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Variable friction drives for mixers and roll mills. (Process Machinery).

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 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 tile pressure in the system, the torque can be controlled.

Typical operating ranges for variable friction drives are as follows:

* 0-80 rpm with up to 1,800 kW installed for the mixers;

* 0-25 rpm and up to 500 kW installed for the roll mills.

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 changes of 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.

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 nor really 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 where 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 1 and 2 illustrate a short comparison between frequency controlled AC and the hydraulic drives solution.


Frequency controlled AC motors

The drive consists of an 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 preset 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.

Roll mills

Some of the features and benefits of using hydraulic drives on a roll mill include:

* Simplicity--The motors are shaft mounted directly on to the rolls. No foundation and alignments are needed. The power unit can be placed at a convenient spot for operation and maintenance.

* Controllability--Control over the whole process can easily be obtained by monitoring the speed of the driven shaft and the system pressure. A 4-20 mA signal plus the speed range can be integrated with the existing process control system.

* Low inertia--Due to the low inertia of the rotating parts inside the hydraulic motor, the startups, speed changes, stops and reversals will be smooth and easy to control.

* Shocks--Because it operates on hydraulic oil, the system is totally self-lubricating, therefore the drive is truly shock resistant and will absorb shocks that a gearbox would not tolerate.

* Safety--With the very low inertia of the hydraulic motor, and a built-in safety stop block in the system, an emergency stop will shut down the drive to a complete stop within milliseconds. The distance traveled by the outer periphery of the roll is only about 25 mm compared to over 300 mm when using a conventional drive.

* Health--The noise level of the whole system is very low. Standard power units emit about 80-85 dB. By using water cooled electric motors and other sound deadening designs, a sound level below 70 dB is possible to achieve for especially demanding environments.

* Maintenance--The only regular maintenance needed is changing the oil filters. No other maintenance on the motors or pumps, etc., is required during normal operations. The designed service life of the system is a minimum of 40,000 hours, based on an operation at half the maximum rated speed and torque of the hydraulic motor.


Basically, the same features and benefits of using hydraulic drives on roll mills also apply to tangential mixer drives. The reason for this solution not having been used earlier is that there were no hydraulic motors large enough to be used as direct drives. This problem has now been overcome by the introduction in the marketplace of larger hydraulic motors.

A number of hydraulic drives on mixers has been installed in Europe during the last five years, and the first one in the U.S. was installed at R&S Processing in Los Angeles, CA in 1999. Additional features that apply to the mixer drive include:

* Maximum rotor control--For each and every molding recipe, users can optimize the speed, friction ratio, rotor offset, etc., thus achieving the best result in the shortest time.

* Ram drive--This can replace the pneumatic operated ram and be hydraulically integrated within the main system.

* Harmonics--No harmonics will be transmitted back into the electrical supply.

* Selectable electric power--Users only need to use the number of electric motor units required for the speed (one electric motor per hydraulic pump). Thus, energy savings are achieved when utilizing the higher efficiency curve.

* Standard components--Major components in the drive system are standard items, therefore no long periods of downtime are noted.

* Reduction of operating costs--Optimized speed/torque control will lead to lower energy costs.

Functional description of a radial-piston type motor

A typical hydraulic industrial motor of the radial-piston type with a rotating cylinder block/hollow shaft and a stationary motor case/housing is the Hagglunds Marathon motor (figure 3). The cylinder block is mounted in fixed roller bearings in the case. An even number of pistons is radially located in bores inside the cylinder block, and the valve plate directs the incoming and outgoing oil to and from the working pistons. Each piston is working against a cam roller.
Figure 3--radial-piston type hydraulic industrial motor

1. Cam ring
2. Cam roller
3. Piston
4. Shaft coupling
5. Cylinder block/
 hollow shaft
6. Guide plates
7. Guide roller
7a. Cylinder block
8. Connection block
9. Valve plate
10. Shaft end
11. Port end
R = inlet or outlet
 port >>R<<
L = inlet of outlet
 port >>L<<
[D.sub.1], [D.sub.2], [D.sub.3] and [D.sub.4]
 = drain ports

* [D.sub.4] = MB 1150-MB 4000


When the hydraulic pressure is acting on the pistons, the cam rollers are pushed against the slope on the cam ring that is rigidly connected to the case, thereby producing a torque. The reaction force is transferred by the guide roller bearings on the cam rollers shaft ends to the two guide plates, which are connected to the cylinder block/hollow shaft. Rotation therefore occurs, and the torque available is proportional to the pressure in the system.

Oil main lines are connected to ports R and L in the connection block and drain lines to ports [D.sub.1] [D.sub.2], [D.sub.3] or [([D.sub.14]).sup.*] in the port cud housing.

The motor is connected to the shaft of the drive machine through the hollow shaft of the cylinder block. The torque is transmitted by using a mechanical shrink disc/ shaft coupling, or alternately by splines.
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Comment:Variable friction drives for mixers and roll mills. (Process Machinery).(Brief Article)
Author:Lattstrom, Lars I.
Publication:Rubber World
Article Type:Brief Article
Geographic Code:1USA
Date:May 1, 2002
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