GB2158011A - Ink detecting device for rotary printer - Google Patents

Abstract

A rotary printing machine has two printing drums (1, 2) receiving ink from an applicator roll (13), which in turn receives ink from a distributer (10) coupled to a pumping unit (11). A sensor roll (15) is mounted in rotatable contact with the printing drums (1, 2) and the number of revolutions of the sensor roll (15) are detected by a generator (18). Similarly a detector (20) detects the number of revolutions of a printing drum (2). The detectors (18, 20) are connected by a comparing circuit to a solenoid (25), which is connected via a link mechanism to the pumping unit (11). The comparing circuit compares the number of revolutions of the sensor roll (15) and printing drum (2) and triggers the solenoid when the result of this comparison falls outside a predetermined range. Thus slip of the sensor roll (15) and printing drum (2), due to the ink between them, may be detected and the amount of ink adjusted accordingly. <IMAGE>

Description

SPECIFICATION Ink detecting device for rotary printer The present invention generally relates to a rotary printing machine and, more particularly, to an ink detecting device for use in the rotary printing machine for detecting the quantity of a printing ink supplied onto an outer peripheral of printing drum.

There is known a rotary printing machine of a type wherein, during the rotation of the printing drum, a printing ink supplied onto the outer periphery of the printing drum is allowed to penetrate through the image area of the stencil to a paper positioned on one side of the stencil opposite to the drum and being moved in same direction in synchronism with the rotation of the drum. With this known rotary printing machine, the printing ink has to be replenished with a predetermined quantity upon a time predetermined number of papers have been printed, or the printing ink has to be manually replenished occasionally depending on the tone or the quality of the printed image on the papers being monitored by the attendant operator.The replenishment of the printing ink against the predetermined number of the papers printed on the manual replenishment of the printing ink against the tone or the quality of the printed image on the paper is disadvantageous not only in that the image can not be uniformly printed on all of the papers, but also in that it is a time-consuming and tiresome procedure.

In view of the above, an ink detecting device for detecting the quantity of ink remaining on the outer periphery of the printing drum has been devised and disclosed in, for example, the Japanese Patent Publication No. 56-41436, published in 1981. According to this publication, the ink detecting device comprises an inking roll, an ink control element for controlling the quantity of a printing ink supplied onto the periphery of the inking roll, said inking roll and said ink control element forming therebetween a pool of ink on the trailing side with respect to the direction of rotation of the inking roll, and a pivotally supported lever operatively positioned in the ink pool for enabling a microswitch to be actuated by said lever in response to the rotational flow of the ink formed the ink pool, thereby detecting the quantity of the printing ink.

With this known ink detecting device, since when the quantity of ink forming the ink pool decreases below a predetermined value, the lever detects the limit of decreasing of the ink quantity and the printing ink is, therefore, replenished with a quantity required to form the ink pool again. Therefore, this known ink detecting device is effective to eliminate the above discussed disadvantages and to permit the machine to produce the substantially uniform printed papers.

However, since the detection of the ink quantity is not directly carried out at the outer periphery of the printing drum, but is carried out indirectly in the ink pool by supplying the printing ink onto the inking roll and detecting the rotational flow of the ink in the ink pool by means of the lever, the prior art ink detecting device is not satisfactory in practice and in precision. In addition, the prior art ink detecting device is operable only with a so-called pasty ink, that is, a printing ink of relatively high viscosity, but not with a low viscosity printing ink and is, therefore, limited in application.

The present invention has been devised with a view to substantially eliminating the above discussed disadvantages and inconveniences inherent in the prior art ink detecting device and has for its essential object to provide an improved ink detecting device for use in a rotary printing machine, which can generate an output signal indicative of the quantity of printing ink when the quantity of the printing ink supplied onto the outer periphery of the printing drum has fallen beyond the predetermined optimum range of quantity which the printing ink must be supplied onto the outer periphery of the printing drum.

Another important object of the present invention is to provide an improved ink detecting device of the type referred to above, wherein the detection of the ink quantity is directly performed on the outer periphery of the printing drum.

