Diametral force sensor

Disclosed is a device for determining the diametral changes in a substantially cylindrical member produced by axial loads imposed on the cylindrical member, such as the valve stem in a motor operated valve assembly. The device is a clamp assembly comprised of a first jaw member and a second jaw member rigidly connected to each other and to the portions of the periphery of the cylindrical member by a pair of studs. The second jaw member has a flexure with a contact surface contacting a peripheral portion of the cylindrical member and a projection extending away from the contact surface to transmit compressive forces representative of diametral changes in the cylindrical member against a compression load cell housed in the second jaw member. The device is useful in the field of valve diagnostics.

BACKGROUND OF THE INVENTION 
This invention relates to an improved device for measuring diametral 
changes in a substantially cylindrical member. It relates particularly to 
a device for measuring slight diametral changes in a substantially 
cylindrical member, such as a valve stem, that can be used for determining 
the axial loading on the substantially cylindrical member. 
In many industries it is important to measure the variable dynamic axial 
loads that may be imposed on a cylindrical member or shaft. This is 
especially true in the nuclear power industry where motor operated valves 
are used extensively and monitoring of the various operating parameters of 
the valves are required by regulatory agencies. Motor operated valves are 
comprised generally of an electric motor driven valve actuator that is 
connected to a valve stem and a valve yoke that partially surrounds the 
valve stem. Rotation of a nut attached to the valve stem by the valve 
actuator will move a valve plug into a closed, open or intermediate 
position with respect to a valve seat in the body of the valve. 
It has been found that one of the best ways to monitor the dynamic forces 
and events that occur during the operation of a motor operated valve is by 
direct measurement of the valve stem axial loads using either axial or 
diametral strain gages. 
It is well known that one can calculate the axial load or strain in a valve 
stem or any other similar substantially cylindrical member, by measuring 
changes in the diameter of the valve stem or cylindrical member. The ratio 
of the diametral change to axial elongation for a material, referred to as 
Poisson's ratio, is known. Therefore, by measuring the diametral change on 
the valve stem or cylindrical member, axial strain and valve stem axial 
load can be easily determined. 
U.S. Pat. Nos. 4,911,004; 4,930,228; 4,936,150 and 5,123,283 describe 
several prior devices and systems that have been developed for measuring 
axial strains in a valve stem or similar cylindrical member. 
Some of these prior devices were a clamp type of strain gage that used two 
clamp assemblies to attempt to define a reference gage length axially 
along the valve stem. Such devices were usually expensive, comprised of 
many parts and often difficult to mount on the exposed portion of a valve 
stem. 
Many of the other prior devices designed as clamps to fit around an exposed 
portion of the valve stem or cylindrical member to measure diametral 
strains were usually designed so that any diametral strains in the valve 
stem would cause the clamp to deflect. This clamp deflection was then 
measured and calibrated to diametric strains. The measurement of clamp 
deflection however, requires the device to be flexible enough to provide a 
reasonable level of output signals. However, the flexible nature of these 
devices often led to undesirable and erroneous output signals due to the 
torsional loads that are also applied to the valve stem during operation 
of the valve. Many of these prior devices were not able to distinguish 
strains induced in the valve stem due to torsional loads from those 
induced in the valve stem through axial loads and thrust. 
SUMMARY OF THE INVENTION 
It is therefore an object of this invention to provide a device for 
determining diametral changes in a valve stem or other cylindrical member 
that is simple, relatively inexpensive and easily installed on commercial 
motor operated valves. 
It is another object of this invention to provide a device for determining 
diametral changes in a valve stem or other cylindrical member that is able 
to provide direct and continuous measurements of the axial loads on a 
movable valve stem. 
It is another object of this invention to provide a device for determining 
diametral changes in a valve stem or other cylindrical member that is 
quite accurate and not subject to errors caused by torsional loads or 
off-center axial loads on the valve stem. 
These and other objects of this invention can be attained by a device for 
determining the diametral changes in a cylindrical member produced by 
axial loads imposed on the cylindrical member, comprising a clamp assembly 
having a first jaw member and a second jaw member which are rigidly 
connected to each other on opposite sides of the cylindrical member. The 
first jaw member has a substantially V-shaped recess adapted to engage two 
peripheral portions of the cylindrical member. The second jaw member has a 
flexure with a contact surface contacting a peripheral portion of the 
cylindrical member and a projection extending away from the contact 
surface to provide the transmittal of compressive forces representative of 
diametral changes in the cylindrical member against a compression load 
cell contained within the second jaw member.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
FIG. 1 is an isometric view of a typical motor operated valve assembly 1, 
partly in section, to illustrate the installation of the diametral force 
sensor 2 of this invention on the cylindrical valve stem 3 of the valve 
assembly 1. The valve assembly 1, generally is comprised of a valve body 4 
which contains a valve plug or gate 5, operated to a closed, open or 
intermediate position with respect to a valve seat by the valve stem 3. 
