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
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.
SU-A-830025 discloses a nut and bolt for applying compressive force
to hold together machinery components. The bolt passes through holes in the components
and applies force to one side of the components to be joined. Compressive force
is applied to the other side of those components through thrust washers with a
conical spring washer outwardly directed from the nut to the thrust washers. The
shank of the bolt has a conical collar fitting between the thrust washer and flexible
tabs on the underside of the nut and gripping the conical collar.
The present invention provides a nut for applying a compressive force
to one or more components mounted on a shaft having a centerline defining an axial
direction, to position the components against a fixed surface, comprising:
- an annular member having means for coupling the annular member to said shaft
on the inner diameter of said annular member; and
- transmitting means for transmitting compressive force from said annular member
to said components.
According to the invention, said transmitting means comprise a compliant
member integral with said annular member and having a first surface for transmitting
said compressive force to said components, said compliant member having a conical
portion extending axially from a region of said annular member to said first surface,
and extending inwardly from said region to a second surface coaxial with said annular
member, and which compliant member is configured to deflect inward to contact said
shaft with said second surface when said first surface is transmitting said compressive
force, whereby said nut is centered to said shaft.
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
BRIEF DESCRIPTION OF THE DRAWINGS
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
- FIG. 1 is a cross section of a gas turbine engine coupling shaft utilizing
a nut according to the present invention
- FIG. 2 is a perspective view of the nut, looking toward the conical portion.
- FIG. 3 is a cross section of the nut.
- FIG. 4 is a side view of the back side of the nut.
- FIG. 5 is a graphical representation of the percent of average thread stress
for a nut according to the present invention compared to a conventional nut.
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
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 821 N (3650 lb) in the preferred embodiment.
As shown in FIG.'s 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
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
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, Jan 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
In FIG. 3, the conical angles C1 and C2 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 C1
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 C1 and C2 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 C1
and C2 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
-18 to +204°C (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.