The Lubricant and the Materials It Touches

Why this matters

A lubricant is picked for the bearing and then it touches four more things: the seal, the plastics and coatings around it, the lubricant already in the housing, and sometimes the water or product the equipment handles. Only the bearing is on anyone's mind at the parts counter. Most lubricant-caused failures in the field are not lubrication failures at all. The oil film was fine and something else the oil touched gave way, which is why they show up weeks later as a leak, a soft seal, a cracked plastic bracket, or a bearing that ran out its own grease.

Before you open a lubricated component

• A pinhole leak in a pressurized hydraulic line, or the nozzle of a grease gun, can inject fluid under the skin. The entry wound looks like a pinprick and the damage keeps developing for hours. Never run a hand along a line to find a leak; use a piece of cardboard held at arm's length. Any suspected injection is an emergency department visit, immediately, with the fluid's safety data sheet in hand, and the person needs to say the words "high pressure injection injury" because it does not look like an emergency on arrival.
• Relieve stored energy before opening a housing. Isolate, lock, tag, and prove zero at a gauge under 29 CFR 1910.147. A gearbox or accumulator that is off is not the same as a gearbox that is drained and vented.
• Hot oil stays a burn hazard after the machine stops. Check with an infrared thermometer before a wrench moves.
• Match the glove to the fluid, not to habit. Hand protection has to be selected on its performance characteristics for that substance under 29 CFR 1910.138(b), and the fluid's safety data sheet, required to be available under 29 CFR 1910.1200, names the class.

Contact 1: the metal it is there for

Viscosity does the actual lubricating, and additives handle what the film cannot. The compatibility issue here is narrow and specific: sulfur-phosphorus extreme pressure additives can attack yellow metals, meaning brass, bronze and copper. That matters wherever a bearing cage, a worm gear, a bushing or a thrust washer is a copper alloy, which is common in gearboxes and older equipment.

The industry check for this is the copper strip corrosion test, ASTM D130, which reports a rating from 1a for a bright strip through to 4c for a badly corroded one. A product data sheet showing a good copper strip rating is telling you it is safe on yellow metals; an aggressive extreme pressure gear oil with no such claim is not something to put in a bronze-geared unit on a hunch.

Contact 2: the seal

This is where most of the damage happens, and it is decided by the base oil family rather than by anything on the front of the container.

Two entries in that table catch people out. Silicone grease on a silicone seal is the like-dissolves-like trap: silicone is the safe default for so many elastomers that it gets treated as universally safe, and the one thing it swells is its own family. Mineral oil on EPDM is the opposite pairing and the more common one, because EPDM is the standard water-service elastomer and petroleum grease is the standard assembly lubricant, so the two meet constantly on valves and fittings.

Use these families to narrow, then confirm against the seal manufacturer's chart for the specific compound. Compounds within one class vary enough that a class-level answer is a screening result, not a decision. The sibling article on elastomer classes covers what each class is at home with from the seal's side.

Contact 3: plastics, paint, and anything to be bonded later

• Silicone contaminates surfaces for coating and bonding. It causes craters and fisheyes in paint applied later and is stubborn to remove even with solvent. A silicone spray used on a hinge, a track, or a fastener near anything that will be painted, sealed, or adhesive-bonded creates a defect that appears at the coating stage and gets blamed on the coating.
• Some oils and greases crack certain rigid plastics under load. Fine cracks radiating from a threaded boss, a clamp point, or a moulded stress riser in a part that is otherwise sound name environmental stress cracking, which needs a chemical and a stress together. Polycarbonate is the classic sensitive material.
• Anti-seize compounds are for metal threads. On a plastic thread they do nothing useful and the metallic solids can act as an abrasive at the flank.
• Plasticizer migration runs both ways. A soft vinyl component in contact with an oil can lose plasticizer to it and go hard, and the oil ends up carrying something it was not formulated with.

Contact 4: the lubricant already in there

Greases mix by thickener, not by base oil, and many thickeners are incompatible. Lithium complex, polyurea, calcium sulfonate, aluminum complex and clay are all common and several pairs of them produce a mixture that is markedly softer than either parent, which then runs out of the housing and leaves the bearing on boundary contact. A few pairs go the other way and harden. Neither outcome is what you were topping up for.

