Apparatus for Automatically Maintaining a Specified Thickness of Rubber Sheet During Calendering

S. S. Kropanin

In accordance with the law on the postwar five-year plan, industrial enterprises have been faced with the question of automating production processes.

Active automatic control devices make it possible to prevent rejects, increase manufacturing accuracy, reduce material consumption, simplify the servicing of working machines, and increase productivity.

This article deals with the application of an active automatic two-limit device, built to an order from the “Sevkabel” plant.

The apparatus serves to maintain the constancy of the thickness of a rubber sheet during calendering. The specified accuracy is \(\pm 0.02\) mm.

Description of the Calender

The calender is a nonreversible rolling mill having three rolls, each 507 mm in diameter and 1400 mm long. The gap between the upper \(A\) and middle \(Б\) rolls (Figs. 1, 2) determines the thickness of the sheet and is regulated by raising and lowering the bearings \(Г\) of the upper roll, driven by an asynchronous squirrel-cage motor \(Д\) with push-button control. The lower roll \(В\) is a cooling roll. After the trimming roll \(Е\), the sheet is fed onto conveyor \(Ж\), which carries it to the receiving—

Fig. 1

—device.

shop 3. Web width 1050 mm with thicknesses: 0.33 mm; 0.4 mm; 0.5 mm; 0.6 mm; 0.75 mm. Web travel speed: 13.25 cm/sec.

Fig. 2

The rubber compound, prepared on mills in the form of a blank (“roll”), is fed by the feeder operator into the gap between the upper and middle rolls, where the rubber calendering process takes place.

Analysis of Calender Operation

Analysis of the technological process made it possible to establish the factors affecting the accuracy of the calendered rubber with respect to thickness:

1. Quality of preparation of the compound on the mills.
2. Temperature of the “roll” before it is fed into the calender.
3. Quality of charging of the “roll” by the feeder into the calender rolls.
4. Temperature of the calender rolls.
5. Condition of the surface of the calender rolls.
6. Condition of the bearings of the upper calender rolls.

Before the introduction of the automatic device, the calender operator had to monitor the work of the mill operator, the feeder operator, and the receiving operator, observe the temperature regime of the calender himself, maintain the specified thickness of the strip, change rolls, and keep the documentation.

To check the web thickness, the calender operator had to systematically cut pieces from the web at three points and measure them with a micrometer and, in the event of deviations from the specified thickness, change the size of the gap by means of the motor for raising and lowering the rolls, switching it by a cam clutch to the left or right end of the roll.

It is clear that with this method of thickness regulation it was not possible to obtain sufficient accuracy over the entire length of the web. The accuracy depended on how often the calender operator made measurements and how quickly he changed the gap between the calender rolls.

Fig. 3 shows the nature of the change in web thickness under manual regulation.

To reduce the percentage of rejects when testing the web for electrical strength, calender operators, as a rule, work with plus tolerances, which the technical department has specified as a value of +0.05 mm. Fig. 3 gives a clear illustration of this.

The calender has been in service for more than two decades and during this time has not undergone a single major overhaul. The surface of the rolls is in especially poor condition. Wear on the surface of the rolls reaches...

is up to 0.04 mm—0.06 mm on one side of the circumference, over a length of 600–650 mm; in one revolution of the calender rolls, over the total web length of 1592 mm, a section 600–650 mm long is obtained with an excessive thickness of 0.04–0.06 mm (Fig. 4).

In addition, on the surface of the rolls there are deep and frequently occurring small and large dents, producing an excess in thickness

Fig. 3

from 0.05 mm to 1.00 mm (Fig. 3). Along the length of the calender rolls there is a large variation in size (Fig. 5), reaching 0.05 mm and 0.02 mm.

The accuracy of the manufactured web is affected by the temperature regime of the calender rolls. According to the production instruction, “the temperature

Fig. 4

of the lower cooling roll must ensure complete cooling of the web going to the conveyor.” These conditions of the instruction are not observed, especially in the summer period, since the water for the lower roll is supplied from an open reservoir at a fairly high temperature. After

Fig. 5

two hours of work on the calender, the lower roll reaches a temperature of 50°C. Such an excess of temperature leads to a large variation in thickness and causes shutdowns for 2–6 hours to allow the rolls to cool.

Large play in the bearings of the upper roll in all links of the kinematic chain of the lifting mechanism (Fig. 6) sharply affects the accuracy of the thickness of the rolled web.

