Sheet meandering movement preventing method

In a printing sheet conveying device in which a sheet bearing a toner image is passed through a heating roll and a pressure roll, and which has means for changing the right and left contact pressures of these rolls to correct the meandering movement of the sheet when conveyance of the sheet is started; in which when conveyance of the sheet is started, the contact pressure of the rolls is made higher on one of the right and left sides than on the other side, and in a predetermined period of time the contact pressures on both sides are made equal to each other.

BACKGROUND OF THE INVENTION 
1. (Technical Field of the Invention) 
This invention relates to a method of preventing the meandering movement of 
a printing sheet in an electrophotographic apparatus. 
2. (Prior Art) 
The operation of an electro-photographic apparatus such as a laser beam 
printer will be described with reference to FIG. 2. A printing sheet 201 
is conveyed by means of upper and lower tractors 204 and 203 at a speed 
equal to the peripheral speed of a photo-sensitive drum 202. A toner image 
(not shown) is transferred onto the printing sheet 201 by means of a 
transfer unit 210. Thereafter, tension is given to the printing sheet 201 
by a buffer as required. Under this condition, the printing sheet 201 
together with the toner image is heated and pressurized by means of a 
heating roll 207 and a pressure roll 208 so that the toner image is fixed 
to the printing sheet 201. The printing sheet 201 thus treated is pulled 
under a predetermined tensile force by a puller 213 and a pressure roll 
212, so that it is delivered into a stacker (not shown). 
As is apparent from the above description, in the above-described 
operation, the pin drive provided by the upper and lower tractors 204 and 
203 and the frictional drive provided by the heating roll 207 and the 
pressure roll 208 are utilized for conveyance of the printing sheet 201. 
When the printing sheet 201 is conveyed by the frictional force provided 
by the heating roll 207 and the pressure roll 208, since no means for 
preventing the widthwise movement of the printing sheet is provided the 
printing sheet meanders right and left, as a result of which stress is 
applied to the printing sheet 201, thus breaking the conveying 
perforations or creasing the printing sheet. 
In order to overcome the above-described difficulty, means 209 for 
correcting the meandering movement of the printing sheet 201 (hereinafter 
referred to as "meandering movement correcting means 209") is provided. In 
this case, the meandering movement of the printing sheet 201 is detected 
by a sensor 211, and the contact pressure of the heating roll 207 and the 
pressure roll 208 is adjusted with respect to the axial direction of the 
heating roll 207. The meandering movement correcting means 209, as shown 
in FIG. 3, comprises: a left arm 301 and a right arm 302 supporting both 
ends of the pressure roll 208 which is abutted against the heating roll 
207; and an electric motor 304 for driving the right and left arms through 
a wire 303. The right and left arms 302 and 301 are swung about their own 
fulcrums in the opposite directions by the electric motor 304 so that the 
contact pressure of the pressure roll 208 against the heating roll 207 is 
changed along the axial direction of the latter 207. That is, with the 
meandering movement correction means, the difference in peripheral speed 
between the ends of the heating roll 207 and the pressure roll 208 which 
is due to the deformation is utilized to correct the meandering movement 
of the printing sheet 201. 
The correction of the meandering movement of the printing sheet 201 will be 
described with reference to FIG. 4 in more detail. When it is necessary to 
bend the direction of movement of the printing sheet 201 towards on side A 
in FIG. 4, the contact pressure of the heating roll 207 and the pressure 
roll 208 is made higher on the side A than on the other side B. In this 
case, in each of the rolls 207 and 208, the radius rA on the side A is 
smaller than that rB on the side B, and therefore the peripheral speed vrB 
on the side B is higher than the peripheral speed vrA on the side A (vrA &lt; 
vrB). Accordingly, the amount of movement of the printing sheet 201 is 
larger on the side B than on the side A; that is, the direction of 
movement of the printing sheet 201 is changed towards the side A. When, in 
contrast, it is required to bend the direction of movement of the printing 
sheet towards the side B, the contact pressure of the heating roll 207 and 
the pressure roll 208 is made lower on the side A than on the side B. In 
this case, the printing sheet 201 is shifted towards the side B. 
The heating roll 207 is a both-ends-supported roll having a drive source, 
namely, an electric motor 101 on one end. Therefore, as shown in FIG. 1, 
for a very short period of time immediately after the printing sheet 201 
is driven by the heating roll 207, because of the torsional twisting of 
the heating roll 207 the drive force is transmitted from the driving side 
A toward the non-driving side B in a twist mode. Accordingly, the angular 
speed of the heating roll 207 is higher on the driving side A than on the 
non-driving side B; that is, a condition wA &gt; wB is established. 