Oils have their own miscibility rules. Polyalkylene glycol and mineral oil do not mix. In refrigeration, oil selection follows the refrigerant chemistry, and mixing families leaves oil logged in the low side rather than returning to the compressor. Polyol ester oils are strongly hygroscopic: an open container absorbs moisture from room air, and the moisture hydrolyzes the oil into acids, which is why the container gets closed between pours and why an open jug that has been on a shelf is scrap rather than stock.

Contact 5: the water or the product

Where equipment touches drinking water, the components and any lubricant in the wetted path are evaluated for that use under NSF/ANSI 61. Where it touches a food process, a lubricant with the potential for incidental contact carries an H1 registration. Neither of these is optional trivia in the trades that do this work, and neither is something you can assess by reading a product name.

Worked: twelve fan bearings and a supply change

A site with twelve identical fan bearings, greased on a six-month interval, historically with a lithium complex grease. Zero bearing failures in the prior five years.

A supply change puts a polyurea grease on the shelf. Both products are the same consistency grade, both are labelled general purpose, and nothing on the job record captured a thickener because the parts field carried a description rather than a chemistry. Over the next eighteen months, three greasing cycles at the standard interval, four of the twelve bearings ran hot and two failed.

Getting the comparison onto one footing. The obvious framing, two failures in eighteen months against zero in five years, compares an eighteen-month window to a sixty-month one and is not a rate. Put both on bearing-months: the prior period is twelve bearings over sixty months, which is 720 bearing-months with zero failures. The recent period is twelve bearings over eighteen months, which is 216 bearing-months with two failures. Now the two figures measure the same thing and the change is unmistakable rather than merely alarming.

What the physical evidence said. The housings on the hot bearings had grease weeping from the seals and very little left inside, which is the signature of a mixture that went soft rather than of over- or under-greasing. Over-greasing produces a packed, churned housing running hot with grease still in it; under-greasing produces a dry housing with no grease anywhere including the seals. Weeping seals plus an empty housing is grease that liquefied and left.

The correction, with its step size stated. One thickener system for the whole site, chosen and written into the parts spec. Every housing purged, meaning fresh grease pumped until what comes out at the relief is clean new product rather than a smear of the old. Then the first interval after the change shortened from six months to three for one cycle only, and back to six after that cycle confirms the housings are holding grease. The shortened interval is not a permanent change; it exists to catch a bad purge while it is still cheap.

What would change this answer. If the bearings were sealed for life with no grease fitting and no relief, purging is not available and the correct call is replacement of the affected units rather than any re-greasing scheme. The whole correction above depends on being able to get the old product out.

The failure mode worth naming. Nobody in this story did anything careless. The tech greased on schedule with the grease on the shelf. The purchase matched on consistency grade and description. The record captured a product description and not a thickener, which meant the change was invisible to every control the shop had, and the first signal was a hot bearing more than a year after the shelf changed.

Verifying a lubricant choice before it reaches the truck

Four questions, all answerable from a product data sheet or a supplier in writing, none of them answerable from the label:

• What is the base oil family and, for a grease, what is the thickener? A marketing name is not an answer. If a supplier cannot state both, the product is not identified.
• What seal class is it going against? Screen with the base oil table above, confirm against the seal manufacturer's chart for that compound.
• Will anything nearby be painted, sealed, or adhesive-bonded later? If so, silicone comes off the list for that job entirely.
• Is there a potable water or food path? If so, the registration is a requirement rather than a preference.

Then standardize. One grease thickener system per site removes the entire class of failure in the worked case above, and it costs nothing except the discipline to write the chemistry into the parts spec instead of a description.

References

• ASTM D130, copper strip test for corrosiveness to copper, for yellow-metal compatibility
• NSF/ANSI 61, drinking water system components, for lubricants and components in a potable water path
• 29 CFR 1910.147 for isolating stored energy, and 29 CFR 1910.138(b) with 29 CFR 1910.1200 for glove selection and safety data sheet availability
• Manufacturer product data sheets for base oil family, thickener type, and seal compatibility charts
• See related: How to Choose a Lubricant That Will Not Attack a Seal; How to Tell Which Seal Material You Are Holding