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Rolling bearing selection: look at the bigger picture

Rolling bearing selection: look at the bigger picture

When selecting rolling bearings for industrial plant, machines and equipment, it is important to analyse the total cost of ownership (TCO) and not just the purchase price, says Dr Steve Lacey of Schaeffler.

Rolling bearings are critical components in rotating plant, machines and equipment, including machine tools, automated handling systems, wind turbines, paper mills and steel processing plants. However, the decision in favour of a specific rolling bearing should always be taken after analysing the whole life costs or total cost of ownership (TCO) of the bearing and not merely on the basis of purchase price alone.

Buying cheaper bearings can often prove more expensive over the long term. Often the purchase price accounts for just 10% of the overall costs. So when it comes to buying rolling bearings, what’s the point in saving a couple of pounds here and there if this means higher energy costs due to higher friction bearings? Or higher maintenance overheads resulting from a reduced service life of the machine? Or a bearing failure that results in unplanned machine downtime, leading to lost production, delayed deliveries and dissatisfied customers?

Today’s advanced high technology rolling bearings offer many improved features that enable TCO reductions to be achieved, providing added value over the complete life of rotating plant, machines and equipment. For a bearing designed/selected for a given industrial application, the TCO is equivalent to the sum of the following:

Purchase price + installation/commissioning costs + energy costs + operation cost + maintenance cost (both routine and planned) + downtime costs + environmental costs + decommissioning costs

Whilst the initial purchase price of an advanced bearing solution will be higher than a standard bearing, the potential savings that can be achieved in the form of reduced assembly times, improved energy efficiency (for example by using lower friction bearing components) and reduced maintenance costs, often more than outweigh the initial higher purchase price of the advanced bearing solution.

The influence of an improved design in reducing TCO and adding value over life can be significant, as designed-in savings are often sustainable and permanent. Sustained reductions over the life of the system or equipment are worth far more to the customer in terms of savings than a reduction in the initial purchase price of the bearings.

To industrial OEMs, the design of bearings can add value to their own products in many ways. By engaging with these OEMs early in the design and development stages, bearing suppliers can customise fully optimised, integrated bearings and assemblies, which meet the specific requirements of an application. Bearing suppliers can add value by, for example, creating and customising internal bearing designs that maximise load carrying capacity and stiffness or minimise friction.

In applications where design envelopes are small, the bearing design can be optimised for ease of assembly and to reduce assembly times. For example, screw threads on assembly mating surfaces can be incorporated into the bearing design. It may also be possible to incorporate components from the surrounding shaft and housing into the bearing design. Features such as these add real value to the OEM customer’s system and can potentially lead to cost savings over the whole life of the machine.

Other features can be added to the bearings that add further value over the life of the machine. These include special sealing technology within the bearing to help save space; anti-rotation features to prevent slippage under the effects of rapid changes in speed and direction of rotation; coating the surfaces of bearing components to minimise friction; and optimising bearing operation under boundary lubrication conditions.

The bearing supplier can examine closely the overall costs of machines, plants and their components – from purchase, energy consumption and maintenance all the way through to repairs, dismantling and disposal. Well-known cost drivers and hidden expenses can therefore be identified, optimised and eliminated.

Let’s consider a specific example, looking at the paper industry. In paper manufacturing, rolling bearings in CD-profile control rolls of calender machines are normally subjected to low loads. The loads are only higher when the gap between the rolls is open. For these applications, machine manufac-turers traditionally chose spherical roller bearings with adequate load carrying capacity for the high-load phase. However, in the low-load phase this led to slippage, resulting in premature bearing failure. By coating the rolling elements and optimising lubrication, these slippage effects could be reduced, but not completely eliminated. For this reason, Schaeffler developed the ASSR bearing (Anti-Slippage Spherical Rolling Bearing). The bearing comprises rings of standard spherical roller bearings, but barrel rollers alternate with balls in each of the two rows of rolling elements. In the low-load phase, the balls ensure slippage-free operation, while the barrel rollers take up the loads in the high load phase.

The benefits for the customer are clear: while the original bearings typically achieved a service life of about one year, the new ASSR bearings are expected to last for up to 10 years. This means fewer rolling bearings are required over the life of the calender machine, a reduction in maintenance requirements and savings of six-digit savings over the entire machine lifecycle. All this was achieved by taking only one single machine position into consideration. Further optimisation and therefore additional significant savings can be achieved by supplementary measures, such as online condition monitoring and vibration diagnosis, temperature monitoring or dynamic/static balancing – all of which can be provided by Schaeffler.

We can also consider the example of bearings for wind turbines and construction machinery. Many rolling bearings from Schaeffler are available in a high performance, premium quality X-life version. For example, when developing the X-life series of tapered roller bearings, particular attention was paid to achieving high reliability and minimising friction, particularly in high load applications and those that require rotational accuracy. This means that manufacturers of hydraulic units or gearboxes (pinion bearing supports) such as those found in wind turbines, agricultural vehicles and construction machinery, can now surpass previous performance limits, whilst significantly improving operational safety. In terms of downsizing, the improved characteristics of the X-life bearings mean that the performance of the gearbox is improved, whilst the design envelope remains the same.

The 20% improvement in dynamic load rating and minimum 70% improvement in basic rating life were achieved by improving the geometry, surface quality, materials, dimensional and running accuracies of the bearings.

The premium bearing material used in the manufacture of the X-life tapered roller bearings is specially adapted to meet the requirements of the rolling bearings and is an important factor in the increased performance of the bearings. The fine grain structure of this material provides high toughness and therefore high resistance to solid contaminants. In addition, a logarithmic profile was developed for the bearing raceways and the outside surface of the rollers, which compensates for high stress peaks under high loads and any “skewing” that may occur during operation. These optimised surfaces assist in the formation of an elasto-hydrodynamic lubricant film, even at very low operating speeds, which enables the bearings to withstand high loads during start-up. Furthermore, significantly improved dimensional and geometrical tolerances ensure optimum load distribution. Stress peaks are therefore avoided, which reduces material loading.

The frictional torque of the new X-life tapered roller bearings has been reduced by up to 50% compared to conventional products. This is due to high dimensional and running accuracy in conjunction with improved surface topography. The revised contact geometry of the inner ring rib and roller end face also assists with the reduction of friction. As a result, bearing operating temperature has also been reduced by up to 20%.

X-life tapered roller bearings are not only more economical, but also result in lower bearing operating temperatures, which in turn, places significantly less strain on the lubricant. This enables maintenance intervals to be extended and results in the bearing operating at reduced noise levels.

Once again, although the purchase price of X-life bearings is higher than standard bearings, the TCO is significantly reduced over the life of the wind turbine, tractor or construction machine.

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