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Thrust bearings Support 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

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Types of thrust bearings Plain thrust: Consists of a stationary flat bearing surface against which the flat end of a rotating shaft is permitted to bear ROTOR Bearing surface Flat end of rotor Axial movement

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Thrust bearing- flat land type They handle light loads for simple positioning of rotors They are usually used in conjunction with other types of thrust bearings They carry 10 to 20% of the overall axial load Bearing surface sometimes incorporated with oil grooves that help store and distribute oil over the surface ROTOR Oil grooves for storing and distributing oil over the surface

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Thrust bearing- step type Step bearing: Consists of a raised or stepped bearing surface upon which the lower end of a vertical shaft or spindle rotates The entire assembly is submerged in lubricant Stepped bearings are either designed to undergo hydrodynamic lubrication or are lubricated hydrostatically (external pump) ROTOR Bearing Wedge formation or pressurized oil supply

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Thrust bearing- hydrostatic type These depend on an external pump to provide oil under pressure to form a load-bearing film between surfaces Used in equipment with extremely low speeds as a hydrodynamic film cannot form ROTOR Oil under pressure, supplied by pump Bearing surface

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Thrust bearing- collar type Collar type Shaft Bearing surface Collar Shaft moves in axial direction too Shaft rotates Loads are borne by the bearing surface that comes in contact with the collar which is attached to the shaft Oil supply

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

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Thrust bearing- Tilting pad type Shaft Collar Tilting pad rotates around the pivot (angle of tilt varies) Pivot Axial loads from machinery being driven In this case thrust from propeller Oil wedge Direction of rotation Back thrust from water to propeller causes axial loading on the shaft Axial loads are opposed by pressure buildup in the wedge Gives a damping effect Passes on thrust to the ship Bearing plate Propeller Pushes ship forward

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Tilting thrust bearings- basic geometry U h1h1 h h2h2 X Z h 1 = distance of separation at leading edge h 2 = distance of separation at trailing edge U = velocity of lower pad in the x – direction B = bearing breadth The film thickness “h” at any point is given by: Leading edge Trailing edge B x

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Height ratios U h1h1 h h2h2 X Z Let or, therefore The expression for pressure gradient was derived earlier as Where p is the pressure is the coefficient of dynamic viscosity h o is the separation distance at max. pressure U is the velocity of the bottom surface Top surface is stationary

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Making the equation non-dimensional Let A = h o /h 2 such that h o = Ah 2 Substituting this and the value of h in terms of x we get On rearranging we get: Let x* = x/B, a dimensionless length, so that

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Pressure distribution equation Now h 2 2 /U B has the dimensions of (pressure) -1 so it is possible to write (h 2 2 /6U B)p as p*, the non-dimensional pressure. The equation therefore becomes This is Reynold’s equation in non-dimensional form applied to inclined pads. Integration gives the pressure distribution. On integration we get:

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Applying boundary conditions A 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 = B Non-dimensionalizing we get, p* = 0 at x* = 0 and x* = 1 (since x* = x/B) First putting p* = 0 at x* = 0, we get:

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

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Maximum pressure The max. dimensionless pressure p o * occurs when dp/dx = 0, h = h o, and x = x o. Now, Therefore and

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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 to Load carried (as x* = x/B)

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Tilting pad bearing- expression for load Now Therefore This equation was first derived by Reynold’s for a fixed inclined surface

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Height variation with pivot point The ratio h 1 /h 2 = (1+K) is determined by the position of the pivot point Velocity U h1h1 h h2h2 X Z Pivot point Upper pad rotates around the pivot point The position of the pivot point is found by taking moments about the leading edge. For stability it should be at the center of pressure x

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