An FEA analysis was applied to the welded assembly o f the seal housing and the outer telescopic component, these being as shown in Figure 5-10. The purpose o f this analysis was to check that stress due to cyclic loading for walking is within the fatigue limit and that the maximum occasional stress anticipated is within the yield strength o f the material. It was also required to determine whether deformation o f the seal housing could cause the bore o f the seal housing to pinch radially inward against the gear output shaft.
The assembly was assumed to be rigidly fixed at the closed end o f the outer telescopic component, i.e. the nodes at this end were fixed in all degrees o f freedom to simulate the closed end o f the telescopic assembly being an interference fit in a relatively rigid femoral knee joint component as would be used for a distal femoral tumour site. The loading was modelled as a uniform pressure applied over the surface of the seal housing which contacts the thrust washer. This pressure was calculated to correspond to a
nominal axial load on the thrust washer o f 1.0 kN and the results o f the elastic analysis were then scaled for the full design axial load. An axi-symetric model of the outer telescopic assembly and the seal housing was generated using the geometric modeller within the Cosmos package. The material was treated as continuous across the regions of electron beam welding, which is realistic for full penetration defect free welds. The model included the whole of the outer telescopic section although much of this structure had negligible effect on the results in the area o f interest. Since both the geometry and the loading were axi-symetric a two dimensional axi-symetric elements could have been used, but it was found to be more straightforward to use the automatic mesh generator within the Cosmos package to produce a three dimensional mesh of ten node tetrahedral elements.
The results showed that both the maximum Von Mises stress and the maximum resultant deformation occurred at the inner rim o f the annular face of the seal housing
Section 5. Telescopic Body and Power Screw
which is in contact with the thrust washer. The Von Mises stress at this point was 27.2 ^N/m^ per kN of axial loading. The design maximum occasional axial load was taken to
be 7kN and this gives a factor o f safety o f 5.1 based on the yield strength o f the titanium alloy.
For cyclic loading, as discussed in Section 5.6, the maximum permissible Von Mises stress for the titanium alloy is 250 N/m^, based on 2x10^ zero minimum fatigue cycles and with stress concentration factor kt o f up to 3.0. The design cyclic axial load range in the femur for walking is considered to be 2.1 kN, as discussed in Section 3.4. Using the FEA results this gives a range o f Von Mises stress of zero to 57N/m" which is well within the maximum for the material even allowing 3.0 for the stress concentration factor. This allowance for stress concentration is conservative since the FEA prediction o f Von Mises stress takes account o f stress concentration due to the details o f the
geometry, although the element size used may not have been small enough to accurately model the peak stress concentrations.
During the extension procedure the patient is relaxed and the axial load is not expected to exceed 700N, as discussed in 3.1. For this axial load the FEA predicted the inward deformation of the seal housing at the point o f greatest deformation to be 3 microns. This is less than the radial clearance at maximum metal condition as indicated on the working drawings used for prototype manufacture and so the seal housing should not pinch the shaft during extension procedures. At the maximum design axial load o f 7kN the radial deformation of the seal housing is 28 microns inwards and the housing will then grip the shaft, but this loading is only expected to occur briefly and occasionally and so contact with the shaft is considered to be acceptable. At an axial load o f 2. IkN, which is typical of walking and the other activities o f daily living, the seal housing should be just clear of the shaft for the maximum metal condition.
Deformation plots for the region of the seal housing were derived from the FEA and are shown in Figure 5-12. These views are half sections through the assembly with the components represented as hollow shells rather than solid bodies.
Section 6: M e th o d o f T o rque A mplification GEAR OU T P U T S H A F T - TORQUE = 1 . 0 NM von Ml ooo 2 6 6 . S 1 8 2 . S 1 5 8 . 0 13® . 0 1 0 4 . 0 78 . S 5 2 . 8 2 6 . 8 0 . ® 8
Figure 5-11: Gear output stubshaft - Contour plot of Von Mises surface stress.
U nloaded assem bly L oaded assem bly
Figure 5-12:Deformation plots of seal housing assembly