Impact of maximum rotor control on mixing.

Conventional tangential mixers are normally run by a DC electric motor and a large gear box. They are often operated in a fixed friction ratio mode and occasionally also with variable speed.

The R&D activities among the tangential mixer equipment manufacturers are usually focused on various ways of improving upon the rotor designs, in order to get better performance out of the mixer.

With tangential mixers that have the maximum rotor control (MRC) drive capability, meaning that the mixers can operate at both fixed and variable speed, fixed and variable friction and also in reverse mode, the possibility to optimize the performance of the mixer is much improved.

Tests conducted on custom compounds have yielded better dispersion and shorter batch times by utilizing MRC. The results are especially noticeable when using silica and other hard-to-mix fillers.

Further benefits of using MRC drives are also covered, e.g., less space requirements for the drives, less equipment maintenance, no harmonics back to the net, etc.

The knowledge accumulated by Hagglunds from over 30 years experience of roll mill drives can now also be applied to mixer drives. The concept of "variable friction drives" is simple. One separate drive per roll is connected to a power unit that creates torque speed/control. The hydraulic motor is then connected directly on to the roll shaft and therefore no gearbox is required. The power unit has a standard fixed speed AC electric motor connected to a hydraulic pump with variable flow in order to regulate the speed of the roll. By monitoring the pressure in the system, the torque can be controlled.

A hydraulic drive is a quiet and efficient solution that requires less space than a conventional gear drive. Moreover, maintenance is kept to a minimum by only requiring checking the oil and changing oil filters.

With the mixer or the roll mill rotors operating individually one can change the friction ratio, the rotor offset, the direction of rotation, the acceleration, speed and deceleration, and one can start and stop under full load for an indefinite time period.

The concept

A hydraulic direct drive system consists of a sound insulated power unit (electric motor, pump, oil tank/filters and a control system) connected by flexible hoses and/or steel pipes to a high torque, low speed (variable) hydraulic motor (figure 1).

[FIGURE 1 OMITTED]

The hydraulic motor is a hydro-statically-balanced radial piston cam curve unit with a mechanical efficiency approaching that of a roller bearing, thus resulting in excellent torque efficiency. A torque arm is used to take up the reaction force, while also eliminating any other unwanted forces on the motor bearings. No foundation and/or alignments are required.

The standard power unit is designed as a closed loop hydraulic system liar continuous industrial drive applications. All electrical connections are made to a junction box and the complete power unit is housed in a sound insulated cabinet ready for positioning on site, therefore no special pump room is required.

The standard AC electric motor starts up in an unloaded condition and runs at constant speed, driving the connected hydraulic pump. The pump is a variable displacement unit and a 4-20 mA signal is normally used to control the displacement/flow of the pump and therefore the speed of the hydraulic motor. This method of controlling the speed means that, for example, on a roll mill application when an emergency stop situation occurs, it is not necessary to shut down the electric motor. Instead, the signal to the pump is zeroed and the mill rolls automatically stop. This feature allows for an unlimited number of starts/stops without over dimensioning the drive system.

Since the hydraulic system is able to maintain pressure at any flow, the hydraulic motor will maintain a torque level that is unaffected by speed. Therefore, by allowing the system pressure to rise above the normal operating level, very high torque levels can be sustained without any time restrictions. Furthermore, due to the fact that the motor is running at low speed, the moment of inertia in the drive is very low, which means that the drive is unaffected by shock loads, starts/stops and reversals.

Maximum rotor control--MRC

Hagglunds has developed the drive control system to include all the possible needs within rubber mixing with tangential type internal mixers. The name of this control system is MRC--maximum rotor control. All the functions in this control system are also very usable for open roll mills, except for the rotor-offset control.

Speed control

The speed control is a function widely used in mixing. With the MRC system, the speed is regulated in a closed loop control. A speed command makes the pumps give out a flow that makes the hydraulic motors rotate. The actual speed is fed back from speed encoders mounted on each hydraulic motor, and the flow is adjusted to give a speed following the command.

The high dynamics in the hydraulic system give a speed that accurately follows the speed command independently of the load. In systems with two motors, one is set as the master and the other as the slave. The speed command sets the master speed and the control system regulates the slave to follow at the exact speed.

