Shock-resistant, flexible hose (Figure 8-7) assemblies are required to absorb the movements of mounted equipment under both normal operating conditions and extreme conditions. They are also used for their noise-attenuating properties and to connect moving parts of certain equipment. The two basic hose types are synthetic rubber and poly(tetrafluoroethylene) (PTFE), such as DuPont’s Teflon fluorocarbon resin.
Synthetic Rubber Hose
Rubber hoses are designed for specific fluid, temperature, and pressure ranges and are provided in various specifications. Rubber hoses consist of a minimum of three layers: a seamless synthetic rubber tube; one or more reinforcing layers of braided or spiraled
cotton, wire, or synthetic fiber; and an outer cover. The inner tube is designed to with stand the attack of the fluid that passes through it. The braided or spiraled layers deter mine the strength of the hose. The more layers there are, the greater the pressure rating is. Hoses are provided in three pressure ranges: low, medium, and high. The outer cover is designed to withstand external abuse and contains identification markings.
The size of a flexible hose is identified by the dash (-) number, which is the ID of the hose expressed in 16ths of an inch. For example, the ID of a -64 hose is 4 inches. For a few hose styles this is the nominal and not the true ID.
Cure date. Synthetic rubber hoses will deteriorate from aging. A cure date is used to ensure that they do not deteriorate beyond material and performance specifications. The cure date is the quarter and year the hose was manufactured. For example, 1Q89 or 1/89 means the hose was made during the first quarter of 1989. The cure date limits the length of time a rubber hose can be safely used in fluid power applications. The normal shelf life of rubber hose is 4 years.
As mentioned earlier, flexible hose is available in three pressure ranges: low, medium, and high. When replacing hoses, it is important to ensure that the replacement hose is a duplicate of the one removed in length, OD, material, type, and contour. In selecting hose, several precautions must be observed. The selected hose must do all the following:
1. Be compatible with the system fluid
2. Have a rated pressure greater than the design pressure of the system
3. Be designed to give adequate performance and service for infrequent tran sient pressure peaks up to 150 percent of the working pressure of the hose
4. Have a safety factor with a burst pressure at a minimum of 4 times the
rated working pressure
There are temperature restrictions applied to the use of hoses. Rubber hose must not be used where the operating temperature exceeds 200″F. PTFE hoses in high-pressure air systems must not be used where the temperature exceeds 350″F.
Flexible hose must not be twisted during installation. This will reduce the life of the hose and may cause the fittings to loosen. You can determine whether or not a hose is twisted by looking at the layline that runs along the length of the hose. If the layline does not spiral around the hose, the hose is not twisted. If the layline does spiral around the hose, the hose is twisted and must be untwisted. Flexible hose should be protected from chafing by using a chafe-resistant covering wherever necessary.
The minimum bend radius for flexible hose varies according to the size and construc tion of the hose and the pressure under which the system operates. Current applicable technical publications contain tables and graphs showing the minimum bend radii for the different types of installations. Bends that are too sharp will reduce the bursting pressure of flexible hose considerably below its rated value.
Flexible hose should be installed so that it will be subjected to a minimum of flexing during operation. Support clamps are not necessary with short installations; but for hose of considerable length (48 inches, for example), clamps should be placed not more than 24 inches apart. Closer supports are desirable and in some cases may be required.
A flexible hose must never be stretched tightly between two fittings. About 5 to 8 per cent of the total length must be allowed as slack to provide freedom of movement under pressure. When under pressure, flexible hose contracts in length and expands in diameter. Examples of correct and incorrect installations of flexible hose are illus trated in Figure 8-8.