Axially compact torque transducer

A high sensitivity torque transducer capable of installation between the motor and the output reduction gear train in the housing of a rotating machine, which has a limited axial space therefor, has a disk shaped first member fixed to a fixed portion of the housing. A disk shaped second member has splined coupling with the first member and the second member to provide sliding coupling with a floating portion of an output drive train. This coupling transfers reaction torque back from the output shaft, through the drive train, and into the second member. Shear webs are formed between the inner and outer diameters of the second member by removing some material to form torsion concentrator voids. Strain gages mounted on the shear webs and bridged with appropriate resistors provide an output signal in response to torsional strains imposed on the second disk shaped member. The output signal is routed to a calibrated signal processor and relay control unit.

BACKGROUND OF THE INVENTION 
This invention relates generally to devices for monitoring stress and 
strain in dynamic structures and more particularly to transducers for 
measuring the output torque of power tools and other rotating machinery. 
Monitoring of dynamic stresses and strains in power tools and other 
rotating machinery is often desirable. The data received from such 
operations permit evaluation of the performance of the equipment in 
question. In the case of power tools, such as nutrunners or screwdrivers, 
the data permit measurement of the torque output and thereby calculation 
of fastener tension produced by the tool. Real time feedback coupled with 
programmable controls makes it possible to automatically deactivate the 
tool power when a desired fastener tension is achieved. This provides the 
benefit of reproducible fastener tensioning in assembly line or other mass 
production operations. A variety of transducers are available for 
measuring torque output of rotating machines. Most such transducers are 
quite satisfactory for their intended applications; however, in some cases 
the axial dimensions of the transducer are such that their use requires an 
unacceptable increase in the size of the tool. When the tool must be used 
in a limited space environment, such size increases may be intolerable. In 
such cases, fastener tension can only be determined by use of tortuous 
tool combinations of questionable accuracy. 
In addition, an excessive increase in size can contribute to increased 
weight and unwieldy proportions which can contribute to premature operator 
fatigue and otherwise degrade operator performance. 
The foregoing illustrates limitations known to exist in present devices and 
methods. Thus, it is apparent that it would be advantageous to provide an 
alternative directed to overcoming one or more limitations set forth 
above. Accordingly, a suitable alternative is provided including features 
more fully disclosed hereinafter. 
SUMMARY OF THE INVENTION 
In one aspect of the present invention, this is accomplished by providing a 
high sensitivity torque transducer for low torque applications capable of 
installation between a drive motor and an output gear reduction drive 
train assembly within a housing of a rotating machine in a limited axial 
space. The transducer has a first substantially disk shaped member rigidly 
connectable to a static reference member of the housing a second 
substantially disk shaped member slidably connectable to the first 
substantially disk shaped member and also slidably connectable to a 
floating portion of an output drive train which experiences reaction 
torque proportional to an output torque. The second substantially disk 
shaped member is rotationally restrained with respect to the floating 
portion of the output drive train but is free to slide axially with 
respect thereto. In addition, the second substantially disk shaped member 
incorporates provision for concentrating torsional stresses in a zone 
between a radially outer portion and a radially inner portion of the 
second disk shaped member. Finally, a means is provided for sensing and 
measuring deflections within the zone of the second substantially disk 
shaped member at which the torsional stresses are concentrated. 
The foregoing and other aspects will become apparent from the following 
detailed description of the invention when considered in conjunction with 
the accompanying drawing figures.

