GEAR WHEEL

Abstract: The invention refers to a gear wheel consisting of plastic material or a similar material having a high coefficient of thermal expansion. It is characterized therein that a helix (3) of steel or another material having a desired coefficient of thermal expansion is embedded in the gear rim (1) of the wheel radially inside the teeth (2), whereby the helix (3) has such a strength and dimension, that it determines the coefficient of thermal expansion of the gear rim (1).

Description: The present invention refers to a gear wheel of plastic material and particularly such wheels intended for cooperation with a toothed driving belt. The invention also incorporates a method at production of such gear wheels. Gear wheels are a commonly known machine member, known in an immense number of different embodiments. They all incorporate a rim of teeth, which are intended to cooperate either with corresponding teeth on adjacent gear wheels or with teeth on a toothed driving belt or the like. In the latter case these teeth are rule shaped and have an axial extension allowing them to cooperate with transverse teeth on the driving belt. The gear wheels are commonly made from cast iron or steel. The requirements on the design and machining of the teeth are always high. In the latter case, as mentioned above, when the teeth are rule shaped, it is necessary that they are precision machined by milling or the like, thus that they are given an accurate shape and thereby in a satisfactory manner can cooperate with the transverse teeth of the driving belt. At gear wheels, which shall cooperate with driving belts, it is often a desire that the gear wheel rim is provided with flange on one or both sides for securing that the driving belt shall not disengage from the the gear wheel. As the teeth one to be precision machined in a milling opertation it is difficult or even impossible to provide the gear rim with the flange prior to the milling operation. In such cases use is therefore made of separate flanges, which after precision machining are fitted in grooves on the side of the the gear rim. Gear produced in this manner and from the above mentioned materials are heavy and expensive to produce. It since long has been a desire to be able to mould gear wheels directly without subsequent machining and from a low weight material. Different types of plastic material then has been considered, but the plastic material has the property that its coefficient of thermal expansion is much bigger than that of steel and cast iron, whereby unacceptable dimension changes will arise in connection with temperature changes, as other parts in the vicinity of the plastic material usually consist of steel or cast iron. This problem has however been solved by the present invention and it has been possible to meet the earlier desires by providing a gear wheel consisting of plastic material or a similar material having a high coefficient of thermal expansion, and which is characterized therein that a helix of steel or another material having a desired coefficient of thermal expansion is embedded in the gear rim of the wheel radially inside the teeth, whereby the helix has such a strength and dimension, that it settles the coefficient of thermal expansion of the toothed wheel rim. According to the invention the hub of the toothed wheel may be axially displaced in relation to the toothed wheel rim, and also the hub may incorporate a helix of steel or another material with a desired coefficient of thermal expansion and such a strength, that it has the ability of determining the coefficient of thermal expansion of the entire hub. The invention further incorporates a method for production of a gear wheel, which involves that a helix of a material having a desired coefficient of thermal expansion is inserted into a mould, having a somewhat smaller circumferential diameter than the helix, whereupon the mould is filled with the moulding material, which is brought to become stiff or to cure. For securing that the helical spring is entirely embedded in the plastic material it is according to the invention appropriate that some bosses in the mould, e.g. every fifth boss, intended to form tooth bottoms in the gear rim are equipped with a guiding rule for guiding the helix during the moulding process, prior to th e embedding of the helix. The invention hereinafter will be further described with reference to the accompany drawings, in which Fig. 1 in an axial section shows a gear according to the invention, and Fig. 2 shows a radial section of a portion of the mould, which gives the gear rim its share. In Fig. 1 is shown a gear wheel incorporating a gear rim 1 with axially extending rule shaped teeth 2. In the gear rim adjacent its outer circumference there is inserted a helix 3 of steel. This helix is entirely embedded in the plastic moulding material 4, but can be seen in some tooth bottoms, e.g. each fifth. The hub 5 of the gear wheel, such as shown in the figure, is axially displaced relative to the gear rim 1. The hub 5 is equipped with a steel helix 6 or the like at the inner surface 7. This allows the hub to be fitted with an invariable firm grip upon a shaft, which almost always is of steel. The connecting portion 8 between the the gear rim 1 and the hub 5 has been given such a shape with two knees 9 and 10, that temperature variations can be easily absorbed without creation of any stresses. The plastic material of this and other portions may be reinforced with glass fibre or with other known material. The gear rim 1 according to figure 1 as seen is also provided with a flange 11. This can be moulded directly in connection with the moulding of the entire wheel. From a manufacturing point of view it is also possible to mould a flange 11 on both sides of the gear rim. In figure 2 is shown a portion of the mould for production of the gear wheel according to the invention. In the outer mould half 12 has been made bosses 13, which bosses are intended to delimit the tooth bottoms in the gear wheel rim 1. Such as shown in the figure each fifth boss 13 is provided with rules 14, which are intended to retain the inserted helix 3 in its position during the moulding operation. The helix 3 shall have a natural bigger diameter than the circumferential diameter delimited by the rules 14. During moulding the helix 3 therefore will engage at a slight pressure against the rules 14, and the helix 3 will be entirely embedded in plastic material except for at the points of engagement with the rules 14. By the present invention it is thus obtained a gear wheel, which has a low weight, which can be moulded directly and which is finished without precision machining and which without extra costs during the moulding can be provided with one or two flanges beside the gear rim. The gear wheel also has the same coefficient of thermal expansion as steel. It is essential that the mutual distances between the teeth are exactly equal. This is according to the invention achieved without any subsequent machining whatsoever. The invention is not limited to the embodiment shown but can be varied in different manners within the scope of the claims. It is thus not necessary that the teeth are rule shaped, but they can also have other design. The moulding material must not consist of plastic material but can be e.g. aluminum.

Claims: 1. A gear wheel consisting of plastic material or a similar material having a high coefficient of thermal expansion, characterized therein, that a helix (3) of steel or another material having a desired coefficient of thermal expansion is embedded in the geard rim (1) of the wheel radially inside the teeth (2), whereby the helix (3) has such a strength and dimension, that it determines the coefficient of thermal expansion of the gear rim (1). 2. A gear wheel as claimed in claim 1, characterized therein, that the hub (5) of the gear wheel is axially displaced in relation to the gear rim (1). 3. A gear wheel as claimed in claim 1 or 2, characterized therein, that also the hub (5) incorporates a helix (6) of steel or another material with a desired coefficient of thermal expans ion and such a strength, that it has the ability of determining the coefficient of thermal expansion of the entire hub (5). 4. A method for producing gear as claimed in claim 1, characterized therein, that a helix (3) of a material having a desired coefficient of thermal expansion is inserted into a mould, having a somewhat smaller circumferential diameter than the helix (3), whereupon the mould is filled with the moulding material (4), which is brought to become stiff or to cure. 5. A method as claimed in claim 4, characterized therein, that prior to the embedding of the helix (3), some bosses (13) in the mould, intended to form tooth bottoms in the gear rim (1) are equipped with a guiding rule (14) for guiding the helix during the moulding process.

Inventors: HALLERBACK, STIG, Hallerbäck, Stig
Applicants: SKF NOVA AB
Application year: 1987
Main classification: F16H55
IPC codes: F16H55