Aramid fiber cord for power transmission belt and method of manufacturing the same

An aramid fiber cord for use as a component in a power transmission belt, which aramid fiber cord has a plurality of aramid fiber filaments of 300 to 3100 denier adhesively treated and formed into a strand with a primary twist factor (Y) in a range of -1 to 1. A plurality of the strands are formed into the cord with a finishing twist factor (X) in a range of 1 to 4. The primary twist factor (Y) and finishing twist factor (X) satisfy the following equations: Y is .gtoreq. to -0.6X+1.3 and Y is .gtoreq. to -1.5x+5.0.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to power transmission belts and, more particularly, 
to an aramid fiber cord for use as a load carrying component in a power 
transmission belt and having excellent resistance to elongation, flexing 
fatigue, and fraying. 
2. Backqround Art 
It is known to use aramid fiber cords as load carrying members in power 
transmission belts. The aramid fiber is an organic fiber that is strong 
and flexible and has good dimensional stability in high temperature 
environments as compared to other organic fibers. One drawback with load 
carrying cords made of aramid fiber is that the fibers have a tendency to 
fray. This is particularly a problem in belts in which the laterally 
oppositely facing drive/driven surfaces are exposed, i.e. do not have any 
covering layer, such as canvas, thereon. This situation is common in 
synchronous belts, V-ribbed belts, and V-belts. Fraying may occur at 
manufacture and/or during use as the exposed cords on the laterally facing 
surfaces repeatedly contact and disengage from a cooperating pulley. 
Attempts have been made to alter the characteristics of the aramid fiber to 
make cords constructed therefrom more suitable as a component in a power 
transmission belt. It is known, for example, to treat the cords with a 
resorcinol formalin rubber latex adhesion liquid (hereinafter RFL liquid). 
Cords so treated have good bending strength, however are still prone to 
fraying. 
It is also known to pre-treat the cords with epoxy or isocyanate compound 
before treatment with the RFL liquid. This results in the hardening of the 
cord, which alleviates much of the fraying problem. However, the hardened 
cord has less resistance to flexing fatigue than the cord does in the 
absence of this treatment. Accordingly, neither of the above treatments 
satisfactorily addresses both the problems of fraying and flexing fatigue. 
One of the inventors herein, together with employees of the assignee of 
this invention, previously invented a manufacturing method in which the 
aramid fiber is treated with a mixture of epoxy compound, including at 
least one epoxy radical, and liquid rubber. The treated fibers are then 
adhesively vulcanized with unvulcanized rubber. While this method 
significantly improves the resistance of the aramid fibers to both flexing 
fatigue and fray, the adhesion between the fibers and rubber layer, in 
which they embed, has not proven as effective as desired. Consequently, 
there remains a need to develop a cord with excellent resistance to both 
fray and flexing fatigue and one which is effectively adhered to the 
rubber component layer of a power transmission belt in which it is 
embedded. 
SUMMARY OF THE INVENTION 
The present invention is specifically directed to overcoming the 
above-enumerated problems in a novel and simple manner. 
More particularly, it is the principal objective of the present invention 
to provide a load carrying cord that can be incorporated into a power 
transmission belt and which has excellent resistance to elongation, 
flexing fatigue and fray, and which is at the same time positively adhered 
to the rubber component layer in which it is embedded. 
To accomplish this end, the present invention comprehends an aramid fiber 
cord for use as a component in a power transmission belt, which aramid 
fiber cord has a plurality of aramid fiber filaments of 300 to 3100 denier 
adhesively treated and formed into a strand with a primary twist factor 
(Y) in a range of -1 to 1. A plurality of the strands are formed into the 
cord with a finishing twist factor (X) in a range of 1 to 4. The primary 
twist factor (Y) and finishing twist factor (X) satisfy the following 
equations: Y is .gtoreq. to -0.6X+1.3 and Y is .ltoreq. to -1.5x+5.0. 
The invention also contemplates that the cord be adhesively treated with 
one of a) a combination of resorcinol formalin rubber latex adhesion 
liquid and rubber cement and b) rubber cement. 
In one form, the aramid fiber filaments are treated with one of a) an epoxy 
compound and b) an isocyanate compound. 
