Image forming apparatus employing led head

An image forming apparatus is described which includes a photoreceptor drum; an LED head for exposure provided along the width of the photoreceptor drum; a pin provided on the LED head; a supporting base substance to which the photoreceptor drum is attached; and a pin admitting groove provided on the supporting base substance for admitting the pin. The LED head is half-fixed on the supporting base substance.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to an image forming apparatus employing an 
LED (light emitting diode). More particularly, the present invention 
relates to an image forming apparatus such as a digital 
electrophotographic copying machine, a printer, etc. 
2. Description of the Prior Art 
For digital image forming apparatuses, an LED head is frequently used as 
exposure means for writing an image onto the photoreceptor surface which 
has already been charged. The LED head has a structure where pairs 
consisting of an LED device and SELFOC lens are linearly arranged in a 
direction parallel to the axis of a photoreceptor drum (that is, in a 
direction along the width of the photoreceptor drum). The number of pairs 
corresponds to the number of picture elements in the direction. Although 
the LED head is arranged in a direction along the axis of the 
photoreceptor drum as described above, the arrangement position of the LED 
head sometimes shifts with respect to the direction along the width of the 
photoreceptor because of a thermal expansion of the whole LED head caused 
by a light emission of the LED device. This means that a formed image is 
shifted from an intended position corresponding to an original, which 
shift is undesirable. However, in a conventional image forming apparatus 
for paper of smaller size, since the number of LED devices and the thermal 
expansion are small so that hardly any problems are caused, both ends of 
the LED head are fixed along the width of the photoreceptor on a 
supporting base substance to which the photoreceptor is attached. 
In an image forming apparatus for paper of larger size, since a greater 
number of LED devices are required because of a large number of the 
picture elements, an expansion and contraction, in actual size, of the LED 
heads and the shift of a formed image from an intended position 
corresponding to an original due to the thermal expansion cannot be 
ignored, so that, for example, there are occasions when the center of the 
original and that of the formed image do not correspond. 
SUMMARY OF THE INVENTION 
An object of the present invention is to provide an image forming apparatus 
where the center of a shift of the LED heads with respect to the 
photoreceptor is maintained fixed to form a stable image. 
According to the present invention, an image forming apparatus is provided 
with a photoreceptor, an LED head for exposure provided along the width of 
said photoreceptor, first fixing means provided on said LED head, a 
supporting base substance to which said photoreceptor is attached, second 
fixing means provided on said supporting base substance and on which said 
first fixing means is fixed, and half-supporting means which half-fixes 
said LED head on said supporting base substance. 
With the above-described image forming apparatus according to the present 
invention, when a fixing position of the first and second fixing means is 
the center of the LED head, although the both ends of the LED heads freely 
expand and contract in a direction of the thermal expansion caused by a 
light emission of the LED device, the center never shifts with respect to 
the photoreceptor. Thus, the center of the shift is fixed even when a 
formed image is shifted from an original due to the thermal expansion of 
the LED heads, so that a stable image is formed.

BRIEF DESCRIPTION OF THE PREFERRED EMBODIMENTS 
An embodiment of the present invention will hereinafter be described with 
reference to the drawings. FIG. 1 shows a main portion of a digital 
copying machine in which the present invention is incorporated. In the 
figure, an original 3 is placed on a contact glass 2 provided on the upper 
surface of a copying machine 1. A scanning optical system 4 is provided 
below the contact glass 2. The scanning optical system 4 moves from a home 
position shown in the figure toward the right while scanning the original, 
and turning around at a predetermined position, it returns to the home 
position. Light L from the optical system 4 is directed to a CCD (charge 
coupled device) 6 through an image forming lens 5. After being converted 
into an electric signal at the CCD 6, the light information is processed 
as image data at an image processing circuit 7. An output of the image 
processing circuit 7 is directed to an LED head 10 for exposure to expose 
the surface of a photoreceptor drum 8. A charging portion 9, the LED head 
10 for exposure, a developing roller 11, a transferring portion 12, a 
separating portion 13, a cleaning portion 14 and a charge removing portion 
15 are provided on the periphery of the photoreceptor drum 8 in the order 
named along a rotation direction thereof. 
