Tubing, when done well, is highly reliable and maintenance-free. The only time you change tube is typically when a modification or upgrade is done to the machine. Tubing resists motion or vibration that, although possibly transmitted, doesn’t cause it to shake or move the assembly itself. However, the inherent flexibility of hose permits chaffing, rubbing and scratching against other surfaces when not well supported. Many failures occur at those contact points on a hydraulic hose, but tubes resist motion altogether.
Technicians spend more upfront work on tube assemblies but they look cleaner and have other advantages. If the attached components require servicing, you may still remove and replace hose sections because tubes are made with semi-flexible bends at strategic points. You rarely see a straight shot of tube, but rather an L or S shape to provide some flexibility so the assembly can be pulled and replaced between two fixed locations.
Tube assemblies also provide long-lasting, leak-free service, especially with newer O-ring face fittings. Although older assemblies with JIC type flare have been in operation for decades, if a tube does leak, it will nearly always be between the seat and face of a JIC connection.
Tubing has a natural high-pressure capacity, and dollar-for-dollar is actually less expensive to manufacture and sell than hose. Expect to pay pennies per foot of small tubing, while any high-quality hose with a reasonable pressure rating is relatively expensive, and the cost rises exponentially with size. For example, a 1 in. ID hose rated for 4,000 psi can be hundreds of dollars, but a tube can be made for a fraction of the cost, even including labour. However, a tube may be more labour intensive and require special machinery. For example, you need expensive bending and flaring equipment, and the investment in those machines may run five digits.
We express tube size by its outside diameter, and the technician must factor the wall thickness to calculate the appropriate flow rate for any given pressure capacity. For example, a 3/8 in. OD tube may come in four wall-thickness variations from 0.028 to 0.065 in. or larger. Although both are the same OD, the 0.028 in. wall tube is “thinner” and offers a maximum pressure rating of only about 1,700 psi. Conversely, the 0.065 in. wall tube easily handles 3,900 psi.
As you’d assume, the above examples experience inversely proportional flow ratings. The effect is the inside diameter of the 0.028 in.-wall, 3/8 in. tube is 0.319 in. and will flow about 4 gpm for work lines. However, when the same 3/8 in. OD tube employs thicker 0.065 in. walls, the inside diameter is now only 0.245 in. and flow is reduced to only about 2 gpm.
Tube sits atop the compatibility pyramid because there is very little in the way of fluid, chemical or temperature that can faze it. However, if caustic or oxidative environments require more protection, stainless steel tube is not uncommon. The fittings used to plumb together a circuit could be simple sleeves and nuts or more expensive swage-type fittings requiring no flare — both options come available in stainless steel.
Source: Fluid Power World