To this end, the present invention provides an improved ink detecting device which comprises a sensor roll rotatable in frictional contact through the printing drum with a layer of printing ink interposed therebetween, a first detector capable of generating a first output signal indicative of the number of revolution of the sensor roll, a second detector capable of generating a second output signal indicative of the number of revolution of the printing drum, and a comparing circuit operable to compare the first and second output signals and to generate a control output signal in the event of the first output signal falls beyond the predetermined range as comparing with the second output signal.

According to a preferred embodiment of the present invention, the control output emerging from the comparing circuit is used to operate the printing ink supply unit to supply the printing ink onto the outer peripheral surface of the printing drum to keep the quantity of the ink retaining on the outer peripheral surface of the printing drum within a predetermined optimum range required to make prints of uniform tones and qualities.

These and other objects and features of the present invention will become clear from the following description taken in conjunction with a preferred embodiment of the present invention with reference to the accompanying drawings, in which: Figure lisa schematic perspective view of a rotary printing machine embodying the present invention; Figure 2 is a circuit block diagram using an ink detecting device according to the present invention; Figure 3 is a graph showing an output characteristic curve of one of the generators employed in the ink detecting device; Figure 4 is a graph showing the relationship between the output level of the generator of the first detector and the quantity of ink supplied with respect to the different number of revolution of a printing drum; and Figure 5 is a schematic diagram showing a different type of rotary printing machine to which the present invention is equally applicable.

Before the description of the present invention proceeds, it is to be noted that like parts are designated by like reference numerals throughout the accompanying drawings.

Referring first to Figure 1 showing a rotary printing machine to which the present invention is applicable, the machine shown therein comprises a pair of juxtaposed, upper and lower drums 1 and 2 supported one above to the other in any known manner by a machine framework (not shown), and a generally endless back-up screen 3 trained and stretched between the upper and lower drums 1 and 2. The lower drum 2 is mounted on a shaft 5 for rotation together therewith, said shaft 5 having a pulley 6 which is rigidly mounted thereon and is drivingly coupled to a drive pulley 8, rigidly mounted on a drive shaft 7 of an electrically operated motor 4, by means of a generally endless belt 9.Thus, it will readily be seen that, when and so long as the drive shaft 7 and, hence, the drive pulley 8, is rotated in one direction shown by the arrow Ar1,the lower drum 2 is rotated in one direction shown by the arrow Ar2 accompanied by the corresponding rotation of the upper drum 1 in the same direction Ar3 as the direction Ar2 of rotation of the lower drum 2 with the back-up screen 3 travelling in one direction around the upper and lower drums 1 and 2.

The machine shown therein also comprises a generally pipe-shaped, perforated ink distributor 10 having a length generally equal to the length of the lower drum 2 and having at least one row of axially equally spaced perforations (not shown). This ink distributor 10 is so supported inwardly of the back-up screen 3 by the machine framework as to permit the row of the equally spaced perforations to confront the outer peripheral surface of the lower drum 2. The ink distributor 10 has one end closed and the other end fluid-coupled to a discharge port of a pumping unit 11 having its inlet port adapted to be removably coupled with a source of printing ink. So far shown, the source of printing ink is constituted by a flexible tube 12 filled with printing ink and having one end adapted to be fluid-connected to the inlet port of the pumping unit 11.While the pumping unit 11 will be described in detail later, the printing ink supplied to the ink distributor 10 from the tube 12 through the pumping unit 11 emerges outwardly from the perforations in the distributor 10 onto the outer peripheral surface of the lower drum 2, the deposits of the printing ink on the outer peripheral surface of the lower drum 2 being subsequently uniformly spread over the outer peripheral surface of both of the drums 1 and 2 by an applicator roll 13 during the continued rotation of the lower drum 2 in the direction Ar2 to form a substantially uniform layer of ink on the outer peripheral surface of both the lower drum 2 and the upper drum 1.The applicator roll 13 is positioned inwardly of the back-up screen 3 and on the leading side of the ink distributor 10 with respect to the direction of rotation of the lower drum 2 and mounted on a support shaft 14 which is journaled at its ends to the machine framework so as to resiliently urge the applicator roll 13 with its outer periphery contacting the outer peripheral surface of both of the drums 1 and 2.Thus, during the rotation of the lower drum 2 in the direction Ar2, the applicator roll 13 rotates in frictional contact with any one of the upper and lower drums 1 and 2, but in a direction counter to the direction Ar2 oral3 of rotation of the upper or lower drum, as shown by the arrow Ar4 with the ink deposits, which have emerged onto the outer peripheral surface of the lower drum 2 from the respective perforations in the ink distributor 10, being consequently uniformly spread by the applicator roll 13 to form the ink layer of substantially uniform thickness over the outer peripheral surface of both of the upper and lower drums 1 and 2, as described hereinbefcre.

