MAINTENANCE TECHNIQUES and GEAR UNIT FAILURE MODES MIKE FIELD DAVID BROWN GEAR INDUSTRIES Revision 1
Maintenance Regular maintenance saves money One hour per week of effort can save millions in lost production Train your staff to know what to look for
Typical problem areas Apart from the gears, other components within the unit give problems.
Blame the manufacturer It is a common misconception that when a unit fails it was incorrectly manufactured. Obviously, there are times when this is a correct conclusion, but independent surveys tell a slightly different story.
Correct installation If the unit is installed incorrectly, problems can be expected. Check for soft-foot or case twisting. Align the couplings. Fill the unit with the correct grade and amount of oil. Check all alarm interlocks are working and connected.
Common gear box problems Minor Oil leaks Ingress of dirt Water contamination Oil starvation Overheating Coupling misalignment Regular on site monitoring should eliminate these problems
Common gear box problems Major Lack of attention to the minor problems can result in the following typical major problems : Bearing failure Gear tooth damage Rust contamination Mechanical looseness Bearing bore wear Shaft damage
Common methods of monitoring gear units Human observation Overall vibration levels Spectral analysis Waveform analysis Oil analysis
Human observation The oldest and probably the most cost effective technique for assessing the condition of a gear unit is by using the three basic human skills of listening, looking and touching. Listen to the unit running, all knocks, rumbles or high pitch whistling should be noted and investigated. Look at the lubrication system. Is the pump working? Has the tank got lubricant in it? Do we have leaking seals or joints? Feel the gear case near the bearing housings. Can you keep you hand on the housing for a count of 10? For years, the mining industry has relied on these human skills and their importance cannot be underestimated.
Weekly routine maintenance - Enclosed gear units Check oil levels Check and clean breathers Check pressure, flow and temperature settings on lube systems - clean filters Check for oil leaks Check cooling fans are working Clean external surfaces of unit
Condition monitoring VIBRATION Overall levels Spectral Analysis OIL Wear Particle count % Water Viscosity & Additive package
Maintenance Vibration analysis using overall levels It is common practice on mechanical installations to use the level of mechanical vibration of the installation as an indication of the condition of the equipment. Various technical methods and equipment exist to undertake these analyses which are outside the scope of this presentation. However, for Gearbox Condition Monitoring, simple vibration measurements recording the velocity of the vibration can give excellent results. The simplest form involves recording the overall level of vibration in three planes (Horizontal, Vertical & Axial) on each bearing housing. The magnitude of these readings, recorded in mm/sec. RMS gives an early indication of a problem. Although various International Standards exist for allowable limits of such vibrations, none are specific to gear units. From experience, until the operator has a better understanding of his equipment, a limit of 5mm/sec RMS can be considered acceptable. Levels greater than this should result in more detailed investigation using Spectral Analysis methods
Maintenance Spectral analysis If the overall levels of vibration are greater than 5 mm/sec RMS, the analyst needs to identify the predominant frequency of this vibration. This can be done by making use of a Spectral Analyzer which breaks the vibration signal down into a series of discreet frequencies which make up the overall vibration. Once these frequencies are known, a simple comparison between them and the known forcing terms within the drive train will identify the cause of the problem.
Maintenance Common sources of vibration When making such comparisons, it is useful to remember that not only the gears contribute to the overall vibration of a mill. The most common sources of vibration are: - Rotational speeds of shafting Meshing frequency of the gears on the mill and the main drive gearbox Vibrational frequencies of the bearings within the drive train (BPFOR, BPFIR, BSF & the cage frequency) Alignment condition of the couplings Mechanical looseness between assembled components Out of balance or bent components
Acceptance criteria for vibration levels The magnitude of any discreet measured frequency should be within the following guidelines : Vibration measured in mm/sec. RMS Less than 2 mm/sec RMS Between 2 5 mm/sec RMS Greater than 5 mm/sec RMS EXCELLENT ACCEPTABLE Monitor for increasing tends UNACCEPTABLE Shutdown unit and eliminate cause of vibration
Maintenance - Checking vibration waveform It is a common misconception that Spectral Analysis can identify all sources of vibration. This is incorrect for a number of reasons, the most common ones being:- The lower frequency range of the accelerometer used for measurement is higher than the units lower rotational speeds. (accelerometer range 0.5 Hz, typical mill speed 0.2 Hz) A joint error, pitch error or broken tooth is expected to give a once per rev signal, which you would expect to be easily detected by Spectral Analysis. In fact these errors create a shock loading to the system which have the same effect as hitting the system with a hammer
Maintenance Checking vibration waveform Pitch errors, joint errors and broken teeth can however be detected by means of the real-time waveform traces.