The invention contemplates the use of 100 to 3000 fiber filaments, of 1 to 
3 denier each, in defining each aramid fiber strand. 
In one form, the aramid fiber filaments are in the form of a ribbon. 
The aramid fiber filaments are preferably made from one of CONEX.TM., 
NOMEX.TM., KEVLAR.TM., TECHNORA.TM., and TOWARON.TM.. 
In one form, the aramid fiber cord is defined by 2 to 5 strands, as 
described above. 
In one form, the rubber cement is one of a) chloroprene rubber, b) 
chlorosulfonated polyethylene and c) alkylated chlorosulfonated 
polyethylene. The rubber cement preferably has a solvent that is one of a) 
nitrile hydride rubber component, b) isocyanate compound, c) methyl ethyl 
ketone, and d) toluene. 
In a preferred form, the resorcinol formalin rubber latex adhesion liquid 
includes latex that is one of a) styrene-butadiene-vinylpyridine 
terpolymer, b) chlorosulfonated polyethylene, c) nitrile hydride rubber, 
d) epichlorohydrine, e) natural rubber, f) butadiene-styrene rubber (SBR), 
g) chloroprene rubber and h) olefine-vinylester copolymer. 
In one form, the resorcinol formalin rubber latex adhesion liquid is a 
mixture of initial condensates of resorcinol and formalin and rubber latex 
and the mole ratio of resorcinol to formalin is 1:0.5 to 3. In one form, 
the initial condensates of resorcinol and formalin are mixed with the 
rubber latex such that a resin portion thereof is present in the amount of 
10 to 100 weight parts to 100 weight parts of rubber latex. 
The invention further contemplates the combination of the aramid fiber cord 
and a belt body so as to define therewith a power transmission belt, with 
the aramid fiber cord being at least partially embedded in the belt body. 
According to the invention, the power transmission belt has a length and a 
plurality of the aramid fiber cords are provided in the belt body and 
extend lengthwise therein to define a neutral axis for the power 
transmission belt. The belt may be any of a V-belt, a V-ribbed belt, a 
synchronous belt, etc. 
In one form, the power transmission belt has laterally oppositely facing 
surfaces and the belt body has a cushion rubber layer in which the aramid 
fiber cords are embedded The cushion rubber layer defines a portion of 
each laterally facing surface and the portion of the laterally facing 
surface is not covered by fabric. 
The invention also contemplates an aramid fiber cord having a plurality of 
aramid fiber filaments of 300 to 3100 denier formed into a strand with a 
primary twist factor (Y), with a plurality of the strands being formed 
into the cord with a finishing twist factor (X). The primary twist factor 
(Y) and finishing twist factor (X) satisfy the following equations: Y is 
.gtoreq. to -0.6X+1.3 and Y is .ltoreq. to -1.5X+5.0. 
The invention further contemplates a method of forming an aramid fiber 
cord, which method consists of the steps of adhesively treating a 
plurality of aramid fiber filaments of 30 to 3100 denier, twisting the 
plurality of filaments with a primary twist factor (Y) in a range of -1 to 
1 into a strand, and forming a plurality of the strands into the cord with 
a finishing twist factor (X) in a range of 1 to 4.

DETAILED DESCRIPTION OF THE DRAWINGS 
A synchronous belt, made according to the present invention, is shown at 10 
in FIG. 1. The belt 10 has a belt body 12 with a compression section 14, a 
tension section 16, and a neutral axis defined by a plurality of 
longitudinally extending, load carrying cords 18. A plurality of laterally 
extending teeth 20 are provided at regularly spaced intervals 
longitudinally of the belt 10. A canvas layer 22 is adhered to and covers 
the inside surface 24 of the belt 10, including the belt teeth 20. 
The compression and tension sections 14, 16, respectively, are made of 
chloroprene rubber (CR), chlorosulfonated polyethylene rubber (CSM), 
alkylated chlorosulfonated polyethylene (ACSM), or hydrogenated 
acrylonitrile-butadiene rubber (referred to also as hydrogenated nitrile 
rubber and hereinafter abbreviated as H-NBR). The H-NBR rubber is made by 
adding more than 80 weight percent of hydrogen to the double bonding part 
of acrylonitrile-butadiene rubber. The H-NBR rubber has good resistance to 
deterioration at high temperatures. 
