The effects of bronze and composite sleeves on trunnion yoke plate stress concentrations

Abstract

Nonlinear simulations were used to predict and further understand the load transfer between trunnion shafts and yoke plates within a tainter gate trunnion assembly. Traditionally, yoke plates have been sized for an average stress based on the projected bearing area between the trunnion shaft and the yoke plate; however, finite element analyses proved that the stress is not uniform across the thickness of the yoke plate. The non-uniform stress distribution is attributed to the transverse shaft rotations that exist at the supports, concentrating load on the inboard edges of the yoke plates. Further study showed that the installation of either a bronze or composite sleeve between the yoke plate and the trunnion shaft will reduce the magnitude of the stress concentrations. A series of finite element models was developed to investigate the effects that shaft diameter, yoke plate thickness, and sleeve material have on the trunnion shaft to yoke plate load path. The finite element models were developed utilizing solid elements in order to capture the stress distribution across the yoke plate thickness by including multiple solution points across the yoke plate thickness. The analyses showed that a trend can be identified between the magnitude of the edge stress and the L/D ratio (shaft clear span to shaft diameter). As the L/D ratio of the system is increased, the magnitude of the edge stress increases; however, when a sleeve with a lower modulus of elasticity is introduced into the system, it is observed that the magnitude of the edge stress is reduced. The results proved that the reduction in stress is sensitive to shaft diameter, sleeve material, yoke plate thickness and L/D ratio. Typically for a yoke-shaft detail, the L/D ratio is designed to be close to 1.0 in order to minimize the inboard edge stress; however, trunnion assemblies with larger L/D ratios are desirable from a tainter gate design perspective. Larger clear spans (L) simplify the connection between the strut arms and the trunnion assembly, and small shaft diameters (D) reduce the trunnion pin friction moment demand on the tainter gate strut arms. By installing a low modulus sleeve between the trunnion shaft and the yoke plate, the magnitude of the edge stress is reduced; therefore, the design can accommodate L/D ratios larger than 1.0 while still keeping the stresses below an acceptable level. The simplified detailing and the reduction in strut arm demand will produce a more cost effective tainter gate design.