Motor bearing lubrication and VFD shaft grounding field guide

Motor bearing maintenance, and why it is most of the battle

Motor bearing maintenance is the work of keeping the bearings that carry the motor shaft clean, lubricated, aligned, and, on drive-fed motors, protected from shaft current. It matters because the bearing is where electric motors die. Industry surveys, including the IEEE motor reliability data, put bearings at roughly half of all motor failures, and some studies push that higher. The windings get the attention, but the bearing is what fails first on most machines.

That changes where you spend your time. A motor with a good winding and a wrecked bearing is down just the same, and the bearing took a fraction of the cost to protect. Two things do most of the protecting: lubrication that is correct in type, amount, and timing, and on any motor fed by a variable frequency drive, a path for shaft current that goes around the bearing instead of through it.

This guide covers both. For how the drive that causes those currents is selected and set up, see the motor starting methods guide. For the relays and devices that protect the winding from overload and fault, see the motor protection guide. The bearing is the third leg, and it is the one that gets neglected until the machine is screaming.

Why do motor bearings fail?

Motor bearings fail from a short list of causes, and on most jobs the same few repeat. Knowing the list lets you read a failed bearing instead of just replacing it.

Lubrication is first, and it cuts both ways. Too little grease and the rolling elements run metal on metal until they overheat and seize. Too much grease, which is the more common field mistake, churns and overheats just as badly. Contamination is next: dirt, water, and washdown getting past the seals into the grease. Then mechanical load, from coupling misalignment or an over-tight belt that pushes the bearing past its rating. Then heat, which thins the lubricant and shortens its life on a curve. And on drive-fed motors, electrical discharge, where shaft current arcs through the bearing and pits the races.

Most failures are a combination. A bearing that runs hot from over-greasing also oxidizes its grease faster, which leaves it starved, which makes more heat. Read the failed bearing for the pattern. The discoloration, the wear track, the kind of pitting, and where the damage sits all point back to a cause, and if you do not fix the cause the new bearing fails the same way.

Bearing types: ball, roller, shielded, sealed, open

Most general-purpose motors ride on deep-groove ball bearings, which carry radial load and a bit of thrust and run with low friction. Larger motors and high-radial-load applications, like belt-driven machines, often use roller bearings on the drive end for the heavier load. The motor nameplate or the bearing journal stamping tells you the bearing number, and that number defines size, type, and clearance class.

How the bearing is closed off decides if it ever gets greased. An open bearing has no built-in shield and relies on the housing and end-bell seals. A shielded bearing has a metal shield on one or both sides that holds grease near the rolling elements but is not a tight seal. A sealed bearing has a rubbing lip seal and is packed and closed at the factory.

Lubricant type follows from size and speed. Grease handles the large majority of motor bearings because it stays in place and seals against contamination. Oil lubrication, by bath, ring, or mist, shows up on large, high-speed, or high-temperature machines where grease cannot shed heat fast enough. Most of what follows is about grease, because that is what most of the trade services.

Grease lubrication: the core of bearing life

Grease is base oil held in a thickener, with additives, and the thickener works like a sponge that releases oil to the contact as the bearing runs. The base oil does the lubricating. The thickener holds it in place and keeps it from running out of the bearing. Most motor greases use a lithium, lithium-complex, or polyurea thickener with a mineral or synthetic base oil at NLGI grade 2, which is the soft-solid consistency you see in a cartridge.

For a regreasable bearing the job has three variables, and all three have to be right. The right grease is whatever the motor or bearing manufacturer specifies, matched on thickener type, base oil viscosity, and NLGI grade. The right amount is a measured volume, not until it purges and not by feel. The right interval is set by the motor size, speed, and operating temperature, again from the manufacturer or a bearing-life calculation.

Get any one of those wrong and the bearing pays for it. The trade tends to obsess over which grease and ignore how much and how often, and the how-much is where most of the damage happens. A premium grease pumped in to excess on a careless interval will still cook a bearing.

How often should you grease a motor?

Regrease interval is set by the bearing size, the shaft speed, and the operating temperature, and the right source is the motor or bearing manufacturer, not a calendar habit. A small motor at 1800 rpm in a clean, cool plant might want grease once a year or less. A large two-pole motor at 3600 rpm running hot can want it every few months. There is no single number, and any guide that gives you one is guessing.