The valve stem 3 is partially enclosed by a valve yoke 6 which supports a 
valve actuator 7 operated by an electric motor 8. 
As shown in FIG. 1, the diametral force sensor 2 of this invention is 
clamped around the valve stem 3. The diametral force sensor 2 has an 
electrical cable 9 leading from the diametral force sensor 2 to an 
electrical signal conditioning module 10, a data recording device 11, such 
as a computer disk, and a computing and display device 12, such as a 
personal computer. 
As shown in FIGS. 2 and 3, the preferred embodiment of the diametral force 
sensor 2 of this invention is a clamp assembly comprised of a first jaw 
member 13 and a second jaw member 14 rigidly connected to each other and 
to the valve stem 3 on opposite sides of the valve stem 3. The jaw members 
13 and 14 are preferably made of steel and joined together by a pair of 
steel studs 22. 
The first jaw member 13 has a substantially V-shaped recess 15 adapted to 
engage two peripheral portions of the valve stem 2. The second jaw member 
14 has a relatively thin flexure 16, having a contact face or surface 17 
that contacts another peripheral portion of the valve stem 3 and a 
projection 18 that extends away from the planar face or surface 17. If 
desired, the rear face of the flexure 16 is provided with several spaced 
shallow grooves 23 designed to control the flexing of the flexure 16 
resulting from the diametral changes in the valve stem 3. The projection 
18 is designed to transmit compressive forces from the back of the flexure 
16 to a compression load cell 19 contained in a recess 20 formed in the 
second jaw member 14. A removable cover plate 21, covers the recess 20 to 
retain and protect the load cell 19. The second jaw member 14 is also 
provided with a wiring port 24 to permit the electrical cable 9 connecting 
the load cell 19 and the signal conditioning module 10 to exit the rear of 
the second jaw member 14. 
A suitable load cell as used in the preferred embodiment of this invention 
is a Model No. 53 compression load cell manufactured by Sensotec 
Corporation. 
In operation, the diametral force sensor 2 of this invention is clamped 
around the valve stem 3 and connected electrically by the electrical cable 
9 to the signal conditioning module and the data recording device 11 and 
the computing and display device 12. 
As mentioned earlier, prior valve stem clamp-like sensors that measured the 
diametral strains were designed so that the diametral strains would cause 
the clamp to deflect. this deflection of the clamp was then related to the 
diametral strain but also required that the clamp-like sensor be flexible. 
These flexible sensors led to errors due to torsional loads also imposed 
on the valve stem by the valve actuator or components of the valve 
assembly. 
The diametral force sensor of the present invention uses a rigid rather 
than a flexible clamp and an internal compression load cell that measures 
directly any increase in the clamping forces due to a "swelling" or 
enlargement of the diameter of the valve stem due to both axial and 
torsional loads. The very rigid nature of the present diametral force 
sensor of this invention minimizes any distortion caused by torsional 
forces and therefore minimizes any torque induced errors. Unlike the prior 
known sensors, the output level of the signals from the sensor of the 
present invention increases as the clamp gets stiffer. 
The preferred embodiment of this invention uses the flexure 16 positioned 
between the load cell 19 and the valve stem 3 to prevent errors due to 
"off center" loads applied by the valve stem 3 to the load cell 19. 
Without the flexure 16, off center loads could be imparted to the load 
cell 19 if it was connected directly to the valve stem 3. The flexure 
prevents errors caused by any such off center loads. 
As described above, the electrical change in voltage signals produced by 
the load cell 19 are transmitted by the electrical cable 9 to the signal 
conditioning module 10 that converts these electrical signals to a digital 
form for output, computations and display at the computing and display 
device 12 and recording in the data recording device 11, which will 
provide a record of the loads on the valve stem 3 under a variety of test 
or operating conditions for further study and analysis. 
The computing and display device 12, such as a personal computer, is easily 
able to compute the actual axial forces in the valve stem 3 using Hooke's 
Law and Poisson's ratio. For a column, such as the valve stem 3, subjected 
to an axial force the axial strain or change in length can be related to 
the applied force using Hooke's Law. 
Delta L=(F*L)/(A*E) 
Where: Delta L=change in length (inches) 
F=axial load (pounds) 
L=gage length (inches) 
A=cross sectional area (square inches) 
E=Young's Modulus (psi.) 
The axial strain discussed above is accompanied by a lateral strain 
reflected in a change in cross section dimensions of the valve stem which 
can be related to the axial force or load applied using Poisson's ratio 
and Hooke's Law 
Delta d=(V*D*F)/(A*E) 
Where: Delta d=change in lateral dimension (inches) 
V=Poisson' ratio for material 
D=lateral dimension (inches) 
F=axial force applied (pounds) 
A=cross sectional area (square inches) 
E=Young's Modulus (psi.) 
It is believed that the present invention and its advantages will be 
understood from the above description and the accompanying drawings and it 
will be apparent that changes may be made in the form, construction and 
arrangement as described without departing from the scope of this 
invention.