1. Field of the Invention
The present invention relates to a power transmission apparatus using gears. More particularly, it relates to a power transmission apparatus having a first shaft and a second shaft each supported by an aluminum housing, and an iron gear and a synthetic resin gear which are mounted on to said two shafts respectively and are engaged with each other.
2. Description of the Prior Art
Previously known materials used for a gear, a power transmission element, include those which comprise a synthetic resin as the main component (see, for example, Japanese Utility Model Application Kokai (Laid-Open) No. 70,702/85). Synthetic resin gears of this kind are used, for example, for camshaft gears of engines. An example of such uses will be described below with reference to the accompanying figure illustrating a working example of that utility model.
In the FIGURE, 1 indicates the crankshaft of an engine (a first shaft), 2 indicates the camshaft (a second shaft), 4a indicates a crankcase (housing) and 4b indicates the crankcase cover (housing). The crankshaft 1 and the camshaft 2 are arranged in parallel with each other and are supported by the crankcase 4a and the crankcase cover 4b. The crankshaft gear 5 made of iron press-fitted to the crankshaft 1 and the camshaft gear 7 made of synthetic resin fixed to the camshaft 2 with rivets 6 engage with each other and transmit the torque of the crankshaft 1 to the camshaft 2. In the prior art, as the material for the camshaft gear 7, synthetic resins containing with glass fibers have been used to improve the mechanical properties of the camshaft gear 7.
Since the camshaft gear 7 is made of a synthetic resin, it is abraded by meshing with the crankshaft gear 5 made of iron even though the resin contains glass fibers, causing the glass fibers to fall off into lubricating oil. Since the glass fibers which fall off are minute, they get into, for example, such sliding parts as the bearing parts 8 and 9 of the crankshaft 1 and the camshaft 2. Since the glass fiber is very hard, the above-mentioned phenomenon brings about the disadvantage of abrasion of the sliding parts. Further, there is another disadvantage in that the glass fibers come up to the surface of the gear teeth on the side of the camshaft gear 7 made of resin, whereby the abrasion of the opposing crankshaft gear 5 of iron is increased.
As a means for solving such problems, it is possible to secure the necessary mechanical properties of the gear, without including glass fibers in the synthetic resin, by increasing the face width, changing to a resin with more enhanced heat resistance, or adding a substance capable of enhancing sliding such as molybdenum. However, the elimination of glass fibers increases the coefficient of linear expansion of the resulting material and markedly increases the change of backlash with temperature. Thus, the coefficient of linear expansion is 2.3.times.10.sup.-5 /deg C for aluminum, 1.2.times.10.sup.-5 /deg C for iron and 9.times.10.sup.-5 /deg C for the synthetic resin. Accordingly, with respect to the distance D between the crankshaft 1 and the crankshaft 2, the sum of the magnitude of expansion (or contraction) of the two gears 5 and 7 is larger than the magnitude of expansion (or contraction) of the crankcase 4, so that the backlash between the two gears 5 and 7 will differ greatly from the set value with changes in temperature.
If the change of backlash with temperature is large, when, for example, the backlash is so set as to be optimum at high temperatures, the backlash will become excessively large at low temperatures, causing emission of noise. On the contrary, when the backlash is so set as to be optimal at low temperatures, the backlash will become small at high temperatures, causing emission of noise and severe abrasion.
On the other hand, phenolic resins and the like containing cloth or wood chips have little of the above-described problems with respect to both contamination and backlash. However, generally they require the process of gear cutting, which markedly lowers the production efficiency.