Self positioning nut

The present invention provides a compliant nut that can provide a more consistent compressive force to engine components by using a compliant section to absorb axial deflections. The present invention also provides a self centering feature that positions the nut upon compression, reduces thread stresses and prevents nut and shaft runnout. The invention consists of an annular member having threads on the inner diameter for mating with a threaded shaft and having a conical portion extending axially and radially inward from the annular member for abutting engine components and the engine shaft. The nut includes a plurality of exciter teeth circumferentially disposed and extending radially from the annular member used to provide shaft speed signal for engine controls and a plurality of axial spline members extending axially from the annular member on the opposite face from conical section for mating with torque applying tools and locking mechanisms.

TECHNICAL FIELD 
This invention relates to a nut used on a shaft to apply a consistent 
compressive axial force on a plurality of components to position the 
components and to self position itself to the shaft. 
BACKGROUND OF THE INVENTION 
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. 
SUMMARY OF THE INVENTION 
An object of the invention is to provide a nut which will provide a more 
uniform compressive load through a plurality of components stacked upon a 
shaft when the components and the shaft have axial deflection mismatch. 
Another object of the invention is to provide a nut which will self center 
itself to the shaft and prevent shaft and nut runnout. 
Still another object of the invention is to provide a nut which will reduce 
maximum thread stresses. 
Still another object of the present invention is to provide a nut that can 
be used to obtain an engine speed signal. 
The present invention meets the above mentioned objects by providing a 
compliant nut that can provide a more consistent compressive force to 
engine components through a compliant section which absorbs axial 
deflection mismatch. The present invention also provides a self centering 
feature that positions the nut upon compression, reduces thread stresses 
and prevents nut and shaft runnout. More particularly, the invention is an 
annular member having threads on the inner diameter for mating with a 
threaded shaft and having a conical portion extending axially and radially 
inward from the annular member for abutting engine components and the 
engine shaft. The conical portion is compliant in the axial direction, and 
thus, deflects when compressed. Also, the conical portion inner surface 
deflects radially inward to contact the shaft when the nut is compressed 
which centers the nut on the shaft, reduces thread stresses and prevents 
nut and shaft runnout. 
Other features present in the compliant nut include a plurality of exciter 
teeth circumferentially disposed and extending radially from the annular 
member used to provide a shaft speed signal for engine controls and a 
plurality of axial spline members extending axially from the annular 
member on the opposite face from the conical section for mating with 
torque applying tools and locking mechanisms.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
The following description of the preferred embodiment of the present 
invention refers to a section of a conventional gas turbine engine that 
includes a compressor, a combuster and a turbine (all not shown) in fluid 
communication for providing shaft power to a load compressor 12 and output 
power at the gearbox shaft 20. Air is compressed in the compressor, then 
combusted in the combustor with fuel and then expanded over the turbine to 
provide the shaft power. The shaft power is transferred to the load 
compressor 12 and shaft 20 through engine shaft 18. 
FIG. 1 shows a rotating coupling shaft 10 coupled at one axial end to the 
load compressor 12 through a curvic coupling 14 and to the engine shaft 18 
through threads 16 and is coupled at the other axial end to a gearbox 
shaft 20 through spline 22. The coupling shaft 10 is radially positioned 
in a gearbox housing 24 through bearings 36. The bearings 36 require 
lubrication which is provided through passages 44 in a support member 42. 
A seal rotor 38 and carbon face seal 40 seal the lubricant from the load 
compressor 12. The carbon face seal 40 is an annular member with a axially 
flat surface for abutting the seal rotor 38 and is positioned by support 
member 42. The seal rotor 38 and bearings 36 are axially positioned by 
abutting against surface 28 on shaft 10 through a compressive force 
applied by the nut 30. The nut 30 is coupled to the shaft 10 through 
threads 32, and when assembled, secures the engine components 36 and 38 
through a force of approximately 3650 lb in the preferred embodiment. 
As shown in FIGS. 2-4, the nut 30 is comprised of an annular member portion 
68 with the threads 32 on the inner diameter. A plurality of axial splines 
54 are located circumferentially on the annular member 68 to mate with a 
tool that torques the nut 30 on the shaft 10 for assembly. A plurality of 
exciter teeth 52 are circumferentially disposed about the outer surface of 
the annular member 68 and extend perpendicularly therefrom. As seen in 
FIG. 1, a sensing device 46 extends radially in from the engine outer 
housing (not shown) and uses the exciter teeth 52 to measure the engine 
rotational speed. By using the nut 30 to provide the engine speed, the 
controls can be moved to the gearbox housing 24 for better accessibility 
and component temperature control. 
