Selecting the ‘best’ bearing

An adequate bearing is not the same as the right bearing. Justin Cunningham finds out how one company wants to change how engineers perceive bearing selection

Most design engineers are pretty confident about selecting such components as bearings and fasteners for the job in hand. And, in most cases, an adequate component is selected that does the job and everyone is happy.

But bearing and fastener manufacturers are having to, on a fairly regular basis, visit companies seeking advice on why a bearing is wearing out too quickly or why a fastener is failing prematurely.

The trouble is there is a difference between 'adequate' and 'optimal' – and the difference is sometimes not subtle. Bearing manufacturer Schaeffler finds this is a particular problem when it comes to gearbox design.

"Like most engineered components and systems today, gearboxes are becoming smaller – especially in automotive applications – but also more powerful," says Dr Steve Lacey, engineering manager at Schaeffler UK.

"In addition, gearbox designers will be looking at such factors as low noise levels and reduced friction, as well as how safe and straightforward the mounting of the bearings will be."

Around 85% of Schaeffler gearbox bearings are standard catalogue products, but Dr Lacey says this is only an optimal process when a gearbox is designed with a clean sheet of paper. "If the bearings are considered much later in the design process, the customer may have already signed off important parts of the machine design without giving much thought to the bearing requirements," he says. "As a result, there may not be sufficient space to mount the ideal bearing or lubrication method. This means the engineer has to select a compromised bearing that may not meet all of the requirements.

The need for gearboxes to be more compact, reliable and durable, often under higher loads, has become an increasing requirement for industries. The drive to reduce energy loss in power transmission is an area of considerable effort for wind turbine manufacturers, tidal energy systems, construction vehicles as well as gearboxes for gas, water and vacuum pump applications.

For many of these larger niche applications, a custom bearing can offer a far superior solution than an off the shelf part, particularly if bearing selection comes as an afterthought to an existing design. Although a more expensive route initially, the lifecycle cost of a custom bearing often is several times less expensive, because early failures, costly retrofits and additional maintenance costs are avoided.

Custom designs bring the need to trade off space availability, load requirements, operational speed, environmental conditions, lubrication needs and life expectancy to find best solution to a problem. However, Dr Lacey is encouraging designers to take this approach at the beginning of new designs.
"The ideal scenario is to invite the bearing supplier into your design process early, in order to avoid potential costly design errors, over or under engineering the product and to eliminate safety or reliability issues that will need rectifying later on," he says. "Product development times will be reduced considerably if bearing design issues are resolved early."

Bearings are the key component for moving parts of a design, so it makes sense to have the best system in place for a design as early as possible to minimise the number of design iteration loops in a process.

But, for those who want to avoid custom bearings – principally due to the cost implications – there are more and more standardised bearings coming to market. German manufacturer Glacier Garlock Bearings is offering its DB self-lubricating, maintenance-free bearing for heavy-duty applications such as the offshore industry, underwater equipment, iron and steel industry equipment, cranes and conveyors, mining equipment and construction and earthmoving equipment.
Made from cast bronze, with a graphite-free solid lubricant insert, the structure provides an ultra low coefficient of friction, high wear resistance and total corrosion resistance, even in wet, dirty environments and exposure to seawater.

The company has also developed filament wound plain bearings. These use high strength epoxy impregnated wound glass fibres to provide the radial and axial strength to support high loads, but give natural lubricity and low friction.

Unlike many conventional non metallic bearing materials, the high-strength composite has a thermal expansion rate comparable to that of steel, assuring maximum dimensional stability and positive housing retention, even at elevated temperatures. These could be used in offshore and corrosive environments but also for mechanical components in such demanding applications as steering linkages, hydraulic cylinder pivots, king pins, boom and scissor lifts and cranes, in addition to high load and harsh environments as the bearings resist misalignment, shock and even chemicals.

Additionally, IBS Precision Engineering has extended its range of air bearings. These provide a significant advantage over traditional roller bearings because they are non-contact, have zero friction and wear, and are silent and smooth in operation. Larger sizes could, for example, offer machine builders more load capacity in a smaller area, while the standardised smaller sizes have been engineered to meet the high precision requirements of an increasing number of micromachining and nano-positioning applications, including biomedical and pharmaceutical.

Modular air bearings provide high repeatability and acceleration, are inexpensive, robust and easy to use. Unlike conventional orifice air bearings, the 'New Way Porous Media' technology controls the airflow across the entire bearing surface through millions of sub-micron sized holes in the material with more consistent air pressure distribution ensures superior performance.

With no friction/wear problems or lubrication requirements, air bearings are ideal for applications in abrasive, dry and demanding environments that would damage traditional contact bearings. They are also approved for use in Class 10 clean rooms where the constant flow of air helps keep the dust and particles found even in that environment, away from the bearing surface.

Pointers

Schaeffler's tandem bearings are often used for supporting very high axial loads

For demanding industrial gearbox applications, Schaeffler offers its premium quality X-life brand of bearings. Because these are designed with the maximum possible number of rolling elements, these bearings offer very high radial load carrying capacity and high rigidity, which makes them ideal for compact gearbox designs. High precision gearboxes, manual gearboxes and some planetary gear units require needle roller bearings

DB bearings by Glacier Garlock Bearings can withstand a maximum static load of 200MPa and a dynamic load of 100MPa and have the recommended maximum dry sliding speed is 0.5m/s. Typical operating temperature ranges between -50 to 350°C and the coefficient of friction dry is 0.05 to 0.18

IBS Precision Engineering's modular air bearings are silent and smooth in operation, provide high repeatability and acceleration, and are inexpensive, robust, and easy to use

Author
Justin Cunningham

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