Bearings: A new roll to play
Rolling bearings in rotating equipment traditionally function to reduce friction with rolling elements, support shaft loads and provide system rigidity and shaft location. But the latest trends underscore a dramatic evolution in technology and significant improvements in design and engineering to enable bearings to contribute much more.
Many advances can be attributed directly to emerging marketplace and application needs. Plants strive to save energy, minimize maintenance and boost productivity. They run machinery faster, quieter and longer; and optimize equipment performance. Rolling bearings can play vital roles in meeting these objectives %%MDASSML%% whether in electric motors, gearboxes, fans, pumps, compressors, tools, robotics or other equipment applications %%MDASSML%% regardless of industry.
As in most areas of plant operations, the need for energy efficiency is having an effect on how bearings are designed and manufactured. The need for multifunctionality requires innovation in bearing design. The use of protective coatings and unconventional materials, and the desire for integral seals are among the guidepost trends making their mark in the world of bearings.
Addressing energy efficiency
Motor-driven equipment, such as pumps, compressors, conveyors and fans, accounts for roughly 16% of all the industrial energy consumed in the U.S. As energy costs continue to rise, plant managers seek effective ways to reduce energy consumption. One path is to increase electric motor efficiency.
According to some industry estimates, motor-driven systems lose up to 15%, or 204 billion kilowatt-hours (kWh), every year due to motor inefficiency. About 20% of this loss (41 billion kWh/year) is due to mechanical losses and 20% (8.2 billion kWh/year) relates to bearings. The very purpose of bearings is to reduce the amount of energy a machine requires.
New designs have increased the potential to deliver increased efficiency. Recent designs of deep-groove ball bearings are engineered to work with less energy. They benefit from an optimized internal geometry, a unique, polymer-cage design and high-performance grease to exhibit significantly less friction torque, or friction loss, compared with conventional bearing types. Friction loss can be reduced by at least 30%, depending on bearing size and application conditions. This leads to reductions in power loss and energy consumption.
Additional strides in seals, lubrication systems and other related technologies further compound the capabilities for plants to use bearings to gain more energy efficiency.
More than one function
A new generation of bearings performs multiple functions and add value in the process. Sensor-bearing units, which integrate a sensor, an impulse ring and a ball bearing, provide precise information on the motion status of rotating or axially-traveling machine parts. These ‘intelligent’ bearings record revolutions, speed, direction of rotation, relative position and/or counting and acceleration or deceleration.
Relevant applications include electric motors, linear actuators, steering systems, conveying and handling systems and automation equipment.
The bearing unit’s impulse ring moves past the stationary sensor ring when the inner ring rotates, generating a magnetic field of changing polarity. The sensor outputs a pulse, based on the number of polarity changes per second. The sensor output signal then is transmitted by connecting cable and used to generate the required application-specific information.
Bearing housings are being integrated with sensor technology for the purpose of reliably tracking machine performance, another trend towards multi-functionality. Machines are usually monitored with external instrumentation and condition-monitoring sensors positioned around bearing housings. The downside of this approach is susceptibility to vibration and a need for additional protection or shielding for effective operation. Bearing housings with built-in sensors are practical solutions.
With smart housing bearing designs, machinery rotational speed, temperature and vibration can be monitored. Signals from the sensors can be processed in a printed circuit board to provide a robust output signal resistant to interference. The board also incorporates power surge protection circuitry to shield the sensors from anomalies in the power supply signal. These signals also can be used as input to a machine control system.
Coating bearings serves, protects
Whether intended to resist wear or insulate, newly formulated coatings can both serve and protect. Most bearing types can be coated and, should pre-existing bearings need to be replaced, coated bearings can be ‘dropped in’ without modifying or reconfiguring equipment.
Wear-resistant, low-friction coatings applied on a bearing’s inner ring surfaces can withstand severe operating conditions due to sudden load variations, high operating temperatures, poor lubrication, vibration, smearing and/or contamination. The coated bearings can optimize bearing speed and life in such plant applications as compressors, fans, hydraulic pumps and electric motors.
