Mill Bearings & Lubrication: Complete Guide

The grinding mills of the cement plant, the ball mills, the vertical roller mills and the roller presses, depend on their bearings and their lubrication systems for the continuous operation: the bearings carry the heavy loads of the mill, the grinding media, the material and the drive, the lubrication protects the bearing surfaces from the wear and the heat, and the bearing and the lubrication failures are among the most common causes of the unscheduled mill stops. This article covers the mill bearings and the lubrication in the professional depth: the bearing types and their applications, the bearing selection and the mounting, the bearing loads and the life, the lubrication principles, the oil and the grease systems, the oil selection and the management, the bearing monitoring, the bearing failures and their causes, and the maintenance and the troubleshooting. The article is written for the mechanical engineers, the maintenance personnel, the lubrication specialists and the operators who work on the mill bearings, and its objective is to give the reader the complete practical knowledge: to select, to mount, to lubricate, to monitor and to maintain the mill bearings professionally, which is the foundation of the mill reliability and the grinding availability.

The Bearing Types of the Grinding Mills and Their Applications

The grinding mills use the different bearing types for the different functions: the main bearings of the ball mill support the mill shell, the bearings of the pinion and the gearbox support the drive, the bearings of the vertical mill support the grinding table and the rollers, and the bearings of the roller press support the rolls, and each application selects the bearing type by the load, the speed, the alignment and the environment.

The ball mill main bearings support the mill shell at the trunnions: the trunnion bearings are the large plain bearings, the hydrodynamic bearings, or the large rolling element bearings, and they carry the full mill load, the shell, the charge and the material, at the low speed. The trunnion bearing design includes the babbitt-lined shells for the plain bearings, the oil bath lubrication and the temperature monitoring, and the trunnion bearings are the critical bearings of the ball mill: the bearing failure stops the mill, and the bearing replacement is the major operation. The drive bearings include the pinion bearings, the gearbox bearings and the motor bearings: the pinion bearings carry the gear loads, the gearbox bearings carry the gear stages, and the motor bearings carry the motor rotor, and the drive bearings are the standard rolling element bearings with the oil or the grease lubrication.

The vertical roller mill bearings include the table bearing, the roller bearings and the internal drive bearings: the grinding table is supported by the thrust or the combination bearings that carry the table and the grinding loads, the grinding rollers are supported by the roller bearings in the roller assemblies that carry the grinding forces and the pressure, and the internal drive carries the table rotation. The vertical mill bearings operate in the dusty and the hot environment, with the lubrication systems that protect the bearings from the contamination, and the roller bearing failures are among the critical problems of the vertical mills. The roller press bearings include the roll bearings, the radial and the axial bearings, that carry the high pressing forces and the roll loads: the roll bearings are the large spherical roller bearings or the cylindrical roller bearings with the heavy-duty designs, and the roll bearing failures are the critical problems of the roller presses.

The bearing types are summarized in the table below, which is the reference of the bearing applications in the grinding mills:

Application	Bearing type	Load and speed	Lubrication	Criticality
Ball mill trunnions	Plain babbitt or large rolling bearings	Very heavy, low speed	Oil bath or circulating oil	Critical: stops the mill
Pinion and gearbox	Spherical and cylindrical roller bearings	Heavy, medium speed	Oil splash or circulating oil	High: drive failures
Motor	Cylindrical and deep groove bearings	Moderate, high speed	Grease or oil	High: motor failures
Vertical mill rollers	Spherical roller bearings	Heavy, low-medium speed	Circulating oil, filtered	Critical: roller failures
Vertical mill table	Thrust and combination bearings	Very heavy, low speed	Circulating oil, filtered	Critical: table failures
Roller press rolls	Spherical or cylindrical roller bearings	Very heavy, low speed	Circulating oil or grease	Critical: roll failures

The Bearing Selection and the Mounting

The bearing selection is the first stage of the bearing life: the bearing is selected by the load, the speed, the alignment, the environment and the life requirements, and the correct selection is the condition of the long life. The bearing load calculation determines the equivalent dynamic load from the radial and the axial loads, the load factors and the application, and the bearing life is calculated with the standard life equations, the basic rating life and the adjusted life with the reliability, the lubrication and the contamination factors. The bearing selection also considers the speed capability, the temperature capability, the mounting and the dismounting, the sealing and the lubrication compatibility, and the selection is verified by the bearing manufacturer’s calculation service or the selection software.

