Bearings Bearings Bearings reduce friction by providing smooth

Bearings

Bearings • Bearings reduce friction by providing smooth metal balls or rollers, and a smooth inner and outer metal surface for the balls to roll against. • These balls or rollers "bear" the load, allowing the device to spin smoothly. • Bearings are usually classified by the rolling element used…. . e. g. Ball Bearings and Roller Bearings. • Ball Bearings use steel balls which rotate in raceways. • Roller Bearings use various types of rollers in suitably shaped raceways

Radial & Axial Loads • The rolling element of a bearing is subjected to two main types of loading : • Radial & Axial • Most bearings are designed to accept one type of loading but some accept both simultaneously. • Ball bearings and tapered roller bearings accept both while other roller bearings only accept radial loads.

Radial and Axial Loads on a Ball Bearing Axial Movement Forces the Inner Race into the Ball Rolling Element Radial Load Axial Load Shaft Outer Race Inner Race Rolling Element

• Bearings contained in cages are generally for high speed engine and gearbox use. (in excess of 100 rpm) • Most other aircraft bearings are for slow rotation, have no cage and are prelubricated and sealed.

Optional Components Shield – A stamped, profiled sheet metal disc. Pressed into a very small groove on the inside edge diameter of the outer ring. Prevents contamination entering the bearing. Does not contact inner ring thereby preventing added friction. Seal – Prevents dust penetration by allowing grease to collect around it.

Types of Ball Bearings • There are four types of ball bearing: • Radial bearing. • Thrust bearing. • Angular-contact bearing. • Instrument precision bearing.

Radial Bearing • Most common type. • Found in many types of transmissions. • Available in single and double rows. • Used for rigid and self aligning aplications.

Radial Bearing • Single row ball bearing Seal Shield Inner ring • Double row self-aligning ball Outer ring

Angular Contact Bearings • Unlike radial or thrust bearings, angular-contact bearings accept both radial loads and also axial loads in one direction. • The load is transferred through the bearing at an angle between the axial and thrust loads. • They are fitted in pairs if the axial loads are in both directions.

Thrust Bearings • Washer-type ball thrust Raceway groove Thrust cage Rolling element Washer • The thrust bearing is designed to accept axial loads only. • Thrust bearings are normally used in conjunction with bearings that take the radial loads. • Best used for high loads at low speeds.

Thrust Bearings Barstools and Lazy Susan turntables use this type of bearing.

Instrument Precision Bearing • Usually small, high quality precision bearings manufactured to a high accuracy and finish. • They are normally radial bearings and used in instruments and communication equipment.

Roller Bearings Roller bearings utilise cylindrical, tapered or spherical rollers running in suitable shaped raceways compared to ball bearings.

Types of Roller Bearing • Cylindrical bearing. • Spherical bearing. • Tapered bearing.

Cylindrical Roller Bearings Rigid roller • They accept greater loads than ball bearings due to the increased surface contact area. • When run in grooves on both the inner and outer race, they will accept, light axial loads.

Cylindrical Roller Bearings

Cylindrical Roller Bearings • Needle roller • If the length is greater than the diameter they are classed as needle roller bearings and are used for oscillating, radial loads. • Used where space is limited.

Needle roller

Spherical Roller Bearings Spherical roller • Can be either single or double row. • They run in spherical raceways that allow for slight misalignment. • They accept high radial and moderate axial loads.

Spherical Roller Bearings

Tapered Roller Bearings • Designed so that the rollers rotate at an angle to the shaft. • This transfers the loads between the radial and axial directions. • They are capable of accepting radial loads and axial loads in one direction. • Used back to accept two way axial loads. Taper roller

The bearing above is like the one in the hub of your car wheel. This bearing has to support both a radial load and a thrust load. The radial load comes from the weight of the car, the thrust load comes from the cornering forces when you go around a turn.

Split Journal Bearings • Split bearings are recommended for application where it is necessary to remove a cap for servicing or replacing a bearing. • This style eliminates removing pulleys and couplings from the shaft when replacement is required. • Loading range is limited to 30° below the joint. The angular parting of the cap is provided with shims to compensate for wear

Bearing Internal Clearances • Bearings are manufactured with varying internal clearances allowing for different working temperatures. • Without this internal clearance, the bearing can be difficult to rotate or may freeze up and be impossible to rotate. • Too much clearance will result in a noisy and wobbly bearing.

Bearing Internal Clearances • A marking system is used to identify the standard for each bearing. • The most common is the dot system, with four different grades of bearing.

Lubrication of Bearings • Bearing failure can be catastrophic, so the schedule for their lubrication and inspection is detailed in the maintenance schedule. • Sealed bearings can not be lubricated and must be replaced if they show signs of wear. • In areas where the lubrication may be lost, then bearings are used that can be re-greased using a grease gun.

Inspection of bearings • Bearing are designed to have the minimum of maintenance, but must be inspected regularly for wear or corrosion. • They are normally inspected still fitted to prevent damage and wear removing them. • They will be inspected for freedom of movement, smoothness (brinelling), corrosion and fouling.

