Tire pressure indicator
This invention relates to a tire pressure indicator for vehicle wheels. 
     Various types of wheel-mounted tire pressure indicators have been proposed heretofor, but, to our knowledge, none has gained wide commercial acceptance. The lack of such acceptance results from the cost, the service problems resulting from such indicators and also the inconvenience involved in their use. 
     The present invention has for its primary object the provision of a tire pressure indicator which is not only of economical construction, but also reliable operation over an extended period of time and convenient to use. 
     More specifically, this invention contemplates a self-contained tire pressure indicator in the form of a housing adapted to be mounted on a vehicle wheel and arranged to illuminate a signal lamp on the housing when the tire pressure is below a selected predetermined value. In addition, the indicator of this invention is adapted to indicate the pressure of the tire relative to any preselected pressure by simply substituting one spring for a differently calibrated spring. 
     The invention likewise comprises a self-contained unit which not only signals under-inflation of the tire by energizing a light, but also indicates by a visual scale the extent to which the tire is either under- or over-inflated. 
    
    
     Other objects, features and advantages of the present invention will become apparent from the following description and accompanying drawings, in which: 
     FIG. 1 is a fragmentary view of a vehicle wheel showing the tire pressure indicator of this invention mounted thereon; 
     FIG. 2 is a view of the tire pressure indicator with the cover removed therefrom; 
     FIG. 3 is a view taken along the line 3--3 in FIG. 2 and showing the position assumed by component parts thereof when the tire is under-inflated; 
     FIG. 4 is a fragmentary view of a portion of FIG. 3 showing components thereof when the tire is properly inflated. 
     FIG. 5 is a fragmentary sectional view along the line 5--5 in FIG. 2; 
     FIG. 6 is a fragmentary sectional view along the line 6--6 in FIG. 2; 
     FIG. 7 is a view similar to FIG. 5 showing a modified form of mounting for the electrical switch; and 
     FIG. 8 is an end view of the mounting illustrated in FIG. 7. 
    
    
     Referring to FIG. 1, there is illustrated at 10 a truck wheel having a tire 12 mounted thereon. The tire pressure indicator of the present invention, which is generally designated 14, is mounted on one of the conventional wheel lugs 16 and locked in place by a lug nut 18. A rubber or other flexible tube 20 is connected at one end to indicator 14 and at its other end to the conventional valve stem 22 on the wheel having the valve removed therefrom. Referring more particularly to FIGS. 2 and 3, indicator 14 comprises a housing 24 which is molded or otherwise formed of a non-metallic material, such as plastic. At one end thereof housing 24 is provided with an apertured extension lug 26 in which a metal bushing 28 is secured for facilitating mounting of the unit securely on a vehicle wheel in the manner illustrated in FIG. 1. 
     The interior of housing 24 is partitioned into a battery compartment 30, an electrical circuit board compartment 32 and a cylinder compartment 34. These compartments are closed by a cover 36 which is partially retained in place by a screw 38 threaded into a lug 40 in the housing as at 42. A gasket 43 forms a dirt and moisture seal between the cover and housing. Within compartment 34 there is arranged a cylinder body 44 which has a threaded fitting 46 formed integrally at one end thereof. Fitting 46 extends through an aperture 48 in one end wall of housing 24 and is secured in place by a nut 50. Cylinder 44 is formed of a non-magnetic material, such as aluminum, and is provided with a central bore 52 and a threaded cross bore 54 communicating with bore 52. A conventional valve stem 55 is threaded into cross bore 54. Valve stem 55 extends upwardly through a shroud 56 formed integrally with cover 36. A valve cap 57 is threaded over the upper end of valve stem 55 and, when tightened, bears down on the upper end of shroud 56 and forms a seal around the upper end thereof as shown in FIG. 3. 
     The end portion of bore 52 opposite fitting 46 is formed with two concentric shoulders 58,60 which define reduced bore sections 62 and 64 respectively. Within bore portion 64 there is arranged a bearing 66 having a flange which abuts against shoulder 60. Bearing 66 is formed of a non-metallic, low-friction material, such as nylon. Within bore 52 there is arranged a lubrication bushing 68 having a flange which abuts against shoulder 58. An O-ring 70 is arranged in bore portion 62 between bushing 68 and bearing 66. Bushing 68, preferably formed of an oil-impregnated, sintered, powdered metal, provides lifetime lubrication to the oil ring. Shoulder 58 prevents O-ring 70 from being squeezed by bushing 68 and thereby reduces the &#34;break-away&#34; friction of the O-ring. 
