Machinery Lubrication

Lubrication is one of the highest-value maintenance tasks there is, and one of the easiest to do badly. A lubricant puts a thin film between moving surfaces. That film cuts friction, heat, and wear. It also carries heat away, flushes out contaminants, and protects against rust. An often-quoted estimate from bearing manufacturers says 40 to 50 percent of bearing failures come from improper lubrication: the wrong type, the wrong amount, or the wrong interval. The exact figure varies by study, but the point holds. Most of these failures can be prevented with care and a good routine.

What You Will Be Able to Do

• Choose between grease, oil, and dry lubricants, and read NLGI and ISO VG grades.
• Grease a bearing using the right method and a calculated quantity.
• Explain why over-greasing and mixing greases damage equipment.
• Label, store, and transfer lubricants so they stay clean.
• Follow and build a lubrication route.

Types of Lubricant

Grease stays where you put it, so it suits points that cannot hold oil. Oil flows, so it can carry heat away and be filtered.

NLGI Grades for Grease

The National Lubricating Grease Institute (NLGI) grades grease by how firm it is, from 000 (nearly fluid) to 6 (a hard block). This number tells you nothing about the base oil or the thickener, only the firmness. NLGI 2 is by far the most common grade for general bearing lubrication.

ISO VG Grades for Oil

ISO viscosity grades (VG) say how thick an oil is at 40°C, in units called centistokes. A higher number means a thicker oil. Common grades are VG 32 and 46 for hydraulic systems, and VG 68 through 460 for gear oils and circulating systems. Viscosity is the most important property of an oil. Too thin, and the film breaks down. Too thick, and the oil runs hot, wastes energy, and may not reach the parts it should.

Never substitute a different grade because it is the one on the shelf. Use what the equipment maker or lubrication chart specifies.

How to Grease a Bearing

1. Check the route sheet for the correct grease, quantity, and method.
2. Confirm you can reach the fitting safely. If the fitting is inside a guard or near moving parts, the machine must be locked out. Greasing a running machine is only for fittings piped outside the guard, where your procedure allows it.
3. Wipe the fitting and the gun's nozzle with a clean, lint-free rag, so you do not push dirt into the bearing.
4. Connect the coupler squarely. Replace a damaged or plugged fitting before going on.
5. Add grease slowly in small amounts, a stroke at a time, while the shaft turns slowly, so the grease spreads evenly around the bearing. On a locked-out machine, turn the shaft by hand if the design and your procedure allow it.
6. Stop at the calculated quantity. On a housing with a purge path, also stop when you see a small bead of fresh grease at the seal or relief.
7. Wipe away any excess, and record the job.

Warning: A hand grease gun can build far more pressure than a seal can hold. If the gun will not pump with normal effort, stop. The fitting or line may be blocked. High-pressure grease can also be injected through skin, which is a medical emergency.

Not every bearing takes grease. A bearing sealed for life has no fitting and does not need one. Do not drill or modify one to add grease.

How Much Is Enough

More grease is not better. Over-greasing packs the bearing so the rolling parts churn the grease. That generates heat, blows seals, lets dirt in, and can cause early failure. In an electric motor, excess grease can also reach the windings.

A widely used starting formula for regreasing comes from bearing makers:

• Grams = 0.005 x D x B, with D and B in millimeters.
• Ounces = 0.114 x D x B, with D and B in inches.

Here D is the bearing's outside diameter and B is its width. This is the amount to add at each regreasing, not the initial fill. Check the bearing maker's data, because the factor depends on how the grease enters the bearing.

Worked example: a bearing has a 62 mm outside diameter and is 16 mm wide.

1. Grams = 0.005 x 62 x 16 = about 5 grams.
2. Weigh the output of your grease gun. Pump 10 strokes onto a scale, and say you get 12 grams. That is 1.2 grams per stroke.
3. Strokes needed = 5 ÷ 1.2 = about 4 strokes.

Grease guns differ, and output changes with wear, so check yours rather than assuming. Write the stroke count on the route sheet so everyone adds the same amount.

How Often to Lubricate

Follow the manufacturer's chart first. Four conditions then move the interval:

A slow, cool, clean conveyor bearing may go a long time between greasings. A hot bearing in a dusty area needs it much sooner. If a bearing keeps running hot or failing, do not simply add more grease each time. Find out what is going on, starting with the interval, grease type, and alignment.

