Magnetically detectable tennis ball

A tennis ball is provided with magnetically detectable properties while still meeting USTA specifications. The magnetic properties are provided by sponge iron powder which is formed from magnetite iron ore. The sponge iron powder is blended with the rubber which is used to mold the core of the ball.

BACKGROUND 
This invention relates to tennis balls, and, more particularly, to a tennis 
ball with magnetic properties which permit an instrument to detect whether 
the ball is inside or outside of a boundary line. 
A persistent problem in the game of tennis is making accurate and 
consistent judgments of whether or not the tennis ball is inside or 
outside of boundary lines on the tennis court. Tennis tournaments use line 
judges who attempt to make a visual determination of whether the ball is 
in or out on the service and during subsequent play. However, any person 
who is even a casual fan of tennis is familiar with the arguments which 
commonly occur between players and line judges over the correctness of the 
judge's call. The problem is exacerbated when a line judge'call is 
overruled by the chair umpire, who presumably does not have as good a view 
of the line as the line judge. 
Attempts have been made to provide automatic detection of whether a tennis 
ball lands inside or outside a boundary line. For example, some tennis 
balls have been provided with a metallic device which is intended to close 
an electrical circuit between wires which are embedded in the court to 
provide an audible signal when the ball is out. More recently, attempts 
have been made to provide a magnetically detectable ball which can be 
sensed by an instrument which measures the magnetic permeability of the 
ball while in motion. 
One such magnetic detection instrument is produced by a company named Tel 
Pty. Ltd., from 26-28 Fitzroy Avenue, Camden Park 5038, South Australia. 
Although the details of the manner in which the instrument operates are 
not known, it is believed that the instrument measures the magnetic flux 
or magnetic permeability of a ball which has ferromagnetic permeability 
incorporated in it. According to published information from Tel, the Tel 
detection system has four components: antenna arrays buried below the 
court lines which transmit and receive data, an instrument box holding 13 
computers (one for each line), a hand-held computer operated by the chair 
empire, and tennis balls which contain metal particles embedded in the 
rubber core. When a moving tennis ball is within about four inches above a 
line, an electronic signal is produced because the magnetic particles in 
the ball disturb the magnetic field above the line. The Tel system 
provides information on ball velocity, approach trajectory angle, 
elevation and position of the centroid of the ball footprint relative to 
the outer edge of a court line. This information is used by the 13 
computers to make in and out decisions, although during play the system 
makes only out decisions audibly. 
One prior art tennis ball which was used with the Tel instrument used an 
iron powder obtained from AEM Cores Pty. Ltd., Bedford Street, Billman, 
South Australia 5013 under the name Telsen. The powder had a specific 
gravity of 7.65. 
Tennis balls which incorporated the Telsen powder did not meet the 
specifications for use with the Tel instrument and did not meet the 
specifications of the United States Tennis Association (USTA). The average 
magnetic reading level met the Tel specification, but the range of the 
readings was too great (88% of the balls failed to meet the 
specification). The Tel specifications are a total magnetic permeability 
of greater than 0.6 with a variance (variation in the uniformity of 
distribution of the magnetic permeability) less than 0.60 as measured by 
the Tel instrument. The balls did not meet USTA specifications because 
their deflection was too soft. 
USTA specifications for a tennis ball provide that the ball shall have a 
uniform outer surface, be white or yellow in color, have a diameter of 
more than 21/2 inches (6.35) and less than 25/8 inches (6.67 cm), and have 
a weight more than 2 ounces (56.7 grams) and less than 21/16 ounces (58.5 
grams). The ball shall have a bound of more than 53 inches (135 cm) and 
less than 58 inches (147 cm) when dropped 100 inches (254 cm) upon a 
concrete base. The ball shall have a forward deformation of more than 
0.220 inch (0.56 cm) and less than 0.290 inch (0.74 cm) and a return 
deformation of more than 0.350 inch (0.89 cm) and less than 0.425 inch 
(1.08 cm) at 18 lb. (8.165 kg) load. The deformation figures shall be 
averages of three individual readings along three axes of the ball and no 
two individual readings shall differ by more than 0.030 of an inch (0.08 
cm) in each case. 
SUMMARY OF THE INVENTION 
The invention provides a magnetically detectable tennis ball which meets 
both USTA specifications and the specifications for use with the Tel 
instrument. The tennis ball uses a sponge iron powder which is obtained 
from magnetite iron ore. The iron powder is incorporated into the rubber 
core of the ball. The iron powder makes the rubber core softer, so only 
No. 3 Ribbed Smoke Sheet Rubber is used for the core. No. 3 Ribbed Smoke 
Sheet gives a lower deflection than Standard Indonesian Rubber, which is 
conventionally used for tennis ball cores alone or in combination with No. 
3 Ribbed Smoke Sheet.

