Belt tension device

A belt tensioning device for use in a copying machine. The tension device includes two pairs of levers each lever having a first and second arm. A pair of floating rollers are provided to tension the belt by formation of loops in front of and behind, respectively, of an intermittent contact point between the belt and a pressure element moving at a different speed. Each floating roller is suspended in the ends of the first arms of the respective levers. The end of other arms of one pair of levers is connected to the end of the corresponding second arm of the other pair of levers by a tension spring system. One pair of levers has a larger transmission ratio than the other, so that the floating rollers return to their old positions after contact between the belt and pressure element.

FIELD OF THE INVENTION 
The present invention relates to a tensioning means and, in particular to 
belt tensioning means for use with apparatus such as photocopiers in which 
a first moving belt is contacted with a second pressure element so that 
the belt is driven at the speed of the second element. 
BACKGROUND OF THE INVENTION 
Belt tensioning means for use in photocopying are know, for example, U.S. 
Pat. Nos. 4,183,658 and 4,592,641. See also U.S. Pat. No. 4,114,536. In 
the photocopier disclosed in U.S. Pat. No. 4,183,658, a photoconductive 
belt on which an image is electrophotographically formed is pressed 
against an intermediate support in order to transfer the formed image to 
the support. At the point where the contact pressure is applied, the belt 
assumes the speed of the intermediate support so that the image transfer 
can take place without smearing. 
In this copier, the intermediate support is driven at a speed slightly less 
than that of the photoconductive belt so that during the image transfer 
the loop formed in the belt by a first floating roller in front of the 
point of contact pressure becomes larger and the loop formed by a second 
floating roller after contact, becomes smaller. If the contact pressure is 
eliminated after the image transfer, the second floating roller, which 
exerts a tensioning force greater than that of the first floating roller, 
enlarges the loop it forms in the belt after the point of contact and at 
the same time reduces the loop that the first floating roller forms in the 
belt in front of the contact point. Thus, after each image transfer the 
original position of the photoconductive belt with respect to the 
intermediate support is restored while the belt remains tensioned. 
In order to avoid disturbing impacts and vibrations in the belt during the 
movement of the floating rollers from one position to the other, the 
difference between the forces exerted by the two floating rollers is 
preferably controlled to be no larger than the force required to move the 
rollers and that part of the belt therebetween. Also, the force which each 
roller exerts separately is preferably made no larger than necessary to 
keep the associated belt loop sufficiently taut. 
One method of controlling such forces has been to use the weight of the 
floating rollers and/or tension forces produced by springs acting on the 
floating rollers. However, in practice, difficulties often occur in the 
operation of such devices due to the cumulative effect of deviations from 
various components due to accepted manufacturing tolerances which result 
in imprecise control of the weight on balancing. Accordingly, it is an 
object of the invention to obviate such difficulties by providing a novel 
tensioning means. 
SUMMARY OF THE INVENTION 
Generally, the present invention provides a belt tensioning means in which 
each of the floating rollers is mounted in associated first arms of a pair 
of levers each having first and second arms. Each end of the second arms 
of the one pair is connected to the end of the corresponding second arm of 
the other pair of levers by means of a tension spring system extending 
between the pairs of levers. The tensioning means provides a transmission 
ratio for one pair of levers which differs from the transmission ratio of 
the other pair. 
The forces involved by the tensioning means of the present invention are 
derived primarily from one tension spring system so that there is no 
cumulation effect caused by manufacturing tolerance deviations. Also, the 
floating rollers can be made very light in weight so that very little 
force is required to move them. Disturbing impacts and vibrations in the 
belts are thus greatly reduced or eliminated. 
Other advantages of the invention will become apparent from a perusal of 
the following detailed description of a presently preferred embodiment 
taken in connection with the accompanying drawing.

PRESENTLY PREFERRED EMBODIMENT 
In FIG. 1, an endless photoconductive belt 1 is trained over a continuously 
rotating drive roller 2, a pressure roller 3, guide rollers 4, 5 and 6, 
auxiliary rollers 7 and 8 and floating rollers 9 and 10. With the aid of 
narrow rollers 11, which only engage the edges of belt 1, belt 1 is kept 
in contact with the surface of roller 2. In this way, belt 1 is driven 
with a speed of for instance 20 m/min. Pressure roller 3 is freely 
rotatably supported in bearings in arms 12 which can pivot about shaft 13. 
Each arm 12 is provided with cam follower 14 which cooperates with a cam 
15 which is rotatably supported in bearings. Springs 16 exert a pulling 
force on the arms 12, so that cam follower 14 is continuously kept in 
contact with cam 15. 