The canvas cover 22 may be a plain weave fabric, twill weave fabric, satin 
fabric, or another fabric that is expandable in the weft direction, i.e. 
the longitudinal direction of the belt. In the case of plain weave fabric, 
the weft yarns 26 and warp yarns 28 are interwoven to pass alternatingly 
one above the other at every point of intersection. Accordingly, wave-like 
crossings are formed in both the warp and weft directions. 
On the other hand, where twill or satin canvas are used, the warp yarns 28 
and weft yarns 26 are alternatingly passed over and under one another at 
larger intervals. This reduces the number of wave-like crossings. 
Resultingly, the rubber used to impregnate the canvas not only fills the 
voids between the yarns but also fills in between the warp yarns 28 and 
weft yarns 26 at certain of the points of crossing. The rubber between the 
warp yarns 28 and weft yarns 26, at certain of these points of crossing, 
thereby prevents direct engagement and abrasion between the warp yarns 28 
and weft yarns 26 during bending. The result is that the twill and satin 
canvases generally have a longer life than the normal plain weave canvas. 
In the canvas cover 22, the weft yarns 26 and warp yarns 28 are initially 
coated with an adhesion layer made of cured of RFL liquid, isocyanate 
solution, or epoxy solution. The canvas cover 22 is made from one of 
polyamide fiber, polyester fiber, aromatic polyester fiber, or the like. 
The load carrying cords 18 are made from aramid fiber. A raw strand of yarn 
of 300 to 3100 denier is made by bundling 100 to 3000 aromatic polyamide 
fibers of 1 to 3 denier. Prior to twisting the individual fibers, the 
fibers are adhesively treated with one of epoxy compound or isocyanate 
compound. The liquid treatment results in a maintenance of the integrity 
of the bound fibers, i.e. prevents fraying of the cord 18. The individual 
filaments are preferably formed in the configuration of a ribbon. 
If the filaments are twisted in a conventional manner, twisting generally 
becomes non-uniform. This results in a weakening of the cords and lowering 
of the resistance to flex fatigue. The present invention overcomes these 
problems by identifying a range of a finishing twist factor (X) and a 
primary twist factor (Y). More particularly, the raw yarns are first 
twisted at the primary twist factor (Y) in a range of 0 to 1 to define a 
strand. At least two such strands, and preferably 2 to 5 such strands, are 
twisted at the finishing twist factor (X) in a range of 1 to 4. 
The twist factors (X, Y) according to the invention, are calculated using 
the following equation: 
EQU TF=3.48.times.0.01.times.T.times..sqroot.D. 
The directions of the primary twist and the finishing twist may be the same 
or opposite. 
Where the primary and finishing twist directions are the same, the primary 
twist factor (Y) is in the range of 0 to 1. In the case where the primary 
twist direction is opposite to the finishing twist direction, the primary 
twist factor (Y) is in the range of 0 to -1. In each case, the number of 
twists is the same. 
After being treated with RFL liquid, the twisted strands are overcoated 
with rubber cement. Alternatively, the strands are treated only with 
rubber cement. The adhesion treatment using rubber cement may be repeated 
several times as need dictates. 
The RFL liquid used is a mixture of initial condensates of resorcinol and 
formalin with rubber latex. It is preferred to set a mole ratio of 
resorcinol to formalin at 1:0.5 to 3 to enhance adhesive strength. The 
initial condensates of resorcinol and formalin are mixed with the latex in 
such a manner that a resin portion thereof accounts for 10 to 100 weight 
parts per 100 weight parts of rubber latex. The total solid concentration 
is then adjusted to be within a range of 5 to 40%. 
The latex is any of a) styrene-butadiene-vinylpyridine terpolymer, b) 
chlorosulfonated polyethylene, c) nitrile hydride rubber, d) 
epichlorohydrine, e) natural rubber, f) butadiene-styrene rubber (SBR), g) 
chloroprene rubber and h) olefine-vinylester copolymer, etc. 
The rubber cement for the overcoat treatment is a rubber component that 
adheres positively to the rubber in the tension section 16. Suitable 
rubbers are chloroprene rubber, chlorosulfonated polyethylene, and 
alkylated chlorosulfonated polyethylene. Suitable solvents include nitrile 
hydride rubber component, isocyanate compound, methyl ethyl ketone, and 
toluene. 
