Vehicle trailer hitch

The ball element of a ball type trailer hitch is separated along a horizontal plane of bisection and into two substantially hollowed out ball halves. The threaded outer perimeter of a wheel type vertical load measuring transducer is threaded to the inside of the ball halves at the horizontal plane of bisection. The base of the trailer hitch has a vertical stud which extends through a vertical cylindrical bore in the lower ball half and is attached to the center of the transducer. The ball element moves vertically downward under loading. This movement causes elastic bending of the spokes of the transducer. Such elastic bending is measured by strain gauges attached to the spokes.

BACKGROUND OF THE INVENTION 
This invention relates to vehicle trailer hitches and more specifically to 
a vehicle ball type trailer hitch which includes a transducer for 
measuring the vertical component of loads induced on the trailer hitch by 
a trailer attached thereto. 
DESCRIPTION OF THE PRIOR ART 
Devices to measure tongue loads induced by a trailer on a vehicle trailer 
hitch must contend with the fact that the vertical component of such loads 
will have a magnitude of perhaps 200 or 300 pounds while the component of 
such loading in the lateral direction may run as high as 900 pounds and as 
high as 4,000 pounds in the fore and aft direction. In order to isolate 
and measure the vertical component of such loads, some way must be found 
to incorporate a suitable transducer into the hitch. Standard ball type 
trailer hitches are typically made of heavy steel castings or machinings. 
While a transducer attached directly to a surface of such a standard 
trailer hitch is sufficiently sensitive to measure the larger, 
non-vertical loads, it will not be sufficiently sensitive to measure the 
smaller vertical loads, and the amplification necessary to make such 
signals readable adds excessive noise. Attempts have been made to locate 
vertical load transducer units between the ball element and base of a 
standard ball type hitch. These units are large and heavy and have so 
changed the configuration, weight and moment arm of the hitch that the 
signals have been unrepresentative of actual conditions. Such units are 
also subject to excessive "cross-talk", that is, mixing of vertical and 
lateral signals. Suitable transducers exist which separate vertical from 
horizontal loads, allow the use of low gain amplifiers, and which are not 
subject to "cross-talk". However, no way has been found to suitably 
incorporate such transducers within a standard ball type trailer hitch. 
SUMMARY OF THE INVENTION 
The subject invention solves the problem of incorporating a suitable 
vertical load measuring transducer within a standard ball type trailer 
hitch. The ball element is horizontally bisected into upper and lower 
halves which are substantially hollowed out. A vertical cylindrical bore 
extends through the lower half of the ball element. A wheel type 
transducer of conventional design is threaded around its perimeter. Each 
half of the ball element is threaded to the transducer to securely and 
removably join the ball halves and transducer together and locate the 
transducer in the horizontal plane of bisection of the ball element. The 
ball element is then slidably mounted on the base by vertically directed 
lands on the base which cooperate with the cylindrical bore in the lower 
half of the ball element. The transducer is rigidly attached near its 
center to a stud which extends upward from the base. Thus, the trailer 
hitch has the same outward configuration as a standard ball type trailer 
hitch. 
In operation, loading on the ball element moves the ball element vertically 
downwardly as the cylindrical bore in the ball element slides on the 
vertical lands on the base. This movement of the ball element elastically 
deflects the perimeter of the transducer relative to the center thereof, 
which is stationary on the base. The elastic bending is measured and 
monitored by a standard strain gauge circuit. The transducer is located at 
the zero moment axis of the ball element and its design and the gauging 
associated with it make it inherently unresponsive to any signals other 
than those caused by the vertical component of loading on the ball 
element. All loading on the trailer hitch is the same as on a standard 
trailer hitch. 
It is therefore an object of the invention to provide a ball type trailer 
hitch which wholly contains therein a load measuring transducer and is yet 
the same size and shape as a standard ball type hitch. It is another 
object of the invention to provide such a hitch wherein the transducer 
will isolate and measure the vertical component of loading on the hitch 
without "cross-talk" or noise. It is a further object of the invention to 
provide such a hitch wherein the transducer is located at the zero moment 
axis of the hitch so that the moment arms of loads on the hitch as 
measured by the transducer are identical to those in a standard ball type 
trailer hitch.

