An Overview of Hydraulic Nuts

Hydraulic nuts are capable of producing high bolt loads in small, compact spaces.   They are designed to replace existing torqued hardware and remain part of the bolted joint after installation. All hydraulic bolt tensioners, including hydraulic nuts, produce purely axial loads that are independent of stud twist and frictional effects that lead to unpredictable bolt loads when using conventional torque methods.   Using hydraulic nuts allows for more accurate preload control and improved repeatability from stud to stud and operator to operator. They can be applied in tightly restricted areas that do not have enough space for standard wrenches or long heater rods.   When proper tensioning practices are used, we have seen preload accuracy within 5% of the target. Conventional torqued hardware can have preload scatter as high as +/-25%.
Hydraulic Nuts from Riverhawk

Hydraulic Nut Installation and Operation

Installation and operation of hydraulic nuts is simple.   In most cases the hydraulic nut will fit within the same space as your existing hardware and does not require any additional thread length. The hydraulic nut is screwed onto the stud and connected to a hydraulic pump (manual or air operated) using flexible high pressure hose.   When hydraulic oil is pumped into the nut it generates a large axial force. The force generated is proportional to the amount of pressure applied, the greater the hydraulic pressure the higher the force generated.   This force compresses the flanges and stretches the stud creating a gap between retainer and the hydraulic nut cylinder.   The operator turns down the retainer against the hydraulic nut cylinder and releases the hydraulic pressure. The retainer prevents the stud from returning to its original length.   Since the stud is stretched within its elastic region it has a natural tendency to return to its original length. This in-turn creates the clamping force that holds the joint together.   The whole tensioning process can be completed in less than 2 minutes.

Advantages of Using Hydraulic Bolt Tensioners

One significant advantage when utilizing hydraulic bolt tensioners is the ability to daisy-chain multiple studs together for simultaneous tightening. This provides several advantages such as even gasket crush for leak free sealing, uniform compression on bearing housing or connecting rods, and limits neighboring stud interactions for improved preload accuracy and decreased assembly times.   Our field service reports have indicated significant time saving when hydraulic bolt tensioning has replaced torqued installation methods (induction heating, torque multipliers, multi-bolt jacknuts, etc.). A splitline casing for a major compressor manufacture routinely took 6-8 hours to close using torqued hardware. Results were highly variable and they often had trouble passing hydrostatic leak testing. Failed hydrostatic tests required an additional tightening pass on all of the studs before the test could be repeated. This caused significant time delays and missed delivery dates. After switching over to hydraulic bolt tensioning the joint was completely closed in less than 2 hours. The increased preload accuracy and repeatability eliminated the leaking issues found during hydrostatic testing.

Custom Engineered Hydraulic Nuts

Riverhawk specializes in custom hydraulic nuts that are specifically designed to meet demanding customer requirements. Hydraulic nut material, thread forms, load capacity, stroke capacity, and corrosion protection coatings are a few of the custom options available. Riverhawk engineers work directly with our customers to determine the best solution for their most challenging applications. Riverhawk has a standard hydraulic nut line capable of producing preloads up to 45,000 psi thread stress for nominal thread diameters from 1.50” (38 mm) up to 7.00” (178 mm).

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— Written by Charlie Heysler —

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