6.8 bearings Flashcards
What are bearings?
They constrain relative motion to desired motion
What are bearings designed to do?
Produce minimum friction whilst having maximum wear resistance
What are gears used in conjunction with?
Shafts and bearings to transmit power
What two groups can bearings be split into?
Plain bearings
Rolling element bearings
What are plain bearings?
They have no moving parts and comprise of a plain cylinder or flat washer surface with a softer material than the shaft they support
What are rolling bearings?
Assembly comprising of hardened and enclosed rolling components which roll in inner and outer casings
What are the inner and outer casings called?
Races
What are the sub categories of rolling and plain bearings?
Roller and Ball bearings
What characteristics must a bearing consist of?
Strong material (to ensure minimum friction and maximum wear resistance
Parts must be held together with tight tolerances (to provide efficiency and quietness)
What forces are the bearings subject to?
Axial thrust loads
Radial/journal loads
What are axial loads?
Acting along the supported shaft
What are radial loads
Acting at right angles to the eg vertically
Can a bearings have a combination of both loads?
Yes
What materials are suitable for bearings?
Lead
Copper
Iron
Synthetics
Dry powdered materials
What good properties do materials suitable for bearings have?
Sliding properties and emergancy properties
What do bearings require to reach a long service life?
Lubrucation
What are synthetic materials lubricated with?
Water
What else does this water act as?
A coolest as they get hot
What can happen if the lubricant is missing?
The journal can cut into the bearing
What problems do machines with sliding bearings face?
How can they be avoided?
Insufficient oil in the bearings
Using grease pumps and pre pumping machine before use
When is grease only used in sliding bearings? (Plain bearings)
Strongly stressed bearings with a large bearing play. Slow moving and swinging shafts and axels
What are plain bearings also known as?
Sleeve or sliding bearings
What are anti friction bearings also known as?
Rolling or roller elemenent bearings
What is a plain bearing designed to take?
Radial loads which act at right angles to the shaft
When are plain bearings used?
When the loads are light and continuous
In order to work, what do plain bearings need?
A gap between the stationary part and the rotating part
What are pivot or axial bearings?
They act longitudunially
What are solid sleeves suitable for?
Radial loads only
What are slot sleeves used for?
To compensate wear
What are split sleeves used for?
Large diameter shafts
What is great about anti friction bearings?
They eliminate any friction
What are four advantages of antifriction bearings?
they have low starting resistance
They are cooler
They can carry heavy loads
Are good for oscillating or vibrations
What does an anti friction bearing consist of?
A inner and outer race way, the rolling element and a rolling element cage
When are roller bearings used instead of ball bearings?
When the stresses are too severe
What are ball bearings best at?
High rotational speeds
What are the major components of a standard ball bearing?
nner and outer raceways, a rolling element (balls) and retainer.
What can aid installation in ball bearings?
Chamfered ends
What can protect the lubricant and bearing?
Shield and seals
What helps the races become self aligning?
The races can be machined internally or externally
when are ball bearings used?
Where a narrow bearing is required
What are ball bearings not good at?
Oscillating loads because of their size
What is the most common use of a cylindrical bearing?
Non locating positions
One must float free, the other must be connected to the shaft
One must float free, the other must be connected to the shaft
Thermal expansion
Are the inner or outer races seperable or inseparable?
Seperable
What helps cylindrical roller bearings bear radial bearings?
A line contact with the race
What is the load capacity of roller bearings?
Low because the cage must align and retain the rollers
What do needle roller bearings have that are beneficial?
A compact cross section
What load capacity does a needle roller bearing have?
Radial. Only light axial loads
What does spherical roller bearings allow for?
Angular displacement between shaft and housing
What is the load like in spherical roller bearings?
They have high radial load capacity but the stress is not equally distributed
What is bad about spherical roller bearings?
They skid because the rolling motion occurs at two contact points
In spherical roller bearings , what feature must the cage have and why?
Must be robust to oppose any skewing effect which increases inertia
What are spherical roller ball bearings best suited to?
Areas prone to misalignment, and where speed and deflection criteria are not extracting
What is good about tapered roller bearings?
They can withstand radial and axial load
Longer service life
Less vulnerable in environments due to natural pumping
What are air bearings?
They use pressurised air to provide zero friction load
Why do air bearings avoid friction etc?
They don’t have contact
How are air bearings fluid film created?
Through airflow through the bearing face and the bearing gap
What are thrust bearings used for?
Combination of radial/thrust loads
Ball, roller and needle bearings are designed for thrust loads only why?
They have the tracks of their races aligned to withstand it
How are thrust bearings mounted?
Vertically
If radial loading is expected, what must be placed next to the thrust bearing?
A normal bearing
What are needle thrust bearings?
They have high load capacity
High radial load capacity when tapered
