Power transmission apparatus

A power transmission apparatus has a first shaft and a second shaft which are supported by an aluminum housing, and an iron gear and a synthetic resin gear which are respectively mounted on the two shafts and are engaged with each other. The synthetic resin used for the synthetic resin gear contains an aromatic polyamide fiber.

BACKGROUND OF THE INVENTION 
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. 
SUMMARY OF THE INVENTION 
According to the present invention, there is provided a power transmission 
apparatus having a first shaft and a second shaft supported by an aluminum 
housing, and an iron gear and a synthetic resin gear which are mounted on 
the two shafts respectively and are engaged with each other wherein the 
synthetic resin used for the synthetic resin gear is incorporated with 
aromatic polyamide fibers. 
Thus, the present invention has been accomplished to solve the 
above-described problems of the prior art and has the object of providing 
a power transmission apparatus in which the change of backlash due to 
changes in temperature is small, in which the abrasion of the iron gear is 
slight, in which the sliding parts are free from abrasion due to 
fallen-off matter from the synthetic resin gear, and which can be produced 
with great efficiency. 
To attain the above-mentioned object, according to the present invention, 
aromatic polyamide fibers are incorporated into the synthetic resin used 
for the synthetic resin gear. 
Materials usable for the synthetic resin gear are polyamide resins such as 
nylon and the like. 
The aromatic polyamide fibers to be incorporated are mixed with the resin 
in a proportion of 15 to 45 wt% to form a uniform mixture. 
According to the present invention, since the resin gear is formed out of a 
synthetic resin containing aromatic polyamide fibers which decrease the 
coefficient of linear expansion of the gear materials, the change of 
backlash due to temperature increases can be decreased and also the noise 
and the abrasion of the gear can be decreased. Further, since an aromatic 
polyamide fiber has a low hardness unlike a glass fiber, there is no 
likelihood of causing the abrasion of the sliding parts even when it falls 
off from the synthetic resin. Further, the abrasion of the iron gear is 
also decreased.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Example 
An example of the present invention will be described below with reference 
to the accompanying FIGURE. 
In the FIGURE, a camshaft gear 7 is formed out of a synthetic resin 
containing aromatic polyamide fibers. An "aromatic polyamide fiber" means 
herein a fiber obtained by subjecting aromatic materials, such as an 
aromatic dicarboxylic acid and an aromatic diamine, to polyamidation and 
then forming the reaction product into fibers. Among aromatic polyamide 
fibers, particularly preferred are aramid fibers. On the other hand, the 
synthetic resin of the main component is preferably a polyamide such as 
nylon-6, which is a linear polyamide. To mention an example of the 
characteristic property of such a composite material, nylon 6 containing 
15 to 20% by weight of aramid fiber having a length of 3 mm and a diameter 
of 12 .mu.m has a coefficient of linear expansion of 4.times.10.sup.-5 
/deg C. 
Numeral 3 indicates a water pump, the details thereof being omitted from 
the FIGURE. The water pump 3 is driven by torque transmitted through a 
pump gear 10 which engages with the camshift gear 7. The pump gear 10 is, 
like the camshaft gear 7, formed out of the synthetic resin incorporated 
with aromatic polyamide fiber mentioned above. Numeral 11 indicates the 
water pump housing. The structure is otherwise similar to that of the 
prior example mentioned above, so that a detailed description is omitted 
here. 
In the above construction, according to the present invention, aromatic 
polyamide fibers are incorporated into the synthetic resin, whereby the 
coefficient of linear expansion of the camshaft gear 7 becomes smaller 
than that of a gear containing no such fiber. Consequently, the sum of the 
expansion of the camshaft gear 7 and that of the crankshaft gear 5 made of 
iron approaches the expansion of the crankcase 4a and the crankcase cover 
4b made of aluminum. Accordingly, the change of the backlash between the 
two gears 5 and 7 with increase in temperature is small, whereby the 
emission of noise due to an excessively large backlash and the severe 
abrasion of the camshaft gear 7 can be avoided. Further, the mechanical 
properties of the camshaft gear 7 and its dimensional accuracy in 
injection molding themselves are improved. Also, since no gear cutting 
process is required, the production efficiency is not adversely affected. 