The analysis presented above made it possible to determine the basic conditions for the installation:

1) to give greater responsiveness to the lifting mechanism, it is necessary to install two motors with independent reduction gears at both ends of the shaft;

2) the installation must be insensitive to short-term (up to 5 sec.) deviations in the thickness of the sheet from the nominal value, and must respond to these deviations when they last more than 5 sec.;

3) the installation must permit two-limit automatic control and maintenance of a constant sheet thickness at the specified dimensions: 0.33 mm; 0.4 mm; 0.5 mm; 0.6 mm; 0.75 mm, within a specified accuracy of 0.02 mm;

4) control of the sheet thickness must be carried out from two points across the width, located to the left and to the right of the center line at a distance of 300 mm;

5) the installation must include an instrument—an indicator of sheet thickness;

6) the layer of chalk dusting and the rubber compound must not affect the operation of the installation;

7) the installation must be simple in design and maintenance, and reliable in operation;

8) the electrical circuit must allow the calender operator to perform manual control of the lifting and lowering motors.

Fig. 6

In accordance with the above, the installation must have the following basic mechanisms:

1) a sensing mechanism;
2) an electrical-contact sensor;
3) an electrical amplification circuit;
4) an ionic time relay;
5) an electrical circuit for controlling the motors for lifting and lowering the upper roll;
6) a mechanism for lifting and lowering the upper roll of the calender.

Description of the Installation Device

Sensing mechanism. The sheet, leaving the lower roll of the calender for the conveyor, is heated; its surface is tacky, and therefore the sensing element must be made in the form of a roller rolling over the surface of the sheet, in order to eliminate the possibility of sticking.

In view of the small magnitude of the measured deviations in thickness from the nominal value, it is necessary to use a kinematic transmission that increases these deviations. As the sensing mechanism satisfying the stated requirements, clock-type micrometers available at the plant were used, intended for continuous monitoring of sheet thickness. The place for installing the “K” micrometers was chosen between the cutting roll and the driving roll of the conveyor (Fig. 1). The distance from the point

1 to point 2 along the length of the sheet being calendered is 2095 mm. The arrangement of the micrometers on the calender is shown in Fig. 2. The accuracy of the micrometers is 0.01 mm.

Electrocontact sensor. To convert the mechanical impulse obtained from the micrometer when the sheet thickness changes into an electrical impulse, we used an electrocontact sensor. The accuracy provided by the electrocontact sensor, as well as the simplicity and reliability of its design, make it possible to choose precisely this type.

The sensor consists (Fig. 7) of an insulating plate (limb) \(A\), built into the micrometer.

The limb \(A\) has 144° along the circumference, which corresponds to 200 divisions of the main scale, or to a gap of 2 mm between the supporting \(L\) and sensing \(M\) rollers of the micrometer (Fig. 1). By means of the setting head through gear \(Б\) and toothed rack \(В\), the zero position \(E\) of the limb can be shifted clockwise or counterclockwise by 72° along the circumference, which corresponds to 100 scale divisions, or to a 1 mm gap between the micrometer rollers. This device makes it possible to set the zero position \(E\) of the limb to the required division of the main scale according to the specified sheet thickness (from 0.33 mm to 1.00 mm).

Fig. 7

To the left and right of position \(Б\), at 0.02 mm, contact tracks \(Г\) are cut into limb \(A\); these are touched by the movable contact \(Д\), located at the end of the micrometer pointer. The contact area is 0.5 mm², the pressure 10–15 g.

The electrical circuit of the sensor is shown in Fig. 7.

In the case of passage of a strip of nominal thickness, the pointer is located on insulator Zh between the contact tracks. With changes in the thickness of the strip, the pointer will move to the right when the web thickness increases, or to the left when it decreases. At deviations of \(\pm 0.02\) mm, the movable contact D touches the right or left contact tracks G, closing the circuit of the intermediate relays.

Electrical amplification circuit. The installation of intermediate relays of the telephone type is due to the need to reduce the current density in the contact sensor. The current density was taken as \(0.06\ a/mm^2\). The coils of the intermediate relays are supplied with direct current from a two-half-period kenotron rectifier. At the same time, the rectifier serves as a source of direct current for supplying the grids of the thyratrons of the ionic time relay (Fig. 8).

Ionic time relay. To smooth the continuously arriving pulses from the intermediate relays, caused by irregularities of the web (Fig. 3), we provided for the installation of an ionic time relay on TG-213 thyratrons (Fig. 8), with time adjustment from 0 to 5 sec, at intervals of one second, by changing the active resistance of the relay oscillatory circuit by means of a switch.

Fig. 8. For the left motor

Electrical motor-control circuit. To control the motors for raising and lowering the rolls, reversing magnetic starters are used, whose coils are connected into the anode circuit of the thyratrons. To smooth current pulsations in the coils, capacitors are connected in parallel with them.

