Expansion or elastic material working element, particularly for operating a thermostatic valve, having a housing and a movable working pin led out of the housing, the housing containing expansion material and a heating device with an electrical resistance element applied to a support. More particularly for increasing the response speed of such a device, according to the invention the support has a cavity and the resistance heating element is a heating conductor wound around the support.

The invention relates to an expansion or elastic material working element, particularly for operating a thermostatic valve, having a housing and a movable working pin moved out of the housing, in which in said housing are located expansion material and a heating device with an electrical resistance element applied to a support.

Particularly in the cold running phase, engines have increased consumption and emission values, so that said phase must be shortened. This is brought about by reducing the circulated coolant quantity during the cold running and warm-up phase. For this purpose the coolant circuit of an internal combustion engine has a main valve blocking the through-flow in the case of cold coolant and a bypass valve which then opens. The regulation of such a valve on the basis of engine and operating data such as exhaust temperature, speed, torque and oil temperature takes place by means of a preamble-basing device for controlling such a thermostatic valve by the supply of electric power to the heating device.

EP 718 738 A2 discloses a heating device with a planar support, to which is applied a thick-film resistor. It is disadvantageous that the thick-film resistor is only in contact on its side remote from the support with the expansion material, such as expansion material wax and through the construction of the support the heating surface is small. As a result of the central, diagonal arrangement of the support the travel or stroke of the operating pin is limited.

DE 197 05 721 A1 discloses a preamble-basing device, in which the PTC element is located on the outside of the housing. The heat transfer to the expansion material in the housing is poor. This fundamentally also applies in the case of DE 42 33 913 A1 and DE 44 09 547 C2, where a PTC element is frontally located in the interior of the housing and only has a small contact surface with the expansion material.

The object of the invention is to further develop a device according to the preamble in such a way that with an inexpensive and reliable manufacture, there is a short response speed (transient response) and intrinsic operating reliability.

In the case of a preamble-basing device, the invention solves this problem in that the support has a cavity and the resistance heating element is a heating conductor wound around the support.

As a result of the construction according to the invention the resistance heating element in the form of a heating conductor has a limited weight and can therefore be very rapidly heated, the construction as a heating conductor permitting a large heat transfer surface to the expansion material, which is assisted by the construction of a cavity in the heater circuit. In addition, said cavity allows a long travel of the operating pin.

Particularly compared with the known thick-film element and for the same dimensions, more particularly the same external dimensions of the housing, the invention gives a six times larger heat emission surface, so that the specific heat output is correspondingly reduced for the same power supply, which significantly decreases the risk of local overheating. As a result of the cavity and for the same casing dimensions, the heating device can have a greater axial length without reducing the travel of the operating pin. This allows a uniform heating of the expansion material due to its poor thermal conductance over a large part of the housing height. The reaction times of the device according to the invention are significantly reduced.

According to a preferred development of the invention the heating conductor has a positive coefficient of the electrical resistance, i.e. the resistance-temperature characteristic increases in monotonic manner in the working range.

This eliminates the risk of an overheating of the expansion material and a destruction of the inventive device through the self-regulating characteristics of the heating element.

According to a preferred development, the heating conductor has a positive temperature coefficient of the electrical resistance of >2×10−3/K. Thus, in the standard temperature range of the coolant of an internal combustion engine, the heat output of the resistance heating element is reduced by roughly a third, so that the heat supply through the heating element is lower at high temperatures than the heat dissipation through the coolant flowing round the expansion material, so that overheating is prevented.

According to a preferred development of the inventive device, the support has a substantially cylindrical outer contour and the support body a many-sided outer contour and in particular the support has a cage-like structure. A cage-like structure can be formed in that the support has longitudinal and transverse ribs. The heating conductor is preferably wound around the longitudinal ribs of the support. This ensures an all-round contact between the heating conductor and expansion material, so that the response speed is increased.

For the all-round insulation of the individual heating conductor turns, said heating conductor is enveloped by a high temperature-resistant insulating layer. The insulating layer or coating can be of polyimide lacquer, ceramic or glass.

According to a preferred development, the support body is made from high temperature-resistant material. The support can in particular be made from heat-resistant plastic, such as polyphenylene sulphide (PPS) or liquid crystal polymer (LCP), a polymer reinforced by fibre formation, but also epoxy resin, such as Araldite(c), and finally also ceramic, particularly injection mouldable polymer ceramic, i.e. polymer-bound ceramic in the starting state.

A heating conductor with a positive temperature coefficient preferably comprises an alloy based on an iron-nickel alloy, which can also contain further elements such as chromium. The nickel proportion is in particular between 65 and 75%, being preferably approximately 70%, the iron proportion is 25 to 35%, preferably being approximately 30% and the chromium proportion is optionally in the range 0.5 to 2%, preferably approximately 1%.

