Using BOLTCALC for a Clevis Joint Analysis

What is a Clevis Joint?

A clevis type joint consists usually of two parts, a rod or member that is trapped between two supports. Such joints are used to transmit primarily shear loads. There are a wide range of uses of such joints, the one shown in the image consists of a spring on a vehicle suspension being secured between two supports. There must initially be a gap between the bush in the spring and the supports to allow it to be installed. The gap is closed by tightening the through bolt so that there is metal to metal contact through the joint. Accordingly, the gap is usually small so that the force needed to pull the plates together is achievable and that the supports are not over-stressed by the clamp load from the tightening process. The advantage of clamping the parts together is that the loading from the spring is transmitted by friction grip rather than by placing the bolt into direct shear. This allows dynamic loads to be transmitted without joint slip or movement occurring. If the direct shear approach is used, the shear load is carried into the supports by a shear stress carried in the bolt. In such circumstances, the hole clearance needs to be minimised and the thread not to be in the shear plane, ideally. Clevis joints in direct shear are extensively used for applications in which the loading is not highly dynamic and having little or no requirement to transmit any side loading.

Clevis joint showing gap present

Fatigue failure of an M8 bolt

The joint that under study transmits the load using friction grip. That is, the force provided when the bolt is tightened clamps the parts together so that frictional resistance is used to carry any shear load between the connected parts. As such, the bolts do not sustain any shear force. In this application, bolts had been found to fail by fatigue, an example of which is shown below. Such failures are often indicative of the friction grip failing. When this occurs, the joint parts will slide slightly so that the bolt then must withstand loading that it is not designed to sustain. The joint analysis performed here looks at the likely cause of the failure and how it may be fixed.

Fatigue failure of a bolt

Double shear

The joint shown is in what is called double shear. That is, the force splits so that there are two shear planes. If the stiffness of each support is the same, each side will sustain half the loading. This is often not the case, and one support may be stiffer than the other. In such circumstances, the stiffer side will carry more of the loading. This can be determined by an analysis such as FEA (Finite Element Analysis). What some experienced Engineers do is to give some allowance for this effect by dividing the loading on each side by a factor less than 2, such as 1.8, to allow for such effects.

Clevis showing load path

The joint consists of an M8 property class 8.8 bolt with a property class 8 nut. The 10 kN loading is based upon the maximum dynamic loading that is likely to be encountered. The loading is fully reversible. There are other load cases, but this is deemed to be the worst case for the bolt to be able to sustain. The dimensions of the assembly are shown below.

Clevis showing dimensional details

Friction grip joint

With a friction grip joint, if slip occurs the bolt will sustain some shear loading and also because of residual friction grip under the bolt head and the nut, will sustain some bending. Under dynamic load conditions, this results in an alternating stress in the bolt. If such alternating stress is sufficient, fatigue failure of the bolt will occur. To complete an analysis, we need to know how much force is needed to close the gap between the supports and the bush eye. This can be computed or assessed by testing if the assembly already exists. A simplified test approach is to measure the torque and angle of rotation of the nut. The typical shape of a graph based upon such measurements is shown in figure below.

Torque-Angle graph for a clevis joint

On initial tightening the joint stiffness is low since the plates are being pulled together. Once metal to metal contact is achieved, the curvature of the slope of the curve changes. The torque can be measured to achieve this pulling together of the assembly and from this the clamp force can be determined using BOLTCALC. For the purposes of this analysis, it was found from such an approach that will be taken that the force needed to close the gap is 3.5 kN. So, a summary of the specific conditions for this joint are shown below:
Thread size: M8 (coarse thread)
Outer bearing dia. of the bolt: 11.6 mm
Clearance hole dia. 8.4 mm
Property class of the bolt: 8.8
Coefficient of friction 0.11 to 0.17
Shear force applied to the joint: 10000 N
Coefficient of friction between the joint plates: 0.25
In order to ensure that the bolt is not overtightened, the lowest value of friction should be used in the calculations. In this case, since the coefficient of friction can vary between 0.11 and 0.17, 0.11 will be used in these calculations.

The screen capture video below shows the program being used to investigate whether the design of the joint is fit for purpose.

Preload Requirement Charts

The preload requirement chart for the original design condition is shown below:

Preload Requirement Chart for the original design

For the design to work, the Total Preload Requirement, shown in the chart, must be less than the minimum anticipated preload value. That is, the red region must be above the grey; as can be seen this is not the case and the design capability is out by a factor of two or so. When an analysis like this is completed on service failures it is not unusual to find that the joint is this inadequate. It often arises when the joint has not been analysed and the product testing was inadequate.
The Preload Requirement Chart for the final design fix utilising friction shims is shown below.

Preload Requirement Chart for the design fix

The design is inadequate and although special measures may be taken to resolve the service problem, such as the use of friction shims, a larger bolt is needed for a cost-effective design to be achieved.

BOLTCALC pdf Datasheet Presented below are links to further information related to the BOLTCALC program:

Web pages giving further details about BOLTCALC

Training Presentations on the BOLTCALC program

Torque Tightening Analysis using BOLTCALC (pdf)

Thread Stripping Analysis using BOLTCALC (pdf)

Joint Analysis using BOLTCALC (pdf)

BOLTCALC Datasheet in Adobe PDF format

BOLTCALC Example in Adobe PDF format

BOLTCALC Demo Program

dBEditor - BOLTCALC's optional database editor

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