Wikov Gear s.r.o. is a leading manufacturer of gearboxes for drives of cement mills and kilns in central and eastern Europe. One of its main products for the cement industry is side drive gearboxes. These gearboxes are in particular demand owing to their compact design with a small footprint, high efficiency and the low wear of the meshing girth gear.
Wikov's side drive gearboxes have gone through extensive development and innovation. Based on new requirements in the market and experience obtained from operation of these gearboxes Wikov implemented major research and development with the objective to offer customers an innovative product with extended output parameters up to 6MW, including innovative design of a separate lubrication system. This project was implemented with financial support from state budget funds through the Ministry of Industry and Trade of the Czech Republic. Research and development was conducted along three lines:
1. Research into the dynamic behaviour of these gearboxes with the objective to be able to predict weak points of the drive in all conditions established in practice and to create an extremely stable and reliable gearbox;
2. Development of a separate lubrication system, so that any gearing and bearings in the gearbox that are very sensitive to oil quality were lubricated by a separate lubrication circuit and sealed from its environment (in particular the area of the gear rim);
3. Identification of an analytical method to simulate the behaviour of large gear wheels during heat treatment.
All of these tasks had only one objective: the design of a reliable and stable gearbox with low maintenance requirements. Let us focus in particular on the first and second targets, which seem to be the most interesting in terms of mill operation.
Dynamics of side drive gearboxes
At the beginning of the project, background research was conducted into cement mill drives, large gearboxes and side drive gearboxes. Based on this background research, parameters were drawn up for a potential gearbox so that the resultant product would be compatible with the majority of existing plants in the global cement industry.
The basic concept of the gearbox shows that the output is divided into two branches, which meet at the meshing of the two outlet gear pinions with the girth gear. Under ideal conditions, i.e. if the whole mill, girth gear, gearbox and the base are ideally rigid and manufactured with absolute precision, it would be enough just to set the two pinions only once in the meshing with the girth gear and not to deal with evenness of the division of the output flow. However, such conditions do not occur in practice. The mill, the girth gear and the base are subject to elastic deformation, have deviations due to manufacture and are also subject to wear. For these reasons, it is necessary to divide the output automatically in the course of operation.
For this reason there is a stage two floating pinion. This pinion, to which the torque is transferred from the inlet shaft via the stage one gear set and on which the output is first divided, has its gearing designed with a relatively high helix, which causes axial forces. Under ideal conditions, these axial forces negate each other. However, if there is a difference in the transfer of output between the branches, for example due to an imprecisely-manufactured rim gearing, the axial power on the meshing of the branch with a greater output is increased and moves the pinion to the side until the output flow is levelled again and axial forces from the gearings are also levelled.
In order that such a shift is possible, the gear pinion needs to be mounted on axially free bearings. Roller bearings are usually used in similar situations but they have one significant disadvantage in that they cannot compensate for deformations in the shafts or the box.
Therefore, a decision was made to use CARB toroidal bearings from SKF. These bearings have a high load capacity (higher than roller bearings of the same size) and tilt, therefore compensating for deformation of the shaft, box or simple production deviations. Moreover, by shifting the outer ring and the inner ring in relation to each other it is possible to set the radial clearance in the bearing within certain limits.
The following stage included research of dynamic behaviour of side drive gearbox based on the calculated model of the existing 2.8MW and 4MW gearboxes. This included:
- Structural analysis of gearbox casing in an ANSYS environment;
- Modal analysis of torsion oscillation of gearboxes in a Matlab environment;
- Sensitivity analysis of torsional oscillation – Change of various parameters;
- Verification calculations of deflection and displacement of shafts for original casing and for the casing after modifications.
First, a 3D model of the casing of the existing 2.8MW and 4MW gearbox was made. The whole casing was discretised using finite element analysis and structural analysis was performed under loading from reactions in the bearings.
When verifying precision and reliability of the calculation, the actual deformations in operation were measured. The calculation performed in this way was declared as sufficiently credible and based on this, the detail design of the casing was performed.
The findings from this calculation were then used for creation of the model of the casing of the innovated side drive 2.8MW and 4MW gearboxes and the completely new 6MW gearbox. All these designs were calculated in the same way. The result is a substantial improvement of rigidity of the design.
Based on the calculation of deformations in the casing, lateral and cross modifications of the gearing were proposed in order to secure maximum meshing of gearing under load and thus to improve the reliability and service life of the gearbox.
