Machinedesign 5878 12355smbearings 0

Bearings designed to multitask

May 22, 2013
Adding new features and capabilities to bearings improves productivity and lowers costs.

Bearings have long been used to reduce friction, support loads, locate shafts, and reinforce system rigidity. Today, bearing designers are adding sensors, signal-processing capabilities, and advanced housings, and combining seals and lubricants in those housings. These smart bearings offer OEMs and engineers longer service lives and more options.

Here’s a detailed look at these next-generation bearings that will provide even higher levels of form and function.

Sensor bearings

“Intelligent” sensor bearings provide accurate information on speed (rpm), direction, and incremental or absolute position of rotating or linearly moving components. They can generate signals that contains data such as the number of revolutions, relative position/counting, and if they are accelerating or decelerating. These intelligent bearings contribute to productivity and reliability, and allow for more-compact equipment.

This cutaway drawing of an ac motor shows the built-in sensor (Circled; red/blue) that generates speed and position data, which can be used as feedback signals.

In a typical scenario, a sensor gets designed into the bearing. The sensor is often an impulse (or code) ring linked to the bearing’s rotating inner ring. The inner ring contains a sequence of north and south-oriented magnets. The sensor detects changes in magnetic fields as the bearing and its magnets spin and converts those changes into digital signals. Each rotation of the bearing generates a predefined number of pulses or magnetic signatures, which can be decoded in real time as rotational speed and position. If there is too much EMI, the bearing and sensor can be shielded.

Intelligent bearings such as these can serve as motor encoders to monitor position, rotational speed, and direction of motor shafts. They mount inside the motor and provide reliable alternatives to conventional encoders, which mount outside the motor. Variations on this setup include end-of-shaft multiturn sensors, absolute through-shaft sensors, and high-resolution absolute-position sensors for measuring position, and roller encoders for making linear measurements.

Steering units

Steering units for steer-by-wire modules on industrial and off-highway vehicles replace conventional steering columns or provide an alternative to joystick-controlled machines, which are becoming more common.

Encoders that are built-in on ac motors can monitor rpm speed, shaft position, and direction of rotation, and come in a variety of sizes and styles.

Either noncontact incremental or absolute position sensors can trace movements of a steering wheel and, in turn, detect steering-wheel position, steering speed, and direction. For many applications, these sensors can be packaged in a basic steering encoder which creates constant steering friction and moderate but acceptable resolution of steering signals. These compact encoders often combine a stainless-steel stub shaft (on which the steering wheel mounts), two noncontact rotary incremental sensors, a friction mechanism that creates the steering “feel,” and rolling bearings inside a compact and durable housing. Units can have standard resolution (256 increments/turn of the steering wheel) or high resolution (1,024 increments/turn of the wheel), depending on the sensors.

For steering units with more functions and performance, smart bearings give drivers a realistically changing steering feel. These units detect steering-wheel position to generate end stops and just the right amount of resistance (or feel) to turning, which depends on current driving conditions. These units add two independent CANbus interfaces and an electromagnetic brake to the previously described basic steering unit.

The vehicle controller (via the CANbus) or a microprocessor in the steering unit controls the brake so that torque resistance on the steering wheel accurately changes to simulate feedback from mechanical steering units. The brake also creates left and right end-stops that keep the steering wheel from turning. The brake lets go when the steering wheel moves in the opposite direction. Unlike mechanical or hydraulic steering systems in which end-stops are fixed, end-stops in these units are programmable. The steering ratio can also be programmed to vary with vehicle speed or other operating parameters.

This SKF sensor includes an impulse or code ring, which links to the rotating inner ring of a bearing to detect and measure rotational speed and absolute position input.

Built-in redundancy and fail-safe operation are critical for safety in steering units. This means that both the absolute position sensor and output channels comprising the microprocessors and CANbus interfaces are designed with internal redundancy. For example, if one of the two CANbus interfaces fails, the second will continue to operate, providing data to the vehicle controller. During normal operations, these two channels of communications are continually compared to each other to ensure both are providing accurate feedback. If the vehicle controller detects a discrepancy between the two signals, the OEM can design a “limp-home” mode that lets the driver use the vehicle , but with reduced functionality. If additional driver locations are required on a vehicle, several steering modules can be added to the system providing a “belt-and-suspenders” approach to vehicle-steering redundancy.

Advanced housings

Bearing housings are stronger and designers are adding new features, including seals, making custom housings largely unnecessary. For example, upgraded SE plummer block housings are now designed to be stiffer and stronger so they better resist deformation. And larger ribs in the center of the base improve heat dissipation by increasing the contact area between the base and support surface, thereby improving heat flow away from the bearing’s outer ring. These upgrades also lengthen the block’s service life.