The bearing mounting is the critical installation stage: the bearing is mounted with the correct fits, the correct tolerances and the correct procedures, and the mounting quality decides the bearing life. The shaft and the housing fits are selected by the load and the rotation conditions: the rotating ring is fitted with the interference fit, the stationary ring with the transition or the clearance fit, and the fits are defined by the tolerance classes. The bearing mounting methods include the cold mounting with the press, the hot mounting with the induction heater, and the hydraulic mounting with the oil injection, and the mounting procedures include the cleanliness, the lubrication of the mounting surfaces and the verification of the seating. The bearing clearance and the preload are set by the application: the normal clearance for the standard applications, the larger clearance for the high temperatures and the preload for the special requirements, and the clearance is measured and verified at the mounting.

The bearing mounting also includes the sealing and the protection: the bearing is sealed against the contamination with the lip seals, the labyrinth seals and the combination seals, and the sealing is the critical protection of the bearing: the contamination is the main cause of the bearing failures, and the sealing quality decides the contamination ingress. The bearing housing design includes the lubrication provisions, the oil inlets and outlets, the vents, the level and the temperature monitoring points, and the housing is the interface of the bearing and the machine, which the design and the maintenance manage together.

The bearing alignment is the condition of the correct load distribution: the shaft and the housing alignments, the coupling alignment and the gear alignment are set within the defined tolerances, and the misalignment causes the edge loading, the uneven load distribution and the premature failures. The alignment is measured and corrected at the mounting and verified periodically, and the alignment program is part of the bearing management.

The Bearing Loads and the Bearing Life

The bearing life is determined by the loads, the speed, the lubrication and the contamination, and the understanding of the life calculation and the life influencers is the foundation of the bearing management. The bearing life is calculated with the ISO standard: the basic rating life is the life in millions of revolutions that 90 percent of the bearings achieve, and the adjusted life accounts for the reliability, the material, the lubrication and the contamination factors, and the calculated life is the design life of the bearing.

The loads of the mill bearings include the static loads, the weight of the components, the grinding loads and the process loads, and the dynamic loads, the imbalance, the impacts and the vibrations, and the load calculation must account for the real operating conditions, the peak loads and the load cycles. The speed of the mill bearings is the low to the medium speed for the main bearings and the higher speed for the drive bearings, and the speed and the load define the bearing operating conditions, the lubrication requirements and the temperature behavior. The lubrication conditions are the dominant life factor: the adequate oil film separates the rolling elements from the raceways, the film thickness depends on the viscosity, the speed, the load and the temperature, and the lubrication conditions are expressed by the lambda ratio, the ratio of the film thickness to the surface roughness, and the lambda ratio above the critical value indicates the full film lubrication and the long life, while the lambda ratio below the critical value indicates the boundary lubrication and the wear.

The contamination conditions are the second dominant life factor: the particles in the lubricant damage the surfaces, the damage accelerates the wear and the fatigue, and the contamination level is expressed by the ISO cleanliness codes, which the lubrication system controls with the filtration. The temperature conditions affect the life: the elevated temperatures degrade the lubricant, reduce the film thickness and accelerate the fatigue, and the temperature control, the cooling and the heat management, is part of the bearing life management. The bearing life management is the management of the load, the speed, the lubrication, the contamination and the temperature, and the professional plant manages the factors with the monitoring and the maintenance, which the article covers in the following sections.

The Lubrication Principles of the Mill Bearings

The lubrication is the protection of the bearing: the lubricant separates the surfaces, reduces the friction and the wear, carries the heat, protects against the corrosion and flushes the contaminants, and the lubrication principles define the lubricant selection and the system design. The lubrication regimes are the hydrodynamic, the elastohydrodynamic, the mixed and the boundary regimes: the hydrodynamic regime, the full film with the separation of the plain bearing surfaces, the elastohydrodynamic regime, the full film with the elastic deformation of the rolling contacts, the mixed regime, the partial film with the surface contact, and the boundary regime, the thin film with the surface protection, and the lubrication design aims at the full film regimes for the long life.

