Sheet transport apparatus

A sheet transport apparatus capable of applying a predetermined force for transporting any kind of paper, including a sheet of thin paper, without forming transport wrinkles in the paper caused by a nipping pressure. The apparatus includes a hard roller 114 and a soft roller 112 for nipping a sheet of paper 10 therebetween. The soft roller is driven to transport the sheet using a frictional force generated from the nipping pressure applied by the soft and hard rollers. The hard roller is in a barrel-shape having a predetermined radial curvature "R", such that the nipping pressure decreases gradually along its axis from the center toward the periphery of a nipping face between two rollers. This causes a compressive stress against the sheet to shift smoothly from a portion of the sheet nipped by the soft and hard rollers to a portion free from the nipping pressure. Thus, the formation of transport wrinkles in the sheet can be prevented.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates generally to a sheet transport apparatus. 
More particularly, the present invention relates to a sheet transport 
apparatus for use in a copying machine, scanner, printer, facsimile 
machine, microfilmer, and the like. 
2. Description of the Prior Art 
Copying machines, printers and facsimile machines are used in a wide 
variety of applications. A mechanism for transporting sheets of paper is 
essential for such machines. 
A sheet transport apparatus used in such machines includes two 
relatively-positioned rollers, or a belt and a roller, for nipping a sheet 
of paper to be transported. Such rollers, or the belt and roller, rotate 
together to transport the sheet of paper. 
FIG. 2 shows such a conventional sheet transport apparatus in which a pair 
of rollers are nipping a sheet of paper 10. A soft roller 12 is made of a 
material, such as rubber having a high coefficient of friction, and is 
connected to a drive source, e.g., a motor, so that soft roller 12 serves 
as a drive roller. In a position relative to soft roller 12 there is 
provided a hard roller 14 made of synthetic resin or the like. Typically, 
soft and hard rollers 12, 14 apply an appropriate nipping force 
therebetween to thereby increase the force of transporting sheet 10. In 
other words, a certain pressure is applied to a nipping area where sheet 
10 is nipped between soft and hard rollers 12 and 14, respectively. 
Thus, soft and hard rollers 12, 14 of the conventional sheet transport 
apparatus apply a high pressure to sheet 10, so that the nipping area of 
sheet 10 is pressed with a given pressure. In the meanwhile, an area of 
the sheet 10 other than the nipping area remains free from pressure. As a 
result, there is generated a difference in pressure between the nipping 
area and the free area, which causes distortion to be developed on the 
inside of sheet 10. This distortion disadvantageously leads to so-called 
"transportation wrinkles" which are indicated by arrow 16 in FIG. 2. 
SUMMARY OF THE INVENTION 
It is an object of the present invention to alleviate the above problem by 
providing a sheet transport apparatus capable of preventing the formation 
of wrinkles in the paper, including thin or soft paper, caused by a 
nipping pressure, while maintaining the apparatus's force of transporting 
the sheets of paper. 
The above object will be realized by a sheet transport apparatus, according 
to one aspect of the present invention, comprising a first friction member 
for transport and a second friction member for transport abutted against 
the first friction member, the first and second friction members nipping a 
sheet of paper therebetween, and rotating together to transport the sheet. 
A nipping pressure applied to a nipping face between the first and second 
friction members where the sheet is nipped gradually decreases from the 
center toward the outer periphery of the nipping face. 
Such a gradual decrease of the nipping pressure contributes to suppressing 
distortion of the sheet caused by the nipping pressure on the inside of 
the sheet in the sheet transport apparatus in accordance with the first 
aspect of the present invention. 
The sheet transport apparatus, according to a second aspect of the present 
invention, includes a transport roller as the first friction member and an 
elastic body as the second friction member. A diameter of the transport 
roller is made larger at an intermediate position along its axis than in 
other positions. 
The nipping pressure applied to the nipping face increases in the center of 
the transport roller and decreases toward both ends thereof along its axis 
due to the design of the transport roller. 
The sheet transport apparatus, according to a third aspect of the present 
invention, includes a transport belt as the first friction member and an 
elastic body as the second transport member. A thickness of the transport 
belt is made larger in a widthwise direction at an intermediate position 
of the transport belt than in other positions. 
The nipping pressure applied to the nipping face is therefore high in the 
center of the transport belt in an axial direction and decreases toward 
the ends of the transport belt due to the design of the transport belt. 
