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Torque talk: A primer on hydraulic torque wrenches and their use. (Features).

In maintaining equipment or machinery, no one would argue that the simplest components are nuts and bolts. Similarly, most recognize that incorrect use of these basic components can lead to maintenance headaches at best and/or disasters at worst. The physics of fastening devices have been studied closely over the past decades, leading to improved methods of measuring and applying correct force. For many years, manual torque wrenches were the primary tool for applying the specified force to bolted connections.

In the early '70s, larger-scale projects and increased interest in improving worker productivity led to the development of the hydraulic torque wrench. The design of this tool translated the power of linear hydraulics into an arcing motion to generate the torque necessary to turn nuts and bolts. Now, maintenance and machine assembly crews can more safely generate thousands of foot-pounds of repeatable torquing force to fasten or loosen very large bolts within a confined area or a precarious location. This same power can be used to break loose heavily corroded bolts for disassembly.

In recent years, demands have increased on the simple bolting operation. There has been increasing focus on record-keeping and traceability to verify safety and environmental reliability of the project. The control and measurability of hydraulic torque wrenches have helped make this level of inspection possible.

Complex or critical fastening applications 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 structural support member to come apart or pipeline flange gaskets to leak. Tighten a bolt too much and the stress can cause the bolt to eventually break.

Recent developments in versatility have made hydraulic torque wrenches an even greater asset for reliable bolt assembly. The inline wrench is one of the newest developments in hydraulic torque wrenches. Basic elements of the wrench are the drive gear, drive pawl, body--which contains the hydraulic cylinder--and reaction device. The in-line wrench also features a ratcheting wheel with a hexagonal hole cut to the size of the nut that it has to fit. The ratchet wheel, drive pawl and lever arm are part of a replaceable cassette that is exchanged for different bolt sizes. The change-out can be accomplished without using tools.

With previous hydraulic wrench design, the reaction arm was clamped onto adjacent bolts during the bolting. The body of the in-line wrench has a reaction pad, which pushes against any adjacent solid object to provide a reaction point.

The centerline of the hydraulic cylinder forms a 90[degrees] angle with the centerline of the wrench arm when the cylinder is at midstroke. This design provides maximum torque at cylinder midstroke for each hydraulic pressure setting. For maximum, safety, the reaction forces are contained in-line to prevent the wrench from pulling or twisting off the bolt under load.

Swivel couplings allow the hoses to rotate for easier positioning of the wrench. Low body height, high torque-to-weight ratios, and flatter designs allow use of the wrench in restricted areas too tight for manual methods. Wrench bodies can fully enclose the wrench arm to prevent objects or debris interference while in operation.

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

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

The power of a hydraulic wrench is determined by three factors: hydraulic fluid pressure in the cylinder, cylinder position arc, and wrench arm length. For example, an Enerpac SQD 50 torque wrench with a maximum torque capacity of 3,550 ft-lb will generate 2,100 ft-lb at 7,200 lb/[in.sup.2]. An SQD 100 using the same pressure will generate 4,600 ft-lb with a maximum of, 7,360 ft-lb at 11,600 lb/[in.sup..2].

Hydraulic-powered torque wrench productivity is based upon the flow of the pumping unit, the volume required to extend and retract the hydraulic cylinder, and the degree of turn of the nut per cylinder stroke. Using a pump with a greater flow rate can increase the speed of operation for a hydraulic torque wrench. In 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 precision called for by today's projects is one of the prime features of hydraulic torque wrenches. At an accuracy of 1-3%, hydraulic torque wrenches are clearly superior to sledgehammers, striking wrenches (which have no control), or even to pneumatic impact wrenches, which have limited torque control.

For most projects, the necessary amount of torque for each fastener will be calculated in advance by the designer. Some cases will need an estimate, such as equipment maintenance, when specifications do not exist. One method is to tighten a sample of bolts, then use a calculator to find the mean and standard deviation.

