An improved understanding of the physics of fastening devices has led to advances in the measuring and application of the rotational force – or torque – required to fasten, and unfasten, them. The chief tool for applying torque has thus far been the manual torque wrench, a tool that converts the power of linear dynamics into an arcing motion to generate the force necessary to turn fasteners. Due to the success of its method, the torque wrench has remained highly popular since its development in the early 70s. Now, as new demands arise concerning bolting operations – including the need for more accurate record keeping and the desire for traceability to verify safety and environmental reliability – the torque wrench is being updated to meet their needs.

The more complex a fastening application, the more critical it is that the fastener be tightened within the strict specifications regarding the material the bolt is made of, the materials that are being bolted together and the function that the fastening application is meant to perform. If they are not sufficiently tight, bolts can be subject to vibrations that could cause the structural support to come apart. If they are too tight, bolts are liable to break. For these reasons, recent advancements in versatility have made the torque wrench more important than ever.

Among these advances is the in-line torque wrench. In-line wrenches contain a drive gear, drive pawl, a body containing a hydraulic cylinder and a reaction device. It also boasts a ratcheting wheel with a hexagonal hole cut to the size of the nut that it needs to fit and which can be modified without the use of tools. In addition, the reaction pad on the body of the in-line wrench pushes against any adjacent edge to provide a reaction point, as opposed to earlier wrenches that simply clamped onto adjacent bolts. This design allows for maximum torque force at mid-stroke for each hydraulic pressure setting. For safety purposes, the reaction forces are contained in-line to prevent the wrench from pulling or twisting off the bolt under load.

The in-line wrench is also a marvel of efficient design. To begin with, it features swivel couplings that lets the hydraulic hoses pivot for easier positioning of the wrench. Furthermore, its lighter weight, high torque-to-weight ratio and flatter design let it get into spaces considered too tight for its predecessor. The latest body designs fully enclose the wrench arm to protect it from debris. The in-line wrench has a greater variety of pump configurations as well. Those with electric-powered pumps are best for interior applications. Those with compressed air are workable where an electric current would be unsafe. Devising lighter pumps has become a focus of torque manufacturers so that they can fasten bolts in tighter, more difficult-to-reach areas.

In the case of hydraulic-powered torque wrenches, productivity is based on flow rate. This is the volume required to extend and retract the hydraulic cylinder and the degree the nut turns with each stroke of the cylinder. In-line wrenches that have pumps with greater flow rates work faster. In choosing the best pump for the application, use the performance curve as a guide in determining the flow rate for estimating torque wrench speed. Sized correctly, a hydraulic torque wrench can trim hours off a typical bolting operation. Hydraulic torque wrenches are also exceedingly accurate. It should also be mentioned that with an exactitude between 1 and 3 percent, hydraulic torque wrenches are more precise than sledgehammers, striking wrenches or pneumatic impact wrenches.

It is best to calculate the amount of torque needed before a bolt is fastened. Tightening a sample of bolts and then calculating the average and standard deviation is the optimal approach. Of course, it pays to have earlier records of the torque used on the same job. If a certain amount of torque worked well on a particular job, it's a good bet that it'll do the trick again. If this amount proves to be less than satisfactory, decrease the torque by 10% and try again. Repeat until the optimum torque is reached. If there are no prior records, consult the table provided by the fastener manufacturer.

Keep in mind that removing the bolt is just as, if not more, important than fastening it and that this operation becomes increasingly problematic once the bolt is corroded. Lubricants make bolt removal easier. About 70% to 90% of the energy required to tighten bolts is just to overcome the friction in the joint. Lubrication lessens this friction and effects 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. A slew 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, just to name a few.

Considering all of the factors affecting torque, hydraulic torque wrenches are still the most accurate and most productive method for tightening bolts, especially large ones. In order to realize the greatest accuracy possible, it's important to adequately train and supervise the workers who use hydraulic torque wrenches. Maintenance procedures are applied to keep fasteners in good shape including lubrication application and thread cleaning. Keep in mind that the torque wrench should be held a vertical position to the axis of the bolts, multiple fasteners should be tightened from the center first of a rectangular pattern and adequate reaction points should be used to prevent tools from twisting and cramping from cocked surfaces. Finally, records should be kept on the torque wrenches, listing their respective operators, procedures and lubricants.
Despite their hefty price tags, the latest in torque wrench technology does make sense economically. The savings that companies realize through greater accuracy and reliability of bolted joints eventually will offset the investment.

Source: Torque Talk
William Raether
Engineering & Mining Journal, Dec. 1, 2002
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