     A piston 72 is slideably arranged in bore 52. Piston 72 comprises a rod extending through bushing 68 and bearing 66. Rod 72 is formed of a non-magnetic material, such as aluminum, and has a very close fit with the bore of bearing 66. One end of rod 72 is provided with an enlarged head 74 which is preferably hex-shaped with rounded corners so as to have a smooth sliding fit with bore 52 and so that air is free to flow across head 74 through the passageways defined between the flat faces of the hex-shaped head and the cylindrical surface of bore 52. The opposite end of rod 72 projects outwardly through the end of cylinder 44 and has a retainer 76 fixedly secured thereto as by a lock screw 78. Piston 72 is urged axially toward the fitting end thereof by means of a light-weight spring 80 which at one end bears against and is centered by bushing 68 and which at its other end is centered by and bears against the underside of head 74. The sole function of spring 80 is to retain bushing 68 against shoulder 58. 
     Retainer 76 is formed as a plastic or aluminum cylindrical sleeve having a centrally apertured partition 82 intermediate its ends. One end of retainer 76 telescopes over the adjacent end of cylinder 44 and abuts against a radially outwardly extending flange 84 on cylinder 44. Within the cavity 86 at the other end of retainer 76 there is located a circular magnet 88. A non-magnetic compression spring 90 which is relatively heavy in comparison to spring 80 has one end bearing against magnet 88 and the opposite end bearing against partition 92 in housing 24. Spring 90 biases the piston 72 with the retainer 76 thereon in a direction toward the left as seen in FIGS. 2 and 3. Over-compression of spring 90 is prevented by a shoulder 94 in housing 24 against which the adjacent end of retainer 76 is adapted to abut when piston 72 is displaced in a direction towards the right as viewed in FIG. 3. 
     Retainer 76 has bonded to the outer cylindrical surface thereof a plurality of color bands 96,98,100,102. These color bands are visible from exteriorly of the housing through a transparent window 104 on cover 36 which overlies retainer 76. Window 104 has a straight indicia line 106 thereon which is aligned with one of the color bands 96 - 102, depending upon the axial displacement of retainer 76. As shown in FIG. 4, these color bands may be arranged successively so that band 96 is red; band 98 is yellow; band 100 is green; and band 102 is red. If desired, numerical values indicating tire pressures may also be spaced along retainer 76 in the color band zone. Spring 90 is calibrated so that, when the tire is inflated to a desired preselected pressure, indicia line 106 will register with green color band 100 to indicate proper inflation of the tire. Accordingly, if the tire is overinflated indicia line 106 will register with band 102. If the tire is slightly under-inflated line 106 will register with band 98; and, if the tire is excessively underinflated, line 106 will register with band 96. It will be realized, of course, that the extent to which spring 90 is compressed and retainer 76 displaced in a direction toward the right as viewed in FIG. 3 will depend upon the differential pressure across piston 72. It will also be appreciated that, if the indicator is set to designate a proper tire inflation of 40 pounds per square inch and it is desired to use it on a vehicle wheel the tire pressure of which should be 60 pounds per square inch, then it is only necessary to select a stiffer or longer spring 90 so that when the tire pressure is 60 pounds per square inch line 106 on window 104 will register with color band 100. 
     As shown in FIG. 2, within compartment 30 there is located a battery 108 which is connected by conductors 110 with an electronic circuit mounted on a circuit board 112 which is sized to be snugly received in compartment 32. The circuit includes, among other things, a signal light 114, a switch 116 and a push button switch 118. Switches 116, 118 are in parallel circuit with light 114 and battery 108 so that if either of these switches is closed signal light 114 will be illuminated. A transparent cap 120 on cover 36 registers with light 114 to visibly indicate when the light is illuminated. Likewise, a depressible rubber cap 122 on cover 36 registers with push button switch 118 so that the switch can be actuated from exteriorly of the housing to determine whether or not the circuit and signal light 114 are operating properly. 