DETAILED DESCRIPTION 
By reference to FIG. 1, many of the features of the present invention can 
be seen. A representation of a portion of the housing of a power tool 100 
is shown at the juncture of a motor housing 10 and a drive housing 15. 
Motor shaft 11 protrudes from motor housing 10 into drive housing 15 where 
it terminates as a sun gear 26 within a planet gear carrier 20. Sun gear 
26 meshes with and drives planet gear 22, which is one of three or more 
planet gears within the planet gear carrier 20. Planet gear 22 precesses 
around ring gear 24 thereby driving planet gear carrier 20 in rotation 
and, thus, output shaft 21. 
The components of the transducer of the present invention are shown near 
the right side of FIG. 1. These include reference disk 14 which is rigidly 
attached to motor housing 10 by means of reference disk fasteners 12. 
Torsion meter disk 16 is slidably engaged with reference disk 14 and with 
torque reaction ring 18. Torque reaction ring 18 is a substantially 
floating extension of ring gear 24. Also seen in this view, are strain 
gages 19 and resistors 17c and 17z which are used to zero and calibrate 
the strain gages, respectively. Resilient ring 33, also shown here, is 
discussed below. 
During operation, motor shaft 11 rotates and causes sun gear 26 to impart 
rolling motion to planet gears 22. The rotating planet gears 22 travel 
around ring gear 24 thereby causing rotation of planet gear carrier 20 and 
consequent rotation of output shaft 21. Any resistance to rotation by 
output shaft 21 is transmitted through planet gear carrier 20 to ring gear 
24 as reaction torque. Torque reaction ring 18, which is merely a splined 
extension of ring gear 24, must necessarily experience the same reaction 
torque. However, torsion meter disk 16 is splined to torque reaction ring 
18 on its outside diameter and to reference disk 14 on its inside 
diameter. Since reference disk 14 is rigidly secured to motor housing 10, 
neither reference disk 14, torsion meter disk 16, nor torque reaction ring 
18 are free to turn. The use of splined couplings between the components 
prevents development of any extraneous bending stresses within the 
members, thereby assuring that only torsional stresses will be measured. 
Thus, by preventing counter rotation of ring gear 24 and torque reaction 
ring 18, torsion meter disk 16 experiences the full reaction torque which 
is measured by strain gages 19. FIGS. 2, 2a, and 2b illustrate further 
detail of the torsion meter disk of the present invention. External 
splines 30 and internal splines 32 of torsion meter disk 16 act as the 
coupling agents between splines 31 of reference disk 14 (shown in FIG. 4) 
and torque reaction ring 18, respectively. Four shear webs 35, defined by 
torsion concentrator voids 36, provide the concentration of torsional 
strains which permits accurate sensing by strain gages 19. This 
concentration improves the sensitivity and accuracy of the strain 
measurements. Zero resistors 17z are also shown here. 
FIG. 3, viewed in conjunction with FIGS. 2, 2a, and 2b, illustrates the 
typical strain gage/resistor bridge arrangement used in such applications. 
Zero resistors 17z are used to adjust the strain gage signals under zero 
load conditions for a correct reading on the output meter (not shown) and 
for errors in alignment and location of strain gages 19 on shear webs 35 
as well as for slight deviations in shear web area caused by slight 
deviations in the size of torsion concentrator voids 36. Calibration 
resistors 17c, shown in FIGS. 1 and 4, are used to adjust the output 
signal from strain gages 19 at selected levels of strain within the 
transducer range of operation. The output signal of the strain gage bridge 
is routed to the calibrated signal processor and relay control (shown 
symbolically) which compares torsional strain output signals, routed from 
the strain gage bridge, to established standards of torque/fastener 
tension values and deactivates the power switch at the desired fastener 
tension value. 
FIG. 4 illustrates further detail of the transducer assembly of the present 
invention. Torsion meter disk 16 is coupled to splines 31 of reference 
disk 14 by means of internal splines 32 on the torsion meter disk Snap 
ring 34 retains torsion meter disk 16 on the projection of reference disk 
14, while resilient ring 33 provides a bias of the meter disk against the 
snap ring. This assures that, although torsion meter disk 16, is free to 
move axially under load, it cannot slide loosely. Strain gages 19 are 
mounted on the axial face of torsion meter disk 16 while resistors 17z are 
mounted on the inside circumference of the meter disk within torsion 
concentrator voids 36. Calibration resistors 17c are mounted on the back 
surface of reference disk 14. 
The minimum sizes for the torsion meter disk 16 and the reference disk 14 
are determined by the anticipated torque range of service for the 
transducer assembly and by the physical sizes of the resistors 17c and z 
and strain gages 19. Thus, this transducer provides the advantage of 
requiring a minimum volume for installation and, consequently, exerting 
minimum impact on the size and configuration of the tool in which it is 
employed.