TORQUE SIGNATURES IN CRUSHING MILLS. A.L. GÓMEZ, C.F. CAÑÓN and L.F. ROSERO. CENICAÑA- COLOMBIA

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1 TORQUE SIGNATURES IN CRUSHING MILLS KEYWORDS: Milling, Torque, Energy, FFT. By A.L. GÓMEZ, C.F. CAÑÓN and L.F. ROSERO CENICAÑA- COLOMBIA Abstract CRUSHING mills consume a considerable amount of energy. To manage that energy consumption, it is advantageous to have a detailed assessment of their performance and to understand how torque, a major factor in energy consumption, is related to mill settings, fibre % cane, size, position in the tandem and other variables. This paper presents torque signatures in milling units under different conditions with the objective of developing a better understanding of the milling process, from a mechanical point of view, and its relation with process variables. Different drives (electric and steam turbine) were also investigated. A RF (Radio Frequency) transmitter was used for torque measurements and recordings. FFT (Fast Fourier Transform) analysis was applied to data in order to isolate different contributions to torque signals and results of this analysis are reported. The Miner s rule cumulative damage criterion was applied to find equivalent torque figures. A strong influence of settings and feeding on torque was found. Crushing mill size affected the specific power consumption. Some effects of geometric conditions (eccentricity and welding irregularities) are also reported. Introduction The significant impact of the energy and maintenance costs of milling tandems on the overall economics of sugar mills is recognised worldwide. From a mechanical point of view, milling tandem performance affects extraction efficiency, maintenance costs and operational costs. Monitoring and analysis of milling process variables is very helpful to milling engineers in order to satisfy the need for reliable, practical methodologies to compare,over extended control periods, different operating strategies for dealing with changes in cane quality, cane preparation and equipment wear. For new milling units or when a drive or transmission is going to be improved or replaced, it is advisable to assess the torque characteristics of the units under study. Improvements in design, operation and maintenance practices can arise from a better understanding of how milling torque is developed. In this paper, some characteristics of torque in milling units are presented and their application in improving design, operation and maintenance practices is discussed. Materials and methods Torque measurements using strain gauges and radio frequency telemetric equipment are becoming common practice in problem solving, especially when dealing with heavy duty equipment. For milling units, strain gauges are installed on the tailbar or on any other available shaft and the transmitter and batteries are secured firmly on the instrumented shaft. The receiver is connected to a data acquisition system with high recording rate (400 Hz). Depending on the type of assessment it is usually necessary to acquire data for the speed of the milling unit, steam chest pressure, top roll hydraulic pressure, top roll lift, chute level and conveyor belt speed. 1382

2 For this study of torque signatures, several crushing units were evaluated in different sugar mills and the results are reported. From torque recordings, it is possible to apply a cumulative damage criterion known as Miner s rule to calculate an equivalent torque for selection or checking of mechanical transmission components such as gear boxes. Miner s rule (Osgood, 1982) was used to calculate equivalent torque from the following equation: T eq m i T where m is the fatigue exponent that depends on the material, the type of fatigue and other conditions. It is advised to use m = 3.3 for contact fatigue like in roller bearings, but planetary gearboxes recommend m = 6.0 to 6.6 for tooth fatigue. α i is the time fraction of events occurring at torque level i, between 0 to 1. Assuming that a new planetary gearbox needs to be selected, for the following analysis an exponent m = 6.6 was used. A service factor for milling operations is calculated from the average and peak readings during a representative recording time. Some gearbox manufacturers recommend multiplying the service factor by the equivalent torque to obtain a design torque. Histograms are statistical tools to show variations and, when applied to torque measurements, are useful to evaluate process performance (Sutherland and Burwinkle, 1995). To obtain more information about torque variations, Fast Fourier Transform (FFT) analysis was used on the processed data. Specific energy consumption is another index that can be used under particular conditions to evaluate the combination of: Equipment in use: mill size, design details such as grooving, scrapers, trash plate, couplings. Process under investigation: knifing, shredding, milling. Raw material quality: fibre, extraneous matter, specific surface. Operational practices: settings, speed, pressures, imbibition rate and temperature. Mechanical conditions: wear, arcing practices, alignment. Sepúlveda (2004) described the main aspects to be considered in the planning and execution of a full scale evaluation for mineral comminution plants, the data to be recorded, and the required calculation routines, including the theoretical framework justifying their applicability. Applying this approach, it has been found that the same cane shredder has a different energy and process performance when hammers are worn. It is necessary to acquire simultaneous data of crushing rate, speed and torque in order to estimate a specific energy consumption index. Only a typical value is required, taken from a steady state portion of the recordings, under normal operation conditions. Results and discussion Service factor for cane crushing units The service factor is a characteristic that can be determined from a torque recording. Other variables such as steam chest pressure or motor active current can be used for approximate service factor determination. Recommended service factors for normal applications are supplied by gearbox manufactures and standardised by the American Gear Manufacturers Association (AGMA). i m 1383

