Drive shaft for automotive vehicle water pump

A drive shaft for an automotive vehicle water pump has an outer race with plural outer raceways formed in an inner peripheral wall thereof, a shaft member disposed for rotation relative to the outer race and defining a like plural number of inner raceways formed in an outer peripheral wall of the shaft member in radial registration with the corresponding outer raceways, and a multiplicity of rolling elements arranged between the inner raceways and the corresponding outer raceways, respectively. The shaft member comprises a large-diameter bearing portion with the plural inner raceways formed in an outer peripheral wall thereof, a small-diameter impeller shaft portion extending from one end of the large-diameter bearing portion, a small-diameter pulley shaft portion, and a continuously-connecting portion extending between an opposite end of the large-diameter bearing portion and a proximal end of the small-diameter pulley shaft portion. The continuously-connecting portion presents a curved peripheral surface with circular arcs in tangential directions of which an outer peripheral wall of the small-diameter pulley portion extends.

BACKGROUND OF THE INVENTION 
1) Field of the Invention 
This invention relates to a drive shaft for an automotive vehicle water 
pump. The drive shaft is employed to rotate an impeller of the water pump 
for recirculating coolant for an engine of an automotive vehicle. 
2) Description of the Related Art 
A water pump is used to recirculate coolant through a cylinder block of a 
water-cooled engine in an automotive vehicle. The water pump is equipped 
with a drive shaft, which has a driven pulley on an end portion thereof 
and an impeller on an opposite end portion thereof. The impeller is 
positioned within a coolant passage. The drive shaft is driven by a belt 
mounted on a drive pulley, which is fixed on an end portion of a 
crankshaft of the engine, and the driven pulley. As a consequence, the 
impeller is rotated by the drive shaft to recirculate the coolant. 
Reference is first had to FIG. 4 which is a side view of a bearing unit 
with a drive shaft member assembled therein. It is to be noted that a 
quarter of the bearing unit has been cut off to show the internal 
structure. Plural (two in the figure) rows of inner raceways 3,3 are 
formed in an outer peripheral wall of a large-diameter bearing portion 2 
which is provided at an intermediate part of the drive shaft member 
designated at numeral 1. A like plural number (i.e., two in the figure) of 
outer raceways 5,5 are formed in an inner peripheral wall of an outer race 
4 in radial registration with the respective inner raceways 3,3. Plural 
ball bearings 6,6 are arranged as rolling elements between the inner 
raceways 3,3 and the corresponding outer raceways 5,5, respectively, so 
that the drive shaft member 1 is rotatably supported inside the outer race 
4. The outer race 4 is fixed on an unillustrated engine cylinder block. 
At opposite end portions of the drive shaft member 1, a small-diameter 
pulley shaft portion 7 and a small-diameter impeller shaft portion 8 are 
provided in continuation with proximal end faces of the large-diameter 
bearing portion 2 so that the small-diameter pulley and impeller shaft 
portions 7,8 extend coaxially with the large-diameter bearing portion 2. A 
driven pulley (not shown), on which the belt driven by the drive pulley is 
mounted, can be fixed on the small-diameter pulley shaft portion 7 (i.e., 
the right-hand, small-diameter portion as viewed in FIG. 4), while an 
impeller (not shown) adapted to produce a flow of coolant through the 
coolant passage can be secured on the small-diameter impeller shaft 
portion 8 (i.e., the left-hand, small-diameter portion as viewed in FIG. 
4). 
In the drive shaft member 1, each of the small-diameter shaft portions 7,8 
and the large-diameter bearing portion 2 are continuously connected via a 
continuously-connecting portion which presents a circular-arc outer 
peripheral surface as depicted in FIG. 5, so that the outer peripheral 
surfaces of the small-diameter shaft portions 7,8 extend in continuation 
with the circular-arc outer peripheral surface of the 
continuously-connecting portion. This continuously-connecting portion is 
formed in the following manner. First, lathe turning is applied by a lathe 
or the like to an outer peripheral wall of the drive shaft member 1, 
whereby a chamfered peripheral edge portion 9 and a first circular arc 
portion 10 are formed with their outer peripheral surfaces extending 
continuously from a peripheral edge of the proximal end of the 
large-diameter bearing portion 2. 
