Shifting to top gear with specialized designs for coal grinding

Approximately 40 years ago, the power limits for large vertical mills driven with bevel-spur-gear arrangements were reached. With these limitations on the spur gear’s capabilities, gear suppliers had to innovate on a new gear arrangement to fulfil the increased demands coming from the industry.

After years of research and development, the resulting planetary stage gear, branded as the now well-known MAAG® WPU, not only allowed for increased power, but also provided the added advantage of a reduced footprint and weight compared to the legacy bevel-helical gears.

This same planetary gear design change was then applied to coal mill gear applications to improve their efficiency and availability. However, the optimal coal mill gear box is not simply a scaled version of the larger vertical mill gear, because the needs and demands of coal mills differ from the vertical mills used in other industrial applications. In general, the design power of gearboxes for coal mills is considerably smaller than in other applications. The coal grinding system must additionally take into account the available heat sources for drying the raw coal as well as the fire and explosion hazards of coal dust. The entire grinding plant must therefore be shock-resistant and equipped with a number of explosion relief valves.

In order to meet the specialized requirements of coal grinding systems, our engineers and product development people at FLSmidth MAAG Gear took on the challenge to create a reliable gearbox design that was easy to install and maintenance-friendly. It had to be durable and efficient, and had to quickly become the smart choice for coal mill customers.

The resulting efforts developed a smaller series of the established MAAG WPU gear with this new design that's specially focused on coal mill applications in the cement, minerals and thermal power plant industry.

Gearbox requirements

Gearbox design for vertical roller mills is engineered to take into consideration these three main requirements:

  1. Transmits the power from the horizontal drive motor to the vertical mill table
  2. Reduces motor speed to mill speed
  3. Absorbs the grinding forces and supports the mill table

The design of the WPU gear successfully addresses these criteria. Requirements 1 and 2 are fulfilled by the specific WPU gear arrangement where in the bevel stage a first reduction of motor speed and the change of load direction are realized. With additional gear stages, the specific mill speed is realized.

For requirement 3, the absorption of the grinding forces is ensured by a hydraulic-tilting-thrust bearing directly located underneath the output flange of the gearbox. The reaction forces from the grinding process flow from the grinding table through the gearbox casing directly into the foundation.

A state-of-the-art gearbox design

The smaller series MAAG WPU gear for coal grinding is built with quality and efficiency in mind. FLSmidth MAAG Gear has designed, produced and maintained gears, gear units and drive systems for the cement and minerals industries for over a century.

“With our extensive experience and understanding of the variety of customers' needs, such as coal grinding, we’ve been able to develop a compact drive that addresses their specific requirements with a number of advantages,” said Martin Baechler, Product Manager, FLSmidth MAAG Gear.

The reduced load and less-severe operating conditions in coal mill applications, compared to other vertical mill applications, allow the consequent use of antifriction bearings for all geared rotation parts. The bevel gear stage is supported by roller bearings located on both sides of the gearing. This special bearing arrangement is possible thanks to a unique design of the gear casing. It ensures perfect alignment of the bevel gear stage, equalized load distribution, and an optimized operating condition for variable loads.

“The lifetime of the antifriction bearing is calculated for 100,000 operating hours, which translates into lower maintenance requirements and reduced parts and service costs,”said Baechler.

The torque is transmitted from the main motor through the bevel gear to the planetary gear stage via a toothed coupling. Three planet wheels are located in the planet carrier. With this arrangement, a perfect load sharing between the planets is realized and each planet is designed for one third of the total torque. The planets are considerable smaller and easier to handle compared to one large spur gear. Spherical roller bearings mounted on planet shafts ensure best possible position of planet wheel during operation in each load condition. The rotating planet carrier is bolted and pinned to the output disc and drives the grinding table.

The grinding table itself is supported by a tilting thrust bearing and a radial slide bearing. The pads of the high-quality tilting-pad thrust bearing are lined with babbitt metal. The purpose of these bearings is to absorb the static and dynamic grinding forces. They are of hydrodynamic type, lubricated by low-pressure pumps or hydrostatic type on demand.

The gear casing consists of a robust, welded-steel construction or casted, well-ribbed structure with a very small interruption in the circumference caused by the bevel stage. The grinding load, including grinding table weight, is transmitted straight and uniformly to the foundation via the tilting-pad thrust bearing and the casing wall.

Advantages of the new design

The table below clearly shows the advantages of the new gear unit. The huge difference in weight points out the compactness of modern coal mill drives. Exchanges or even power upgrades of the coal mill drives therefore can be realized without complex modification of the foundation or the mill pedestal.

The smart-designed bevel-planetary gearbox also simplifies the maintenance: large circular openings allow easy access for inspection of the gear parts. Shims in the bevel gear arrangement allow dismounting of bevel pinion for bearing changes without opening the entire gearbox. The bevel gear is designed in two parts to simplify the exchange and reduce cost of spare parts.

Benefits for customers

Thanks to the elaborate design concept of the WPU bevel-planetary gear with a special bearing arrangement, customers benefit from a number of advantages. The state-of-the-art gear units can be installed quickly and are maintenance-friendly. The planetary gear stage and the thrust bearing for the grinding table have a very long service life. Not to mention the available customer-specific design of the gear unit, especially in case of retrofit and replacement projects.

"Customers can see how these gear units can benefit their operations. The high operational reliability and the very high efficiency of the WPU gear units are already being proven in installations all over the world,” Baechler said.

Assembly and testing in Bawal

In a continuing effort to reduce costs and proactively pass along the savings to customers, FLSmidth has established centres of excellence for manufacturing and assembly in a number of low-cost countries around the world. On April 28, 2014, FLSmidth MAAG Gear inaugurated its first assembly and service unit in Bawal, India where WPU gear units can now be built and assembled.

The concept of this new facility in Bawal is as simple as it is efficient. During the preparatory supply-chain initiative, FLSmidth MAAG Gear was looking for local suppliers for all non-rotation parts. After conducting intensive audits and adhering to stringent quality standards, it was determined which facilities would be able to optimize the costs and production without sacrificing quality. Today, only the toothed parts have to be imported from FLSmidth MAAG Gear’s Poland facility, where recent major investments have been made in top-level, bevel-gear cutting and testing equipment.

“With this concept, we have the possibility of building around 30 gearboxes a year in the new workshop in India,” Baechler said. “In the assembly shop, we instituted the latest in quality control to test in-depth all final assembled gearboxes before they are shipped to customer sites.”

A state-of-the-art test bed located in the workshop utilizes modern control panels to allow monitoring and recording in real time all relevant operating parameters of the gearbox during the no-load test. Within these facilities, the new gearbox assembling unit follows the business and market requirements of India and neighboring countries.

During the autumn of 2014, the first two gearboxes with a bevel and a planetary gear stage were built and assembled in India. The two gearboxes of type WPU-19 and WPU-27 are typically used for vertical coal mills. The local staff of FLSmidth MAAG Gear assembled the gearboxes under the supervision of Polish and Swiss engineers and service technicians. The concluding no-load test of both gearboxes showed no irregularities and proved the habitual high quality of FLSmidth MAAG gearboxes that customers have come to expect. More units are in production, and because of the many benefits offered by this technology, we already see greater acceptance of WPU-type gearboxes within coal mill applications.

CONTACT: Martin Baechler