluid operating conditions below the threshold temperature when the bypass valve can be in a bypass mode due to viscous fluid, thereby distinguishing between the two bypass modes and reducing wear of the movable member.

Brief Description of the Drawings

Fig. 1 is a block diagram of a fluidic system.

Fig. 2 is a cross-section of an embodiment of the present invention.

Best Mode for Carrying Out the Invention

Fig. 1 shows a fluidic system 10 having a fluid filter 12 which filters fluid being received at an inlet 14 and flowing out an outlet 16. A fluid bypass valve 18 is coupled in parallel across filter 12 to transfer fluid from inlet 14 to outlet 16, bypassing filter 12, in response to a ^' predetermined differential fluid pressure across filter 12. Such a differential fluid pressure can occur typically under two conditions. One condition is when the

filter is plugged, causing a higher pressure at the inlet 14 in relation to the outlet 16. The other is during cold fluid operating conditions when the fluid is viscous, causing a similar differential fluid pressure across filter 12.

Bypass valve 18 has a movable control member 20 which responds to this differential fluid pressure to open valve 18 and transfer fluid from inlet 14 to outlet 16, bypassing filter 12. Bypass valve 18 can be, for example, a spool valve or a check valve. If, for example, bypass valve 18 is a spool valve, then control member 20 can be spool spring biased to a closed state of valve 18. A switch 22 of the present invention can be operatively coupled to bypass valve 18, particu¬ larly control member 20, to monitor the bypass valve 18. Switch 22 is constructed to distinguish between a bypass mode of valve 18 due to a plugged filter 12 and a bypass mode of valve 18 due to viscous fluid at cold fluid temperatures. ,

As illustrated in Fig. 2, switch 22 has a housing 24 which supports a movable member 26. As an example, movable member 26 can be a hollow piston 28 that is slidable along housing 24 and that has an end 30 which can be positioned or operatively coupled adjacent control member 20. Piston 28 has at least one aperture 32 through which some fluid can flow into a chamber 34 for reasons to be described, and is retained in housing 24 by a retainer 36.

A means 38 for moving the movable member 26, specifically piston 28, is supported in housing 24. Means 38 includes a washer 40 secured to piston 28 and a spring 42 disposed between washer 40 and

housing 24. Spring 42 is disposed to bias or move washer 40 and, hence, piston 28, to the right, as viewed in Fig. 2, i.e., towards or against control member 20 to follow the travel of member 20. The aperture 32 produces a fluid path to prevent any differential pressure from developing across piston 28, which differential pressure otherwise might prevent spring 42 from displacing piston 28.

Switch 22 also includes means 44 for generat- ing a signal in response to a predetermined movement of the piston 28 to the right as viewed in Fig. 2, i.e., in response to movement of the piston 28 a certain distance. Signal generating means 44 includes a magnet 46 that is supported by and movable with piston 2-8, and that has north and south poles as shown. A magnetically operated switch such as a normally open reed switch 48 is disposed in a bore 50 of housing 24 and has an electrical connection 52 which can lead to an alarm such as a light (not shown) . As shown in Fig. 2, magnet 46 is in a position to close reed switch 48 to prevent generating the alarm signal. Should the piston 28 be moved a sufficient distance to the right by spring 42 and washer 40, magnet 46 will be sufficiently displaced so that reed switch 48 will open to generate the alarm signal.

Switch 22 also includes means 54 for sensing the temperature of the fluid and inhibiting movement of the piston 28 in response to the fluid temperature being below a preselected temperature or temperature threshold. Means 54 includes wax motor 56 ^• that has a piston 58. A plug 60 is secured to the housing 24 by a screw and lock washer assembly 61. Piston 58 is coupled to or received by plug 60 in a

notch 62. Means 64, such as an extension spring 66, is disposed between wax motor 56 and housing 24 to bias wax motor 56 to the left, as viewed in Fig. 2, i.e., towards plug 60 and against the bias of spring 42 by acting on washer 40.

When the temperature is below the tem¬ perature threshold, e.g., 125°F, wax motor 56 will be in the position shown in Fig. 2. When the tem¬ perature is above the temperature threshold, the wax in wax motor 56 will melt, resulting in the wax motor 56 moving to the right in hollow piston 28 as piston 58 remains stationary, extending spring 66. As wax motor 56 moves to the right, spring 42 becomes extended to move washer 40 and, hence, piston 28 to the right.

Industrial Applicability

The fluidic system 10 can be part of a lubricating circuit of a vehicle such as an earth- working vehicle. Three operating conditions will be discussed to explain the operation of switch 22, including a cold fluid operating condition, a warm fluid operating condition in which filter 12 is not plugged and a warm fluid operating condition in which filter 12 is plugged. During cold fluid operating conditions when the lubricating fluid is viscous, e.g., below 125°F, a differential fluid pressure can build up across filter 12, causing control member 20 to move to the right as viewed in Fig. 1 and Fig. 2. Conse- quently, bypass valve 18 opens to conduct fluid from inlet 14 to outlet 16. Wax motor 56 senses the cold fluid temperature and remains in the position shown in Fig. 2 so that reed switch 48 remains closed and

OMPI

the alarm signal is not generated. In other words, piston 28 does not move to the right to follow the movement of control member 20.

During warm fluid operating conditions in which filter 12 is not plugged, the differential fluid pressure across filter 12 will not build up so that bypass valve 18 remains closed with control member 20 adjacent piston 28. Wax motor 56 senses the warm fluid temperature and moves to the right as shown in Fig. 2. Spring 42 will provide a force tending to move piston 28 to the right; however, the control member 20 overcomes this force to act on piston 28 and prevents piston 28 from moving to the right. Therefore, magnet 46 remains in the position shown and the alarm signal is not generated.

During warm fluid operating conditions in which filter 12 is plugged, the differential fluid pressure across filter 12 will build up so that by¬ pass valve 18 opens, control member 20 being moved to the right. Wax motor 56 senses the warm fluid temperature and moves to the right. Spring 42 can now move piston 28 to the right to follow control member 20. Consequently, magnet 46 is moved sufficiently to the right to open reed switch 48 and generate the alarm signal.

In summary, switch 22 can distinguish between bypass modes occurring due to a plugged filter 12 at warm fluid operating conditions and viscous fluid at cold fluid operating temperatures. The wax motor 56 inhibits movement of the piston 28 during cold fluid operating temperatures, thereby limiting wear and increasing the lifetime of piston 28.

Furthermore, wax motor 56 is highly accur-

ate to about + or - 1% of the temperature threshold for which it is designed. Also, because of the north-south orientation of magnet.46 in relation to reed switch 48, vibration, which might move piston 28 a short distance in housing 24 from the Fig. 2 position, normally will not move magnet 46 so far as to cause reed switch 48 to open and produce a false warning signal. Still furthermore, with reed switch 48 being normally open, a fail-safe circuit is provided. For example, with magnet 46 in the position shown and reed switch 48 closed, a loose connection 52 will open the circuit to generate the alarm signal though this would not result from a plugged filter 12.

Other aspects, objects and advantages of this invention can be obtained from a study of the drawings, the disclosure and the appended claims.