The film formation depends on the viscosity, the speed, the load and the geometry: the higher viscosity forms the thicker film at the lower speeds and the higher loads, and the lower viscosity forms the adequate film at the higher speeds and the lower loads, and the viscosity selection is the balance of the film formation and the friction and the heat: the too-high viscosity increases the friction and the temperature, and the too-low viscosity reduces the film and the protection. The viscosity is selected by the bearing type, the load, the speed and the temperature, with the viscosity-grade charts and the calculation methods, and the operating viscosity is the key parameter: the viscosity at the operating temperature is compared with the required minimum viscosity for the bearing, and the selection ensures the operating viscosity above the minimum.

The additives of the lubricants provide the additional protection: the anti-wear additives protect the surfaces in the boundary regimes, the extreme pressure additives protect against the scuffing and the welding under the high pressures, the anti-oxidants protect the oil from the oxidation, the anti-rust and the anti-corrosion additives protect the surfaces, the anti-foam additives control the foam, and the pour point depressants and the viscosity index improvers adjust the low and the high temperature behavior. The additive package is selected for the application, and the lubricant specification defines the base oil, the viscosity grade and the additives.

The lubrication regimes and the film conditions are summarized in the reference table below, which is used for the lubrication diagnosis:

Regime	Film condition	Occurs when	Consequence	Prevention
Hydrodynamic	Full film, surfaces separated	Plain bearings, adequate viscosity and speed	Low wear, low friction	Correct viscosity, supply
Elastohydrodynamic	Full film, elastic contacts	Rolling bearings, adequate lambda ratio	Long life, low wear	Lambda ratio above 1.5–3
Mixed	Partial film, some contact	Start-up, load peaks, viscosity shortage	Wear, surface damage	Viscosity, additives, start-up practice
Boundary	Thin film, surface protection by additives	Very low speed, high load, starved supply	Rapid wear, scuffing	Anti-wear additives, supply assurance

The Oil Systems of the Mill Bearings

The oil systems deliver the oil to the bearings and manage the oil condition: the oil bath systems, the splash systems, the circulating systems and the injection systems serve the different bearing applications, and the system design and the maintenance are the part of the bearing management. The oil bath system is the simplest: the bearing housing contains the oil bath, the bearing dips into the oil, and the oil is changed periodically, and the oil bath is used for the moderate applications, the pinion bearings and the gearboxes. The splash system lubricates by the splashing from the rotating elements: the gears splash the oil, the bearings are lubricated by the splash, and the splash system is used for the gearboxes and the moderate applications.

The circulating oil system is the professional system of the critical bearings: the oil is pumped from the tank, filtered, cooled and distributed to the bearings, the oil returns to the tank, and the system maintains the oil level, the temperature and the cleanliness. The circulating system components are the pump, the filters, the coolers, the valves, the distribution lines, the return lines, the tank and the instrumentation, and the system design includes the redundancy for the critical mills, the standby pumps and the emergency supply. The circulating system is used for the trunnion bearings, the vertical mill bearings, the roller press bearings and the gearboxes, and the system is the interface of the bearing and the maintenance: the system condition, the oil condition, the filtration and the cooling, is managed by the maintenance program.

The oil injection and the mist systems serve the special applications: the oil injection lubricates the bearings at the defined intervals with the metered quantities, and the oil mist lubricates with the atomized oil in the air stream, and the systems are used for the moderate and the difficult applications. The oil systems are designed with the monitoring: the oil temperature, the oil pressure, the oil flow, the oil level and the oil quality are monitored, with the alarms and the interlocks, and the monitoring is connected to the control room, and the system failures, the pump failures, the filter blockages and the oil loss, are detected and responded to.

The oil management is the continuous practice: the oil is sampled and analyzed on the defined cycles, the oil condition is assessed, the contamination, the degradation and the wear metals, and the oil is changed or treated on the evidence. The oil management includes the oil storage and the handling: the oils are stored cleanly, the containers are identified, the oil transfer is clean, and the wrong oil mixing is prevented, and the oil management is the discipline that the lubrication program institutionalizes.

