Unearthing root causes

Advances in bearing designs and technology take aim at common reasons for failures.

by Daniel R. Snyder

In theory, a rolling bearing can exceed the life of machinery under good operating conditions and provided the fatigue load limit is not exceeded. In practice, bearing failures will occur, but rarely due to inherent manufacturing defects.

Other factors more typically are to blame and such “root causes” of failures beyond the control of the manufacturer can significantly shorten bearing service life. The good news is that bearing designs and related technology have kept pace to help keep root causes of bearing failures in check.

Users can optimize the service life of bearings by first adhering to the basics. Bearing selection must meet the specifications of operating conditions involving speed, load, and temperature, among others; bearings must be stored properly and installed with care and precision; and proper lubrication in the correct amounts is essential.

Recently conducted damage analysis has offered clues about common operational causes of bearing failures. Among these are poor lubrication, mounting and dismounting errors, contamination and electrical damage.

PROBLEM: Inadequate lubrication
The term “lubrication failure” too often is taken to imply there was no oil or grease in the bearing and that it only happens occasionally. It’s not usually so simple. Many cases require a thorough examination of the lubricant’s properties, the amount of lubricant applied to the bearing and the operating conditions. All these factors contribute to “inadequate lubrication,” which fosters surface damage leading to bearing failure.

SOLUTION: Proper greases and application
For rolling bearings to operate reliably under heavy loads at high speeds, they must be lubricated to prevent metal-to-metal contact (and resulting friction) between the rolling elements, raceways and cages. The lubricant chemistry must be capable of generating an elastro-hydrodynamic film in the high contact pressures seen in rolling bearings.

Lubricants also inhibit wear and protect bearing surfaces against corrosion. The choice of a lubricant depends primarily on operating conditions such as the temperature range, speeds and surrounding influences. The lubricant’s additive content is also a consideration.

Huge strides have been made in grease offerings and grease-life calculation models. Users can choose from a wide range of high-quality greases developed specifically to meet the needs of rolling bearings and their application conditions, based on extensive research, grease performance testing and field experience.

Today, synthetic oils and urea type greases are becoming more common to obtain longer relubrication intervals at higher temperatures. Solid lubricants have been introduced specially for extreme temperature conditions.

Delivery systems have advanced beyond conventional grease guns, manual and air-driven grease pumps and grease filler pumps. Automatic lubricator systems can now dispense lubricant at pre-set and predetermined intervals (24 hours a day, seven days a week) to  avoid potential over- or under-greasing, minimize time-consuming  maintenance oversight and reduce unwanted lubricant consumption.

Over time, the lubricant in any bearing arrangement gradually loses its lubricating properties as a result of aging, mechanical work and contamination. This underscores a necessity for grease to be renewed or replenished and for oil to be regularly filtered and changed to promote maximum bearing life.

PROBLEM: Contamination
Although foreign matter can enter a bearing during mounting, its most frequent  area of entry is through the bearing or housing seals. Abrasive particle wear and particle denting in bearings can prove disastrous.

SOLUTION: Integral seals
Seals prevent contaminants from infiltrating bearings and minimize grease leaks. A reliable seal option exists for virtually every type of rolling bearing and should be specified in accordance with application parameters.

Sealed-for-life bearings feature three main components: the bearing, seal or shield and lubricating grease. Once limited to consumer products, sealed-for-life bearings have been engineered for a broader range of applications and  much larger equipment, including pumps, conveyor systems, air conditioning units and construction equipment.

These bearing systems would not be relubricated during the expected machine life cycle. If, due to adverse conditions, the grease needs to be replaced, then the entire bearing assembly is replaced.

Another notable development in seals is tailor-made for widely used deep-groove ball bearings. Specialized nitrile rubber, low-friction seals promote lower energy consumption and offer superior grease retention.

PROBLEM: Mounting/dismounting errors
If a bearing is subjected to loads greater than those calculated for its life expectancy, premature fatigue can result. Unanticipated or parasitic loads can arise from faulty mounting practices; never subject  bearing rings, cages and rolling elements or seals to direct blows and never direct mounting force through the rolling elements.

SOLUTION: Suitable tools and techniques
Because they are precision components, rolling bearings should be handled carefully and mounted with the proper equipment and installed correctly to realize maximum service life.

An estimated 16 percent of all premature bearing failures are caused by poor fitting, usually using brute force, and being unaware of  proper mounting tools and methods. Individual installations may require mechanical, thermal or hydraulic methods for correct and efficient mounting, depending on the bearing type and size.

A main reason for dismounting an “old” bearing is to replace it with a new one. When proceeding, take care not to damage the shaft in the process; this can compromise a machine’s efficiency. A damaged shaft can greatly influence the service life of the new bearing.

Bearings are also dismounted for maintenance or replacement of other machine components. Since these bearings will be mounted again (unless they are damaged during dismounting), proper dismounting methods and tools are required. Tool selection depends on bearing type, size and fit.

PROBLEM: Electrical damage
In some machinery applications, there is the possibility that electric current will pass through a bearing. Current that seeks ground through the bearing can be generated from stray magnetic fields in the machinery, voltage buildups or with  variable-speed AC motors using high-speed frequency converters.

Currents can even be caused by welding applications where a  ground is attached that forces the circuit to pass through the bearing.

Damage occurs when electrical current passes through a bearing (flows from one bearing component to another). The result is washboarding, craters on the rings and rolling elements and premature aging of the lubricant.

Another form of electrical damage occurs as current passes during prolonged periods and the number of individual pits accumulates drastically. The result is fluting, and flutes can develop considerable depth, producing noise and vibration during operation and eventual fatigue from local overstressing.

SOLUTION: Hybrids
Hybrid ball bearings were designed to resolve electrical damage issues. These types combine traditional steel rings matched with silicon nitride (ceramic) balls to provide natural insulating properties and prevent electrical arcing and fluting-pattern surface damage to raceways. They have further demonstrated additional benefits.

Tests have shown superior scuffing resistance of silicon nitride-metal contact under pure sliding, due to the smoother surface and hardness of the ceramic rolling element. Scuffing, or smearing, would cause vibrations in the bearing and eventually lead to bearing failure.

Daniel R. Snyder, P.E., is director of applications engineering for SKF Industrial Division, SKF USA Inc; phone: 215-513-4680; e-mail: daniel.r.snyder@skf.com; Web site: www.skfusa.com.

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