Production I- Production I-
Chapter 3 Part B
S.4. Chassis Design Considerations
5.4.3. Co mputational Finite Element App roach To Design
5.4.3.1. Option I
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Note: The scale (Pa) applies to the stress contour plot which is depicted in Figure 5.4.4
Figure 5.4.3. Structural cross sections for option one
Figure 5.4.3. serves to depict the cross sectional make up of option 1. The contour plots for the stress and deflection, together with their respective scales appear in Figure 5.4.4. and Figure 5.4.5. Option 1 had the following cross sectional make up : (dimensions in mm)
Member I rectangular
Member 2 and 3 square tube
height J 25 width 25
side 76 thickness 3
The following is a maximum stress contour plot obtained from the computational analysis. The author was aware that utilization symmetry was possible in modcling. However, as a full model using beam elements requires relatively
minute number of nodes as compared to other modeling elements, the author sought to use a full model for the graphic advantage of the reader. The use of symmetry was however used in the plate models. The exaggerated deflection contour appears in Figure 5.4.5.
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Figure 5.4.4. Maximum stress contour plot (unit; Pascals)
As is evident fTom the stress contour plot in Figure 5.4.4 the maximum stress is concentrated at the center of M I and at the end of M3. The value of the maximum stress which occurred at the end of M3 was computed to be 83 MPa.
The deflection of less than I mm was well within that which was expected.
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Figure 5.4.5. Deflection of the chassis frame (unit : metres)
5.4.3.2. Option 2
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Figure 5.4.6. The structural cross section for option 2 (unit : Pascals)
Figure 5.4.6. depicts the structural make up of the chassis for option two. The dimensions, in millimeters, for the difTerent sections are listed below.
Member I rectangular
Member 2 and 3 I section
height 125 width 25
height 100 width 50
web and nange 5
The analysis produced the result that was a reduction in the maximum stress from option 1, to a value of 60 MPa. The general stress contour was similar to that in Figure 5.4.4. with the exception that the maximum stress occurred just before the point of contact between M I and M3. The author reserved the displaying of the stress contour plot for this option as the plate model that was employed would be used for the purpose, however, the maximum stress contour scale, is available in Figure 5.4.6. The maximum defection remained essentially the same. The option viability was however evaluated in the light of design for manufacture and assembly discussed in Chapter 6.
S.4.3.3. Option 3
Member I
Member 2 and 3
rectangular
square section
height 125 width 25
side 50 thickness 3
The maximum stress was found to at the fixed constraint, the point of attachment to the hoisting unit, la be 133 MPa. The increase in the stress could have been
an'ributed to the decrease in the section modulus of M3 and M2.
5.4.3.4. Option 4
Member I
Member 2 and 3
rectangular
square section
height 125 width 20
side 50 thickness 3
With a decrease in the moment of inertia of M 1 as a result of the decrease in width the stress pattern remained essentially the same with a slight overall increase in the stress levels. The maximum stress was found at the end of M3 to be 137 MPa. Again the viability of the model was examined in the light of total design as ease of manufacturing and assembly were key specifications.
5.4.3.5. Option 5
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Figure 5.4,7, The structural cross section for option 5 (unit: Pascals)
Figure 5.4.7. is only a graphical description of the structural cross sections In option 5. The dimensions, in millimetres, for option 5 are listed below. The scale shows the stress values in Pascals.
Member 1 rectangular height 125, width 20
Member 2 and 3 round section diameter 40
The results of the analysis on option 5 produced results that were much higher than any of the other options. A maximum stress of 180 MPa was computed at the end afM3.
5.4.3.6. Thc Employmcnt of Plate Elements fOI' Modcling
Options I through 5, were verified successfully using plate elements. The author has modeled the following two options with plate elements. From the above computations, attractive aspects of the models were selected to be modeled using plate elements. As mentioned the model made use of two planes of symmetry and was 1110deled as a quarter of the actual chassis. The plate model had the following cross sections
Member I rectangular
Member 2 square
Member 3 I section
height 125 width 25
side 50 thickness 3
height 100 width 50
web and flange 5
The resulting maximum von Mises stress contour plot appears in Figure 5.4.8.
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Figure 5.4.B. Maximum von Mises stress contours (unit : Pascals)
The result of the static analysis was that a maximum stress, indicated by the red contour in Figure 5.4.8 was found to be 87 MPa. This was a relatively small area
and most of the material was stressed to an average value of 60 MPa. The deflection was very much the same as all of the other models «2 mm).
The author then investigated changing the I - section at the center to a 100 nominal side square tube of 5 mm thickness, using plate elements. The computational output was a stress reduction to 65 MPa. The stresses were concentrated in the same positions as in Figure 5.4.8. However, a larger area, relative to that in Figure 5.4.8. was exposed to the stress concentration.
The designer also sought to look at the outcome of the analysis when the structural tie, was included in the design. The model whose analysis produced a maximum stress of 65 MPa, appears in Figure 5.4.9. As mentioned, the use of the ties or possibly wire rope, would act as a safety measure in the event of the joint between the chassis and pulley housing failing. This was considered to be justified by the fact that all the models tested identified this area as one of high stress concentration.
Figure 5.4.9. Plate model with tie (unit: Pascals)
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