Two rules hold across the variation. Higher speed and higher temperature both shorten the interval, and temperature shortens it fast: a common rule of thumb is that grease life roughly halves for every 15 degrees C of bearing temperature rise above its rating. Bigger bearings hold more grease but turn slower, so the interval stretches with size at a given speed.

Most motor and bearing makers publish a lubrication interval chart or a formula keyed to bearing bore and speed, and many newer motors carry a regrease interval and grease amount right on a lubrication plate. Use that. If there is no plate and no record, get the bearing number and pull the manufacturer chart before you guess. And resist the urge to grease more often than the schedule says, because too frequent is its own failure mode, covered next.

Over-greasing: the most common way to kill a bearing

Over-greasing is the single most common lubrication mistake on motors, and it kills bearings as surely as no grease at all. The instinct is that more grease is safer. It is the opposite. A bearing cavity is meant to run roughly a third to half full, and the rolling elements need room to move the grease out of the way.

Pack the cavity full and the elements have to churn through grease that has nowhere to go. Churning makes heat. The heat oxidizes the grease, bleeds the oil out of the thickener, and bakes what is left into a hard crust that no longer lubricates and blocks fresh grease from reaching the contact. So the over-greased bearing ends up starved, after running hot the whole time.

Then there is the pressure. A grease gun can put out thousands of psi, easily enough to blow out the bearing seal or the slinger. A blown seal lets contamination straight in, so over-greasing causes the very contamination you were trying to keep out. On a motor, excess grease that gets pushed past the inboard bearing can reach the windings, coat them, and cook them. The grease gun in the hands of someone who thinks more is better does more bearing damage than almost anything else in the plant. Measure the amount, open the relief, and stop when the math says stop.

The right way to regrease: amount and procedure

Regrease by volume, with the old grease given a way out. The amount is a calculated quantity, commonly figured from the bearing outside diameter and width, and many manufacturers print the grams per service on the lubrication plate. If you have to estimate, a widely used formula is grease quantity in grams equals bearing outside diameter in mm times width in mm times 0.005. Convert that to pumps by knowing your grease gun output, because guns vary from roughly 0.5 to over 1.5 grams per stroke. Check yours by pumping ten strokes onto a scale.

The procedure matters as much as the number. Wipe the fitting clean before you couple the gun, so you are not pumping grit into the bearing. Remove the relief or drain plug, or confirm the relief valve moves freely, so old grease and excess can purge instead of building pressure against the seal. Add the measured amount slowly. Many manufacturers say to grease with the motor running so fresh grease distributes and old grease purges, but follow the motor maker, because some call for it stopped and locked out.

After greasing, leave the relief plug out and let the motor run for a while so it can expel any excess on its own, then reinstall the plug. Skipping the relief is how people over-pressurize a bearing even with the right amount of grease in the gun.

• Wipe the grease fitting clean before coupling the gun.
• Remove the relief or drain plug, or confirm the relief moves freely.
• Verify your gun output in grams per stroke against the calculated amount.
• Add the measured grease slowly, running or stopped per the motor maker.
• Run with the relief open to purge excess, then reinstall the plug.

Grease compatibility: do not mix thickeners

Mixing incompatible greases can ruin a bearing faster than running it dry, and the trap is that the bearing was just serviced. The fight is between thickeners, not base oils. Polyurea and lithium are the classic incompatible pair, and they sit in many of the greases the trade actually stocks for motors.

When two incompatible thickeners mix, the thickener structure breaks down. The grease either hardens into a plug that blocks the lines and starves the bearing, or it softens into a liquid that bleeds its oil and runs out of the housing. Either way the bearing loses its lubricant, and because the failure shows up days after a fresh greasing, it gets blamed on everything but the grease change. A bearing that fails right after service almost always points back to either over-greasing or a grease mismatch.

Treat the thickener type as part of the spec. Record the product, the thickener, the NLGI grade, and the base oil viscosity for every regreasable position, and do not substitute a different thickener because the shelf was empty. If you have to change grease types, especially anything involving polyurea, the safe path is to pull the bearing or end bell, clean the old grease out with solvent, and repack with the new grease. Pumping new grease in to flush the old does not guarantee they will not mix.