Friction control

The speed command sets the speed of the master rotor, and the friction is given by a different speed of the slave rotor that is separately adjustable. The friction command gives the speed of the slave in comparison to the speed of the master. The friction ratio is of great importance for good mixing and high productivity. The temperature rise in the compound depends on the friction ratio, and this function can control the temperature in the mix.

Offset control

Similar to the friction mode, the master sets the speed, but here we are looking at the rotors' speed at 1:1 ratio. Offset controlling utilizes the digital speed encoders with some 2,500 to 5,000 pulses per revolution. The offset setting gives the number of pulses that correspond to the angular difference between the rotors. The high accuracy of the control system minimizes the deviation from the command value to a few pulses, typically less than three pulses. When the internal mixer is started up, the system senses mechanical markings on the rotors and zeroes the control system counters for each rotor. The offset is set as a positive or negative angle between the master rotor and the slave rotor.

By setting the rotors in positions that give a maximum opening between each other, the material will be fed into the mixing chamber fast. This function can shorten the mixing cycle time, especially when the recipes include large, hard pieces of material. The mixing process accelerates with a higher temperature of the materials, but a good temperature rise can only be achieved with the material fully in the mixer chamber. The offset control function shortens the time needed for shredding the material and quickly brings the process into real homogenous mixing.

Rotation direction control

This function is quite similar to the friction mode, but gives the possibility to change the rotation direction from forward to reverse and vice versa at any time. This control mode is quite new and has not been available in internal mixer drives before. Its utilization has to be further examined for each type of process. The development of this function resulted from requests to regulate the mixer rotors to ensure that all material, in all parts of the mixer chamber, is mixed together to a homogeneous compound.

This feature can also be utilized in open roll mills when there is a problem with bringing the material down between the rollers because the friction between the compound and the rollers is too low. When this problem occurs, it is easy to switch to controlling both rotors following the normal rotation direction of the rear rotor for a short while. By utilizing this, the whole compound can be turned "upside down" on top of the rollers. This provides a possibility of a higher friction so that the compound will go into the mill.

Productivity benefits

Low current spikes at start

The drive system is always started unloaded. The starting current is very low because it is only the AC-electric motor itself and the pump that has to be accelerated at start. Often when two or more electric motors/pumps are utilized, these can also be started one-by-one, minimizing the starting current even further. Once the electric motors are started and running at their asynchronous speed (i.e., about 1,475 rpm for a four-pole electric motor at 50 Hz) the pumps will be controlled to give flow, causing the hydraulic motors to rotate.

Less power installed

As further explained below when comparing different drive alternatives, the Hagglunds drive system gives full possibility to transform the installed power to any torque and speed combination without any time restriction. With this, the maximum available torque can always be reached more or less independently of the power installed. The limits for the system are determined by the maximum available flow, giving maximum rotating speed, and the maximum allowed/set pressure, giving maximum torque.

Very high torque peaks do occur in rubber mixing for a very short time. By utilizing the installed power for torque (instead of speed) during the short load peaks, and to keep the average speed high enough, the installed power can be utilized better and the mixing cycle time can be minimized as well. Following this, the installed power can mainly be based on the production needs instead of minimizing the risk for stalling (when hitting the load peaks) at the fixed mixing speed.

Utilization of installed power

Rubber mixing in internal mixers is very much a question of putting energy into the mixed material as efficiently as possible. With a highly efficient mixer one also needs higher power. The most essential objective is, in other words, to utilize the consumed energy for heating and mixing the material with good dispersion in as short a time as possible without overheating the compound.

Compound temperature and the cycle time are two important parameters for the mixing result. Both parameters should be kept as low as possible for low energy consumption. On the other hand, one normally gets a better mixing result with long mixing time and high temperature, as long as it is below the vulcanization temperature. Temperature and material flow in the mixer can be controlled with the drive parameters. In a tangential mixer, not just the speed and friction ratio, but also the rotor's offset and the rotation direction can easily be set differently during each part of the mixing cycle, optimizing the utilization of the input power.

