When you need to secure a nut onto a bolt or tie rod that’s more than 2 ft in diameter, you don’t just pull a wrench out of your tool cabinet. Even if you could, using screw threads to apply tension isn’t the most efficient or effective method. However, hydraulics is so adept at transmitting huge linear forces, it’s a no-brainer for this application.
Tentec Ltd. designs and produces hydraulic bolt tensioning equipment in West Bromwich, West Midlands, UK. Tentec won a contract to supply four massive hydraulic nuts for a major forging press refurbishment project by Forge Systems Inc., Spring, Texas. Used instead of familiar hex head nuts, hydraulic nuts use high-pressure fluid acting against a large piston area to produce axial force for tensioning bolts. Once the hydraulic nut has stretched the bolt to the specified amount, shims are installed to maintain bolt tension. Hydraulic pressure is then released, and the bolt tension is retained.
These four hydraulic bolts have been installed on a forging press and are the largest ever made by Tentec — and possibly by anyone. They fit a 25-in. threaded shaft and produce 8000 tons of axial force from 1500 psi hydraulic pressure. Tentec designed seals to deal with the challenge of deflection from high pressure applied to large surface areas.
Paul Egginton, Engineering Director at Tentec, said a hydraulic nut is essentially a hydraulic ram cylinder with a hollow piston rod and a threaded inside diameter. “Each hydraulic nut required for the project has 25-in. buttress thread forms, weighs 4 tons, and has an 8000-ton bolt load capacity. They are by far the biggest hydraulic nuts we ever manufactured.”
Four tie rods are fitted through top and bottom press platens, and the hydraulic nuts squeeze the assembly to hold the press together. The nuts are rotated on the tie rods, and the threads move them axially into place. Once the nuts are in place, pressure is applied, and the inward surface of each nut pushes on the press housing, applying tension to each tie rod in the process.
Egginton explained that each hydraulic nut has a maximum axial stroke of 38 mm, and it took about 15 min to reach an applied pressure of 1500 bar using an air-driven hydraulic pump. “The high pressure hoses and hydraulic chamber of the hydraulic nuts were already pre-filled with hydraulic oil, so the pump did not have to fill the nuts; it just needed to pressurize the hydraulic oil that was already in the chamber and hoses.”
A sealing challenge
Egginton said tensioning tools must work at high pressure in order to be effective within the constrained space of bolted flanges. “The design of these large hydraulic nuts is no different from our more standard sized hydraulic nuts, and our in-house seal technology was scaled up with very successful results. Our seal technology has been derived over 20 years and is extremely reliable. The seals are machine cut from polyurethane and are fitted with an anti-extrusion ring to prevent the seal from extruding down the side of the ram-cylinder interface.”
Egginton noted a particular challenge in dealing with the deflection caused by applying high pressure to large areas. “As the cylinder walls deflect under pressure, the anti-extrusion ring follows the wall, effectively filling the gap between the ram and cylinder wall to keep the polyurethane seals from extruding and failing.”
For here or to go?
For this application, the hydraulic nuts remain in place, although Tentec also makes a detachable version. This type is attached to a standard threaded stud or bolt fitted with a conventional nut. The conventional nut is first threaded onto the bolt, then the hydraulic nut is threaded onto the portion of the bolt protruding past the hex nut. The hydraulic nut is pressurized, effectively stretching the bolt. With pressure maintained on the bolt, the hex nut is rotated down onto the bolted joint face and hand tightened. Hydraulic pressure is then released, and the hydraulic nut can then be removed and used for other bolted applications.
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