Development of high-performance brake hose for automotive use.

Automotive brake hose is built up by three rubber layers and two reinforced braided layers as shown in figure 1, and is installed beneath the body under severely bent conditions. With this hose, brake fluid pressure is transmitted to the wheel by a pedal stroke. It should not only be durable for tire braking, but also soft and flexible enough to absorb vibration. Moreover, it is continuously affected by high humidity, high pressure, repeated bending and twisting (ref. 1). Since its operating conditions are so severe as mentioned, a considerably high level of fatigue resistance and extremely low level of water permeability are required.

[Figure 1 ILLUSTRATION OMITTED]

Water permeability

Water permeation depends mainly on each condition of three rubber layers. And water permeation on the hoses consisting of three rubber layers is given by:

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII]

Where

Q : quantity of water permeation, g;

[P.sub.1] ~ [P.sub.3]: water permeability coefficients of rubber

g x cm / [cm.sup.2] x sec x atm;

?? : hose length, cm;

[P.sub.1] ~ [P.sub.3]: pressure differences across layers, atm; [r.sub.1] : inner diameter of inner rubber layer, cm; [r.sub.2] : outer diameter of inner rubber layer, cm; [r.sub.3] : inner diameter of interply rubber layer, cm; [r.sub.4] : outer diameter of interply rubber layer, cm; [r.sub.5] : inner diameter of outer rubber layer, cm; [r.sub.6] : outer diameter of outer rubber layer, cm.

By substituting values of each diameter for this equation, we tried to calculate each layer's contribution to water permeation. From the results, it is obvious that when the contribution of inner rubber layer is defined as 100, 20 is given for the contribution of interply rubber, and 75 for outer cover rubber.

Experiments

We measured water permeation through various kinds of rubbers. Ten mm long tubular test samples were prepared. The rubber tube was filled with distilled water and we measured the change of its weight in a normal atmosphere at a constant temperature.

Method and procedure of the water permeation test of hose has long been discussed by the SAE moisture transmission task force. According to the current SAE method (ref. 2), a bent brake hose is to be filled with brake fluid and immersed in distilled water at 70 [degrees] C for three days. The amount of water in the brake fluid is then measured by the Karl Fischer method.

Results

Water permeability coefficient of various kinds of rubbers at each temperature was calculated from the test results and is shown in table 1.

Table 1 - water permeability coefficients of
various kinds of rubbers
        Permeability coefficient (g cm/[cm.sup.2] sec atm)

Temp.([degree] C)          20                 40

Rubber

CR                  3.63 x [10.sup.-9]   1.02 x [10.sup.-8]
NR                  5.68 x [10.sup.-9]   6.72 x [10.sup.-9]
SBR                 6.02 x [10.sup.-9]   7.10 x [10.sup.-9]
EPDM               7.95 x [10.sup.-10]   1.54 x [10.sup.-9]
IIR                1.14 x [10.sup.-10]  3.84 x [10.sup.-10]

Temp.([degree] C)          70

Rubber

CR                  1.41 x [10.sup.-7]
NR                  1.03 x [10.sup.-7]
SBR                 9.94 x [10.sup.-8]
EPDM                2.95 x [10.sup.-8]
IIR                 6.55 x [10.sup.-9]

Furthermore, by substituting each rubber's water permeability coefficient values for the equation, we tried to calculate the water permeation of hoses at various kinds of rubber materials combinations.

The results are shown in figure 2. From the results, for example, hose built up by IIR inner and interply rubber and EPDM outer rubber is expected to reduce water permeation to 1/10 of hose built up by SBR inner rubber, NR interply rubber and CR/EPDM outer cover rubber. As shown in figure 3, the measured value of hose almost coincides with calculated value. IIR is the superior in water permeability, but the brake fluid resistance is poor. Therefore we select EPDM for the inner rubber of brake hose.

[Figure 2 - 3 ILLUSTRATION OMITTED]

Fatigue resistance

Experiments

Brake hoses are exposed to outer complex force such as up-and-down motion from road surface, twisting motion from steering operation, and high pressure by braking operation. Since hose is required to move in three dimensions, stress on hose needs also to be analyzed three dimensionally (ref. 3). In that case, we presume that resultant force made up by counter forces at three axes is reflected by all the stress. We, therefore, tried to measure three axis forces loaded to both end-fittings, and synthesized them (the three axis resultant force) in order to compare with bench fatigue test results. On this trial, we defined the condition of bench steering-motion fatigue test as: temperature 100 [degrees] C, pressure 9.8 MPa, pressure cycle 42 cycles/minute, steering motion cycle 100 cycles/minute.

Results

Results of measurement are shown in figure 4. In this case, the three axis resultant force varies with much complexity together with steering angle, and forms hysteresis loops. Also it remarkably increases when pressure is added.

[Figure 4 ILLUSTRATION OMITTED]

When these three axis resultant forces of various hose routing are compared with bench fatigue test results, (figure 5), both data are unified on a single line on a graph. Namely we found that our treatment of the three resultant forces correctly reflects the stress on hoses dominating the hose fatigue life, and could be used as a basis for a safety standard of hose routing design.

[Figure 5 ILLUSTRATION OMITTED]

Conclusion

We clarified that inner rubber tube of the brake hose influences mainly the water permeability, and found out that EPDM is the best material to be built up. As the best method of all to analyze the hose stress under steering and stroke motion on hose routing, we established the measurement method of the three axis resultant force and clarified the correlation with hose fatigue.

Acknowledgements

"Compatibilization of CR/EPM blends for power transmission belt applications" is based on a paper presented at the October, 1996 Rubber Division meeting. "Development of high-performance brake hose for automotive use" is based on a paper given at IRC '95. "Clearing the fog about fogging effects of liquid fire retardants in flexible foam" is based on a paper given at Polyurethanes Expo '96. "Analysis of rubber materials by pyrolysis GC" is based on a paper presented at the October, 1996 Rubber Division meeting.

References

(1.) Hiroshi Fujita, Auto-Chemicals, Vol. 7, No. 2 (1983), "Non-mineral brake fluid for automotive."

(2.) SAE J1873 "Moisture transmission test procedure hydraulic brake hose assemblies."

(3.) Y. Kobayashi, Journal of the Society of Rubber Industry, Japan, 63, 552, (1990), "Recent developments in automotive pressure hoses."