The analysis of the dynamic behaviour of gearboxes proved to be an interesting issue. Based on the acquired knowledge and experience with practical operation of existing gearboxes, a decision was made to create a complex dynamic model of the gear train, which would include both the dynamic model of the casing and the dynamic model of internal rotor built-in section. The modelling methodology was based on:
- Discretisation of the spatial model of the casing using finite elements and using selected modal properties (natural frequencies and mode vectors);
- Discretisation of individual shafts with gear wheels using shaft finite elements and suitably selected solid bodies;
- Modelling of flexible tooth couplings between the gear wheels;
- Modelling of flexible bearing couplings respecting the actual number of rolling elements.
The processed modelling methods were implemented according to actual computing software in MATLAB. ANSYS software was used for modelling the casing and export of requisite results of the modal analysis. The whole model of the gearbox with a reduced number of free degrees, which is suitable for subsequent dynamic calculations was compiled in MATLAB.
The 4MW gearbox was selected as a suitable representative of the existing gearboxes for testing the new methodology. A model of the casing was prepared in ANSYS and the parameters of the shaft assembly, gearing and bearing couplings were processed according to drawings and support documentation for the given gearbox. With regard to the complexity of such a model, it is necessary to check the selected concept of modelling based on experimental measurement of an actual gearbox.
The calculation model of the gearbox was then experimentally checked on a 4MW side drive gearbox in a mill in Turkey. A modal analysis of the free gearbox was performed and measuring of multiple values during start-up of the mill was performed including acceleration of the casing, torque, shift of axial shaft, casing loading using tension meters on anchor bolts and the position of girth gear teeth. An MKP model in Cosmos M was created for calibration of the torque.
It was ascertained that the measured relative deformations on the casing achieve very low values. The amplitude of the countershaft in stable condition was surprisingly low at 1.7mm. The torque measured at the inlet shaft (even before the coupling) during start-up is 58% higher than in a stabilised condition.
A significant finding is that the measured torque is significantly affected by the meshing forces - outlet pinion versus mill girth gear. In the chart, it is shown by the red (torque) and green (laser sensor of girth gear) time line. This fact might be used for diagnostics of engagement forces. This time line corresponds to a stable operation after the mill had started up (see torque spectrogram) in the right half of the picture.
In order to perform experimental modal analysis of the gearbox casing in Turkey, the gearbox was set and mounted to the base but the driving motor was not connected. The excitation force was realised using an impact hammer and a response was detected by three-axis acceleration sensors. The 'rowing accelerometer' method was used, i.e. the excitation force was applied in one point in three mutually perpendicular directions, acceleration sensors were gradually transferred to 167 different measuring points on the casing surface.
Unfortunately, the analysis of the acquired data did not result in credible identification of modal parameters and modal shapes of casing oscillations. There are several reasons for this. The relatively small excitation force applied for a short time was not able to excite the actual shape of oscillation of the casing at a sufficient intensity. This is closely related to the fact that no clearances were set in the load-free gearbox (between teeth, in bearings) which resulted in problematic transfer of energy between the casing and the shafts. The gearbox was in contact with the rim, so the measured responses were also affected by the reaction of the mill plant to the excitation force. To conclude - the acquired transfer functions included responses of a large amount of actual oscillation shapes which could not have been 'separated' within the identification process.
The second method to get an idea of the dynamic characteristics of the casing was the determination of the operational shape of oscillations. Casing vibrations were detected by three three-axis accelerometers at 167 points. One single-axis accelerometer was used for reference.
The frequency spectrum of vibrations contains responses to excitation by the forces formed in the gearings. The gearing frequency of the transfer between the outlet shaft of the gearbox and the mill girth gear has the greatest number of harmonic multiples.
Radial vibrations of outlet shaft and countershaft were also measured. Schenck contactless position sensors were used. The frequency spectrums of measured vibrations show that the oscillation on the speed frequency of shaft and the first tooth frequency of the gear between outlet shaft and the girth gear are dominant.
After the first measurements were taken, a report was processed including comments for utilisation of measuring results for verification of comprehensive dynamic model of a 4MW side drive gearbox. The operational shapes of oscillations were selected as the most suitable values, which, with regard to the present conditions of excitation, to best document the dynamic characteristics of the gearbox.