The Oil Selection and the Oil Analysis

The oil selection defines the oil grade for the application: the viscosity grade, the base oil type and the additive package are selected by the bearing type, the load, the speed, the temperature and the manufacturer’s recommendations, and the selection is documented in the lubrication plan of the plant. The viscosity grade is selected with the calculation: the required viscosity at the operating temperature is determined from the bearing parameters, the viscosity grade is chosen to provide the operating viscosity above the minimum, and the selection is verified with the oil supplier’s recommendations. The base oil types include the mineral oils, the synthetic oils, the PAO and the ester oils, and the synthetic oils offer the wider temperature range, the longer life and the better performance, with the higher cost, and the selection balances the performance and the cost.

The additive package is selected for the application: the anti-wear and the extreme pressure additives for the loaded bearings, the anti-oxidants for the high temperatures, the rust and the corrosion inhibitors for the moisture, and the anti-foam and the demulsifiers for the systems with the water, and the oil specification includes the additive package. The oil selection is reviewed periodically: the oil technology advances, the operating conditions change and the failure evidence accumulates, and the oil selection is reviewed with the evidence, the oil analysis and the supplier support.

The oil analysis is the condition monitoring of the oil and the bearing: the oil samples are taken on the defined cycles, the samples are analyzed in the laboratory, and the results are interpreted for the oil condition and the machine condition. The oil analysis parameters include the viscosity, the acid number, the water content, the particle counts, the wear metals, the additive depletion and the contamination, and the trends of the parameters are the diagnostic data: the viscosity change indicates the oxidation, the contamination or the mixing, the acid number rise indicates the oxidation, the water indicates the ingress, the particle counts indicate the contamination, and the wear metals, the iron, the copper, the lead and the tin, indicate the wear of the bearing components. The oil analysis is performed on the defined schedule, and the results are reviewed with the action limits: the normal results continue the schedule, the warning results trigger the repeat sampling and the investigation, and the alarm results trigger the oil change and the bearing inspection. The oil analysis is the professional tool of the bearing management, and the plant that analyzes the oil regularly detects the bearing problems before the failures, which is the practice that the article recommends.

The Grease Lubrication of the Mill Bearings

The grease lubrication serves the bearings where the oil systems are not practical: the motor bearings, the auxiliary bearings and the moderate applications use the grease, and the grease selection, the greasing practice and the grease management are the part of the lubrication program. The grease is the oil with the thickener: the thickener, the lithium, the polyurea, the calcium and the complex soaps, holds the oil, and the grease properties, the consistency, the dropping point, the base oil viscosity and the additives, are selected for the application. The grease selection considers the speed, the load, the temperature, the sealing and the relubrication interval, and the selected grease is specified by the NLGI consistency grade and the base oil properties.

The greasing practice is the disciplined application: the bearings are relubricated on the defined intervals with the defined quantities, the fresh grease flushes the old grease and the contaminants, and the excess grease is avoided, because the excess grease raises the temperature and damages the seals. The relubrication intervals are defined by the bearing size, the speed, the temperature and the grease type, and the intervals are adjusted on the evidence, the temperature and the grease condition. The grease management includes the storage, the handling and the identification: the greases are stored cleanly and identified, the grease guns and the fittings are clean, and the wrong grease mixing is prevented, because the incompatible thickeners degrade the grease.

The grease lubrication of the mill auxiliaries, the motors and the small bearings, is the routine maintenance, and the professional practice defines the greasing plan, the intervals, the quantities and the responsibilities, and it documents the greasing records. The grease problems, the hardening, the separation, the contamination and the mixing, are detected by the inspection and the analysis, and the corrections include the purging, the regreasing and the grease change, which completes the grease management.

The Bearing Monitoring: Temperature, Vibration and Condition

The bearing monitoring is the early detection of the bearing problems: the temperature, the vibration and the oil condition are monitored, the trends are reviewed and the alarms are configured, and the monitoring data is the basis of the bearing management. The temperature monitoring is the basic monitoring: the bearing temperatures are measured with the sensors, the normal temperatures are defined, and the rising temperatures indicate the lubrication problems, the overload, the misalignment or the bearing damage, and the temperature alarms trigger the response.

