Hydraulic torque wrenches improve bolting

The simplest components to maintain on equipment and machinery are nuts and bolts. However, the incorrect installation of these basic components can lead to maintenance headaches at best and disasters at worst. Through most of the industrial revolution, as machinery and the structures that housed them grew more complex, the technology of bolting stayed the same.


Key Concepts


  • Hydraulic torque wrenches can safely generate large bolting forces.

  • Bolting torque is repeatable and accurate when using hydraulic wrenches.

  • Lubrication can have a dramatic effect on bolt clamp loads when torque is consistently applied.

    Wrench power
    Repeatable torque

    Anatomy of bolting

    The simplest components to maintain on equipment and machinery are nuts and bolts. However, the incorrect installation of these basic components can lead to maintenance headaches at best and disasters at worst.

    Through most of the industrial revolution, as machinery and the structures that housed them grew more complex, the technology of bolting stayed the same. Over the past several generations the physics that explain these fastening devices were closely studied, resulting in improved means of measuring and applying force consistently and correctly with manual torque wrenches.

    When hydraulic torque wrenches were developed, construction, maintenance, and machine assembly crews could safely generate thousands of foot-pounds of repeatable torquing force. They can tighten or loosen very large bolts within a confined area or a precarious location (Fig. 1). This same power can be used to break loose heavily corroded bolts for disassembly.

    Complex and critical machine and process functions depend upon bolts tightened within the specifications of the bolt's material, the material being joined, and the function to be performed. If a bolt is tightened too little, vibration can cause a member on a machine to come apart or pipeline flange gaskets to leak. Tighten a bolt too much and the stress can cause the bolt to eventually break.

    The variety of pump designs to deliver pressure to these wrenches results in versatile positioning and usage. Electric-powered pumps are the common choice, especially for interior applications. In dangerous environments compressed air can be used. Lighter weight pumps are continually being developed for use in hard-to-reach areas.

    Regardless of the power source, a pump-mounted relief valve enables accurate torque adjustment and precise repeatability. Digital gauges provide the highest level of accurate measurement of pressure delivered by the pump for greater control.

    Wrench power

    The power of a hydraulic wrench is determined by three factors: hydraulic fluid pressure in the cylinder, cylinder arc position, and wrench arm length.

    Hydraulic-powered torque wrench productivity is based on the flow of the pumping unit, the volume required to extend and retract the hydraulic cylinder, and the degree of turn-of-nut per stroke of the cylinder. Using a pump with a greater flow rate can increase the speed of operation for a hydraulic torque wrench.

    When selecting a pump, use the performance curve as a guide in determining the flow rate for estimating torque wrench speed. When sized correctly, a hydraulic torque wrench can cut many hours from typical bolting operations.

    The hydraulic torque wrench is the tool to use for applying the specifications for torquing bolts. The precision called for by today's projects is one of the prime features of hydraulic torque wrenches (Fig. 2). At an accuracy of

    Removal of corroded bolts is often an inexact science, all too frequently approached with tools of questionable performance and safety. Significant torque, generally 150% to 200% of tightening torque or more, is needed for bolt removal as opposed to fastening. This torque is easily generated by hydraulic wrenches.


    Torque preload and accuracy can be impacted by what has been referred to as the "nut factor." One of the major determinants is the lubrication of the nut surface. Steel-on-steel can create tremendous friction and resistance, versus a nut that has been lubricated.

    About 70% to 90% of the energy required to tighten a bolt is to overcome friction in the joint. Lubricants have a dramatic effect on this friction value. Tremendous contact pressures, up to 250,000 psi, can be reached to overcome friction.

    Small changes in friction result in large changes in bolt tension. Changes can be large enough to cause bolts to be tightened below specification or past safe load limits despite the application of the same torque for the same design fasteners. Hole sizes and burrs on the threads also affect friction.

    Friction is just part of the picture. Other variables exist, ranging from the parts' surface finish, the bolt hole, or the fit of the wrench on the nut or bolt, to the number of times a fastener has been used. Some variables affecting torque are associated with lubricants, including the type used as well as its temperature.

    With all of these factors affecting torque, hydraulic torque wrenches generally remain the most productive, accurate method of tightening bolts.

    Repeatable torque

    The key to achieving hydraulic torque wrench accuracy is to reduce the effect of variables by following some basic steps.

    • Adequately train and supervise crews.

    • Ensure fasteners are in reasonable shape. Wire-brush threads if dirty and rusted. Chase threads with a tap or die if damaged.

    • Use hardened washers between the nut or bolt head and joint members.

    • Use clean and fresh lubricants. Apply consistently, the same amount to the same surfaces by the same procedure.

    • Clean or chase threads if the nut or bolt cannot be run down by hand.

    • Hold wrenches perpendicular to the bolt.

    • Use adequate reaction points to prevent tools from twisting or cramping as a result of cocked or yielding reaction surfaces.

    • Tighten multiple fasteners from the center of the pattern toward the free edges. Work in a cross-bolting pattern on circular or oval joints (Fig. 3).

    • Keep records of the tools, operators, procedures, and lubricants used. This is important for maintenance purposes, both in terms of consistency and clarity.

      • More Info: The author is available to answer questions about hydraulic torque wrenches and bolting. He can be reached at 414-247-5412. Article edited by Joseph L. Foszcz, Senior Editor, 630-288-8776, jfoszcz@reedbusiness.com

        Anatomy of bolting

        The purpose of a bolt is two bring two pieces of material together. The clamping force a bolt exerts on the joint is the preload in the bolt, generated by torquing the bolt, causing the bolt to tighten down or loosen up. Because of the resistance of the bolt's threads against the threaded grooves, the bolt is literally stretched. Since it wants to return to its original condition, the bolt, clamps components together.

        Stress increases in a straight line for the bolt and must stop when yield is reached, or the point at which permanent deformation in the bolt takes place, possibly leading to breaking of the bolt. Not enough clamping force allows nuts to vibrate loose. Too much clamping force leads to gasket damage, bolt galling, and flange damage.

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