Compound quality

One can fine-tune mixer drive settings for optimum dispersion by utilizing the Hagglunds drives concept. Today's fast development of materials ensures that new fillers, elastomers and additives, etc., often will be introduced into the recipes and formulae for the compounds. Maximum productivity in mixing can be achieved with this drive system, as one can easily adjust all the available drive settings for any needs and formulae.

All the settings can, without limitations, be adjusted during the mixing cycle, which makes the optimization a bit complex. But with a good product quality feed-back system, such as dispersion control equipment, the settings for each type of compound and recipe can be developed fast and easy.

Drive setting examples

Features that can be adjusted and set in the drive system during running operation have been described. There are no limitations in the control of the drive of each rotor for a tangential type of mixer. Any of the described modes can be utilized at any time and as long as appropriate during the mixing cycle. Following are some examples to give ideas of how the Hagglunds drive concept, including MRC, can be used in the mixing process for optimizing the productivity.

The speed can be kept low when loading the material in the mixer to avoid dust, etc. At the start of the mixing it is important to bring all the material into the chamber in as short a time as possible and break up all the material into smaller parts. The next task is to mix everything together well enough to be able to raise the temperature and open up the molecules so they can mix and bind together. With a good material flow in the mixer, the compound should be homogenous alter some final mixing time before dumping.

Figure 2 shows the utilization of the available functions in the MRC control system. It starts with rotor offset control to bring all material quickly into the mixer by giving the maximum opening between the rotors at 1:1 ratio. After this, both the speed and the friction ratio are higher to raise the temperature. The friction ratio is also changed during the cycle so that either the rear or the front rotor have a higher speed. The change of the friction ratio from, i.e., 1:1.15 to 1:0.85, can give a better flow through the whole mixer chamber volume.

[FIGURE 2 OMITTED]

One time during the mixing cycle (not shown in figure 2), one of the rotors can also go in the opposite direction for a short time. With one rotor in the opposite rotation direction to the other, it is possible to ensure that all the material is brought into the mix.

As the flow in the mixer normally follows the shape shown in the left half of figure 3, some badly mixed material could remain for a time just under the hopper. With a change of rotation direction, the flow in the mixer will follow the right half of figure 3 and bring all material under the hopper and on top of the opening door into the mix.

[FIGURE 3 OMITTED]

As the Hagglunds concept is a drive concept and not a process concept, driving mode examples have to be tested out and implemented to fit to each machine and process individually.

Reliability and maintenance

The total drive system, including hydraulic motors, power unit, etc., is designed for heavy-duty installations and for continuous operation. Reliability of the drive systems is well known and they are used in many other types of key installations, such as cement kilns, chemical agitators, pulp digesters, etc. The service life is good and a minimum of maintenance is needed. All parts in the system are standard products from actual serial production.

Drives comparison

Traditionally, a thyristor-controlled DC motor or a frequency-controlled AC motor has been used as the prime mover in drive systems for rubber mixing machines (roll mills and internal mixers). Gearboxes have been used to go from the prime mover rotational speed range to the needed roll or rotor speed range. The hydraulic drive solution from Hagglunds has also been utilized for roll mills for 30 years. The product developments at Hagglunds have for l0 years also given the possibilities to utilize hydraulic drives for internal mixers.

These various types of drive systems have different characteristics of torque and speed. The differences are small between DC and AC drives, but the hydraulic drive system has some outstanding performance characteristics that fit into the needs within rubber mixing (figure 4). The following is a short comparison between a frequency controlled AC drive system and the Hagglunds drive hydro-mechanical solution.

[FIGURE 4 OMITTED]

Frequency controlled A C with gearbox

This drive system consists of an alternating current (AC) induction electric motor and a frequency converter. At lower speeds, this drive system can usually operate continuously up to 40% of rated torque with standard motors and up to 70% of rated torque with motors that have forced cooling. At nominal speed, the available continuous operating torque is approximately 90% of rated torque, within 610% of the nominal speed/frequency. The drive can intermittently operate up to 150% of rated torque for a few minutes and can, during start, operate up to some 180% of rated torque for a few seconds.

When selecting a frequency controlled AC drive, one must look at the continuous torque capacity over the entire working speed range (figure 5). This is especially important due to limitations at lower speed/frequencies. For high torque, low speed (variable) applications, a speed reducer and a gearbox, with primary and secondary couplings, have to be added to the system.