Sealed lubed-for-life bearings vs regreasable

A sealed bearing is packed and closed at the factory and is not meant to be regreased. There is no fitting and no relief, and the grease in it is what it gets for its service life, which is why these are called lubed-for-life. They show up on smaller motors and on motors where a clean, no-maintenance bearing is worth more than the ability to add grease. When a sealed bearing reaches the end of its grease life, you replace the bearing, not the grease.

A regreasable bearing has a fitting and a relief and is meant to be serviced on a schedule. Larger motors and continuous-duty machines usually run these because the grease would otherwise age out long before the bearing wears out, and adding grease is cheaper than pulling the motor.

Do not try to force grease into a sealed bearing through a housing fitting that was added later, hoping to extend it. You either blow the seal or pack the surrounding cavity without getting grease where it needs to go. Know which kind you have before you reach for the gun. If the motor has no fittings, it was built to run on its factory charge, and the maintenance is replacement, not lubrication.

Contamination: the leading non-electrical killer

Contamination is the top non-electrical cause of bearing failure after lubrication itself, and it is mostly dirt and water getting past the seals into the grease. A few microns of hard grit between a rolling element and the race is enough to start the pitting that ends the bearing, and water destroys grease by displacing the oil and rusting the steel.

The defenses are the end-bell seals and, on many motors, a shaft slinger that throws liquid away from the seal before it can wick in. Washdown environments are the hard case. A motor that gets hit with a hose on a daily sanitation cycle needs seals rated for it, and even then water finds a way if the grease is run too long between changes. In dusty plants the grit comes in on the shaft and through the breather.

The field tells are rust-colored grease, gritty feel when you rub it between two fingers, or water beading when you wipe a sample. Catch any of those and the cause is a seal that has failed or a service interval that let the grease age out and lose its barrier. And remember that over-greasing blows the seal and lets contamination in, so the over-greaser and the contamination failure are often the same person.

Misalignment and belt tension: the mechanical load

A bearing fails fast when it carries more load than it was sized for, and the two usual sources are coupling misalignment and over-tight belts. Both put a force on the bearing the motor designer never planned for, and the bearing answers with a wear track that tells you exactly what happened.

Coupling misalignment, whether the shafts are offset or angled, loads the bearings cyclically every revolution. A flexible coupling hides small misalignment from your eye but not from the bearing, which sees the force anyway. Align the coupling with a dial indicator or a laser alignment tool to the manufacturer tolerance, not by straightedge and feel, because the tolerance is tighter than you can see. On belt drives, over-tensioning is the killer. A belt cranked tight to stop it slipping puts a large constant radial load on the drive-end bearing and crushes its life. Set belt tension with a tension gauge to the drive maker number, and no tighter.

Read the failed bearing. An even wear track around the race points to normal load. A track that loads one spot or one side points to misalignment or a cocked bearing. Heavy single-direction loading on the drive end points to belt tension. Fix the mechanical cause or the replacement bearing fails the same way on the same schedule.

Catching bearing wear early with vibration analysis

Vibration analysis is how you find a failing bearing months before it seizes, while there is still time to plan the swap. A worn bearing produces vibration at specific frequencies set by its geometry and speed, the ball-pass and ball-spin frequencies, and a spectrum analyzer separates those bearing frequencies from the rest of the machine's vibration. A rising peak at a bearing defect frequency is an early, specific warning that the race or an element is starting to spall.

You do not always need a full analyzer. A trained ear and a hand on the housing catch a lot. A healthy bearing is quiet and smooth. A failing one growls, ticks, or runs rough, and you can often feel the roughness through the frame before a meter would flag it. Overall vibration level, measured in velocity, is the simple screen, and the bearing-frequency spectrum is the diagnosis when the overall climbs.

ISO 10816, and its successor ISO 20816, give vibration severity zones for evaluating machines by measurements on the non-rotating parts, which is the framework most programs use to set alarm and trip levels. Set a baseline when the machine is healthy, then trend against it. The change from baseline tells you more than any single reading, because every machine has its own normal.

Bearing temperature, and what hot means

Bearing temperature is one of the most useful health signals a motor gives you, because almost everything that goes wrong with a bearing shows up as heat. Many larger and critical motors carry a bearing RTD or thermocouple wired to the protection system or the plant historian, and the trend on that sensor is worth watching closely.

A bearing that runs hotter than its baseline is telling you something. The two usual causes are over-greasing, where churning grease makes the heat, and mechanical or lubrication failure, where the bearing itself is starting to fail. New grease often raises the temperature for a while right after service as the excess works out, which is normal and settles. A temperature that climbs and stays up is not.

