Bearing Lubrication Basics

A lubricant is as much a part of a rolling bearing as its races and rolling elements. It reduces friction and wear, provides a barrier to contamination, cools the moving parts and protects against corrosion, and the heavily loaded contact between rolling element and raceway is where its properties matter most. Most bearing problems are not fatigue but contamination and lubricant starvation, so how a mounted bearing is greased, with what, and how often, decides how long it lasts. This guide covers grease against oil, what a grease is made of, the thickener types and their temperatures, mixing greases, the relubrication schedule, and what a sealed unit gives up.

Grease or oil: which lubricates a mounted bearing?

Grease is the primary lubricant in most industrial mounted bearing units, and every Sealmaster Gold and Browning mounted ball bearing is delivered filled with a lithium complex thickened, NLGI grade 2 mineral oil grease with an EP additive, ready to run with no initial lubrication. Oil is used when speeds and temperatures are high or the machine has a central oil supply, and oil systems are generally applied to unmounted bearings.

Grease is chosen where a fluid lubricant cannot be retained: it stays in the housing far longer than oil and acts, to some extent, as a seal against dirt entering the bearing. Oil is chosen where heat must be carried away; cooled oil is sometimes circulated through a bearing to remove the heat of high speeds and heavy loads. With oil, the level is held at the center of the lowest rolling element while the bearing is stationary, because an oversupply churns and generates heat, and the oil should be a well refined mineral oil with high film strength and resistance to oxidation, never an animal or vegetable oil. As a general rule the following viscosities are maintained at the bearing operating temperature.

Either way the aim is a thick film: the speed of the bearing and the viscosity of the lubricant together separate the rolling elements from the raceways. Boundary lubrication, with metal on metal contact, is the most severe condition and distresses the rolling elements and races; thin film leaves some asperities in contact and some distress. A higher viscosity or a higher speed helps reach the thick film condition, but an excessively high viscosity raises the operating temperature through churning or skidding of the rolling elements, which is why high speed applications use the lowest viscosity that still achieves a thick film.

What is grease made of, and what does NLGI 2 mean?

Grease is three things: a base oil, a thickener and additives. The oil does the lubricating; the thickener immobilizes the oil and acts as a reservoir, releasing it slowly and reabsorbing it, so it is the oil bleeding from the bulk of the grease that reaches the rolling contact. NLGI is the consistency grade, from 0, the softest, to 6, the stiffest, and standard bearings are packed with grade 2 because it stays in the bearing, pumps through most grease guns and works at most speeds.

The grade is measured by how far a weighted cone penetrates the grease, and most greases soften with working, so the worked consistency is the one that counts. A grease of grade 2 or 3 is a channeling grease: when the bearing first turns it purges the excess and clears a path for bleed oil to reach the rolling elements, and the grease that stays in the housing feeds that path for the rest of the interval. Once the oil has depleted to about half the weight of the grease, its lubricating ability is doubtful, which is what a relubrication interval is set to stay ahead of. Additives raise load capacity, resist corrosion, oxidation and temperature extremes, and adjust viscosity and consistency; EP additives and solid additives such as molybdenum disulfide or graphite are a matter for application engineering rather than a routine choice.

The base oil is petroleum or synthetic. Petroleum oils are the usual choice, synthetic hydrocarbons behave as synthetic petroleum oils, and the other synthetics are esters, silicones and fluorinated hydrocarbons. Every mounted bearing product page names the factory fill under Lubricant: a NP-16 pillow block and a VPS-216 both list Lithium Complex NLGI 2 Grease, and the food grade, high temperature and PTFE thickened fills on other units are listed the same way.

Which grease thickener suits which temperature?

Lithium soap is the most common thickener, is insoluble in water and covers -30 to +250 F, which is why the standard fill is a lithium complex grease. Calcium soap is water resistant but stops at +150 F; sodium soap tolerates a little water but is fibrous and limited to lower speeds; polyurea and clay thickeners reach +350 F. The table gives the typical range for each.

Above the standard range the mounted ball lines move to a different fill rather than a different thickener alone. The Sealmaster HT, HTA and HTC high temperature options are filled with a lithium complex thickened synthetic hydrocarbon grease, NLGI 2 with an ISO 220 viscosity oil, for 200 to 400 F; the HI option uses Krytox GPL 226, a perfluorinated oil with a PTFE thickener, whose service life diminishes the relubrication frequency at high temperature. A food grade grease is an option in every Sealmaster bearing, and the product pages list it as a calcium sulfonate NLGI 2 NSF H1 food grade grease on the units that carry it.

What is polyurea grease and when is it used?

Polyurea grease is thickened with a polyurea compound rather than a metal soap. It is smooth textured with good mechanical stability and very good resistance to oxidation and water, and that oxidation resistance is what makes it suitable for higher operating temperatures: its typical range is -30 to +350 F (-35 to +175 C), a hundred degrees above a lithium soap grease at the top end.

