Apparatus for measuring torque

Apparatus for measuring the torque transmitted across coupled rotating shafts using load cells located between power imparting surfaces and power receiving surfaces of shafts rotating jointly about a common axis of rotation for directly determining the tangential driving force and, hence, the torque therebetween. The apparatus includes tubular spacers journaled together and supporting the shafts, load cells and power surfaces. In one form concentric overlapping spacers are provided with one spacer having a plurality of radially extending fingers projecting into corresponding slots in the other spacer. Load cells at the interface between the fingers and sides of the slots provide signals representative of torque.

BACKGROUND OF THE INVENTION 
This application is related to commonly assigned Application Ser. No. 
06/926,941 filed concurrently herewith. 
This invention relates to an apparatus for measuring torque transmitted 
across coupled rotating shafts and, more particularly, to torque measuring 
using load cells located between power couplings of coaxially rotating 
shafts for directly determining the tangential driving force at such 
couplings. 
DESCRIPTION OF THE PRIOR ART 
Many approaches have been attempted in an effort to improve torque 
measuring couplings that would be applicable to conventional designs of 
mechanical drive turbines and other machinery. In general, prior art 
equipment is either difficult to calibrate, has short periods of accurate 
service, or is of low accuracy although having a long service life. 
Furthermore, such prior art torque measuring devices measure torque 
indirectly, e.g., by determining either the angular deflection of a driven 
shaft or by measuring the torsional stress in a torque transmitting 
member. Since the measured values are very small numbers, inaccuracies may 
occur when attempting to measure relatively large force and torque values. 
Further, since one of the requisites of a successful design is to maintain 
a high torsional stiffness in order to minimize the impact on torsional 
resonance, it is obvious that the attempt to measure infinitesimal 
deflections is difficult. Attempts to measure torque to within .+-.1/4% 
accuracy, using such small deflection, has generally been difficult. 
SUMMARY OF THE INVENTION 
Among the objects of the present invention is the provision of a method and 
apparatus for measurement of torque transmitted through a rotating 
coupling with high accuracy, sensitivity and reliability. This and other 
objects are attained in an illustrative embodiment in a torque coupling 
mechanism between a driving shaft and a driven shaft. The driving and 
driven shafts are axially aligned and rotatable together about a common 
axis of rotation. A first hollow cylindrical coupling member is axially 
coupled to the driving shaft and a second hollow cylindrical coupling 
member is axially coupled to the driven shaft. The hollow coupling members 
are of different diameters and are concentrically positioned one partially 
within the other. Bearings are concentrically secured to the interior 
surface of the exterior coupling member and the exterior surface of the 
interior coupling member to support the exterior member about the interior 
member for coaxial rotation with minimum wobble. A plurality of torque 
transmitting lugs or fingers extend from one of the coupling members with 
each finger extending into a corresponding slot formed in the other of the 
coupling members. The fingers exert a tangential force against sides of 
the slots to cause common rotation of the coupling members. Load cells are 
positioned at the interfaces between the fingers and sides of the slots to 
directly sense the tangential driving force required to cause the driven 
member to rotate. An electronic sensor responsive to the radial position 
of the cells, the number of load cells, and the sensed tangential driving 
force determines the torque developed between the coupling members during 
their rotation.

DETAILED DESCRIPTION OF THE INVENTION 
Referring generally to FIGS. 1, 2 and 3, two shafts 10 and 12 are coupled 
for conjoint rotation about a common axis of rotation 14. Shaft 10 
represents a driving shaft extending from a turbine 16 or other prime 
mover. Shaft 12 represents a driven shaft coupled to a load 18 to be 
powered. The shafts 10 and 12 are coupled together by an intermediate 
member or coupling apparatus 20. Within the coupling apparatus are a 
plurality of symmetrically positioned load cells 24, two in the preferred 
embodiment, although any number could be utilized, for sensing and 
determining the circumferential or tangential driving force between the 
shafts during rotational operation. The load cells 24 are commercially 
available sensors which generate an output signal proportional to 
compressive force applied thereto. Any of the well known types of sensors 
can be used, including those requiring electrical excitation. The load 
cells 24 may be coupled to a rotating transmitter 26 and power source, if 
required, such as a rotating battery pack or rotating transformer fitted 
with a centrifugal switch within the coupling apparatus 20 for operation 
during rotation of the shafts. Remote electronic transducing equipment, 
not shown, but of a type well known in the art, receives the variable 
output information from the load cells 24 through radio telemetry 
equipment or other electrical coupling components such as slip rings, also 
well known in the art. The transducing equipment integrates such variable 
information from the load cells with the number of load cells, the 
distance from the load cells to the axis of rotation, and an appropriate 
mathematical constant and provides a readout representing the torque 
generated between the shafts. Shaft rotational speed can be measured by 
any conventional method, such as, for example, by use of a pulse counter, 
a tachometer coupled to the shaft or other means well known in the art. 
