INDUSTRIAL BOLTING

Background:

A Bolted Joint is the most widely used method for fastening industrial components together. Attributes of bolted joints include ease of assembly, high load-carrying capacity, relatively low cost and facilitation of part replacement and repair. Because of their prevalence, proper Bolted Joint implementation is of paramount importance.

Bolted Joints are formed using a threaded fastener which captures and attaches two or more components together by mating either with an internally threaded (tapped) hole or a washer and nut combination. An externally threaded fastener whose function requires it to be assembled with a nut is technically referred to as a “bolt”. One which threads into a tapped hole is a “screw”.

When creating a Bolted Joint, a process called controlled bolting is used to correctly apply a known load to a mechanical joint to ensure its integrity. Torquing and Tensioning are the two most widely used techniques to tighten a bolt. Both techniques work by elastically stretching the fastener which in turn exerts a clamping force to the components being joined. Achieving the required clamping force is key to obtaining an optimum bolted joint with the desired characteristics. Fasteners manufacturers provide the required bolt torque and tension specifications to achieve the required clamping force.

Torquing:

The Torquing method develops this stretching force indirectly by applying a twisting load to the bolt head and nut. Only a small fraction of the applied Torque, typically 10 – 15%, contributes to the generation of the clamping force. The remaining 90 – 85% is required to overcome friction between the contacting components (bolt head, contact area, nut, and washer). Because Torquing is an indirect method, an equation relating friction, screw diameter and applied torque is required to calculate the clamping force.

Tensioning:

Alternatively, Tensioning directly applies the tensile load to the fastener and circumvents the torsional effects caused by Torquing. Since the clamping force is equal to the tensile load on the fastener, the clamping force is readily obtained by subtracting load losses due to thread deflections and radial expansion of the nut from the measured applied load.

Bolted Joint Degradation:

There are several factors which contribute to the degradation of a Bolted Joint:

• Vibration may result in relative motion between the clamped components leading to loosening of the fastener.
• Embedment of the bolt head into the substrate material could relax the clamping (preload) force.
• Bolted Joints employing multiple fasteners could give rise to Elastic Interactions causing a decrease in the preload of a previously tightened bolt by compression of the joint elements due to subsequently tightened bolts.
• Component Expansion and contraction caused by Temperature Variations.
• Inadequate initial preload applied during Installation.

Proper design of the Bolted Joint and the correct clamping force or preload can mitigate these adverse effects.

Controlled Bolting:

As previously mentioned, Controlled Bolting, using Torquing or Tensioning techniques, aims to ensure that a bolt achieves a predetermined optimal tension to produce a clamping force which yields a desired Bolted Joint.

For fastening bolts using the Torquing method, a torque wrench – mechanical (manual), electronic (with display), hydraulic or pneumatic – is normally used to apply the manufacturer specified torque tightening value. Mechanical and electronic torque wrenches have a comparable range of 0-300 ft-lbs while pneumatic types can reach 6,000 ft-lbs; and, hydraulic torque wrenches may surpass the tens of thousands of ft-lbs.

On average, the accuracy and range for each type is as follows:

• mechanical (manual): ± 3-4% 0-300 ft-lbs
• electrical(with electronic display): ± 1-2% 0-300 ft -lbs
• hydraulic: ± 3% > 10,000 ft-lbs
• pneumatic: ± 5% 120-6,000 ft-lbs

Regardless of their type, industry standards require torque wrenches to be calibrated once a year or every 5,000 cycles.

The Tensioning bolting technique typically utilizes hydraulic pressure to axially stretch the bolt to achieve the desired preload. During the tensioning operation, a bolt tensioner or hydraulic nut is fitted over the bolt. Hydraulic pressure is applied to the tensioner to stretch the bolt. The bolt’s nut is threaded down to meet the joint face. The pressure is then released and the tensioning tool is removed. A significant advantage of this approach is the ability to simultaneous tighten multiple bolts by using an equal number of bolt tensioners connected to a single pump. This effectively ensures a uniform clamping force across the joint by delivering the same hydraulic pressure to each tool to develop the exact same load at each fastener.

Sensing Systems Bolting Measurements:

Proper and accurate implementation of either bolting technique, Torquing or Tensioning, requires a measurement to be performed; either the torque exerted to the nut or bolt head, or the tensile force applied to the fastener.

To measure applied torque to a bolt, Sensing Systems designs and manufactures highly accurate, ≤ ± 0.25% of Full Scale, electronic torque wrenches as well as torque sensors integrated into torque guns.  These sensors surpass typical electronic torque measurement accuracies of ± 1-2%.

To measure the tensile force on a fastener, Sensing Systems designs and performs machining modifications to off-the shelf bolts which are instrumented with strain gages to convert them into load measurement sensors. Sensing Systems also designs, manufactures, and calibrates custom machined bolts instrumented with strain gages to create fastener load cells. Sensing Systems is one of the nations leading load cell manufacturers. 

Due to space constraints, small instrumented bolts typically have two axial strain gages wired into quarter bridges. Larger bolts can accommodate axial and transverse gages configured into a full Wheatstone bridge to perform the measurement. During the bolting operation, the actual tensile force developed on the instrumented fasteners can be monitored and logged. Measuring the tensile load eliminates the inherent inaccuracies from applied torque to clamping force calculations or loss load equations.