Friction material

A composition of material for use as a friction material for a brake lining wherein steel fiber is the only metallic ingredient and the holding matrix is formed by rubber particles and phenolic resin. The holding matrix being strengthened by a reaction of the resin with calcium hydroxide. When the steel fiber comprises from 25-48% by weight of the total mixture and the rubber particles and phenolic resin comprise 17-25% by weigh of the total mixture, a sufficient structural unity is produced for the resulting friction material to withstand dynamic loading experienced during a brake application.

BACKGROUND OF THE INVENTION 
This invention relates to a composition of material for use as a friction 
material in a brake. This composition uses steel fiber as the reinforcing 
material. The steel fiber is combined with fillers, a lubricant, and 
friction modifiers which are held in a binder made up of rubber and 
phenolic resin. The resulting friction material has a substantially 
uniform and predictable coefficient of friction after repeated brake 
applications. 
Many different fibers have been suggested as a substitute for asbestos in 
reinforcing a brake lining, for instance U.S. Pat. No. 3,967,037 discloses 
the use of glass fiber, U.S. Pat. No. 3,896,075 discloses the use of 
basalt fiber, U.S. Pat. No. 4,019,912 discloses the use of carbon fiber, 
and U.S. Pat. No. 4,119,591 discloses the use of a mixture of cellulose 
and steel fibers. Unfortunately such substitutions have resulted in some 
undesirable characteristics such as rotor wear, low temperature 
coefficient of friction stability, noise and high temperature structural 
unity. 
SUMMARY OF THE INVENTION 
The composition of matter for the friction material of this invention is 
reinforced by steel fibers having a length of 3-15 mm. The steel fibers 
make up from 25-48% by weight of the total composition. The steel fibers, 
lubricant, fillers and friction modifiers are held together by a binder 
made up of approximately equal amounts of rubber and phenolic resin. The 
binder comprises from 17-25% by weight of the total composition. The 
phenolic resin is modified by a reaction with calcium hydroxide during 
curing to stiffen the binder and with the steel fiber develops sufficient 
strength in a resulting friction material to withstand dynamic loading 
experienced during a brake application. 
In performance test, the friction material exhibited a move stable 
coefficient of friction during repeated brake applications than a 
reference friction material that included asbestos fibers. 
This composition of material when used as a friction lining provides an 
advantage over friction linings that have asbestos therein since the 
coefficient of friction is substantially stable over the operating range 
of a brake. 
It is an object of this invention to provide reinforcement for a brake 
lining through the use of steel fiber held in a binder matrix stiffened by 
a reaction of calcium hydroxide and phenolic resin. The strength of the 
binder is enhanced by curing rubber in the composition by sulfur such that 
the total strength is sufficient to withstand dynamic loads experienced 
during a brake application.

DETAILED DESCRIPTION OF THE INVENTION 
In order to evaluate the composition ot matter developed by this invention, 
a typical composition of matter having asbestos reinforcement which meets 
the current stopping requirements of customers was selected as a base line 
composition, this base line is disclosed in FIG. 1 as composition X. The 
ingredients in composition X were placed in a mixer and after uniformly 
dispursing the ingredients throughout, a portion of the mixture was placed 
in a mold to produce a brake lining. Thereafter, the brake lining was 
placed in an oven and resin therein cured to establish a matrix for 
holding the ingredients in a fixed relationship. 
The asbestos fiber in composition X was removed and replaced with steel 
fiber having a nominal length of between 3-15 mm to produce composition A. 
As shown in FIG. 1, composition A also includes carbonaceous materials, 
friction modifiers, fillers and a binder system. The binder system 
includes both rubber and phenolic resin. The rubber was cured by the 
addition of sulfur at about 20% by weight of rubber and the resin was 
modified by the addition of calcium hydroxide. After the mixture was 
uniformly mixed, a portion was removed and placed in a mold to produce a 
brake lining. The curing of the resin as modified by the calcium hydroxide 
makes the resin matrix stiffer and as a result the physical strength of 
the brake pad is increased. The inclusion of silica and magnesium oxide 
enhances the coefficient of friction while whiting adds to the friction 
stability. In order to provide for minimum wear between the brake pad and 
a corresponding mating surface, graphite carbon coke or a combination 
thereof provides for some lubrication. 