3 An evaluation of service factor was performed on a turbine driven # 4 mill (1.1 m x 2.13 m) with restricted top roll lift. Figure 1 shows a histogram of the torque recordings and Table 1 presents some results from the data analysis. A service factor of 1.42 is a high value for this application and only cane feeding can be blamed for this performance. A heavy duty shredder (a more impulsive machine) has been reported with a measured service factor of 1.65 (Gómez, 2005). High amplitude variations shorten the fatigue life of mechanical components. It is recommended to have the feeding rate as constant as possible. Fig. 1 Torque histogram. Table 1 Mill # 4 with restricted top roll hydraulic lift. Torque Steam chest Hydraulic pressure Power R/min kn-m pressure kpa kpa kw Max Min Average Standard deviation c.v. 13.6% 1.70% 9.40% 1.60% 13.10% Service factor Equivalent torque for milling units Using Miner s rule, an equivalent torque analysis was performed on first and last mills in two factories. All units were 1.16 m x 2.13 m and a fatigue exponent m = 6.6 was used. It can be observed that both mills at Factory B have higher torque than those of Factory C. It was determined that setting practices led to the different results about which mill is taking more load. Factory B has a #6 mill more loaded than the #1 mill and the inverse situation was observed in Factory C. Results are shown in Table

4 Table 2 Equivalent torque calculations. Factory Crushing Crushing rate TFH Equivalent Equivalent unit (TCH) torque lb*in torque kn*m B Mill C Mill B Mill C Mill Torque measurements for assessment of electric motor selection. In torque measurements at Factory B, it was additionally required to develop tests to define the size for an electric motor to replace the steam turbine. The tests included starting under load and operation under three conditions: current, with increased crushing rate, and with lower crushing rate when cleaning one evaporator series. Figure 2 shows a torque- frequency plot where a possible AC electric motor curve has been superimposed (Rosero and Ramirez, 2006; Gómez et al., 2006). The frequency is a direct indicator of the speed of the drive. Fig. 2 Torque-frequency plot with maximum values under four conditions (Rosero and Ramirez, 2006). Comparison of units of different size from the point of view of specific energy Specific energy consumption is another index that can be used under particular conditions to evaluate milling units but care should be taken because, as discussed earlier, it can be affected by mill size, design details, cane quality, operational practices and mechanical conditions. Tests have been performed during 2006 at different sugar mills and significant differences have been found in specific energy consumption (Figure 3). Some of the variables mentioned above can explain those differences. It is advised to measure torque in different units of the same tandem under different conditions in order to assess requirements (for example, to select new prime movers). 1385

5 Fig. 3 Comparison of specific energy consumption in different units. FFT analysis of torque recordings Using high speed data acquisition recorders, it is possible to show that the milling torque signal is composed of different inputs that can be isolated in the frequency domain using tools such as FFT (Fast Fourier Transform) analysis. In the time domain, recordings for Mill #1 (Figure 4) and Mill #6 (Figure 5) of the same factory show the variability in the torque signal. Figure 6 shows a FFT plot for a 0.92 m x 1.8 m last mill driven by an electric motor (Rosero, 2005; Gómez et al., 2005). Peak 1 represents the rotational frequency and peaks 2 and 3 coincide with harmonics of crown gear mesh and bagasse carrier feeding frequency respectively. More work should be performed to understand the sources of high frequency contributions to overall torque signals. An interesting analysis is reported by Peng et al. (2005), mentioning rubbing as a possible source of peaks in torque recordings. In cane crushing, rubbing is surely present, particularly in journals but also in the trash plate top roll region, and could be present in other interactions inside the milling unit, probably because of shear due to the different peripheral speed of the rolls. The rubbing often occurs where there are small clearances in rotating machinery and its impacts lead to an increase of magnitudes, not only at the fundamental rotational frequency and its harmonics, but also at some higher frequencies. Peng et al. (2005) also suggest the use of wavelet transforms (WT) based scalograms to provide another alternative for revealing these contributions, because even the FFT is not enough to identify rubbing in torque spectra. 1386

