COPE Drive Systems: The right solution for large VRM drives

Vertical Roller Mills (VRM) represent the most widely spread milling technology for raw materials in the cement industry and for many years also for clinker and slag grinding.

The availability of a VRM largely depends on the driving gear unit which features the transmission between the motor and the VRM as well as the VRM axial thrust and radial bearings. LOESCHE and RENK focus on offering the most reliable VRM drive system at the lowest possible total cost of ownership (TCO).

The COPE drive system for LOESCHE Mills (LM) is the next stage in a development which started with bevel/parallel shaft gear units nearly a century ago. In the 1980s, those were gradually replaced by bevel/planetary gear units – first introduced by RENK – as the output torques increased and required a power split gear train.

Before the turn of the century, mill sizes had increased further and mill speeds had been reduced as a consequence. RENK was the first in the market to launch a three stage gear unit featuring a bevel stage and two planetary stages with an output stage of six planetary wheels.

With mill powers exceeding 6 MW and torques exceeding 3 million Nm, the demands on the drive system have increased considerable.COPE is the solution that meets these demands.

From a component to a system

A major change over conventional VRM drives is the reduction of interfaces: From the power supply line to the mill table, everything is from a single source.

As a result, COPE is the first VRM drive to be tested on the RENK factory test stand as a complete system, including the Sub Control System (SCS), the motors and the power supply. This reduces the set-up time and the commissioning risk.

The vertical axis torque path

Six to eight water jacket cooled cage induction motors drive a central bull gear. This parallel shaft stage has a high ratio. The bull gear is linked to the planetary output stage. This planetary stage has been used for more than 15 years in VRM drives with output torques of more than 5 million Nm.

These two stages are sufficient to cover all ratios required for LOESCHE mills.

The radial mill bearing is arranged next to the mill table to transmit the radial load directly into the casing. The rating of this bearing allows the operation of mills with 4, 5 and 6 rollers even if one or more rollers are out of service.

Fixed speed or variable speed

Depending on the application of the LM, COPE can be ordered with a power supply system for constant speed (for optimum efficiency) or variable speed (for adaption to varying materials). As the power of an individual motor is only a fraction of the total mill power, the power supply system allows the direct on line (DOL) starting of medium voltage motors.*

Smaller mills are normally driven by low voltage motors.*

If required by the grinding process, COPE can be operated at variable speed. The motor power allows the use of low voltage frequency converters.*

*) In specific mill designs and applications, the grinding track is repaired by welding in abraded materials which requires a very slow rotation of the mill. Up until now, a specific drive system has been mounted to drive the gear input shaft and has been removed after the welding work. COPE now integrates this function which can be selected via the controller in any of the above configurations.

Maintenance under the mill

Major maintenance of conventional VRM drives involves the removal of drive motor, coupling guard, coupling and gear unit from under the mill. After maintenance, the motor has to be realigned.

COPE opens a new horizon in the maintenance of compact gear drives under VRMs: If one drive is inoperable, the operator has four options:

1. disconnection of the motor and letting it idle until the operational requirements allow its removal
2. quick drive disengagement where the drive unit remains passively in the gear unit and is just radially moved to disengage
3. removal of the drive unit and operation with less motors
4. replacement of the drive unit.

In any case, the gear unit is left under the mill which can continue operating at close to nominal load as one motor just represents 12.5 % (8 motors) to 17 % (6 motors) of the total installed power.

Through the grinding table of the LM, the central sun gear and the coupling between the two stages are accessible.

This way, more than 50 % of the geared components and more than 70% of the bearings are accessible without requiring a major stop, on-site work by gear specialists and shaft alignment. For conventional drives, this figure is around 10 % for both the geared components and the bearings. This is a big step forward in maintainability.

All bearings which are not easily accessible are fatigue-proof sleeve bearings. Gears (except the driving pinion) and couplings are designed in a way that their back flank can become the load flank – i.e. COPE is the first large size drive with the “spares built in”.

The reasons we “COPE” with