Bearings

Why it matters

Almost every rotating machine in a plant, from a conveyor pulley to a pump to a fan, spins on bearings. When a bearing fails, the machine stops, and a seized bearing can wreck a shaft, a housing or a motor along with it. Mechanics who can name the bearing in front of them, read its part number and spot early warning signs catch problems during a planned shutdown instead of at 2 a.m.

What a bearing does

A bearing supports a load while letting one part move relative to another with little friction. Loads come in two directions:

• Radial load pushes at right angles to the shaft, like the weight of a pulley hanging on it.
• Axial (thrust) load pushes along the shaft, like the push from a helical gear or a vertical pump impeller.

Choosing a bearing is mostly about matching its design to those loads, the speed and the space available.

The main types

Rolling-element bearings have an inner ring, an outer ring, rolling elements and a cage that keeps the elements spaced.

• Deep groove ball bearings are the most common. They take moderate radial loads, some thrust in both directions, and high speeds. Electric motors usually use them.
• Angular contact ball bearings have raceways set at an angle so they carry heavy thrust in one direction. They are often mounted in pairs.
• Cylindrical roller bearings carry heavy radial loads but little or no thrust.
• Tapered roller bearings carry combined radial and thrust loads. Wheel hubs and gearbox shafts use them, usually in opposed pairs that must be set with the right end play.
• Spherical roller bearings have two rows of barrel-shaped rollers and tolerate some shaft misalignment. They are common on heavy conveyors and fans.
• Needle bearings are thin cylindrical rollers for tight spaces.
• Thrust bearings (ball or roller) carry axial load only.

Plain bearings (bushings and sleeve bearings) have no rolling elements. The shaft slides on a lubricated surface of bronze, babbitt or polymer. Large turbines and compressors run on oil-film plain bearings.

Mounted bearings such as pillow blocks and flange units put a bearing in a housing that bolts to the frame, which is what you will replace most often on conveyors.

Reading a bearing number

Most manufacturers follow a similar numbering pattern for common ball bearings. Take 6205-2RS:

• 6 is the type: single-row deep groove ball bearing.
• 2 is the dimension series (a light series).
• 05 is the bore code. For codes 04 and above, multiply by 5: 05 × 5 = 25 mm bore. Codes 00, 01, 02 and 03 mean 10, 12, 15 and 17 mm.
• 2RS means rubber seals on both sides (2Z or ZZ would mean metal shields).

So a 6308 is a deep groove ball bearing with a 40 mm bore in a medium series. Suffixes for internal clearance (such as C3) and cage material vary between brands, so always confirm with the manufacturer's catalog or interchange tool before ordering.

How bearings fail

A bearing that is clean, lubricated, aligned and properly loaded usually outlives its rated life. Most early failures trace back to a few causes:

• Lubrication problems: too little, too much, the wrong grade, or old grease that has broken down.
• Contamination: dirt or water past a damaged seal scratches and corrodes the raceways.
• Misalignment and poor fits: a shaft or housing out of tolerance, or a bearing hammered on through the rolling elements.
• Overload and imbalance: a belt tensioned far too tight is a classic one.
• Electrical damage: stray current, often from a variable frequency drive, arcs across the bearing and leaves a washboard pattern called fluting.
• False brinelling: vibration while the machine is stopped dents the races, common in standby equipment.

Warning signs

You usually get warnings before a bearing seizes:

1. Vibration rises first, often detected by a route-based vibration program long before you can feel it.
2. Noise changes: a rumble, growl or rhythmic clicking.
3. Heat rises at the housing. Compare it with an identical bearing nearby rather than guessing.
4. Grease changes: discolored, burnt-smelling or gritty grease, or grease purging past the seals.
5. Finally, visible damage: looseness, smoke or a locked shaft.

When a failed bearing comes out, keep it. Spalling (flaking metal), discoloration from heat, corrosion and fluting each point to a different root cause, and fixing the cause is what stops the next failure.

Common mistakes

• Pressing a bearing on by pushing on the outer ring when the inner ring has the tight fit. Force must go through the ring being fitted.
• Spinning a bearing with compressed air. It can overspeed and burst, and it damages the raceways.
• Ordering by bore size alone. Width, outside diameter, seals and clearance all matter.
• Replacing the bearing without asking why it failed.

Safety

Bearing work happens on stopped, locked-out equipment. Bearing heaters, pullers and presses store a lot of force and heat. Follow your site's lockout/tagout procedure and get hands-on training from your employer before doing this work. OSHA's lockout/tagout page is the place to start: https://www.osha.gov/control-hazardous-energy (Checked on 2026-10-23.)

Recap

• Radial loads push across the shaft; thrust loads push along it.
• Ball bearings suit speed, roller bearings suit heavy loads, tapered and angular contact bearings handle combined loads.
• In a number like 6205, the last two digits × 5 give the bore in millimeters.
• Vibration, noise, heat and grease condition warn you before failure.

Try it

Open the Do tab and take the quiz on bearing types, numbers and failure signs.