Heat also feeds back on the grease. Higher temperature thins the oil, accelerates oxidation, and shortens the regrease interval, which is why a hot-running motor wants grease more often than the same motor running cool. If a bearing temperature trends up with no change in load or ambient, treat it as a bearing or lubrication problem developing, not as noise. On motors without a sensor, a simple infrared check on a route catches the same thing, as long as you compare to a known-good baseline on that machine.

What causes bearing currents on a VFD motor?

A variable frequency drive can destroy a perfectly good bearing in months through electrical discharge, and this is the modern bearing problem the trade is still catching up to. The drive does not make a clean sine wave. It switches DC on and off thousands of times a second to synthesize the output, and that fast switching produces a common-mode voltage that capacitively couples across the air gap from stator to rotor and charges the shaft.

When that shaft voltage builds past what the thin grease film can insulate, it discharges through the bearing to the grounded frame. The discharge is an electrical arc, the same physics as electrical discharge machining, and each arc melts a microscopic pit in the race and the rolling elements. It happens thousands of times a second. The grease film is only microns thick, so it is a poor insulator against these high-frequency voltages, and the bearing becomes the path of least resistance to ground.

This is purely a drive-fed problem. A motor across the line does not see it. The faster switching and longer cable runs that make modern drives efficient also make the shaft voltage worse, so it has grown as a failure mode, not shrunk. For how the drive is applied and set up, see the motor starting methods guide. The bearing damage it causes, and the two fixes for it, are covered next.

Fluting and EDM damage: the signature

Electrical discharge through a bearing leaves a signature you can learn to recognize. It starts as frosting, a dull matte gray finish on the race where thousands of microscopic discharge pits have roughened a surface that should be mirror-smooth. A frosted bearing already runs rougher and hotter than a healthy one, which speeds its own decline.

Left alone, frosting develops into fluting, a pattern of evenly spaced ridges and valleys across the race that looks like a tiny washboard. The fluting forms because the discharge pitting and the bearing's own vibration interact and reinforce a regular spacing. By the time you can see fluting, the bearing is near the end, and it usually announces itself with a rising whine or growl that climbs over weeks.

When you pull a bearing off a drive-fed motor and find frosting or fluting on the races, the diagnosis is electrical, not mechanical. Replacing the bearing without addressing the shaft current just resets the clock on the same failure. The damage pattern is the evidence, and the cure is to give the shaft current a path that does not go through the bearing, which is what the next two sections are about.

The shaft grounding ring: divert the current

A shaft grounding ring gives shaft current a low-impedance path to the frame that goes around the bearing instead of through it. The common type uses a ring of conductive microfibers, sold under names like AEGIS, that ride on the shaft and continuously bleed the shaft voltage to ground before it can build high enough to discharge across the bearing. The bearing stops being the path to ground, so the arcing stops, and the fluting with it.

The ring mounts on the shaft, usually at the drive end or the non-drive end depending on the motor, and the install is mechanical: clean the shaft to bare metal where the fibers ride, mount the ring concentric to the shaft, and confirm the fibers make contact all the way around. The shaft contact surface has to stay clean and conductive, so paint, grease, or corrosion under the fibers defeats it. Some motors come from the factory with a ring already fitted when they are sold for drive duty.

This is the most common retrofit fix for a VFD motor that keeps eating bearings. If you find frosting or fluting on a drive-fed motor, fit a grounding ring when you replace the bearing, or you will be back. A grounding ring without a clean shaft contact does nothing, so the prep is the job.

The insulated or ceramic bearing: block the path

An insulated bearing breaks the electrical path through the bearing rather than diverting it. Two forms are common. A hybrid ceramic bearing uses ceramic rolling elements that do not conduct, so the discharge cannot cross the bearing. An insulated-housing or coated bearing puts an insulating layer on the outer race or in the housing so the circuit through that bearing is open.

Insulating one bearing is the usual move for circulating bearing currents, which flow in a loop through both bearings and the frame. Break the loop at one end, typically the non-drive end, and the circulating current has nowhere to go. The standard pairing for a motor that sees both discharge and circulating currents is an insulated bearing on the non-drive end and a grounding ring on the drive end, so one path is blocked and the discharge path is diverted.

