Bolt Tightening Methods , Applications , and Devices
Introduction
Bolting assemblies and joints, one of the most common methods to fix together two or more parts, is used throughout mechanical equipment. For many applications, tightening bolts evenly to the appropriate level of tension is extremely important, requiring adequate equipment and tools.
While bolted joints are regarded as a simple and straight forward method of connection, they are often subject to conditions that are quite challenging.
Traditional torque methods using torque wrenches or spanners suffer limitations (Figure 1), as it is not possible to know the preload tension with any accuracy due to the influence of the friction coefficients in the threads and contact surfaces, which are quite impossible to define exactly and change continuously. Using either of these methods can be inconsistent as it is not possible to reproduce the exact tightening actions each time.
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| FIG 1 |
Hydraulic bolt tensioning essentially induces a predetermined axial load into the bolt or stud. This load is commonly called the applied load and is calculated to make sure that the bolt (stud) retains the desired residual load.
- No torsional loading of fasteners
- Direct loading - no damage to assembly.
- Easy and fast operation.
- Very high accuracy and repeatability.
- Automation feasible and can be used for critical applications.
Bolted assemblies are the most commonly used joints in mechanics. These types of assemblies employ two basic elements:
• on the one hand, some kind of threaded component:
- screws and nuts,
- studs with nuts on one end,
- studs with nuts on both ends.
These components are sometimes used with different kinds of washers (Fig.2).
• on the other hand, some means for tightening.
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| FIG 2 |
Correct tightening of a bolt means making the best use of the bolt’s elastic properties. To work well, a bolt must behave just like a spring. In operation, the tightening process exerts an axial pre-load tension on the bolt. This tension load is of course equal and opposite to the compression force applied on the assembled components. It can be referred to as the “tightening load” or “tension load”.
Depending on the application, the purpose of the tightening load is multiple:
- ensure the rigidity of the whole assembly and make it capable of supporting external loads due to traction, compression, bending moments and shear;
- prevent leakage at seals
- avoid shear stresses on the bolts
- resist spontaneous loosening effects
- reduce the influence of dynamic loads on the fatigue life of the bolts (Figure 3)
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| FIG 3 |
1- Traditional tightening methods
There are several methods of tightening bolts. The respective principles are quite different , as are the quality and accuracy levels achieved.
The following is a summary (FIG 4) of the most commonly used methods .
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| FIG 4 |
where: 𝛾 = F₀ max /F₀ min.. : Uncertainty factor on tightening load
F₀ = tightening load in the bolt
Characteristics of torque tightening
- High amount of uncertainty as to the final bolt tension load
- Incorporation of additional “parasite” torsion stress
- Damage to bearing surfaces (fig 5 )
- Difficulties in untightening
- Problematic tightening of large bolts
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| FIG 5 |
2- Tightening with heater rod
This method consists of elongating the bolt by heating it with a heater rod inserted down the bolt centre. It then suffices to turn the nut under low torque force until it is in contact with the flange.
Upon cooling, the bolt will contract lengthwise, thereby tightening the nut. Simultaneous tightening of several bolts is theoretically possible. The method is theoretically accurate but in fact has several disadvantages:
- A hole must be drilled down the centre of the bolt to receive the heating rod.- Heating systems, electrical connections, temperature-control devices and handling means are required, especially in the event of simultaneous tightening.
- The method is exceedingly slow, due to the time required to heat the bolts, and the final tightening load can only be checked after the bolts have cooled down, which takes even longer.
The process cycle includes: heating the bolt, advancing the nut, cooling down the parts, and measurements. This cycle must be repeated several times in order to adjust the tightening.
The temperature required to reach suitable elongation is often so high that it could modify the mechanical properties of the equipment. As a result, when thermal elongation is insufficient, additional torque tightening must be performed and verified by measuring the nut angle.
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| FIG 6 |
3- Hydraulic tightening methods
Square Drive Torque wrench
This type of hydraulic torque wrench utilizes industrial impact sockets to apply force on fasteners for a precision mechanical fit. A square drive torque tool is very similar to your typical hand operated torque wrench. (figure 7)
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| FIG 7 |
this hydraulic torque wrench uses individual hex links to force down pressure on a nut within a tight or confined space.(figure 8)
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| FIG 8 |
Tightening with hydraulic bolt tensioners
Cold extension is applied to the bolt by means of an annular hydraulic cylinder placed around it.The bolt undergoes an axial traction load only.