6 Fig. 4 Torque recording of a first mill. Fig. 5 Torque recording of a last mill. 1387

7 Fig. 6 FFT plot of an electrically driven last mill. Conclusions It is advised to measure torque in different units of the same tandem under different conditions in order to assess requirements (for example, to select new prime movers). High amplitude variations shorten the fatigue life of mechanical components, and it is recommended to have the feeding rate as constant as possible. Specific energy consumption is another index that can be used under particular conditions to evaluate milling units but care should be taken because it can be affected by the combination of mill size and design details such as grooving pitch and angle, scrapers, trash plate and couplings. Also mechanical conditions, operational practices and cane (or bagasse) quality can affect this index. More work should be performed to understand the sources of high frequency contributions to overall torque signals. REFERENCES Gómez, A.L. (2005) Análisis de capacidad modificada (derating) para un reductor de velocidad de dos etapas aplicado al accionamiento por turbina de vapor de una desfibradora de trabajo pesado. CENICAÑA Reporte interno, 2 3. Gómez, A.L., Cañón, F.C. and Rosero, L.F. (2006) Reporte de mediciones de torque en molinos, Ingenio Providencia. CENICAÑA Reporte interno Gómez, A.L., Carvajal, A. and Cañón, F.C. (2005). Reporte de mediciones de torque en molinos, Ingenio Pichichí. CENICAÑA Reporte interno Osgood, C.C. (1982). Fatigue Design, 2 nd Edition, Pergamon. Peng, Z.K, Chu, F.L. and Tse, P W. (2005). Detection of the rubbing-caused impacts for rotor stator fault diagnosis using reassigned scalogram. Mechanical Systems and Signal Processing, 19: Rosero, E. (2005). Informe de Avance. M.Sc. Thesis. Grupo GICI, Universidad del Valle. 1388

8 Rosero, E. and Ramirez, J.M. (2006). Selección de Motores Eléctricos para molinos en Ingenio Providencia. Informe Técnico. Grupo GICI, Universidad del Valle. Sepúlveda, J.E. (2004). Methodologies for the evaluation of grinding media consumption rates at full plant scale. Minerals Engineering, 17: Sutherland, H.J. and Burwinkle, D.P. (1995). The spectral content of the torque loads on a turbine gear tooth. ASME Sol. Energy Div. Publ. SED, 16: SIGNATURES DE L EFFORT DE TORSION DANS DES MOULINS DE MACHACAMIENTO Par A.L. GÓMEZ, C.F. CAÑÓN et L.F. ROSERO CENICAÑA- COLOMBIA MOTS CLÉS: Broyage, Couple, Énergie, FFT. Résume Les moulins consomment beaucoup d énergie; pour bien gérer cette consommation d énergie, il faut évaluer la performance aux moulins. Il faut donc bien cerner le couple qui demande une grande partie de l énergie. Le réglage des moulins, la fibre, la taille du moulin, sa position dans le train de moulins et beaucoup d autres variables affectent le couple. Ce papier présente des couples mesures avec différentes conditions opérationnelles, pour entreprendre une étude mécanique du broyage. On a étudie les commandes électriques et les turbines a vapeur. Une sonde radio fréquence a transmit les couples et on s est servi des transformations de Fourrier (FFT) et de la règle de Miner pour analyser le data. On trouve que le réglage et l alimentation ont un effet important sur le couple; la taille du moulin affecte la demande d énergie. Certains effets mécaniques (excentricité et soudures) sont aussi donnes. FIRMAS DEL ESFUERZO DE TORSIÓN EN MOLINOS DE MACHACAMIENTO A.L. GÓMEZ, C.F. CAÑÓN y L.F. ROSERO CENICAÑA- COLOMBIA Resumen LOS TRENES de molinos son intensivos en consumo de energía y es necesario disponer de evaluaciones de detalle de su comportamiento y como, por ejemplo, el torque de molienda se relaciona con los ajustes, la fibra % caña, tamaño y posición del molino en el tándem y otras variables. Este artículo trata con las firmas de torque en unidades de molienda bajo diferentes condiciones operativas, buscando mejor comprensión del proceso desde el punto de vista mecánico y su relación con las variables de proceso. Diferentes accionamientos (eléctrico y de turbina de vapor) fueron también investigados. Un transmisor RF (Radiofrecuencia) fue utilizado para el registro del torque y se aplicó la FFT para aislar diferentes contribuciones al torque reportando algunos resultados. El criterio de Miner para daño acumulativo se usó para el cálculo de torques equivalentes. Ha sido encontrada relación entre tamaño del molino y la energía específica de molienda así como algunos efectos atribuidos a geometría de mazas (excentricidad e irregularidades de soldadura). 1389