Another aspect of the present invention is a copying machine utilizing a 
sheet transport apparatus in accordance with the above first through third 
aspects. 
A further aspect of the present invention is a scanner utilizing a sheet 
transport apparatus in accordance with the above first through third 
aspects. 
Still another aspect of the present invention is a printer utilizing a 
sheet transport apparatus in accordance with the above first through third 
aspects. 
A still further aspect of the present invention is a facsimile machine 
utilizing a sheet transport apparatus in accordance with the above first 
through third aspects. 
A still further aspect of the present invention is a microfilmer utilizing 
a sheet transport apparatus in accordance with the above first through 
third aspects. 
By adapting a sheet transport apparatus according to the first through 
third aspects of the present invention, it is possible to realize a 
copying machine, for example, having an improved force for transporting 
sheets of paper.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
FIG. 1 illustrates a sheet transport apparatus according to a preferred 
embodiment of the present invention, in which a soft roller 112 and a hard 
roller 114 nip a sheet of paper 10 therebetween. 
A feature of this embodiment is that hard roller 114 is barrel-shaped, 
rather than having a cylindrical shape of hard roller 14 in FIG. 2, such 
that a diameter of hard roller 114 changes along its axis, a radial 
curvature of the diameter change being shown by "R" in FIG. 1. Thus, 
barrel-shaped hard roller 114 applies a greater pressure (or a nipping 
pressure) on sheet 10 in a position where the diameter is large than in a 
position where the diameter is small. In other words, a compressive stress 
against the thickness of nipped sheet 10 decreases smoothly from the 
center toward both ends of hard roller 114 about its axis. Formation of 
transport wrinkles is thereby suppressed to the least possible degree. 
Like the conventional soft roller, soft roller 112 is made of an elastic 
material, such as rubber. During rotation, soft roller 112 changes its 
shape in compliance with the barrel-shape of hard roller 114. 
The embodiment shown in FIG. 1 comprises hard roller 114 and soft roller 
112. It is also preferable to apply other conventionally-known structures, 
such as the combination of a transport belt and a roller, or the 
combination of transport belts, to the principles of the present 
invention. 
In the case where a transport belt is used, it is advantageous to form the 
belt so that the center of the belt is thicker in the width direction than 
other portions. With such a structure, it is possible to decrease the 
nipping pressure toward the periphery of the belt, while maintaining a 
high nipping pressure in the center of the belt. The barrel-shaped hard 
roller 114 can thus be applied to other structures such as the transport 
belt. 
It is also preferable to connically taper the rollers or belts, instead of 
shaping them like a barrel. In either case, the distortion stress 
developed on nipped sheet 10 can be dispersed smoothly, thereby 
effectively preventing the formation of transport wrinkles in sheet 10. 
It can be understood from the above description that, according to the 
first aspect of the present invention, the sheet transport apparatus 
capable of preventing the formation of transport wrinkles in the sheet is 
provided, in which the nipping pressure changes smoothly from the nipping 
face toward other areas of the sheet. 
According to the second aspect of the present invention, the sheet 
transport apparatus of the first aspect can be realized with a simple 
structure by changing the diameter of the transport roller along its axis 
so that the diameter is larger in the center than at other portions, 
thereby facilitating setting the nipping pressure to a high level in the 
center of the nipping face. 
According to the third aspect of the present invention, as illustrated in 
FIG. 3, a transport belt 120 is made thicker in the center of the belt 
than in other portions. Belt 120 is supported on a roller 122 and, with a 
second transport member 124, forms a nip for sheet 10. As a result, it is 
also possible to realize the sheet transport apparatus with a simple 
structure by setting the nipping pressure to a high level in the center of 
the nipping face and decreasing the nipping pressure smoothly toward the 
outer periphery. 
Referring to FIG. 4, another aspect of the present invention provides a 
copying machine 130 by adapting the first, second or third aspect above. 
The copying machine has an increased force of transporting sheets of 
paper, so that the formation of transport wrinkles can be prevented. 
Referring to FIG. 5, still another aspect of the present invention provides 
a scanner 140 having an increased force of transporting sheets of paper. 
Other aspects of the present invention provide a printer 150 (see FIG. 6), 
facsimile machine 160 (see FIG. 7), and microfilmer 170 (see FIG. 8) 
having the same effect as the first through third aspects of the present 
invention.