Anchor bolt

Why Bolt Preload is Important?

The diagram shown above presents the way that the basic joint diagram is constructed. As a nut is rotated on a bolt's screw thread against a joint, the bolt is extended. Because internal forces within the bolt resists this extension, a tension force or bolt preload is generated. The reaction to this force is a clamp force that is the cause of the joint being compressed. The force-extension diagram presented above shows the bolt extension and the joint compression. The slope of the lines represents the stiffness of each part. The clamped joint usually being stiffer than the bolt.

The basic joint diagram is formed by moving the compression line of the joint to the right. A triangle is formed because the clamped force tending to compress the joint is equal to the bolt preload. Positive extension is to the right such as that sustained by the bolt, negative extension (compression) is to the left and is sustained by the joint

Joint Diagrams with External Forces Applied

When an external tensile force is applied to the joint it has the effect of reducing some of the clamp force caused by the bolt's preload and applying an additional force to the bolt itself. This is illustrated in the joint diagram shown above. The external force acts through the joint material and then subsequently into the bolt. At first sight it may seem a bit strange to place the applied force in the position shown in the diagram. However, it should be realised that the load on the bolt cannot be added without decreasing the clamp force acting on the joint. As can be observed from a study of the diagram, the actual amount of increase in the bolt force is dependent upon the relative stiffness of the bolt to the joint.

As an illustration of the importance of the relative stiffness of the bolt to the joint, presented above is a joint diagram for a 'hard' joint (a low stiffness bolt with a high stiffness joint). In this case, because of the steep stiffness slope of the joint, the bolt will only sustain a small proportion of the applied force.

With a 'soft' joint (a high stiffness bolt with a low stiffness joint), because the stiffness slope of the bolt is greater than that of the joint, the bolt would sustain the majority of the applied force. Study of these diagrams provides understanding of why high performance bolts have shanks that have been reduced to a diameter below that of the outside diameter of a thread. By reducing the shank diameter in this manner the stiffness of the fastener is reduced so that it will not sustain as much of any applied force that it would otherwise do. If the shank diameter is not reduced to a diameter below that of the stress diameter (see stress area in the glossary) then the strength of the fastener will not normally be impaired.

The Effect of a Large Applied External Force

As the external force is increased the force acting on the bolt is proportionally increased. At the same time the clamp force acting on the joint is decreased. If the external force continues to increase then either:

1) The proportion of the external force acting on the bolt together with the bolt's preload, results in the yield of the bolt material being exceeded with the imminent likelyhood of bolt failure. Even if failure does not immediately occur when the external force is removed, the preload will be reduced. The joint diagram showing an external force causing the bolt to yield is illustrated below.

2) The clamp force acting on the joint will continue to decrease until it becomes zero. Any further increase in the applied force will result in a gap forming between the plates comprising the joint and the bolt sustaining all of the additional force. This is illustrated in the joint diagram below.

If a gap does form between the plates comprising the joint then the bolt or bolts are almost always subjected to non-linear loadings from bending and shear forces acting. This usually quickly leads to bolt failure. Hence it is normal to set a design criteria that the applied forces must not under any circumstances result in a gap forming within the joint.

The Effect of a Compressive External Force

If the joint experiences a compressive external force this has the effect of increasing the clamp force acting on the joint and decreasing the tension in the bolt. This is illustrated with the joint diagram shown below. If the compressive external force is great enough then either:

1. The tension in the bolt can be reduced to a low value - if the external load is cyclic then the bolt could fail due to fatgue (since it is experiencing tension variations under a compressive external force). Also the bolt is more susceptable to vibrational loosening.

2. The yield limitations of the clamped material may be exceeded since the joint is sustaining a compressive force in addition to that provided by the bolt's preload. This will result in some permanent deformation that upon the release of the external force a loss of bolt preload would result.

The Effect of Joint Deformation loss due to Embedding

A joint diagram showing the effect of embedding is presented below.

When a bolt is tightened, very high local pressures can exist in the contact areas on the threads and under the nut/bolt. Local plastic deformation can occur at these interfaces by flattening of surface roughness. This plastic deformation has the effect of reducing a bolt's preload. Research has been completed to establish guide values for the amount of embedding that typically occurs within joints. The amount of embedding determined is a loss of joint deformation. This can be converted into force by calculation or by the aid of a joint diagram.

Bolt Preload Variation due to the Tightening Method

The effect that the method of tightening has on determining what size of bolt is required to fulfil a specific function is largely underestimated. If several bolts of the same size are tightened by the same method then there will be variation in the bolt's preload - they won't have all the same value. This variation is influenced by such factors as variation in friction characteristics in the thread and under the nut face, thread form and pitch variations, variations in the surface flatness etc. Hence for any particular tightening method there will be a maximum anticipated preload and a minimum given a set of conditions.

The tightening factor is a measure of the scatter in a bolt's clamp force as a result of the tightening method used to tighten the fastener. It is defined as the maximum bolt clamp force divided by the minimum value anticipated for that tightening method. For tightening with a torque wrench the tightening factor is usually taken as 1.6; i.e. the maximum preload value is 1.6 times the minimum.

A joint diagram showing the effect of preload variation and embedding is presented below.

Since the bolt is not to be broken by overtightening on assembly, it must be selected for the maximum initial preload. Hence for a given bolt size, the smaller the tightening factor, the larger the residual preload is remaining to sustain the applied forces to the joint.