The vibration monitoring is the professional monitoring of the rolling element bearings: the vibration is measured at the bearing housings in the three directions, the vibration spectra are analyzed, and the bearing defect frequencies, the outer race, the inner race, the rolling element and the cage frequencies, are identified. The bearing defect frequencies appear in the spectra with the characteristic patterns, and the rising amplitudes at the defect frequencies indicate the developing damage, the pitting, the spalling and the cracks, and the vibration monitoring detects the damage at the early stages, before the failure. The vibration analysis is performed with the portable instruments on the defined cycles or with the online systems continuously, and the vibration data is trended and interpreted with the envelope analysis for the early detection.

The oil analysis completes the monitoring: the wear metals in the oil indicate the bearing wear, the particle counts indicate the contamination and the oil degradation indicates the lubrication problems, and the oil analysis is the third monitoring dimension, together with the temperature and the vibration. The condition monitoring program combines the three dimensions: the temperature, the vibration and the oil analysis, with the defined frequencies and the action limits, and the monitoring data is reviewed by the condition monitoring specialist, who issues the recommendations, the monitoring, the lubrication correction, the inspection or the bearing replacement. The condition monitoring is the foundation of the predictive maintenance, and the mill bearing program that monitors the temperature, the vibration and the oil is the program that replaces the bearings on the condition, not on the failure, which is the professional practice.

The Bearing Failures and Their Causes

The bearing failures are the end of the bearing life, and the failure analysis identifies the causes and directs the corrections: the failures are classified by the appearance, and the appearance reveals the cause. The main failure modes are the fatigue, the wear, the contamination damage, the lubrication failures, the misalignment damage, the electrical damage, the corrosion and the incorrect mounting damage, and each mode has its characteristic appearance and its causes.

The fatigue failures are the spalling and the flaking of the surfaces: the fatigue cracks initiate below the surface under the repeated contact stresses, the cracks propagate and the surface flakes, and the fatigue is accelerated by the high loads, the inadequate lubrication, the contamination and the misalignment. The fatigue appearance, the spalled areas, is the evidence of the overload or the life exhaustion, and the correction includes the load, the lubrication and the contamination management. The wear failures are the gradual loss of the surface material: the abrasive wear by the contamination, the adhesive wear by the boundary lubrication, and the correction includes the contamination control and the lubrication improvement.

The contamination damage is the surface damage by the particles: the particles indent the surfaces, the indentations create the stress concentrations and the premature fatigue, and the contamination damage is the most common bearing failure cause. The contamination enters through the seals, the oil and the mounting, and the correction includes the sealing improvement, the filtration and the clean handling. The lubrication failures are the failures from the inadequate lubrication: the starved lubrication, the wrong oil, the degraded oil and the blocked supply cause the overheating, the wear and the seizure, and the correction includes the lubrication system maintenance and the oil management. The misalignment damage is the edge loading damage: the misaligned rings and the shafts load the bearing edges, the edges wear and fail, and the correction includes the alignment and the mounting corrections.

The electrical damage is the fluting and the pitting by the electrical currents: the stray currents pass through the bearings, the currents cause the electrical discharge machining of the surfaces, and the correction includes the current insulation, the grounding and the shaft grounding brushes. The corrosion damage is the surface corrosion by the water and the acids: the water enters with the oil and the condensation, the corrosion attacks the surfaces, and the correction includes the water control and the corrosion protection. The incorrect mounting damage is the damage from the mounting errors: the force applied through the rolling elements, the wrong fits, the wrong clearances and the wrong preload cause the damage, and the correction includes the mounting procedures and the training. The failure analysis is the evidence-based investigation: the failed bearing is examined, the appearance is classified, the operating data is reviewed and the cause is confirmed, and the corrections are implemented and verified, which is the professional practice of the bearing management.

The Maintenance and the Troubleshooting of the Mill Bearings

The maintenance of the mill bearings includes the routine servicing, the inspections, the lubrication tasks and the planned replacements, and the maintenance is planned with the mill stops and managed with the records. The routine servicing includes the lubrication tasks, the greasing, the oil checks, the filter and the breather maintenance, the seal checks and the visual inspections, and the routine servicing is the daily and the weekly work of the maintenance team. The inspections include the periodic bearing inspections at the stops: the bearing is inspected for the wear, the clearances are measured, the lubrication condition is assessed and the bearing condition is graded, and the inspection results are recorded in the bearing register.