Because the stocked mounted ball bearings leave the factory with a lithium complex fill, switching to a polyurea grease at the first relubrication mixes two thickener types in one housing, and that is a compatibility question before it is a temperature one. Where a higher temperature is the reason for the change, the high temperature fills above are already matched to the seal and internal clearance of the unit they ship in.

Can different greases be mixed?

Not without checking. Combining greases with different thickeners can produce a mixture with poorer performance or physical properties than either grease on its own, and because a grease is oil, thickener and additives, any of the three can be incompatible with its counterpart in the other grease. Relubricate a mounted ball bearing with a lithium complex thickened grease of comparable NLGI consistency and base oil properties, and confirm compatibility with the grease supplier before changing to anything else.

Petroleum oils and synthetic hydrocarbons are, generally speaking, compatible with each other; the other synthetic oils are more often than not incompatible with other oils, and additives can cause problems of their own. The high temperature fills are the strict case: an HT, HTA or HTC unit is relubricated with a lithium complex NLGI 2 grease with a synthetic hydrocarbon oil of ISO 220 viscosity, and an HI unit with Krytox GPL-226 and no substitution. Greases are stored clean and dry and protected from contaminants, and the fitting and the grease gun nozzle are wiped clean before every charge.

How often should a mounted bearing be regreased?

The interval depends on speed, temperature and contamination. In a clean environment below 125 F, a bearing running at up to a quarter of its catalog maximum speed is greased every 4 to 10 months, and one above three quarters of maximum every day to a week; in a dirty environment, or above 175 F, the interval is a day to a week at any speed. Sealmaster Gold and Browning mounted ball bearings state the same schedule.

The speed percentages are against the maximum listed for a single lock setscrew insert with felt seals; Skwezloc and contact seal units have their own maximums, listed as Maximum Speed on each product page.

Where it is safe to do so, the bearing is greased while rotating until grease purges from the seals. Otherwise the equipment is stopped, half of the recommended charge is added, the bearing is run for a few minutes, and the second half is added. A temperature rise after relubrication, sometimes 30 F, is normal and falls once the excess has purged; the bearing should then run below 200 F and not above 250 F for intermittent operation. Near the top of the speed range, grease is added more often and in smaller amounts. The charges below are for a lithium complex NLGI 2 grease.

Relubricatable units carry a grease fitting, listed on the product page as Grease Fitting Thread Size: 1/4-28 on Sealmaster Gold units such as the NP-16 and 1/8 NPT on Browning units such as the VPS-216. On a Sealmaster Gold bearing the fitting also positions the lock pin that stops the outer ring rotating in the housing, so it is factory adjusted and replaced only with a fitting made for the bearing; 90 degree, 45 degree and lube line adapter fittings are factory options. Hanger bearings for screw conveyors have their own arrangement: SEHB units such as the SEHB-16 carry the fitting on the outside of the body, and SCHB units carry it inside the threaded shank so a grease line can be run through the pipe.

Sealed or relubricatable: what does Ability to Relubricate mean?

Ability to Relubricate on a product page says whether the unit is meant to be regreased. Yes means it has a grease fitting or zerk and follows the schedule above; No means a reduced maintenance or lubricated for life bearing, supplied with a plug instead of a fitting and designed to run on its factory fill, so its life is bounded by that grease and by the seals that keep contamination out.

The two are otherwise the same unit. The NP-16 is relubricatable, and the NP-16 RM is the same 1 inch pillow block with the RM suffix and Ability to Relubricate: No. The RM bearing has a Tapered Lands race profile, tapered in a radial direction toward the raceway so lubricant flows back to it more easily, which is what lets it run its service life on the original fill with no reduction in radial or thrust capacity; for a bearing that is maintained and regreased on schedule anyway, there is no significant difference in expected service life. Some inserts are sealed the same way: the Browning LS-112 light duty insert is listed with Ability to Relubricate: No, where the Sealmaster ER-16 cylindrical OD insert in the same insert bearings category is listed as Yes.

Two related options change the rule. The Sealmaster LO and XLO low drag units are built to run with less torque than a standard bearing, and adding lubricant increases drag, so they are relubricated only if necessary, with a very small amount, and the rotational torque is checked afterwards; the XLO is oil lubricated and shipped with an adapter and plug for field relubrication. Where a bearing cannot be reached at all, a solid lubricant replaces grease: an oil saturated polymer fills the bearing cavity, releases oil during operation and reabsorbs it at rest, keeps contaminants out, and is limited to slower speeds and temperatures below 200 F. Graphite and molybdenum disulfide dry films bonded to the bearing surfaces serve high temperatures, oscillating rotation and maintenance free service.

Choosing between a sealed and a relubricatable unit is part of choosing the bearing, covered in the mounted bearing selection guide. For a speed, temperature or environment outside the schedule above, or a grease you need to confirm against the factory fill, send the details through the lubrication support form. Ready to buy? Browse pillow blocks, mounted ball bearings and mounted spherical roller bearings and filter by the Ability to Relubricate and Lubricant specifications.