Common failure modes of gear units We now have various methods of monitoring the condition of a unit whilst in operation. Once a problem is detected prompt action can avoid catastrophic failure. To insure corrective action is taken, the maintenance foreman needs to know what caused the problem.
Common failure modes of gear units Gear failure Mechanical looseness Bending fatigue of shafts Bearing bore wear Rust Bearing failure
Common gear failure modes Although there are many different modes of failure of a gear, they can generally be classed into four categories: SCUFFING or SCORING ABRASIVE WEAR SURFACE FATIGUE (Pitting) BENDING FATIGUE
Abrasive wear Abrasion : When foreign material of an abrasive nature enters between the meshing teeth, the resulting lapping or grinding action may either polish the surfaces or scratch them. In either case this is abnormal wear. This type of wear is caused by ingress of abrasive material into the mesh. Typical abrasives are dirt, grit, mill slime & metal particles. Although similar in appearance to scuffing, the lines are usually less ragged and the tooth surface usually appears polished. The tooth flank along the pitch line is usually unmarked.
Surface fatigue Pitting Sub-surface cracking propagates to the surface until the piece falls out creating a pit.
Non-destructive, initial or pitch line pitting With this type of pitting, after a short operating period the pitting stops and the unit will operate satisfactory for its working life
Destructive pitting With this type of pitting, the contact stress generated during the meshing action is greater than the material endurance limit. The gear will continue to deteriorate.
Spalling Although similar to pitting, spalling is more commonly found on case hardened gears. Large pieces of the tooth surface break out.
Bending Fatigue The most common form of tooth breakage is by fatigue. If a particular tooth is subjected to a higher load than it is designed to transmit, either by shock loading, misalignment or bad manufacture, the tooth is subjected to a higher bending moment as it passes through mesh.
Bending fatigue The most common form of bending fatigue which occurs on gears is known as a hump-back fatigue failure A crack occurring on the surface of the tooth will propagate along the line of maximum bending until it reaches the root zone. The point of maximum bending moment of a gear tooth is at the radius in the root of the tooth just below the working contact zone.
Mechanical looseness A common problem with older gear boxes is looseness between the shaft and the wheel. Wheel / shaft assemblies are usually a combination of interference fit and keyways. The amount of interference fit to be used is specified by International Standards (BS 1916 : 1953) The recommended fit for wheel / shaft assemblies in BS 1916 is H7/p6. This allows for dismantling of the assembly for handing changes and gear rotation.
Mechanical looseness Note loss of material caused by fretting corrosion In some cases, when the components are manufactured at the extremes of their tolerance bands, this need for assembly / disassembly can result in insufficient interference to transmit the load
Mechanical looseness Note eye of fatigue With time, this results in fretting corrosion between the surfaces. This corrosion results in the wheel working loose on the shaft. If left uncorrected the corrosion accelerates until the wheel is no longer held in position on the shaft and an overload condition occurs on the keyway. Continued operation in this mode can result in keyway damage or shaft breakage.
Detecting looseness using spectral analysis Fortunately mechanical looseness can be detected using spectral analysis before catastrophic failure occurs. NOTE multiple harmonics of the pinion frequency detects mechanical looseness of the mating wheel assembly.
Bending fatigue of Shafts Input and output shafts of gear units are not only subjected to torsional stresses. If incorrectly installed or aligned the shafts can also be subjected to rotational bending stresses, with catastrophically effects. A very common mode of failure particularly on conveyor drives where the unit is shaft mounted via a rigid coupling is rotational bending fatigue.