The brake pads in compositions X & A were molded into a shape corresponding 
to the rear brake used on the Topaz vehicle manufactured by Ford Motor 
Company. This brake has a drum diameter of 8" and a weight of 576# of the 
vehicle assigned thereto. In order to determine the effectiveness of the 
brake linings made from compositions A & X the torque required to 
decelerate a vehicle at 6.4, 19.2 and 32.0 ft./sec..sup.2 was calculated. 
In this analysis, torque is defined as a ratio of the average torque to 
line pressure times the area of the wheel cylinder. Thus, a higher value 
of torque indicates a more effective lining material. Differences in 
torque from stop to stop reveal changes in friction lining, such that, as 
a lining fades, the torque decreases. A total of 108 stops were made to 
simulate pre-burnish, post burnish and final effectiveness of the lining 
at an initial brake temperature of 0.degree., 100.degree. and 200.degree. 
F. for a vehicular speed of 30 and 60 mph. 
FIG. 2 illustrates the torque factor calculated to stop a vehicle under 
pre-burnished conditions, FIG. 3 illustrates the torque factor calculated 
to stop a vehicle under post-burnish conditions, and FIG. 4 illustrates 
the torque factor calculated to stop a vehicle under final effectiveness 
conditions for composition X. From the total 108 stops, it was determined 
that composition X had an average torque factor of 9.103, a minimum torque 
factor of 5.422 and a maximum torque factor of 13.262. The minimum and 
maximum torque factors can be used in calculating the best and worst case 
condition for a brake. On any given day, on any given test, with any given 
brake lining there is a chance that the effectiveness will fall outside a 
range defined by a standard deviation. Statistically for a torque factor 
range of from 5.422 to 13.262, it can be shown that a standard deviation 
is 1.966. An important ramification of the standard deviation is that the 
smaller it is, the greater the friction stability of the brake lining 
under test. An ideal brake system would have all its components operating 
such that the system would operate smoothly without lockup with the torque 
factor of the rear lining having values at a range defined by the average 
torque factor +/- a standard deviation. 
Since the test for composition X was separated into pre-burnish, 
post-burnish and final effectiveness, a statistical analysis for each 
segment was performed. 
For the pre-burnish segment the overage torque factor was calculated to be 
10.485 with a minimum of 5.422 and a maximum of 13.262. The standard 
deviation was found to be 1.818 which results in a range indicated by 
lines 20 and 22 in FIG. 2. 
For the post-burnish segment, the average torque factor was calculated to 
be 9.381 with a minimum of 6.752 and a maximum of 11.919. The standard 
deviation was found to be 1.613 which results in a range indicated by 
lines 24 and 26 in FIG. 3. 
For the final effectiveness segment, the average torque factor was 
calculated to be 7.444 with a minimum of 5.816 and a maximum of 8.844. The 
standard deviation was found to be 0.976 which resulted in a range 
indicated by lines 28 and 30 in FIG. 4. 
When the averaqe torque for the entire test for composition X is compared 
with the average torque for the segments, it is apparent that the 
effectiveness of the lining diminished as the test progressed from 
pre-burnish through final effectiveness for composition X. The standard 
deviation of the torque factor decreased as the test progressed for 
composition X. This decrease indicated that composition X had a decreasing 
sensitivity for initial brake temperature and line pressure. 
The friction lining made from composition A was tested in the same manner 
as composition X to produce the 108 stops: 36 stops for pre-burnish 
illustrated in FIG. 5; 36 stops for post-burnish illustrated in FIG. 6; 36 
stops and for final effectiveness illustrated in FIG. 7. The average 
torque factor for the 108 stop was calculated to be 6.819 with a minimum 
of 4.695 and a maximum of 8.457. The standard devialion was found to be 
0.857. 
In the individual segments of the test, the average torque for pre-burnish 
was calculated to be 6.712 with a minimum of 4.797 and a maximum of 7.956. 
