The cushioning seal is active only at the end of the stroke. A pressure chamber, which is formed between cushioning seal and piston seal, works like an air pillow to absorb the kinetic energy smoothly. To ensure that the cushioning seal doesn&#;t affect movement when the cylinder reverses direction, the seal has an integrated check-valve function that forces the pressure to act on the full cylinder area. If O-rings are used as cushioning seals, the movement is delayed because the air pressure can&#;t act on the full cylinder area until the cushioning rod has moved out of the O-ring.
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A test program performed with commercially available pneumatic cylinders confirmed the material and design-development efforts. The cylinders were equipped with a rod seal-scraper, two piston seals and two cushioning seals. The test program included:
The test procedures reflected harsh, but realistic, operating conditions using oil-free compressed air with an oil content < 0.01 mg/m3 (class 1 to ISO -1). Testing monitored two indicators of seal functionality: the leak rate at two pressure values (29 psi and 145 psi) and the break-off pressure in both directions. The break-off pressure is the minimum pressure needed to move the cylinder. It&#;s different for outstroke and return stroke because the pressurized area of the cylinder is smaller on the rod side.
High-pressure test: This test reflects the situation at peak pressure during the cushioning cycle. To avoid having to produce and dissipate excessive energy, the test equipment included a pressure intensifier to generate an operating pressure of 360 psi. The cylinders have a short stroke of 0.59 in. to simulate only the cushioning cycles. Special high-pressure lines and high-pressure valves are used.
After 100,000 cycles, the sealing function wasn&#;t compromised by physical damage, seal wear or leakage. Break-off pressure was the same as when new. The sealing system proved its robustness and reliability even under extreme pressure peaks during the pneumatic end-cushioning cycle.
Bursting pressure test: This test confirms seal robustness and safety in case of abnormally high pressure. For the rod seal, this test shows the safe fixation by the retainer nose. The cylinders functioned at the target pressure of 725 psi without problem. The first leakage occurred at 942 psi, when the rod seal was partly extruded out of the housing, but no seals were damaged. After re-installing the rod seal, the cylinder was fully functional with a leak rate and break-off pressure the same as new.
Low-temperature test: This test shows the minimum temperature under which the cylinder functions properly. The cylinders were installed in a temperature chamber and connected to an external measuring station to enable measuring of leak rates and break-off pressure. An additional air dryer helped avoid water accumulation in the system. After cooling down the system, the measuring was done quickly to avoid frictional heating that would affect the result.
The low temperature limits of the piston seals depend on their location on the piston, although the two piston seals are identical. The piston seal at the bottom of the cylinder is the most critical position because it is measured with the rod extended, when the guidance clearance causes a misalignment between the piston and the cylinder tube. The piston seal at the rod side is measured with the rod retracted, when piston and cylinder tube are in good alignment. Under such good conditions, the piston seal works well at lower temperatures. So, the degree of misalignment leads to a low-temperature limit between -4°F (piston seal bottom side, rod extended) and -22°F (piston seal rod side, rod retracted). The rod seal is tight down to -40°F because the volume shrinkage from temperature reduction helps to keep the lip in sealing contact.
To summarize, testing showed that the pneumatic cylinder remains fully functional down to -4°F under worst-case conditions, and down to -13°F under normal conditions.
Temperature cycling test: This test cycles between minimum and maximum operating temperature to show the seal material&#;s ultimate behavior. The test equipment was the same as for low-temperature testing. Temperature cycling revealed no leakage and no change in break-off pressure, which proves that frequent changes of operating temperatures don&#;t affect the service life.
Minimum speed test: This test explores a cylinder&#;s operation at creep velocity. The cylinder, equipped with a side-load weight, is pressurized on one side while a throttle valve on the other side is closed until the cylinder begins to exhibit stick-slip mode.
This test is relevant to polyurethane because its higher modulus gives higher stick-slip tendency compared to nitrile-butadiene rubber. The cylinder is connected to a position-measuring system, which records the movement . After five cycles of running-in, the air outlet throttle is closed stepwise until the minimum speed without stick-slip is found.
For a moment, the cylinder stopped (v = 0). That&#;s already understood as stick-slip, although usual stick-slip effects are found as a permanent alternation between stick and slip and produce a vibration noise.