The specific electrical resistance of such a PTC wire rises from approximately 0.3 Ωmm2m−1at room temperature of 200 in substantially linear manner with the factor Cδ=1+αΔ T for determining the resistance at different temperatures, the temperature coefficient α in the largely linear application range being approximately α=3000×10−6/C°.

According to a preferred development of the invention, the support is sealed by an O-ring in the housing of its contact part-carrying ends, because in particular the O-ring is positioned between the housing and support in a ring step of the support body or the O-ring is located in a circumferential annular groove positioned laterally on a base of the support. Thus, the support can be sealingly connected to the housing. This construction is particularly advantageous if, as in accordance with a further development, contact parts are moulded into a base of the support. A rational manufacture of the support body with contact parts therein and therefore the inventive device is achieved with a method for the manufacture of a support for such a device, in which contact parts are supplied to the injection mould for a support for a heating conductor in the form of a blanking skeleton.

A device1according to the invention has a substantially cylindrical housing14with a cup-shaped lower part14aand an upper part14b, which by beading the free front side of the housing lower part14ais firmly connected therewith to the housing14.

In the area of the housing lower part14aremote from the housing upper part14bis provided a heating device1a. An operating pin1bsurrounded by a membrane1cwithin the housing is passed out of the housing upper part14b. The further inner space of the housing14is filled by an expansion or elastic material element1d, such as an expansion material wax.

The heating device1ahas a support2, which has a bobbin7formed in cage-like manner from longitudinal and transverse ribs8,9. The transverse ribs9, only one being shown in the embodiment, stabilize the bobbin7and prevent a constriction of the longitudinal ribs8during the winding of the heating wire. A solid ring10is formed on the upper end of the support body. The ring10has lugs11, whose external dimensions correspond to the internal diameter of the cylindrical housing lower part14a. On the side of the support2remote from the ring10is formed a base4, whose diameter corresponds to the internal diameter of the housing lower part14b. The base4and ring10with lugs11in this way centre the support2in housing14. Within the base4is formed a ring step16which, for sealing purposes, receives an O-ring15. An alternative sealing of the support2is shown inFIG. 3. In the latter case laterally on the base4of support2is formed a radially circumferential annular groove17, which also receives an O-ring for sealing purposes. This leads to a large bearing surface20between support2and housing14and consequently high security is provided also with respect to the high internal pressure prevailing in the housing14. In addition, in the case of the construction according toFIG. 3, in the area of a keyway18between base4and inner wall of housing14is introduced a sealing compound19, which also has a sealing action through the high internal pressure. This sealing compound19can also be provided in the construction according toFIG. 1.

Into the base4of support2are moulded contact parts5,6with inner connection ends5a,6aand outer connection ends5b,6b, which in each case project out of the support2. A heating wire3is wound in single layer form around the bobbin7and to its two ends are connected in each case with inner connection ends5a,6acontact parts5,6.

InFIG. 6, which shows section I—I ofFIG. 5through support2, it can be gathered that an inner boundary line of longitudinal and transverse ribs8,9of this exemplified embodiment form a circle, so that the interior of the support body2is cylindrical. The outsides or outer edges of the in this embodiment longitudinal ribs8project over the external dimensions of the in this case transverse rib9in such a way that the outer connecting line of two adjacent longitudinal ribs8is located outside the external dimensions of the transverse rib9. Thus, a heating wire3wound around the outsides of the longitudinal ribs8does not touch the transverse ribs9and therefore, as in the area of the openings between longitudinal and transverse ribs8,9, can be flowed around by the expansion material wax in all-round manner also in the vicinity of the transverse ribs9. As a result of the provision of the longitudinal ribs8, in the exemplified embodiment shown the heating wire is in the form of a regular hexagon. A different number of longitudinal ribs8, e.g. between 3 and 10, preferably between 4 and 8, is also possible.

The moulding of the contact parts5,6in support2is indicated inFIG. 7.FIG. 7makes it clear that the contact parts5,6are in the form of a blanking skeleton and during the injection moulding process the parts shown in dot-dash line form inFIG. 7of said skeleton are still present. As is clear fromFIG. 7, simultaneously several supports2are moulded in the mould. After the removal of support2and the blanking skeleton from the injection mould the dot-dash line-shown parts are removed, so that all that remains connected to the individual supports2are the contact parts5,6shown in continuous line form. This ensures a rational and good sealing connection of the contact parts5,6to the outside.

In an alternative construction of the contact parts5,6they are given a round construction, instead of being made from flat material. The inner ends5aand6aare pressed flat and shaped as forks21. The heating wire is fixed thereto in such a way that it is firstly wound behind the fork and the end is then passed through the gap in said fork21. By compressing the fork tips, the end is reliably fixed by melting the insulating material layer during soldering at this point. The electrical connection is made (cf.FIG. 8).

FIG. 9shows the monotonic increasing resistance-temperature characteristic of the heating conductor.