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     I like recalling the following episode as another good example of how bench tests sometimes fail to simulate real life conditions.

    At that time I was employed at a company that sold (among other brands) losi (www.loesi.de) steering units and hydraulic motors. A client was claiming that a brand new steering unit was causing the oil of his tractor to overheat, although the power steering function was ok.  He brought the unit over to our shop, and it was immediately hooked up to a bench test. The steering was of a very basic type - simple open circuit, non-reaction, with a relief valve and no anti-shocks - the type any hydraulic shop sells a dozen a week... The test itself was also the classic no-brainer - the steering  was connected to a fixed displacement pump (P port), tank (T port), and a hydraulic jack (ports A and B). The unit functioned flawlessly - everything worked one hundred percent ok without anything even remotely suggesting overheating. The client, as it would be expected, was advised to look for a problem somewhere else in the hydraulics of his machine.

   A couple of days later the client sent the unit over again and said that it was still overheating the oil. The power steering function was fine, meaning that the wheels turned and the effort on the steering wheel seemed to be normal, yet the oil would overheat. When the new unit was replaced with the old one - the overheating problem disappeared. Understandably the man was pretty upset, because in an honest effort to "fine-tune" his tractor he'd paid good money for the new steering, which was now causing him downtime without any fault from his side. Anyway - the unit was reconnected to the test bench, a steering wheel was attached to it, and a mechanic spent a good hour "playing Schumacher" in attempt to see if any signs of malfunction or overheating would appear - and yet again without any result - the damned steering was working fine... Working pressure was normal, pressure limiter was functioning fine, the delta P between the P port and the T port was normal - nothing, absolutely nothing suggested that the unit was problematic in any way. Still the owner was ardent that it was the steering that was causing overheating. The fact that when he replaced the unit with the old one the overheating problem went away, and when he re-installed the unit back the problem reappeared, was pointing to the unit as the source of all evil.

   Luckily fate intervened - the frustrated Schumacher gave the steering wheel a violet kick, and then suddenly the unit went "s-s-s-s" and the stand-by P to T free flow differential of 0.5 bar suddenly went as high as 40 bar!!! Obviously something was strangulating the oil passage. When the pump was stopped and re-started - the "s-s-s-s" disappeared, and when the wheel was turned violently again - reappeared! Hurray! If you see the problem you can solve it!

    It turned out that the P line check valve, which was of an "inverted umbrella" design,  was not in the correct position (too much in - see schematics) which allowed it to function as a some king of a hose rupture protection valve (speed fuse, if you will). When the oil was flowing through it at normal speed, the valve was opening normally, but when the flow would  increase abruptly, the poppet would "glue" itself against the bottom oil passage, thus strangulating the oil flow and causing the overheating, without jeopardizing the assisted steering function, as the oil was still passing through the valve. Normally, when the operator started the tractor, he would accelerate it for a short time, thus causing the increase of flow big enough to put the check valve in the "heating mode", which then would stick in that position till the engine was turned off. The problem was quickly solved by fixing the check valve in correct place.

   Moral of the story (yet again):

- even the simplest component (like a check valve) can malfunction
- sometimes it is next to impossible to simulate machine working conditions on a test bench
- even a brand new component can be faulty
- want to be a good hydraulic technician? - Start opening your mind!    
steering overheating