By Mike Santora
Bearings are ubiquitous and essential components for reducing friction between moving parts. Their primary function is to give moving parts something to roll on rather than slide over. Their secondary function is to transmit loads. Bearing loads are either radial (perpendicular to the shaft), axial (parallel) or a combination of the two. Depending on the type of loads expected and the likelihood of extreme temperature, pressure and contaminant exposure, there are different bearings specialized for certain applications.
Consider the bearings and slewing rings in wind turbines. These can reach 2 m in diameter and must, in some cases, bear hundreds of tons. At the opposite end of the size spectrum (but only slightly less demanding) are bearings for medical applications. These often take the form of compact equipment necessitating bearings of just a couple millimeters in diameter, with rollers that are smaller. Some going into equipment subject to washdown have treated steel sub-components; others for disposable or quiet designs are made of plastic. Likewise, industrial machines that are particularly hot, fast, or starved of lubrication under normal design conditions benefit from the use of hybrid bearings, which are those that use steel raceways and rollers or balls made from ceramic. No matter the design, each bearing type satisfies specific parameters.
Ball bearings use balls to provide a low-friction means of motion between two bearing races. Ball bearings are usually inexpensive and, when selected properly, require little maintenance. These characteristics, along with limited heat generation make them one of the most common types of bearings. They are found in everything from skateboard wheels to turbine engines in aerospace applications, medical equipment and home appliances.
Roller bearings use rollers of varying shape (spherical, spherical thrust, cylindrical) instead of balls to transmit loads and reduce friction. Thanks to this shape, roller bearings have greater surface contact than ball bearings, so handle larger loads without deforming. Their shape also allows for a moderate amount of thrust load because the weight is distributed across cylinder line contacts instead of sphere point contacts.They are common in conveyor belt rollers, turbine engines, transmissions and gearboxes, as well as the mining, food and beverage, pulp and paper, waste-water and marine industries.
A needle roller bearing is a roller bearing with rollers whose length is at least four times their diameter. The large surface area of a needle roller bearing allows them to accommodate high radial loads in a limited ammount of space. Needle bearings often go in compressors, rocker-arm pivots, pumps, transmissions and other automotive designs. The drive shaft of a rear-wheel drive vehicle typically has at least eight needle bearings (four in each U joint) and often more if it is particularly long, or operates on steep slopes. They are also found in construction and agricultural equipment, two-cycle engines, outboard engines and paper moving equipment.
Thrust ball bearings go in applications with mostly axial loads and can handle shaft misalignment. These bearings also excel in high-speed applications in aerospace, automotive, and oil and gas industries. Thrust ball bearings are also often found in machine tool tables, fluid control valves and spindles.
Thrust roller bearings transmit load from one raceway to the other to accommodate radial loads. Bearings like these also have a self-aligning capability that makes them immune to shaft deflection and alignment errors. Typical applications include: crane hooks, pulverizers, cone crushers and other heavy-load, medium-speed applications.
Tapered roller bearings have tapered inner and outer ring raceways with tapered rollers between them. The rollers are angled so their surfaces converge at the bearing’s axis. These bearings are unique in that, unlike most bearings that can handle either axial or radial loads, they can handle large amounts of load in both directions. Common applications include helicopter transmissions, fuel pumps and agricultural uses.
The principle difference between plain bearings and roller bearings is that plain bearings do not have any rolling elements. Plain bearings are usually compact, lower-cost bearings that are relatively simple in design. They can be found in many applications, but automotive and construction are some of the most common. Plain bearings are used in steering cylinders, front and rear struts, dump cylinders and on hydraulic excavators.
Intelligent bearing technology trialed in railway and wind energy sectors
SKF is now using a bearing health-management technology called Insight in several industry applications. The design puts a self-powered intelligent wireless sensor in a bearing to get condition-monitoring data through the Internet. In fact, the setup is already undergoing trials in challenging wind-turbine and railway applications.
“It goes beyond current sensorised bearing technology by integrating more sensors and … self-powering and wireless technology,” said Ronnie Spolidoro, business development manager, SKF Insight. “These bearings connect to the Cloud, giving customers access to a range of diagnostic and support services.”
SKF Insight monitors dynamic parameters such as temperature, lubrication condition, vibration and load, and informs operators when conditions are abnormal or threatening. When launched at Hannover 2013, SKF Insight was a just a concept for maintenance engineers to track bearing health. But now it’s a functioning solution running in customer pilots to supply data directly to diagnostic centers for condition monitoring using SKF aptitude. From here, dashboards and reports go to plant operators, machine manufacturers, SKF and anyone else authorized online. Users can even access the data on mobile devices.
Now using dynamic bearing data provided by SKF Insight, bearing health management will let operators see how actual conditions are affecting bearing health and spur corrective action—automatic lubricant application or shifting speed, for example.
One promising application for SKF Insight is wind-turbine monitoring. Sometimes changing a turbine’s main bearing is so expensive that it negates the turbine’s overall viability. Here, Insight could monitor loads and lubrication conditions in service, giving plenty of time to prevent damaging conditions.
In fact, SKF is already helping a wind turbine customer develop such a system to measure dynamic bearing information in the true operating state, then wirelessly communicate it to monitoring centers and maintenance crews. Ultimately, the system will monitor bearing speed, vibration, temperature and lubrication—even as a retrofit to enhance operational potential of thousands of turbines already in operation worldwide.
The company is developing a similar solution in the railway sector for wheel-end bearings—critical components usually changed at set intervals, regardless of condition. Here, Insight could soon let operators change the bearings based on real conditions.