For oil systems, check the level at the sight glass under the conditions the maker specifies, top up with the same oil, and watch for changes in color or clarity. Change oil at the set interval or when analysis says to. Drain it warm so contaminants come out with it, and replace any filters at the same time.

Compatibility: Do Not Mix Greases

Greases use different thickeners, such as lithium, lithium complex, calcium sulfonate, and polyurea. Some do not get along. Mixing incompatible greases, such as a lithium grease with a polyurea grease, can make the mix soften and run out, harden, or separate. In any of those cases, it stops protecting the bearing.

• Treat every grease as incompatible until a compatibility chart or the supplier says otherwise.
• Use one gun per grease, and never refill a gun with a different product.
• If you must change products, clean out or purge the old grease as the equipment maker directs, then shorten the interval for a while.
• The same caution applies to oils, since additive packages can react.

Labeling Lubrication Points

Most wrong-lubricant mistakes happen because two products look alike. Label every point, gun, container, and storage slot with the same code, using color, shape, or both. A blue square tag on the bearing housing matches a blue square tag on the gun.

There is no universal color code for lubricants. ISO 6743 is a classification system for lubricant types, not a color standard. Your site's lubrication chart is the key, and the code only works if everyone uses the same one.

Storing and Handling Lubricants

Clean lubricant is cheap protection. Dirty lubricant becomes a grinding paste, and water speeds up wear and rust.

• Store lubricants indoors in a cool, dry, clean room. Heat speeds up oil separating from grease.
• Keep sealed grease cartridges upright, following the supplier's directions.
• Depressurize a grease gun with a cartridge in it and store it on its side in a clean spot, so oil does not drip out of the nozzle.
• A small amount of oil on top of stored grease is normal. Stir it in if the supplier says to, and reject grease that has hardened or badly separated.
• Keep drums indoors. If one must stay outside, lay it on its side with the bungs at 3 and 9 o'clock so water cannot pool over them.
• Use sealed, labeled transfer containers, never open buckets or food containers. Filter oil going into sensitive systems, since new oil may not be as clean as the system needs.
• Use oldest stock first, and watch the shelf life on the label.
• Clean up spills at once. Oily floors are a slip hazard. Dispose of used oil and rags as your site requires.

Lubrication Routes

A route is a planned walk that covers every lubrication point. Route sheets stop points from being missed and keep the work efficient. Each line tells the technician exactly what to do.

To build one:

1. Walk the area and list every lube point on every machine.
2. Record the lubricant, the amount, the interval, and whether the machine must be stopped.
3. Group the points so one trip covers a sensible path, and match stops to planned downtime where you can.
4. Load the route into the maintenance system so it generates work orders.
5. Have the person who does the route record what was done and what was noticed, such as a hot bearing, a leak, or a damaged fitting.

Common Mistakes

Check Your Understanding

1. A bearing is 80 mm in outside diameter and 21 mm wide. Your gun delivers about 1.5 grams per stroke. How many strokes should you add?

0.005 x 80 x 21 = 8.4 grams. Then 8.4 ÷ 1.5 is about 6 strokes, added slowly.

2. A coworker is out of the usual grease and offers a tube of a different brand with the same NLGI grade. Is that fine?

Not by itself. NLGI grade only tells you firmness. The thickener could be incompatible. Check the compatibility chart or ask the supplier before you use it.

3. A motor bearing runs hot after a new technician greased it for the first time. What might have happened?

It may be over-greased. The excess churns and creates heat. Stop adding grease, report it, and let maintenance decide whether to purge it.

4. You find a gearbox labeled for ISO VG 320, and the only oil nearby is ISO VG 68. Can you top it up for now?

No. VG 68 is much thinner and could lower the film strength. Get the correct oil, and report the level if it is low.

Key Takeaways

• Use the lubricant the equipment maker or chart specifies, because NLGI and ISO VG grades describe only part of a product.
• NLGI 2 is the most common grease for general bearing service.
• Add grease in small amounts while the shaft turns, and stop at the calculated quantity.
• Over-greasing generates heat, blows seals, and can cause early bearing failure.
• Incompatible greases can soften, harden, or separate, so use one gun per grease.
• Keep lubricants clean, dry, and labeled, and follow a route so no point gets missed.