CORROSION AND LINEAR
Getting a linear solution that is corrosion resistant can be a real challenge. Part of the problem is the additional difficulties that a linear application creates but also a great deal of misunderstanding in the wider engineering community.
The first thing to understand is there is no such thing as universal corrosion resistance. You just need to find the right chemicals and some sort of reaction will start. Halogens such as fluorine, chlorine and iodine will corrode stainless and this is exactly why chemical laboratories use glass equipment – unless they are handling molten sodium hydroxide as that will dissolve glass. Fortunately applications with these sort of chemicals aren’t common and don’t often involve bearings.
More common environments where corrosion may be an issue is general atmospheric, especially near or on the sea, or factories where moisture or wash down might be present such as food manufacture. As long as abrasive or chlorine based (bleach) cleaners are not used then stainless steel especially the 300 series will show excellent corrosion resistance especially the “marine grade” 316L.
The problem with 300 series stainless for bearing applications is they are soft being typically Rockwell RC25. This is well bellow the RC60 of the high carbon steels recommended for a bearing surface. The softer steel will indent easily, drag will increase and flaking of the bearing surface will occur in short order. Alternatively 400 series stainless especially 440C can reach the recommended RC60 hardness. If you have a set of quality chefs knifes you’ll usually see 440C etched in the blade and the use of this sort of steel allows the knife to keep it’s edge without constant sharpening.
Unfortunately 440C can be susceptible to pitting corrosion in some environments especially salt. Not a problem for the chefs knife which is carefully cleaned every day but more difficult in many food or marine situations. In additional 440C is a specialty steel, made in much lower volumes and very expensive. The price difference can be economic for an expensive knife, the ball of a rod end or a smaller linear bush but becomes prohibitive in larger sizes and longer lengths of rail or ball screws.
As a consequence most manufactures of rail and ball screw systems don’t offer stainless but focus more on coating or treatment solutions. Coating systems such as chrome or nickel can be effective provided they are of similar hardness to the base material which will reduce the risk of the coating flaking off. Our standard round shaft is chrome plated induction hardened bearing steel and has shown excellent corrosion and mechanical performance over many years. Rollon offer Nickel coating of Compact Rail and this is especially effective for Compact Rail as the preload system can adjust for the dimensional change of the coating simply and easily. For profile rail or ball screws the dimensional change must be compensated for in the factory with either ball size, grinding tolerances or a combination of both. This can add considerably to the cost.
We have had great success over the years with Raydent treatment offered by SBC with their profile rail and ball screws. Raydent treatment is a low temperature chromium diffusion process. Being low temperature there is little dimensional distortion. The diffusion of chromium into the surface layer means the base material itself develops the corrosion resistance whilst retaining its basic properties, in particular hardness. As there is very little dimensional change or coating build up the bearing system doesn’t require specialised grinding or set up. They retain their inter changeability which means you can mix and match bearings and replace a bearing without having to replace the rail and bearing as a factory set.
Another item to consider is lubrication as even with a corrosion resistant system lubrication is still required. This is another large topic so maybe best left to another newsletter. One tip however – using good trade practice and keeping a layer of grease and oil on a linear bearing will provide a suitable level of corrosion resistance for most environments where moisture and liquids aren’t in use.
We specialise in linear applications only and would be happy to discuss any application requirements.