Mechanical Elements Of Tube Mills: Complete Guide

The mechanical elements of tube mills are the steel anatomy of the largest rotating machine of the cement plant: the shell that carries the charge, the bearings that carry the shell, the liners that protect the steel, the diaphragms that separate the compartments, the grinding media that do the work and the drive that turns the whole assembly: each element is engineered for the decades of service and each fails in its own characteristic way: the mill mechanic who knows the elements knows the mill.

The Complete Cement Technical Package (931 files including the books, the courses, the Excel tools and the presentations: $249.99 one-time: instant download via the PayPal payment) includes this mechanical elements guide with the design tables, the materials specifications and the maintenance checklists: this article walks the file: every major element of the mill from the feed end to the discharge, its function, its materials and its failure modes: the reader closes the page with the complete mechanical anatomy of the tube mill.

The tube mill is deceptively simple in appearance: a steel cylinder that turns on its bearings: the engineering inside that cylinder is the accumulated knowledge of a century: this page is organized so the reader meets the static parts first (the shell, the liners, the diaphragms), the moving parts second (the bearings, the media, the drive) and the maintenance last: the file itself follows the same order: the breakdowns of the plant are usually the breakdowns of the elements, and the element knowledge prevents them.

1. The Mill Shell: The Pressure Vessel of the Grinding Charge

The shell is the cylindrical body of the mill, the tube that carries the material and the media through all the grinding duty:

• The construction: the rolled steel plates of the structural low-carbon steel, welded into the ring sections and joined with the longitudinal and the circumferential welds: the shell thickness of the large cement mills sits between 25 and 60 millimeters depending on the diameter and the charge weight: the plates are stress-relieved before the machining;
• The heads: the cast or the welded end plates that close the shell and carry the trunnions: the hollow trunnions through which the material enters and leaves: the heads of the modern mills are welded to the shell with the full penetration welds, the classical mills used the bolted flanges;
• The stresses of the shell: the shell bends under its own weight and the charge weight between the supports, and the bending stress cycles with every revolution: the fatigue of the bending is the lifetime enemy of the shell: the maximum bending stress sits at the center of the span, and the design keeps it well below the endurance limit of the steel;
• The fatigue history: the stress cycles of the order of 10 million revolutions per year accumulate until the crack initiation: the cracks of the shell start at the weld toes, the manhole corners and the repairing zones: the shell inspection with the ultrasonic and the dye penetrant methods discovers the cracks while they are still small;

The shell must also resist the local loads of the liners, the wear of the internal surface and the temperature gradients of the grinding: its design margins decide the decades of the service: the file documents the shell design calculations, the weld procedures and the shell inspection program of the mills: the shell is the element that can never be replaced cheaply, so the plant protects it religiously.

2. The Bearings: The Trunnion and the Slide Shoe Supports of the Mill

The mill rotates on its bearings, and the two bearing families of the industry carry the huge weights with the different philosophies:

• The trunnion bearings: the classical supports of the mill heads: the white-metal lined bearings that sit under the trunnion journals of the hollow shafts: the trunnion diameter of the large mills reaches 1000 to 1800 millimeters and the total load of the mill on its two bearings reaches several hundred tons: the bearing pressure is moderate and the white metal runs on the hydrodynamic oil film;
• The slide shoe bearings: the modern supports of the large mills: the mill shell runs directly on the bearing shoes without the trunnion heads: the discharge end remains free, simplifying the civil construction and the maintenance: the slide shoe mills of the two-compartment and the single-compartment designs carry the same huge loads on the hydrostatic oil films;
• The hydrodynamic lubrication: the oil wedge that forms between the journal and the bearing surface at the running speed: the oil film thickness of the order of 0.05 to 0.1 millimeters separates the metal surfaces completely: the hydrodynamic regime is the design condition of the white metal bearings: the start and the stop pass through the boundary regime where the wear risk is highest;
• The hydrostatic lubrication: the high-pressure oil injected into the bearing pockets before and during the rotation: the oil lifts the journal from the bearing surface at the standstill: the standard feature of the slide shoe and the large trunnion bearings: the hydrostatic jacking avoids the scoring of the surfaces at every start;

The bearing temperatures are the daily telemetry of the element condition: the normal bearing runs at the oil temperature below 60 °C, and the alarm limits at 65 to 70 °C guard the white metal: the abrupt temperature rise announces the oil film failure minutes before the damage: the file documents the bearing types, the oil film calculations, the jacking systems and the temperature monitoring of the mill bearings.