Aromatic polyamide fibers have, unlike glass fibers, a low hardness. 
Therefore, when the aromatic polyamide fibers fall off from the camshaft 
gear 7 or the pump gear 10, there is no possibility of the fallen-off 
fibers causing the abrasion of such sliding parts as the bearing parts 8 
and 9 of the crankshaft 1 and the camshaft 2 even when the fibers get into 
these sliding parts. The abrasion of the directly engaging crankshaft gear 
5 made of iron is also decreased. 
Although this example was described above with reference to the crankshaft 
gear 5 and the camshaft gear 7 of an engine, the present invention may 
also be applied, instead of the camshaft gear 7, for example to a balance 
gear, oil pump gear and the like. Further, the present invention may be 
applied to power transmission apparatuses for other uses than engines, 
which are also within the scope of the present invention. When the 
invention is applied to an internal combustion engine which undergoes a 
large temperature change, such as an automobile engine, the aforesaid 
effect of decreasing the change of backlash is of great significance. 
The words "aluminum" of "iron" used herein for the materials of housings, 
gears, etc. mean metals comprising aluminum or iron as the main component. 
The present invention can be applied not only to those gears which are 
mounted on shafts that are parallel with each other, such as spur gears, 
helical gears and double helical gears, but also to those gears which are 
mounted on two shafts perpendicular to each other, such as worm gears, 
bevel gears and the like, to take advantage of the small thermal expansion 
of resin gears. 
To make the aforesaid effect of decreasing the backlash more clearly 
understood, some examples of calculations are shown in Tables 1 and 2. In 
the Tables, a resin gear having a coefficient of linear expansion of 
9.times.10.sup.-5 /deg C is used in Comparative Examples, whereas a resin 
containing an aromatic polyamide fiber is used in the Examples of the 
invention; the aromatic polyamide fiber used is an aramid and the amount 
used is 18% by weight. The iron gear and the aluminum housing have a 
coefficient of linear expansion of 1.2.times.10.sup.-5 /deg C and 
2.3.times.10 .sup.-5 /deg C, respectively. 
Table 1 shows the results of calculation conducted for primary balancer 
gears wherein the pitch circle raddi A and B of the resin gear and the 
iron gear are both 100 mm and the distance D is 200 mm. Table 2 shows the 
results of calculations for camshaft gears wherein the pitch circle radii 
A and B of the resin gear and the iron gear are 100 mm and 200 mm, 
respectively, and the distance D is 300 mm. 
TABLE 1 
__________________________________________________________________________ 
(Unit: mm) 
Resin gear pitch 
Iron gear pitch 
Distance between 
Change with 
circle radius 
circle radius 
shafts temperature 
(A) (B) (D) (A + B - D) 
__________________________________________________________________________ 
20.degree. C. 
Standard 
100 100 200 -- 
100.degree. C. 
Comparative 
100.72 100.096 200.368 0.448 
Example 1 
Example 1 
100.32 100.096 200.368 0.048 
__________________________________________________________________________ 
TABLE 2 
__________________________________________________________________________ 
(Units: mm) 
Resin gear pitch 
Iron gear pitch 
Distance between 
Change with 
circle radius 
circle radius 
shafts temperature 
(A) (B) (D) (A + B - D) 
__________________________________________________________________________ 
20.degree. C. 
Standard 
200 100 300 -- 
100.degree. C. 
Comparative 
201.44 100.096 300.552 0.984 
Example 2 
Example 2 
200.64 100.096 300.552 0.184 
__________________________________________________________________________ 
As described above, according to the present invention, in a power 
transmission apparatus in which an iron gear and a synthetic resin gear 
which engage with each other are mounted on two shafts supported by 
aluminum housings, the synthetic resin contains aromatic polyamide fibers, 
whereby the change of backlash due to temperature change is decreased and 
the noise and the abrasion of the gears can also be lowered. Further, the 
abrasion of sliding parts including bearings, etc. caused by fallen-off 
fibers and the abrasion of the directly engaging iron gear can be 
decreased. Moreover, the production efficiency is not adversely affected.