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.… continue reading »

Pipe threads are a lot like suction strainers in that they have no place in a modern, properly designed hydraulic system. HOWEVER, pipe threads are a legacy item, and as such, are still a feature of many (too many) hydraulic systems still in operation today. So every hydraulics user needs to know about them.

National Pipe Tapered (NPT – U.S.) and British Standard Pipe Tapered (BSPT – UK) were originally designed for water pipe plumbing (60 psi rating). Both NPT and BSPT thread forms engage on the sides (flanks) of the threads. This leaves a spiral leak path at the root and crest of the threads, which makes the use of a sealing tape or compound essential for any chance of a leak-free connection to be achieved.… continue reading »

The term pilot ratio refers to the ratio of two, discrete areas of a piston, spool or poppet on which opposing pressures act. The pilot area, on which pilot-pressure acts, is always the larger of the two areas. Working or system pressure acts on the smaller, opposing area. This means a relatively small pilot-pressure can overcome a much larger working or system pressure. In other words, pilot ratio is the hydraulic equivalent of mechanical advantage.

For example, consider a pilot-operated check valve with a pilot ratio of 4:1. If working or system pressure acting on the smaller poppet area, which opposes the larger pilot-piston area, is 3,000 PSI, it means pilot pressure of just greater than 750 PSI is required to overcome the working pressure and open the poppet (3000 / 4 = 750 – this assumes the pressure in the downstream port in the reverse flow direction is negligible, otherwise the effective pilot area is reduced).… continue reading »