Methods of Bolt Tightening

There are six main methods used to control the preload of a threaded fastener. Specifically:

1. Torque control tightening.

2. Angle control tightening.

3. Yield controlled tightening.

4. Bolt stretch method.

5. Heat tightening.

6. Use of tension indicating methods.

One of the major problems with the use of bolted joints is the precision, with regard to achieving an accurate preload, of the bolt tightening method selected. Insufficient preload, caused by an inaccurate tightening method, is a frequent cause of bolted joint failure. It is important for the Designer to appreciate the features and characteristics of the main methods employed to tighten bolts. Presented below is a brief summary of the major bolt tightening methods. Note however that whatever method is used to tighten a bolt, a degree of bolt preload scatter is to be expected.

There are six main methods used to control the preload of a threaded fastener. Specifically:

1. Torque control tightening.

2. Angle control tightening.

3. Yield controlled tightening.

4. Bolt stretch method.

5. Heat tightening.

6. Use of tension indicating methods.

Torque Control Tightening 

Controlling the torque which a fastener is tightened to is the most popular means of controlling preload. The nominal torque necessary to tighten the bolt to a given preload can be determined either from tables, or, by calculation using a relationship between torque and the resulting bolt tension.

When a bolt is tightened the shank sustains a direct stress, due to the elongation strain, together with a torsional stress, due to the torque acting on the threads. Most tables of bolt tightening torques ignore the torsional stress and assume a direct stress in the threads of some proportion of the bolts yield stress, usually 75%. For high frictional conditions the magnitude of the torsional stress can be such that when combined with the direct stress, an equivalent stress over yield can result, leading to failure. A more consistent approach is to determine the magnitude of the direct stress which, when combined with the torsional, will give an equivalent stress of some proportion of yield. The proportion commonly used with this approach is 90%.

Torque prevailing fasteners (such as Nyloc, Cleveloc nuts etc.) are often used where there exists a risk of vibration loosening. The prevailing torque has the effect of increasing the torsional stress in the bolt shank during tightening. This affects the conversion of the tightening torque into bolt preload and should be allowed for when determining the correct torque value for this type of fastener.

As can been seen by study of the above chart, a fundamental problem with torque tightening is that because the majority of the torque is used to overcome friction (usually between 85% and 95% of the applied torque), slight variations in the frictional conditions can lead to large changes in the bolt preload. This effect can be reduced by the use of so called friction stabilisers. These are substances which are coated onto the fasteners to reduce the frictional scatter. Other ways to improve the accuracy of the method are:

1. Do not use plain washers; their use can result in relative motion to change from the nut to washer, to washer to joint surface, during tightening. This as the effect of changing the friction radius and hence affects the torque-tension relationship. If, because of excessive bearing pressure, a larger bearing face is required, thought should be given to the use of flanged nuts and bolts.

2. Determine the correct tightening torque by the completion of tests. Strain gauges can be attached to the bolt shank and tightening completed on the actual joint. A load cell under the bolt head can be used, however it is not as accurate as strain gauging, since the joint characteristics have been changed.

3. If it is not feasible to establish by testwork the actual tightening torque, determine the tightening torque using the best information available i.e. fastener finish, nut head bearing surface size and prevailing torque characteristics, if applicable. (The computer program TORQUE developed by Bolt Science can allow for all these effects.)

4. Ensure that the tightening torque value is specified on the assembly drawing. Quotation of a plus or minus 5% tolerance is good practice. More unusually, quote that a calibrated torque wrench is to be used to check the torque after installation. The method used to tighten the bolt has a significant influence on the preload scatter (see below).

Angle Controlled Tightening 
This method, also known as turn of the nut method, was introduced for manual assembly shortly after the second World War when a certain tightening angle was specified. The method has been applied for use with power wrenches, the bolt being tightened to a predetermined angle beyond the elastic range and results in a small variation in the preload due, in part, to the yield stress tolerance. The main disadvantages of this method lie in the necessity for precise, and, if possible, experimental determination of the angle; also the fastener can only sustain a limited number of re-applications before it fails.Yield Controlled Tightening 
This method, developed by the SPS organisation, is also known under the proprietary name "Joint Control Method". Very accurate preloads can be achieved by this method by minimising the influence of friction and its scatter. The method has its roots in a craftsman's "sense of feel" on the wrench which allowed him to detect the yield point of the fastener with reasonable precision. With the electronic equivalent of this method, a control system is used which is sensitive to the torque gradient of the bolt being tightened. Rapid detection of the change in slope of this gradient indicates the yield point has been reached and stops the tightening process. This is achieved by incorporating sensors to read torque and