The aramid fiber making up the cords 18 may be an aramid fiber having an 
aromatic ring in a principal chain in the molecular structure. 
Commercially available aramid fibers, suitable for this purpose, are sold 
under the trademarks CONEX.TM., NOMEX.TM., KEVLAR.TM., TECHNORA.TM., and 
TOWARON.TM., etc. 
According to the invention, the desired strength and resistance to 
elongation and flexing fatigue for the cords 18, and the belt 10 in which 
the cords 18 are incorporated, are realized with the finishing twist 
factor (X) and primary twist factor (Y) satisfying the following two 
equations: Y is .gtoreq. to -0.6X+1.3 and Y is .ltoreq. to -1.5x+5.0. 
It has been found that if Y is less than -0.6X+1.3, the flexing fatigue 
resistance of the belt deteriorates undesirably. On the other hand, the 
strength of the belt is diminished and the elongation thereof increased if 
Y is greater than -1.5X+5.0. If, in the equations, Y becomes a negative 
number, it indicates that the primary twist direction is opposite to the 
finishing twist direction. 
The inventive cord 18 is particularly desirable where it is exposed at the 
laterally oppositely facing surfaces 30, 32 of the belt 10. 
The inventive cord 18 is equally effective when incorporated into a 
V-ribbed belt as shown at 36 in FIG. 2. The belt 36 has a tension section 
38, a compression section 40 and a neutral axis defined between the 
tension and compression sections 38, 40 by a plurality of load carrying 
cords 18, as previously described. The cords 18 are embedded in a 
cushion/adhesive rubber layer 42. A plurality of longitudinally extending, 
laterally spaced ribs 44 are defined in the belt compression section 40. 
The ribs 44 are defined by V-shaped cut-outs 46 between adjacent ribs 44. 
The ribs 44, which have discrete, reinforcing fibers 48 embedded therein, 
are relatively thin in a vertical direction and are therefore prone to 
stretching during use, which stretching is resisted by the load carrying 
cords 18. Rubberized canvas layers 50, 52 are adhered to the outer surface 
54 of the cushion/adhesive rubber layer 42. Laterally oppositely facing 
side surfaces 56, 58 are not covered by any fabric so that the load 
carrying cords 18 may be exposed thereat with the belt 36 manufactured by 
conventional techniques. 
A still further type of belt, made according to the present invention, is 
shown at 60 in FIG. 3. The belt 60 is a V-belt having tension and 
compression sections 62, 64, respectively, with the netural axis of the 
belt defined by the cords 18. The cords 18 are embedded in a 
cushion/adhesive rubber layer 66, the outer surface 68 of which is covered 
by rubberized canvas layers 70, 72, 74, adhered to the layer 76 and each 
other. A rubberized canvas layer 76 is also provided on the bottom surface 
78 of the compression section 64. 
The side surfaces 80, 82 are defined by the exposed rubber making up the 
cushion rubber layer 66 and the compression section 64. Consequently, the 
cords 18 may be exposed, as with the belt 10 shown in FIG. 1. 
To construct a belt, according to the present invention, the aramid fibers, 
in the form of filaments, are adhesively treated with either epoxy 
compound or isocyanate compound. This secures the individual filaments 
positively in bundles/strands. By so doing, the problem of fray is 
substantially eliminated. The adhesively treated filaments harden in the 
form of a ribbon. 
After being treated with RFL liquid, the individual strands are overcoated 
with rubber cement. Alternatively, the strands are treated only with 
rubber cement. This improves adhesion between the cords 18 and the rubber 
layers in which they are embedded and further eliminates the tendency of 
the cords 18 to fray. 
The cords 18 are then incorporated into a belt body to define therewith a 
power transmission belt having any desired configuration. The cords 18 are 
very strong and resist elongation and flexing fatigue. Further, the cords 
18 can be positively adhered to the rubber sections in which they are 
embedded. At the same time, the problem of fraying is overcome, 
particularly at the exposed lateral faces of the belt. The result of this 
is that the life of the inventive belt is substantially extended. 
Embodiments of the present invention, as described below, were tested to 
demonstrate the superiority of performance of the inventive belts. 