Accordingly, with the heating roll 207 uniform in diameter, the peripheral 
speed is higher on the side A than on the side B (vA &gt; vB). Under this 
condition, the amount of movement of the printing sheet 201 being conveyed 
is larger on the side A than on the side B, and therefore the direction of 
movement of the printing sheet 201 is bent towards the side B. 
Thereafter, in a very short period of time, the drive force is transmitted 
uniformly in the heating roll 207, and the entire surface of the heating 
roll 207 becomes constant in peripheral speed. Under this condition, the 
amount of movement of the printing sheet 201 on the driving side A is 
equal to that of the printing sheet on the non-driving side B. Therefore, 
as shown in FIG. 5, a returning force which the puller 213 and the 
pressure roll 212 provide in response to the bending of the direction of 
movement of the printing sheet towards the non-driving side B and the 
rigidity of the printing sheet 201 in the widthwise direction shift the 
printing sheet 201 towards the driving side A. 
On the other hand, the meandering movement correcting means 209 utilize the 
difference in peripheral speed between the right and left ends of the 
heating roll 207 and the pressure roll 208, to make the amount of movement 
of the printing sheet on the driving side equal to the amount of movement 
of the printing sheet on the non-driving side or vice versa. Therefore, in 
order to correct the meandering movement of the printing sheet, it is 
necessary to operate the meandering movement correcting means 209 for a 
period of time for which the printing sheet 201 is shifted to cover the 
difference in the amount of movement between the two sides A and B. 
Therefore, even if, when at the start of the conveyance of the printing 
sheet the sensor 211 detects the meandering movement of the printing 
sheet, the meandering movement correcting means is operated, it is 
impossible to completely eliminate the meandering movement of the printing 
sheet. In other words, immediately after when the conveyance of the 
printing sheet is started, the printing sheet is unstable being swung 
right and left. 
SUMMARY OF THE INVENTION 
Accordingly, an object of this invention is to eliminate the 
above-described difficulty accompanying a conventional method of 
correcting the meandering movement of a printing sheet in an 
electro-photographic apparatus. 
More specifically, an object of the invention is to provide a method of 
preventing the meandering movement of a printing sheet in an 
electro-photographic apparatus in which a printing sheet is prevented from 
meandering at the start of the printing operation. 
The foregoing object and other objects of the invention has been achieved 
by the provision of a method of preventing the meandering movement of a 
printing sheet in a sheet conveying device in which a sheet bearing a 
toner image is passed through an electric heating roll rotatably supported 
and having a drive source and an elastic pressure roll held in roll 
contact with the elastic heating roll by a frictional force provided 
therebetween, and which has means for changing the right and left contact 
pressures of the heating roll and pressurizing roll to correct the 
meandering movement of the sheet when conveyance of the sheet is started; 
in which when conveyance of the sheet is started, the contact pressure of 
the heating roll and pressure roll is set higher on one of the right and 
left sides in the axial direction of the heating roll than on the other, 
and in a predetermined period of time the contact pressure on the one side 
is made to equal to that on the other side thereby to prevent the 
meandering movement of the sheet in the axial direction of the heating 
roll at the start of the conveyance of the sheet. 
In a roll which is supported at both ends and having a driving source at 
one end, according to this invention, when the conveyance of the printing 
sheet is started, the pressure is applied to the roll on the driving side, 
and thereafter the pressure thus applied is made to be in balance with the 
pressure on the non-driving side after an appropriate period of time 
elapses, paying attention to the fact that (1) the driving force is 
transmitted from the roll on the driving side at the time of the start of 
conveyance of the printing paper due to the torsional twisting of the roll 
so that the peripheral speed of the heating roll near the driving side is 
higher than that far from the driving side; (2) in the case where the 
angular speed is constant, the roll with its smaller diameter is lower 
than that with its larger diameter in peripheral speed; and (3) the roll 
is compressed by application of pressure thereby to decrease its diameter 
in dimension. 
The nature, principle and utility of the invention will become more 
apparent from the following detailed description when read in conjunction 
with the accompanying drawings.

DETAILED DESCRIPTION OF THE INVENTION 
One example of a method of preventing the meandering movement of a printing 
sheet will be described with reference to the accompanying drawings. 
Components employed in the method of the invention which are functionally 
equal to those in the abovedescribed conventional method are designated by 
the same reference numerals or characters. 
