Of all industrial sectors, aircraft engine manufacturing and maintenance have possibly the most stringent requirements and specifications for rolling bearings and related components. A number of advanced bearing maintenance products, such as customized induction heaters and sophisticated thermal cameras, can help aircraft engine OEMs and maintenance providers meet such exacting standards.
Aircraft engine applications typically call for specially designed rotating components, including deep groove ball bearings, spherical roller bearings, and cylindrical roller bearings. The bearings’ rings and rolling elements are made from corrosion-resistant steels with high levels of cleanliness and homogeneity. The bearings are also heat-treated under close metallurgical control.
To prevent damage to new bearings before mounting, engine OEMs and maintenance providers are urged to follow these well-established bearing handling practices:
• Store bearings in locations where the relative humidity does not exceed 60%.
• Protect bearings from excessive vibrations emanating from operating machinery that can cause raceway damage.
• Store larger bearings lying down, with proper support for the bearings’ entire side faces.
• Keep bearings in their original packaging until just before installation.
Bearing heating and installation
Selecting the right bearing installation method is key to prolonging life expectancy and reducing the risk of premature failure. Although other methods such as ovens and hot plates are still used, induction heating is generally considered the most modern and efficient way to mount mid-size and larger bearings. Heating expands the bearings’ rings and allows them to slide easily onto shafts, developing a secure interference fit when the bearings cool.
Modern heaters generally have advanced energy-saving features and the capacity to handle bearings of different bore sizes.
Larger engine bearings can also be mounted using oil injection. With this method, engine shafts are machined beforehand with oil ducts and grooves. During installation, a pressurized oil film is injected between the shaft and bearing inner ring, minimizing the mounting force required.
Bearings should be installed in a dry area protected from contamination by dust, dirt, and moisture.
An engine OEM converts to induction heating
In a recent example involving a small engine for civilian aircraft, an aircraft engine OEM wanted to gain greater control over its bearing installation process by converting to induction heating. Previously, the engine’s bearings were heated for installation using hot plates, but the method proved slow and failed to keep up with the OEM’s production goals.
The OEM began researching induction heating, and decided upon a heater with a preset temperature cycle to exert more control over the process and prevent overheating. It also wanted a heater capable of preparing bearings more quickly than previous methods.
Engineers from the bearing manufacturer devised a solution by modifying an existing induction heater’s software. The modifications equipped the heater to heat bearings to the optimal temperature every time without operator intervention. This eliminated the risk of bearing damage due to operator error.
The customized heater also met the OEM’s production requirements. The heater’s induction coil is located outside the heater’s body, allowing it to prepare bearings weighing up to 40 kg. The heater automatically demagnetizes bearings after heating them, reducing the risk of bearing contamination by small, potentially damaging particles.
After the customized heater proved successful in the small aircraft engine application, the OEM acquired additional heaters and implemented induction-heating practices at its other engine production and repair facilities.
Monitoring engine bearings
Recent advances in digital technology and instrumentation have resulted in new generations of condition-monitoring devices, including IR thermometers, data collectors, and electrical erosion detectors. These instruments facilitate noncontact, noninvasive inspection of aircraft engine bearings and other rotating components during engine testing or maintenance.
A notable example is thermography, which detects thermal energy in the spectrum’s IR band. Handheld thermal cameras have become more sophisticated, allowing users to find hot spots in operating engines from a distance of 10 ft or more. The temperature variations are depicted as different colors or shades on a display screen.
One newly introduced camera has a 160- by 120-pixel detector and a measurement range of -20 to +350°C. Hot and cold extremes and temperature differences between any two points can be instantly displayed on screen.
A new type of spring-operated mechanical pullers is now available that efficiently removes aircraft engine bearings without damaging other components, such as shafts and housings. The pullers’ spring-operated arms can be positioned behind the targeted component and allow users to remove bearings even in extremely tight quarters.
An alternative to pullers is oil injection, especially for larger bearings. This versatile method can safely remove engine bearings mounted on either tapered or cylindrical seatings. An injection of pressurized oil reduces the removal force needed by 90% or more depending on the seating. To use the oil injection method of dismounting, engine shafts must be prepared in advance with the necessary oil ducts and grooves.
This article was written for Aerospace Engineering by Paul Michalicka, SKF USA Inc.