Gearbox plays an important role in the system of load transmission, and is available in a variety of types, sizes and shapes. ICR feature takes a view on gearbox and its applications.
In the production process of cement manufacturing, there are many hardware which have a very important role to play. Moving equipment like rotary kiln, ball mill, roller press, vertical roller mill, conveyor belts are a few such examples. All of these use gearbox and gear drive. Since the equipment is continuously moving to carry out assigned jobs, the gearbox attached to the equipment requires proper and timely attention for maintenance. Out of these, the most important and critical gearbox is that of rotary kiln and next to that is of vertical roller mill. Among the maintenance fraternity of the cement industry, the subject of failure of a gearbox is largely debated. There have been several case studies inked and distributed across the industry which largely cover various kind of failures of gearbox says Jacob Thomas, Deputy General Manager with TECHPORT-Thane, the support organisation of LafargeHolcim, and a specialist in gearbox.
Incidentally Thomas has a very wide exposure of witnessing different kinds of failure across industry. He initially worked with Flender before joining ACC Ltd, the group company of erstwhile Holcim. He further says that the incidences of breakdown of gearbox have come down drastically in the last 3-4 years, since his peers are doing right job at plants.
Worldwide there are a very few manufacturers of gearbox and gear drive. For cement industry, Flender is a very common name, the other is Renk AG. Both these German companies have presence all across globe including China. One more multinational player that is active is FL Smidth MAAG Gear. Indian companies like Elecon (a very old and notable manufacturer), Vulcan Engineers from Western India, New Allenberry Works from East and Shathi Gears from Southern India also figure in the list. Indian manufacturers are much smaller as compared to that of their multinational counterparts, and their product range is also quite limited. Particularly, manufacturing a gearbox and gear drive for rotary kiln and for vertical roller mill is a super specialty area and requires understanding of user industry and the conditions in which the product is set to be used. An in-depth knowledge of metallurgy is a basic requirement. The product range of these multinational companies not only include gearbox for cement industry, but also includes automotive industry, shipping industry, power generation business, etc.
The first generation of gearbox for vertical roller mills was designed on the basis of a bevel-spur-gear arrangement. The challenge of this design was to balance the ratio of the two-gear stages equally. This allowed keeping the dimensions of the bevel wheel and the spur gear within a manageable range. In order to fulfill the demand of the cement industry, gear suppliers have since chosen a new gear arrangement. The spur gear design was replaced by a spiral bevel helical with multi planetary stage and consequently it was possible to reduce the overall dimension (compact design) and weight compared to the bevel-helical gears.
Nobody realises that a gearbox can have such a complicated design and performance, looking at it from the outside. It is a fine piece of sophisticated equipment produced based on the specifications provided by the client, under stringent conditions and strict quality control.
Sizable capital and efforts are put in by plant personnel in smooth running of rotary kiln and vertical roller mill. Engineers are even sent abroad for training, consultants are hired to have optimum performance of these equipment. Sprat Consultancy based at Kolkata is one such outfit which offers its services right from training to failure analysis to retro fitting of gearbox. The team members of Sprat had earlier worked at Flender and were part of the design and commissioning. They have a vast experience of troubleshooting the gearbox problems not only for the cement industry but also for the other sectors. The article written by Sengupta of Sprat Consultancy imposes the responsibility of smooth running on both - user and supplier. Thomas says that failure occurring at the stage of commissioning is to be treated differently as it relates to the design process of a gearbox. A different approach is to be taken to understand such failures. Failures during running and operation of the plant will altogether need different methodology. The failure of a gearbox cannot be attributed to only manufacturer. Before the order is finalised, the supplier asks for the details of the properties of the product to be processed in the mill and then suitably designs the machine. Any subsequent change in the input material can vary the process parameters and/or lead to improper functioning of the mill or the kiln. Today, due to pressure on becoming competitive, the design margins of the equipment are the least which must be borne in mind by the user. Therefore the saying-´something worked in the past should work now also´-is not acceptable.
By and large these failures press a panic button for the top management, as generally there is no ´spare gearbox´ available for either kiln or VRM except in case of planned maintenance.
While investigating the failures-which starts right from the foundation of the machine in case of replacement of old gearbox with the new one and if old foundation is used-proper study must be done before final decision is taken. Another pointer as Thomas says is environment or surroundings of the gearbox. The dusty environment always leads to failure of bearings or motor.
The root-cause analysis is a perfect method to follow in case of failure of a gearbox. Many plants have their own format to carry out the root-cause analysis. Proper recording of the performance parameters is necessary. Debate and discussion on the failure is the first step to follow. Engineers have to update the history sheet of the machine including past failures, change of oil done, replacement of bearings or any other part, etc. The history sheet and the operating manual are two very important documents of the machine. For better results, the teamwork between the process personnel and maintenance engineers is another requirement. Their sharing of information will definitely lead to healthier work environment but can save plant from untimely failures.
Lubrication oil is too important and a challenging subject. Many a times, decisions on lube purchase are commercially driven and not on the basis of the properties of the bought-out items. Present generation of oils have many additives which enhance the performance of the oil and have less moisture absorption. Lower quality or cheaper oil need to be replaced more frequently and wastes precious productive hours which commercial-minded managers rarely appreciate. Experience suggests such decisions can be proved costly in the long run. The money spent on lubrication need not be based on price alone but on performance and price together. The decision on lubes has to be taken after consulting the OEM or the oil manufacturer. Proper documentation on replacement, partial replacement or testing must be maintained. It has been observed that analysis of used oil has been found a reliable source of understanding failures, Thomas says.
There are a good number of software packages available for predictive maintenance, which are purchased by the plant management that need to be used in the best possible way. This feature also covers a glimpse about a company called ´DALOG.´ It is a company operating in a very niche area of condition monitoring of a gearbox and charging hefty fees. But industry still prefers to hire its services. Today there is no competitor for DALOG. It also offers service to the gearbox located at any plant from their office in Germany. It is a perfect marriage of instrumentation and information technology.
Involving and motivating technicians who are working with the plant for reasonable length of time have been found to be additional resource. If these people are vigilant and taken into confidence, they can immediately report the abnormalities of the gearbox like excessive vibration, noise or temperature which are the signals given by the machine before breakdown.
In short, gearbox and gear drives are very sophisticated piece of equipment manufactured at world-class manufacturing plants strictly as per the inputs given by the user and have to be used as per the instructions given and understood by the plant personnel. The available latest technology to access the health of the gearbox has to be at the disposal of the plant engineer, and then the incidence of failures will be minimal.