As shown in FIG. 1, when driving the heating roll 207 is started, because 
of the torsional twisting of the heating roll 207 the drive force is 
transmitted from the driving side A to the non-driving side B in a twist 
mode, and therefore the angular speed wA of the heating roll 207 on the 
side A is higher than that wB of the heating roll 207 on the side B, or 
the angular speed of the heating roll 207 is lower towards the side B; 
that is, wA &gt; wB. Accordingly, the peripheral speed vA on the side A is 
higher than that vB on the side B, and the amount of movement of the 
printing sheet 201 is larger on the side A than on the side B. As a 
result, the direction of movement of the printing sheet 201 is bent 
towards the side B. 
In order to eliminate this difficulty, it is essential that the heating 
roll 207 is uniform in peripheral speed when driven; that is, the 
peripheral speed of the heating roll 207 on the driving side A should be 
equal to that of the heating roll on the non-driving side B (rA wA=rB wB). 
That is, when the heating roll 207 is driven, because of wA &gt; wB, rA &lt; rB 
should be established. 
Therefore, the meandering movement correcting means 209 as shown in FIG. 3 
is used to make the contact pressure of the heating roll 207 and the 
pressure roll 208 higher on the driving side A than on the non-driving 
side B. In this case, the heating roll 207 being compressed, the apparent 
radius rA on the driving side A becomes smaller than that rB on the 
non-driving side B as shown in the middle part of FIG. 1, and therefore 
when driving the heating roll 207 is started, the peripheral speed vA on 
the driving side A can be made equal to that vB on the non-driving side B, 
and accordingly the amount of movement of the printing sheet 201 on the 
driving side A is equal to that of the printing sheet on the non driving 
side B. Thus, the printing sheet 201 will not meander. 
Thereafter, in a very short period of time .increment.t, the drive force 
being uniform in the heating roll 207 both on the sides A and B, the 
angular velocity wA on the driving side A becomes equal to that wB on the 
non-driving side B (wA=wB). Accordingly, with the pressure maintained 
applied on the driving side A, the peripheral speed is higher on the 
non-driving side B because of the larger diameter, and therefore the 
amount of movement of the printing sheet 201 is larger on the non-driving 
side B. Thus, the direction of movement of the printing sheet 201 is bent 
towards the driving side A. Therefore, as indicated by the curve (from (1) 
to (2)) in FIG. 7, the pressure applied to the heating roll 207 on the 
driving side A is returned to zero in pressure difference during the 
period of time .increment.t mentioned above, so that the radius rA of the 
heating roll on the driving side A is made equal to that rB on the 
non-driving side B. Thus, the peripheral speed of the heating roll 207 on 
the driving side A is equal to that of the heating roll 207 on the 
non-driving side B; that is, the amount of movement of the printing sheet 
201 on the driving side A is equal to that of the printing sheet on the 
non-driving side B. 
Accordingly, the printing sheet 201 is allowed to move straightly, and 
there is no phenomenon that the printing sheet 201 is returned because of 
its rigidity in the widthwise direction as indicated at (3) in FIG. 6. 
That is, the printing sheet 201 is conveyed without any meandering 
movement. 
In the above-described embodiment, the period of time which elapses from 
the time instant when the conveyance of the printing sheet is started with 
a pressure applied to the heating roll 207 on the driving side A until the 
pressure thus applied is made to be in balance with that on the 
non-driving side B is 0.5 second. 
In the above-described meandering movement correcting means 209, the left 
and right arms 301 and 302 coupled through the wire 303 are operated in 
association with each other to provide the difference in contact pressure 
between the two ends of the heating roll 207, thereby to prevent the 
meandering movement of the printing sheet. The same effect can be obtained 
by using meandering movement correcting means having a mechanism for 
applying pressures separately to the right and left ends of the heating 
roll which is operated as follows: 
(1) The pressure applied to the heating roll 207 on the driving side A is 
reduced until it becomes equal to that applied thereto on the non-driving 
side B after the conveyance of the printing sheet is started. 
(2) The pressure applied to the heating roll on the non-driving side B is 
increased until it becomes equal to that applied thereto on the driving 
side A after the conveyance of the printing sheet is started. 
As was described above, in the above-described embodiment, the period of 
time which elapses from the time instant when the conveyance of the 
printing sheet is started with a pressure applied to the heating roll 207 
on the driving side A until the pressure thus applied is made to be in 
balance with that on the non-driving side B is 0.5 second; however, the 
period of time may be set to one or two seconds depending on the 
materials, structures and configurations of the heating roll 207 and the 
pressure roll 208. Furthermore, the period of time may be changed for 
instance with the variation in friction coefficient of the heating roll 
207 or the pressure roll 208. 
As is apparent from the above description, when the conveyance of the 
printing sheet is started, the meandering movement of the printing sheet 
is cancelled out by application of the corresponding force according to 
the invention, so that the printing sheet is conveyed straightly at all 
times.