Patent GB-1061813-A (GB)
Title: Improvements in or relating to devices for detecting pressure losses in pneumatic tyres
Inventor: BULGIN DOUGLAS
IPC: B60C23/04
Filed: 1962-11-05
Abstract: 1,061,813. Indicating pneumatic tyre pressures. DUNLOP RUBBER CO. Ltd. Oct. 24, 1963 [Nov. 5, 1962], No. 41729/62. Heading B7C. Apparatus for detecting loss of pressure within a pneumatic tyre comprises magnetizable members within which a magnetic flux can be set up, at least one magnetizable member being mounted on the vehicle chassis, together with means operable according to change of pressure with the tyre for varying a flux set up in the magnetizable members and means for detecting a change in the magnetic flux in one of the members. In the embodiment shown in Fig. 1, a member 1 comprises two coaxial coils 2, 3 mounted in annular recesses 4 formed in a radially extending surface of a magnetizable core 5 of mild steel. The member 1 is coaxially mounted on a radially extending flange 6 of a brake drum 7 for rotation therewith. The coils 2, 3 are connected in series to a switch 10 which is operated by the pressure within a tyre, the switch being held closed until the pressure falls below a predetermined minimum value, when the circuit is broken. A member 11 is secured to a non-rotatable backplate 9 of the vehicle chassis and comprises an exciter coil 12 facing the coils 2, 3 and located in two recesses formed in a rectangular magnetizable core 14 of mild steel. A similar member 15 is secured in a diametrically opposite position on the back-plate and comprises a pick-up coil 16 for detecting changes in magnetic flux. The coil 16 is connected in series to an amplifier, and a relay circuit for operating warning means which comprises a buzzer located on the vehicle dashboard. The exciter coil 12 is connected to an alternating current supply so that when the wheel is rotating and the switch 10 is closed a current is induced in the coils 2, 3 which in turn induce a current in the pick-up coil 16. As soon as the switch 10 is opened by a drop in pressure the circuit is broken so that no current is set up in the coils 2, 3 or in the pick-up coil. The relay circuit is arranged to operate the buzzer when the signal from the amplifier ceases. In modifications of this embodiment the coils 2, 3 are received in recesses in the brake drum flange 6, or a single rotatable coil may be provided. In a second embodiment (Fig. 9, not shown), a U-shaped permanent magnet (26) is secured to the back-plate. Two coils (28), Fig. 8 (not shown), surrounding the arms of the magnet are connected in series to an amplifier (29), a trigger circuit (30) and a relay circuit (31) which, when the coils emit a signal of less than a predetermined strength, is operated by the trigger circuit to sound a buzzer (32). A ferro-magnetic rod (43) is slidably mounted for axial movement upon the brake drum and is held against the action of a compression spring (44) by a pressure capsule (46) mounted on the brake drum and abutting the rod (43). The rod (43) is positioned so that its end remote from the capsule (46) is flush with the surface of the brake drum facing the magnet at the operating pressure of the tyre. The capsule (46) is connected to the inflationary chamber of a tyre so that a decrease in pressure causes the rod (43) to move away from the magnet. Thus, for each revolution of the wheel, the magnetic flux is reduced. This results in a reduction of current induced in the coils (28), which if sufficiently great causes the trigger circuit to operate the buzzer. In a modification the capsule (46) may be suitably coupled to the rod (43) so that a reduction in tyre pressure causes the rod to move towards the magnet (26). In a second modification a ferromagnetic bar, which is operable by the pressure capsule, is pivotally mounted on the brake drum axle, while its free end is movable in an axial slot in the brake drum. In a further modification; a ferromagnetic rod is rigidly secured at one end to the brake drum flange by means of a non-magnetizable member. When the tyre contains the correct pressure, the pressure capsule holds a plate in position to connect the free end of the rod to the brake drum surface and thus complete a magnetic circuit around one arm of the magnet, the rod, the plate, the brake drum and the other arm of the magnet. A pressure decrease causes the plate to retract thus reducing the current induced in the coils connected to the buzzer.