3. The Mill Liners: The Armor of the Shell and the Lifters of the Charge

The liners are the replaceable armor plates bolted to the inside of the shell: the shell never meets the material directly:

• The protection function: the liners absorb the impact and the abrasion of the media and the material: the shell thickness is preserved for the decades while the liners are replaced several times: the liner thickness of 50 to 100 millimeters provides the sacrificial layer of the steel;
• The lifting function: the profile of the liner controls the motion of the charge: the lifting liners with the high wave profiles carry the media up the shell wall for the cataracting impact; the classifying liners with the declining wave heights grade the media along the mill length: the profile of the liner is a grinding parameter, not only a protection plate;
• The materials: the high-chromium white iron for the first compartments and the impact duty: the alloyed chrome-molybdenum steel for the second compartments: the rubber liners for the fine grinding compartments with the low impact and the superior wear life: the composite and the magnetic liners of the modern designs: the metallurgy of the liner is matched to the duty of its compartment;
• The lifting angle: the angle of the liner wave face directs the charge trajectory: the liner design adjusts the charging behavior, the power draw and the grinding efficiency: the worn liners lose the lift angle and the mill performance drifts down until the relining: the liner condition is one of the first checks of the mill performance investigation;

The liner is the element that the plant replaces most frequently: the complete relining of a large mill is a major campaign with the heavy lifting and the working time of weeks: the material choice and the profile selection decide both the performance and the campaign life: the file of the package documents the liner profiles, the materials tables and the relining procedures with the bolt torque, the lifting equipment and the spare parts management.

4. The Diaphragms: The Internal Screens of the Compartments

The diaphragms are the perforated partitions inside the mill that separate the grinding compartments and control the material flow:

• The intermediate diaphragm: the partition between the first (coarse) and the second (fine) compartments: the grate plates with the tapered slots of 8 to 12 millimeters that pass the material and hold back the media: the diaphragm keeps the large balls of the first compartment from entering the fine compartment: the material level control of the compartments: the slots prevent the overloading and the back-flow;
• The discharge (outlet) diaphragm: the grate at the discharge end that retains the final compartment media: the radial or the concentric slot patterns with the lifters that sweep the product into the discharge chamber: the outlet diaphragm defines the material level of the last compartment and the retention time of the mill;
• The materials and the wear: the wear-resistant alloy castings for the grate plates and the hub segments: the slot wear widens the passages and disturbs the material hold-up, the tapered slot design keeps the passage dimensions stable through the wear life: the diaphragm segments are replaceable from the manhole without the full relining;
• The ventilation role: the diaphragm open area is the gas passage of the mill ventilation: the air sweeps through the slots and carries the fines and the moisture: the clogged diaphragms strangle the ventilation and overheat the mill: the slot blockage inspection is part of the maintenance routine;

The diaphragms are the throttles of the material flow inside the mill: the worn or the clogged diaphragm changes the retention time, the power draw and the product quality without any visible change outside the mill: the file explains the diaphragm designs, the slot geometries, the wear patterns and the maintenance of the partitions: the internal condition of the mill is readable through the diaphragm state.

5. The Grinding Media: The Balls and the Rods that Do the Work

The grinding media are the actual grinding machines inside the mill: the cascading and the cataracting balls that break the material:

• The ball grades: the forged and the cast steel balls of 25 to 100 millimeters in the coarse compartments and the smaller media (15 to 40 millimeters) in the fine compartments: the graded charge fills about 28 to 35 percent of the mill volume: the ball charge weight of the large finish mill reaches 250 tons and more;
• The charge gradation: the graded distribution of the ball sizes along the compartments: the large balls break the coarse feed in the first compartment and the small balls finish the fine grinding in the second: the ideal grading follows the falling curves of the charge theory: the wrong grading wastes the energy and fails to reach the fineness;
• The media wear: the abrasion of the balls during the service: the media consumption of the cement mills runs between 300 and 1000 grams per ton of the product depending on the hardness of the clinker and the ball quality: the wear rate is logged per charge and per period, feeding the replenishment schedule and the quality evaluation of the suppliers;
• The charge monitoring: the mill power draw indicates the charge level: the standard practice of the media level measurement with the charge level probe and the power curves: the periodic sampler of the charge and the weight check of the balls upon the relining: the charge management is the daily economy of the grinding department;

The media are the replaceable currency of the mill: the correct gradation and the adequate level are the two knobs of the mechanical grinding efficiency: the file of the package carries the ball grading tables, the charge calculation methods, the wear accounting sheets and the charge level measurement procedures that the mill departments use month after month.