The catch with insulating both ends and stopping there is that you have blocked the bearings but left the shaft floating with nowhere to discharge. That can push the discharge to whatever is coupled to the shaft, like a pump or gearbox bearing downstream. Insulation usually wants a grounding ring with it so the shaft still has a deliberate path to ground.

Shaft grounding ring vs insulated bearing: which when

The grounding ring and the insulated bearing solve the same problem two different ways. The ring diverts shaft current to ground around the bearing. The insulated bearing blocks current from passing through that bearing at all. The right answer depends on the size of the motor and the kind of current.

For most small and medium drive-fed motors, where the damage is discharge current arcing to ground, a single grounding ring is the simpler and usually sufficient fix, and it has the advantage of giving the shaft a real path to ground rather than just blocking one. For larger motors, roughly the bigger frames where circulating currents become significant, the common approach is to insulate the non-drive-end bearing to break the circulating loop and fit a grounding ring on the drive end for the discharge current.

What you do not do is insulate both bearings and call it done, because the shaft still needs a discharge path or it will find one through the coupled equipment. The decision tracks the motor size and what the failed bearings show. Frosting and fluting at one end say discharge, fit a ring. Damage at both ends on a large motor says circulating current, insulate one end and ground the other. When in doubt on a large or critical machine, follow the motor manufacturer and the drive maker recommendation for that frame.

Bearing replacement: heat it on, never hammer

Replacing a motor bearing is straightforward, but the way the bearing goes on and comes off decides whether the new one lasts. The rule that matters most: never drive force through the rolling elements. A bearing pounded on with a hammer, or pressed on the wrong race, has its elements brinelled before it ever turns, and it fails early no matter how clean the rest of the work was.

To remove, use a bearing puller that pulls on the inner race, and if it is tight, an induction heater on the inner race eases it off without shock load. To install, heat the bearing in an induction heater to expand the bore so it slides onto the shaft, commonly held to around 110 to 120 degrees C and never over the limit the bearing maker sets, because too much heat changes the steel. A bearing heated correctly drops onto the shaft and clamps as it cools.

If you must press a bearing on cold, press only on the race that is an interference fit, the inner race for a shaft fit, using a sleeve that contacts that race and nothing else. Force routed through the outer race and across the balls is how a brand-new bearing arrives already damaged. No hammer, no chisel, no driving on the cage.

Installation: cleanliness, fit, and the small things

A bearing install is won or lost on cleanliness. The bearing, the shaft, the housing, your hands, and the grease all have to be clean, because the grit you trap on assembly is grit the bearing grinds for the rest of its life. Keep the new bearing in its wrapper until the moment it goes on, and do not wash the preservative out of a greased bearing.

Check the shaft and housing fits before anything goes together. A shaft worn undersize lets the inner race spin and fret, a classic repeat failure. A housing bore worn oversize lets the outer race turn. Measure them against the bearing maker fit tolerance, and if they are out, the shaft or end bell needs repair, not a fresh bearing on a bad fit.

Pack the right amount of grease into an open bearing on assembly, roughly a third to half the free space, not full, for the same reason you do not over-grease in service. Seat the bearing fully against its shoulder so it sits square, and confirm it turns freely by hand before the motor goes back together. A bearing that feels rough or notchy by hand will not get better under power.

Commissioning: set the baseline

The most useful thing you can do for a motor's bearings happens right after it is back in service: record a baseline. Every later diagnosis is a comparison against normal, and if you never captured normal, you are guessing about whether a reading is bad.

Take the baseline once the machine has run long enough to reach steady temperature. Record overall vibration at each bearing in velocity, a bearing-frequency spectrum if you have the tool, bearing temperature at each end, the grease type and amount and date, and the load the machine was carrying when you read it. On a drive-fed motor, note whether a grounding ring or insulated bearing is fitted, because that is the record the next tech needs when a bearing fails again.

File it where the next person finds it, not in a notebook that leaves with you. A baseline that nobody can locate is a baseline that does not exist. The point of the whole exercise is that six months out, when a bearing starts to growl, someone can pull the baseline, see how far it has drifted, and plan the repair instead of waiting for the seizure.

