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Bolt Tensioning & Hydraulic Stud FAQ

Common Bolt Tensioning & Hydraulic Stud Questions

  1. Answer: HTI's Hydraulic Stud and Bolt Tensioning Systems are the simple and efficient answer to common stud and bolt tightening problems, and provide:

    • Accurate and Repeatable Stud Loading – Engineers design a flange or joint knowing the stud tension necessary for it to function as intended. Under–tightened, over–tightened or unevenly–tightened studs can lead to the failure of the joint. Bolt tensioning produces a known tension in the stud.

      Accurate stud tension is difficult to attain using torque, because friction has a significant effect on the torque–tension relationship. For example, the torque required to stretch a 2"–8UN stud to 35% of its yield strength varies between 1,400 ft–lbf (using a moly–based lubricant) and 8,360 ft–lbf ("dry", with no lubricant). If the operator failed to lubricate the threads and nut surface and torqued dry, the resulting stud tension would be only 6% of yield (Tension = 17,100 lbf) instead of 35% of yield (Tension = 102,300 lbf)!

    • Fast Operation – Using Hydraulic Stud Tensioners is usually faster than using hydraulic torque wrenches. Compared to torque, using one Tensioner for every four studs on the flange, or "25% cover" will typically cut the tightening time by fifty percent (50%). With one Tensioner for every two studs, or "50% cover", the time can be cut by an additional one–third (33%).

    • Even Flange Closure – Using multiple Hydraulic Stud Tensioners on a bolt pattern allows even flange closure and even gasket loading. No pinched gaskets or over–stressed studs, which is possible when using one torque wrench and a star torque pattern.

    • Gall–Free Operation – High levels of torque, especially in studs over 2–1/2" diameter, can produce thread galling and nut–surface galling. In the Hydraulic Stud Tensioning process, the nut is not under load and turns freely. Galling is not a problem.

    • Conformation to the Specification and Design Intent – If there is extra thread protruding above the nut (for example, for a 2"" stud there would be about 2" of thread) the application was intended to be tensioned. Applications with either round nuts or drilled, hexagon nuts are also designed for Hydraulic Stud Tensioning.

    • Ease of Use – HTI's Hydraulic Stud and Bolt Tensioners have a very high power to weight ratio, which is especially important for larger stud sizes. The operator handles the Hydraulic Stud Tensioner parts (Load Cell, Puller Bar and Bridge) separately. The weight of any one part can be significantly less than the weight of the hydraulic torque wrench for the same stud size.

  2. Answer: Hydraulic Stud Tensioners are frequently used to loosen, or breakout, studs. It is important to protect the stud threads from damage and from corrosion during the time between makeup and breakout. The Hydraulic Stud Tensioner eliminates nut–to–flange friction, but you can only apply a limited torque to break the friction between the stud and nut threads. The easiest way to protect the exposed threads is to lubricate them and to then thread an additional nut over them.

  3. Answer: You can tension most applications with one Hydraulic Stud Tensioner. On applications with several studs; however, HTI recommends using enough Hydraulic Stud Tensioners to provide between 25% and 50% "cover". This means one Hydraulic Stud Tensioner for every fourth, or every other, stud.

    On circular flanges, HTI recommends using at least four Hydraulic Stud Tensioners and, preferably, 25% or 50% cover. For example, on a flange with 36 studs, use four, nine (25% cover) or eighteen (50% cover) Hydraulic Stud Tensioners. Using this number of Tensioners gives the benefit of even flange closure and speeds up the job. Some critical applications require 100% "cover".

  4. Answer: Plan to have at least one (1) diameter (Example: 2" of threads above the nut for a 2" stud) of thread protruding above the nut. In some applications we can get by with less than one (1) diameter, but seldom with less than three-quarters (3/4) of a diameter.

  5. Answer: When you tension a stud with a Hydraulic Stud Tensioner, the stud acts like a spring and gets longer. For example, the 2"– 8 studs in an ANSI 20 x 900 flange have a grip length of 11" (279 mm). At 50% of yield in a B16 stud, the residual tension in this stud is 146,200 lbf (649 kN). This tension produces an elongation of 0.019" (0.48 mm).

  6. Answer: Most of HTI's Hydraulic Stud Tensioners have dual Hydraulic Ports even though they are single–acting, because Dual Ports simplify the Hydraulic Hose connections. Typically, a longer Hose (15' or 5m) connects from the Hydraulic Pump to the first Tensioner. Additional Hydraulic Hoses interconnect the Hydraulic Stud Tensioners. These Hydraulic Hoses are available in any length, are supplied complete with Hydraulic Quick–Disconnect (QD) Fittings and are pre–charged with hydraulic oil. They have rated working pressures consistent with the working pressure of the Hydraulic Stud Tensioner.

    Single Hydraulic ports are sometimes an advantage. For these applications, HTI will supply the Hydraulic Stud Tensioners with a single port and the Hydraulic Hoses with Tees.

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