Request: Customer needs to validate that the pneumatic clamps are well designed and there is no plastic deformation at working surfaces.
Strength analysis of pneumatic clamps - linear static analysis
Strength analysis of pneumatic clamps - linear static analysis
Solution:
- Obtain a 3D CAD model
- Simplify model to save time and costs
- Load validation
- Proposed new design
The pneumatic clamps have to be analyzed in three separate studies.
The first study is a linear static analysis. Aim of the linear static analysis is to identify stresses which should not exceed allowable values of each component. It is the yield strength of a material in this case.
The second study is a linear buckling analysis. This analysis validates that clamps are rigid enough, that buckling does not occur under the maximal force exerted by a pneumatic cylinder.
The third study is a non-linear static analysis. As clamps are designed to plastically deform metal pin, analysis is performed to verify, that no plastic deformation occurs on clamps working surfaces.
This post describes the procedure of the first study – linear static analysis.
The clamps CAD model is shown in Figure 1 and supplied by a customer. The peripherals are removed as they have no impact on results. The pneumatic cylinder is also removed and it is replaced by the force acting to the clamp arms. As the model is symmetric it is convenient to only consider half of the model to save costs and time. Figure 2 shows the half-symmetry model suitable for analysis.
Fig. 1 – Customer CAD model
Fig. 2 – Half-symmetry CAD model
The Finite Element Method is suitable to analyse complex shapes of prototypes and products. The regular mesh of elements is created by meshing the half-symmetry CAD model. The linear Tetra elements (4 nodes) should not be used as they are too stiff. Complicated model shapes are usually easier meshed by tetra elements than by hex elements. Quadratic Tetra elements with 10 nodes and quadratic Hexa elements with 20 nodes are used in this case.
Fig. 3 – Meshed half-symmetry CAD model
Fig. 4 – Mesh element type, HEX8, HEX20, TETRA4, TETRA10
Boundary conditions are shown in Figure 5: Lower flange of basement profile is fixed in all 6 degrees of freedom (C). The symmetry plane is constrained by frictionless support (A). The clamp working face is also constrained by frictionless support (B). The maximum force (produced by the operating pressure of the pneumatic valve) acts to the lower part of the arm shown in Figure 6. The surfaces between components are in No Separation contact.
Fig. 5 – Boundary conditions application – constraints
Fig. 6 – Boundary conditions application – force
The material properties are defined and assigned to the parts. Then the linear analysis is performed. If linear stresses exceed the yield strength of the material, it is crucial to run analysis non linearly. The linear analysis does not consider plasticity. Therefore redistribution of load within all components does not occur. Figure 7 and Figure 8 show Von Mises stress distributions. The Von Mises stress is compared with material characteristics defined by the customer or with material properties obtained from the physical test. Figure 9 shows the total displacement caused by force from the pneumatic cylinder. Von Mises stresses are lower than yield strength and therefore no plasticity occurs.
Fig. 7 – Stress distribution – Von Mises
Fig. 8 – Stress distribution – Von Mises – detail
Fig. 9 – Total Displacement
Conclusion: Analysis has shown that no plastic deformation occurs and component Von Mises stresses are lower than component yield strength. The second step is to evaluate linear buckling behaviour.