The standard deviation was found to be 0.868 which resulted in a range 
illustrated by lines 32 and 34 in FIG. 5. 
For post-burnish the average torque was calculated to be 6.984 with a 
minimum of 4.695 and a maximum of 8.244. The standard deviation was found 
to be 0.888 which resulted in a range shown by lines 36 and 38 in FIG. 6. 
For final effectiveness, the average torque factor was calculated to be 
6.761 with a minimum of 4.821 and a maximum of 8.457. The standard 
deviation was found to be 0.877 which resulted in a range shown by lines 
40 and and 42 in FIG. 7. 
When the average torque factor for the entire test is compared with the 
torque factor for the individual segments, it was found that they were 
within 5% of the average. This indicates that the friction lining of 
composition A has substantially the same effectiveness throughout the 
entire test. This is also reflected in the calculation of the standard 
deviation which is relatively constant. 
Since parking brakes for most vehicles are only connected to the rear 
wheels, a static friction analysis of compositions X and A was performed. 
In this analysis the minimum holding torque for a vehicle on a grade is 
measured. In FIG. 8, curves 44 and 48 illustrate the holding torque for 
composition X. Curve 44 represents forward motion and curve 48 represents 
reverse motion on a slope. Curves 46 and 50 in FIG. 10 represent the 
static friction produced by composition A. From a review of FIG. 10, 
composition A produced more static friction and thus be capable of holding 
a vehicle on a steeper incline under the same condition. 
Since vehicles often operate under conditions where the brake linings are 
wet, as for example after a vehicle has passed through a puddle of water. 
For this reason a water recovery analysis was performed on compositions X 
& A. In performing this analysis, several stops are made to establish a 
base line and then the brake lining submerged in water. Thereafter, the 
test is resumed. When the torque factor is equal to the baseline, a water 
recovery is considered completed. Curves 54 and 56 in FIG. 9 illustrate 
the baseline and recovery for composition X and curves 58 and 60 represent 
the baseline and recovery for composition A. Composition X recovered to 
the baseline torque factor at the fourth stop while it is projected that 
composition A would recover after about 7 stops. 
From a customer standpoint, wear of a brake lining is one of the most 
important factors in selecting one lining over another lining. Before the 
test began, the thickness of the brake lining of composition X was 
measured at 7 different points on the brake lining for both the primary 
and secondary shoe. The average wear for composition X on the primary shoe 
was 0.002371 inches and on the secondary shoe was 0.000858 inches. The 
average wear for composition A on the primary shoe was 0.001479 inches and 
on the secondary shoe was 0.000729 inches. 
Visual inspection of composition X after the 108 stops revealed a smooth 
surface glaze with some cracking adjacent one rivet hole while the 
corresponding drum was dull and showed some scoring. 
Visual inspection of composition A after 108 stops revealed a polished 
surface, which was dusty and faintly scored while the corresponding drum 
was polished with water spots, some scoring and light dust. 
From the above test, it was evident that composition A was an acceptable 
substitute for composition X. 
To further evaluate composition A, a series of compositions B-L shown in 
FIG. 1 were compounded and placed on a vehicle. The material compositions 
B-L were evaluated for noise generated during braking since this is a 
primary consideration of an acceptable production lining after friction 
stability and wear. Compositions B-L were based on composition A with 
materials added and substracted in an effort to attenuate noise. 
Composition B differs from composition A in that the liquid resin was 
eliminated and the dry resin increased, the filler (Whiting) was reduced, 
the friction modifier (silica) increased and the calcium hydroxide 
increased. The calcium hydroxide reacts with the phenolic resin to provide 
physical strength for the resulting brake lining. During this test no 
unacceptable noise was produced. The low noise level of composition B is 
attributed to the high level of calcium hydroxide in this formula. 
Composition C differs from composition A in that the silica was removed and 
the other ingredients proportionally increased. This material when tested 
did not produce unacceptable noise all through the friction level or 
efficiency seemed to be decreased as a result of the total removal of the 
silica. 