There is is no question that pipe threads should not be specified in new equipment deisgns. Pipe threads are prone to leakage, especially after being disassembled and reassembled. Furthermore, many more-modern thread forms are widely available that offer long-term, leak-free performance, even after being assembled and reassembled several times. Still, despite their poor perfomrance, pipe threads continue to be used throughout a variety of industries. So accepting that pipe threads will still be encountered, this discussion reviews methods for reducing shortcomings of pipe fittings.
Four methods are commonly used to seal pipe threads:
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Yielding metal. The sealing interface is limited in area and unlimited in force so that yielding takes place. Metal flow fills misalignment and leak paths. These dryseal joints can be effective, but they usually cannot be disassembled and reused without leaking.
Trapped dope. The use of drying or non-drying dopes is the oldest and least costly thread-sealing method. Made from ingredients ranging from crushed walnut shells in shellac to other fillers and oils, usually with some thinning volatiles, they are inherently weak, and will shrink when the volatiles evaporate.
Trapped elastomer. Confined O-rings can seal effectively, but also can suffer from sloppy assembly. Damaged threads or pinched rings also can contribute to leakage. O-rings typically are used in high-pressure fluid power systems where the extra cost is more easily justified and freedom from contamination is especially desirable.
Curing resins. Sometimes called machinery adhesives, these anaerobic acrylic materials develop strength by curing. They are very forgiving of tolerances, tool marks, and slight misalignment. They make tapered fittings as effective as O-rings at a fraction of the cost. They lock free-standing fittings &#; such as gauges. They can also improve the 98% effectiveness of yielding-metal joints to 100%. The correct grade must be selected because of their wide range of strengths so that disassembly will not be hampered.
Curing materials are so effective in sealing threads that they are often used on straight threads that enter or plug pressure vessels. In addition, the curing materials are effective even when tapered threads are lightly torqued. Lightly torqued threads (straight or tapered) do not leave high residual stresses in housings or valves that can distort valve bodies to the point of inoperation or long-term fatigue failure.
Probably the most significant event in sealing fittings has been the development of anaerobic pipe sealant with TFE materials. Since the first appearance of these materials, many companies have added anaerobic thread sealants to their lines. The new sealant technology offers a variety of benefits:
Convenient curing. Being anaerobic, it cures in the absence of the air, remaining uncured until the parts are assembled. There is no evaporation, hardening beforehand, or other work-life problems.
Lubricity. Containing TFE filler, the material eliminates galling or other component-assembly problems. These products prevent over-torquing to affect a seal.
Fills threads. Due to high wetting ability, the material fills threads so well that leakage past nicks, scratches, and dents does not occur.
Fitting movement. Systems being assembled with anaerobic sealant can be initially readjusted without breaking the seal in the threads.
Vibration resistance. Anaerobic sealant does not permit a fitting to be loosened by vibration. Reusability. Fittings sealed with acrylic and latex-based materials can be disassembled and reused with sealant in the field without danger of leakage.
Freedom from contamination. Unlike the tape most often replaced by the anaerobic material, sealant does not break up to contaminate lines and valves.
A review of the important performance properties of compounds of tetrafluoroethylene (TFE) resin and filler materials shows that the resin performs well in many applications without fillers. In fact, fillers can lessen TFE's outstanding electrical and chemical properties. In mechanical applications, however, compounds of TFE and inorganic fillers offer improved wear resistance, reduced initial deformation and creep, and increased stiffness and thermal conductivity. Hardness is increased, and the coefficient of thermal expansion is decreased. These compounds can make it possible to gain the advantages of TFE in applications where the unfilled resin cannot be used.
Many different fillers can be blended with TFE, but most application requirements have been met with five filler materials: glass fiber, carbon, graphite, bronze, and molybdenum disulfide. The properties of any compound depend on filler type and concentration, and processing conditions. Compounds &#; such as plain TFE &#; are made into finished parts by molding, extrusion, or machining.
One example of the application of TFE resin and fillers is O-rings made of TFE. They are used where resistance to solvents and other chemicals, or extremely high- or low-temperature resistance is required. These are applications where elastomeric materials are not suitable. An additional benefit of TFE O-rings, in certain applications, is the material's low coefficient of friction and anti-stick properties. Typical applications are rotary, piston, and valve seals, and gaskets.
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