Hose is widely used in applications where lines must flex and bend. System pressure, pressure pulses, fluid velocity, temperature, fluid, and environmental conditions form important factors in the use of hydraulic hose.
Pressure: Hoses are impulse tested from 100 to 133% of rated pressure, to allow for transient pressures. Proof pressures are 50% of burst pressure. The SAE recommends that the operating pressure be no more than 25% of minimum burst pressure. This is an acceptable safety margin for industrial use, and it also allows the hose to accept some surge pressures.
Size: Required hose size depends on volume and velocity of fluid flow. If fluid velocity is too high, flow is turbulent and energy is lost.
Sizes available for fluid-power applications range from 3/16 to 3-in. ID. Sizes are designed in 16ths of an inch by using a "dash" equivalent to the numerator of the fraction. Thus, "--10" is 10/16 or 5/8-in. size. These dash sizes are marked on the hose.
Temperature: Hydraulic or pneumatic hoses must be able to tolerate both the external ambient and the internal fluid temperatures. A hose with a broad recommended operating range -- for example, -40 to 200°F -- can accommodate most operating conditions. Some hose elastomers permit operation at rated pressure and temperatures up to 300°F; others are rated for temperatures down to -65°F.
Reinforcement type: The reinforcement may be a natural or synthetic yarn or fiber, a metal wire, or combinations. The reinforcement may be braided, spiral wound, or both. Wire-braid reinforcement, commonly used for hydraulic applications, provides good service life at moderate cost. The wire is usually braided with 2 plaits over 2, which is economical and reliable with good dimensional stability.
Spiral wire-wound hoses are often used for applications with high-frequency surges in high-pressure systems. As pressure surges, normal braided reinforced hoses tend to fail at the wire crossover points in the weave. These points cause bending and shear in wires that are already highly stressed. The advantage of spiral wire reinforcement is that it eliminates touchy crossover points permitting up to 97% of theoretical total coverage.