Machine element or mechanism – Gearing – Follow-up mechanism
Reexamination Certificate
2002-02-14
2004-10-19
Pecci, David A. (Department: 3682)
Machine element or mechanism
Gearing
Follow-up mechanism
C464S030000, C180S444000, C180S443000
Reexamination Certificate
active
06805017
ABSTRACT:
TECHNICAL FIELD
The present invention relates to an electrically power assisting steering apparatus.
BACKGROUND ARTS
In the operation of an electrically power assisting steering apparatus (hereinafter called as an EPS apparatus) wherein turning force of an electric motor becoming auxiliary steering torque is decelerated by a gear device to transmit it to an output shaft, thereby to assist a driver's power for rotating a steering wheel to steer wheels, e.g., when the steering wheel is rotated up to a stroke end of a rack and the steering is abruptly stopped, the motor can not stop immediately due to inertia force and occasionally, excessive torque is produced in the decelerating gear portion. In order to protect decelerating gears against the excessive torque, conventionally, a torque limiter using a friction plate is provided between a motor and a worm, or a torque limiter using a ring member for applying elastic force is provided between an output shaft and a worm wheel. Thereby, at the time of generation of such excessive torque, slip is developed between both of them to prevent transmission of the excessive torque.
In an EPS apparatus, generally, a worm is formed of iron material, and a worm wheel is made by forming a gear portion of resin material integrally on the outer peripheral surface of a core portion of solid iron material by way of adhesion, fusion or injection molding. Also, an output shaft having the worm wheel mounted thereon is formed of iron material the same as the core metal.
In this field of the art, the ring member for applying elastic force used for the torque limiter provided between the output shaft and the worm wheel is called as a rotation transmitting member or a torque setting member, etc., or is known as a tolerance ring (trade name: Rencol Tolerance Rings).
As mentioned above, in the conventional EPS apparatus, since the core metal portion of the worm wheel constituting the gear device is formed of solid iron material, there are the following problems. First, when attempting to correspond to achieving high output of the EPS apparatus, it is necessary to make the diameter of the worm wheel large for high strength and high module ability. However, in that case, the weight of the core metal portion is increased and inertial force in steering is increased, so that a driver's feeling in rotating the steering wheel and cutting of the steering wheel are deteriorated. Also, in compliance with the enlargement of the diameter of the core metal portion, when the diameter of the resin gear portion is enlarged, accuracy of the gears might be worsened to cause increase of noise of the gears in operation since dimensional change of resin due to moisture absorption and temperature is large. Also, the strength of the gear might be lowered since there is the danger of occurring voids in the resin portion due to injection molding. Thus, in the conventional EPS apparatus, there are problems in making the diameter of the worm wheel large.
Next, by comparison on the basis of ordinary temperature, the resin gear portion has characteristics in that its strength is lowered under high temperature and is heightened under low temperature. However, the conventional output shaft and the core metal portion are made of iron material and their linear expansion coefficients are approximately the same, so that the limit torque of the torque limiter is constant to temperature. Therefore, the limit torque of the torque limiter needs to be set at the maximum operating temperature, and needs to be set low, whereby the difference between the limit torque and the transmission torque was small. When the ring is worn and the limit torque is lowered because of long-term use of the EPS apparatus, the torque transmission might not be carried out, and thereby the designing is difficult.
For example, a worm wheel is disclosed in the Japanese Utility Model Publication No. 2556890 and the Japanese Patent Application Laid-Open No. 7-215227. As shown in
FIGS. 15A and 15B
, the worm wheel has a core metal X
19
and a resin ring X
20
. The entire outer peripheral surface of the core metal
19
X is formed in the shape of a gear with irregularities X
19
a
constituted of rotational direction stopping serrations X
19
b
and axial direction stopping projections X
19
c
. The resin ring X
20
with teeth on its outer peripheral surface is fitted on the irregularities X
19
a
of the core metal X
19
. As the thickness of the resin ring X
20
is properly set, it is possible to maintain proper backlash between the gears even though atmospheric temperature is changed.
Also, in a worm wheel disclosed in the Japanese Patent Application Laid-Open No. 11-192955, an approximately gear-shaped core metal is used for enhancement of the strength of the teeth, and the gear-shaped rim portion of the core metal is covered by a resin by way of injection such that the cross section of the resin in the axial direction becomes a rectangle. Then, the binding of the rim portion and the resin depends on the strength of the cooled and hardened thin resin layer.
However, in the conventional worm wheel of the Japanese Utility Model Publication No. 2556890 and the Japanese Patent Application Laid-Open No. 7-215227, the resin ring X
20
is fitted on the irregularities X
19
a
formed on the outer peripheral surface of the metallic core metal
19
X, but since the mechanical strength of the resin is inferior to that of metal, the module needs to be enlarged in comparison with a metal gear, and the thickness of the teeth needs to be made larger than that of the metallic portion, whereby there occurs a problem that the decelerating mechanism is enlarged.
Also, the resin with the large thickness is inferior in heat-dissipation ability, and so the thickness of the resin is set appropriately, but there is a problem in that wear of the resin is increased due to heat produced in the engagement of the gears. Especially, as shown in
FIG. 16
, when the worm wheel is incorporated in the EPS apparatus and disposed in an engine room, the temperature condition is severe and it is difficult to secure the durability under high temperature environment. (Description of the structure of an EPS apparatus in
FIG. 16
will be made later in the description of the preferred embodiments and then omitted here).
Further, since the binding power of the core metal X
19
and the resin ring X
20
depends on mechanical catching of the irregularities
19
a
, it is necessary to form the thickness of the resin ring X
20
large so as to make the resin ring X
20
withstand the slipping-off force. In case of using nylon family resin, as it has water absorptive property, the larger the volume of the resin becomes, the larger its dimensional change caused by expansion due to water absorption becomes. However, in the EPS apparatus, as a driver rotates a steering wheel, a steering assisting force is transmitted via the decelerating mechanism, whereby the rotational directions in the decelerating mechanism are reversed intricately. Therefore, in order to prevent noise from the teeth surfaces of the gears, it is necessary to control such that the backlash becomes extremely small. As mentioned above, when the volume of the resin is large, its dimensional change due to water absorption is considerably large. Therefore, there is a problem in that when the resin absorbs water and expands to the extent that the backlash is lost, the operability is deteriorated.
Further, in the worm wheel disclosed in the Japanese Patent Application Laid-Open No. 11-192955, external force generated due to the engagement of the gears acts on the thin resin layer as tensile stress, so that the resin is easily broken and it is difficult to obtain strength. Also, in a case where the core metal is used as the insert in an injection molding, when the high temperature resin is cooled and contracted, residual tensile stress is produced. In this case, the joining of the resin and the core metal relies on mechanical catching, and if only a one portion of the resin is broken, the broken-out surface is
Chikaraishi Kazuo
Ohshima Masamichi
Miles & Stockbridge P.C.
NSK Ltd.
Pecci David A.
Van Pelt Bradley J.
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