Race-Tec: Using FEA software to design superior and long lasting CV boots

3
Feb

Race-Tec Sealing supplies a large proportion of constant velocity CV boots into motorsports and for high-performance military vehicles. Sports cars, military and off road vehicles – which are often steered at sharper angles, at high speeds, and in the case of the latter, on uneven ground – are quite demanding on CV boots. The boots must be designed to withstand a huge amount of deformation as the vehicle is steered and the suspension moves up and down; withstand articulation angles without excessive stress; and prevent contact with itself or the vehicle body. The CV boots also needs to be as small as possible to reduce the risk of being struck by flying objects.  

 

To meet these challenges, Race-Tec uses Marc to simulate designs. This enables them to quickly evaluate designs, develop alternatives and tweaks to arrive at the best possible solution. The team has found that results from Marc’s simulations closely match results from physical tests, providing a high degree of confidence in the design process. Marc is unique in that it is able to consistently and reliably solve problems involving highly complex frictional contact, large deformation, large strain and hyperelastic material behavior.

Race-Tec engineers prepare for simulation by creating a computer aided design model of the proposed boot design, which they import into Marc. They define the mesh manually in the most critical areas and use the preprocessor’s automeshing capabilities to define the remainder of the mesh. Centrifugal forces are applied to the boot to account for the fact that it is rotating and further boundary conditions are applied using rigid contact bodies. In this way, very complex translational and rotational deformations can be readily applied to the boot to simulate shaft loading behavior identical to that experienced in the real world.

The boot is made from a hyperelastic material that exhibits a nonlinear elastic response up to a very large strain. Such materials are challenging to represent mathematically because their behavior depends on a large number of variables. Race-Tec uses its on-site facilities for tensile and compression testing of hyperelastic materials at large strains at temperatures ranging from -30 deg C to 175 deg C.

The Marc solver performs the nonlinear analysis in load steps called increments. The user defines acceptable tolerances for force, displacement, strain energy and other parameters in seeking equilibrium for each increment. Within each increment, the program seeks a solution by iterating until equilibrium is achieved before proceeding to the next increment. Depending on the level of nonlinearity  experienced at each increment, Marc automatically modifies the step size in order to achieve a  converged solution using the minimum number of increments.

 

The simulation typically begins with the shaft at a zero angle of rotation and ends with the shaft at the maximum angle. In the case of the CV boot for the off-road vehicle, Race-Tec engineers simulated the existing design at worst case conditions – 37 degrees of steering, 19 degrees of suspension travel and 3 degrees of camber for a combined 3D angle of 42 degrees.

Marc predicted the displacement, stress levels, strains and the points where the CV boot contacts itself and where it contacts the steel shaft. The first simulation established a base understanding of how the boot was deforming in response to the movement of the joint. Over another 60 iterations, the team was able to make drastic improvements, most significantly reducing the height by 82% – a key design goal. They were also able to predict and eliminate “popping” – where extreme angular articulation caused the CV boot to come away from the shelf and start to form out on itself.

All in, Marc helped the team make a superior boot that substantially outlasts competitive designs, enabling Race-Tec to increase their market share and even enter new markets.

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