Cementos La Cruz Azul was experiencing ongoing problems with a raw mill gear box at its Hidalgo plant. An unreliable lubrication unit necessitated regular repairs, leading to significant downtime and expense. But was it worth replacing the whole gearbox?
Cementos La Cruz Azul is a company belonging to the cooperative La Cruz Azul, a traditional Mexican cement producer and owner of four cement factories with a total cement production of 8 million t/a of. Cementos La Cruz Azul, situated in the state of Hidalgo, uses three MAAG® Gear units for driving cement and raw mills that had been delivered by FLSmidth The first ones replaced the gearboxes of a horizontal cement mill and a horizontal raw mill in 1993. Both new gearboxes were of CPU type. The latest installation was a WPU type gear unit for another vertical raw mill in 2005.
The gearbox of (the raw) mill had required major repairs every five years since its start-up due to problems with the lubrication unit. The problems with the high-pressure lubrication system, supplying oil to the axial bearing, which supports the grinding table of the raw mill had even caused damage to the mill itself. MAAG Gear is familiar with problems regarding lubrication units.
The harsh environment and rough operating conditions for vertical roller mills place the highest demands on the reduction gears themselves as well as on the ancillary equipment, such as the lubrication unit. As a result, the installation of a MAAG lubrication system is advantageous due to its higher level of operational reliability and lower maintenance requirements.
An obvious and significant difference is that the gearboxes from MAAG cannot be misused as oil tanks. The lubrication units are mounted on a separate tank, thus forming a self-contained system. This lubrication unit can then be installed at a distance from the mill, preferably in an enclosed space, where it is protected from the dusty environment of the mill. The tank volume and design are chosen so that a sufficiently long outgassing time for the oil can be achieved. Heat exchanger, filter, pumps and control valves are mounted on top of the tank, creating a compact unit that is easy to install.
The high-pressure pumps are among the critical components of the lubrication unit. They are necessary to raise the grinding table of the mill hydrostatically. Usually, piston pumps are used for this task. However, the narrow tolerances of the delivery pistons make them highly sensitive for oil contamination. The lubrication system has been designed for optimum operating reliability and low maintenance in this regard, too. The high-pressure pumps do not suck in dirty and hot oil straight from the tank but are purposely supplied with cool, filtered oil.
Implementing a gear box replacement
Three years ago, these benefits led Cementos La Cruz Azul to the decision to replace the gear unit of raw mill No. 5 with a MAAG® WPU gearbox. Following the positive experience with the first replacements, especially in 2005, the company ordered a gearbox with the product designation WPU and all the necessary ancillary equipment rated for the installed motor power of 2 100 kW with a ratio from 890 to 25.73 rpm. The WPU gear unit is a two-stage planetary gearbox.
A 3D sectional drawing of the WPU gearbox for driving the table of vertical roller mill
Maintaining the existing dimensions of the connections is a great challenge when replacing a reduction gear from another manufacturer. The compact design of the MAAG WPU series makes it easily adaptable to individual conditions in a cement plant. The dimensions of the new gearbox were built identically to those of the existing one. Therefore, there was no need for any on-site adjustment either to the mill or its foundations.
Conversion work for gearbox and the ancillary lubrication system started in the middle of February 2013. When the service engineer from arrived on site, the mill had already been taken out of service and the old reduction gear removed. Thus, the service engineer inspected the foundation straight away, and the base plate for the reduction gear could be cleaned. On the second day, the reduction gear was already connected to the mill table. Figs. 4 to 5 show the gearbox replacement. At the same time, the best location for the lubrication system was identified. The local conditions made it impossible to install the tank exactly as originally planned and the foundation had to be slightly adjusted. However, that was relatively easy to do during the first week of the replacement. At the same time as fitting and aligning of the drive engine, the pipework between oil tank and reduction gear was adjusted to the new conditions.
The pipes were adjusted, welded and cleaned in parallel with the wiring of the instruments and the power supply of the engine. After only two weeks, the oil tank could be filled, and the first rotation direction tests of the lubrication drive system were carried out. Simultaneously, the mill and its equipment were also tested. These short tests thus served as the launch of the commissioning phase in the last two days of February. On 1 March 2013, the new drive system ran for the first time at full load for approximately three hours. Following the successful functional test of the instruments, the mill went immediately into normal operation. One week later, after being in operation for 40 hours, the service engineer was able to carry out the final on-site tests and check the contact pattern of the bevel gear.
Investing in more money in new drive systems is a good way to improve operational reliability and maintenance
Alejo Cruz SalasChief of Service and Maintenance
About the customer
The management of Cementos La Cruz Azul was completely satisfied with the replacement work. Following the slogan that a drive unit is only as good as its weakest piece of ancillary equipment, the management was not afraid to replace the existing drive of the vertical roller mill No. 5.