Brass is best for split bearing cages
02 August 2012
The advantages provided by split roller bearings in delivering greatly reduced downtime compared with the performance of solid bearings are well documented: savings of 80 percent or more are achieved with SRB types on installation/disassembly due to their split-to-the-shaft configuration.
However, savings in downtime can be forfeited if split roller bearings fail to provide the levels of reliability demanded by many tough user industries, says Adrian Menzies.
In industries such as quarrying, mining, water treatment, steel and paper making, the choice of cage material for the bearings is critical to ensuring reliability; Revolvo's Adrian Menzies argues that brass is always best for these tough duties.
“We designed our ranges of SRBs from blank sheets of paper and, after considering the alternatives, we decided that brass was overwhelmingly the best cage material for use in our bearings," he says. "Several recent studies have shown that machined high tensile brass cages have several advantages over pressed steel, aluminium and non-metallic bearing cages.
"Brass cages offer high rigidity, high strength and can be used at high temperatures. They also offer the key features of absorbing lubricant and performing well in aggressive environments where lubrication is marginal. In contrast to some non-metallic cage materials, such as polyamide, they are unaffected by, and can be used with, most common bearing lubricants, including synthetic oils and greases.
“Brass is also first choice in industries such as petrochemical where unexpected failures can be catastrophic. Brass lends itself to preventative maintenance programmes, where vibration monitors are used to get early warning of potential failures. This is possible because brass fails very gradually, giving indication of failure along the way.
"In contrast, polyamide cages do not lend themselves to vibration monitoring programmes and pressed steel cages can fail catastrophically without warning. This is the major reason why operators of petrochemical plants have replaced pressed steel and polyamide cage bearings with brass caged split roller bearings.
"The machined brass cages in our ranges of split roller bearings also deliver improved bearing reliability by ensuring accurate guidance of the bearing rollers. The Revolvo cage design has CNC bored pockets that are optimised to give very accurate roller guidance that prevents skewing: a condition that can lead to edge loading on the bearing and reduced bearing life."
In addition to providing excellent roller guidance, the brass cage also allows operation at higher temperatures up to 140 degrees Celcius without problems, and operating speeds up to 25 percent higher, as long as thrust loads are within design limits.
“To the above advantages should also be added the non-flammable characteristics of brass, which enable the material to be used for the cages of bearings employed underground in bearing mining applications, Mr Menzies continues. "In contrast, bearing cages manufactured from aluminium, light metal alloys or non-metallic materials cannot be used because they are flammable.
“Designing a standard range of bearings with brass cages has resulted in Revolvo winning many prestigious contracts, among them a large order for the Hatfield Colliery near Doncaster. One of the major investments when the mine was reopened several years ago was a huge belt conveyor for coal extraction. The conveyor is equipped with Revolvo’s split roller bearings, including heavy duty 240mm series units and various medium series units. All of these bearings are manufactured in cast iron with brass cages as standard; this means that they can be used underground without modification.
“The standard material specification of the SRB bearings was a major factor in their specification; equally important was the bearings ease of maintenance – particularly important in the cramped conditions underground – and their superior sealing systems. SRB also offer the benefit of inspection in situ, as part of a planned maintenance schedule; and have longer life, because they cannot be cross-located.”
Adrian Menzies is with Revolvo
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