I was recently consulted about a situation where the glass-reinforced nylon, rod guide-bush in a hydraulic cylinder was scratching the hard chrome plated rod. So in effect, the guide-bush was harder than the hard chrome plating. And this should NOT be the case. In other words the hard chrome plating on the rod was soft. Or more correctly, not hard enough. The hardness of the chrome plating should be Rockwell C (Rc) 65 minimum, and preferably higher.
So how can the hard chrome plating end up not being hard enough? This can happen when the surface area of the cylinder rod to be plated, is too big for the electrical power available to the chroming tank. If this happens, the rod still gets electroplated, but the chrome is not as hard is it should be. Obviously, the bigger and longer the rod, the more power required to chrome plate it with sufficient hardness.
One way chrome-plating companies get around this issue, is to ‘double-dip’ the rod. To do this the plating plant has to have vertical tanks, as opposed to horizontal. Vertical tanks are superior by the way, because they result in more concentric application of the chrome. The piston-rod is lowered vertically into the plating solution to only half its length. Then, when that half of the rod has been plated to sufficient depth, the rod is flipped over and the other half is lowered into the plating solution for chroming.
Doing this halves the surface area to be plated at one time. And this enables the plating plant to handle rods that they don’t have the power to electroplate with sufficient hardness otherwise. The obvious negative to double-dipping the rod is that there is a join in the chrome. Done properly, you shouldn’t be able to see it. But it will always be there.
But is ‘double-dipping’ something to be avoided? Ideally, yes. If you need to get a large cylinder rod hard chrome plated, I recommend you try to find a supplier who can do it – without compromising hardness, by NOT double-dipping. But if that’s not possible, then you really don’t have a choice. In other words, pursuit of a continuous chrome layer at the expense of finished hardness is a mistake. And to discover 6 other costly mistakes you want to be sure to avoid with your hydraulic equipment, get “Six Costly Mistakes Most Hydraulics Users Make… And How You Can Avoid Them!” available for FREE download here.