The comprehensive model is based on decomposition of the gearbox to rotor subsystems interlinked by tooth couplings and to stator subsystem interlinked with rotor subsystems by means of bearing couplings.
The results of calculations and numerical calculations were compared with the results of experimental measurements in the mill plant in Turkey. Some results of the comparison have been presented earlier.1 Analysis of dynamics of the gearbox was performed using the Advanced Rotating Machinery Dynamics programme (ARMD) from RBTS, Inc. This programme allows comprehensive calculation of dynamic behaviour of gearbox rotors by connecting eight programme modules together to share data.
The two main modules, 'Rotor Dynamics' and 'Torsional Vibration', allow computing of flexural and torsional oscillation of branched rotor systems. It is possible to calculate natural frequencies, mode shapes, perform stability analyses and calculate transient and stabilised oscillations. The other modules allow performance of calculations of all types of sleeve and roller bearings. The programme package also includes support modules allowing graphic and text output and a module for calculation of lubrication characteristics.
In order to prevent contingent resonance oscillation, an analysis of resonance frequencies was performed on torsional oscillation of the internal rotating built-in structure. The model created in the previous stage of research was extended by a driving motor, massive coupling on the motor side and the coupling of the auxiliary motor as these elements substantially affect the size of some natural frequencies and the character of oscillation of the whole gearbox.
A sensitivity analysis was performed for the identified natural frequencies which may resonate with the exciting tooth frequencies (for five variants of the gearbox), whereas based on this analysis changes were recommended in the construction of the gearbox design in order to remove the resonance. Suitability of the performed changes was verified by a modal analysis with a modified model according to design documents.
A model for torsional oscillation analyses was created for the new 6MW gearbox. The natural frequencies of the linked shaft system were compared with the theoretical tooth excitation frequencies and possible changes to parameters were suggested in order to remove resonation conditions.
Separated lubrication system
The second line of development was planned with the objective of removing one of the greatest issues of current side drive gearboxes, i.e. their easy contamination by cement dust and impurities from the girth gear. Despite the fact that the rim is thoroughly sealed during installation, practical experience shows that this sealing is far from perfect and it wears very quickly. Conventional side drive gearboxes, which are open in the direction of the rim, are extremely polluted by cement dust and other impurities. This results in rapid wear of gears and bearings, contamination of oil and clogging of filters and the lubrication system, resulting in early faults with the gearbox.
Wikov focused on this problem and designed a gearbox that is closed and sealed so that contact with the rim is limited only to outlet tilting pinions.
The gearbox space is divided by a system of partitions into the area where gear wheels and gearbox bearings operate and the external area of meshing of outlet pinions with the girth gear. Sealing in the area of shafts at outlets is implemented using labyrinth seals and scraper rings.
The great advantage of this system is the significant increase in gearbox reliability, in particular that of the bearings, which are very sensitive to cleanliness and quality of lubricating oil. In order to achieve oil cleanliness fully conforming to the operation of the bearings in the original design, we would have to include in the lubrication system a full-flow filter, filtering at about three microns. This is not realistic for practical applications and therefore we have to ensure that the lubricating circuit of the bearings and the transmissions is as insulated as possible from the external environment of the casing. The time between repairs or replacement of bearings in the new gearbox can be doubled through using this system.
Finally yet importantly, the system of partitions significantly increases the rigidity of the casing. In the case of conventional side drive gearbox, the side adjacent to the rim is open and therefore not reinforced, whereas with the innovative gearbox the partition system closes the gearbox from this side as well and significantly increases rigidity of the whole casing.
The result of the research is the building of prototypes of two sizes of gearboxes - 2.8MW and 6MW - with one common property: they have extremely rigid gearbox casings on which the effect of all negative static and dynamic phenomena has been removed. The casing is adjusted to dampen vibrations at all prevailing frequencies and to exclude any resonation phenomena. As a bonus for customers a fully-enclosed internal section is offered, which is thus isolated from the adverse and difficult-to-seal girth gear environment. Therefore, these gearboxes have high resistance, long service life and great reliability.
References
1. Hajzman, M.; Polach, P.; Pasek, R. & Vaclavik, J. 'Presentation at Applied Mechanics conference 2010,' presentation at 'Proceedings of Applied Mechanics conference 2010,' and Hajzman, M. & Polach, P. Engineering Mechanics, Vol. 18, No. 5/6, 2011.