Bending fatigue of Shafts Shaft mount assemblies are connected to the drive pulley of the conveyor by a rigid coupling. The alignment of this coupling is critical to the installation. Any angular mis-alignment between the coupling faces will cause the installation to oscillate at a frequency equal to the gear box output speed. This oscillation introduces an oscillating bending moment on the output shaft. The situation is commonly worsened by the erector forcing the base plate to align with the torque reaction point at the opposite end of the base plate. External bending moment applied to output shaft Baseplate forced to align with reaction pin
Bending fatigue of Shafts The introduction of this oscillating bending moment results in crack propagation normally emanating from changes in section in the shafting i.e. fillet radii, keyways, etc. With continued operation these cracks propagate radially inward toward the centre of the shaft until the shaft is no longer able to transmit the normal torque loads and failure due to a brittle fracture. Common origins of failure
Bending fatigue of Shafts Examination of the fracture surface after failure often reveals the cause of the failure 1. Moderate bending moment combined with high stress concentration. 2. Excessive bending moment with moderate stress concentration.
Bending fatigue of Shafts This mode of failure results on the fracture site at right angles to the shaft axis with fatigue beach marks propagating inwards from a single (keyway) or multiple (fillet radii) origins.
Bearing bore creep Oscillating load causes the bearing to creep within its housing resulting in fretting corrosion. Commonly cause by coupling mis-alignment. Note early stages of fretting corrosion
Note rust damage at roller spacing
Bearing failure modes There are several modes of failure found on bearings, which can generally be categorised into three groups : False brinnelling Lubricant starvation Abrasive wear Fatigue Water contamination
False brinnelling The most common cause of false brinnelling in bearing is caused by shock loading or bad fitting practice by either the assembler or installer of the unit. If bearings or couplings are not correctly installed and force is used to position the bearing on its shaft, this force results in a depression onto the inner and outer raceways of the bearing caused by the bearing rollers. This can easily be identified by axial lines across the raceways at roller spacing.
False brinnelling These markings are effectively undulations across the raceways which cause localised overload as the rollers pass over the markings. With time, this localised overload causes fatigue damage to the raceways at roller spacing. Ultimately this will result in premature bearing failure.
Lubrication starvation This is by far the most common cause of bearing failure in gear units. Bearings require oil for two reasons, firstly to lubricate the bearing and secondly to cool the bearing. Inadequate cooling results in a differential expansion of inner race relative to the outer race. This reduces bearing clearance which further contributes to the heat input. Eventually the lubricate can no longer maintain an effective oil film and the bearing begins to seize. Once this occurs the failure is catastrophic.
Abrasive wear When foreign particles (grit, slime, coal dust, etc.) enters a gear unit, it mixes with the oil and forms an effective lapping paste. As the rollers pass across the bearing load zone this abrasive mixture causes excessive wear to all the rotating elements within the bearing (rollers, cages, raceways). This excessive wear results in excessive bearing clearances and misspacing of the rollers within the cage, which causes the rollers to slide across the raceways rather than rolling. This sliding action accelerates the rate of wear and also results in fatigue damage to the rolling elements.
Abrasive wear Although it is difficult to visually detect early stages of abrasive wear on rollers and raceways it can commonly be detected by inspection of the roller cage. Inspection of the cage at the ends of the rollers typically show early stages of abrasive wear. On split cage bearings, when the unit is stripped the contacting surfaces between each half cage shows severe scoring on the contacting surfaces.
Fatigue When Engineers talk about bearing life, they are talking of the number of operating hours a typical bearing can be subjected to a given load before the bearing commences to fail by the mechanisms of fatigue. Provided a bearing is correctly installed, lubricated, kept clean and not subjected to excessive overload or shock loading, the bearings can typically be expected to greatly exceed the designed life. Most fatigue failures therefore can be directly attributed to one or more of the above causes of failure.
Fatigue Typically the fatigue commences on the outer raceway of the bearing in the load zone. This can be expected since the outer raceway remains stationery and the same portion of its raceway is continually loaded. Inner raceways and rollers (by the fact that they are rotating) are less susceptible to localised overload. It must be recorded however that fatigue to rollers and inner raceways without damage to the outer raceway is not uncommon.
Water contamination Water entering the gear box either via the seals or breathers mixes with the lubricant and spreads throughout the unit. Once the unit is stopped and remains stationery for a period, the water separates from the lubricant and typically lies on the rollers and between the rollers and raceways. This results in rust corrosion of the bearing element which is effectively surface corrosion. Further operation of the bearing over these surface corrosion marks will result in the commencement of fatigue failure.