The pumping unit 11 so far shown is of a design that the reciprocal movement of a piston rod 11 a can result in the flow of the ink from the inlet port to the outlet port, the reciprocal movement of the piston rod 11 a being effected by the rotation of the lower drum 2 through an actuating mechanism when and so long as a solenoid unit 25 is, for example, energized electrically in response to a control output signal generating from a control circuit shown in Figure 2 and as will be described later, Referring still to Figure 1, the actuating mechanism comprises a generally V-shaped rocker arm 33 having one end loosely coupled with the piston rod 11a and the other end formed with an inwardly extending guide groove 33a, a substantially intermediate portion thereof being rotatably mounted on a bearing pin 32 fast with the machine framework, a drive gear 29 operatively coupled with one end of the shaft 5 remote from the pulley 5 through a clutch, and a driven gear 30 constantly held in mesh with the drive gear 29 and having a transmission pin 31 extending outwardly therefrom in a direction parallel to the axis of rotation of said gear 30 and terminating in loose engagement in the guide groove 33a in the rocker arm 33.The actuating mechanism so far described operates in such a manner that, when the drive gear 29 is coupled with the shaft 5 through the clutch, as will be described later, and is therefore, rotated together with the lower drum 2, the driven gear 30 is rotated in a direction counter to the direction of the drive gear 29 with the transmission pin 31 moving within the guide groove 33a while, consequent upon the revolution of the transmission pin 31 about the axis of rotation of the driven gear 30, the rocker arm 33 swings about the bearing pin 32 accompanied by the linear reciprocating motion of the piston rod 1 lea.

The clutch referred to above and so far shown is of any known construction, for example, a slip-spring clutch including a coil spring 28 having one end secured to and retained on the hub of the drive gear 29 and the other end 28a extending radially outwardly therefrom for selective engagement with a trigger lever 27.

The trigger lever 27, pivotally supported at a substantially intermediate portion thereof by a bearing pin 26 fast with the machine framework, has one end pivotally coupled with a plunger 25a of the solenoid unit 25 and is formed with a pawl 27a adjacent the other end thereof, said pawl 27a protruding laterally outwardly therefrom for engagement with the end 28a of the coil spring 28. The clutch is of such a design that, when and so long as the end 28a of the spring 28 is trapped by and engaged with the pawl 27a of the trigger lever 27, the clutch is in a decoupling position with no drive being transmitted from the lower drum 2 to the drive gear 29, but when and so long as the pawl 27a is disengaged from the end 28a of the spring 28, the clutch is in a coupling position with the drive being transmitted from the lower drum 2 to the drive gear 29.

The plunger 25a of the solenoid unit 25 is movable between retracted and projected positions, but is normally biased to assume the projected position during the absence of the control output signal from the control circuit. Thus, when no contol output signal is applied to the solenoid unit 25, the pawl 27a of the trigger lever 27 is held in position to engage the end 28a ofthe spring 28, and the clutch consequently held in the decoupling position.

Although not shown, the machine further comprises a presser roll positioned beneath the lower drum 2 to provide a nipping area at which an actual printing work takes place as a paper is fed therethrough. As is well known to those skilled in the art, the presser roll (not shown) is in practice supported for movement in a direction close to and away from the lower drum 2 and can be brought close to the lower drum 2 at the timing the paper is to pass through the nipping area, for the purpose of avoiding any possible contamination of the outer periphery of the presser roll which would occur when it contacts the lower drum 2.