Guide roller 4 is continuously driven in the direction indicated with an 
arrow with a circumferential speed which is slightly lower than the speed 
of movement of belt 1, for instance with a circumferential speed of 19.8 
m/min. Because of the difference in speed, belt 1 slips over the surface 
of roller 4. While using this slip, belt 1 is aligned with the aid of the 
guide plates 17 present on roller 4 at either side of belt 1, as more 
fully described in Dutch Patent Specification 148 418. 
The invention provides two pairs of two-armed levers 19 and 21 having first 
arms 19a and 21a, respectively, and second arms 19b and 21b, respectively. 
Levers 19 and 21 are pivotably mounted to shafts 20 and 23, respectively. 
Guide rollers 5 and 6 and auxiliary rollers 7 and 8 are freely rotatably 
and supported in bearings. Floating roller 9 preferably consists of two 
narrow discs, engaging only the edges of belt 1. These discs are rotatably 
supported in bearings on rod 18, which is fixed to first arms 19a of 
levers 19. Floating roller 10 comprises similar discs which are freely 
rotatably and supported in bearings on shaft 22, which at either side is 
supported in bearings in first arms 21a of levers 21. 
Second arms 19b of levers 19 are connected by tension springs 24 to the 
second arms 21b of levers 21. For technological reasons associated with 
the construction it is possible to provide tensioning wires, which may run 
over pulleys, between the levers and the springs. Since the dimensions of 
levers 21 preferably differ from those of the levers 19 (the first arms 
21a and 19a are, for example, each 50 mm long, the second arms 19b is 
preferably 95 mm and the second arm 21b is preferably 100 mm), the 
transmission ratio of levers 21 is thus greater than that of levers 19 
(about 5% for the given lengths). 
Since tension springs 24 pull on second arms 19b and 21b with equal force, 
a greater force is exerted on floating roller 10 than on floating roller 
9. In order to prevent floating roller 10 from completely cancelling the 
belt loop formed by floating roller 9, first arms 19a of levers 19 are 
limited in their movement by stop 25. Since the forces for tensioning the 
belt are derived from the tension spring system, floating rollers 9 and 10 
can be made very light in weight. The tension spring used in the 
above-described example has a spring constant of 0.15 N/ mm, a 
prestressing of 2.35 N and a length, in the unloaded state, of about 70 
mm. Any deviation within the tolerance limits does not affect operation. 
Above pressure roller 3 an intermedate support is installed consisting of 
an endless belt 26 which is provided with a thin top layer which is 
manufactured of soft silicone rubber. Belt 26 is driven in the direction 
of the arrow by drive roller 27 with a speed which is slightly lower than 
that of belt 1, for example, at a speed of 19.8 m/min. Belt 26 is guided 
over a roller 28 which is freely rotatably supported in bearings. At the 
inside of roller 28 a heating element 29 is installed with which the 
cylinder of roller 28 and, consequently, belt 26 coming in contact with it 
is heated. With the aid of a generally known means (not shown) the energy 
supply to element 29 is controlled in such a way, that the temperature at 
the surface of belt 26 is constantly kept at, for instance, 105.degree. C. 
At the side of driver roller 27 a pressure roller 30 is freely rotatably 
supported in bearings on shaft 31, which is connected to arms 32. These 
arms forms a rigid unit which is rotatably supported in bearings on shaft 
33. As shown in FIG. 1, arm 32 is provided with a cam follower 34 which 
cooperates with rotatable cam 35. 
In operation, the copying machine is provided with a supply of copy paper 
which includes tray 37 for receiving a stack of sheets 38. Tray 37 is 
provided with a friction roller 39 with the aid of which the sheets can be 
fed from stack 38 one after another. A spring 40 presses against tray 37 
so that the top sheet of the stack is always head against roller 39 with a 
predetermined pressure while tray 37 swivels about shaft 41. A sheet fed 
from stack 38 is guided by guide plate 42 into the nip between the 
continuously rotating roller 43 and a disc roller 44 resting freely on 
roller 33. Into the joining passage between guide plates 45 and 46 extends 
a stop 47 which is connected to arms 48 fixed on a shaft 49 which is 
rotatably supported in bearings. The arm 48 is held continuously in 
contact with a cam 51 which is rotatably supported in bearings by the 
force exerted by springs 50. 
Beyond stop 47 is an endless belt 52 which is tightened about freely 
rotatable rollers 53 and 54 and about a continuously rotating drive roller 
55. Roller 55 drives belt 52 with a speed which is as high as or slightly 
higher (for instance 5% higher) than the speed of movement of belt 26. 
Inside roller 53 a known heating element 56 is provided with which the 
cylinder of roller 53 and, consequently, belt 52, is heated. Element 56 is 
controlled so that the temperature at the surface of belt 52 is constantly 
kept at a predetermined temperature, for instance, of 105.degree. C. On 
belt 52 rests a flexible pressure plate 57, which is fixed to shaft 58 
which is rotatably supported in bearings. 
