In many applications, nuts and bolts are used to apply compressive forces on multiple components, securing them in a stacked relation. The compressive force through the components is equal to the tensile force in the bolt which stretches proportionally to the bolt length. A problem occurs when the bolt is placed in a hot environment where it grows due to thermal expansion and relieves the compressive force. This problem can be further compounded by vibration which can help loosen the nut. This is particularly evident when the compressive force is minimal because the friction holding the nut in place is minimal. These detrimental conditions occur in gas turbine engines and other applications and must be overcome because securing the components is critical.
In gas turbine engines, a nut is often used on the end of a threaded shaft to secure and position engine components relative to the shaft. The shaft traditionally has a radial flange extending outward at one end to provide an abutting surface and threads for the nut at the opposite end. The engine components are stacked along the shaft such that the shaft extends through the center of the components. The nut is threaded to the shaft to apply a compressive force through the components which secures them in place relative to the shaft, and thus, pilots the components.
In some engines, such as the one in FIG. 1, the shaft is relatively short, and thus, has little axial deflection when pulled on by the nut. This presents several problems. First, different coefficients of thermal expansion can make the thermal growth of the shaft greater than that of the engine components during hot, operating conditions. Second, the engine components are subject to dynamic radial forces which results in a Poisson axial contraction in the components. These phenomena tend to relieve the securing force and pilot of the engine components. Also, a large axial force is required to maintain the engine components in compression which can create high stresses in the nut threads. Because the shaft and nut threads are at an angle other than 90 degrees to the nut and shaft centerline, the compressive load tends to be unevenly distributed circumferentially on the engine components and the threads tend to axially align at 90 degrees to the centerline. Another serious problem is the entrapment of debris which can cause runnout in both the nut and shaft end such that the nut and shaft end are no longer centered to the engine centerline.
Accordingly, a need exists for a nut that can apply a consistent compressive force on piloted components to allow for shaft and engine component axial deflection mismatch and can reduce thread stresses. A need also exists for a nut that can apply a circumferentially uniform compressive force to the engine components and will not create nut and shaft runnout and will self center the nut in the event of entrapped debris.
USPTO Backgrounds
USPTO Backgrounds