Housings have also been designed to make serving easier by adding a grease-guiding flange that sends grease directly from the fitting to the bearing, which lets technicians lubricate the bearing efficiently from the side. Housings can also have grease-level markings at each corner inside the base that show when the proper amount of grease has been applied, and an off-center hole in the housing cap relative to the shaft axis permits a higher flow of grease during servicing.

Combining intelligent bearings, advanced housings and seals, and the proper grease helps keep bearings operating in harsh environments such as mines.

Bearing housings are also designed to accommodate sensors that measure operating parameters such as bearing rotation and speed, temperature, and vibration in real time. Technicians tracking these parameters can detect and diagnose operating problems before they escalate into failures.

Some of these features can work to minimize environmental impact, as well. For instance, the previously mentioned grease-guiding flange ensures no lubricant is wasted and only the minimum is used; and better heat conduction in housings reduces grease consumption, which also reduces the amount of grease that must be properly disposed.

Bundled bearing packages

Adding lubrication and sealing to bearings can reap rewards in performance and reliability by extending service life, reducing grease consumption, and protecting components. Plus, from an OEM designer’s perspective, precisely “bundled” bearings can be smaller, making them well matched for today’s ever-smaller design envelopes.

Bearings and efficiency

The engineering behind bearings has changed over the years, letting designers realize greater efficiencies and more performance. Yet the main task of bearings remains the same: reduce friction and, in the process, reduce energy consumption.

While developing new bearing designs, researchers rely on sophisticated computer modeling and proprietary software to attack virtually every source of bearing friction. With roller bearings, for instance, engineers have tightened bearing specifications and refined the internal geometry of the polymer cage, roller, raceway, and guiding flange. And bearing companies make these designs using precise manufacturing techniques.

For energy-efficient (E2) bearings, engineers focused on getting the number of rollers just right. They also wanted to modify the raceway to cut the weight of rotating parts by about 10% because lighter bearings take less power to move, making them more efficient. The lower-weight moving parts also have less inertia and are, therefore, less likely to skid or smear, which reduces performance and service life. E2 bearings also generate less heat and run cooler, extending the service life of the lubricants.

Relative to comparably sized standard bearings, energy-efficient versions can reduce frictional losses by 30% or more and are well suited for grease-lubricated, light-to-normal loads such as electric motors, pumps, conveyors, and fans. And shielded versions have longer service lives than standard bearings.

In one case, bearings were needed on mining equipment that could survive the harsh environment. The goal for the new design was that it last longer than current versions and help prevent premature bearing failures — without relying on complicated and relatively expensive auxiliary taconite seals or consume large quantities of grease. The design team’s “three-barrier solution” included the housings with seals, grease, and sealed spherical roller bearings — all in a single package.

The final bearing, a proprietary design, consists of split pillow-block housings with PosiTrac Plus seals, factory-sealed high-performance spherical roller bearings prepacked with specially formulated grease, and then more grease between the bearing and seals inside the housing. This effectively delivers three layers (“three barriers”) of protection for the bearing during assembly and operation. The PosiTrac Plus seals in the housing are the first line of defense against extremely fine contaminants. The second barrier is the grease packed into the housing on both sides of the sealed bearing at installation. And the third layer of protection are the bearing seals which keep clean lubricant inside the bearing and keep contaminants out.

Sealed spherical roller bearings are well suited to applications where uptime is critical and bearing maintenance must be minimized. These bearings also mitigate grease leakage, especially important where environmental concerns are an issue. In one case, a coal-mine operator replaced open cylindrical-roller bearings with sealed spherical-roller bearings in conveyor impact idlers and saw service life double. Installing the spherical-roller bearings took half the time of conventional bearings and, being greased for life, the new bearings dramatically reduced relubrication costs and disposal of used grease. In addition, maintenance staff could focus on more critical mining operations.

Another mining operation reported that its annual grease use was cut by more than 220 lb at each bearing position in a conveyor by replacing open spherical-roller bearings with sealed versions. The investment paid off in a year, while operational costs associated with each bearing position were halved. This can add up to considerable savings as disposing of grease can cost more than the purchase price.

PosiTrac Plus seals offer distinct advantages over traditional taconite seals. The contact lip seal keeps out debris and is protected by the bearing housing. And the Plus seals cost less than taconite alternatives.

In short, by adopting a “three-barrier solution,” end users ultimately could increase a bearing’s service life by up to a factor of three, compared with traditional arrangements, and cut lubrication by 90%.

Combining bearings with sensors, specialized housings, seals, or lubricants, along with advances in the bearing design, delivers more functions with fewer components in smaller packages for more productivity.

Mark Hinckley, Director of Strategic Projects, SKF USA Inc., Landsdale, Pa.

Edited by Stephen J. Mraz, [email protected]

Resources: SKF USA Inc.,

Click here for some apps from SKF, including a Bearing Calculator

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