Vibratory finishing applications and parameters
This time last year I wrote an article talking about the basics of vibratory finishing but did not mention many applications, and without writing about the specific instances where we might want to use low- or high-energy processes. In this article therefore I shall go into more detail and look at some cases where we ought to be careful to specify to the supplier exactly what we need.
There is a multitude of applications for vibratory finishing in engines and transmissions, and the main thing they have in common is that most of the treated components are in some form of sliding or rolling contact with other components. The merits of improved surface finish in such contacts are widely understood: by removing the high spots on any surface through a surface treatment such as vibratory finishing, we reduce friction and increase reliability by giving the oil film a higher margin of safety. Until the oil film is sufficiently developed – as in it becomes thick enough to separate the components – parts of the components are in contact, and friction is a function not only of the partially developed oil film but also of the solid contact, hence the high friction.
With poor lubrication conditions such as this, wear can rapidly cause a component to fail. The smaller the surface finish height, the thinner the oil film can be. So, particularly at start-up or low speeds, the improved surface finish is a real bonus. Components such as gears, cams and cam followers benefit from such treatments. Vibratory finishing is also used sometimes on crankshafts, again improving surface finish on bearing surfaces.
In terms of energy, there are some components where we definitely want a high-energy process capable of removing burrs and taking off sharp edges. For example, the edges of cam lobes and gear teeth are better when the edges are thoroughly de-burred and given a small radius.
However, there are applications in engines where we definitely want to retain sharp edges. On some machined edges on engine poppet valves, the sharp edges on the back of the valve are important, and choosing to put a nice radius on the wrong one of these can bring a measurable performance penalty.
The reason is that some sharp edges are carefully developed features to enhance the energy in the flow over the valve, acting in a similar way to a flow trip. By increasing turbulence in intake valve flows, for example, there are circumstances where the flow through the valve seat is greater because there is a reduced degree of separation. There are specific designs of valves to create these conditions, and if the critical sharp edges are turned into small radii then the valves can represent a step backwards compared to a conventional valve design, rather than giving the intended improvement.
There are some extremely high-energy vibratory finishing processes that are characterised as peening techniques, whose aim is to create significant residual compressive stresses in component surfaces, but the energies involved are far beyond those that can be imparted by conventional vibratory finishing treatments.
Written by Wayne Ward