7.11 Bearings Flashcards
What must all aircraft bearings be inspected in accordance with?
All aircraft bearings must be inspected in accordance with the appropriate maintenance manuals.
What problems can visual examinations detect in aircraft bearings?
Visual examinations can detect surface flaws, over-heating, lack of lubricant, and other defects.
What types of bearings are predominantly used on aircraft?
The majority of bearings used on aircraft are either sealed or shielded.
What is the purpose of sealed or shielded bearings?
These bearings are designed to prevent the ingress of foreign particles, which could cause Foreign Object Damage (FOD) or contaminate the bearing contact surfaces.
What is usually tested in a bearing?
Testing a bearing is usually restricted to rotational checks for excessive backlash or free play.
What indicates roughness in a bearing during testing?
Slow rotation of the rolling elements and raceway will highlight roughness due to damage, corrosion, or wear.
What must a serviceable bearing exhibit?
A serviceable bearing must have a smooth actuation and movement.
How should free play be tested in a bearing?
Free play must be tested in both a radial and axial direction.
What is the consequence of excessive backlash in a bearing?
Any wear that leaves excessive backlash in the system is unsatisfactory.
How does the speed of rotation affect bearing wear?
The rate of wear depends on the speed the bearing is rotated at, with high-speed bearings failing quicker than those that rotate slowly.
What must be done if a bearing shows any indication of a fault?
A bearing that shows any indication of a fault or defect must be removed and replaced.
What is the minimum cleaning procedure for bearings?
The bearing must be cleaned of any preservative oil in a solvent bath approved for bearing cleaning purposes.
What is used to remove heavy grease, carbon, or oil from bearings?
Heavy grease, carbon, or oil is removed in a soak bath with chemicals to suit the requirements.
What is an ultrasonic bath used for in bearing cleaning?
Ultrasonic baths are used as a final cleaning method to shake off any stubborn particles or contaminants.
What should be done after cleaning a bearing?
When the bearing has completed its cleaning process, it is rinsed and then coated in light preservative oil if it is not to be immediately inspected.
How should cleaned bearings that are not installed immediately be stored?
Cleaned bearings must be coated in a rust-preventing oil, wrapped in greaseproof paper, boxed, and labelled, and stored horizontally in a clean and dry atmosphere.
What happens after a bearing has been cleaned?
Once the bearing has been cleaned, it can be inspected for unacceptable conditions.
What grounds for rejection or replacement exist for bearings?
Nearly any flaw or defect detected in a bearing assembly is likely to be grounds for replacement.
Why is the correct maintenance of bearings important?
The correct maintenance of bearings is important to prevent defects and shorten service life caused by foreign object contamination, moisture, and vibration.
What is required on a scheduled basis for bearing maintenance?
The application of a lubricant is required on a scheduled basis, as recommended by the manufacturer.
What should be done before lubricating bearings?
Thoroughly clean the lubrication equipment and fittings before lubricating to prevent dirt from causing bearing failures.
What can happen if too much grease is used in bearings?
Too much grease can overpack the bearing, causing it to run hot and shortening its service life.
What is a lubricant?
A lubricant is a fluid or grease introduced between two moving surfaces to reduce friction, improving efficiency and reducing wear.
What do lubricants typically consist of?
Lubricants contain 90% base oil and less than 10% additives.
What are the effects of effective lubrication on rolling element bearings?
Effective lubrication reduces friction and abrasion, reduces heat generated by friction, prolongs service life, prevents rust, and keeps foreign objects away.
What is a suitable lubricant for low rotational speeds?
Grease is a suitable lubricant for low rotational speeds or oscillating functions.
How are bearings in engines and gearboxes generally lubricated?
Bearings are generally lubricated by oil spray, splash, mist, drip-feed, or a controlled level oil bath.
What prevents the loss of lubricant in bearings?
Loss of lubricant is prevented by oil retaining devices such as labyrinth seals, felt, or rubber washers.
What type of grease is often used in airframe applications?
Most bearings in airframe applications are packed with anti-freeze grease due to low temperatures.
What should be done if the lubricant in sealed bearings is lost?
Sealed bearings must be replaced if it is evident that the lubricant has been washed out or lost.
What is the purpose of grease nipples in open bearings?
Grease nipples allow for the replenishment of grease at specified intervals.
What is required when repacking wheel hub bearings?
Wheel-hub bearings must be repacked with the correct grease, usually done by hand.
What are the four basic types of lubricants used for bearings?
The four basic types of lubricants are hydrodynamic lubrication, hydrostatic lubrication, boundary lubrication, and solid film lubrication.
What characterizes hydrodynamic lubrication?
Hydrodynamic lubrication separates load carrying surfaces with a thick film of lubricant to reduce friction.
What is hydrostatic lubrication?
Hydrostatic lubrication uses air and oil, with air forced into the bearing area at high pressure to separate surfaces.
What is boundary lubrication?
Boundary lubrication has an oil film that is only a few molecules thick, where lubricity and adhesion are more important than viscosity.
What are solid film lubricants?
Solid film lubricants are paint-like coatings of fine particles that repel water, reduce friction, and increase wear life.
What are the leading causes of bearing failure?
Bearing fatigue and contamination are the leading causes of bearing failure.
What substances can cause damage to bearings?
Abrasive substances like sand, grit, or dust can cause dents or scratches in the bearing race or rolling elements.
What are common sources of contamination in bearings?
Common sources of contamination include poor handling procedures, dirty hands or tools, and foreign matter in the lubricant or cleaning fluid.
What is essential for a bearing to provide long and reliable service?
A clean, properly lubricated, and installed bearing provides long and reliable service.
What should be done after a bearing has been removed?
After the bearing has been removed and cleaned, it is now ready for inspection.
What are some apparent defects in bearings?
Defects include broken or cracked races, dented or damaged seals, cracked or broken separators, damage to the balls or rollers, and discolouration due to overheating.
What are the common defects in bearings?
Common defects include Brinelling, True Brinelling, False Brinelling, Burnishing, Overheating, Galling, Spalling, Wear, Rust & Corrosion, and Electric current damage.
What causes True Brinelling?
True Brinelling is caused by excessive impact.
How is True Brinelling identified?
True Brinelling is identified by regularly-spaced indentations in the bearing raceway and rolling elements, and an increase in vibration.
What causes False Brinelling?
False Brinelling is caused by vibrations of the bearing while in a static state.
How is False Brinelling identified?
False Brinelling is identified by hollow spots that resemble brinelling dents due to wear caused by vibration.
What is Burnishing in bearings?
Burnishing is a smoothing of the original surface roughness in the bearing races as a result of being mechanically rolled over during normal running.
What causes overheating in bearings?
Overheating is often the result of excessive operating temperatures and improper lubrication.
What is Galling in bearings?
Galling occurs when the rolling element of the bearing skids, leading to metal transfer due to welding or tearing action.
What is Spalling in bearings?
Spalling is the breakdown of surfaces due to cyclic loading, resulting in fatigue of the metal at localized points.
What causes Wear in bearings?
Wear is caused by dirt and other hard foreign objects getting into the bearing ring ways.
What causes corrosion in bearings?
Bearing corrosion is caused by a chemical attack on the bearing materials, with moisture being the main cause.
What is Electrical Current Damage in bearings?
Damage occurs when an electric current passes through a bearing at the contact surfaces, similar to electric arc welding.