Embodiment 1 
After being immersed in a treatment liquid including the components A, as 
shown in Table 1, below, untwisted aramid fiber filaments (TECHNORA.TM., 
made by Teijin Corp.) of 1500 denier were heated at 200.degree. C. for one 
minute. 
TABLE 1 
______________________________________ 
Component A B 
______________________________________ 
PAPI-135 *1 -- 5 
EPICOAT 828 *2 4 -- 
DMP-30 *3 1 -- 
Toluene 95 95 
Total 100 100 
______________________________________ 
*1 M. D. Kasei Corp. polyisocyanate compound 
*2 Shell Chemicals Corp. epoxy compound 
*3 2,4,6trisdimethylaminomethylphenol 
The untwisted filaments were twisted at a primary twist factor (Y) of -1 to 
1 to define individual strands (the negative factors indicating that the 
finishing twist direction is opposite to the primary twist direction). Two 
of the strands were bundled and twisted at a finishing twist factor (X) of 
1 to 4 into a cord. The cord was then immersed in RFL liquid including the 
components shown in Table 2. 
TABLE 2 
______________________________________ 
Component Parts by Weight 
______________________________________ 
CR latex 100 
Resorcin 14.6 
Formalin 9.2 
Caustic soda 1.5 
Water 262.5 
Total 387.8 
______________________________________ 
The resulting cord was heated at 200.degree. C. for two minutes. 
Thereafter, the cord was immersed in the rubber cement having components 
shown in Tables 3 and 4, and then heated at 200.degree. C. for two 
minutes. 
TABLE 3 
______________________________________ 
Component Parts by Weight 
______________________________________ 
Chloroprene rubber 
100 
Magnesia 4 
Zinc oxide 15 
Accelerator *4 2 
Sulfur 0.5 
Antioxidant *5 2 
Carbon black 65 
Oil 8 
Total 196.5 
______________________________________ 
*4 NN'-diethylthiuramthiourea 
*5 octylateddiphenylamine 
TABLE 4 
______________________________________ 
Component Parts by Weight 
______________________________________ 
Rubber Composition 
100 
(Table 3) 
Methylenediisocyanate 
20 
Toluene 1080 
Total 1200 
______________________________________ 
Combinations of the primary twist factor (Y) and finishing twist factor (X) 
for the test cords in this embodiment were: (0, 1), (0.5, 1.5), (-0.5, 
1.5), (1, 2), (0, 2), (-1, 2), (0.5, 2.5), (-0.5, 2.5), (1, 3), (0, 3), 
(-1, 3), (0.5, 3.5), (-0.5, 3.5), and (0,4). 
The cover canvas was twill weave fabric using 6.6 nylon wooly finished 
yarns as wefts and 6.6 nylon yarns for industrial use as warps and having 
a thickness of 0.25 mm and 0.30 mm in cross section. An adhesive 
treatment, suitable for the rubber in the teeth, was applied to the cover 
canvas. The tooth rubber and tension section rubber were made of rubber 
components shown in Table 3, with chloroprene rubber as the main material. 
A synchronous belt was fabricated using the aforementioned materials in 
accordance with a conventional press-fit method. The resulting belt was an 
STPD tooth type, with a tooth pitch of 8 mm. The number of teeth on the 
belt was 99, and the width of the belt was 19.1 mm. 
The elongation of the belt was measured under a load of 5g/d. The tensile 
strength was measured before and after a running test. 
The belt was tested on an apparatus with a drive pulley having 24 teeth and 
three driven pulleys, each having 24 teeth, arranged up and down and left 
and right so as to oppose each other. An idler pulley having a 32 mm 
diameter was placed between the drive and driven pulleys. The belt was 
trained around the four pulleys and was subjected to a tensile force by 
placing a load of 40 kgf on one of the driven pulleys. The drive pulley 
was operated at 5500 rpm at room temperature for 200 hours. The tensile 
strength of the belt was measured after 200 hours of running. Various 
measurements were taken, including the primary twist factor (Y), finishing 
twist factor (X), the elongation of the belt under 5g/d load, the tenacity 
utilization rate, and the tenacity retention rate of the belt. In 
addition, contour lines were derived for certain of these measurements in 
relation to the primary twist factor (Y) and finishing twist factor (X) 
according to the Box Wilson method. The results are shown in the graphs in 
FIGS. 4-7. 