Compared with standard uncoated types, coated bearings are harder, generate less friction and heat, and can better tolerate potential damage from contamination and/or marginal lubrication.
Another coating alternative offers excellent insulating properties. The coating is applied to the exterior of a bearing’s outer or inner ring and can resist potential damage from the passage of electric current through the bearing %%MDASSML%% notably present in variable-speed electric motors and generators. A thin aluminum-oxide layer forms the barrier against potential electric arcing, which can cause fluting damage to rollers and raceways, or grease oxidation. This electrical erosion can potentially result in premature bearing failure.
Using unconventional materials
Materials used in the manufacture of rolling bearing rings, rolling elements and cages impact bearing performance, reliability and service. New materials are always being evaluated, as they can help make or break an application’s success.
In the case of bearing rings and rolling elements, newer process developments for through-hardening bearing steels have allowed for reduction of oxygen and non-metallic elements, realizing improved properties. For bearings operating at temperatures higher than 250 C for extended periods, highly-alloyed bearing steels retain hardness and bearing performance characteristics.
In applications where corrosion can develop, stainless bearing steels such as chromium or chromium/molybdenum stainless introduce a measure of protection. However, their reduced hardness lowers load-carrying capability compared with conventional steels, and corrosion resistance is only effective when the entire surface is perfectly polished and undamaged during mounting.
Bearing-grade ceramics, such as using silicon nitride for rolling elements, have solved problems in many applications due to the material’s high hardness, low density, low thermal expansion, high electrical resistivity, low dialectric constant and lack of response to magnetic fields. Ceramics, in turn, have helped create the category of ‘hybrid bearings,’ which combine the silicon nitride rolling elements with steel rings to exhibit demonstrable advantages compared to conventional, all-steel bearing counterparts. They are lighter, harder and more durable; can run at higher speeds and with lower operating temperatures; and are highly resistant to wear %%MDASSML%% even under poor lubrication conditions.
The ceramic balls are roughly 40% less dense than steel balls, reducing centrifugal force and friction and enabling them to run faster and cooler. The relatively greater hardness yields enhanced wear resistance against hard particles, contaminants and vibration; and their highly inelastic nature means increased bearing stiffness and reduced deflection under load to promote reliable performance. Their natural insulating properties prove particularly advantageous in variable-speed motor and generator applications prone to the potential for electric arcing. In contrast to coatings, hybrid bearings insulate ‘from the inside.’
Turning to bearing cages, newer polymer types have demonstrated superior thermal, friction and wear properties compared with conventional steel or brass counterparts. Polyetheretherketone (PEEK) cages can operate at higher speeds, perform at higher temperatures and provide enhanced resistance to aggressive agents. They have exhibited excellent mechanical properties, even at temperatures up to 200 C. They resist heat aging in aggressive media and demonstrate good tribological properties relative to wear resistance and low friction. Compressor applications are among those that can clearly benefit.
In general, the primary purpose of seals is to retain lubricant and protect against contaminants %%MDASSML%% including mineral-oil based lubricants, fuels, oils and greases, dust and water. An increasing number of bearing types have been engineered with pre-lubricated seals that are ‘greased for life,’ which makes external seals unnecessary, simplifies bearing arrangements, minimizes maintenance requirements by eliminating any need for on-site lubrication and promotes ‘green’ operation on the plant floor. The nitrile rubber, low-friction seals further can promote lower energy consumption, offer ideal grease retention and help extend bearing service life.
Perhaps an entirely new performance class of rolling bearings has been pioneered. Unprecedented improvements relating to service life, load-carrying capacity, friction, noise and vibration, wear resistance and speed capabilities have been realized.
The new designs encompass most sizes and types of rolling bearings found in plant machinery across industries, including spherical roller bearings, angular contact ball bearings, cylindrical roller bearings, spherical roller-thrust bearings, tapered roller bearings, deep-groove ball bearings and four-point contact ball bearings.
More are on the way.
|Daniel R. Snyder, P.E., is director of applications engineering for SKF Industrial Division, SKF U.S.A., Inc.|