GB2024422A

GB2024422A - Temperature-responsive actuating-element

Info

Publication number: GB2024422A
Application number: GB7918824A
Authority: GB
Original assignee: Delta Materials Research Ltd
Current assignee: Delta Materials Research Ltd
Priority date: 1978-05-31
Filing date: 1979-05-30
Publication date: 1980-01-09

Abstract

A trip mechanism, which can be used to actuate a control device, consists of a spring member 13 which is normally held in a stressed position by a latch member 12 made of a shape memory effect alloy. As the temperature of the latch member increases through the transition temperature range of the alloy, the latch member releases the spring member which then moves toward its unstressed position 13' and actuates the control device. <IMAGE>

Description

SPECIFICATION Trip mechanism This invention relates to a trip mechanism which is operated when the temperature exceeds or falls below a given value. There are many applications for such a trip mechanism, as when an actuating function is to be performed when the temperature exceeds a safe value; an example is the automatic cut-out on an electric kettle, where the supply of current to an immersion heating element is to be discontinued when boiling occurs or when the kettle boils dry. In this instance and in others, it is beneficial if the mechanism does not automatically reset when the temperature reverts from beyond the given value, e.g. falls back to a safe value, but instead requires to be reset manually.

Trip mechanisms of the present invention make use of materials displaying shape memory effect (SME). An SME material as used in this specification is defined as a material which exhibits, over a transition temperature range determined by the composition of the material, a significant and reversible change in modulus of elasticity with temperature; in effect, the stiffness of the material increases with temperature. The material is preferably an alloy which has been treated to a condition in which it undergoes a reversible martensitic transformation during change of temperature through the transition range. A copper-zinc-aluminium alloy is preferred.

The present invention resides in a temperatureresponsive trip mechanism comprising a latch member made at least in part of SME material as defined herein and a cooperative spring member, the latch member being arranged to alter its disposition, on change of temperature in the transition temperature range, between a first disposition, in which the latch member can retain the spring member in a stressed condition, and a second disposition in which the spring member is released by the latch member releasing the spring member on change of temperature. Preferably, the trip mechanism does not reset automatically on reverse temperature change. The trip mechanism may be employed to actuate a control device, such as an electric switch, the spring member on release actuating the control device either directly or through a remote control link.Both the latch member and the spring member may be made of SME material and may be integral with one another. It is however preferred to have the spring member made of steel of other suitable metal which does not display the shape memory effect, since its mechanical properties do not need to be thermally dependent.

The latch member may be secured in heattransmitting contact with a surface, the temperature of which is to be sensed.

The invention will be more readily understood by way of example from the following description of trip mechanisms in accordance therewith, reference being made to the accompanying drawings, Figures 1 and 2 of which illustrate alternative forms.

The trip mechanism of Figure 1 consists of a latch member 12 and a spring member 13 which are secured together to a wall 14, as by a rivet 1 5.

The spring member 1 3 is in the form of a generally U-shaped strip of steel having an end portion 1 6 bent away from the wall 14. The latch member 12 when in the condition shown has a portion 1 7 directed away from the wall 14 and terminates in a downwardly directed lip 18 forming a catch with which the free end of the spring member 13 engages and which can retain the spring member in a stressed condition, as shown.

The latch member 12 is made of an SME alloy which originally had the shape indicated by chain line at 12', but which was elasto-plastically deformed to the shape shown in full line at a temperature below the transition temperature range. When the temperature of the spring member rises through that range, as by heat conduction from the heated wall 14, it returns towards its shape prior to deformation and, in so doing comes out of engagement with the spring member 1 6 and allows that member to spring forwardly to the position shown in chain line at 13'. Relatching can only be performed manually and when the temperature of the latch member has fallen sufficiently for the latch member to return to the latching disposition, the resilience of the spring member enabling it to be forced past the lip 18.

The composition of the SME alloy is so chosen that the upper part of that range encompasses the temperature at which the trip mechanism is to operate. If for example, 14 is the wall of an electric kettle and the trip mechanism is to operate when a boil-dry condition exists, the trip mechanism should operate at a temperature in excess of 1 00 C, such as 1 300C. A preferred SME alloy is a copper-zinc-aluminium alloy having the following approximate composition by weight: Copper 70% Aluminium 4% Zinc 26% the precise composition varying to a small degree according to the required transition temperature range. Tripping of the mechanism may operate a rod 20, or other remote control link, to actuate a control device such as an electric cut-out switch.

It is not necessary for the entire latch member 12 to be made of an SME alloy although it is practically advantageous that it should be. For proper operation, it is necessary that only that part which bends away from the wall should be of SME alloy, and the catch part 18 may be made of another material.

In the construction of Figure 2, the latch member and the spring member are made integrally of SME alloy. In that case the spring part 21 as weil as the latch part 22 may tend to change shape as the temperature rises through the transition range. However, the spring part should be in an elastically stressed state in the latched position at least immediately before release, so that, when released, it moves immediately away from the latched position as indicated in chain line.

Claims

1. A temperature-responsive trip mechanism comprising a latch member made at least in part of SME material as defined herein and a cooperative spring member, the latch member being arranged to alter its disposition, on change of temperature in the transition temperature range, between a first disposition, in which the latch member can retain the spring member in a stressed condition, and a second disposition in which the spring member is released by the latch member.

2. A trip mechanism according to claim 1, in which the spring member is U-shaped with one leg secured in position and the other leg resilientiy movable, and, in its first disposition, the latch member lies in the path of movement of that other leg of the spring member.

3. A trip mechanism according to claim 1 or claim 2, in which the spring member is integral with the latch member.

4. A trip mechanism according to any one of the preceding claims, in which the latch member is secured to a surface, the temperature of which is to control the release of the spring member.

5. A trip mechanism according to any one of the preceding claims, in combination with a control device which is arranged so as to be actuated by the spring member on release by the latch member.

6. A temperature-responsive trip mechanism, substantially as herein described with reference to the accompanying drawing.