When the rotary printing machine of the construction described hereinbefore is in use for making prints of an image, a stencil paper having a replica of the image to be printed has to be applied over the back-up screen 3 in any known manner although means for supporting the stencil paper to the back-up screen 3 is not illustrated. With the image bearing stencil paper so applied to the back-up screen 3, and when the motor 4 is driven to rotate the drums 1 and 2 in the same direction Ar3 and Ar2 and assuming the respective layers of ink have been formed on the outer peripheral surfaces of the drums 1 and 2, the printing ink retaining on the outer peripheral surface of any one of the drums 1 and 2 penetrates through the screen 3 and then through the image area of the stencil paper.The printing ink running outwardly through the image area of the stencil paper is subsequently transferred onto a paper as the latter passes through the nipping area between the lower drum 2 and the presser roll while having been pressed to contact the lower drum 2 through the stencil paper and the back-up screen 3. In this way, the image can be printed on the paper.

Eventually, as a certain number of papers have been printed, the quantity of the ink retaining on the outer peripheral surface of any one of the drums 1 and 2 decreases and the image being printed then becomes thin, diverting from the tone of the same image printed on the paper at the beginning of the printing work.

Once this occur, the printing ink has to be replenished from the tube 12 onto the outer peripheral surface of the lower drum 2 through the ink distributor 10 by means of the pumping unit 11. According to the present invention, the replenishment of the printing ink can be performed automatically in response to the decrease of the quantity of the printing ink retaining on the outer peripheral surface of at least the lower drum 2 and, for this purpose, the ink detecting device is employed which will now be described with particular reference to Figures 1 and 2.

The ink detecting device comprises a sensor roll 15 rotatably supported by the machine framework so as to extend in parallel to the longitudinal axis of any one of the upper drum 1, the lower drum 2 and the applicator roll 14 and positioned inwardly of the back-up screen 3 at one side of any of the drums 1 and 2 opposite to the applicator roll 14. This sensor roll 15 is mounted on a shaft 16 for rotation together therewith and is resiliently urged in a direction to the outer periphery both of the upper drum 1 and the lower drum 2 so that the sensor roll 15 can rotate about the shaft 16 in a direction shown by the arrowAr5 in contact with both of the drums 1 and 2.The shaft 16 is drivingly coupled through a universal joint 17 with a generator 18 for generating a speed signal indicative of the number of revolution of the sensor roll 15, the voltage of which is generally proportional to the number of revolution of the sensor roll 15. A similar generator 20 is also drivingly coupled through a universal joint 19 with the shaft 5 for generating a speed signal indicative of the number or revolution of the lower drum 2, the voltage of which is generally proportional to the number of revolution of the lower drum 2.

Referring particularly to Figure 2, the generators 18 and 20 are electrically connected through a comparing circuit CC to the solenoid unit 25. The comparing circuit CC is so designed as to generate the control output signal only when the level of the speed signal E2 from the generator 18 exceeds a first predetermined level of the speed signal El from the generator 20, but to cease the generation of the control output signal when the level of the speed signal E2 falls below a second predetermined level of the speed signal El which is lower than the first predetermined level. As hereinbefore described with reference to Figure 1, the control output signal from the comparing circuit CC is utilized to energize the solenoid unit 25 to bring the plunger 25a into the retracted position.It is to be noted that, since the generators 18 and 20 are identical in performance, and assuming that no slip takes place between the sensor roll 15 and any one of the drums 1 and 2, the rotation of the drums 1 and 2 results in the rotation of the sensor roll 15 at a speed n-times that of the drums 1 and 2 and, thus, the voltage of the speed signal E2 will theoretically be n-times the voltage of the speed signal El, wherein n represents the ratio of the diameter of any one of the drums 1 and 2 to that of the sensor roll 15.