The planned replacement is the replacement of the bearings on the condition or the life data: the bearings are replaced when the monitoring indicates the damage, when the life data indicates the exhaustion or when the inspection grades the bearing for the replacement, and the replacement is planned with the spare parts, the tools and the procedures. The bearing replacement is the critical operation: the bearing is removed with the correct tools, the induction heater for the dismounting, the new bearing is mounted with the correct procedures and the fits are verified, and the replacement is documented with the new bearing data and the mounting records.

The troubleshooting of the bearing problems follows the systematic method: the symptom, the temperature, the vibration, the noise and the oil data, is defined, the data is collected, the possible causes are listed and the evidence is evaluated, and the cause is confirmed and corrected. The common troubleshooting cases are the overheating, the vibration, the noise, the oil contamination and the premature failures: the overheating is diagnosed by the temperature, the load and the lubrication data, the vibration by the spectral analysis, the noise by the listening and the inspection, the oil contamination by the analysis and the filter inspection, and the premature failures by the failure analysis, and each case follows the defined diagnostic path. The troubleshooting is documented, and the solutions are shared with the maintenance team, which builds the knowledge base of the bearing management.

Frequently Asked Questions

What is the most common cause of the mill bearing failures?

The contamination damage is the most common cause: the particles enter the bearing through the seals, the oil and the mounting, the particles indent the surfaces and cause the premature fatigue, and the contamination control, the sealing, the filtration and the clean handling, is the main prevention.

How is the bearing life calculated?

The bearing life is calculated with the ISO standard life equations: the basic rating life from the load and the speed, adjusted for the reliability, the material, the lubrication and the contamination factors, and the adjusted life is the design life that the application requires.

Why is the operating viscosity of the oil critical for the bearings?

Because the operating viscosity determines the oil film thickness: the film thickness above the surface roughness, the lambda ratio above the critical value, gives the full film lubrication and the long life, and the too-low operating viscosity causes the boundary lubrication and the wear.

What does the oil analysis detect in the mill bearing oil?

The viscosity, the acid number, the water content, the particle counts, the wear metals and the additive depletion, and the trends indicate the oil condition and the machine condition: the wear metals indicate the bearing wear, the particles the contamination, and the viscosity and the acid the degradation.

How often should the mill bearings be relubricated with grease?

The relubrication intervals are defined by the bearing size, the speed, the temperature and the grease type, typically from weeks to months for the mill auxiliaries, and the intervals are adjusted on the evidence, the temperature, the grease condition and the manufacturer’s recommendations.

What is the vibration signature of the bearing damage?

The vibration spectra show the bearing defect frequencies, the outer race, the inner race, the rolling element and the cage frequencies, with the characteristic sidebands and the harmonics, and the rising amplitudes at the defect frequencies indicate the developing damage.

Summary and Final Recommendations

This article has covered the mill bearings and the lubrication in the professional depth: the bearing types and their applications, the selection and the mounting, the loads and the life, the lubrication principles, the oil systems, the oil selection and the analysis, the grease lubrication, the bearing monitoring, the failures and the causes, and the maintenance and the troubleshooting. The article has provided the reference tables of the bearing applications and the lubrication regimes, and it has given the reader the complete practical knowledge of the mill bearing management. The recommendations for the plant are these: select the bearings professionally, with the load and the life calculations, the correct fits and the correct mounting, because the bearing life starts with the selection and the mounting; lubricate correctly, with the correct oils and the greases, the correct systems and the correct intervals, because the lubrication is the protection of the bearing; control the contamination, with the sealing, the filtration and the clean handling, because the contamination is the main killer of the bearings; monitor the bearings, the temperature, the vibration and the oil, with the defined frequencies and the action limits, because the monitoring is the early detection; analyze the failures, identify the causes and correct them, because the failure analysis is the improvement engine; and plan the maintenance, the inspections, the lubrication tasks and the replacements, with the records and the training, because the planned maintenance is the reliable maintenance. The mill bearings are the silent carriers of the grinding loads, and the lubrication is their protection, and the professional bearing and the lubrication management that this article describes keeps the mills running, the bearings living their full life and the grinding availability at the design, which is the foundation of the plant reliability and the profitability.

This file is part of the Complete Cement Technical Package (931 files) available from cementequipment.org. Respective rights holders; library copy for the licensed single user.