The torque may be multiplied by the rotational velocity of the load cells 
obtained from the measured shaft speed and divided by an appropriate 
mathematical constant to provide a readout representing the horsepower 
transmitted. The electronic equipment may comprise a receiver, amplifier 
and other circuitry necessary to convert the signal from the load cell 24 
into indications of torque. A microprocessor based or microcomputer system 
is readily implemented and allows variation and selection of parameters to 
accommodate different load cells and different distances between the axis 
of rotation and load cells. The transmission and collection of data from 
the load cells 24 and the conversion of such data to measurement of torque 
can be implemented by several conventional techniques which will be 
readily apparent to those ordinarily skilled in the art. 
Any modulation of the signal, i.e., the instantaneous torque signal, 
obtained from this system is indicative of torque or horsepower 
transients, and spectrum analysis of the data may be used to detect 
torsional resonant conditions. The average value of the signal is 
representative of the steady state torque. 
Adjacent the load cells 24 are a plurality of power imparting surfaces 30 
securable with respect to the drive shaft 10 and a plurality of power 
receiving surfaces 32 securable with respect to the driven shaft 12. In 
the illustrative embodiment, the surfaces 30 are faces on a plurality of 
lugs or fingers 36 extending from a cylindrical coupling member 38. The 
member 38 is transitioned into any conventional flexible coupling 40, such 
as a gear or diaphragm type which attaches to a mating flange 42 on the 
driving shaft 10. The member 38 is sized to fit within a slightly larger 
hollow cylinder 44. The cylinder 44 may have a flange 46 at one end 
allowing it to be coupled to a corresponding flange 48 on the driven shaft 
12. Alternatively, cylinder 44 may be transitioned into any conventional 
coupling which attaches to the driven shaft 12. 
The cylinder 44 is provided with slots 50 into which the fingers 36 fit. 
The fingers 36 can be seen protruding radially from the slots 50. The 
surfaces 32 are surfaces along the slots 50 which contact the surfaces 30 
of fingers 36. Rotation of shaft 10 causes rotation of member 38 and, by 
virtue of the contact between fingers 36 and slots 50, rotation of 
cylinder 44 and driven shaft 12. 
It is preferred that the power imparting surfaces 30 be formed with a 
recess 52 (best seen in FIG. 3) in the faces of fingers 36. The load cells 
24 are positioned in the recesses. In this manner an unanticipated 
overload condition will not harm the load cells since the surface of the 
surrounding face would contact the adjacent power surfaces 32 and limit 
excess compression of the load cells to thus preclude damage to the load 
cells through crushing. However, it will be appreciated that since power 
is transmitted through the load cells 24, the only portion of surface 30 
which transmits force during normal operation is that portion at the 
bottom of the recesses 52. The remaining portion of the surface 30 only 
transmits power in the event of a sudden overload which compresses the 
cells 24 below the upper or nonrecessed portion of surface 30. 
As will be appreciated from the above description, the coupling apparatus 
20 comprises a first cylindrical coupling member 38 axially coupled to the 
drive shaft 10 and to similarly shaped second hollow cylindrical coupling 
member 44 axially coupled to the driven shaft 12. The cylindrical coupling 
member 38 is preferrably a hollow cylinder as shown in FIGS. 2 and 3. The 
members 38 and 44 act as concentric overlapping spacers for transmitting 
rotational torque while permitting some degree of longitudinal flexure. 
The hollow coupling members are of an essentially tubular shape for the 
majority of their extents but are of different diameters. They are 
concentrically positioned one partially within the other. In order to 
facilitate concentric rotation with a minimum of deflection, the members 
38 and 44 are journaled for rotation by one or more bearings, such as, for 
example, bearing members 54 and 56. The bearings also eliminate frictional 
engagement between member 38 and 44. 
The bearing members 54, 56 are anti-friction roller bearing assemblies, two 
in the preferred embodiment, designed and positioned to align the joined 
coupling members and shafts during rotation between these members. The 
bearing assemblies are cylindrical in shape with their outer races secured 
to an inner surface of the exterior or second coupling member 44. Their 
inner races are secured to an exterior surface of the interior or first 
coupling member 38. 
