Bearings Flashcards
What is the definition of a bearing and what is the important relationship that it plays?
A bearing is a contacting surface within a machine through which a load is transmitted. It is important as it maintains a spacial relationship between the two surfaces.
What is tribology?
it is the study of friction, wear and lubrication and the design of bearings. The science of relative surfaces in motion.
What are the operating considerations we must take into account?
Minimise or control of friction
Minimisation of wear or damage.
Provide heat transfer
Prevent contaminants from entering the system
how do we minimise friction and wear?
Lubrication
What is a sliding/Journal bearing?
Involves direct contact and sliding of the bearing surfaces. They permit motion by incorporating surfaces with low coefficients of friction. Lubrication is key as it prevents the surfaces coming in contact with each other.
What is a Rolling Bearing?
Motion is aided by a combination of rolling motion and lubrication. While very low friction can be achieved, their performance is limited by large contact stresses between the supporting elements and rolling elements.
How do we classify lubrication and What are the three lubrication classifications for Sliding Bearings?
The degree that the surfaces are in contact.
Separated - Hydrostatic or hydrodynamic.
Partial contact - Mixed film
Substantial Contact - Boundary
What is hydrodynamic lubrication?
The surfaces are completely separated by pressure in the fluid film. This pressure arises from the relative motion of the surfaces that move in such a way that the fluid film thickness will vary.
What is hydrostatic lubrication?
Fluid is forced into the space between surfaces. The high pressure of the feed forces the surfaces to be separated.
What is Mixed Film?
The surfaces are intermittently in contact and there is partial hydrodynamic support.
What is Boundary?
The surfaces are constantly in contact and so lubrication is continuously smeared over the surfaces.
What is the typical film thickness for hydrodynamic bearings?
0.008 - 0.02mm
What is Viscosity
Symbol: mew.
Resistance of fluid to velocity gradients
What is the bearing unit load?
The load divided by the projected area. (Journal diameter / Bearing length.
What is the equation for the fluid motion resisting force?
F = (mew x v x L x B) / h mew = viscosity v = velocity of the moving surface L = length of stationary surface that the moving surface covers B = Breadth h = diameter.
What is the effect of fluid viscosity equation?
Force per unit area:
P = F/A = F/LB = mewU / h
This force will be in the direction that opposes motion.
How do we generate a perpendicular force between bearing surfaces?
The separation of the surfaces , h, must vary along the direction of movement.
What is the working principle behind hydrodynamic lubrication?
The moving surfaces force the fluid into a gap which is ever decreasing in size. This produces the required separation force. The wedging action is provided by the shaft eccentricity. Also need a suitable fluid.
What is the equation for circumferential velocity?
U = 2piR x n where n is the rotating speed of the shaft.
What is the equation for torque?
T =R x Ff where Ff is the frictional force.
What is the Power equation?
Power = Circumferential velocity x torque P = UT
What is the equation for the average film pressure?
P = W / DL, W is the applied radial load.
What is the equation for the coefficient of friction?
f = Ff / W = Ff / PDL
Can use this to find the torque needed to drive the bearing as: Tf = fWR
If there is no or a very small radial load, then there will be little eccentricity so what will the equation for the friction force be?
Ff = mew x (surface area of the bearing) x U / c
Ff = 4 x mew x R^2 x pi^2 x L x n / c
This is known as Petroff’s equation
What is the equation for the Sommerfield Number, S, and when do we use it?
S = (R/c)^2 x (mew x n / p)
We use it when there is an applied radial load.
What are the 3 phases of lubrication when a journal bearing starts up?
At rest, the load presses the load against the bearing surface so there is boundary lubrication. As rotation slowly increases, the hydrodynamic forces aren’t great enough to fully separate the two surfaces so we have mixed film. Eventually at a great enough speed the two surfaces will separate and true hydrodynamic lubrication will occur.
Is the starting torque in a journal bearing high or low?
High
What is a Plain Bearing?
Uses hydrodynamic loading to carry heavy loads. Used in crankshafts, camshafts, gearboxes….
What is a plastic bushes bearing?
They are formed from a solid piece of material. Applications include low speed moving parts, motor vehicle linkages, hinges….
They can have very low coefficients of friction even when in direct contact with metal.
They are a poor heat conductor so not suitable for high velocity applications.
What is a thrust bearing
?
Designed to locate a shaft longitudinally. They resist movement along the axis of the shaft. Have a tapered inner to resist falling out.
How do Roller bearings overcome friction?
By having rolling elements that are in direct contact with each other.
Which bearing will have greater load carrying capabilities?
Journal
What are the advantages of roller bearings over sliding bearings?
Low friction at start up
The shaft can be accurately positioned, doesn’t have hydrodynamic lubrication
Shaft isn’t subjected to wear
Easily to replace
Suitable for high speed and temperature applications.
How do we classify rolling bearings?
By the type of rolling element, say a ball or roller.
Of a ball or roller bearing, what will have the higher speed capacity?
And what will have the higher load carrying capacity?
Ball will have higher speed capacity
Roller will have higher load.
What is a grease a mixture of?
Lubricating oil, soap which is used as a thickening agent..
What are the problems that can occur with grease?
Overheating which can lead to melting\
overpacking which can then lead to overheating if the grease cannot escape
Grease has a limited life, and if it needs to be replaced then it all must be replaced as can’t have a mixture of new and old.
What are the 5 steps of bearing assembly?
The inner race is positioned inside the inner race
The balls are inserted
The balls are placed uniformly around the bearing
The ball separators are inserted
The bearing is packed with grease and sealed.
Why do we need hard, high strength material for the balls or rollers>
Due to the very small point of contact there will be high contact stresses.
What are the ways to shorten the life of a bearing?
Damaging components on assembly
Inappropriate storage that leads to corrosion
Miss alignments of journals or mounts
Overloading the bearing
Wrong lubricant used
Ineffective seals that let in contaminants.
What is a tracking band?
It is the slight wear of the running path due to the rolling elements. Will appear even after brief use. Can indicate how the bearing is loaded.
How do we define roller bearing life?
The number of revolutions that a bearing can complete before fatigue develops
