Aerospace roller bearings
The term roller bearing in aerospace terms usually depicts either a cylindrical roller bearing, taper roller bearings or a specialist needle roller device such as can be found on flap and slat tracks.
Cylindrical roller bearings
Aerospace cylindrical roller bearings are usually found in one of three places, pumps, engines, engine gearboxes or auxiliary power units (APUs). The cylindrical roller bearing is designed for medium speed applications where heavy radial loads or shock loads can be encountered. Being designed with cylindrical rolling elements, the ability of the cylindrical roller bearing to take axial loads is at best minimal and depending on internal ring shoulder design, in some cases no existent. Cylindrical roller bearings if fitted and maintained properly will normally return large many hours of operation life. Their large roller to raceway contact zone (normally a line as apposed to a ball bearings point contact zone) gives them a large capacity but at the sacrifice of high speeds. Roller bearing construction consists of an inner ring, outer ring, a collection of rollers and normally a cage (although for very low speed/oscillation situations and high loads, a full complement bearing can be used). With cylindrical rollers, cage construction is normally vital. Due to the simple design of the other components within a cylindrical roller bearing, the cage is usually the first area to fail in a normal fatigue situation. Cage materials can vary from steel, brass, phenolic, polyamide and other more exotic self-lubricating impregnated designs. The rings and rollers can also be made from various grades of steel, not just 52100 bearing steel. For example M50 steels have been used with great success in vital areas such as helicopter gearboxes. Like their commercial counterparts, careful attention is paid to roller and raceway design with radii'd rollers and recessed track corners releaving both roller and raceway of any edge loading during normal and extreme running conditions.
Taper roller bearings
Aerospace taper roller bearings are normally associated with wheel applications for virtually all aircraft. Their ability to take radial and axial loads (both of which are experienced in appreciable amounts on landing and take off) is unmatched. The Taper roller bearing essentially consists of two elements. A Cup (the outer portion) and the cone (the inner portion). The cone is an integral inner ring with tappered rollers usually spereated by a special cage. The cup is simple by comparison, simply letting the cones rollers sit on its inner track. Correct fitting of taper roller bearings is vital as too much preload (pushing the cone into the cup) will cuase enormouse heat build up and potentially catastrophic failure. Too little preload and the bearing could literally explode if it's natural frequency was found during use. Taper bearings can also be found in aerospace gearboxes. Again, their ability to take axial and radial loads in a reasonably compact space has yielded significant life time figures.
Specialist roller bearingsMany specialist aerospace roller bearings can be found on aircraft. For example,m aircraft door mechanisms have simple rollers which follow a cam profile when the door is being opened or shut. Some track and slat devices use specialist roller bearings to move the flaps or slats in or out. Undercarriages can also use special rollers in the uplock mechanisms.
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