INTERNATIONAL DESIGN CONFERENCE - DESIGN 2012Dubrovnik - Croatia, May 21 - 24, 2012.

ANGULAR TIGHTENING

K. Lindblad

Keywords: angular tightening, turn of the nut method, bolt tightening

1. IntroductionIn order to decrease the clamping force variation in a bolted joint a new way of specifying tightening of bolts was introduced shortly after the Second World War. This method is called Angle Controlled Tightening, also known as turn of the nut method, which results in a stretching of the bolt beyond the elastic range into the plastic range. These types of bolts are called Torque To Yield (TTY) bolts, also referred to as stretch bolts, and are used predominantly for head bolts in modern engines, main bearings and big end caps. This method can be applied with either an ordinary torque wrench or a more sophisticated DC-tool, often used in modern assembly plants. The benefit from using this method is its insensitivity to friction variation, the main parameter being the yield stress tolerance. Friction is a parameter which naturally suffers from relatively high variation and is very sensitive to contamination, such as thread lock or other liquids, while the yield stress varies very little from bolt to bolt. Angle Controlled Tightening is a way of utilizing the bolt to its maximum in a controlled fashion, giving the designers more control over the preload scatter, enabling weight and cost savings as well as adding more design freedom as joints may require fewer bolts.

2. BackgroundOne of the most common machine elements in the world is bolts and nuts, but the knowledge on how to use these optimally is very low. The most important parameter in the design of a robust bolted joint is the preload, and the factor that influences this the most is the control of the tightening process. The main objective of a bolted joint is to fasten parts together; it has to ensure fatigue resistance and friction grip between the part surfaces. A bolted joint with an incorrect preload will cause problems, no matter if it’s too low or too high. Some of the problems are joint separation, joint slip, static failures in either fastener or the joint parts, bolt fatigue failure, vibration loosening of the fastener etc. Quality problems in products due to poor joint design and assembly processes is something engineers in different kinds of industry branches face all over the world. A joint that is dimensioned with high variances in preload during the assembly process only adds unnecessary weight and cost to the joint. Saving both weight and cost in the motor industry is very important due to the high number of engines being built and their ambition to be as light as possible. This is why engines most often are designed utilising TTY head bolts, as they require fewer head bolts to achieve the desired clamping loads then those using conventional bolts. Fewer bolts also give the manufacturer more flexibility in the design of the cylinder head and engine block as well as reducing the cost of the engine.

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3. Two ways of tighteningThere are two basic ways of categorizing different tightening methods. Either the bolt is utilized in its elastic region or in its plastic region. The most common way of tightening a bolt is using a torque control method, having the bolt utilized in only the elastic region. This method generates high scatter in the preload values due, mostly due to the high scatter in the friction coefficient between the threads and bolt head. Using air driven tools can also increase the scatter on top of that, as the torque applied by the tool can differ +/- 15%, which is considered normal deviation in an air tool. This leads to a total preload scatter up to +/- 33%, with lower accuracy assembly tools the preload scatter can increase up to +/- 50% of the nominal value. In order to increase the preload accuracy a different method is used, called the torque and angle method, which tightens a bolt into its plastic region. Since the main parameter of the preload scatter is the yield characteristic of the bolt instead of friction, the preload scatter is greatly reduced. The downside to this method is that since the bolt is deformed plastically it is not recommended to retighten the same bolt several times. In some cases no retightenings are allowed at all, to be on the safe side. It is also very difficult to define the tightening angle.

2.1 Using DC-tools

Sophisticated DC-tools have the ability to automatically tighten to a pre-set torque value and then continue to tighten the bolt to a certain angle. This can be very useful in environments where numerous bolts have to be tightened by the operator, such as assembly lines in a large factory.

2.2 Digital torque wrench

Some modern digital torque wrenches have a built in "torque and angle mode" where the torque wrench only starts to measure angle after a pre-set torque is reached. An example of this is the Snap On ATECH3FR250 which can be seen below.

Figure 1. Digital torque/angle wrench

Since this requires the operator to tighten the bolt manually this method of tightening is more used in small workshops or places where you don’t need to tighten a large number of bolts.

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2.3 Analogue method

A low cost method is using an ordinary analogue torque wrench to the specified torque and then measuring the angle either by making a small mark both on the bolt head and the joint surface and tightening it further by approximating the angle between the marks or by some kind of analogue angle measurement device (Figure 2).

Figure 2. Example of an analogue angle measuring device fitted on a ratchet.

3. Tightening strategy In order to ensure that all gaps are closed and gaskets or other parts have been fully compressed a certain “snug torque” is applied before applying the tightening angle. If there were to be gaps or still compressible gaskets the tightening angle would not be used correctly and wasted on just clamping the parts together instead of plastically deforming the bolt. It is also important to set a low enough snug torque in order to avoid plastic deformation before the angle is applied. In cases of low friction contamination a high snug torque is most likely to cause plastic deformation in the bolt.

4. Tightening angle In general, the snug torque constitutes the starting position of the angular part of tightening. An appropriate tightening angle should be chosen so that it in all cases brings the bolt over its yield point. The applied angle must not be too high, as this will decrease the reusability and also increase the risk of fracture in the bolt. Since the torque is influenced by friction, while the geometry dependent

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tightening angle is said to be almost immune to frictional effects, it is suggested that the snug torque should be chosen as low as possible.The snug torque (Msnug) is the initial torque before applying the tightening angle. A figure illustrating the torque against tightening angle is shown in Figure 3. The torque required closing the gap (Mgap) and the torque to begin plastic deformation (My) of the bolt is shown in the figure below.

Figure 3. Torque vs. angle

Friction and poor tightening accuracy are parameters that greatly influences the tightening torque, but the tightening angle is mostly dependant on the yield stress tolerance of the bolt and almost independent of friction and tightening accuracy. Therefore, an appropriate level of the minimum snug torque, and consequently the minimum snug preload, should be set somewhat higher than the torque and force required to clamp the parts together. This can result in a relatively low snug torque and a relatively high tightening angle. The tool used to apply the snug torque must have high enough accuracy to not deviate to higher or lower torque than the maximum or lower snug torque respectively.

5. Conclusions Using the torque and angle method of tightening bolted joints is a very good and robust way to ensure a proper preload with low deviation. It can be applied both in small workshops with simple analogue tools and in automotive assembly plants with sophisticated DC-tools. When designing a joint, knowing that the clamping force will not scatter significantly, both weight and cost can be lowered. Bolts that have been tightened into the plastic range are not recommended to be retightened too many times. In critical joints these bolts should never be retightened.

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6. References [1] http://www.hexagon.de/tasignat.htm (2011-11-30)[2] http://www.sbi.se/omraden/o_dokument.asp?mId=3&kId=72&subKId=0&mgrp=&dId=144 (2011-11-28)[3] Göran Toth, 2006, Torque and Angle Controlled Tightening of Bolted Joints, Chalmers University of Technology, Sweden[4] http://en.wikipedia.org/wiki/Torque_wrench (2011-11-29) [5] http://www.enginebuildermag.com/Article/2343/threaded_fasteners_torquetoyield_and_torquetoangle.aspx (2011-11-27)[6] http://www.rv8.ch/article.php/20051227112926716 (2012-01-13)[7] http://www.precisionenginetech.com/tech-explained/2011/07/04/snap-on-torqueangle-wrench-review/ (2012-01-13)

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