DESCRIPTION OF SPECIFIC EMBODIMENT 
Referring to FIG. 1, a tennis ball 10 comprises a core 11 and a cover 12. 
The core 11 is hollow sphere which is molded primarily from rubber and 
which includes sponge iron powder formed from magnetite iron ore to 
provide the ball with ferromagnetic properties. The cover 12 is 
conventional and includes a pair of dumbell or FIG. 8 shaped pieces of 
felt 13 which are adhesively secured to the core. A seam 14 of adhesive 
surrounds the peripheries of the felt pieces. 
The preferred composition of the magnetic core 11 is set forth in Table I 
and is compared with a typical prior art ball which does not have magnetic 
properties. 
TABLE I 
______________________________________ 
Magnetic Core 
Non-Magnetic Core 
Ingredient (parts by weight per 100 parts of rubber) 
______________________________________ 
No. 3 Ribbed Smoke 
100 
Sheet Rubber 
Standard Indonesian 100 
Rubber 10 
stearic acid 1.50 1.38 
retarder W 1.00 0.75 
4,4-dithiodimorpholine 
1.00 0.75 
Rubber maker's sulfur 
3.60 3.00 
sulfenamide 2.25 2.25 
90% methyl zimate 
0.15 
butaraldehyde aniline 
0.25 
antioxidant 0.50 0.50 
process oil 1.00 11.00 
precipitated silica 
3.00 
zinc oxide 4.00 22.75 
modified kaolin clay 
72.00 
metal powder 29.00 
diorthotolyl guanidine 0.10 
magnesium carbonate 29.00 
precipated hydrated 2.50 
amorphous silica 
kaolin clay 30.00 
Mercapto-terminated 20.00 
kaolin clay 
______________________________________ 
With the exception of the metal powder, the foregoing ingredients are 
conventional and well known to manufaturers of tennis balls. Some prior 
art tennis ball cores also use No. 3 Ribbed Smoke Sheet rubber in 
combination with Standard Indonesian Rubber. 
The specific metal powder used was obtained from Hoeganaes Corporation of 
Riverton, N.J. under the name Ancor MH-100. Ancor MH-100 is a sponge iron 
powder which is made from magnetite iron ore. The iron ore is reduced 
directly at elevated temperatures to obtain sponge iron which is 
disintegrated into powder. Final properties are obtained by annealing. 
Sponge iron powder has very high surface area and exhibits high green 
strength. Ancor MH-100 sponge iron powder has the properties listed in 
Table II. 
TABLE II 
______________________________________ 
Apparent Density 2.50 g/cm.sup.3 
(weight of a unit volume 
of powder) 
Chemical Analysis % 
Fe 98.2 
SiO.sub.2 0.20 
C 0.02 
H.sub.2 - Loss 0.35 
S 0.01 
P 0.01 
Flow (Hall Flowmeter) 
30 seconds for 50 gm. 
______________________________________ 
Sieve Analysis, % 
U.S. Standard Mesh % 
______________________________________ 
+80 (177 micron) 1 
-80 + 100 (149 micron) 4 
-100 + 140 (106 micron) 20 
-140 + 200 (074 micron) 27 
-200 + 325 (044 micron) 24 
-325 24 
______________________________________ 
Compacting at 
30 tons per square inch 
(with 1% zinc stearate added) 
Briquette Strength 
Density Green Strength 
Newtons per square 
g/cm.sup.3 psi millimeter 
______________________________________ 
6.4 2000 (13.8) 
______________________________________ 
The process of manufacturing the magnetic tennis balls is illustrated in 
FIG. 2. Except for the addition of the iron powder, the manufacturing 
steps are conventional. 
The rubber is loaded first into a Banbury mixer 18, and one minute later 
the other ingredients of the core are loaded into the Banbury. The 
ingredients are mixed for an additional 5 minutes, and the speed of the 
Banbury is adjusted to maintain the temperature at a maximum of 
220.degree. F. 
The sheets of rubber compound formed by the Banbury are broken down and 
blended on a rubber mill 19, and thereafter the material is fed through an 
extruder which forms rubber slugs 20. The slugs are molded into sheets 21 
which contain hemispherical half shells 22 at the stage labeled First 
Cure. 
The next step is Shell Trim where the flash is cut away from the half 
shells. At Buff and Cement the edges of the half shells are sanded, and 
adhesive is applied. At Second Cure the half shells are joined to form 
cores 23. The cores are abraded and sanded and then dipped in adhesive at 
Core Coating. The coated cores then go to Ball Covering where the FIG. 8 
pieces of felt are applied to the cores. 
The felt processing is shown in the upper left of FIG. 2. Adhesive is 
applied to a felt sheet 24 at Felt Backing, and the FIG. 8 pieces are cut 
at Felt Cutting. For ease of illustration the FIG. 8 pieces are shown as 
ovals in FIG. 2. A plurality of FIG. 8 pieces are clamped together and 
dipped in felt edge adhesive in dip tank 25. 
The cores are covered with felt at Ball Covering, and after Ball Repair and 
Ball Inspection the covered core is placed in a press at 3rd Cure which 
applies heat to cure the adhesives. The felt is fluffed at Ball Fluffing, 
markings are applied at Logo, and the finished balls are packaged at 
Canning and Packing. 
Balls made in accordance with the invention meet all USTA specifications 
and also meet the specifications for use with the Tel instrument. The 
magnetic permeability of the balls can be detected by the instrument to 
provide an automatic indication of whether the ball lands outside of a 
service line, base line, or side line. 
Adding the iron powder to the core makes the ball softer. Accordingly, the 
preferred embodiment uses only No. 3 Ribbed Smoke Sheet Rubber, which is 
harder than Standard Indonesian Rubber. 
Although the preferred composition of the core uses 29 parts of sponge iron 
powder per 100 parts of rubber, we have had successful results using 
between 29 and 39.08 parts of sponge iron powder per 100 parts of rubber. 
While in the foregoing specification a detailed description of a specific 
embodiment of the invention was set forth for the purpose of illustration, 
it will be understood that many of the details herein given may be varied 
considerably by those skilled in the art without departing from the spirit 
and scope of the invention.