The flanges 40 and 46 are provided with a plurality of apertures which 
align with corresponding apertures in flanges 42 and 48 at the free ends 
of the shafts 10 and 12. Bolts and nuts 58 extending through the apertures 
effect the removable coupling of the shafts to the coupling members and, 
hence, to each other. While the coupling members 38 and 44 are shown as 
being fixedly coupled to corresponding shafts 10 and 12, there are many 
applications in which the shaft connections are desireably made through 
flexible couplings which can accomodate angular deflection and axial 
shifts. Many different types of such couplings are commercially available 
and their description is beyond the scope of this invention. A description 
of many types is given in the October, 1981 edition of DESIGN ENGINEERING 
magazine at pages 64-66. 
Considering FIG. 3, it can be seen that the fingers 36 are attached to and 
extend from the inner cylindrical member 38. While only two fingers are 
shown, the number may be varied as necessary to support various different 
torque loads. The fingers 36 extend into slots 50 and the engagement 
between fingers 36 and slots 50 transmit torque. The notches or slots 50 
are of such size as to receive the remote ends of the arms or fingers 36 
and their supported load cells. The circumferential extent of the slots 50 
is such as to allow for a limited rotational oscillation between the 
coupling members and, therefore, between the shafts. The slots 50 each 
have one flat face 32 extending radially with respect to the axis of 
rotation and which is positioned to contact a load cell 24. Such flat face 
constitutes a power receiving surface. Rotation of the drive shaft rotates 
the power imparting surfaces 30 and, through the rotating load cells 24, 
rotates the power receiving surfaces 32 and, hence, the driven shaft 12. 
The coupling members are located partially one within another to allow for 
the above-described coupling relationship. 
To insure accurate torque measurements, it is common practice to apply 
torque meters between 70% and 100% of full capacity. With current torque 
meter designs, this requires entire coupling replacement to match the 
actual equipment torque to the useful range of the torque meter. The 
present system facilitates the matching of the torque meter to the 
equipment because the entire coupling is maintained and only the load 
cells selected from a family of load cells, are changed to adjust the 
system for accommodating different operating conditions. 
One of the problems confronting a user of such above-described equipment is 
that of calibration drift, or change of reading, due to centrifugal 
effects when the unit is at speed. Calibration at standstill may be 
accomplished with a simple balanced lever arrangement which applies a true 
torque of known magnitude. A simulated torque that deflects the load cell 
can be developed by compressing the load cell to produce a load cell 
distortion comparable to that developed by a torque load while unloaded 
operation at speed can indicate the magnitude of the centrifugal effect, 
if any. Such methods for calibrating load cells are well known in the art. 
The above described apparatus with its shafts is illustrative of a system 
capable of carrying out a new method of measuring torque between the drive 
shaft 10 and the driven shaft 12 rotating together about a common axis of 
rotation 14. Such method comprises the steps of providing a plurality of 
power imparting surfaces 30 and power receiving surfaces 32 secured with 
respect to the driving and driven shafts 10 and 12 at an equal 
predetermined distance from the axis of rotation 14. Each such power 
imparting and receiving surface is positioned or located essentially 
parallel with, radially with respect to, and at a predetermined distance 
from, the axis of rotation and is thus adapted to tangentially transmit a 
driving force with respect to the axis of rotation during operation and 
use. Tubular coupling members 38 and 44 are provided to couple together 
the power imparting surfaces 30 with their associated shaft. The method 
further includes the step of journaling the tubular members 38 and 44 by 
bearing assemblies 54 and 56, one within the other, and positioning a 
predetermined number of load cells 24 at the interfaces between power 
imparting surfaces 30 and power receiving surfaces 32 to transmit the 
driving force and directly sense the tangential driving force 
therebetween. The positioning step includes removably inserting load cells 
24 into recesses adjacent the power imparting surfaces 30. The load cells 
24 are selected to have a predetermined characteristic as a function of 
the torque anticipated to be transmitted. The method further includes the 
step of determining the torque at the interface between the drive shaft 10 
and the driven shaft 12 during their rotation as a function of the 
predetermined distance, the predetermined number of load cells, and the 
sensed tangential driving force. 
While the present invention has been described with respect to a particular 
embodiment, many modifications and variations will become apparent to 
those skilled in the art. For example, while the coupling apparatus 20 has 
been shown as coaxial, concentric members with fingers and slots, it will 
be appreciated that coupling could be achieved through a pair of U-joints 
such as that shown in FIG. 4. Accordingly, all such variations and 
modifications are intended to be included within the scope of the appended 
claims.