Motor having worm and worm wheel

Details Motor having worm and worm wheel Motor having worm and worm wheel

2001-03-09

2003-01-07

Mullins, Burton S. (Department: 2834)

Electrical generator or motor structure

Dynamoelectric

Rotary

C310S07500D, C310S090000

Reexamination Certificate

active

06504277

ABSTRACT:

CROSS REFERENCE TO RELATED APPLICATION
This application is based on and incorporates herein by reference Japanese Patent Application No. 2000-97653 filed on Mar. 31, 2000.
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a motor having a worm and a worm wheel.
2. Description of Related Art
As shown in
FIG. 5
, one previously proposed motor includes a motor main body
81
and a speed reducing unit
82
secured to the motor main body
81
. The motor main body
81
includes a generally cylindrical yoke housing
83
. The motor main body
81
further includes a rotor
84
substantially contained in the yoke housing
83
. The rotor
84
includes a rotatable shaft
85
, an armature
86
secured to the rotatable shaft
85
, and a commutator
87
. A base end portion of the rotatable shaft
85
is rotatably supported by a radial bearing
88
arranged at a base of the yoke housing
83
. In a base end surface of the rotatable shaft
85
, there is formed a ball receiving recess
85
a
in which a metal ball
89
is partially received. Part of the ball
89
that protrudes from the ball receiving recess
85
a
engages a bottom wall of the yoke housing
83
to act as a thrust bearing that supports a load exerted in an axial direction of the rotatable shaft
85
. A worm
90
is provided at a distal end portion of the rotatable shaft
85
.
The speed reducing unit
82
includes a gear housing
91
secured to an open end of the yoke housing
83
, a worm wheel
92
contained and rotatably supported in the gear housing
91
, and the like. In the gear housing
91
, there is formed a shaft receiving recess
91
a
that receives a portion of the rotatable shaft
85
(worm
90
) extending out from the yoke housing
83
. A radial bearing
93
is arranged at an open end of the shaft receiving recess
91
a
. A middle portion of the rotatable shaft
85
is rotatably supported by the radial bearing
93
. The worm wheel
92
is meshed with the worm
90
of the rotatable shaft
85
.
When the rotor
84
is rotated, a rotational speed of the rotor
84
is reduced through the speed reducing unit
82
(worm
90
and worm wheel
92
). Then, the rotational force of the speed reducinq unit
82
is outputted from the motor through an output shaft
94
connected to the worm wheel
92
.
The above motor is designed to rotate the worm wheel
92
shown in
FIG. 5
only in a counterclockwise direction and is used, for example, as a motor of wiper system.
By way of example, during operation of the wiper system, if there is some resistance, such as a resistance exerted by snow located on a wiping surface of a vehicle front glass, a large load larger than a load normally applied on the output shaft
94
is quickly applied on the output shaft
94
via a link mechanism of the wiper system which links the output shaft
94
to wipers of the wiper system. Thus, a large reaction force is applied on the rotatable shaft
85
, thereby causing movement of the rotatable shaft
85
toward its base end side (right side in FIG.
5
). However, this movement of the rotatable shaft
85
is limited by the ball
89
, so that a large force (rotational force or inertial force of the rotor) is applied on a connection (teeth) between the worm
90
and the worm wheel
92
. As a result, in the described motor, each one of the worm
90
and the worm wheel
92
needs to have a large strength that is larger than a normally required strength to avoid damage to the connection (teeth) between the worm
90
and the worm wheel
92
. This results in an increase in a material cost of the worm wheel
92
. This is especially true when the worm wheel
92
is made of a resin material. Also, this results in a decrease in a motor efficiency due to an increase in the weight of each rotating component. Furthermore, this results in an increase in a gear module, so that the worm wheel becomes bigger. Thus, a size of the entire gear housing needs to be increased.
SUMMARY OF THE INVENTION
Accordingly, it is an objective of the present invention to provide a motor that has a worm and a worm wheel and that can prevent application of a large force on a connection between the worm and the worm wheel.
To achieve the objective of the present invention, there is provided a motor including a motor case, a rotor and a worm wheel. The rotor is contained in the motor case and includes a rotatable shaft. The rotatable shaft has a worm that is meshed with the worm wheel. The motor further includes frictional braking force generating means for braking the rotatable shaft with friction when a thrust load applied on the rotatable shaft exceeds a predetermined value.


REFERENCES:
patent: 3088043 (1963-04-01), Graham
patent: 4129797 (1978-12-01), Lindner
patent: 6215209 (2002-05-01), Yamamoto
patent: A-5-56601 (1993-03-01), None
patent: A-10-164796 (1998-06-01), None
patent: A-10-210708 (1998-08-01), None
patent: A-11-98765 (1999-04-01), None
patent: A-11-178277 (1999-07-01), None

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