The stress-free nut is then turned down with very little effort and does not transmit any torque to the bolt. When the fluid pressure is released in the tensioner, the major part of the hydraulic load on the tensioner is transferred into the nut, and tightening is completed
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| FIG 9 |
Methods and devices for measuring tightening torque
It is possible to reduce the deviation on the final tightening load by using an instrument to measure either the torque or the resulting bolt elongation. But whatever the means of control, is must not be forgotten that any torque tightening method increases the equivalent stress level because of the “parasite” torsion stress.
1- Monitoring the torque value
This is the simplest method. However, as described above, even where the accuracy of the applied torque value is good, a great deal of uncertainty still remains as to the final tension load in the bolt
2- Checking by the angle of rotation of the nut
There are two steps to this method. First, the nut is tightened to a torque value which is slightly lower than the required final torque. Then, a further, specific angle of rotation is apllied.
This slightly reduces the deviation in the final tension load. However, the uncertainty remains high, and the “parasite” torsion stress can be significantly increased.
3- Bolt-elongation measurement methods
The accuracy is significantly improved when direct bolt-elongation measurements are taken. Several methods can be used:
- Rod and knurled-wheel method
- Measurement by dial gauge or LVDT
- Ultrasonic measuring (US) method
- Strain-gauge method
Measurement devices
for hydraulic
bolt tensioning
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Measuring the hydraulic pressure
- The “double pressurisation” method
- Elongation measurement
- The “sensor washer”
Tightening by mechanical elongation
With this method, the tension load is directly applied to the bolt (Fig. 10 below). In general, the body of the nut is provided with a set of small thrust screws located symmetrically around the main threaded hole. These screws apply - either directly or through a washer - a bearing pressure on the contact surface of the flange. They are turned one by one and step by step using a low torque load until a suitable tension load for the bolt is reached.
The bolt elongation is most often measured using one of the previously mentioned methods. In spite of the fact that this method eliminates torsion stress in the bolt, it has several drawbacks:
- Simultaneous tightening is not easy to carry out: only a step-by-step tightening process is reasonably possible, from one bolt to the next. This is both tedious and time-consuming, and the result is pseudo-simultaneous tightening.
- To precisely determine whether tightening was carried out correctly, an additional measurement means must be provided, such as the elongation method or the use of load - measuring washers .
- The nuts are generally expensive, since they are bigger and require several small thrust screws and machining of several threaded holes
- When professionally applied, this method is the best way to achieve the quality criteria of proper tightening as described in the introduction.
- The process is very slow because the small screws have to be hand-tightened several times.
For all of these reasons, the mechanical elongation method is not used frequently.
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| FIG 10 |
Calculation notes on using bolt tensioning tools.
The relation between applied torque and axial force - or load - in a bolt can be calculated as
T = K F d
where
T = wrench torque (Nm, in lb)
K = constant that depends on the bolt material and size
d = nominal bolt diameter (m, in)
F = axial bolt force (N, lb)
Typical values for K with mild-steel bolts in range 1/4" to 1":
normal dry: K = 0.2
nonplated black finish: K = 0.3
zink-plated: K = 0.2
slightly lubricated: K = 0.18
cadmium-plated: K = 0.16
Residual Bolt Load = Bolt Stress x Bolt Tensile Stress Area
where :
Residual Bolt Load = (N or Tons)
Bolt Tensile Stress Area = (mm² or In²)
Load Transfer Factor(LTF) = 1.01 + (D / C)
D = Nominal Thread Diameter (mm or In)
C = Bolt Clamp Length (mm or In)
Note: If the calculated LTF is less than 1.1, then use a 1.1 LTF
Required Bolt Load = Residual Load * L.T.F
Tool Pressure = (Required Bolt Load / Tool Hydraulic Pressure Area
Tool Pressure = (N/mm² or Ton/In²)
Required Bolt Load = (N or Ton)
LTF = (No Units)
Tool Hydraulic Pressure Area = (mm² or In²)
*To convert N/mm² to bar: Multiply by 10
* To Convert Ton/In² to psi: Multiply by 2240
Bolt Stress = Bolt Load / Bolt Tensile Stress Area
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