DETAILED DESCRIPTION OF THE DRAWINGS 
Referring to FIG. 1, a vehicle generally designated 10 is shown with a 
towing bar 12 attached thereto. The ball type trailer hitch 14 of the 
invention is conventionally attached to the towing bar, and the tongue 16 
of a trailer sits on the trailer hitch 14. The vertical, fore and aft, and 
lateral component directions of a typical load on the trailer hitch are 
respectively designated by arrows L.sub.V, L.sub.F-A, and L.sub.L. 
Referring to FIG. 4, a trailer hitch base designated generally at 18 is 
machined from high strength material, such as stainless steel. The base 18 
includes a threaded lower stud 20 which terminates in plinth 22. Squared 
off shank 24 includes conventional strain gauges 26 for measuring the fore 
and aft and lateral loads. A wiring passage 28 for the wiring for strain 
gauges 26 opens to the shank at the juncture thereof with the plinth. 
Cylindrical vertical land 30 merges into an offset cylindrical vertical 
land 31 which in turn terminates in an outer radial shoulder 32 and an 
inner offset radial shoulder 33 of a smaller diameter. Another wiring 
passage 34 opens at the juncture of shoulders 32 and 33. An upper threaded 
stud 36 projects from the shoulder 33. A metal pin 37 is included in stud 
36 and extends outwardly on each side thereof serving to locate elements 
described below. 
A ball element 38 of stainless steel is bisected along a horizontal plane 
into lower ball half 40 and upper ball half 42. As seen in FIG. 2, the 
interior of ball element 38 is machined to provide three vertical, 
concentric and intersecting cylindrical bores of varying diameters. Lower 
bore 44 extends through lower ball half 40 and has a diameter closely 
conforming to that of vertical land 30 and an axial length equivalent to 
the height of lands 30 and 31. Lower bore 44 opens into an internally 
threaded bore 46 which opens to a like bore 48 of ball half 42. Bore 48 in 
turn opens into an upper closed bore 50. 
Referring now to FIGS. 3 and 4, a conventional vertical load measuring 
transducer 52, such as shown on pages 222, 223 of The Strain Gauge Primer 
by C. C. Perry and H. R. Lissner, measures the vertical component of 
loading thereon without responding to horizontal loading. Transducer 52 
includes a journaled hub 54 having a bore matching the diameter of stud 
36, and an externally threaded perimeter or rim 56. Hub 54 has described 
therein a two sided slot 57 which matches the ends of pin 37 which serves 
to locate it on stud 36 as described below. Four identical spokes or beams 
58 connect hub 54 and rim 56. Eight strain gauges 60 are attached one each 
to the upper and lower surface of each beam 58 and connected together in a 
full bridge circuit. Such a circuit with eight gauges is sensitive enough 
to require only low, if any, amplification and its electrical output is 
independent of the point of load application, as outlined in The Strain 
Gauge Primer, referred to above. 
Referring now to FIG. 2, to assemble the trailer hitch, transducer 52 is 
threaded into lower ball half 40 to about half its thickness, and the 
wiring from strain gauges 60 is fed into wiring passage 34 which connects 
with a central wiring passage 62 extending through the base 18. This 
wiring and the wiring from strain gauges 26, which feeds into passage 62 
from wiring passage 28, designated 64, exits from the lower end of stud 
20. The lower ball half 40 is slidably mounted on land 30 as the upper 
threaded stud 36 is inserted through hub 54. Pin 37 slides in slots 57 to 
locate hub 54 so that it does not turn with respect to stud 36, thus 
protecting the wiring from gauges 60. When hub 54 seats on shoulder 33, 
the lower ball half 40 is threadedly adjusted relative to the transducer 
to align the opening of bore 44 with the lower radial shoulder of land 30. 
A nut 66 is then threaded on stud 36 to fix hub 54 to shoulder 33. The 
upper ball half 42 is then threaded onto the transducer until it seats on 
the lower ball half 40. This locates the transducer 52 in the horizontal 
plane of bisection of ball element 38 and connects the ball element to the 
transducer to ensure that all loads on the ball element are transferred to 
the transducer when the ball element moves with respect to the base 18 as 
lower bore 44 moves vertically on land 30. While the transducer 52 is the 
only connection between the ball element 38 and base 18, its strength is 
sufficient to withstand the lateral and vertical loading. The hitch 14 is 
mounted on the bar 12 by inserting stud 20 through an opening in the bar 
and securing hitch 14 by nut 68. The wiring 64 is connected to a 
conventional monitor which receives the signals from gauges 60 in a bridge 
circuit arrangement as well as gauges 26. 
The operation of the trailer hitch will now be described with reference to 
FIGS. 1 and 2. A load generally designated by the arrow "L" has vertical, 
fore and aft and lateral components designated by the arrows "L.sub.V," 
"L.sub.F-A " and "L.sub.L ". The component L.sub.V will elastically bend 
beams 58 as the ball element and rim 56 of the transducer move relative to 
hub 54 and base 18, with this movement being guided by bore 44 and land 
30. This elastic bending is measured by the gauges 60. Components L.sub.L 
and L.sub.F-A, while larger in magnitude, cause little deformation of 
beams 58. Such deformation is essentially symmetrical about the horizontal 
plane of bisection of the ball element and any signals generated by beams 
58 are cancelled out by the bridge circuit. Thus, the signals from the 
transducer 52 corresponding to the vertical component L.sub.V are free 
from "cross-talk" and the low amplification eliminates excessive noise. 
The components L.sub.L and L.sub.F-A are adequately measured by the gauges 
26 on the heavier shank portion 24. 
Thus, this invention provides a ball type trailer hitch which incorporates 
vertical load measuring transducers capable of isolating and measuring the 
vertical component of loading on the trailer hitch.