     In the arrangement illustrated, switch 116 is a magnetically responsive reed switch which is closed by magnet 88 when the magnet is displaced by piston 72 to a position directly adjacent the switch. More specifically, switch 116 is located on circuit board 112 so that when the tire is inflated to a pressure below a predetermined selected value magnet 88 will be located as shown in FIG. 3 directly adjacent switch 116 to close the same and thereby energize light 114. As shown in FIG. 5, switch 116 has stiff wire leads 124 projecting from its opposite ends which are bent downwardly and secured to the underside of circuit board 112 in the electrical circuit. If necessary, switch 116 can be adjusted axially of retainer 76 by simply bending the leads 124 so that the switch will be properly located relative to magnet 88 when the piston is displaced to a position to indicate under-inflation of the tire. 
     In the alternate construction shown in FIGS. 7 and 8 switch 116 is connected into the circuit by means of a pair of upstanding slotted terminals 126 in the circuit which are secured to the underside of circuit board 112. The upper ends of terminals 126 are apertured and notched as at 128 to slideably receive the leads 124 of reed switch 116. With this arrangement switch 116 can be shifted axially of retainer 76 to the proper position and thereafter the upper ends of terminals 126 can be crimped and/or soldered to firmly secure the leads 124 thereto. 
     In operation the proper spring 90 is assembled in the indicator and the indicator with the cover 36 removed is placed in a fixture designed to displace piston 72 such that a pair of transversely aligned notches 130 in the upper edges of housing 24 will be aligned with the color band 100 on retainer 76. Notches 130 are located such that they will align with the color indicia line 106 on window 104 when the cover is applied to the housing. With the piston 72 displaced by the fixture in the manner described, switch 116 is shifted axially in a direction such that signal light 114 is de-energized and will again be energized when retainer 76 is shifted slightly in a direction towards the left as viewed in FIG. 3. After the indicator is so assembled and switch 116 so adjusted the circuit board is secured in place on four stand offs 132 molded in the bottom of cavity 32 and the entire cavity 32 is filled with an electrical potting material. The cover 36 is applied to housing 24 and secured thereto by screw 38 and valve cap 57. The unit is then applied to a lug of the wheel as shown in FIG. 1 in a position such that piston 72 extends generally circumferentially of the wheel. Tube 20 is then connected to valve stem 22 and fitting 74. The indicator is then operative to indicate whether or not the tire is over- or underinflated. 
     To initially determine whether the entire circuit is in working order, rubber cap 122 is depressed to actuate push button switch 118. This will illuminate signal light 114 if the unit is in proper working order. Thereafter, if the tire is inflated to the desired preselected pressure, signal light 114 will not be illuminated since magnet 88 will have been displaced to a position out of the range of actuating switch 116. By the same token, indicia line 106 will register with color band 100 on retainer 76. When the tire pressure diminishes below the pre-selected value, spring 90 will shift retainer 76 to the left to a position wherein magnet 88 will shift retainer 76 to the left to a position wherein magnet 88 will close switch 116 and illuminate signal light 114. Light 114 will be visible through the transparent cap 120 even when the vehicle is operating. When the vehicle operator, a truck driver for example, stops the vehicle, he can walk around it and determine which, if any, of the tires are under-inflated by simply determining whether any of the signal lights 114 are illuminated. If the signal light of a particular wheel is illuminated, he can determine the extent of under-inflation by visually observing the position of indicia line 106 relative to color bands 98,96. If indicia 106 registers with color band 98 he can assume that the tire is only slightly under-inflated and may decide to continue his trip to the next service station. On the other hand, if indicia line 106 registers with color band 96, he may determine that the tire needs further inflation or replacement at that point. Inflation can be effected by applying air to the tire through valve stem 55. 
     Thus, the indicator of the present invention provides a very useful instrument for vehicle operators which is convenient to use. It is constructed so as to be very reliable over an extended period of time. The axial displacement of piston 72 is substantially unaffected by the centrifugal force resulting from the rotation of the wheel since it extends circumferentially rather than radially. In addition, retainer 76 is formed as an elongated sleeve and is arranged in overlapping relation with the cylinder body 44 not only to minimize the length of the unit while permitting visual indication of a wide range of tire pressures, but also to maintain the center of mass of the movable portion of the indicator relatively close to the bearing 66.