Sole plate temperature control including differential expansion

Details Sole plate temperature control including differential expansion Sole plate temperature control including differential expansion

1992-08-20

1994-06-28

Crowder, Clifford D.

Textiles: ironing or smoothing

Smoothing implements

Flatirons

219253, D06F 7526, H05B 102

Patent

active

053235509

DESCRIPTION:

BRIEF SUMMARY
BACKGROUND OF THE INVENTION

1. Field of the Invention
The present invention relates to electric irons, and more particularly to thermostatic controls for electric irons.
2. Description of Prior Art
A typical iron control consists of a bimetal actuator which actuates a microswitch which controls the electrical supply to the heating element. The bimetal is usually mounted by a screw on a boss on the sole plate, and is heated by conduction of heat from the sole plate. This results in a thermal lag which leads to the iron temperature overshooting its nominal, set temperature when heating up particularly on initial turn on. This is obviously undesirable as clothing may be damaged. Also, the sole plate temperature undershoots when the iron is cooling down before power is resupplied to the element. Thus the temperature variation of the sole plate is considerably larger than that measured for the thermostat alone.
Furthermore, depending on where the bimetal is mounted on the sole plate, it may experience a higher or lower temperature than the average sole plate temperature. For example if the bimetal is too close to the element, it will experience too high a temperature and the control will therefore initially trip at lower sole plate temperatures than desired and clothing will not be properly ironed.
Various steps have been taken to try to improve such controls, particularly with the advent of cordless irons where high wattages are employed to quickly bring the iron back to temperature when it is replaced on its stand, and where the problems of temperature overshoot are therefore greater.
One approach has been to introduce electronic controls. However these controls are inherently unsuitable for high temperature and high humidity conditions which are typical in steam irons. Other attempts have been made to improve the performance of the bimetals, but these have not been particularly successful.


SUMMARY OF THE INVENTION

The present invention therefore seeks to provide an improved thermostatic control which does not suffer from the inherent draw backs associated with the controls described above.
From a first aspect the present invention provides an iron comprising a sole plate, a low expansion reference member which is fixed at one end with respect to the sole plate and extends over a major portion of the length of the sole plate, and is connected at its other end to a switch means which is activated by the relative movement of said reference member with respect to said sole plate which occurs as a result of the expansion and contraction of the sole plate.
Thus in accordance with the invention, the thermal expansion of the sole plate itself is used to actuate a switch controlling the power supply to the element. The invention uses the critical member, the sole plate, directly and does not rely on conduction into a second member such as a bimetal. This leads to a control with a much improved response to temperature changes. The expansion and change in length of the sole plate is converted into an actuating movement by virtue of its expansion relative to a reference member of a material having a relatively low coefficient of thermal expansion such as Invar, glass or carbon fibre.
By using the expansion of the sole plate over a major portion of its length, not only is a larger movement obtained for actuation of the switch means, but also, the control will not be sensitive to local hot or cool spots.
Sole plates are normally constructed from aluminium or aluminium alloys. Typically therefore, extensions in sole plate length of the order of 1 mm are produced in conventionally shaped sole plates over the normal operating temperature range. This easily provides sufficient movement to actuate a switch, and a reference member of moderate cross-section will enable considerably more force to be provided than a normal bimetal.
The reference member may be a rigid member, such as a rod, but preferably it is a flexible member, which is held in tension with respect to the sole plate. In particularly preferred embodiments the

REFERENCES:
patent: 1535751 (1925-04-01), Turenne
patent: 1558077 (1925-10-01), Cheeseman
patent: 2024856 (1935-12-01), Graham
patent: 2154042 (1939-04-01), Gough
patent: 2201115 (1940-05-01), Reingruben et al.
patent: 2797292 (1957-06-01), Polivka
patent: 4345389 (1982-02-01), Balchunas
patent: 4433231 (1984-02-01), Balchunas

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