6. The Girth Gear and the Pinion: The Torque Transmission of the Mill

The rotation of the shell is delivered through the rim gear and the pinion of the peripheral drive, the classical transmission of the tube mills:

• The girth gear: the split ring gear bolted to the shell flange near the center of the mill: the gear tooth count runs from 200 to 400 teeth in the large mills: the pitch circle diameter reaches 7 to 9 meters: the gear is split into two or four segments for the handling and the replacement;
• The pinion shaft: the small gear driven by the motor or the gearbox that meshes with the girth gear: the spring-mounted pinion carrier compensates the shell ovality and the gear run-out: the pinion tooth of the case-hardened steel transfers the full motor torque to the girth gear;
• The tooth contact: the alignment of the gear axes, the backlash and the contact pattern determine the tooth load distribution: the tooth contact inspection with the blueing method is the classical adjustment procedure: the misaligned gears run noisily, heat up and fail with the pitting and the tooth breakage;
• The lubrication of the open gears: the open gear spray lubrication with the heavy gear oils sprayed onto the gear meshing zone: the lubricant film protects the tooth flanks against the pitting and the wear: the spray nozzles, the oil quality and the dosage are the maintenance elements of the drive;
• The wear and the failure: the pitting of the flanks, the tooth bending fatigue and the wear of the profile: the inspection with the tooth profile measurements and the crack detection: the spare girth gears are ordered years ahead because the delivery of the large castings takes the long lead times;

The girth gear drive is the visible heart of the mill transmission: its health is audible in the gear mesh sound and readable in the vibration spectrum of the pinion bearings: the file documents the gear geometry, the alignment procedures, the lubrication schedule and the failure analysis of the rim gear drives: the classical drive remains the standard of thousands of mills worldwide.

7. The Mill Drives: The Central, the Peripheral and the Dual Systems

The mill drive is the complete power train from the motor to the shell, and its architecture defines the mechanical character of the mill:

• The central drive: the motor connected through the gearbox to the hollow trunnion of the feed end: the mill turns from its axis: the classical drive of the medium mills with the girthless construction: the central gearbox carries the full torque of the mill in the compact train: the alignment of the gearbox to the mill axis is the critical mechanical discipline;
• The peripheral drive: the motor and the pinion outside the girth gear: the mill turns from its girth: the classical arrangement described in the previous section: the standard of the large mills because the girth gear and the pinion replace the enormous central gearboxes of the biggest machines;
• The dual and the multiple drives: the large mills driven by the two pinions on the opposite sides of the girth gear: the torque split between the pinion drives: the synchronization of the drives with the control systems: the drive redundancy of the big finish mills: the two motors run the mill at the half torque each and the plant survives the single motor failure at the reduced capacity;
• The gearless drives: the wrap-around ring motor with the rotor built into the mill shell: the shell itself is the motor rotor: the gearless drive of the giant mills and the modern large installations: the ultimate drive architecture with the precise speed control and the elimination of the gear train: the technology of the biggest tube mills of the industry;
• The motor family: the synchronous motors with the fixed speeds of 150 or 167 rpm, the wound-rotor motors and the modern variable speed drives: the motor starting with the reduced voltage, the couplings and the brakes of the drive trains: the power factor and the efficiency of the drive complete the electrical picture of the mechanical element;

The drive selection follows the mill size and the plant preference: the peripheral drives dominate the classical fleet, the central drives the medium mills and the gearless motors the largest machines: the file compares the drive families with their efficiencies, their alignments and their maintenance profiles: the drive train of the mill is the biggest mechanical investment after the shell itself.

