The Route To Bearing Reliability

There are no shortcuts, but lubricating correctly makes it easy to follow. The right materials and procedures are key.

Our recent “State of the Lubrication Nation Survey” revealed several roadblocks that prevent many North American industrial sites from moving toward implementation of Good Lubrication Practices (GLP) in their maintenance programs. When such practices aren’t implemented, the reliability of a plant’s equipment and processes can suffer—starting, in many cases, with bearings.

This article addresses several crucial issues involved in the quest to extend bearing life. These issues begin to surface before a bearing even goes into service. For example, when an engineer designs a machine that involves moving parts, he or she is expected to choose the appropriate bearings. Several factors affect this selection, including:

• Budget
• Application (radial, axial and planular)
• Load
• Speed
• Space
• Clearance and fit
• Length of machine warranty
• Bearing reliability specification
• Lubrication entry design and method
• Expected operating conditions

When a machine goes into service, the reliability baton passes to the end-user maintenance department that must work with the engineer’s final design and an often-vague machine operations and maintenance (O&M) instruction manual that rarely spells out good lubrication instructions. Many maintenance planners will recognize the catch-all instruction “lubricate as necessary,” placing responsibility on the end-user to develop a lubrication strategy suitable for the ambient conditions in which the machine is expected to perform.

Unless the maintenance department literally starves the bearing of lubricant in the first year, most bearings will surpass their warranty period, and the manufacturer probably won’t learn if their design is truly robust and reliable. Ultimately, the design conditions are set, and machine availability and reliability depend on how well the maintenance department understands: 1) bearing design and their lubrication requirements; 2) how machine bearings fail in their working environment; and 3) their ability to design, implement and execute a strategic asset-lubrication program that adequately meets bearing needs.

A noisy bearing
contaminated with paint (Courtesy ENGTECH Industries, Inc.)

What’s in a bearing?

As highlighted in Part 1 of the article “The Inner Life of Bearings” by reliability expert Neville Sachs, when we think of a bearing, various shapes, sizes and materials come to mind. These components can take on many forms in performing their duty to support sliding or rotating parts.

To recap, sliding contact bearings (commonly known as plain, sleeve or journal bearings) allow full sliding contact between mating surfaces in three specific ways: radially, in which the bearing provides a 360˚support for a rotating shaft or journal; axially, in which the bearing supports any side thrust load from the end of the rotating shaft; and planular, in which a flat bearing surface acts like a slipper to guide moving parts in a straight line.

To help carry the impressed load with minimized friction and wear, a lubricant is introduced into the engineered clearance between the mating surfaces via a specially cut channel to generate hydrodynamic/hydrostatic full fluid film separation of the surfaces to allow them to slide freely over one another. Sliding bearings are most commonly manufactured in yellow metal or composites of brass, bronze and copper, the bearing material designed to be softer than the supported component.

Rolling contact bearings (commonly known as ball bearings, roller bearings and needle bearings) provide a rolling contact that supports both radial and axial thrust—often simultaneously. These bearings are often termed “anti-friction” bearings due to their point contact area, where lightly lubricated rolling elements (balls, rollers and needles) carry the impressed load under an elastohydrodynamic lubricant film.

Rolling-element bearing manufacturers measure the reliability of their bearings using a Load-Life calculation rating, which is known as the L10 rating life. To achieve its reliability design rating, the manufacturer assumes the bearing will be operated within its load-and-speed design limits in a clean operating environment, and that an adequate lubricant film of the correct viscosity is applied on a regular basis (“adequate” meaning a film equal to or greater than the composite roughness of the two mating surfaces). As a result, reliability is the minimum percentage probability of 90% of a group of identical bearings achieving their L10 design life expectancy (operated under identical operating and load conditions). With the advent of cleaner and degassed steels used in rolling-bearing manufacturing, manufacturers are now able to upgrade the L10 rating to L10a designations that promise even higher reliability percentages.

The reality is, many bearings lead a less than ideal life, often subjected to the severest conditions and many forms of abuse within industrial-plant environments. Many maintenance and reliability departments don’t take time to understand the root cause of bearing failures, nor do they implement life-cycle management strategies for these key components.

