BEARINGS, SEALS & GASKETS FEATURE Improving performance levels
To help ensure a bearing is capable of withstanding the conditions it may face in use, coatings can be applied, the geometry can be altered, and the bearing materials themselves can be replaced, as The Barden Corporation explains
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f a bearing is to be used in high speed, high temperature applications,
steel cages can be coated with silver to improve lubrication performance and reliability. Silver plating improves the lubricity of the bearing, making it more robust and resistant to oil-off events. In the case of lubricant failure/ starvation, the plating acts like a solid, dry lubricant, allowing the bearing to continue running for a short period of time. Applications include aerospace starter generator bearings and those for air conditioning units on trains. Replacing the steel balls with ceramic (silicon nitride) balls can improve bearing performance in high speed applications. The reasons for this are that ceramic balls are much lighter, reducing centrifugal forces and improving dynamic conditions. Their surface finish is almost perfectly smooth, and they exhibit vibration levels two to seven times lower than conventional steel ball bearings. At higher speeds, internal loading in the bearing is also reduced. Added to this, ceramic hybrid
bearings run at lower operating temperatures which, in conjunction with the lower mass, allows running speeds to increase by as much as 40-50%. Applications for ceramic hybrid bearings include high-speed vacuum pumps, medical/surgical hand tools and aerospace fans and generators.
IMPROVING PERFORMANCE The bearing material will also help to maximise its performance. Special ring materials, which offer superior corrosion and wear resistance with the ability
to withstand higher dynamic loads than conventional bearing steels, are beneficial. When used in conjunction with ceramic balls, significant gains in bearing life and performance can be achieved. The four predominant ring materials used by Barden are AISI 440C (corrosion- resistant steel), SAE 52100, AISI M50 and Cronidur 30. For applications up
induction case hardened. It can enhance corrosion-resistance and improves the fatigue life and wear resistance. Full complement bearings capitalise on the space normally occupied by the ball retainer. This allows for more balls, which in turn provides an increase in load capacity, either predominantly radial, in the case of filling notch designs, or axial and in the case of angular contact designs. The use of preloaded angular contact pairs can also allow bi-directional axial loads to be applied. Applications here range from high
temperature valves for aerospace applications, to missile fin supports and emergency touch down bearings.
MEETING APPLICATION DEMANDS Although getting smaller, bearings are still required to run faster, at higher temperatures, carry higher loads and operate reliably for longer periods. Here, advanced coatings and surface treatments can be applied that combat friction, prevent corrosion and reduce wear. Benefits here include higher power density, improved performance, more predictable/consistent bearing behaviour (particularly in harsh environments), lower running costs and longer service intervals. Multi-layer sub-micron (sputtered)
coatings, for example, can be employed to enhance the physical and tribological characteristics of bearing surfaces. The
“Although getting smaller, bearings are still required to run faster, at higher temperatures, carry higher loads and operate reliably for longer periods. Here, advanced coatings and surface treatments can be applied”
to 345˚C, AISI M50 tool steel or a special tempered version of Cronidur 30 are suitable. These are widely used in high temperature aerospace accessory applications, such as bleed valve systems on aircraft. Cronidur 30 is a martensitic
through-hardened, high nitrogen, corrosion-resistant steel that can also be
Selecting the right bearing material will help to maximise its performance in use
success of such techniques relies on the avoidance of distinct layers by generating a graduated or diffused interface between different materials. Similarly, keying layers such as nickel or copper are frequently used to improve the adhesion of soft films to hard or passivated substrates. These coatings can be applied to the
internal and external surfaces of bearing rings and rolling elements if required. For high speed applications, the
ball separators or cages can be supplied in special polymer materials which retain the oil in a controlled manner when vacuum impregnated. When the bearings require high resistance to chemicals or thermal attack, other polymers are available. Furthermore, it is also possible to
incorporate special design features into bearings to improve their performance – including flanges, shafts and housings.
The Barden Corporation
www.bardenbearings.co.uk
DESIGN SOLUTIONS | MAY 2016 35
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