Steel tubing is used in hydraulic systems when rigid lines are required. They are easier to assemble and don’t require welding in order to achieve leak-proof connections. Seamless steel tubing is the most widely used conductor type for hydraulic systems as it has significant advantages over pipes. The tubing can be bent into any shape thereby reducing the number of fittings in a system. Tubing is easier to handle and can be reused without any sealing problems. For low-volume systems, tubing can handle the pressure and flow requirements with less bulk and weight. However the flip side to tubing and their fittings is that that they are very expensive.
Steel tubes are measured and specified by their outside diameter and wall thickness. The tubing grade and wall specification determine the pressure ratings. As shown in Figure 7.32, one piece of tubing is connected to another component through a tube connector and fastening nut. The tube is often pre-flared to an angle of 37° to accept a 37° flare connector as shown in Figure 7.32.
The most widely used material for steel tubing is SAE1010 dead soft, cold drawn steel. This material has a fairly high tensile strength and is quite easy to work with. In order to obtain higher tensile strengths, the tubes are made of AISI 4130 steel.
The fittings required for tubing are slightly different to those required for piping. Although the concept of fittings remains the same, the method of joining and sealing are different. A few fittings along with the sealing methods adopted for tubing have been illustrated in Figure 7.33.
Some of these fittings are known as compression fittings. They seal by metal-to-metal contact and may be either a flared type or flare-less type. Other fittings may use 0-rings for sealing purposes.
The sleeve inside the nut, supports the tube, to dampen vibrations. When the hydraulic component has straight threaded ports, straight thread type 0-ring fittings can be used. This type of sealing is ideal for higher pressures, because as the pressure increases the seal gets tighter.
The tubing is supported ahead of the ferrules, by the fitting body. Two ferrules grasp tightly around the tube without causing any damage to the tube wall. There is virtually no constriction of the inner wall, ensuring minimum flow restriction.
Figure 7.34 shows a Swage tube fitting which can sustain any pressure up to the bursting strength of the tubing, without leakage. This type of fitting can be repeatedly taken apart and re-assembled.
The secret of the Swagelok fitting is that all the fitting action is along the axial direction of the tube and not rotary. Since no torque is transmitted from the fitting to the tubing, there is no formation of initial strain that may weaken the tubing.