Moreover, when the printing ink forms a layer on the outer periphery of each of the drums 1 and 2, the slip will takes place between the sensor roll 15 and any one of the drums 1 and 2, the slippage of the sensor roll 15 being generally a function of the thickness of the printing ink layer (namely, the quantity of the ink) on each of the drums 1 and 2. In the event of any slippage of the sensor roll 15 has occurs owing to the presence of the printing ink layer, the voltage E2 will deviates, i.e., decreases, from the value n-times of the voltage El.

Therefore, the quantity of the printing ink retaining on the outer periphery of each drums 1 and 2 can be detected in terms of the slippage of the sensor roll 15 relative to the respective drum 1 and 2 by comparing the voltage E2 to the voltage El as a reference.

The comparing circuit CC comprises a first comparator 21 having a non-invertiiig input connected to the generator 18 through a variable resistor VRO and an inverting input connected to the generator 20 through a variable resistor VR1, a second comparator 22 having a non-inverting input connected to the generator 20 through a variable resistor VR2 and an inverting input connected to the non-inverting input of the comparator 21, and a flip-flop 23 having set and reset terminals connected respectively to the outputs of the comparators 21 and 22 and an output terminal Q connected to the solenoid unit 25 through an amplifier 24.

The variable resistor VRO is utilized to substantially eliminate the difference in output voltage between the generators 18 and 20 which would result from the difference in diameter between the sensor roll 15 and each drum 1 and 2 during the absence of the slip of the sensor roll 15 relative to any one of the drums 1 and 2.

Therefore, the position of the movable element of this variable resistor VR0 is generally fixed at such a position of the diameter ratio of the sensor roll 15 and the drums 1 and 2 as to permit a 1/n of the voltage E2 to be supplied to both of the non-inverting input of the first comparator 21 and the inverting input of the second comparator 22.

The variable resistors VR1 and VR2 are utilized to provide the different reference voltages for the first and second comparators 21 and 22, it being, however, to be noted that the reference voltage applied to the inverting input of the first comparator 21 should be higher than that applied to the non-inverting input of the second comparator 22 such that the difference between these reference voltages can represent the range of the control output emerging from the flip-flop 23.The position of the movable element of the variable resistor VR1 is preferably adjusted to such a position asto permit the inverting input of the first comparator 21 to receive the reference voltage which is 80 to 90%, preferably 85%, of the voltage El whereas that of the variable resistor VR2 is preferably adjusted to such a position as to permit the non-inverting input of the second comparator 22 to receive the reference voltage which is 60 to 70%, preferably 65%, of the voltage El, the reason therefor being described later.

The comparing circuit CC shown in Figure 2 operates in the following manner. Assuming that the printing work is being performed, the printing ink retaining on the outer peripheral surface of each of the drums 1 and 2 is consumed and the quantity thereof decreases with increase of the number of prints being made. As the quantity of the retaining ink decreases in the manner described above, the slippage of the sensor roll 15 relative to each of the drums 1 and 2 decreases.While the generator 20 generates a substantially constant voltage El at all time during the rotation of the drums 1 and 2, the output voltage E2 from the generator 18 associated with the sensor roll 15 increases with decreasing of the slippage of the sensor roll 15, accompanied by the corresponding increase of the voltage across the variable resistor VR0, that is, the voltage E2ln. Should the reference voltages across the associated variable resistors VR1 and VR2 be selected to be 85% and 65% of the voltage El, respectively, the first comparator 21 generates a high level output to the set terminal of the flip-flop 23 in the event that the voltage E21n exceeds the reference voltage 0.85E1.At this time, although the voltage E2/n has already exceeded the reference voltage 0.65E1 fed to the second comparator 22, the latter keeps generating a low level output to the reset terminal of the flip-flop 23 because the voltage E2/n is fed to the inverting input of the second comparator 22.

In response to the high level output from the first comparator 21, the flip-flop 23 is set to generate the control output signal with which the solenoid unit 25 is energized to effect the pumping of the printing ink from the tube 12 onto the outer peripheral surface of the drum 2 in the manner as hereinbefore described with reference to Figure 1.