At the time right after the formation of the first circular arc portion 10, 
the outer diameter of the small-diameter pulley shaft portion 7 is still 
greater than a desired value as indicated by a two-dot chain line a in the 
same figure. An outer peripheral wall of the small-diameter pulley shaft 
portion 7 is therefore subjected to grinding after heat treatment, so that 
the outer peripheral wall is removed as thick as .delta. to reduce the 
outer diameter of the small-diameter pulley shaft portion 7 to the desired 
value. By such grinding, a second circular arc portion 11 is formed at a 
radius of curvature, which is determined by the profile of a grinding 
stone employed in the grinding work, at the continuously-connecting 
portion between the first circular arc portion 10 and the outer peripheral 
surface of the small-diameter pulley shaft portion 7, whereby a connecting 
peripheral outer edge 12 remains as a boundary. 
As a result, the large-diameter bearing portion 2 and the small-diameter 
pulley shaft portion 7 are continuously connected via the chamfered 
portion 9 and the first and second circular arc portions 10,11. 
The conventional drive shaft member for an automotive vehicle water pump, 
in which the large-diameter bearing portion 2 and the small-diameter 
pulley shaft portion 7 are continuously connected in such a form as 
described above, may not exhibit sufficient strength in some instances. A 
driven pulley is fixed on the small-diameter pulley shaft portion 7 and a 
drive belt is mounted on the driven pulley. Substantially large tension of 
the belt is therefore applied to the driven pulley. In addition, a cooling 
fan for an engine is also mounted on the small-diameter pulley shaft 
portion 7. As a consequence, significant bending load is applied to the 
drive shaft member 1. 
Under such significant bending load, substantial stress is applied to the 
first and second circular arc portions 10,11. There is hence the potential 
danger that, in the course of use of the drive shaft member 1 over a long 
time, a crack may occur in at least one of the circular arc portions 10,11 
and the drive shaft member 1 may be broken there. 
Carburizing may be applied to the surface of the drive shaft member 1 
especially to improve the durability of the rolling bearing unit. When 
such carburizing is applied, intergranular oxide layers may be formed in 
black scales in the surface of the first circular arc portion 10 so that 
the strength of the drive shaft member 1 may be reduced considerably 
there. This makes the drive shaft member 1 more susceptible to breakage. 
SUMMARY OF THE INVENTION 
An object of this invention is to provide a breakage-resistant drive shaft 
for an automotive vehicle water pump by reducing the concentration of 
stress on the continuously-connecting portion between the large-diameter 
bearing portion 2 and the small-diameter pulley shaft portion 7. 
In one aspect of the present invention, there is thus provided a drive 
shaft for an automotive vehicle water pump, said drive shaft having an 
outer race with plural outer raceways formed in an inner peripheral wall 
thereof, a shaft member disposed for rotation relative to the outer race 
and defining a like plural number of inner raceways formed in an outer 
peripheral wall of the shaft member in radial registration with the 
corresponding outer raceways, and a multiplicity of rolling elements 
arranged between the inner raceways and the corresponding outer raceways, 
respectively. The shaft member comprises: 
a large-diameter bearing portion with the plural inner raceways formed in 
an outer peripheral wall thereof; 
a small-diameter impeller shaft portion extending from one end of the 
large-diameter bearing portion; 
a small-diameter pulley shaft portion; and 
a continuously-connecting portion extending between an opposite end of the 
large-diameter bearing portion and a proximal end of the small-diameter 
pulley shaft portion, said continuously-connecting portion presenting a 
curved peripheral surface with circular arcs in tangential directions of 
which an outer peripheral wall of the small-diameter pulley portion 
extends. 
In the drive shaft of this invention having the construction as described 
above, the curved surface portion via which the large-diameter portion and 
the small-diameter pulley shaft portion are continuously connected to each 
other continues smoothly with the small-diameter pulley shaft portion. As 
a result, even when bending load is applied to the small-diameter pulley 
shaft portion, localized excessive stress is no longer applied to the 
continuously-connecting portion between the small-diameter pulley shaft 
portion and the large-diameter bearing portion so that the drive shaft is 
made resistant to breakage. In particular, black scales can be removed 
from the surface of the curved surface portion by conducting the surface 
grinding of the curved surface portion and the small-diameter pulley shaft 
portion after their heat treatment. This makes it possible to avoid the 
above-described strength reduction due to the formation of intergranular 
oxide layers, so that the prevention of breakage of the drive shaft can be 
ensured further. A water pump with the drive shaft of the present 
invention assembled therein can therefore exhibit improved durability and 
reliability.

