Bearing the strain

Bearings may not be the most glamorous of mechanical components, yet they perform a vital role in a host of innovative applications — from the enormous bearings which drive wind turbines and open roofs at sports stadiums to tiny ones within electronic devices.

It is often the large, high-profile engineering achievements that put bearings in the spotlight.

Scotland’s Falkirk Wheel, a giant rotating boatlift, is one example. It is the only structure of its kind in the world, transferring boats between the Forth and Clyde Canal, and the Union Canal between Glasgow and Edinburgh, over a vertical gap equivalent to the height of eight double-decker buses.

The wheel, the centrepiece of the Millennium Link, a £78m British Waterways-led project, measures 35m in diameter, with an axle length of 28m.

The idea of connecting the canals via a rotating boatlift was put forward and originally conceived as a giant Ferris wheel with suspended gondolas. For this design, SKF proposed large, double row, spherical roller bearings and specially-designed bearing housings to support the wheel.

The design however, evolved over the years of planning into the radical concept that can be seen today. It takes the shape of a Celtic-inspired, double-headed axe, in which two axeshaped arms rotate in a continuous circle, 180º at a time. It simultaneously lifts and lowers two 22m-long caissons, that each hold a payload of 300 tonnes, comprising water and up to four boats, and uses a series of synchronous gears to positively keep the caissons horizontal.

To support the wheel a solution was developed that uses a pair of four-metre diameter, three-row, slewing bearings, one positioned at either end of the wheel, with outer rings bolted to the support structure and inner rings bolted to the arms. The inner ring of one of the bearings is equipped with gear teeth to transmit the drive to the wheel.

The use of the slewing bearings was an unusual solution, as these are normally used in applications with heavy axial loads, such as in the rotational movement of large cranes. However, the bearings were specially designed to be positioned on a horizontal axis and to cope with the specified combination of radial and axial loads.

When the wheel is fully loaded, it weighs 1,800 tonnes, which means it has a a radial load of 9095 kN/bearing. Each slewing bearing has three rows of cylindrical rollers, one for the radial load and two with smaller rollers for the axial loads.

The wheel is rotated by 10 hydraulically- driven gearboxes, via the geared slewing bearing. It turns at a rate of around 0.125 rpm, which sees it lift and lower boats at an average rate of four metres/ minute.

The wheel drive system has been designed to handle a degree of imbalance due to differing water levels in the caissons. However, even allowing for this, the very low friction torque of the antifriction bearings means that a rated torque of only 2972 kNm is required to rotate the wheel.

Another equally impressive engineering feat is the London Eye, where for four years huge bearings in excess of 2.5m have ensured that it continues to rotate smoothly.

The two heavyweight large spherical roller bearings from INA are incorporated in the hub of the Wheel. The locating bearing has a weight of 6.3 tonnes and is firmly mounted on the shaft in axial direction. The floating bearing weighs 5.2 tonnes and can move on the shaft in order to compensate for longitudinal expansion.

As the bearings are considered as safety relevant components, they had their second routine inspection at the beginning of last year and passed with flying colours. The two bearings are double row radial spherical roller bearings with pintype cages.

The bored-through barrel rollers were made in Germany’s FAG plant at Eltmann and then fitted with pins and welded to the cage side washers at Wuppertal.

This design feature ensures their extremely high load capacity as a greater number of rollers can be mounted than in bearings with standard-type cages. Each of the two spherical roller bearings, which are positioned at a distance of nine metres, has a width of 400mm. The manufacture of these highprecision bearings took around six weeks.

The Eye’s safe foothold is ensured by two large maintenance-free radial spherical plain bearings from Elges. These (bore diameter 440mm, outside diameter 600mm, width 218mm) were essential for slewing the wheel from its horizontal mounting position across the Thames into its vertical operating position, with resulting contact pressure in excess of 450 N/mm.

Since then, the bearings have had the task of compensating for wind-induced micro-movements that would otherwise have an adverse effect on the comparatively stiff steel construction.

The success of the Eye has inspired imitators worldwide, and some 30 Ferris wheels are currently in their concept stage, providing great opportunities for high-profile bearing placements.

But not all bearing applications are on such a vast scale. Applications from an effector head for invasive surgery to dental turbine headpieces feature miniature specialised bearings from Myonic.

Another company that specialises in miniature bearings is BSA, which produces small bearings for the aerospace and medical sectors with diameters starting at between 6mm and around 7mm — the smallest having an internal diameter of just 2mm. But one of the most interesting small bearing applications comes from fishing reels manufacturer ABU Garcia.

The company faced a dilemma. It was keen to improve its spools’ 30-second free spinning time. The solution had to be acoustically pleasing and sound the part as well as be able to cope with potential corrosion problems.

In the initial stages of the design, SKF demonstrated that it was possible to have a free spinning time in excess of 210 seconds, but the hardware would be too expensive. So the company decided to go for a time of 90s ±30. It also set its sights on a sound level of 52-62dB, high-corrosion resistance to ISO standards, a weight of just 7.5g and a lifetime in excess of 50,000 casting cycles.

A few hours into the first meeting, it became obvious that suitable hybrid or stainless bearings were not available in the normal range. The end result was a customised, ready-to-mount mini spindle, which simply inserts into the spool.

As well as replacing four parts with one, the spindle is claimed to offer superior performance, quieter operation and provides a strong sales message for ABU Garcia in what is an incredibly competitive market.