Assignment Brief: Energy reduction in Tilting-pad thrust bearings
This assignment assesses the following Learning Outcome.
“Synthesise designs of … lubricated bearings using an understanding of the fundamental operating principles and aided by computerised design guides.”
Describe the principle of operation of a tilting-pad thrust bearing 10 marks
A horizontally-mounted, steam turbine in a power station has a shaft diameter of 600 mm and generates an axial thrust of 0.8 MN when operating at 1,500 rpm. The thrust is 200 kN at zero speed. The allowance for misalignment is known to be 40 µm. The runner has a roughness of 0.25 µm Ra. Oil supply temp is 50OC. The bearing material is whitemetal.
Design a uni-directional, directed-lubrication, full-complement, tilting-pad thrust bearing according to the design criteria in ESDU 83004 (using the ESDU spread sheet). The lubricant is an ISO 68 oil. The inside diameter of the pads is initially fixed at 800 mm. Place the final design variables, to 3 significant figures, in the row for Design 1 in the table provided overleaf. 10 marks
Demonstrate, below the results of Design1, that your chosen design passes all the required criteria in the design guide. 10 marks
Using the same lubricant, determine the power loss if the outside diameter remains the same as in part (a) and the inside diameter is reduced to the minimum allowed without exceeding a mean actual running pressure of 2.1 MPa. Enter these in the table as Designs 2 up to 4. Produce a graph that shows how the power loss varies with pad mean diameter and explain the significance of this curve for the design of energy-efficient bearings, linking your commentary to the equation for power loss in the Guide. Insert the graph below Design 4 in the table. And your commentary below that. 20 marks
Refine the design (full-complement only) to achieve the lowest power loss you can achieve without exceeding the mean pressure constraint described in (a). You are free to change all the bearing’s dimensions and lubricant (ISO VG 22, 32, 46 and 68 oils are available), but you cannot change the load, speed and shaft diameter. Insert any of the results of your study, to 3 significant figures, into the remaining rows of the table to demonstrate that your final design does consume the minimum amount of power. Clearly explain why the power loss is a minimum, relating your answer to the equation for power loss in the ESDU guide 10 marks
Using the data calculated in part (a), estimate the minimum film thickness that would exist if the pad had no side leakage, and the effective viscosity is as calculated in part (a). Why the difference? 20 marks
In all parts above, you should not exceed the maximum number of pads allowed in the ESDU 83004 design guide, nor exceed by more than 5% the 2 MPa actual, maximum mean pressure for white-metal bearings. The allowance for misalignment is known to be a fixed amount of 40 µm.