The nut 30 also has an annular compliant section 60 which is conically 
shaped and extends axially and radially inward toward the engine 
centerline from the annular member 68 so as to form an inverse V shape 
therewith and forming a circumferential channel 66. The compliant section 
60 has an axially flat end with surface 62 at the radially inner end for 
contacting the engine components 36 and 38 and applying the compressive 
force thereon. When the nut 30 is threaded onto the shaft, the compliant 
section 60 acts as a spring and deflects axially toward the annular member 
68 more than the shaft 10 stretches. Thus, the difference in deflection of 
the engine components 36 and 38 and the shaft 10 during engine operation 
can be reacted through deflection in the compliant section 60 and the 
compressive force exerted on the engine components 36 and 38 will remain 
proportional to the spring rate of the compliant section 60. In the 
preferred embodiment, the thickness t of the compliant portion 60 is 
approximately 1/3 of its axial extension 1. This thickness provides a 
sufficiently stiff compliant section 60 to apply the compressive force 
desired and provides a small radial deflection to prevent gouging of the 
shaft 10. The stiffness or spring rate of the compliant portion 60 can be 
tailored by simply thinning or thickening the compliant portion 60. 
The compliant portion 60 also has an inner surface 64 that encircles the 
shaft 10 with little or no clearance therebetween before any load has been 
applied. In the preferred embodiment, the inner surface 64 is sized with a 
line-to-line clearance with the shaft 10, but greater compliancy in the 
compliant portion 60 would require greater clearance between the inner 
surface 64 and the shaft 10 to prevent gouging into the shaft 10. When the 
nut 30 is torqued onto the shaft 10, the inner surface 64 deflects 
radially inward to contact the shaft 10. By clamping the inner surface 64 
on the shaft 10, the nut 30 is self centered on the shaft and any 
misaligning moment, caused by the nut threads 32 or any entrapped debris, 
is reacted through the radial loads in the inner surface 64 rather than 
through the axial loads in surface 62 which would create runnout in the 
nut 30 and shaft 10. 
The clamping of the inner surface 64 also creates a radially outward force 
that is transferred through the compliant section 60 to the annular member 
68. This force pulls radially outward on the annular member 68, relieving 
the compressive force in the first few threads of threads 32 as 
illustrated by FIG. 5. FIG. 5 graphically depicts the percent of average 
stress for stresses in a conventional nut, line 100, and stresses in the 
present invention nut, line 101. A conventional nut is used in the 
comparison as the closest prior art the applicants are presently aware of. 
The stresses were calculated for the conventional nut using data from 
"Controlling Fastening Reliability and Cost", Assembly Engineering, 
January, 1973, p 27 and the stresses in the nut 30 were calculated using a 
finite element model of the nut. Each point on the lines depicts the 
percent of average stress for a thread root where the first thread root is 
represented by the furthest point to the right. FIG. 5 indicates that the 
present invention tends to decrease the stresses in the first few thread 
roots by more evenly distributing the compressive load over the threads. 
In FIG. 3, the conical angles C.sub.1 and C.sub.2 in the preferred 
embodiment are approximately 30 and 44 degrees respectively from the 
radial direction RD which is perpendicular to the nut and engine 
centerline. As one skilled in the art can appreciate, as the angles 
C.sub.1 and C.sub.2 approach 90 degrees, the axial stiffness of the 
compliant section increases and as the angles approach 0 degrees, the 
axial stiffness of the compliant section 60 decreases as long as the 
compliant section 60 is spaced apart from the annular member 68. 
Conversely, as the angles C.sub.1 and C.sub.2 approach 0 degrees, radial 
stiffness increases so that the radial force through the inner surface 64 
to center the nut 30 becomes increasingly stiffer. Thus, angles C.sub.1 
and C.sub.2 must be less than 90 degrees and are preferably between about 
45 degrees and 0 degrees. 
Preferably, surface 62 has a slight angle A as shown in FIG. 3 that is less 
than about 1 degree. The angle A allows the surface 62 to align flat 
against the compressed engine components, namely the bearing 36 inner 
race. Similarly, surface 64 also has a slight angle B that is less than 
about 1 degree so that the surface will lay flat against the shaft 10 upon 
nut compression. Both angles A and B should be changed inversely 
proportionally to changes in the stiffness of the compliant section 60. 
The nut 30 is preferably made from stainless steel 17-4 which provides good 
material properties through the gearbox operating temperatures of less 
than 0 to 400 degrees Fahrenheit. Of course, other stainless steels or 
A286 could be used in this application and Inco 718 could be used for hot 
applications. The nut 30 is preferably manufactured by bottle boring such 
that material is removed between the annular ring 68 and the compliant 
section 60 creating the annular channel therebetween. However, alternative 
processes are available, such as casting an integral nut 30 or bonding a 
compliant section 60 to an annular member 68. 
As one skilled in the art can appreciate, the compliant section 60 could be 
comprised of a conical section extending radially outward if the engine 
components had larger diameters. While this would provide the desired 
axial compliance, it would not center the nut to the shaft or relieve the 
thread stresses as the preferred embodiment does. 
Other modifications and alterations to the above described preferred 
embodiment will be apparent to those skilled in the art. Accordingly, this 
description of the invention should be considered exemplary in nature and 
not as limiting to the scope and spirit of the invention which should be 
determined from the following claims.