With the printing ink so supplied, the ink layer on each ofthe drums 1 and 2 increases in thickness consequent upon the spread of the replenished printing ink which is effected in the manner as hereinbefore described. As the ink layer increases, the relative slip takes place between the sensor roll 15 and any one of the drums 1 and 2, the slippage increasing with the increase of the thickness of the ink layer which is representative of the quantity of the printing inkretaining on the outer peripheral surface of any one of the drums 1 and 2. As a result the voltage E2 from the generator 18 and, hence, the voltage E2/n across the variable resistor VRO decreases to a value lower than the reference voltage 0.85E1.When this happens, the first comparator 21 starts generating a low level output to the set terminal of the flip-flop 23, it being, however, that because of the operating characteristic of the flip-flop 23 once set is held in the set state unless a high level output from the second comparator 22 is applied to the reset terminal thereof. The generation of the high level output from the second comparator 22 to the reset terminal of the flip-flop 23 to reset the latter take place when the voltage E2/n decreasing with increase of the slippage of the sensor roll 15 subsequently falls below the reference voltage 0.65E1. The flip-flop 23 when reset in response to the high level output from the comparator 22 ceases the generation of the control output signal and the solenoid unit 25 is therefore deenergized to interrupt the pumping of the printing ink onto the lower drum 2.

While the ink detecting device according to the present invention is constructed as herein before described, it is to be noted that the slippage of the sensor roll 15 relative to any one of the drums 1 and 2 is in practice affected by the following parameters and is not, therefore, fixed at all times: (a) The condition of the outer perhipheral surface of any one of the sensor roll 15, the upper drum 1 and the lower drum 2.

(b) The biasing force used to urge the sensor roll 15 to contact the drums 1 and 2.

(c) The resistance against to the rotation of the sensor roll 15 by various bearing elements as well as imposed by the generator 18.

(d) The viscosity of the printing ink used.

(e) The rotational speed of the lower drum 2, that is, the number of prints to be produced per minute.

Of these parameters, the parameters (a) to (d) are generally fixed within a predetermined range depending on the design of the printing machine and/or the type of the printing ink actually used. With respect to the parameter (e), the adjustable range of the number of prints to be produced per minute is 1:3 to 1:4.

Therefore, even though the change in slippage of the sensor roll 15 resulting from one or a combination of these parameters is taken into consideration, the ink detecting device according to the present invention is effectively utilizeable in practice with the retaining ink quantity being satisfactorily controlled to the optimum value.

By way of example, a series of experiments conducted have shown the following results.

When an AC generator capable of generating a voltage of about 2 to 12 volts when driven at a speed of 100 to 1300 rpm (manufactured by Oriental Motors Co., Ltd. of Japan, the operating characteristic of which is shown in the graph of Figure 3) was used for the generator 18, and when the quantity of printing ink supplied to the outer periphery of the printing drums 1 and 2 in the rotary printing machine was adjusted to zero to 269 one at a time, the AC generator generated such output voltage as shown in Table 1 for a different number of revolution (rotational speed) of the shaft 5.

TABLE 1 Rotational Speed of Shaft 5 {rpm) Ink Quantity 35 75 125 Og 4.5 volts 9.4 volts 12.9 volts x x x x 4 4.2 8.9 13.8 6 3.5 8.6 13.4 8 3.5 7.9 12.7 10 3.1 7.3 11.8 12 2.9 6.9 11.3 14 2.4 6.2 10.4 16 2.1 5.7 10.2 18 2.0 5.5 9.8 20 1.95 5.3 9.5 x x x x x x x x 26 1.6 4.6 9.1 When these numerical values representative of the output voltage from the AC generator for the different rotational speed of the shaft 5 and the different quantity of the ink supplied are plotted on a graph wherein the axis of abscissas and the axis of ordinates represent the quantity of the ink supplied and the output voltage of the AC generator, respectively, the result is shown in Figure 4 wherein the curves VO, V1 and V2 represent 35 rpm, 75 rpm, and 125 rpm of the shaft 5.