The charging portion 9 charges the surface of the photoreceptor drum 8 so 
that its potential is approximately 700 V. The LED head 10 forms a latent 
image on the surface of the photoreceptor drum 8 by exposing the charged 
surface of the photoreceptor drum 8 based on the above-described image 
information. The developing roller 11, which constitutes a part of the 
developing apparatus, provides toner onto the surface of the photoreceptor 
drum 8 and develops the latent image formed on the surface of the 
photoreceptor drum 8. The transferring portion 12 transfers the image on 
the surface of the photoreceptor drum 8 onto a sheet of paper provided by 
a paper feeder (not shown) so as to contact the surface of the 
photoreceptor 8. The separating portion 13 separates the sheet of paper on 
which an image is transferred from the surface of the photoreceptor drum 8 
and directs it to a conveying path leading to a fixing portion (not 
shown). 
FIG. 2 is an enlarged view of the photoreceptor drum 8 and the LED head 10. 
In the figure, the numeral 16 represents an optical system where zooming 
is not performed such as a SELFOC lens, etc., the numeral 17 represents a 
substrate where the LED device is attached, and the numeral 18 represents 
a heat radiating plate serving as a heat sink and made of aluminum. The 
optical system 16 and the LED device make a pair. Many such pairs are 
arranged in an array in a direction perpendicular to the surface of FIG. 2 
(that is, a direction along the axis of the photoreceptor drum 8). The 
numerals 19 and 20 represent pins attached to the LED head 10, and they 
are engaged with pin admitting grooves 37 and 38 of stays 21 and 22 to be 
subsequently described (see FIG. 5). 
FIG. 3 is a plan view showing a condition of an engagement of the LED head 
and the stays 21 and 22. FIG. 4 is a a cross-sectional view taken on a 
part thereof. In this embodiment, an engagement position of the LED head 
10 and the stays 21 and 22 is the center of the LED head 10, that is, the 
center of the photoreceptor drum 8 along the axis thereof. The stays 21 
and 22 are fixed with screws 29, 30, 31 and 32 on side plates 27 and 28 on 
which the photoreceptor drum 8 is fixed. As shown in FIG. 5, the pin 
admitting grooves 37 and 38 for admitting the pins 19 and 20 provided on 
the LED head are formed on the stays 21 and 22. Both ends of the LED head 
are fixed with pins 23 and 24 through elliptic holes 25 and 26 on L-shaped 
fixing plates 33 and 34 fixed on the side plates 27 and 28 with screws 35 
and 36. 
In the above structure, the LED head 10 is heated by the turning on of the 
LED device thereof and is expanded when the heat is high. Relating an 
expansion in a lateral direction in FIGS. 3 and 4, since a correspondence 
of the center of the LED head and the centers of the stays 21 and 22 is 
maintained by the engagement of the pins 19 and 20 of the LED head 10 and 
the pin admitting grooves 37 and 38 of the stays 21 and 22, the center of 
the LED head 10 never shifts with respect to the center of the 
photoreceptor drum 8. Thus, an image formed on transferring paper (not 
shown) through the photoreceptor drum 8 is relatively clear, since the 
image, although shifted on both sides, is not shifted in the center (that 
is, the center of the shift is fixed). In this embodiment, the positions 
of the center of the LED head 10 and the centers of the stays 21 and 22 
correspond, and the positions of both sides of the LED head and both sides 
of the stays 21 and 22 shift. The shift becomes permittable by moving the 
pins 23 and 24 in the elliptic holes 25 and 26. 
Although the pins 19 and 20 are provided on the LED head 10, and the pin 
admitting grooves 37 and 38 are formed on the stays 21 and 22 in this 
embodiment, it may be possible to provide the pins on the stays 21 and 22 
and to form the grooves on the LED head. 
Moreover, although the engagement position of the LED head 10 and the stays 
21 and 22 is the center of the LED head so that the center corresponds the 
center of an original in this embodiment, it may be possible, for example, 
to provide the pins 19 and 20 and the pin admitting grooves 37 and 38 on 
one end of the LED head and one end of each of the stays 21 and 22, 
respectively, so that the engagement position corresponds to one end of an 
original. The above one-end engagement can also be realized by, without 
providing any of the pins 19 and 20, the stays 21 and 22 and the pin 
admitting grooves 37 and 38, fixing one end of the LED head 10 and 
half-fixing the other end on the fixing plates 33 and 34 through either of 
the elliptic holes 25 and 26. 
Obviously, many modifications and variations of the present invention are 
possible in light of the above teachings. It is therefore to be understood 
that within the scope of the appended claims, the invention may be 
practiced other than as specifically described.