Fight Back Against Bearing Failure

The Enemy: Friction. The Solution: Bearings.

Machines from oxcarts to space shuttles have used antifriction bearings to keep rotating elements working smoothly in mechanisms. Friction is enemy number one to machine designers and MRO professionals alike, but reducing it to a minimum is about more than pumping grease into a pillow block.

New to bearings and antifriction technology? There are two common types:

Ball bearings use spherical, commonly steel rolling elements caged between inner and outer “races” that control movement between stationary and rotating machine elements, like shafts or gears. Balls carry loads through point contact, minimizing friction, but limiting the maximum allowable load.

Roller bearings are cylindrical and transfer the load over a larger area, described as line contact. This means they can operate under heavier loads, but are more limited in their speed.

Most bearing solutions are comprised of an inner ring that fits around the shaft or axle and an outer ring that fits inside the housing. Interior grooves on each ring provide the raceway that the bearings roll in. Lastly, a cage or retainer keeps the bearings separated and in the proper operation location.

Separated view of a rolling-element bearing. (1) Outer Ring. (2) Ball bearings. (3) Cage or retainer. (4) Raceway. (5) Inner ring. (Image courtesy of Niabot/Wikimedia Commons.)

Service Life and Bearing Failure – Why It Matters

No bearing will last forever. Even the highest quality steel or the lowest possible friction cannot prevent eventual bearing failure.

“Bearings have a life cycle and it’s a matter of understanding this cycle and how to prevent this failure from happening prematurely,” explains Steve Katz, president of Emerson Bearing.

Wear is the condition that a machine operator has the most control over, by ensuring that the bearing type and size, weight tolerance and lubricant are all the right choices for the machine and job at hand. With proper inspection and preventive maintenance, damage from wear can be minimized in order to maximize the service life of bearings.

The life cycle of a bearing, or service life is a sum of the number of operating hours a bearing can operate before material fatigue and wear damage prevent it from performing its function properly

Service life is usually cut short by excessive clearance in a critical operating parameter, such as end lash or runout in a supported shaft application. It may be limited by excessive running temperatures or noise in some applications. In some conservative and critical applications, bearings may be “lifed” and are simply replaced at recommended intervals regardless of performance.

In every case, catastrophic bearing failure is expensive and unnecessary.

How and Why Bearings Fail

Eventually, all bearings fail; but they should not fail prematurely. Identifying the signs of wear and bearing failure and determining the cause of the problem is essential to getting the most out of bearings and the machines that use them.

There are many ways bearings can wear and both visual and acoustic indicators will help determine the cause of the failure – suggesting the right course of action to mitigate future problems.

Acoustic indications of bearing failure. (Courtesy of Emerson Bearing, Inc.)

Failure	Cause	Appearance
Flaking Surface of the raceway and rolling elements peel away in flakes, leaving hills and valleys.	Excessive load Improper handling, mounting or precision in shaft Insufficient clearance Improper lubrication
Peeling Patches of minute flaking or peeling. Multiple hairline cracks visible.	Infiltration of bearing by foreign matter Insufficient lubrication
Spalling Score accompanying seizing. Scores on roller end face. Scratches in spinning direction of raceway and rolling surfaces	Poor mounting and removing practice Oil film discontinuation on contact surface Slippage or poor lubrication of rolling elements
Smearing/Scuffing Surface becomes rough and small deposits form. Roughness on race collar and ends of rollers.	Inadequate lubrication Entrapped foreign particles Roller skewing due to misaligned bearing Bare spots in collar oil film due to large axial loading
Wear Surfaces wear and result in dimensional deformation. Often accompanied by roughness and scratches.	Entrapment of foreign particles in the lubricant Inadequate lubricant Skewed rollers
Speckles/Discoloration Surfaces wear and result in dimensional deformation. Often accompanied by roughness and scratches.	Infiltration of foreign matter into bearing Insufficient lubrication
Dents/Scratches Scoring during assembly and surface denting due to mechanical shock.	Entrapment of foreign objects Bite-in on the flaked-off side Dropping due to careless handling Misaligned assembly
Chipping Partial chipping of inner ring, outer ring or rolling elements.	Impacts or excessive load Poor handling Trapped large foreign objects
Cracking/Notching Localized flaking, cracks and notches appear.	Excessive shock loads Improper handling Formation of decomposed surface layer due to improper lubrication Frication cracking
Rust/Corrosion Raceway surface lacks luster, is rough or matted.	Poor storage conditions Poor packaging Insufficient rust inhibitor Penetration by water, acid, etc.
Seizure Scoring during assembly, gouges and surface denting.	Insufficient clearance Insufficient or improper lubrication Excessive loads Skewed rollers
Electrical Pitting/Electrolytic Corrosion Localized flaking, small cracks and notches.	Electric current flowing through the rollers
Cage/Retainer Damage Scoring during assembly, surface denting due to mechanical shock.	Excessive moment loading High speed or excessive speed fluctuations Impact with foreign objects Excessive vibration

Examples of bearing failure. (Images courtesy of Emerson Bearing.)

The Solution to Premature Bearing Failure

Once you know the type of bearing failure you are dealing with, it’s time to determine the fix. Often the solution is relatively simple, such as switching to the correct lubricant for the speed, load and type of bearing you’re using.

Other times the solution can be more complex, with failures caused by several contributing factors related to lubricants, exposure to debris, load, operating speed and bearing type. An effective solution can involve adjustments to one or more of these factors.

“Sealed” bearings, for example, may exclude debris and retain lube in harsh environments, but in some cases may trap contaminants in the bearing and actually shorten bearing life. Similarly, pressure-fed lubrication might solve premature wear in some applications or exacerbate wear in poorly maintained, contaminated systems.

Breakdown of common bearing failure modes. (Image courtesy of Emerson Bearing.)

A bearing specialist such as Emerson Bearing can examine damaged and worn bearings and rollers, determine the cause of the problem and assist in designing a solution that will improve performance of bearings and avoid premature failure in the future.

For a complete rundown of bearing failures, causes and solutions, check out the Bearing Failure Analysis eBook from Emerson Bearing.

Emerson Bearing has sponsored this post. They have no editorial input into this post. All opinions are mine. James Anderton.