Founded in 1979, Orient Cement pursues a clear growth strategy. Between 1990 and 2009, the company steadily increased its total production capacity to 5Mta by upgrading the existing cement lines at its Devapur cement works in Adilabad, central India, and its split-grinding unit in Jalgaon, Maharashtra. By 2020, the India-based cement producer expects to further raise its production capacity to 15Mta. However, the company’s focus is not only on the construction of new production sites. It also recognises the importance of securing established output capacity through the effective maintenance of existing systems. For this reason, Orient Cement recently contacted FLSmidth MAAG Gear to completely overhaul the gear unit of a raw meal mill.

 

Successful adaptation

The gear unit, of type CPU-30-2, was designed and produced in 1987 by  FLSmidth MAAG Gear in Switzerland. As motors with the required 3800kW output were not available at the time, the design differs from the standard CPU series. Standard CPU gears consist of two series-connected planet stages, each with three planets and a rotating planet carrier. When the first two-stage planet gear unit for the cement industry was developed in 1966 by MAAG Zahnräder AG (now FLSmidth MAAG Gear), it replaced traditional, multistage spur gear units for the central drive of ball mills. Thanks to their higher power density and better efficiency, these gear units are able to transfer more power and take up less space. The bearings, monitoring system and oil system have, of course, been upgraded constantly over the years to remain state of the art. However, the basic principle of the current CPU gears is still based on the original 1966 design. To comply with the 3800kW output required, a drive with two parallel motors, each with an output of 1900kW, was developed for Orient Cement. As its first reduction, this CPU does not have a planet stage. It has two input pinions that mesh with a common gearwheel. The accumulated power is transferred to the second stage, a planetary gear with  rotating planet carrier, via a gear coupling. The input speed of 992rpm is reduced to the mill speed of 14.76rpm in the two gear stages. 

Gear operating conditions
Figure 1: Gear operating conditions with and without service

Benefits of overhaul

While the gear design is very robust and durable – the first two units built in 1967 are still in use in Belgium after over 46 years of operation – to operate a gear unit beyond its normal service life under the tough operating conditions of a cement plant, more is required than just periodical nspection and maintenance work. The service life can only be extended if the entire gear unit is completely overhauled. Overhauls reduce the overall wear in the gear unit. The wear parts are replaced and the gear unit is serviced so that it is ready for the next operating period. If overhauls are planned in advance, they can be carried out in periods of low demand and also ensure that the gear unit continues to operate without problems. They help reduce maintenance costs, extend the service life of the plant and help avoid unexpected downtime. Orient Cement recently benefitted from all of these advantages.

 

Complete overhaul of a two-stage central drive

After first operation, the gear unit was inspected during the warranty period after 6000 and 12,000 operating hours. It then ran for more than 10 years without problems, maintained by the local maintenance team. The first major inspection by FLSmidth MAAG Gear took place in 2005 and the gear unit was found to be in good condition. As recommended by FLSmidth MAAG Gear, a complete overhaul was carried out in September 2013 after around 134,000 operating hours or a service life of 26 years. Roughly six months before, a service engineer from FLSmidth MAAG Gear produced an initial analysis of the gear unit on site and established that all necessary spare parts for the overhaul were available locally.

Ultrasonic inspection
Ultrasonic inspection on planet axle

When the service engineers arrived in Devapur, the gear unit had already largely been prepared for the overhaul. The raw mill was stopped, and the auxiliary drives and the power supply of the main motors had been removed by Orient Cement employees. The overhaul work was completed in just 10 days. The oil pipes and connection couplings had to be removed first to allow the gearing and the bearings of the two gear stages to be inspected. The three planet axles in the second stage also serve as journals and are therefore coated with a layer of white metal. As part of this overhaul, Orient Cement inspected the journals using ultrasound to ensure that the white metal layer was still in perfect condition. The gearing of the sun pinion of the second stage was also inspected using ultrasound as it is the gearing subject to the highest load in the gear. All sliding bearings in both gear stages were completely replaced. The gear unit was also cleaned and small parts and wearing parts were replaced. When the gear stages were assembled, the internal alignment of the gearwheels and the tooth contacts were reset.


The gear unit was completely assembled in the last three days of the mill’s downtime. The service engineers first assembled the two gear stages again and set the tooth contact and backlash correctly. The gear unit was provisionally aligned and braced on the foundation. Wearing parts such as O-rings and coupling bolts in the output coupling were replaced before the coupling was installed; The cleaned and inspected couplings were filled with new lubricating grease and the alignment between the motors, the gear unit and the mill was restored. The oil pipes were then connected and flushed with the oil. The rotational direction test n the motors was the last job before the load test and the resumption of operation of the raw meal mill.

Disassembly of planet carrier bearing
Disassembly of planet carrier bearing

Fit for the next stage

After this 10-day complete overhaul, the Orient Cement gear unit has new bearings, all wearing parts have been replaced and the alignment of the gearing has been restored to the original setpoint. The alignment with the drive motors and the mill has also been checked and is within the permissible range. The gear unit is not new but is fit for the next stage of its life. With good care and the necessary maintenance, it will continue to operate well until the next complete overhaul is due, scheduled after 80,000h or 12 years of further operation.

Martin Baechler

FLSmidth MAAG Gear AG
Switzerland
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