GB2085999A - Valve gear for internal combustion engines - Google Patents

Abstract

Valve gear for internal combustion engines comprises toothed wheels (2) fixed to the engine crankshaft and camshaft, and a toothed belt (1) effecting driving connection therebetween. The relative dimensions of the teeth on the wheels and the teeth (4) on the belt are so chosen that backlash (X) between the teeth on the belt in the recess (5) between adjacent teeth on the wheels is minimised e.g. less than 0.0125 inches (0.3 mm). <IMAGE>

Description

SPECIFICATION Valve gear for an internal combustion engine This invention relates to a valve gear for internal combustion engines.

It is known, for example from German Patent Specification DE-AS 29 06 619 and DE--OSS-1957922 to drive valve timing gears of internal combustion engines with a toothed belt.

Such drive mechanisms frequently present difficulties resulting from the fact that both the crankshaft of an internal combustion engine and also the camshaft of the internal combustion engine do not rotate uniformly but are subject to torsional vibrations. Resonance in the valve gear can cause undesirable relative movements between gear wheel and toothed belt leading to premature wearing of the gear wheel and belt.

According to the invention there is provided a valve gear for an internal combustion comprising toothed wheels adapted to be fixed on the one hand to the crankshaft and on the other hand to the camshaft of an internal combustion engine and a toothed belt for effecting driving connection between the toothed wheels characterised in that the relative dimensions of the teeth of the belt and the teeth of the wheel are so chosen that backlash between the teeth on each wheel and the teeth on the belt is minimised.

By reducing the clearance between the teeth on the toothed belt and the adjacent teeth of the toothed wheel with which the belt engages vibrations of the toothed belt which produce noise and wear are avoided. This reduction of the normally provided clearance leads not only to the avoidance of vibrations of the toothed belt drive but also additionally to an absorption of the torsional vibrations of the shafts of the valve gear connected by the toothed belt.

Preferably the drive belt and the drive gear are provided with teeth having profiles as described in German Patent Publication DE-B-1 9 57 922.

Such belts comprise a tensile member and teeth secured thereto, the teeth being curvilinear in profile, the tip of each both having a longitudinal cross sectional contour composed of two interesting areas of equal radii, the arcs being concave towards the tensile member.

The invention is illustrated with the aid of a preferred embodiment shown, by way of example only, in the drawing which is a schematic side view of one tooth of a toothed belt engaged between two teeth of a toothed wheel.

Referring to the drawing, a toothed belt 1 consists basically of an endless tensible member 3 and a plurality of moulded-on teeth 4 which is in driving engagement with two toothed wheels 2 connected respectively to the crankshaft and camshaft of an engine. The toothed wheels are similar in configuration. The recesses 5 between adjacent pairs of teeth are of similar shape and receive the teeth 4 of the tooth belt 1.

The teeth 4 and the recesses 5 have complementary profiles, and the teeth 4 are shaped in accordance with German Patent Specification DAS 1 957922. Thus, each tooth has a tip which has a longitudinal cross-sectional contour composed to two intersecting arcs of equal radius, said arcs being concave towards tensile member 3. Further details of the construction of the belt are set forth in German Patent Specification No. 1 294 103 and British Patent Specification No. 1294103 and will not therefore be repeated here.

The interengagement of the tooth 4 with the recess 5 provides a backlash or clearance X, which is the distance through which the centreline of the tooth on the belt can undergo movement relative to the wheel 2 from the position in which one flank of the tooth on the belt is engaged with one wall of the recess 5, to the position in which the other flank of the tooth is engaged with the opposite wall of the recess 5. In conventional valve drive gears, this distance has been about 0.020 inches (0.5 mm). According to the present invention however, this distance is reduced, preferably by about half, either by increasing the size of the tooth 4, or by decreasing the size of the recess 5. Typically, in accordance with the present invention the distance X/2 is about 0.0125 inches (0.3 mm) or less, e.g. 0.010 inches (0.25 mm).

1. Valve gear for an internal combustion engine comprising toothed wheels adapted to be fixed on the one hand to the crankshaft and on the other hand to the camshaft of an internal combustion engine and a toothed belt for effecting driving connection between the toothed wheels characterised in that the relative dimensions of the teeth of the belt and the teeth of the wheel are so chosen that backlash between the teeth on each wheel and the teeth on the belt is minimised.

2. A valve gear according to claim 1 wherein the toothed belt comprises a tensile member and teeth secured thereto, the teeth being curvilinear in profile, and the tip of each tooth having a longitudinal cross-sectional contour composed of two intersecting arcs of equal radii, said arcs being concave towards the tensile member.

3. Valve gear according to claim 1 or claim 2 wherein the distance by which a tooth on the toothed belt having one flank engaged with one wall of the recess between two adjacent teeth on the toothed wheel may move before the opposite flank of the tooth engages the opposite wall of the recess is less than 0.020 inches (0.5 mm).

