Trends in bearings 2014

Trends in the design of main shaft and gearbox bearings are driven by unexpected failures in these units. Unplanned main-shaft bearing replacement costs operators up to $450,000 and have an obvious impact on financial performance.

One gearbox service company says certain gearbox models show a particular design weakness: use of a four row, cylindrical bearing with a through hardened race in the planet gears. This is not an optimal bearing configuration for the application.

The company adds that when remanufacturing such a gearbox, it is best to use a case carburized, inner-race bearing with coated rollers. In addition, altering clearance across the individual rows improves load sharing, thereby providing a longer performing bearing. This can also prevent common case-crack failures due to debris that collects in the bottom of a gearbox. It is also necessary to use case carburized and coated bearings in the high speed and intermediate positions. These simple upgrades can greatly enhance the overall performance of the gearbox, leading to significant life extension.
In main-shaft bearings, modular wind-turbine designs commonly use spherical roller bearings (SRB) to support and carry main-shaft loads. Designers often select single SRB designs, one supported by a single main bearing and two torque arms that carry gearbox reaction loads.

At least two factors contribute to early failures. One is high thrust load on a radial SRB bearing. While there is no official maximum limit, a conventional ratio of permissible thrust-to-radial load for two-row spherical roller bearings is between 0.15 and 0.20. The second factor is inadequate lube film generation. Generally, operating conditions for the main shaft’s bearing are not ideal for lubricant-film generation. With a max operating speed of ~20 rpm, the bearing surface speed and lube-film generation may be insufficient to separate the roller-to-race asperities.

After several years of analyses, experimentation, and design work, upgrades are now available in the market for existing turbines, and more sophisticated engineering designs for newer turbine platforms.

A direct interchange to existing fleets is an ideal solution. At least one company offers a wear-resistant SRB that uses engineered surface technology in combination with enhanced surface finishes. Wear resistant bearings increase raceway protection against micropitting by reducing shear stresses and asperity interactions. The engineered surface is a durable, proprietary tungsten carbide, amorphous-hydrocarbon coating. Generally, such coatings are moderately harder than HRc60 steel, 1 to 2-µm thick, and have low friction coefficients when sliding against steel. The engineered surface on the rollers polish and repair raceways damaged during operation. With enhanced surface finishes, the lubricant film increases thickness, helping reduce asperity contacts. The engineered surface also reduces asperity interactions and surface-shear stresses that cause wear. The benefits lead to an increased calculated bearing life and a reduction in rolling torque.

A tapered roller bearing (TRB) main-shaft design with preload characteristics can also improve the powertrain performance. TRBs help ensure system stability and rigidity, load sharing between rows, and predicted roller-to-race interactions. This design allows several configurations.

For one, a widespread single tapered roller bearing offers an economical design that can preload an entire system with two dissimilar TRBs. An upwind and downwind bearing series can then accommodate the application load by adjusting the contact angle and bearing capacity as needed. With a widespread effective center, the bearings are usually more compact.

Another configuration on the large diameter, tapered-roller bearing use a spacer between cone races. This has become an appealing option based on its field performance and ease of assembly. Steep race angles create high-tilting stiffness in a short axial space to counteract applied pitch and yaw moments. Separate bearing components can include seals and grease to simplify handling and installation. A factory set preload ensures a properly mounted setting.

In a third configuration, a single preloaded tapered roller bearing, offers a high load capacity and manages the combination of radial and thrust loads as compared to a single SRB. The single preload ensures load sharing across both bearing rows and tolerates greater system misalignment as compared to the design with the space between cone races.

Reducing Linear Bearing Wear

To get the longest working life out of a linear bearing, keep it clean and well lubricated. This commonsense advice may sound easy enough to follow, yet in the real world of round-the-clock, high-cycle manufacturing operations, bearings do get dirty and dry.

And when either of these conditions happens, linear bearings will wear prematurely. In the worst-case scenarios, contamination and inadequate lubrication can create metal-on-metal contact between the bearing’s rolling elements and raceway. This can cause excessive wear in the form of denting, pitting, or galling.

This warning about contaminants and the importance of lubrication will not come as news to anyone who has designed or worked around industrial machines. When using linear guides on medical, food, packaging, semiconductor, or other sensitive equipment, machine builders often take extraordinary measures to keep the contaminants out and the oil in. They may add expensive bellows to cover the guides, or they may opt for a pricey automatic greasing system.

Yet in their zeal to keep linear motion systems running smoothly, machine builders can overlook less expensive design solutions to contamination and lubrication issues.

Combating contamination
Contamination comes in many forms, some more aggressive than others. Metal chips from machining operations, for example, qualify as one of the biggest offenders from a wear perspective. Silicon dust produced during semiconductor manufacturing can also be tough on linear bearing surfaces. Modern manufacturing processes can throw off a long list of other abrasive, wear-inducing contaminants.

Design Decisions: A Comparison of 3 Different Ball-Bearing Materials

Ball bearings can be used in a variety of applications from medical and aerospace technology to packaging equipment, electronics, office technology, and even high-end yo-yos. Since these components are available in different types of materials, each with its own set of features and benefits, weighing the pros and cons of a specific kind of ball bearing can become an important part of the design process.

Three main types of ball bearings are steel, ceramic, and polymer. While every ball bearing is comprised of four main parts -- an outer race, an inner race, a cage, and balls -- each has its own set of characteristics and benefits.

xiros plastic ball bearings open new fields of application for plastic bearings. The inner and outer races are made from high-performance iglide materials. The corrosion-free balls are made from stainless steel. Glass balls are also available for maximum corrosion resistance.
xiros plastic ball bearings open new fields of application for plastic bearings. The inner and outer races are made from high-performance iglide materials. The corrosion-free balls are made from stainless steel. Glass balls are also available for maximum corrosion resistance.

Steel ball bearings
Partly because they are an older technology, steel ball bearings are a trusted solution for many design engineers. Typically, these types of bearings are comprised of all-steel parts, but are available with different types of steel races and balls, or with a phenolic cage. Steel ball bearings are ideal for robust applications handling extremely high loads and fast rotations per minute (RPMs), specifically when compared to polymer ball bearings, and some feature a radial load capacity of up to 30,000 lb. Another advantage is that steel ball bearings tend to be more precise due to the clearance that can be achieved.

However, steel ball bearings do have some disadvantages. They are heavy, tend to deliver noisy operation, and depending on the grade of steel, lack chemical resistance. They require constant lubrication, which increases the time and cost spent on maintenance. Steel is also susceptible to corrosion in humid or wet environments, and specifically in medical applications, its magnetic properties can cause problems.

In addition, there is such a high number of steel ball bearing manufacturers that prices can greatly vary, ranging from downright inexpensive to very costly. This could be perceived as a pro or a con, but either way, the options can be overwhelming at times.

Ceramic ball bearings
The most common type of ceramic ball bearing is often considered a "hybrid," which indicates the outer race, inner race, and cage of the bearing is comprised of steel, while the balls are made of ceramic. The ceramic material enables the bearing to run faster while maintaining a cooler operational temperature and simultaneously reducing noise and vibration.

Ceramic ball bearings tend to be more corrosion-resistant, more rigid, and lighter weight than most steel ball bearings. Lower coefficients and higher RPMs are also possible, and since they are nonconductive, ceramic ball bearings can be used in electrical applications. In addition, most ceramic balls bearings can operate in temperatures up to 1,800F.