Controlled Connections: Torque measurements are more than just a 'click'

Bolts, screws, and other forms of threaded fasteners are among the most common forms of fastening devices. Often great care is made in the selection of fasteners to enable the desired characteristics for an "engineered connection." However the integrity of the connection lies not only on the fastener and its characteristics but also in the correct installation and final application of torque to the fastener and clamped materials.

A secure but releasable connection is the primary task of a threaded fastener. Along with this, the fastener and connection must be able to withstand all specified or foreseeable operational and environmental loads and other such relevant factors. In simplest terms a fastener such as a nut and bolt must be tightened to provide adequate clamping force on the materials being joined in the connection. The tension load on the fastener must be high enough to prevent itself from loosening but not so tight that the fastener experiences deformation.

Many users of torque wrenches undoubtedly know how to adjust or use a torque wrench to reach a specific value. Few consider how that value was determined and that it is not a single value but the sum of multiple sources of resistance and friction coefficients. In order to establish a torque setting the sum of the following must be determined: the friction incurred by the movement of the fastener parts being mated, the friction incurred from movement of the fasteners against the materials being joined, and the energy required to load the fastener into its elastic range and adequately provide the ideal clamping force to the materials being joined.

There are a variety of recognized methods to properly secure or "torque" a fastener. The most widely specified and practised method is by tightening to a predetermined value. Another recognized method is that of angle-based fastening. This is where a connection is preloaded to a specified torque value and then an additional amount of angular rotation is applied to the connection to obtain the final setting. For the most demanding or critical applications, variations of a fastener elongation measurement method can be used. With this method the actual elongation of the fastener being tightened is measured and can be logged. This can be accomplished through the combined use of electronic sensors and modern software in the wrench or through the use of an external ultrasonic measurement means. Through such means and measurement techniques a greater portion of and a more accurate setting inside of the elastic range of the fasteners can be utilized.

The most common types of hand-operated torque wrenches are classified in the DIN EN ISO 6789 International Standard.

Indicating torque tool (Type I)

This form of torque wrench or torque screwdriver indicates a value equivalent to the torque being applied to a fastener. This form of indication can be accomplished in multiple ways. Some utilize a dial indicator. Others provide indication by means of a digital display.

Setting torque tool (Type II)

This classification covers the common "click" type torque wrench. These torque wrenches will signal the user that the preset value has been reached in a manner that can be heard, felt, or seen. This type of torque wrench can either be preset to a single value or adjustable within a range. Such torque wrenches are simple to operate and are commonly used in maintenance and repair environments.

When working with torque wrenches with preset or adjustable release mechanisms ("click type"), the accuracy of the final result can also be dependent on the proper application technique of the user. It is important that the user not only pulls the torque wrench smoothly but also with a consistent speed and minimal reaction time between the release or "click" and the point at which no additional load is applied to the fastener to avoid overtightening.

The allowable deviation between the preset or adjusted torque values must be inside of the tolerances as defined by the DIN EN ISO 6789 International Standard or the relevant National Standard.

Torque wrenches: proper characteristics and function

A torque wrench must be able to fulfill a variety of tasks. The ability to measure and verify fixed or varying torque values is just the beginning. Given the vast arena of possible operational environments a torque wrench should not only be precise but robust and easily adaptable to a wide variety of fastening applications.

An ever-increasing importance and value is being laid on increased accuracy and the ability to document the measurements being made. These higher requirements can be met with an advanced electronic torque wrench which can provide not only a high level of accuracy but also the ability to store, administer, and provide a platform for additional evaluation of the measurement settings and results.

Calibration

In order to assure that a torque wrench is capable of functioning within the required level of accuracy, it must be calibrated at regular intervals.

Calibration is often mistaken for the adjustment of a torque wrench. The calibration of a torque wrench is in simplest terms the operational comparison of measurements between a subject torque wrench against a master which is traceable to a known standard. The accuracy deviation also needs to be measured multiple times at several predefined locations if the torque wrench has an adjustable range to determine the overall deviation accuracy.

DIN 51309 further defines a process to calculate the calibration procedures measurement uncertainty. A periodic basic accuracy check of a torque wrench can take place between regular calibration intervals with the use of an appropriate and calibrated tester. The calibration of a torque wrench at regular intervals of time or usage in a nationally or internationally accredited calibration laboratory is of absolute importance. The accreditation of the laboratory by a National Authority in accordance to the DIN EN ISO 17025 standard (or equivalent) assures traceability compliance in accordance to the DIN EN ISO 9001 standard. AMT

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