Acquiring a bearing for a mechanical system on a military aircraft can be a challenging process, particularly if that aircraft is reaching a late stage in its lifecycle. The aircraft itself may have been in operational service for more than 40 years and may also be projected to remain in service for some years to come. Should the original bearings need replacement, the first port of call is the original supplier.
But the company may have ceased to exist, re-located overseas, or perhaps is no longer willing to manufacture the equivalent bearing in such small volumes (less than 1,000 units). This means that many aircraft (and other mechanical) system operators and managers are now facing significant bearing obsolescence issues, particularly if the quantity of bearings they require is relatively low.
In addition, there may be other design features about the bearings that make it equally unattractive, or even impossible, for a supplier to adhere to. For example, the bearing may require special materials, special lubrication, certification, special features such as tapered outside diameters, or may need to fit an Imperial chassis size.
There are similar issues in the nuclear industry too. Bearings are commonly found in nuclear power plants as part of actuation systems, which position the control rods into the fuel bundle. In an emergency situation, the control rods are dropped into the fuel bundle to absorb the reactor heat, which dictates that the bearings must not, under any circumstances, fail. This means bearing suppliers must offer full manufacturing traceability, controlled lubrication and complete retention of records, similar to the aerospace and defence industry.
The solution for the system operator is to find a bearing supplier who is able to manufacture the specified bearing to the same (or higher) quality standards as the original and who is also willing to supply in small batch sizes, anything between ten and 500 units.
In the UK and most of Europe, the problem of obsolete bearings is predominant in the military aerospace and nuclear sectors. The problem usually arises when the mechanical system is very old, usually more than 25 years. In this case, many customers approach companies that are willing to manufacture direct replacement bearings in relatively small batch sizes and which have the manufacturing capability to produce special engineered bearings. Companies with a strict non-obsolescence policy are also often sought.
Often the bearing drawings on legacy products will define only the chassis size, with little or no information regarding the internal design, materials or tolerances. With such a basic specification, a wide range of bearings could meet the requirements, ranging from low precision, commercially produced units to high-precision bearings with full traceability and inspection.
Some of these bearings may be relatively inexpensive to buy, but they were never designed to operate in these types of environments or applications. Often, the buyer doesn’t actually realise that the bearing they have just purchased is not a like-for-like replacement. Certification, full traceability and quality should be the key issues here.
The author’s company is one that manufactures bearings under strict aerospace procedures, providing full traceability and retention of records. Some customers actually visit the manufacturing plant to witness the correct lubrication of a bearing. Considering the potential risks of fitting an incorrectly lubricated bearing for a nuclear or aircraft application, it is easy to understand why a customer should demand this facility. Customers may pay more for such bearings, but they also get a guarantee of bearing quality and know that small batch sizes can be supplied.
Robin Kyte is with Barden Corporation (UK)
Barden Facts
Barden’s manufacturing plant in Plymouth boasts more than 1,700m² of clean rooms for contaminant-free assembly and inspection of bearings. The company designs and manufactures super precision bearings to a minimum of P4/ABEC 7 quality standards. Bearings are manufactured to a geometric tolerance of P2 or better, and envelope dimensions to P4 or better. Raceway roundness is better than 0.5 of a micron. Bearing sizes range from 5mm to 250mm outside diameter and most are either angular contact or deep groove types. The company can also now produce roller bearings.
Bearing rings are available in SAE 52100, AISI 440C, Cronidur 30 (high corrosion resistance and high temperature operation), M50 and BG42 materials. Balls can be made from silicon nitride, TIC-coated, zirconium dioxide or tungsten carbide. Cage materials can be specified in steel, bronze, phenolic, polyamide, polyimide, PEEK or PTFE. Lubricants include hydrocarbon, synthetic esters and hydrocarbons, silicone and perfluoralkypolyether.