Power transmission mechanism

Details Power transmission mechanism Power transmission mechanism

2000-06-06

2004-02-03

Binda, Greg (Department: 3679)

Rotary shafts, gudgeons, housings, and flexible couplings for ro

Shafting

With disparate device for coupling shaft to additional shaft...

C403S359600

Reexamination Certificate

active

06685572

ABSTRACT:

BACKGROUND OF THE INVENTION
The present invention relates to a power transmission mechanism for transmission of torque between two members.
Transmission shafts for transmission of power (torque or rotation) are used in many machine parts in automobiles and industrial machines. Some shafts are solid and others hollow, these being produced by direct cutting or plastic working of bar or pipe material or, in recent years, by the sintering of powder.
Spline shafts or serrated shafts for transmission of high torque are generally formed by subjected medium carbon steel or low alloy steel (case hardening steel, nitrided steel or the like) to a heat treatment, such as surface hardening process or tempering, for example, carburization hardening, high frequency hardening or nitriding, so as to increase the shaft strength in consideration of plastic workability, machinability and cost, it being only after such treatment that these shafts are put to use. Further, recently, use has been made of non-refine steel to dispense with refining, or a material subjected to high alloying or high purifying (reduction of inclusions, reduction of P, i.e., phosphorus, etc.) to increase strength, or a material subjected to shot peening to increase fatigue strength.
FIG. 5
shows an example of a machine part having said transmission shaft, which is a constant velocity joint used in the drive shaft of an automobile. This constant velocity joint includes a shaft member
11
having an inner ring
12
fitted thereon through splines
13
formed on the outer periphery of said shaft member
11
, the torque in the shaft member
11
being transmitted to the inner ring
12
through the groove-and-ridge fit of the splines
13
.
In this connection, there are various types as to the shaft of the spline terminal end side (C in the figure) of the shaft member
11
—the “terminal end side” means the opposite side when the shaft end surface which, when the shaft member
11
is inserted in the inner ring
12
, is the first to fit in the inner ring
12
is taken as the inlet side.
FIGS. 6 through 9
show examples thereof,
FIG. 6
showing a first type (tentatively referred to as the “cut-through type”) in which the spline trough
11
a
of the spline
13
is directly cut through the outer peripheral surface of the shaft member
11
,
FIGS. 7 through 9
showing a second type (tentatively referred to as the “cut-up type”) in which the spline trough
11
a
is smoothly diametrically increased until it connects to the outer peripheral surface of the shaft member
11
. Different forms of the cut-up type are known: one in which the diametrical increase is effected by an arc with a radius R
1
(FIG.
7
), one in which the diametrical increase is effected by an arc with a greater radius R
2
than in
FIG. 7
(R
2
>R
1
) (see FIG.
8
), and one in which the diametrical increase takes a spherical form with a radius SR (see FIG.
9
).
FIG. 10
shows a conventional fit between said shaft member
11
and inner ring
12
, wherein a relief region T′ where the inner diameter is increased is defined in the inner ring
12
on the terminal end side of the spline ridge
12
b
, and the portion of the ridge
12
b
excluding the relief region T′ is fitted in the portion of the trough
11
a
excluding the diametrically increased region S′ of the shaft member
11
, it being arranged that such fitting portion F′ (marked with dots) does not enter the diametrically increased region S′ of the trough
11
a
of the shaft member
11
.
In recent years, with the global environmental problem being highlighted, it has been required in the automobile industry to tighten emission control and improve fuel efficiency, and as a measure therefor, lightening has been promoted. In automobiles, splines and serrations (hereinafter represented by the term spline shaft) have been used abundantly in such parts as transmissions, differentials, drive shafts, and propeller shafts, it being noted that since the reduction of the weight of the spline shaft contributes much to the lightening of the automobile, there has been a strong need to increase the spline shaft strength, i.e., to increase the strength in two aspect: static strength and fatigue strength.
As for the measures for strengthening and lightening the spline shaft, the aforesaid high alloying or high purifying may be contemplated, but these would not be advantageous from the viewpoint of production cost since they are attended by an increase in the cost of material or a large decrease in workability. Further, shot peening, though effective in improving the fatigue strength, is not observed to provide sufficient merits as to static strength; rather, it leads to high cost.
A spline shaft whose terminal end is increased in diameter in a large arc form (
FIG. 8
) or in a spherical form (FIG.
9
), though improved in static strength as compared with the type shown in
FIG. 7
, is not observed to provide sufficient merits in increasing the fatigue strength, as can be seen from the test results shown in FIG.
13
. Further, since working tools (hob cutters, rolling racks, etc.) have to be newly produced, a cost increase is incurred. On the other hand, the cut-through type shown in
FIG. 6
is not suitable for weight reduction measures, since it is inferior to the cut-up type shown in
FIG. 7
in both static strength and fatigue strength, as is clear also from the experimental results shown in FIG.
12
.
As described above, the conventional measures for weight reduction are confronted with problems in either cost or strength and there has been no measure that has successfully satisfied both of the requirements at one time.
Accordingly, an object of the present invention is to make it possible to achieve improvements in the static strength and fatigue strength of a spline shaft or serrated shaft without incurring an increase in costs.
SUMMARY OF THE INVENTION
The boss of the inner ring was fitted on a spline shaft of the type shown in
FIG. 7
whose troughs were diametrically increased by an arc (for the specifications of the spline shaft, see FIG.
14
), and this assembly was put to torsion test to examine and analyze the fracture mode. As a result, it was found that as shown in
FIG. 11
, the fracture comprised two main fracture planes A and B: a first fracture plane extending along the bottom of the trough
11
a
of the shaft member
11
(A: axial fracture plane), and a second fracture plane inclined at an angle of 45° with respect to the axis (B: main stress fracture plane). It is believed that the axial fracture plane A is a shear fracture plane due to an axially-acting shearing force and that the main fracture plane B is a tensile fracture plane due to torsional main stress.
Next, the fit position of the boss was axially stepwise shifted and the strength of the spline shaft was measured at each shifted position; the results shown in FIG.
15
(A) were obtained. The horizontal axis in this figure represents the position X [mm] at which the boss is fitted, and the left-hand vertical axis represents a ratio Y
1
of repetition till the occurrence of fatigue fracture (the load shear stress is set to ±665 MPa [67.8 kgf/mm
2
] and the right-hand vertical axis represents the rate of increase Y
2
[%] in torsional break strength. The X on the horizontal axis, as shown in (B) of the same figure, indicates the distance from the terminal end
11
a
1
of the trough
11
a
of the shaft member
11
to the point (&Circlesolid;) where the terminal end outline
12
b
1
of the ridge
12
b
of the boss
12
crosses the outer periphery level L of the shaft member
11
. Measurements were conducted at positions X=a, b, . . . e, the repetition ratio Y
1
and rate of increase Y
2
being determined with the position a (X=6 mm) used as a reference (Y
1
=1, Y
2
=0). Further, (
2
), (
4
), (
6
), (
10
), and (
12
) in FIG.
15
(A) indicate axial shear crack lengths [mm].
It is seen from FIG.
15
(A) that the more the fit position of the boss is

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