The conducting system, part II: hydraulic hoses and fittings - Hydraulic Systems Trends - Brief Article

Diesel Progress North American Edition, July, 2002 by Russ Henke

As we indicated in the first part of this discussion (see April 2002, Diesel Progress), the conducting system -- how hydraulic fluid moves between pumps, motors, actuators, etc. --- is a critical part of any hydraulic circuit. Looking at mobile hydraulic applications, it is generally the case that piping and tubing are rigid conductors and are not suitable, since the two ends of a conductor typically move relative to one another. In such applications, flexible conductors such as hoses are used.

There are more hoses to choose from than pipe or tubing. They come in many types and are designed for a broad spectrum of uses. In a hydraulic system, flexible conductors are called hose assemblies. An assembly includes a length of flexible hose to each end of which an appropriate fitting has been attached.

In addition to the freedom and convenience of relative motion between components, hoses provide other advantages. They help overcome severe vibration, compensate for manufacturing tolerances in piping, provide freedom in routing conductors on a machine, absorb hydraulic impulse shocks and smooth fluid flow.

Rigid tube dash sizes (tube o.d.) form the basis for the dash sizes used to identify single-wire braid (SAE 100R5) hose and fittings. Sizes for most other standard hose styles (fabric braid, two-wire braid and multiple-spiral wire wrap) are also expressed by a dash number, but in these cases, the dash number represents the number of 1/16th of an inch of the hose i.d.

Table I is a comparison of working pressures for four types of wire braid hoses. Table I is based on SAE 517 standard for the hoses listed. Note that working pressures listed decrease as the size of the hose increases. As the inside diameter increases, the area exposed to pressure also increases, causing stress in the body of the hose to rise. In the U.S., hydraulic hose specifications are set by SAE J517; fittings and couplings by SAE J514 and SAE J518.

The five basic types of industrial hose are classified by the amount of fluid pressure they will withstand. The type of reinforcement used is the chief difference between those hose types because high-pressure hoses require stronger reinforcement to handle the pressurized fluid in a system.

Low-pressure hose is generally reinforced with a fabric braid; medium- and high-pressure hoses have single- and multiple-wire braid reinforcement depending on need. Spiral-wire wrap hoses were developed for special, high-impulse pressure applications.

Fig. 1 illustrates a typical low-pressure hose showing a fabric braid reinforcement, a synthetic rubber inner tube and either a cotton or rubber cover. This hose is used largely in the automotive and trucking industries and is not recommended for hydraulic applications.

Fig. 2 illustrates four methods of attaching fittings to a low-pressure hose. In Fig. 2a, a barbed nipple is pushed into the hose and held with a clamp. This is seen often in industrial applications. A screw-together fitting, in which the nipple is screwed into a socket attached to the hose is shown in Fig. 2b. A crimped-on fitting, which is pressed firmly on the hose by a machine is shown in Fig. 2c. The socketless fitting in which a ridged nipple grips the interior of the hose and holds it fast, Fig. 2d, is engineered to require no exterior clamp as does a barbed nipple.

A cap fits over the end of the hose to protect the raw end and give the assembly a finished appearance. The cap determines the position of the hose over the fitting. It can be assembled manually by pushing the fitting into the hose.

Fig. 3 is a cutaway drawing of a medium-pressure hose. SAE specifications 100R1 and 100R5 describe this type hose as having cotton or rubber covers; most manufacturers conform closely to these specifications. The hose is reinforced with a single layer of carbon steel wire-braid, and has a cotton inner braid between the wire and the synthetic rubber inner tube. Available in -4 through -48 sizes this hose is suitable for many industrial applications with hydraulic oil, crude oil, lubricating oil, air and water.

Typical constructions of wire braid hoses are illustrated in Fig. 4, SAE 100R2, high-pressure hose; Fig. 5, SAE 100R 10, very high-pressure hose; and Fig. 6, SAE 100R 11, super high-pressure hose.

Two types of fittings are manufactured for medium-pressure hoses. One is a pressed-on type, (Fig. 7a) non-reusable, usually swaged-on or crimped-on. These fittings must be installed with a special machine and discarded with the hose if the assembly fails.

The second is the detachable, reusable type which has a socket and a threaded nipple (Fig. 7b). This type fitting can be reused when hose lines wear out or become damaged. Assembly and disassembly is done with simple hand tools. The socket grips the hose carcass, the nipple is inserted into the hose and screwed into the socket. This design provides a firm, strong attachment of hose-to-fitting; the assembly will not leak or wear at connecting points.

One distinct advantage of reusable fittings other than the ability to re-use them with different hose lengths is their versatility. With the reusable fitting, one socket can mate with a variety of nipple assemblies (Fig. 8). Thus one thread type can be replaced by another simply be changing nipple assemblies in the socket.