8. The Feed and the Discharge Elements: The Material Gates of the Mill

The material enters and leaves the mill through the mechanical elements of the feed and the discharge ends:

• The feed spout: the cast steel chute that delivers the material into the feed trunnion: the feed chute with the abrasion-resistant linings passes into the rotating hollow shaft: the feed spout of the mill is also the inlet of the mill ventilation: the seal between the rotating and the static parts is the dust gate of the mill inlet;
• The feed tube and the lifter spirals: the internal spiral vanes inside the feed trunnion that push the material into the first compartment: the gravitational fall of the feed must overcome the rotation of the shell: the spirals are the mechanical solution of the continuous feeding;
• The discharge arrangement: the hollow outlet trunnion with the discharge chamber and the return spiral: the overflow discharge larries and the grates of the different mill types: the material exits through the discharge trunnion into the product duct or the classifier feed;
• The seals and the distortion: the flexible seals of the feed and the discharge spouts that follow the shell movement without the dust leakage: the kiln-like spring seals of the big mills: the seal replacement is the small maintenance item with the large environmental effect: the dust escaping at the mill ends is the visible failure of the sealing elements;

The feed and the discharge elements are the gates of the mill: their wear and their clogging disturb the material flow and the mill levels: the file documents the feed spout geometries, the seals, the spirals and the discharge arrangements of the classical and the modern mills: the ends of the mill carry the same engineering attention as its middle.

9. The Lubrication Systems: The Oil Circuits of the Mill

The lubrication of the mill elements is organized in the separate oil circuits that protect each critical component:

Oil circuit	Component protected	Typical oil
The bearing oil circuit	The trunnion or the slide shoe bearings	The ISO VG 320–460 mineral and synthetic oils
The gear spray circuit	The girth gear and the pinion	The heavy open gear oils with the tackifiers
The gearbox circuit	The central drives and the reducers	The EP gear oils of the factory specification
The hydraulic circuits	The hydrostatic jacking and the auxiliary motions	The hydraulic oils ISO VG 46 or 68

The bearing oil circuits run continuously with the main oil pumps and the standby pumps, the oil coolers and the filters: the oil pressure and the temperature are interlocked with the mill drive: the mill cannot start without the confirmed oil flow: the oil analysis program of the monthly samples measures the wear metals, the viscosity and the contamination, converting the oil itself into the diagnostic sensor of the machine: the oil of the mill tells the condition of the bearings and the gears before the vibration does.

10. The Ventilation and the Cooling Elements: The Air Path of the Mill

The mill elements also include the ventilation and the cooling systems that carry the heat and the moisture away:

• The mill ventilation: the airflow through the mill that removes the fine particles, the moisture and the heat: the air enters with the feed and exits through the discharge into the dust collector: the air velocity of the closed circuit mills runs at 1 to 2 meters per second through the shell: the ventilation rate is an operating parameter of the grinding;
• The cooling elements: the water spray systems inside the mill shell and the cooling of the heat exchange: the finish mills inject the fine water mist into the second compartment to control the cement temperature: the jacket cooling of the shell in the special designs: the temperature control of the mill protects the gypsum from the dehydration and the media from the excessive wear;
• The temperature effects on the elements: the material temperature above 110 to 120 °C starts the gypsum dehydration and the quality loss: the bearing temperatures rise with the shell heat: the ventilation and the water sprays are the mechanical countermeasures of the thermal management: the mill temperature instrumentation closes the control loop;

The air and the water paths are as mechanical as the steel elements: the fans, the ducts, the spray nozzles and the control valves are maintained with the same discipline: the file covers the ventilation calculations, the spray systems and the temperature control practice of the finish mills: the heat management of the mill protects the product and the machinery at once.

11. The Alignment, the Vibration and the Condition Monitoring of the Mill

The health of the mechanical elements is monitored with the precision measurement program of the modern plants:

• The shell ovality measurement: the out-of-roundness of the shell cross-sections measured at the bearings and the center: the ovality of the changed cross-sections indicates the structural weakening: the periodic measurement of the mill shell follows the deformation history of the element;
• The bearing vibration: the vibration measurements on the bearing housings: the velocity and the acceleration spectra: the gear mesh frequencies of the pinion, the bearing defect frequencies and the imbalance components: the vibration baseline of the mill and its alarm thresholds give the early warning of the element failures;
• The gear and the alignment checks: the pinion-bearing alignment, the gear contact pattern and the axial float of the mill: the thermal growth corrections of the alignment: the precision measurements of the rotating machine follow the maintenance calendar of the mill;
• The oil analysis and the wear particle analysis: the monthly oil samples and the ferrography of the wear particles: the metal traces of the bearings, the gears and the shell bolts in the oil: the condition monitoring combines the oil and the vibration data into the complete element diagnostics;

The condition monitoring converts the mill from the periodic repair machine into the predictable asset: the measurement trends allow the plant to schedule the repairs before the failures and to order the parts ahead of the campaigns: the file of the package includes the measurement procedures, the baseline templates and the alarm philosophy of the mill condition monitoring: the elements of the mill are watched continuously.