Preload Requirement Charts

Joint diagrams can display a significant amount of information about the joint but in our experience many people find them difficult to interpret and understand. Preload Requirement Charts are a way to graphically display the results of a joint analysis in a clear and understandable manner.

By way of example, consider the joint shown below that is subject to combined axial and shear loading. For information, the bolt is M12 property class 10.9, the joint thickness is 20 mm with an axial load of 15 kN and a shear force of 4 kN being applied. (If the joint consists of several bolts, it is first necessary to determine the loading on an individual bolt.)

One key aspect to appreciate is that the root cause of the majority of bolt/joint failures is due to insufficient preload. It is unusual for the bolt to be overloaded. If the preload provided by the bolt is insufficient, joint separation and movement can occur resulting in possible bolt fatigue and self-loosening issues. In order that such problems do not occur it is vital that there is sufficient residual clamp force acting on the joint interface after accounting for the effects of the applied forces and embedding losses. A Preload Requirement Chart graphically illustrates this point as it looks at the forces acting on the joint interface. Such a chart is shown below for the above joint.

The above chart was produced by the BOLTCALC program, but such charts can be produced manually. Explaining each of the parts of the chart in turn:

Embedding Loss: Embedding is localised plastic deformation that occurs under the nut face, in the joint faces and in the threads as a result of flattening of the surface roughness. Embedding results in a loss of clamp force acting on the joint. If the joint and bolt stiffness can be established, the amount of this force loss can be quantified if the surface roughness of the contact surfaces is defined. In the above chart, a loss of 10 kN is anticipated. Large amounts of embedding loss can occur in joints with a short grip length consisting of many interfaces.

Axial Force Requirement: In a preloaded joint, the majority of the applied axial load reduces the clamp force on the joint interface rather than increasing the load in the bolt (see an earlier tutorial for an explanation). The amount of the axial load that unloads the joint interface can be determined from the joint/bolt stiffness calculations. In this example, of the 15 kN applied force, 13.8 kN reduces the clamp force on the interface (the remaining 1.2 kN increases the load in the bolt). To simplify, when hand calculations are being completed, the conservative assumption is often applied that all the applied axial load reduces the clamp force on the joint interface.

Shear Force Requirement: The majority of joints in mechanical engineering use clearance holes and any shear load is transmitted by friction grip. That is, the clamp force on the joint interface generates a friction force that resists any applied shear loading. On such joints, if slippage is prevented, the bolts do not directly sustain any shear loading, however they have to provide sufficient clamp force to prevent joint movement. To achieve this, the clamp force required is the shear force divided by the coefficient of friction present between the joint surfaces (for the single shear plane present in the joint shown above). Since the coefficient of friction is usually significantly less than 1, this requirement results in a significantly larger clamp force being required than the magnetude of the shear force. In this example, the applied shear is 4 kN which, if a coefficient of friction of 0.2 is assumed between the joint plates, results in a minimum clamp force of 20 kN (i.e. 4/0.2).

Total Preload Requirement: This represents the minimum preload required to be provided by the bolt. It is the sum of the embedding loss, the amount of the applied axial force that reduces the clamp force on the joint and the clamp force needed to prevent slippage of the joint due to a shear loading.

Preload Variation: In an ideal world the preload provided by the bolt would be known to an exact value and would be the same for every bolt tightened. Unfortunately there is no low cost means of tightening a bolt and knowing, precisely, the preload value. Techniques such as tightening the bolt to a specific torque value results in variation in the preload between, apparently, identical bolts. This is as a result of not being able to apply the torque to the same exact value each time, variation in the hole and bolt tolerances but more importantly, variation in the coefficient of friction present in the threads and under the nut/bolt face. To design a joint successfully this scatter in the preload must be taken into account. This can be done in a number of ways but usually either by determing the minimum/maximum preloads from knowledge of the friction variation or by the use of a tightening factor.

The problem: In the above chart the total preload requirement exceeds the minimum preload. What this means is that on some, but not all joints, the preload will be insufficient to resist the applied forces. In such cases, joint failure can be anticipated. The failure islikely to be by either bolt fatigue (due to bending due to the joint slipping and separating) or by self-loosening (due to joint movement).

The solution: In general, changes can be made to increase the minimum preload value (by using a stronger or larger bolt or changing the tightening method) or by reducing the applied forces (by using more bolts in the joint, or by increasing the friction between the joint interface and so reducing the shear force requirement etc.) Shown below is the chart for changing the tightening method to torque and angle. If applied correctly. this method will consistently provide a high preload value.

Factor of Safety: A question which often arises is how much of a gap there should be between the total preload requirement and the minimum preload value. This depends essentially upon engineering judgement. If the applied forces are accurately know, if product testing is going to be completed, then the gap can be small. If the forces are not known accurately, and the consequences of failure disastrous, then a larger gap would be sensible. The consequence of having a generous factor of safety is that a larger bolt size (or higher strength bolt or better tightening method etc.) would be needed then which would otherwise be the case. This can result in a more expensive and less competitive product.

Preload Requirement Charts can be developed to include other effects such as the effect on bolt loading of differential thermal expansion. They are a useful method for joint analysis and solving bolting issues.

http://www.boltscience.com/pages/basics1.htm

Bolt Calculation 3D Animation with Blender 3D (English Version)