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Low tire pressure detector

  • US 4,134,102 A
  • Filed: 11/14/1977
  • Issued: 01/09/1979
  • Est. Priority Date: 11/14/1977
  • Status: Expired due to Term
First Claim
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1. In a low tire pressure warning device, a combined low pressure detector and magnetic signal sending unit having a magnet assembly movable within a housing to an outer signal emitting position to generate a minimum strength magnetic flux field outside the housing and to an inner shielded position for which the magnetic flux field outside the housing is substantially below the said minimum strength, an expansible bellows responsive to tire air pressure for moving the magnet assembly to the outer position when tire pressure decreases below a set value and to the inner position when tire pressure increases to a value above the set value, the bellows having a spring force urging bellows expansion and charged inside with a gas at a reference pressure and exposed outside to tire air pressure, one end of the bellows secured relative to the housing and the other end of the bellows being free to move to effect bellows expansion upon decrease of tire pressure relative to the reference pressure, a slug of ferro-magnetic material secured to the free end of the bellows, such that the magnet assembly is attracted to the slug to form a releasable connection between the magnet assembly and the slug, a magnetic flux shunt element carried by the housing and positioned to contact the slug to provide a limit stop for bellows expansion, the flux shunt element including a tubular shield having its axis aligned with the direction of bellows movement, the magnet assembly being partially disposed in the said tubular shield and including a magnet having at least a minimum strength and a polepiece secured adjacent to one pole of the magnet, the polepiece having a radially extending circumferential flange disposed outside the tubular shield to abut the end of the shield to provide a limit stop for the magnet when in shielded position and to provide a flux path from the magnet and through the flux shunt element, the circumferential flange having a preset axial dimension sufficiently thin that it is saturated by the magnetic flux from the magnet when the flange is in contact with the tubular shield to thereby provide a present constant magnetic latching force between the flange and the shield, the bellows when contacting the magnet assembly in the inner shielded position having a spring force greater than the said preset latching force so that in the event of a loss of pressure differential across the bellows the bellows force alone is sufficient to overcome the preset latching force and move the magnet assembly to signal emitting position.

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