Reaping productivity rewards with precision bearings
01 September 2014
Making machines run faster means hardware wears faster, loses positional accuracies and can also increase ambient noise levels. For Mike Chipchase, 'precision' bearings offer a solution to these problems.
In most industries, managers face a shortage of qualified engineers and maintenance staff to properly support and maintain equipment. Consequently, their engineers face multiple challenges when designing rugged, reliable, and quiet machines that meet these new motion control demands.
One way design engineers are meeting these challenges is with components that the motion control industry refers to as precision bearings. Although no one type of bearing is specifically labelled 'precision', some bearings require little maintenance and offer features that deliver low noise and long life at high operational speeds.
These features may add about ten percent to the cost of such a bearing, but the return on that investment is longer life. Depending on the precision-bearing type, its life could be eight times longer than non-precision type bearings.
One factor that categorises bearings as 'precision' is the tolerances held throughout the manufacturing process. Close tolerances result in a higher quality product, one that can withstand greater speeds and stresses, and other operational demands.
Less chatter in the workplace
A factory, warehouse, or distribution centre is inherently a noisy environment. Over the last few years, though, noise has become a big issue – simply increasing the speed of a few motion systems can increase the noise to a level where it is detrimental to communication and wellbeing in the workplace. As a result, noise dampening is a priority for most new equipment.
A properly selected bearing can greatly reduce the noise problem. And while, by their nature, bearings aren’t silent components, newer designs incorporate features that help lessen noise. Bearings that would fall into the precision category are also able to run at high speed - typically from 1.5 to 3m/s - and they tend to eliminate bearing binding or chatter.
Bearings manufactured more precisely usually have a coefficient of friction of 0.001 to 0.002. A low coefficient of friction also reduces stick-slip, which is another factor to consider when selecting precision-type bearings.
To help dampen noise, precision-type bearings generally have smaller ball elements. The smaller the balls, the less contact area is available among the balls and the less space there is between them, all of which helps reduce chatter and thus reduce noise levels. Moreover, much of the steel that’s not used for transfer of load is replaced with a polymer, such as Delrin (Polyoxymethylene).
The ball return portion of the bearing is one such area where polymers are used instead of steel because the balls aren’t under load and polymers can help reduce noise. The main steel components of such bearings are usually confined to the shaft, the balls and the bearing plate.
In conveying equipment, not every roller needs to have a precision-type bearing to reduce noise or lengthen life. Conveyor manufacturers will often put rollers with precision bearings in about one-quarter to one-third of the conveyor rollers. This strategy can also be used on existing equipment, replacing old, worn rollers with ones using higher quality bearings.
For those applications where acceleration is important, precision-type bearings can offer significant advantages over standard bearings. Indeed, some precision bearings can handle accelerations of up to 140m/s/s.
Some bearings come with dual plates, reducing the total number of plates compared with single-track products. Two plates bear the load concurrently, which reduces the effect of minor variations in housing bore on load carrying capacity. In addition, this design also helps increase bearing life. Dual plates in Thomson’s Super Smart bearing, for example, result in a bearing travel life some eight times that of a standard bearing.
Misalignment
Proper alignment of all the elements, both in the bearing case and in the equipment, is crucial with respect to bearing life, noise emissions, and load carrying capacity. Precision-type bearings have inner tolerances that eliminate misalignment and reduce play among their components. Hardened steel bearing components react like elastic bodies under load, which means that the bearing components flex as the load- bearing balls move though them. Over time, material fatigue can set in, and they eventually fail. The manufacturing tolerances of precision-type bearings should be much higher than in other bearings as this will help in terms of ‘tolerance stack-up’.
Bearings installed on equipment will often be subject to tough alignment challenges, either dynamic or static, including a warped roller shaft or a deflected head shaft on a loaded belt conveyor. Chatter is often an initial sign that there’s misalignment in the system. Essentially, there are three types of misalignment: pitch (shaft angular deflection or misaligned housing bore); roll (distributed load on the ball tracks), and yaw (skewing between ball tracks and shaft).
Torsional alignment inaccuracies are often to be found in the base carriage of many machines. However, some bearing features handle such challenges well. Precision hardened rings enable a bearing to find its own proper position under load, which evens out wear and, as a result, extends the life of the bearing. Self-alignment enables a bearing to compensate for misalignment that results from imperfections in housing-bore roundness and parallelism, deviations in flatness of mounting surfaces, imperfect system assembly, or deflection at load.
Maintenance
Maintenance is a huge issue with motion equipment, and in 24/7 operations it is often difficult to schedule; equipment tends to run until it fails. Most bearing failures are due to improper installation, lack of lubrication, and contamination, with lack of lubrication probably the most common issue. In some applications - medical equipment, for example - lubrication may be particularly undesirable for sanitary reasons. While bearings can run without lubrication under certain conditions, it is not recommended. And this is where precision type bearings can once again come to the engineer’s aid.
But while precision type bearings offer the advantage of longer wear, lubrication and reducing incidences of contamination, either of the bearing parts or of its external environment, are still issues that have to be addressed. As a consequence, most precision bearing assemblies will include a lubrication port or pillow block for oil.
Contamination can be tackled by appropriate seal design. Thomson’s Super Smart, for example, has a double-lipped wiper that forms an integral part of the bearing. It retains lubrication with one lip while the other lip acts as a scraper to deter the ingress of contaminants. Thomson also uses a Buna-N seal with every pillow block suppled.
When an installation cannot be oil lubricated regularly according to a schedule, grease offers a viable alternative as it allows a longer interval between lubrications.
Bear in mind, however, that for high-speed applications, oil is recommended. Grease is more generally used in high-load, low-speed-applications, from 1 to 1.5m/s. It is also worth noting that grease tends to dampen noise rather better than oil. Ultimately, it is proper installation that will help extend the intervals between maintenance interventions.
Customers changing their own bearings are advised to pay attention to maintaining tight mountings and alignment, and eliminating vibration. All bearing manufacturers have written procedures and recommendations that should be followed to ensure correct shaft seating, mounting tightness, and unit alignment.
At the beginning of a machine building project, you might do well to consider bringing in a bearing manufacturer at an early stage in the design process. Bearing manufacturers typically look at load, speed, and life, along with other critical factors affecting the system design. They can also often help with drive system selection as well as other system components, which can be a real advantage to design teams. The manufacturer can advise on tolerances so that there is less play among components, and therefore more system accuracy and reliability.
Precision bearings may be commonplace in critical medical applications or in semiconductor manufacturing, where positional accuracy is essential. Now, there is an increasing tendency for OEMs to turn to precision bearings for less demanding applications because they not only offer the advantages outlined above, but also a way to differentiate their products by way of extended warranties and brand recognition.
Mike Chipchase is a product specialist at Thomson Linear Components Group, Europe
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