What is in this article?:
- Engineering Essentials: Hydraulic Fittings and Flanges
- Hydraulic flanges
Among the basic elements of virtually every hydraulic system is a series of fittings for connecting tube, pipe, and hose to pumps, valves, actuators, and other components.
Fittings for tubing larger than 1-in. OD have to be tightened with large hexnuts which, in turn, require larger wrenches to enable workers to apply sufficient torque to tighten the fittings properly. To install such large fittings, system designers must provide the necessary space to give workers enough room to swing large wrenches. In addition, worker strength and fatigue could be factors affecting proper assembly. Wrench extensions (cheater bars) might be needed for some workers to exert an applicable amount of torque.
Fittings manufacturers have designed split-flange fittings so that they overcome both of these problems. Split-flange fittings, Figure 6, use an O-ring to seal a joint and contain pressurized fluid. An elastomeric O-ring rests in a groove on a flange and mates with a flat surface on a port - an arrangement similar to the FFOR fitting. The O-ring flange is attached to the port using four mounting bolts that tighten down onto flange clamps, thus eliminating the need for a large wrench when connecting large-diameter components. When installing flange connections, it is important to apply even torque on the four flange bolts to avoid creating a gap through which the O-ring can extrude under high pressure.
The basic split-flange fitting consists of four elements: a flanged head connected permanently (generally welded or brazed) to the tube, an O-ring that fits into a groove machined into the end face of the flange, and two mating clamp halves with appropriate bolts to connect the split-flange assembly to a mating surface.
All mating surfaces must be clean and smooth. Joints are more likely to leak if either of the mating surfaces are scratched, scored, or gouged. Additionally, wear tends to accelerate on O-rings which are assembled against rough surfaces. Where perpendicular relationships are critical, all parts must meet appropriate tolerances. While 64-µin. surface finishes are acceptable, most flange manufacturers prefer and recommend 32-µin. finishes on mating surfaces to ensure leak-free connections.
In a properly designed split-flange assembly, the flange shoulder protrudes approximately 0.010 to 0.030 in. beyond the clamp face to ensure adequate contact and seal squeeze with the mating face, Figure 6. However, the clamp halves do not actually contact the mating surface. The most critical operation during assembly of a split-flange fitting to its mating surface is to make certain that the four fastening bolts are tightened gradually and evenly in a cross pattern. Air wrenches should not be used because they are difficult to control and can easily over-tighten a bolt.
Fully tightening one of the bolts while the others are still loose will tend to cause the flange to tip upward, Figure 8. This action pinches the O-ring, and the joint can then be expected to leak. When the bolts are fully tightened, the flanges sometimes bend downward until they bottom on the port face, and the bolts bend outward, Figure 8. Should flanges and bolts bend, they tend to lift the flange off the shoulder; once again, the result will be a leaking joint.
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