Investigation in the Eccentricity of Spoke Centered Turbomachinery Components

A spoke centered fixing arrangement is a typical design element in gas turbine machinery. It is used to mount two annular components concentrically allowing for free relative movement in radial direction due to different elongations caused by temperature gradients. In most cases the outer part represents the absolute coordinate system the inner ring has to be centered in reference to. Several tab slot combinations are used to provide the desired degree of freedom while guaranteeing the inner ring’s concentrical position. Typically the inner ring is equipped with pins sliding in the opposite lying slots of the outer ring. The arrangement requires at least three tab slot combinations, called spokes, between fixed and floating rings. A typical application is for example an inner air seal ring mounted concentrically to the guide vane ring of a gas turbine. Mountability demands clearance between tab and slot surfaces. Associated manufacturing tolerances lead to a certain eccentricity bandwidth for the floating inner ring. The eccentricity was investigated using commercial statistical tolerance analysis software. Since contact between the two rings is only established at three spokes according to acting forces and geometric proportions influenced by tolerances, a contact definition can’t be made before the actual simulation. Therefore, the software’s functionality had to be extended to allow automatic contact definition according to the above mentioned influences. An adaptable simulation model was developed in order to provide a basis of decision making regarding future spoke design parameters. Based on a typical spoke centering design a parameter study with respect to the number of spokes and with respect to a variation of tab and slot dimension tolerances was accomplished. The parameter study showed that the eccentricity mean value decreases with an increasing number of spokes. The mean value was found to approximate the nominal circumferential gap between tab and slot already at low spoke numbers. Lower mean values are hypothetically possible down to the minimum gap between tab and slot surface pairings. The number of spokes would therefore have to be infinite. The corresponding eccentricity standard deviation was also found to decrease with an increasing number of spokes. The investigation revealed that the eccentricity mean value interestingly also declines at higher spoke numbers with increasing tab and slot dimension tolerances. The standard deviation however increases in that case. This opposed effect may be neutralized, such that the upper limit of the eccentricity bandwidth and therefore the maximum eccentricity remains almost constant while broadening the tolerance bandwidths.