On the other hand, in the case of the rotary printing machine of the construction shown in Figure 1, experience has shown that the optimum quantity of the printing ink is generally within the range of 6 to 1 29 for the printing work with B-4 sized papers and with the stencil paper bearing the image area of usual pattern in the characters and figures. Therefore, it will readily be seen from the graph of Figure 4 that, for 6g in ink quantity, the output voltage from the AC generator obtained when the rotational speed of the shaft 5 is 35 rpm, 75 rpm and 125 rpm, is 80 to 90% of the maximum output voltage outputed by the AC generator when no ink is supplied.Similarly, for 129 in ink quantity, the output voltage from the AC generator at 35 rpm, 75 rpm and 125 rpm is 60 tO 70% of the maximum output voltage outputted from the AC generator when no ink is supplied.

In view of the foregoing, if the variable resistors VR1 and VR2 are so adjusted as to provide a voltage of (0.6-0.7)xE1 as the reference voltage and a voltage of (0.6-0.7)xE1 to the first and second comparators 21 and 22, respectively, any possible adverse influence which may be brought about by the change in any one of the parameters (a) to (e) discussed above can substantially be eliminated, and the quantity of the printing ink supplied on the outer periphery of the drums can be controlled and maintained within the optimum range of 6to 129.

The ink detecting device of the present invention capable of providing the control output signal which is used to operate the ink supply unit in the manner as hereinbefore described is applicable to any type of rotary printing machine. More specifically, should the rotary printing machine to which the present invention is to be applied differ from the construction described with reference to and shown in Figure 1, the optimum range of the ink quantity to be replenished may vary and, correspondingly, one or both of the reference voltages to be fed respectively to the first and second comparators 21 and 22 may also vary. In such case, because of the employment of the variable resistors VR1 and VR2, the reference voltages can readily be adjusted to suit to the different design of the rotary printing machine.

From the foregoing description, it has now become clearthatthe present invention utilizes the change in number of revolution of the sensor roll resulting from the relative slip between the sensor roll and any one of the drums, the slippage of the sensor roll being a substantial function of the quantity of the retaining printing ink on the drums, for enabling the ink detecting device to generate the control output signal necessary to operate the ink supply unit to compensate for the reduction in quantity of the printing ink standing on the drums.Accordingly, with the ink detecting device according to the present invention, since the detection of the ink quantity is performed direct at the outer peripheral surface of any one of the drums and since the control output is generated when the quantity of the retaining ink decreases below the lowermost limit of the optimum range of the ink quantity, the printing ink retaining on the outer peripheral surface of any one of the drums can advantageously be kept within the optimum range of the ink quantity with the consequence that the image can be printed uniformly on the papers.

Moreover even though the optimum range of the quantity of the printing ink on the outer peripheral surface of any one of the drums is changed by various external conditions, the ink. detecting device according to the present invention can cope with change of such external conditions merely be effecting the adjustment to the variable resistors used to provide the respective reference voltages to the first and second comparators.

Although the present invention has fully been described in detail in connection with the preferred embodiment with reference to the accompanying drawings, it is to be noted that various changes and modifications are apparent to those skilled in the art. By way of example, although the use of the sensor roll has been made separate from the applicator roll 13, the applicator roll employed may be concurrently used as a sensor roll with its shaft 14 drivingly coupled to the generator 18. Separately therefrom, the drums 1 and 2 may not be always of the same diameter and the drum 1 may be smaller in diameter than the drum 2, in which case both of the applicator and sensor rolls 13 and 15 are not required to contact the drum 1.

Moreover, the drum 1 may be omitted where the screen 3 shown in Figure 1 is constituted by a self-supporting screen, such as shown by 3' in FigureS, made of a perforated, mesh-like metal sheet having its opposite ends welded to provide a seamless joint.

Furthermore, the generators 18 and 20 may be employed in the form of either an AC generator or a DC generator. In addition, any other signal generator, for example, an electro-optical generator for generating a train of pulses, may be employed for each of the generators 18 and 20 if the load imposed thereby on the sensor roll 15 is small. In this case, the comparing circuit may be required to be modified to operate on a different comparing scheme that can deal with the pulses outputed from the electro-optical generator.

Yet, although the generators 18 and 20 have been described as identical in performance, they may have different performances because the difference can be compensated for by adjusting the variable resistor VR0.

