1. Field of the Invention
This invention relates to power transmission belts and, more particularly to a power transmission belt having discrete reinforcing fibers embedded therein and projecting from pulley-engaging side surfaces on the belt.
2. Background Art
It is well known to construct power transmission belts with one or more V-shaped fibs to engage a cooperating pulley having complementary grooves. It is also known to embed short fibers in the compression rubber layer which has exposed surfaces that engage cooperating pulleys. These short reinforcing fibers are typically oriented laterally of the belt body to give the compression rubber layer the necessary amount of lateral rigidity. It is also known to cause portions of the short fibers to protrude from the belt at the side surfaces which frictionally engage the pulleys, to thereby improve wear resistance. These protruding fibers change the frictional characteristics of the side surfaces and additionally reduce noise that is generated between the belt and pulleys attributable to the tackiness of the belt side surfaces.
Japanese Patent Laid-Open No. 164839/1989 discloses a power transmission belt with aramid fibers embedded therein and protruding slightly from the pulley-engaging side surfaces on the belt. Because the aramid fibers are known to be highly durable, wear resistance at the side surfaces is enhanced.
Because aramid fibers are highly resistant to wear, they have been widely used for reinforcement purposes in power transmission belts. Only a small amount of the aramid fibers is necessary to provide the requisite reinforcement. However, when the amount of aramid fibers is small, and there is little protrusion of the fibers at the belt sides, there is a substantial contact area between the belt rubber and the cooperating pulleys. As a result, in mounting the belts, as on an automotive engine without a belt tensioner, problems with wear and sticking between the compression rubber layer and pulleys become significant.
To overcome this problem, it is known to cause the short aramid fibers embedded in the belt body to protrude substantially in the region that contacts the cooperating pulleys. As the belt is operated on a pulley system, the pulley causes the protruding portions of the aramid fibers to bend against the side surfaces of the body. As this occurs, the protruding fiber portions cover a substantial portion, if not all, of the pulley-engaging side surfaces. This protects the side surfaces against wear after long periods of use. The problem of sticking between the belt and pulley is alleviated, even at high tensions. These benefits are realized even though the amount of short, aramid fibers may be relatively small.
However, after the belt is run for a long period of time, the protruding portions of the aramid fibers tend to embed and become buried in the side surfaces by reason of repeated contact with the pulleys. As a result, parts of the embedded fibers may remain exposed at the pulley-engaging side surfaces through a substantial portion of the belt life.
With the belt highly tensioned, and there being no tensioning device, the tension on the belt reduces over time. The belt then becomes prone to slippage. The embedded and exposed aramid fibers result in a reduced coefficient of friction between the belt and pulley compared to a belt in which the robber on the side surfaces is fully exposed. There results a detrimental synergistic effect. That is, the belt is prone to slipping, has reduced power transmission capability, and at the same time generates noise in operation.
The above slippage problem can be avoided by preventing burying of the bent, protruding portions of the fibers in the side surfaces. This can be achieved by increasing the amount of short aramid fibers, which in turn increases the concentration of fibers exposed at the side surfaces. The exposed fibers prevent burying of the protruding fiber portions in the robber. With this arrangement, the protruding fibers ultimately are cut off due to the friction between the belt and pulley. As this occurs, the area of exposed robber increases, as does the coefficient of friction, so that slippage is prevented. However, simply increasing the amount of short aramid fibers results in an increase in the hardness of the compression robber layer, which in turn leads to slippage.