Design Application

Run quiet, run smooth brings profits from precision

Tom Shelley reports on ways in which added value from bearing makers can help engineers meet more demanding environmental requirements and increase sales.

New, more wear-resistant precision bearings can make a substantial contribution to noise reduction by allowing gears to be run closer together with less backlash. They can also be incorporated into integrated assemblies, which reduce both weight and cost. Both advances contribute to the improvement of the environment in different ways, and also offer opportunities to achieve sales advantages over competitors.

Noise reduction is crucial, particularly in construction and agricultural equipment. Noisy running can get equipment thrown off an urban building site, especially at night. And the drivers of agricultural tractors are often their owners. Farmers appreciate low noise levels in their working environment as much as they do in their cars.

Michael Trimm, a senior engineer at Perkins, says that one of the factors which has contributed to halving the noise output of their new, four-cylinder, 1100 series engines, is reducing backlash between gears. Now that engineers have learned to reduce engine noise from internal combustion detonations, one of the major remaining areas to be tackled is gear noise. Part of the solution has been to increase the number of gear teeth – a crank gear now has 40 teeth instead of 28, but it has also been necessary to decrease backlash by over 50% to 0.1mm. This requires much more accurate machining of the centres of the positions carrying the bearings and, although Mr. Trimm did not mention it, a very high degree of precision in bearings plus very low wear rates so that the tight tolerances are maintained.

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Bearings come in a very wide range of precisions. Standards, set by the Annular Bearing Engineers Committee (ABEC) of the Anti-Friction Bearing Manufacturers Association (AFBMA) and through international agreements, conform essentially with standards for precision ball bearings developed by the International Standards Organisation (ISO).

Trevor Morris, product engineering manager at Barden UK, pointed out at a recent conference that one of the most common bearing sizes used is the 608 (Barden 38) whose external dimensions are 8 x 22 x 7mm. This bearing size is used in applications as diverse as skateboards, domestic appliance motors, oven cooling fans, aircraft generators, vacuum pumps and deep space instrumentation. At one end of the spectrum, skateboard bearings would not meet the requirements of the lowest ABEC 1 or ISO P0 normal grades. At the other end of the scale, the bearings used in deep space instrumentation may well exceed the requirements of ABEC9/P2. As the standards cover only size and geometric accuracy, it is difficult to visually distinguish a skateboard bearing from one used in an aircraft generator.

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Barden UK boasts a production set up to produce a minimum quality level of ABEC 7/P4. Automotive production normally requires only ABEC 1/P0, but as we have seen, a case can, in some circumstances, be made for going to tighter tolerances. ABEC 5/P5 would typically be required by blowers, pumps, generators and synchro motors, while aircraft engines, machine tool spindles, aircraft accessories, computers and dental equipment would typically require ABEC7/P4.

As well as covering the dimensional tolerances of the bearing envelope sizes, bore outside diameter and width, the ABEC/ISO standards control geometry features such as: raceway run out to bore, raceway run out to face, face run out to bore and parallelism on faces. The standards do not affect parameters such as: raceway curvature, raceway depth, pitch circle diameter of balls, the number and size of balls, ring, ball and cage materials, or cage type. All these parameters can have an enormous effect on the running performance, wear rates and working life of the bearing.

One of the ways which Barden offers a special service, is in developing bearings as parts of larger fabrications. Related assembly parts can be integrated. These may include mounting flanges, gear teeth, puller grooves and threaded shafts. Benefits said to be gained include: improved assembly reliability; enhanced rigidity or stability; better location control through proper bearing orientation; reduced handling operations and contamination; greatly improved alignment of the rotating assembly; weight reduction; and improved resistance to temperature extremes.

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Morris added that during the first six months of this year, over one hundred special bearing designs were put into production at Barden UK. Customers included machine tool, dental, Formula 1, TMP vacuum pumps and Aerospace.

Other bearing makers too have been following a similar path. NSK has developed a Hub III automotive bearing unit, with a self-contained inner ring held by swaging. Over the years, these units have evolved from assemblies of discrete components, made up from tapered roller bearings and single-row deep-groove ball bearings, to the fully integrated one-piece hub units which are a feature of vehicle design today.

Hub III consists of a flanged outer ring, a flanged inner ring, a smaller inner ring, ball elements, ball cages and a clamp nut. In the assembly, both the outer and inner rings are flanged for attachment to the vehicle. The outer ring flange is fixed to the vehicle body; the brake assembly and the wheel are fixed to the inner ring flange.

In previous design versions, the flanged inner ring and another inner ring were unified. This tended to complicate assembly. A more recent design overcomes the problem by press-fitting a second, smaller ring onto the larger, flanged inner ring and fixing them together with a clamp nut. In the latest design, the clamp nut, which fastens the inner and outer rings together, is replaced by a swaged edge. Swaging harnesses axial forces, which deform the flanged inner ring to capture the smaller inner ring. This removes the need for a retaining nut and helps reduce both weight and size and improve reliability.

The same technology is applicable to both driven and non-driven wheels, Hub III applications on driven wheels normally maintain a nut to provide bearing retention in combination with a constant velocity joint. The same driven wheel application can also be achieved using swaged bearing technology. NSK considers that reliability is increased by swaging, as the bearing itself provides pre-load assurance in the event of the constant velocity joint connecting nut coming loose. The company does not mention how such units might be maintained. Presumably they are now throw away and replace, like almost everything else on a modern car.

The swaging process used is rocking die forging, which differs from conventional die forging. If conventional forging were to be applied to a bearing, deformation would occur beyond the local area and balls may be pressed against raceways, possibly damaging them. Rocking die forging, on the other hand, locally deforms an area using less pressure and is therefore less intrusive.

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