The tenacity utilization rate (%) is a value obtained by dividing the 
tenacity of the strength of the belt (kg) before the running tests by the 
tenacity of the filaments (g/d). The tenacity retention rate (%) is the 
value obtained by dividing the strength of the belt (kg) after the running 
test by the strength of the belt (kg) before the running test. 
Embodiment 2 
After being immersed in a treatment liquid shown in Table 5 below, 
untwisted aramid fiber filaments (TECHNORA.TM., fabricated by Teijin 
Corp.) of 1500 denier were heated to 200.degree. C. for one minute. The 
untwisted filaments were twisted at a primary twist factor (Y) of 0.5 in 
the same direction as the finishing twist direction to define a strand. 
Two of the strands were bundled and twisted at the finishing twist factor 
(X) of 2.5 to define a cord. The cord was immersed in an RFL liquid as 
shown in Table 5, below, and then treated with rubber cement. 
TABLE 5 
__________________________________________________________________________ 
Example Comparison 
Cord No. A B C D E 
__________________________________________________________________________ 
Form of fiber in first 
Filament Twisted Cord 
treatment liquid 
Treatment condition in 
Table 1A 
Table 1B 
Table 1A 
Table 1A 
RFL 
first treatment liquid 
200.degree. C. .times. 
200.degree. C. .times. 
200.degree. C. .times. 
200.degree. C. .times. 
200.degree. C. .times. 
1 min. 
1 min. 
1 min. 
2 min. 
2 min. 
Treatment condition in 
RFL RFL Rubber 
Rubber 
Rubber 
second treatment liquid 
200.degree. C. .times. 
200.degree. C. .times. 
Cement 
Cement 
Cement 
2 min. 
2 min. 
200.degree. C. .times. 
200.degree. C. .times. 
200.degree. C. .times. 
2 min. 
2 min. 
2 min. 
Treatment condition in 
Rubber 
Rubber 
-- Rubber 
-- 
third treatment liquid 
cement 
cement cement 
200.degree. C. .times. 
200.degree. C. .times. 
200.degree. C. .times. 
2 min. 
2 min. 2 min. 
Tenacity utilization 
77 77 78 76 79 
rate (%) 
Peeling (Kg/25 mm) 
35.6 35.2 33.4 34.7 15.1 
Fray A A A B E 
Tenacity retention 
84 84 85 62 85 
rate (%) 
__________________________________________________________________________ 
Fray A (good) . . . E (bad) Alternatively, the cord could be treated 
with only rubber cement. Cords A, B and C were obtained. 
The same filaments were twisted at the primary twist factor (Y) of 0.5 in 
the same direction as the finishing twist direction to be formed into a 
strand. Two of the strands were bundled and twisted at the finishing twist 
factor (X) of 2.5 to define a cord. The cords were treated in the manner 
as shown in Table 5 to obtain cords D and E. 
As in the first embodiment, described above, a synchronous belt was 
fabricated with a tooth pitch of 8 mm, 99 teeth, and a width of 19.1 mm. 
Test results for this belt are shown in Table 5. 
The flat peeling strength shown in Table 5 was measured in accordance with 
JISK6854 (T-type peeling test). The fray of the cord was determined in 
five stages (A, B, C, D, E) by inspecting how loose the cord exposed at 
the side surfaces of the belt was. 
As can be seen from the results, the cord and belt demonstrated 
satisfactory flexing fatigue resistance. The belt fabricated by adhesively 
treating the twisted cords did not demonstrate the improved fray. 
As described above, an aramid fiber cord, according to the invention, is 
fabricated by adhesively treating aramid fibers in the form of filaments 
with liquid that is one of an epoxy compound and isocyanate compound. With 
this treatment, the filaments are firmly bundled, thereby remedying the 
fray of the cord. At the same time, the cord performs satisfactorily in 
terms of strength and elongation and flexing fatigue resistance so long as 
the primary twist factor (Y) and finishing twist factor (X) lie within the 
range as previously described. 
Because transmission belts using the inventive cords also have an improved 
flexing fatigue resistance, the life of such belts is extended. 
The foregoing disclosure of specific embodiments is intended to be 
illustrative of the broad concepts comprehended by the invention.