Vibration reducing plastic gear

Details Vibration reducing plastic gear Vibration reducing plastic gear

1998-12-29

2001-01-30

Grainger, Quana M. (Department: 2852)

Electrophotography

Having particular structure

Modular or displaceable

C074S431000, C074SDIG001

Reexamination Certificate

active

06181899

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to a gear made of plastic (hereinafter referred to as a plastic gear), which includes a ring-like hub forming a core portion, a concentric gear ring which has a larger diameter than that of the hub, on an outer surface of which gear teeth are arranged, and a plate-like web which connects the hub with the gear ring in a body. In particular, relates to a plastic gear having high productivity and capable of avoiding a vibration in a transmission device.
2. Discussion of the Background Art
It is well known in a transmission apparatus to have at least one plastic gear which transmits a rotation force of a driving motor to a driven member. For example, in an image forming apparatus such as a copier, a printer, a facsimile or a multi-functioned machine having a plurality of functions, a rotation force of a driving motor is generally transmitted to an image carrier through a driven member contacting the surface of the image carrier or the like for forming a toner image on the surface of the image carrier during its rotation. The conventional plastic gear may also be used in a duplicator, a camera, a video deck or a compact disk player and so on to transmit a rotational force to a driven member thereof.
In recent years, such plastic gears have tended to be rotated at higher speeds and so have been subjected to higher external forces. Since the conventional plastic gear is simply constituted by a hub, a gear ring and a web connecting the hub with the gear ring, it has been difficult to meet the necessary level of rigidity and strength required for the plastic gear. It is of course possible to increase both the rigidity and the strength to meet the prescribed level if both a thickness and a size of the gear are increased. However, this is costly and the transmission apparatus unavoidably becomes bulky.
To increase the rigidity, a plurality of ribs may be symmetrically mounted on both front and rear surfaces of the web in a body in a manner such that one edge connects to the hub and another edge connects to the gear ring. However, the diameter of such a plastic gear generally varies during its molding process due to the so called a shrink phenomenon of the plastic. As a result, a peripheral speed of the plastic gear periodically changes when it rotates, and accordingly, unevenness of the rotational speed of the driven member may arise.
The present inventor has determined that the shrink phenomenon occurs for the reason hereinbelow explained in detail referring to
FIGS. 10 through 12
. A conventional plastic gear
14
includes a cylindrical hub
25
disposed as a core portion and supported by the shaft
15
illustrated in FIG.
2
. The plastic gear
14
further includes a gear ring
27
which is substantially concentric with the hub
25
, has a larger diameter than the hub
25
, and is disposed outside of the hub
25
.
The gear ring
27
includes a plurality of gear teeth
26
on an outer circumferential surface thereof. The plastic gear
14
further includes a circular plate-like web
28
which connects the hub
24
with the gear ring
27
to form a single body. A plurality of ribs
29
A and
29
B are integrally formed respectively on a front side and a rear side of the web
28
. Each of the ribs extends radially from the hub
25
to the gear ring
27
.
The ribs
29
A formed on a front surface of the web
28
are arranged at a prescribed angular interval around the hub
25
. The ribs
29
B formed on a rear surface of the web
28
are arranged in a same way as the ribs
29
A. The ribs
29
A and
29
B are disposed at the same angular positions on the front and rear surfaces of the web
28
. As a result, a perpendicular cross section of the web
28
intersects the cross sections of both the ribs
29
A and
29
B, as illustrated in FIG.
7
. Since a partial shrink phenomenon occurs at each of portions of the plastic gear
14
where the ribs
28
A and
29
B are symmetrically formed on the front and rear side surfaces of the web
28
during a cooling process of molding, diameters of these portions decrease to be less than that of other portions.
When producing a gear made of a metal by cutting a metal material, such a partial shrink phenomenon, of course, does not occur. Such a partial shrink phenomenon may occur only in a case that a pair of ribs
28
A and
29
B are symmetrically formed on the front and rear side surfaces of the web
28
. The eccentricity of a gear periphery of the conventional plastic gear that includes six pairs of ribs
29
A and
29
B respectively formed on the front and rear side surfaces
28
A and
28
B of the web
28
is illustrated in FIG.
14
. As there seen, the gear diameter changes six times corresponding to the number of the ribs. As a result, the rotational speed of the plastic gear varies six times; thereby unevenness of the rotational speed arises when the conventional plastic gear rotates.
A possible cause of the change in rotational speed of the plastic gear is explained below. A portion of the gear ring
27
and gear teeth
26
A,
26
B and
26
C each mounted on the circumference of the gear ring
27
are typically illustrated in FIG.
15
. As there shown, ends of the ribs
29
A and
29
B are connected to the same portion (shown enlarged for ease of illustration) of the gear ring
27
between the teeth
26
B and
26
C. The portion of the gear ring
27
between the teeth
26
B and
26
C is more indented toward a rotational center of the plastic gear than other portions thereof, since the partial shrink occurs when the plastic gear is molded. Thus, the tooth
29
A positioning at a left side of the ribs
29
A and
29
B inclines to the right and the tooth
29
B positioning at a right side of the ribs
29
A and
29
B inclines to the left as illustrated in FIG.
15
.
A gear tooth
26
D of another gear meshes with the plastic gear
14
as illustrated in FIG.
15
. If a pressure angle at a gear connecting portion at which the gear tooth
26
D meshes with the gear tooth
26
A is &agr;
0
, a pressure angle &agr;
1
of the gear tooth
26
B inclining on the right is larger then &agr;
0
. A pressure angle &agr;
2
of the gear tooth
26
C inclining on the left is smaller than &agr;
0
.
If angular velocities are &ohgr;
0
, &ohgr;
1
and &ohgr;
2
correspond to gear portions having the angles of &agr;
0
, &agr;
1
, and &agr;
2
, then the larger the pressure angle, the smaller the angular velocity and the smaller the pressure angle, the larger the angular velocity. Thus, the following relation is established around the ribs
29
A and
29
B:
&ohgr;
1
<&ohgr;
0
<&ohgr;
2
Thus, when ribs
29
A and
29
B extend in a radial state, for example, from the rotational center of the gear and are each disposed in the same angular interval, a rotational speed of the gear periodically varies when the plastic gear rotates.
Further, a rotational speed of the conventional driving motor
10
generally varies once per one revolution thereof. Thus, a rotational velocity of the PC drum
1
remarkably changes at a prescribed timing, if a frequency of a change in rotational speed of the conventional driving motor
10
is almost coincident with that of the plastic gear
14
. This is because cyclical peaks due to the change in rotational speed of the driving motor
10
and that due to the plastic gear
14
coincide with each other. As a result, unevenness of a toner image (so called “jitter”) arises on the surface of the PC drum
1
, and the image quality is inferior.
For example, if the driving motor
10
rotates at 1,800 rpm, a frequency of a change in rotational speed is 30 Hz (obtained by dividing 1,800 rpm by 60 second). If the number of teeth of the output gear
13
of the driving motor
10
is ten, a number of teeth of a plastic gear
14
that meshes with the output gear is seventy, and a number of ribs
28
A and
28
B mounted on each of the surfaces of the web
28
of the plastic gear
14
is seven, a frequency of a change in rotational speed of the gear
14
becomes 30 Hz, as obtained by the following formula

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