Data-center, pump, and fan motor bearing reliability

The motors that run continuously and feed critical loads are where bearing discipline pays off most, and they are increasingly drive-fed. Data-center cooling is the clear case: the CRAC and CRAH fan motors, the chilled-water pump motors, and the cooling-tower fan motors run nearly all the time, and almost all of them are on variable frequency drives for capacity control. That combination, continuous duty plus a drive, is exactly the recipe for shaft-current bearing damage.

On those machines the grounding ring or insulated bearing is not optional hardware, it is part of keeping the cooling online. A flooded bearing failure on a CRAC fan at two in the morning is a thermal event in the white space, not just a maintenance ticket. The same logic covers process pump and fan motors in any plant that cannot take an unplanned motor outage.

For these motors, build the shaft grounding into the spec when the motor is bought, set vibration and temperature baselines at commissioning, trend them, and hold the lubrication schedule. The motor protection that watches the winding is covered in the motor protection guide, and the drive that runs them is covered in the motor starting methods guide. The bearing is the piece those two do not cover, and on continuous critical motors it is the piece most likely to take the machine down.

What to document

Bearing maintenance lives or dies on records, because the failures play out over months and the person who finds the failure is rarely the person who did the last service. Capture the grease specification down to the thickener type, the amount and interval, the baseline vibration and temperature, and whether the motor has shaft-current protection fitted.

The grease record is the one people skip and regret. Write down the product, thickener, NLGI grade, base oil viscosity, the grams per service, and the interval, so the next tech does not grab an incompatible grease off the shelf or guess the amount. The table below is a workable minimum.

Common mistakes

• Over-greasing: pumping until it purges, churning the bearing hot, blowing the seal, or pushing grease into the windings.
• Mixing incompatible greases, especially polyurea with lithium, which breaks the thickener and starves the bearing within days.
• Running a VFD motor with no shaft grounding ring, so shaft current flutes the races in months.
• Letting contamination in through failed seals or a service interval that let the grease age out.
• Coupling misalignment or an over-tight belt loading the bearing past its rating.
• Greasing more often than the schedule calls for, which is just slow over-greasing.
• Hammering a bearing on or off, or pressing on the wrong race, which brinells a new bearing before it turns.

Field checklist

Standards and references

The first and most specific reference is the motor and bearing manufacturer. The bearing number, the grease type, the amount per service, and the regrease interval all come from them, and no general standard overrides the maker's lubrication plate for a given motor. Get the bearing number and the lubrication data before you service a motor you do not know.

NEMA MG-1 is the standard for motors and generators in North America, covering construction, performance, and mechanical vibration limits, and it points to the ISO vibration framework for evaluation. For vibration severity, ISO 10816, now being superseded by ISO 20816, sets the zones used to judge a machine by measurements on its non-rotating parts, which is what most condition-monitoring programs build their alarm and trip levels on.

For shaft current and grounding, the drive manufacturer and the grounding-ring maker, including the AEGIS bearing protection literature, give the application guidance on when a ring, an insulated bearing, or both are needed for a given frame size. Treat regrease intervals and grease amounts as manufacturer-specific values to verify against the equipment, not as universal numbers. The points to hold onto across all of it: do not over-grease, do not mix incompatible thickeners, and put a shaft ground on any motor running off a drive.

Units and terms

Bearing and lubrication work carries its own vocabulary, and the same idea reads differently across a bearing catalog, a grease data sheet, and a motor manual.

Grease consistency is graded by NLGI number, with NLGI 2 the common motor grade. Base oil viscosity is given in ISO VG or in centistokes at 40 degrees C. Bearing size is the bore, outside diameter, and width in millimeters, packed into the bearing number. Vibration is measured in velocity, mm/s or in/s, for severity, and in acceleration, g, for bearing-defect frequencies. Temperature shows up in both degrees C and degrees F.

NLGI grade

Grease consistency scale; NLGI 2 is the common soft-solid motor grade

Thickener

The structure that holds the base oil, such as lithium, lithium-complex, or polyurea; mixing incompatible types fails the grease

EDM / discharge

Electrical discharge machining; here the arc of shaft current through a bearing that pits the race

Frosting

Matte gray finish from many microscopic discharge pits, the early stage of shaft-current damage

Fluting

Washboard pattern of ridges across the race, the late stage of shaft-current damage

Shaft grounding ring

Conductive-microfiber ring that diverts shaft current to the frame around the bearing

L10 life

The rating life at which 10 percent of a bearing population is expected to have failed

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