Thrust bearingsSupport the axial thrust of both horizontal as well as vertical shafts

Functions are to prevent the shaft from drifting in the axial direction and to transfer thrust loads applied on the shaft

Vertical thrust bearings also need to support the weight of the shaft and any components attached to it

The moving surface exerted against a thrust bearing may be the area of the end of the shaft or the area of a collar attached at any point to the shaft

Types of thrust bearingsPlain thrust: Consists of a stationary flat bearing surface against which the flat end of a rotating shaft is permitted to bear

ROTORBearing surfaceFlat end of rotorAxial movement

Thrust bearing- flat land typeThey handle light loads for simple positioning of rotorsThey are usually used in conjunction with other types of thrust bearingsThey carry 10 to 20% of the overall axial loadBearing surface sometimes incorporated with oil grooves that help store and distribute oil over the surfaceROTOROil grooves for storing and distributing oil over the surface

Thrust bearing- step typeStep bearing: Consists of a raised or stepped bearing surface upon which the lower end of a vertical shaft or spindle rotatesThe entire assembly is submerged in lubricantStepped bearings are either designed to undergo hydrodynamic lubrication or are lubricated hydrostatically (external pump)ROTORBearingWedge formation or pressurized oil supply

Thrust bearing- hydrostatic typeThese depend on an external pump to provide oil under pressure to form a load-bearing film between surfacesUsed in equipment with extremely low speeds as a hydrodynamic film cannot formROTOROil under pressure, supplied by pumpBearing surface

Thrust bearing- collar typeCollar typeShaftBearing surfaceCollarShaft moves in axial direction tooShaft rotatesLoads are borne by the bearing surface that comes in contact with the collar which is attached to the shaftOil supply

Thrust bearing- tilting pad type (Michell type)The surfaces are at an angle to each otherOne surface is usually stationary while the other movesUndergoes hydrodynamic lubrication, therefore formation of a wedge of lubricant under pressureThe amount of pressure build up depends on the speed of motion and viscosityThe pressure takes on axial loads

Thrust bearing- Tilting pad typeShaftCollarTilting pad rotates around the pivot (angle of tilt varies)PivotAxial loads from machinery being drivenIn this case thrust from propellerOil wedgeDirection of rotationBack thrust from water to propeller causes axial loading on the shaftAxial loads are opposed by pressure buildup in the wedgeGives a damping effectPasses on thrust to the shipBearing platePropellerPushes ship forward

Tilting thrust bearings- basic geometryUh1hh2XZh1 = distance of separation at leading edgeh2 = distance of separation at trailing edgeU = velocity of lower pad in the x directionB = bearing breadthThe film thickness h at any point is given by:Leading edgeTrailing edgeBx

Height ratiosUh1hh2XZLet or , thereforeThe expression for pressure gradient was derived earlier asWhere p is the pressureh is the coefficient of dynamic viscosityho is the separation distance at max. pressureU is the velocity of the bottom surfaceTop surface is stationary

Making the equation non-dimensionalLet A = ho/h2 such that ho = Ah2Substituting this and the value of h in terms of x we getOn rearranging we get:Let x* = x/B, a dimensionless length, so that

Pressure distribution equationNow h22/UhB has the dimensions of (pressure)-1 so it is possible to write (h22/6UhB)p as p*, the non-dimensional pressure. The equation therefore becomesThis is Reynolds equation in non-dimensional form applied to inclined pads. Integration gives the pressure distribution. On integration we get:

Applying boundary conditionsA and C are constants of integration. In order to evaluate them the value of pressure is required at two specific positions. This, in the case of a pad, is taken as the ambient pressure at the leading and trailing edges, where the pressure curve starts and stops. These pressures are usually considered as zero. Therefore the conditions are:p = 0 at x = 0, and x = BNon-dimensionalizing we get, p* = 0 at x* = 0 and x* = 1 (since x* = x/B)First putting p* = 0 at x* = 0, we get:

Obtaining the constants of integrationThen putting p* = 0 at x* = 1, we get:The above two equations can be solved to give:andThus:Which can be simplified to give:

Maximum pressureThe max. dimensionless pressure po* occurs when dp/dx = 0, h = ho, and x = xo.

Now,Thereforeand

Integration of the pressure across the bearing gives the load carried per unit length, W/L

So which can be defined as the non-

dimensional load W*.

Thus

Which reduces toLoad carried(as x* = x/B)

Tilting pad bearing- expression for loadNowThereforeThis equation was first derived by Reynolds for a fixed inclined surface

Height variation with pivot pointThe ratio h1/h2 = (1+K) is determined by the position of the pivot pointVelocity Uh1hh2XZPivot pointUpper pad rotates around the pivot pointThe position of the pivot point is found by taking moments about the leading edge.For stability it should be at the center of pressurex

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