Inspection of Bearings

Inspection of Bearings The bearings can be subjected to very heavy loads especially during landing, take-off, towing and taxi-ing

Inspection of bearings Zero to maximum speed in 0. 6 seconds

Inspection of Bearings Wheel failure possibly caused by: v Incorrectly fitted to aircraft. v Poor condition bearing fitted. v Inadequate lubrication. v Poor condition seals. v Massive overload.

Inspection of Bearings Roller Ends Good roller end Scored roller end

Inspection of bearings Look for polishing or wear on cage Bent cage. Burr or wear at pocket end.

Inspection of bearings Corrosion - Staining vs. Etching u u Clean surface with ‘Scotchbrite’ “If you can feel it Use ball point pen SCRAP it”

Inspection of Bearings Grease Staining vs Temperature Colouring Staining is generally confined to u roller contact surfaces (OK to re-use) u Temperature colours spread to other surfaces (scrap)

Inspection of Bearings Fill all the air space inside the bearing

Bearing Failure: Causes and Cures

Excessive Loads • Excessive loads usually cause premature fatigue. Tight fits, brinelling and improper preloading can also bring about early fatigue failure. • The solution is to reduce the load or redesign using a bearing with greater capacity.

Overheating • Symptoms are discoloration of the rings, balls, and cages from gold to blue. • Temperature in excess of 400 F can anneal the ring and ball materials. • The resulting loss in hardness reduces the bearing capacity causing early failure. • In extreme cases, balls and rings will deform. • The temperature rise can also degrade or destroy lubricant.

True Brinelling • Brinelling occurs when loads exceed the elastic limit of the ring material. • Brinell marks show as indentations in the raceways which increase bearing vibration (noise). • Any static overload or severe impact can cause brinelling.

False Brinelling • False brinelling - elliptical wear marks in an axial direction at each ball position with a bright finish and sharp demarcation, often surrounded by a ring of brown debris – indicates excessive external vibration. • Correct by isolating bearings from external vibration, and using greases containing antiwear additives.

Normal Fatigue Failure • Fatigue failure - usually referred to as spalling - is a fracture of the running surfaces and subsequent removal of small discrete particles of material. • Spalling can occur on the inner ring, outer ring, or balls. • This type of failure is progressive and once initiated will spread as a result of further operation. • It will always be accompanied by a marked increase in vibration. • The remedy is to replace the bearing or consider redesigning to use a bearing having a greater calculated fatigue life.

Reverse Loading • Angular contact bearings are designed to accept an axial load in one direction only. • When loaded in the opposite direction, the elliptical contact area on the outer ring is truncated by the low shoulder on that side of the outer ring. • The result is excessive stress and an increase in temperature, followed by increased vibration and early failure. • Corrective action is to simply install the bearing correctly.

Contamination • Contamination is one of the leading causes of bearing failure. • Contamination symptoms are denting of the bearing raceways and balls resulting in high vibration and wear. • Clean work areas, tools, fixtures, and hands help reduce contamination failures. • Keep grinding operations away from bearing assembly areas and keep bearings in their original packaging until you are ready to install them.

Lubricant Failure • Discoloured (blue/brown) ball tracks and balls are symptoms of lubricant failure. • Excessive wear of balls, ring, and cages will follow, resulting in overheating and subsequent catastrophic failure. • Ball bearings depend on the continuous presence of a very thin -millionths of an inch - film of lubricant between balls and races, and between the cage, bearings, and balls. • Failures are typically caused by restricted lubricant flow or excessive temperatures that degrade the lubricant’s properties.

Corrosion • Red/brown areas on balls, raceway, cages, or bands of ball bearings are symptoms of corrosion. • This condition results from exposing bearings to corrosive fluids or a corrosive atmosphere. • In extreme cases, corrosion can initiate early fatigue failures. • Correct by diverting corrosive fluids away from bearing areas and use integrally sealed bearings whenever possible.

Misalignment • Misalignment can be detected on the raceway of the nonrotating ring by a ball wear path that is not parallel to the raceways edges. • If misalignment exceeds 0. 001 in. you can expect an abnormal temperature rise in the bearing and/or housing and heavy wear in the cage ball-pockets. • Appropriate corrective action includes: inspecting shafts and housings for runout of shoulders and bearing seats; use of single point-turned or ground threads on non hardened shafts and ground threads only on hardened shafts; and using precision grade locknuts.

Loose Fits • Loose fits can cause relative motion between mating parts. • If the relative motion between mating parts is slight but continuous, fretting occurs. • Fretting is the generation of fine metal particles which oxidize, leaving a distinctive brown colour. • This material is abrasive and will aggravate the looseness. • If the looseness is enough to allow considerable movement of the inner or outer ring, the mounting surfaces (bore, outer diameters, faces) will wear and heat, causing noise and runout problems.

Tight Fits • A heavy ball wear path in the bottom of the raceway around the entire circumference of the inner ring and outer ring indicates a tight fit. • Where interference fits exceed the radial clearance at operating temperature, the balls will become excessively loaded. • This will result in a rapid temperature rise accompanied by high torque. • Continued operation can lead to rapid wear and fatigue. • Corrective action includes a decrease in total interference.

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