Monday, March 10, 2014

Welded Rotating Components

We see a lot of welding in Formula Student and Baja Cars, which is not surprising. However, design judges always come across a few horror welds that are easy to label as 'GTB' or 'Going to Break'. Here is one example -

Both the half shaft and the universal joint are off the shelf components which are not designed to be mated with one another. The universal joint on the welded end has an internal spline and a mechanism where one can use a threaded bolt to tighten the grip of the universal joint on the splined shaft. Ideally there was no need for welding, if the shaft could have had splines of the correct size.

This team for some reason has decided to attempt a "butt-weld" to couple the two. Butt-welded shafts have been a source of debate many a times on technical forums, but they are risky but if done correctly there are championship winning cars out there with butt-welded half-shafts. However, the weld pictured above is probably not going to make it for very long.

When rotating parts are welded together such that they need to have their individual axis of rotations collinear, extreme care is required to ensure that the components being welded are held together tightly in place via rigid fixtures and jigs. Slight nonlinearity can cause many issues. If for example, the co-linearity is off by half a degree on a meter long half shaft then the other end of the shaft would move around ~9mm either way during rotation causing, compliance in the joints, vibration and high cyclic loading.

The shaft in the first picture above is clearly misaligned by more than a couple of degrees. In fact even when care is taken when doing such a weld there is invariably going to be some misalignment because of the welding process not being symmetrical (You have to start somewhere). In this video a shaft is welded and it is interesting to observe the amount of effort and precision put into making sure the shafts are aligned after the welding is complete.
This is not only true about half shafts. Any part welded about a rotating axis must take this tolerance into consideration. This includes wheel hubs, steering columns, their connection to joints, brake disk mounts and other rotating components. These welds are many times unavoidable as may have been in this case. here are some ideas to improve the quality of the design -
  • Rigid and tight fixtures set up during the welding process. The weld should be completed fully before the components are removed from the fixtures to avoid bending and deformation during the welding process.
  • In the case of chrome alloy and there metal being used that require heat treatment after the welding is complete. the component should be heat treated with the fixtures such that the reheating or cooling does not cause any deformation.
  • If possible especially for welded hubs, have some excess material on the welded on bits such that after the welding is complete the entire component can be placed on a turning machine and made radially symmetric. A welded wheel hub like in the picture below is bound to have some misalignment.

  • Very small misalignments (not visible without measurement) can still cause vibration and large cyclic loading of the component.
  • It is a good idea to have grooves and steps in the weld areas such that the components fit into one another when on the welding fixture. This drastically reduces the chances of misalignment
Apart from misalignment weld strength is also questionable in some of these designs. Specially for rotating parts the weld area see high shear forces, which is not the best place to have them. Special care must be taken to ensure the filler material used during welding is the correct one. Small differences in the material composition in the weld area can cause minute differences in the coefficient of Thermal expansion that can cause tiny cracks to be formed in the area. These cracks will propagate during cyclic loading.

A couple of ideas to improve the weld strength -

  • Find a method to increase the weld area. This can be done by having grooves and flanges or have the 2 components to be joined to be cut at an angle. Cutting at an angle not only takes the weld seam out of plane with the shear forces but also increase the weld area significantly.
  • Support the butt-joint with an internal or an external sleeve. This distributes the shear force over multiple welds thereby effectively increasing the welded region and the strength of the component.
This blog post is originally written for the Formula Student India website and has been cross posted from here.


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