The best approach, when possible, is through prior experience. Continue to use the same level of torque if it has proven to be satisfactory. If not, increase or decrease the torque by 10% and record the amount. Repeat these incremental changes until the proper level of torque is reached. If there is no prior experience to draw from, refer to the fastners table available from the manufacturer.

To achieve the highest level of accuracy, there are a number of factors that can affect hydraulic fastening inherent to threaded bolts. Before covering these, first consider the basic anatomy of a bolting operation.

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 generated by torquing the bolt, causing it to rotate, tighten, or loosen. Because of the bolt's threads' resistance against the threaded grooves in its hole, the bolt is literally stretched. Since it wants to return to its original condition, the bolt, with the help of its head or nut, clamps both materials together.

Stress increases on a straight line for the bolt and must stop when the yield is reached--the point at which permanent deformation in the bolt takes place, possibly leading to breaking of the bolt. Insufficient clamping force allows the nut to vibrate loose, causing flanges to leak or structural parts to detach. Excessive clamping force leads to gasket or flange damage or bolt galling.

Fastening is one part of the bolting equation. The formidable challenge of diassembly for maintenance operations is as (or more) important as fastening.

Removal of corroded bolts is often an inexact science, all too frequently approached with tools of questionable performance and safety. Often, frustration leads to cutting the bolt head or nut. Significant torque--generally 150-200% of tightening, sometimes more--is needed for bolt removal, as opposed to fastening.

Torque calculations run up against the real world, and in this world the torque-preload relationship and accuracy can be impacted by what has been referred to as the "nut factor." About 70% to 90% of the energy required to tighten a bolt is to overcome friction in the joint. One major determinant is lubrication of the nut surface. Steel on steel can create tremendous friction and resistance, compared to a bolt assembly that has been lubricated. Small changes in friction will result in large changes in bolt tension. Changes can be large enough to cause bolts to be tightened below specification or beyond safe load limits despite the application of the same torque to the same design fasteners. Hole sizes and burrs on the threads also can affect friction.

Lubrication not only can play an important role in overcoming friction, but also will have a direct effect on the bolt's ductility. More ductile bolts can be stretched beyond yield before failure tension, a situation that calls for a reduction in torque. Friction is just part of the picture. Dozens of variables exist, including the surface finish, the bolt hole, the fit of the wrench on the nut or bolt, and the number of times a fastener has been used.

With all of these factors affecting torque, hydraulic torque wrenches generally remain the most productive and accurate method of tightening bolts, particularly large bolts. Bear in mind hydraulic wrenches can handle bolts as small as 1/2-in, hex. The key to achieving hydraulic torque wrench accuracy is to reduce the effect of variables with a basic list of operating steps, which are as follows:

* Adequately train and supervise the work crew when hydraulic torque wrenches are in use;

* Be sure that fasteners are in reasonable shape. (Wire-brush threads if they are dirty and/or rusted. Chase threads with a tap or die if damaged. Use hardened washers between the nut or bolt head and the joint members.);

* Use clean, fresh lubricant, and apply it consistently. (Apply the same amount to the same surfaces of each part);

* Clean or chase the threads if the nut or bolt cannot be run down by hand;

* Hold wrenches perpendicular to the axis of the bolts;

* 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 first and then toward the free edges of a rectangular pattern. Work in a cross-bolting pattern on circular or oval joints; and

* Most importantly, keep thorough records of the tools, operators, procedures, and lubricants. This is particularly important for maintenance purposes, both in terms of consistency and clarity.

Though the above indicates that bolting is still an art, hydraulic torque wrenches are 30% more accurate than mechanical torque multipliers, air tools, electric nut runners and other more traditional torquing technologies. While the primary complaint against using torque wrenches is their cost, the accuracy the user buys pays off in greater reliability of bolted joints and greater peace of mind. Moreover, hydraulic torque wrenches provide your project the power, versatility, productivity and documentation to meet the demands of construction, equipment assembly and maintenance.
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Author:Raether, William
Publication:E&MJ - Engineering & Mining Journal
Geographic Code:1USA
Date:Dec 1, 2001
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