Mixer drives solve problems for compounder.

Over the past 40 years, R&S Processing has remained competitive by always looking for process improvements. The Paramount, CA compounder supplies more than 6,000 custom formulations to rubber product manufacturers around the U.S.

In 1998, R&S began planning for the installation of a Number 11 internal mixer. The company operates three other tangential rubber mixers, all of which use electro-mechanical drive systems to power the twin rotors. These drives, with their massive gearboxes and extensive foundations, consume significant floor space. Because the fourth mixing line had to fit within the confines of the existing plant, space conservation became a critical priority.

For this reason, R&S accepted a proposal from Hagglunds Drives to switch to an electrohydraulic drive system. These drive motors use oil pressure at varying flow rates to control torque speed, and direction of rotation. The MRC (maximum rotor control) system utilizes compact drive motors which mount directly onto each of the two mixer rotor shafts, and require neither a gearbox nor a supporting floor structure. Remote mounted electric motor/pump/reservoir units supply hydraulic power to the drive through high-pressure pipes and hoses, and thus could be positioned anywhere within reasonable distance of the mixer.

Added benefits

Beyond the issue of physical dimensions, the new drives promised improvements in process efficiency. The direct-mounted hydraulic drive motors allow independent control of the speed, torque, phase angle and rotational direction of each rotor. This was not possible with the mechanical drives, which applied a single power source to both shafts through a gearbox and connecting gears.

Independent rotor control produces additional turbulence within the mixing chamber as the friction ratio is increased. The higher friction ratio can significantly improve particle dispersion, producing a higher quality compound with greater production efficiency. As the friction ratio is increased, more energy is put into the batch in a shorter period of time, thus shortening the cycle time.

Conversely, the friction ratio can be instantly eliminated by equalizing the speed of the two rotors. This maintains and even reduces the temperature inside the chamber, thus helping to eliminate double-pass mixing.

The chemists at R&S also discovered that the individually controlled rotors presented the opportunity to create new and modified formulations. Through skillful programming of the PLC that controls the power to the drives, technicians can minutely adjust the speed and friction ratio of the mixing process, yielding a variety of new blends with specific characteristics. With the previous drives, the primary variables were the type and amount of ingredients.

"With the Hagglunds drive, we have much more flexibility now in creating various recipes," explained John Turner, one of the three chemists at R&S. "This means that we can meet the customer demands more precisely than ever before."

Unique characteristics of the MRC drive

In addition to space and efficiency considerations, R&S cited several other advantages to support their decision to use the MRC drive. It delivers full rated torque from standstill to maximum operating speed, and up to 200% of rated torque for unlimited time periods. The drive is highly resistant to shock loads. Hydraulic motors have very low inertia and can thus be stopped almost instantaneously, minimizing the risk of personal injury or equipment damage.

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 2).

[Figure 2 ILLUSTRATION 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 for 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.

Comparison of high torque-low speed (variable) drives

Traditionally, a frequency controlled AC motor or a thyristor controlled DC motor has normally been used in conjunction with a gear box in order to create variable speed.

The various types of drive systems that exist on the market all have different characteristics of torque and speed. Figures 3 and 4 illustrate a short comparison between frequency controlled AC and the hydraulic drives solution.

[Figures 3-4 ILLUSTRATION OMITTED]

Frequency controlled AC motors

The drive consists of an alternating current AC induction electric motor and a frequency converter. This unit can usually operate continuously at lower speeds up to 40% of rated torque for the standard motors and up to 70% of rated torque for motors with forced cooling. At nominal speed, the continuous operating torque is approximately 90% of rated torque and within [+ or -] 10% of the nominal frequency. The drive can intermittently operate up to 150% of the rated torque for a few minutes, and can, during start, operate at 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 speed range. This is especially important due to the limitations at lower frequencies.

For high torque, low speed (variable) applications, a gear reducer with primary and secondary couplings, aligned on a foundation, has to be added to the system.

Direct hydraulic drives

A direct hydraulic drive can operate continuously throughout the entire speed range up to 200-300% of the normal operating torque from zero to full speed, determined by the displacement of the pump and the installed electrical power.

The drive can operate above the installed power rating with the same limitation as any fixed speed electric motor with equivalent service factor. Moreover, this drive solution has a distinct advantage during extended acceleration cycles or during overload conditions. A pre-set pressure limiting function, which de-strokes the pump, effectively avoids loads that exceed the maximum torque requirement.

The machine is also well protected from shock loads due to the low kinetic energy of the low speed motor, and unlimited starts/stops and reversals can be performed without any restrictions.

Conclusion

Interestingly, it is often smaller manufacturers who first discover the benefits and competitive advantages generated by employing MRC drive systems. Not surprisingly, increasing numbers of rubber manufacturers, their customers and equipment suppliers are becoming aware of the scope and importance of the benefits provided by this technology. No matter whether they are seeking a reliable drive system for their calenders, open roll mills, extruders or internal mixers, more and more companies are turning to MRC drives to improve quality, productivity and profits. R&S production manager Jerry Naylor said, "We reduced batch cycle times, have better temperature control, and have improved dispersion. The net result is higher product quality and increased production."

[Figure 1 ILLUSTRATION OMITTED]