Field of the invention.

The present invention relates to a timing or transmission belt having teeth which is reinforced by an elongated steel element, such as a steel filament or a steel cord.

Within the context of the present invention the terms"timing belt"and the terms"transmission belt"refer to the same thing.

Background of the invention.

Timing belts run under a constant axial tension ranging from 10 to 100 Newton and undergo a great number of rotations around relatively small wheels. Hence the timing belts and the reinforcing members undergo a great number of bendings under a relatively small radius of curvature.

This is the reason why steel is chosen as a suitable material for reinforcing timing belts, since, unlike synthetic yarns such as aramide, steel does not exhibit the phenomenon of creep.

Steel reinforcement, however, suffers from corrosion. The prior art has already taken a number of measures to mitigate the drawback of corrosion.

A first series of measures provides the elongated steel elements with a corrosion resistant metallic coating such as zinc or a zinc aluminium alloy.

In a second series of measures the surface area of the timing belt is provided with a fabric which prevents the reinforcing elongated steel elements from being exposed to the environment. This second series of measures has, however, the drawback of complicating the manufacturing process of timing belts.

Summary of the invention.

It is an object of the present invention to further mitigate the corrosion problem of steel without complicating the manufacturing process of timing belts.

According to the present invention there is provided a timing belt having teeth. The timing belt is reinforced by an elongated steel element such as a steel filament or a steel cord. This steel element is coated by a first synthetic material. The coated steel element is exposed to the neighbourhood or environment at least at one spot between two successive teeth of the timing belt.

As will be explained hereinafter, the coating in advance of the steel element with a synthetic material avoids the later use of a fabric in the manufacture of the final timing belt.

Conveniently, the timing belt is made of a second synthetic material.

Preferably the melting zone or melting point of this second synthetic material lies under the melting zone or melting point of the first synthetic material so that the molding or extrusion temperature of the timing belt can be chosen so that the initial coating of the first synthetic material on the steel element is not influence by the molding or extrusion process of the final timing belt. Most preferably, there is at least a difference of 10 °C to 15 °C between the melting zone of the first synthetic material and the melting zone of the second synthetic material.

The second synthetic material may be conveniently a rubber or, most frequently, a polyurethane.

Preferably the first synthetic material is of the same nature as the second synthetic material and may react with the second synthetic material in order to obtain a strong adhesion or bond to the second synthetic material.

The first synthetic material may be a thermoplastic elastomer such as ARNITELe UM551, a thermoplastic elastomer based on polyether esters.

Its melting temperature is about 200 °C and its Vicat softening point 10N

is 170 °C.

Brief description of the drawings.

The invention will now be described into more detail with reference to the accompanying drawings wherein -FIGURE 1 shows a cross-section of a timing belt according to plane 1-1 of FIGURE 2; -FIGURE 2 shows a cross-section of a timing belt according to plane 11-11 of FIGURE 1; -FIGURE 3 schematically illustrates the first step of manufacturing a timing belt according to the invention.

Description of the preferred embodiments of the invention.

Referring to FIGURE 1 and FIGURE 2, a timing belt 10 made of polyurethane is reinforced with a steel cord 12 which was coated in advance with a first synthetic material 14 such as a thermoplastic elastomer. Between two successive teeth 16 of the timing belt 10 there is, as a consequence of the manufacturing mold, an indentation line 18 where no polyurethane is present and where the coating 14 of the steel cord 12 comes to the surface. Without the coating 14 the steel cord 12 would be exposed to the environment and would be susceptible to accelerated corrosion.

FIGURE 3 schematically illustrates a first step in a process of manufacturing a timing belt 10 according to the present invention. A steel cord 10 is coated in advance with a thermoplastic elastomer 14 and the thus coated steel cord 12,14 is wound on an inner mold 20 in the direction of the arrow. The inner mold 20 has the form of a toothed wheel where the teeth 22 form the final teeth 16 of the timing belt 10.

Each tooth 22 has a projection 24 which causes the indentation line 18 in the final timing belt 10. An outer mold (not shown) is placed around the

inner mold 20 and polyurethane is fed under appropriate pressure and heat between the inner mold 20 and the outer mold. If the temperature of the polyurethane between the inner mold and the outer mold is about 200°C, then the melting point or melting zone of the thermoplastic elastomer 14 is preferably higher than 200°C in order to prevent the thermoplastic elastomer from flowing away from the steel cord 12.

A suitable composition for the steel cord is a plain carbon composition and is along the following lines: a minimum carbon content of 0.65%, for example 0.80% or 0.90%, a manganese content ranging between 0.30% and 0.70%, a silicon content ranging between 0.15% and 0.30%, and a maximum sulphur content of 0.03% and a maximum phosphorous content of 0.03%.

Micro-alloying elements such as chromium, vanadium or boron which limit the degree of drawing required for determined tensile strengths are possible. The micro-alloying elements are, however, each limited to less than 0.4 %.

With respect to the small diameters of the filaments, the number and size of the non-metallic inclusions is preferably limited, e. g. sizes below 0.10 pm.

The diameter of the individual steel filaments of a steel cord suitable for reinforcement of timing belts ranges from 0.03 mm to 0.20 mm.

Before the twisting of the individual steel filaments into a steel cord, the individual steel filaments may be coated with a metallic coating such as brass, tin, zinc or a zinc-aluminium alloy (95% zinc, 5% aluminium).

An example of a steel cord reinforcement is as follows : 3 x 3 x 0.08 (zinc coated filaments, filament diameter 0.08 mm) 4.5/4.5 S/Z (strand pitch: 4.5 mm; cord pitch: 4.5 mm) diameter of steel mm first synthetic material =.....

diameter of the coated steel cord: 0.335 mm.

Other suitable constructions are : <BR> 3 x 0.08<BR> 3x3x0.04<BR> 7 x 3 x 0.06<BR> 7 x 4 x 0.08<BR> 3 + 5x7x0.15 A supplementary advantage of the coating of the steel cord with a first synthetic material is that it does no longer exhibit a sharp drop in the values of the wet fatigue limit in comparison with the values of the dry fatigue limit. This may be derived from the following table which summarizes tests that have been carried out in this respect.

Table: steelcord3x3x0.15 steelcorddiameter: fatigue level 0.63mm(N/mm2) steelcordnotcoatedcoatedsteelcord withsyntheticmaterialdiametercoatedcord = 1.58mm drywetdrywet messingcoated 1000 <250 1000 900 hotdipzinccoated 650 450 750 700 verticalexitfrombath hotdipzinccoated 650500700700 horizontalexitfrombath electrolytically zinc 7506501050900 coated stainlesssteel600 700 800 850

With respect to reinforcement of timing belts, a steel cord with a low cost plain carbon composition and coated with a synthetic coating is a proper alternative for a steel cord made of stainless steel.