Composition D is similar to composition C in that rather than remove the 
silica, the magnesium oxide was removed from composition A and the 
remaining ingredients proportionally increased. Composition D when tested 
did produce noise and as a conclusion, it is presumed that the silica in 
composition D may be responsible for noise. 
In composition E, the silica and magnesium oxide were removed from 
composition A and the remaining ingredients proportionally increased. When 
tested on a vehicle composition E failed due to cracking prior to 
completion of the noise evaluation. However, prior to termination of this 
test, a reduction in noise levels was observed. 
In composition F, the silica and magnesium oxide of composition A was 
reduced by one half and the other ingredients proportionally increased. 
Composition F was installed on a vehicle and was acceptable since no 
objectional noise was observed during the major portion of the noise test. 
In composition G, the silica and calcium hydroxide were removed, graphite 
was doubled and the other ingredients proportionally increased. When 
placed on a vehicle, the noise level of composition G was unacceptable. 
In composition H, the silica and magnesium oxide were reduced, the graphite 
increased and the calcium hydroxide eliminated. When composition H was 
placed on a vehicle, hill noise was observed throughout the test. 
In composition I the silica, magnesium oxide and calcium hydroxide were 
removed, and the remaining ingredients proportionally increased. When 
composition I was placed on a vehicle, this test was terminated due to 
high temperature inbalance between the primary and secondary shoes. 
In composition J the filler whiting was reduced, while the graphite and 
calcium hydroxide and steel fiber content were increased. When composition 
J was tested on a vehicle, very little noise was observed. It is believed 
that the high calcium hydroxide content in composition J which reacts with 
the resin therein to provide a stiffer structure is a primary factor in 
reducing noise. 
In order to evaluate the effect of fillers on the composition A, the 
carbonaceous ingredients were increased by the addition of coke and 
carbon, the calcium hydroxide removed, and the remaining ingredients 
proportionally increased to produce composition K. When a brake lining of 
composition K was placed on a vehicle, the brake lining exhibited good 
friction stability and very little noise with the exception of one brake 
application on a hill. 
To set a range for the steel fiber, fillers and calcium hydroxide, these 
ingredients were selected to fall within what could reasonably be expected 
the limits for such ingredients in a brake lining to produce composition 
L. Composition L was placed on a vehicle and possessed sufficient strength 
to complete the noise test. Noise was within an acceptable level, however, 
corresponding drum was polished but not scored. Unfortunately, it was felt 
that wear for composition L may be unacceptable. 
In conclusion, it is my opinion from the test performed on the composition 
A and the various modifications thereof wherein the binder therein makes 
up from 46-60% by volume which comprises from 17-25% by weight of the 
total mix a good strength can be achieved. The binder can be a mixture of 
liquid resin, dry resin and a chemical resistant rubber wherein the rubber 
makes up at least 50% of the total binder. The rubber being cured by the 
addition of sulfur at about 20% by weight of the rubber in the mixture and 
the resin being modified by a reaction with calcium hydroxide to improve 
the physical strength of a resulting brake pad. The primary reinforcement 
strength of the brake pad being provided by steel fibers, having a length 
of 3-15 mm. The steel fiber which makes up from 7-10% by volume comprises 
from 40-48% by weight of the total mixture. In order to prevent excessive 
wear at high temperature, it is suggested that a lubricant such as 
petroleum coke, graphite, carbon or a combination thereof be added to the 
basic mixture. While the percentage of lubricant can vary from 7-30% by 
volume, it is preferred that the lubricant be limited to from 6-8% by 
weight of the total mixture. The remaining ingredients in composition A, 
which are fillers and friction modifiers, whiting, barytes, talc, etc., 
can be adjusted to change the friction level, however, the friction 
stability and overall effectiveness appears to be optomized when the 
fillers are at about 20% by volume or weight of the total composition and 
the friction modifiers, silica and magnesium oxide, are held to about 5% 
by volume or 10% by weight of the total composition. 
Thus, the evaluation of composition A with the asbestos based composition X 
should provide a customer with evidence that composition A would perform 
in an acceptable manner as a substitute for asbestos based friction lining 
.