Abstract
The wheels of steam turbines are subjected to three separate forces, namely (1) the body forces due to the inertia of the wheel itself, (2) the force applied radially to the rim due to the inertia of such attached masses as blades, spacers, etc., and (3) the force applied to the bore of the wheel due to force fitting or shrinking the wheel on the shaft. The method of calculating the stresses in turbine wheels which is usually explained in textbooks and published papers takes account of these forces simultaneously. The calculations are tedious, and if it is desired to analyse the effects of changing the speed of rotation of the wheel, or of changing the magnitude of the attached masses, or of varying the degree of force fit on the shaft, then the calculations must be repeated for each combination of conditions.
In the method given in the paper, the stresses in the wheel are calculated (1) for a rotating wheel free from rim loading and radial stress in the bore due to force fitting, (2) for a stationary wheel subjected only to the rim load, and (3) for a stationary wheel subjected only to the force fit pressure. The resultant stress in the wheel is then simply the sum of the three stresses so obtained.
Since the wheel inertia stress varies directly as the square of the speed, and since the stresses in the stationary wheel are directly proportional to the boundary stresses, the effects of (1) varying the speed of rotation, (2) varying the magnitude of the attached masses, and (3) varying the degree of force fit may be analysed with very little effort.
The paper shows how the method of superposition may be employed in the calculation of (1) the stresses in a turbine wheel, (2) the amount of force fit necessary to satisfy specified conditions, (3) the effect of varying force fits on the maximum stress in the rotating wheel and in the stationary wheel, and (4) the distribution of stress in the stationary wheel.
The paper concludes with an estimation of the localized stresses in the neighbourhood of pressure-balancing holes.
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