DETAILED DESCRIPTION OF THE INVENTION AND PREFERRED EMBODIMENTS 
The drive shaft according to the first embodiment of this invention will 
now be described with reference to FIG. 1. A chamfered portion 9 is formed 
at a peripheral outer edge of an end of a large-diameter bearing portion 
2. Formed in continuation with a peripheral outer edge of the chamfered 
portion 9, said peripheral outer edge being on the side of a 
small-diameter pulley shaft portion 7, is an inclined surface portion 13 
having a conical convex outer surface whose inclination is more acute than 
the peripheral surface of the chamfered portion 9. An outer peripheral 
edge of the inclined surface portion 13, said outer peripheral edge being 
on the side of the small-diameter pulley shaft portion 7, and a proximal, 
i.e., inner peripheral edge of the small-diameter pulley shaft portion 7 
are continuously connected together via a curved surface portion 14 which 
presents a circular arc in cross-section. 
To smoothly and continuously connect both peripheral edges of the curved 
surface portion 14 with the proximal peripheral edges of the inclined 
surface portion 13 and small-diameter pulley shaft portion 7, 
respectively, the inclined surface portion 13 and the small-diameter 
pulley shaft portion 7 extend in continuation with and in tangential 
directions relative to the outer peripheral surface of the curved surface 
portion 14. These inclined surface portion 13 and small-diameter pulley 
shaft portion 7 can be formed by grinding the outer peripheral walls of 
the inclined surface portion 13, curved surface portion 14 and 
small-diameter pulley shaft portion 7 with a single piece of a grinding 
stone, which rotates about an axis of rotation extending in parallel with 
the drive shaft member 1, while rotating the drive shaft member 1. 
In the drive shaft of this invention having the construction as described 
above, the curved surface via which the outer peripheral surface of the 
large-diameter bearing portion 2 and that of the small-diameter pulley 
shaft portion 7 extend continuously includes the curved surface portion 14 
whose outer peripheral surface extends in a tangential direction relative 
to the outer peripheral surface of the small-diameter pulley shaft portion 
7. The outer peripheral surface of the curved surface portion 14 and that 
of the small-diameter pulley shaft portion 7 therefore smoothly continue 
so that, even when bending load is applied to the small-diameter pulley 
shaft portion 7, no excessive stress is exerted on the connecting portion 
between the small-diameter pulley shaft portion 7 and the large-diameter 
bearing portion 2. As a consequence, the drive shaft member 1 is resistant 
to breakage. 
Reference is next had to FIG. 2 which illustrates the drive shaft according 
to the second embodiment of the present invention. As opposed to the 
provision of the inclined surface portion 13 between the curved surface 
portion 14 and the chamfered portion 9 in the first embodiment described 
above, the inclined surface portion 13 is omitted in the second embodiment 
so that the curved surface portion 14 and the chamfered portion 9 are 
directly and continuously connected together. The remaining construction 
and the advantage are similar to those of the first embodiment described 
above. 
In the drive shaft depicted in FIG. 3, the outer peripheral surface of the 
curved surface portion 14 is partly recessed relative to the outer 
peripheral surface of the small-diameter pulley shaft portion 7. A drive 
shaft of such a profile has greater strength than the conventional drive 
shaft illustrated in FIG. 5 but its bending strength has been reduced by 
as much as the reduction in diameter at the recessed part. The profile 
shown in FIG. 3 is therefore not fully preferred. 
Incidentally, when the curved-surface limit space L (see FIG. 2) is the 
same, the present invention becomes more effective as the ratio (d/D) of 
the diameter d of the small-diameter pulley shaft portion to the diameter 
D of the large-diameter bearing portion increases, in other words, the 
difference in diameter between the small-diameter pulley shaft portion and 
the large-diameter bearing portion becomes greater. At d/D .gtoreq.0.88, 
for example, even the conventional profile shown in FIG. 5 does not 
develop any particular problem provided that the radius of curvature of 
each portion is properly chosen. At d/D .ltoreq.0.75, on the other hand, 
the conventional profile involves the potential danger that the drive 
shaft may be broken.