By Josh Cosford, Contributing Editor
The metal tube assemblies used in hydraulic plumbing applications offer one of the most reliable methods to transfer fluid under pressure. Still, their factors for design and installation are sightly more complicated than hose assemblies, which are traditionally more popular.
Mills make seamless tubing used for hydraulic systems by pushing a hot slug of steel bar around a circular mandrel to create a tube before this still-hot product is sent through various procedures before quenching. For example, a relatively small slug of hot-rolled steel may be drawn into dozens of meters long tubes.
The primary pressure holding capacity is a combination of tube diameter and wall thickness. It’s a mistake to assume any given wall thickness will offer the same pressure holding capacity across all tube diameters. Hydraulic pressure acts at perpendicular force vectors against tubing wall, and larger diameter tube has more surface area for pressure to act against. The ratio of tensile strength to the interior surface becomes unfavorable as diameter increases, thereby reducing strength.
To combat the limitations in pressure capacity, tubes are manufactured in a limited range of wall thicknesses. It is also essential to understand that you define tubing against its outside diameter, not its inside diameter. For example, ¼ in. tube comes in standard wall thickness ranging from 0.028 to 0.065 in., offering a range of pressure capacity from approximately 2,500 to 5,800 psi.
You must subtract the wall thickness from the OD when sizing tube for your application because a thicker wall reduces the internal diameter, and therefore reduces flow. For example, 1 in. OD tube with 0.035 in. wall thickness has an internal diameter of 0.93 in.. However, this tube is only rated for 788 psi, which may only be appropriate for return lines. Jumping up to 0.120 in. wall tube is required for 2,700 psi sees the ID shrink to 0.76 in. Be aware of how much wall thickness affects tube ID and its subsequent effect on maximum flow capacity.
Tubing rarely follows a straight line, and often assemblies are bent in various lengths and radii to suit the plumbing requirements of the machine. Each increasing tube diameter also sees an increase in the minimum bend radius possible. Not only does the larger tube become more susceptible to kinking when bent too narrow, but the flow characteristics also become more turbulent, increasing backpressure in the plumbing.
Commonly, hydraulic power units, control systems and actuators are happy with standard steel construction, where paint often coats the entire machine to offer protection from any typical ambient condition. However, stainless steel offers the technician a popular option for ultimate corrosion protection.
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