12. The Typical Failures and the Maintenance Campaigns

The experienced mill maintenance is organized around the known failure modes of the elements:

• The liner failures: the broken bolts, the worn profiles and the liner displacements: the signs are the unusual mill noise and the power fluctuations: the bolted liner inspection and the retorque are the quarterly routines: the relining campaigns replace the complete armor at the planned intervals;
• The diaphragm blockages and the breakages: the clogged slots with the sticky materials and the broken grate segments by the tramp metal: the internal inspection during the relining periods: the tramp metal protection of the mill feed is the prevention of the diaphragm damage;
• The bearing damage: the wiping of the white metal from the oil film failures, the starting damages and the contamination: the bearing re-white-metaling is the major repair of the classical mills: the oil discipline prevents the bearing damage better than any inspection cures it;
• The gear problems: the pitting, the tooth breakage and the mesh misalignments: the gear repairs with the profile grinding and the partial replacements: the gear protective margin and the maintenance of the teeth monitoring prevent the catastrophic tooth loss;
• The shell cracks: the fatigue cracks at the welds and the manholes: the periodic shell inspections with the non-destructive testing: the crack repair with the welded patches and the fatigue life management: the shell is the element that the plant monitors most carefully because its failure stops the mill for the longest campaign;

The failure modes of the elements are the curriculum of the mill maintenance: each failure has its early signs and its economical prevention: the file documents the failure cases, the inspection frequencies and the repair procedures with the parts lists of the typical campaigns: the mill maintenance engineer of the plant works from this knowledge.

13. The Often Asked Questions

What is the operating lifetime of the tube mill shell?

The shell is the most durable element: the well-designed and weld-repaired shells serve for 40 to 60 years in the industry, while the liners are replaced every few years and the diaphragms every decade: the shell lifetime depends on the crack management: the regular inspections catch the fatigue cracks while the repair is still easy.

Why do the tube mills run on the two different bearing types?

The trunnion bearings are the classical solution with the white metal journals, while the slide shoe bearings release the mill heads and simplify the large mill construction: the central shaft mills with the slide shoes dominate the modern large installations because the heads are eliminated and the maintenance of the drive end is easier.

How often is the ball charge of the mill replenished?

Continuously and periodically: the small additions of the balls compensate the daily wear (the top-up practice), and the complete re-grading of the charge happens at the relining campaigns: the media level is measured with the mill power and the charge probes, and the magazine reports track the consumption per ton of the product.

What are the first signs of the gear damage in the girth gear drive?

The audible mesh noise changes, the vibration spectrum of the pinion shows the gear mesh frequency sidebands, the gear tooth temperature rises and the oil analysis shows the increased iron: the inspection of the tooth flanks with the blueing and the profile measurements confirms the diagnosis before the tooth breakage.

Is the rubber lining suitable for the first compartment of the mill?

Not normally: the first compartment suffers the impacts of the large balls on the coarse feed, and the rubber is damaged by the sharp impacts: the rubber liners serve the second compartments and the fine grinding duties where the impact energy is low: the steel and the chrome-alloy liners protect the first compartment, and the composite designs extend the rubber to the moderate duties.

Does the guide include the calculations of the liners and the drives?

Yes: the file carries the element design calculations with the worked examples: the liner bolt forces, the shell bending, the gear sizing and the drive power: the Excel tools of the package complement the text with the ready calculation sheets that the mechanical department uses for the modifications and the spare part reviews: the package documentation includes the full mechanical engineering of the mill elements.

14. Conclusion

The mechanical elements of the tube mill form the complete anatomy of the workhorse of the cement industry: the shell that carries, the bearings that support, the liners that protect, the diaphragms that separate, the media that grind and the drive that transmits: each element has its materials, its design calculations and its failure modes, and the mill mechanic who masters the elements masters the machine: the condition monitoring, the lubrication and the campaigns of the maintenance keep the elements alive for the decades: the guide of the package documents the complete mechanical engineering of the tube mill element by element.

The Complete Cement Technical Package includes this mechanical elements guide with the design tables, the material specifications and the maintenance checklists: the one-time 249.99: the instant download: the 931 files of the library of cement: the anatomy of the mill, drawn element by element: the knowledge of the package, the availability of the machine.

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.