Accordingly, such changes and modifications are to be understood as included within the scope of the present invention.

Claims (13)

1. An ink detecting device for a rotary printing machine, comprising: a sensor roll for rotation in frictional contact with a printing drum with a layer of printing ink between the sensor roll and the printing drain; a first detector for detecting the number of revolutions of the sensor roll and generating a first output signal indicative of the number of such revolutions; a second detector for detecting the number of revolutions of the printing drum and generating a second output signal indicative of the number of such revolutions; and a comparing circuit adapted to compare the first and second output signals, and to generate a control output signal in the event that the result of the comparison falls outside a predetermined range.

2. A device according to claim 1, wherein the first and second detectors each comprise a voltage generator, and the first and second output signals are represented by the voltage outputted from the respective voltage generator.

3. A device according to claim 2, wherein the voltage of the first output signal varies as a function of the thickness of the layer of printing ink in the outer periphery of the printing drum.

4. A device according to any one of the preceding claims, wherein the comparing circuit comprises two comparators operable to define the predetermined range, and a switching circuit adapted to receive an output signal from the comparators and to be set and reset in response to that output signal.

5. A device according to any one of claims 1 to 3 wherein the switching circuit comprises: a first comparator for comparing the first output signal with a first predetermined value and generating a first comparator output when the first output signal exceeds the first predetermined value; a second comparator for comparing the first output signal with a second predetermined value and generating a second comparator output when the first output signal falls below the second predetermined value; and a switching circuit adapted to be set and reset in response to the first and second comparator outputs respectively, the switching circuit generating a control signal only when the switching circuit is set in response to the first comparator output, the control signal being suitable for actuating ink supply means to supply a quantity of printing ink to the outer periphery of the printing drum, thereby to keep the quantity of ink on the outer periphery of the printing drum within a predetermined range.

6. A device according to claim 5, wherein the first predetermined value is between 80% and 90% of the level of the second output signal, and the second predetermined value is between 60% and 70% of the level of the second output signal.

7. An ink detecting device substantially as herein described with reference to Figures 2 to 4 of the accompanying drawings.

8. A rotary printing machine comprising: at least one printing drum supported rotatably; means for supplying a suitable quantity of printing ink onto the outer periphery of the printing drum; and an ink detecting device according to any one of the preceding claims, the sensor roll of the ink detecting device being in fictional contact with the or a printing drum.

9. A rotary printing machine according to claim 8 further including an image carrier medium having an image area bearing an image to be printed on an image supporting material, the printing ink on the outer periphery of the printing drum being allowed to penetrate through the image area of the image carrier medium to the image supporting material during rotation of the printing drum.

10. A rotary printing machine according to claim 8 or claim 9 having two rotatably supported printing drums adjacent each other, the sensor roll being resiliently urged to contact both printing drums.

11. A rotary printing device comprising: at least one printing drum rotatably supported; means for supplying a quantity of printing ink to the outer periphery of the printing drum; and an ink detecting device having a sensor roll rotatably supported in contact with the printing drum via a layer of printing ink between the sensor roll and the printing drum, a first detector for detecting the number of revolutions of the sensor roll and generating a first output signal indicative of that number of revolutions, a second detector for detecting the number of revolutions of the printing drum and generating a second output signal indicative of that number of revolutions, and means for detecting the slippage between the sensor roll and the printing drum by comparison of the first and second output signals, the slippage depending on the quantity of ink between the sensor roll and the printing drum.

12. A rotary printing machine substantially as herein described with reference to Figures 1 or 5 of the accompanying drawings.

13. A method of detecting the quantity of ink on the periphery of a printing drum, comprising: providing a sensor roll rotatably supported in contact with the printing drum via a layer of printing ink between the sensor roll and the printing drum; detecting the number of revolutions of the sensor roll and generating a first output signal indicative of the number of such revolutions; detecting the number of revolutions of the printing drum and generating a second output indicative of that number of revolutions; and detecting the slippage between the printing drum and the sensor roll by comparing the first and second output signals, the slippage depending on the quantity of ink between the sensor roll and the printing drum.