Mechanism for raising and lowering the calender rolls. Control point 2 (Fig. 1) is separated from the point of defect formation 1 by a distance of 2095 mm. It becomes necessary to strictly calculate and experimentally confirm the rate of raising and lowering the bearings of the upper calender roll. In the event that the thickness of the strip is excessive or insufficient, a command is given to lower or raise the corresponding end of the upper calender roll.

The rate of change of the web thickness must be such that, over the path of 2095 (15.8 sec), the strip thickness becomes no less (or greater) than the minus-

tolerance (plus tolerance). Thus, the speed of displacement of the calender roll should be (Fig. 9):

\[ V=\frac{0.04}{15.8}=0.00252\ \text{mm/sec}. \]

In practice the speed had to be increased by using a large amount of play in the kinematic links of the lifting mechanism. Fig. 6 shows the kinematic diagram of the mechanism for lifting one of the ends of the calender roll.

Fig. 9

Description of the Operation of the Electrical Circuit of the Installation

Fig. 8 shows the schematic diagram of the installation. The letters \(B\) or \(H\) designate the apparatus serving to carry out the “up” or “down” operating mode. The letters \(L\) or \(P\) designate the apparatus serving to carry out the operating mode of the “left” or “right” end of the calender roll.

Normal position of the circuit. The manual-control pushbuttons are not pressed; the movable contacts of the sensors are at the zero position of the limb.

In the normal position of the circuit, the normally closed contacts of the intermediate relays (\(PR\)) close the circuit of the thyratron grids. With the thyratrons locked, the coils of the magnetic starters are de-energized and the raising and lowering motors are switched off.

Manual control of the circuit. When one of the buttons is pressed, for example \(B_l\) (up, left end of the roll), the coil circuit of the intermediate relay \(PRB_l\) (up left) is closed; the relay opens its normally closed contact \(PRB_l\). With a time delay determined by the resistance value of the oscillatory circuit of the time relay, the negative potential is removed from the grid of the thyratron \(TB_l\). The thyratron ignites, closing the coil circuit of the magnetic starter \(B_l\).

Motor \(DL\) receives power and raises the left end of the calender roll until the button is pressed.

Automatic control of the circuit. To carry out automatic control of the circuit and maintain a constant web thickness within the specified tolerance limits, it is necessary first to set the zero position of the limb opposite the corresponding division of the main micrometer scale.

After adjustment and after the web is inserted between the micrometer rollers, the micrometer control-circuit switch (\(VCUM\)) is turned on.

At the nominal web thickness, the movable contact of the sensor will be between the contact tracks and the entire circuit is in its normal position.

When the web thickness deviates, the movable contact of the sensor comes into contact with the right or left contact tracks, thereby closing the circuit of the intermediate relay. The relay, operating with a time—

with a time delay, removes the negative potential from the grid of the thyratron. The thyratron, igniting, closes the circuit of the coil of the corresponding magnetic starter, thereby setting in motion the motor for raising or lowering the end of the calender roll. The motor will continue to operate until a sheet of nominal thickness passes between the micrometer rollers and the movable contact of the sensor moves to the zero position of the limbus.

To limit the lowering of the rolls, limit switches \(KBN_l\) (down left) and \(KBN_p\) (down right) are included in the circuit of the intermediate relays; these limit the lowering of the rolls to a minimum gap between them of \(0.2\) mm.

To prevent the possibility of simultaneous energizing of coils \(B_l\) and \(H_l\), normally closed interlock contacts \(H_l\) and \(B_l\) are included in the circuit.

Testing of the installation and conclusions

Experimental testing of the installation showed that, with proper observance of the technological conditions (temperature regime of the rolls, temperature of the “calender bowls,” good preparation of the rubber compound on the mills, uniform feeding), good results are obtained even when the calender rolls are in poor condition.

Comparison of the curves in Figs. 4 and 10 makes it possible to give a positive evaluation of the operation of the installation.

For the calender operator, time and attention are freed for observing the work of the mill operator, the feeder, and the receiver, and for maintaining

Fig. 10

a better temperature regime of calender operation, which to a considerable extent improves product quality, reduces time standards, and lowers excess consumption of rubber to a minimum.

LITERATURE

1. Bushtuev and Gulyaev.—Rubber Calenders.
2. Ginkin.—Handbook of Radio Engineering, Gosenergoizdat, 1948.
3. Temnikov.—Electrical Measurements of Non-Electrical Quantities, Gosenergoizdat, 1948.
4. Temnikov. Remote Control in Industry, Gosenergoizdat, 1940.
5. Trapeznikov.—Automatic Size Control, Oborongiz, 1947.