[FIGURE 5 OMITTED]

Hagglunds direct drive

A direct hydraulic system like the Hagglunds concept can operate continuously throughout the entire speed range, up to some 200% of normal operating torque. The maximum torque and speed are determined by the displacements of the hydraulic motor and the pump, plus the installed power.

The Hagglunds drive system can operate above the installed power rating with the same limitations as any fixed speed AC electric motor with equivalent service factor. Moreover, this drive solution has a distinct advantage during extended acceleration/deceleration cycles and during overload conditions. The built-in pre-set pressure limiting function, which de-strokes the pump very fast, effectively avoids loads exceeding the maximum allowed torque.

With this concept one also protects the machine well from shock loads due to the low kinetic energy of the low speed motor (figure 6). Frequent start/stop sequences and reversals can also be performed by the system without any restrictions.

[FIGURE 6 OMITTED]

Drives benefits

Briefly, the drives benefits with the Hagglunds concept can be summarized as follows:

* Simplicity--the motor is mounted directly onto the driving machine shaft without any need for foundation and/or alignments. The power unit can be placed at a convenient spot for operation and maintenance.

* Small dimensions--dimension of the hydraulic motor is very small compared to a conventional gearbox. The power unit, including the electric motors and the pumps, etc., is designed to take up a minimum of floor space and can easily be placed anywhere in the production facility. Nevertheless, we recommend as close a distance between the power unit and the motors as possible.

* Controllability--full and total control over the whole process can be obtained by monitoring the speed and the torque of the driven shaft. The closed loop speed controlling is very accurate. The torque is equal to the system pressure, which easily can be monitored and controlled. See also the explanation of maximum rotor control.

* Low inertia--due to the low moment of inertia of the rotating parts in the hydraulic motor, the start-ups, accelerations, decelerations, stops and reversals can be smooth and easily controlled whenever needed.

* Shocks -the hydraulic transmission has a very low moment of inertia and operates with hydraulic oil pressure and oil flow, without any gear clearance. This makes the system totally self-lubricating and truly shock resistant. It can, without over-dimensioning, absorb shocks that a gearbox would not sustain.

* Safety--with the low moment of inertia of the hydraulic motor and the system controllability, it can go from driving mode to braking and stopping smoothly and very fast. With the Hagglunds certified safety stop valves, the drive will stop the machine shafts within milliseconds. The benefits from this in open roll mills are obvious--the rolls will stop within some 25 mm compared to some 300 mm with a conventional drive.

* Health--the noise level of the hydraulic motors and the whole system is very low. The hydraulic motor normally emits less than 70-75 dBA and standard power units emit about 75-85 dBA. By utilizing some optional sound deadening designs, a noise level below 70 dBA can be achieved for especially demanding environments.

* Maintenance--the only regular maintenance needed is the changing of oil filters. No other maintenance is required during normal operations and conditions. The service life of the system depends on requirements and the system design, but is often laid out to be some 80,000 hours or longer for heavy duty drive applications.

* Harmonics--because the control of the drive is made in the transmission with the prime mover, the electric motor, always rotating at its optimal speed, will not transmit harmonics back into the electrical supply net.

* Selectable power--by utilizing more than one electric motor/pump unit for the drive only to reach the needed speed, fewer units have to be started and running. Thus, energy savings can be achieved.

* Standard parts--major components and parts in the drive system are easily accessible and are standard items with short delivery times and short periods of down time.

* Accessory drives--drive systems for accessories such as the ram, the mixer door opening, etc., can easily be integrated into the main drive system.

Conclusion

The Hagglunds concept with direct hydraulic drives makes it possible to run the internal mixer at any speed from zero to the maximum allowed speed. The only limits are the maximum available/allowed torque, the installed power and flow. For tangential types of internal mixers, each rotor can be independently controlled with regard to both speed and rotational direction. Here also the offset position between the rotors can be controlled at 1:1 friction ratio.

MRC maximum rotor control, includes adjustable speed, adjustable friction ratio and rotor offset, which many users have found highly valuable. Dispersion has significantly improved after installing a Hagglunds drive with MRC, and improvements of 8-10% or better have been reported.