Along track followed by belt 1 means of the usual kind are installed in 
order to form powder images on the photoconductive surface of belt 1 via 
the xerographic process. These means comprise a light-source 59 with which 
any charges present on belt 1 are removed, a cleaning brush 60 with which 
any powder residues present on belt 1 are removed and a charging device 61 
with which a uniform electrostatic charge is applied to belt 1. A 
projection station 62 is provided in which a light image of an original 
lying on an exposure plate is projected onto belt 1 with the aid of flash 
lamps, mirror(s) and an objective lens (not shown) during which projection 
of an image-wise charge pattern is formed on belt 1. A developing device 
63 is provided to develop the charge pattern of belt 1 into a powder 
image. A light-source 64 is provided with which belt 1 is radiated for 
reducing the adhesion between image powder and belt 1. 
Finally, the copying machine is provided with control means (not shown) 
with which the operation of the above-mentioned image-forming means 59-64, 
as well as the operation of cam 15, cam 35, friction roller 39 and cam 51 
are synchronized. Control systems which can be used for this, are known in 
many kinds and many embodiments; a very suitable embodiment of such a 
system is for instance described in Dutch Patent Application 7311992. 
When a powder image which was formed on belt 1 by successively charging, 
image-wise exposing and developing, approaches pressure roller 3, cam 15 
is rotated through 180.degree., under influence of a signal emitted by the 
control system. Consequently arms 12 pivot and roller 3 is moved upwards. 
Belt 1 is thus pressed between the rollers 3 and 28 against belt 26. As a 
result of the friction then occurring, belt 1 at the place of the pressure 
zone accepts the slightly lower speed of belt 26. As belt 1 is driven by 
roller 2 with a constant and slightly higher speed, the loop in which 
roller 9 is positioned will increase. However, by the force exerted by 
spring 24 via lever 19, roller 9, pivoting swivels with arms 19a, will 
keep the loop tightened. 
Since belt 1 is carried away with unchanged speed slipping over roller 4, 
the loop in which roller 10 is positioned will decrease and roller 10 will 
swivel with arms 21a. By the force exerted by spring 24 via lever 21 
roller 10 will keep the belt tightened in the loop. 
While the powder image passes through the pressure zone, it is pressed into 
the soft rubber layer of belt 26, and in this way it is transferred from 
belt 1 to belt 26, and is carried along by the latter. Although during 
this transfer a very great part (90-95%) of the image powder is 
transferred to belt 26, a residue is normally left on belt 1. This residue 
is removed later in the usual way by the operation of lamp 59 and brush 
60. While being carried along by belt 26, the transferred powder image is 
heated from belt 26. During this heating the powder grains are softened 
and start coalescing, so that the image becomes sticky as it approaches 
pressure roller 30. 
In the meantime the control system of the copying machine has also emitted 
signals with which first friction roller 39 has been activated in order to 
feed a sheet from stack 38 against stop 47, and with which subsequently 
cam 51 has been rotated through 180.degree.. During rotation of the cam 
arms 48 swivel upwardly so that stop 47 is lifted. The sheet supplied is 
then pushed forward by rollers 43 and 44 and fed between belt 52 and plate 
57. The sheet is then further conveyed by belt 52, during which it is 
heated so that upon approaching pressure roller 30, at least at the side 
with which it makes contact with belt 52, it has a temperature which is 
almost equal to the temperature of the image material supplied by belt 26. 
When the leading edge of the image present on belt 26 and the leading edge 
of the copy material have come in the neighborhood of roller 30, the 
control system of the copying machine emits a signal with which cam 35 is 
rotated through 180.degree.. 
As a result of the rotation of cam 35, arms 32 swivel about shaft 33 and 
roller 30 is pressed against belt 26. The image and the sheet of copy 
material pass through the pressure zone between roller 30 and belt 26, the 
softened and sticky image material is pressed onto the copy material. 
(This is the reason why when passing the pressure zone the whole image is 
separated from belt 26 and transferred to the copy material.) After 
cooling the image will have been firmly attached to the copy material and, 
thus, fixed thereon. 
After the image formed on belt 1 has been transferred to belt 26 or to the 
copy sheet respectively, the control system of the copying machine emits 
signals with which the cams 15, 35 and 51, and, consequently, rollers 3 
and 30, as well as stop 47, are again returned into their original 
positions. When roller 3 is released from belt 26, the force exerted by 
roller 10 will exceed the force of roller 9 since the transmission ratio 
of the levers 21 is greater than the transmission ratio of the levers 19 
and as a consequence of this the loop formed by roller 10 will increase, 
and the loop formed by roller 9, will decrease. Roller 9 is moved upwards, 
until its movement is limited by stop 25. At that moment the two loops 
have again gone back to their original dimensions. 
While a presently preferred embodiment of the invention has been shown and 
described in particularlity, it may be otherwise embodied within the scope 
of the appended claims.