4. Valve gear according to claim 3 wherein the said distance is not more than 0.0125 inches (0.3 mm).

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (6)

**WARNING** start of CLMS field may overlap end of DESC **. SPECIFICATION Valve gear for an internal combustion engine This invention relates to a valve gear for internal combustion engines. It is known, for example from German Patent Specification DE-AS 29 06 619 and DE--OSS-1957922 to drive valve timing gears of internal combustion engines with a toothed belt. Such drive mechanisms frequently present difficulties resulting from the fact that both the crankshaft of an internal combustion engine and also the camshaft of the internal combustion engine do not rotate uniformly but are subject to torsional vibrations. Resonance in the valve gear can cause undesirable relative movements between gear wheel and toothed belt leading to premature wearing of the gear wheel and belt. According to the invention there is provided a valve gear for an internal combustion comprising toothed wheels adapted to be fixed on the one hand to the crankshaft and on the other hand to the camshaft of an internal combustion engine and a toothed belt for effecting driving connection between the toothed wheels characterised in that the relative dimensions of the teeth of the belt and the teeth of the wheel are so chosen that backlash between the teeth on each wheel and the teeth on the belt is minimised. By reducing the clearance between the teeth on the toothed belt and the adjacent teeth of the toothed wheel with which the belt engages vibrations of the toothed belt which produce noise and wear are avoided. This reduction of the normally provided clearance leads not only to the avoidance of vibrations of the toothed belt drive but also additionally to an absorption of the torsional vibrations of the shafts of the valve gear connected by the toothed belt. Preferably the drive belt and the drive gear are provided with teeth having profiles as described in German Patent Publication DE-B-1 9 57 922. Such belts comprise a tensile member and teeth secured thereto, the teeth being curvilinear in profile, the tip of each both having a longitudinal cross sectional contour composed of two interesting areas of equal radii, the arcs being concave towards the tensile member. The invention is illustrated with the aid of a preferred embodiment shown, by way of example only, in the drawing which is a schematic side view of one tooth of a toothed belt engaged between two teeth of a toothed wheel. Referring to the drawing, a toothed belt 1 consists basically of an endless tensible member 3 and a plurality of moulded-on teeth 4 which is in driving engagement with two toothed wheels 2 connected respectively to the crankshaft and camshaft of an engine. The toothed wheels are similar in configuration. The recesses 5 between adjacent pairs of teeth are of similar shape and receive the teeth 4 of the tooth belt 1. The teeth 4 and the recesses 5 have complementary profiles, and the teeth 4 are shaped in accordance with German Patent Specification DAS 1 957922. Thus, each tooth has a tip which has a longitudinal cross-sectional contour composed to two intersecting arcs of equal radius, said arcs being concave towards tensile member 3. Further details of the construction of the belt are set forth in German Patent Specification No. 1 294 103 and British Patent Specification No. 1294103 and will not therefore be repeated here. The interengagement of the tooth 4 with the recess 5 provides a backlash or clearance X, which is the distance through which the centreline of the tooth on the belt can undergo movement relative to the wheel 2 from the position in which one flank of the tooth on the belt is engaged with one wall of the recess 5, to the position in which the other flank of the tooth is engaged with the opposite wall of the recess 5. In conventional valve drive gears, this distance has been about 0.020 inches (0.5 mm). According to the present invention however, this distance is reduced, preferably by about half, either by increasing the size of the tooth 4, or by decreasing the size of the recess 5. Typically, in accordance with the present invention the distance X/2 is about 0.0125 inches (0.3 mm) or less, e.g. 0.010 inches (0.25 mm). CLAIMS

1. Valve gear for an internal combustion engine comprising toothed wheels adapted to be fixed on the one hand to the crankshaft and on the other hand to the camshaft of an internal combustion engine and a toothed belt for effecting driving connection between the toothed wheels characterised in that the relative dimensions of the teeth of the belt and the teeth of the wheel are so chosen that backlash between the teeth on each wheel and the teeth on the belt is minimised.

2. A valve gear according to claim 1 wherein the toothed belt comprises a tensile member and teeth secured thereto, the teeth being curvilinear in profile, and the tip of each tooth having a longitudinal cross-sectional contour composed of two intersecting arcs of equal radii, said arcs being concave towards the tensile member.

3. Valve gear according to claim 1 or claim 2 wherein the distance by which a tooth on the toothed belt having one flank engaged with one wall of the recess between two adjacent teeth on the toothed wheel may move before the opposite flank of the tooth engages the opposite wall of the recess is less than 0.020 inches (0.5 mm).

4. Valve gear according to claim 3 wherein the said distance is not more than 0.0125 inches (0.3 mm).

5. Valve gear according to claim 3 wherein the said distance is not more than 0.010 inches (0.25 mm).

6. Valve gear substantially as hereinbefore described and as illustrated in the drawing.