An over-greased bearing. Maintenance chose not to reset the auto lubricator but to put in place two grease-catchment devices and a PM to empty the grease regularly. (Courtesy ENGTECH Industries, Inc.)

The many causes of bearing failure

If asked to project which bearing is most likely to achieve L10 life status in the following scenarios, which would you choose?

Scenario 1: A pillow-block bearing is placed in service in a HEPA-filtered clean-room manufacturing environment. The bearing runs under light load conditions for eight hours per day, is set up using a laser-aligned and balanced drive shaft, and is continually lubricated using an engineered automatic oil lubrication-delivery system.

Scenario 2: The same pillow-block bearing is placed in service in a hot, dirty foundry operating two full shifts per day. The machinery is set up using manual “eye-ball” alignment techniques, and is manually lubricated with a grease gun on a PM schedule with a subjective job task that states “lubricate as required.” 

If you are like most, you will have voted scenario #1 as the likely winner. In reality, both bearings are likely to prematurely fail if maintenance has not understood how failure can occur and planned accordingly to prevent it! The top 10 causes of bearing failures—confirmed through this author’s 40+ years of investigating real-world failures—are as follows:

• Lack of lubrication training
• Lack of lubrication-application engineering
• Poor housekeeping (lack of order and cleanliness)
• Over-lubrication of bearings
• Under-lubrication of bearings
• Use of dirty or contaminated new lubricants
• Infrequent oil/filter changes
• Bearing lubricant contaminated with an incompatible lubricant
• Bearing lubricated with the incorrect lubricant
• Bearing mounted out of square or misaligned when set up

Note that nine out of 10 items on this list are due directly or indirectly to ineffective lubrication practices.

Taking the path to bearing reliability

The first step to reliability is to stop reacting to failure symptoms. For example, a simple oil leak is not always a failure if the type of shaft seal used is designed to leak. If this is not the case, other causes could include a cross-threaded drain plug, a drain plug refitted with no washer, a plugged breather, a damaged seal or even a porous casting.

By contrast, an oil leak in a hydraulic system can be caused by dirt-contaminated hydraulic oil scoring lapped spools and nicking seals. Both show the same oil-leak symptom, but can have very different root causes. Implementing a root cause analysis of failure (RCAF) program to weed out the real reasons your bearings are failing, and using those findings to implement a strategic lubrication management program will make an excellent first step. Here are other steps to take:

Training

Use your RCAF findings as a kick-start to educate your maintenance staff on the value of good lubrication practices soon to be adopted.

Lubricant choice

Solicit the assistance of a reputable lubricant consultant and/or lubricant manufacturer/supplier to facilitate an engineered lubricant-consolidation program. The purpose will be to determine the most suitable lubricant selection for all your bearings, and picking the correct oil viscosity and additive package designed to work in your ambient conditions that will reduce lubricant degradation and the number of oil changes.

Lubricant contamination

Contaminants are literally “bearing assassins,” the biggest culprits being dirt and water. Contamination avoidance is achievable by implementing a simple housekeeping program designed to keep gearboxes clean of dirt and shielded from water; and dedicated transfer and delivery equipment for each lubricant type, clearly labeled to eliminate dirt contamination and lubricant cross-contamination.

Also, ensure all grease guns and nipples are cleaned before and after use with lint-free rags. And test all bulk fluids to determine their fluid cleanliness and additive package formulation before use to determine if they’ve been delivered in a clean-state specification.

Engineered application

Excessive heat is a common problem found in many manually greased bearings that have received too much grease. Virtually no two grease guns are alike in their grease displacement; a single shot in one can amount to five or more shots in another. Yet a PM task may ask for two shots of grease, which results in over-greasing. Furthermore, a good-hearted maintainer may contribute to the problem by choosing to add an extra “shot or two” in the mistaken belief that more is better!

Bearing cavities are designed to operate with a grease charge of less than 50%.  Filling the cavity until the grease passes by the seal is more than two times the required amount of grease. Excess grease causes internal fluid friction in the bearing. In turn, this creates a significant rise in bearing temperature, resulting in reduced bearing life and a greater increase in energy use. Ensure all grease guns in the plant deliver the same amount of grease displacement.

Navigating your way to bearing reliability is not expensive or difficult. You only need to recognize the value of effective lubrication practice and choose to do it.