Insane Hydraulics

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The Importance of Knowing Where The Grub Screw Plugs and Orifices Are Located in Your Hydraulic Pump

Grub screws and small orifices are a common part of hydraulic pumps and motors. Whenever there is an internal oil channel that needs to be plugged or throttled - these tiny but very important parts come into play. Usually, they are held in place with a thread-locking agent and aren't supposed to come out, like ever. But they do come out, sometimes, and cause trouble and even damage. I can't even say why a glued screw can leave its place - but they do, on rare yet still existent occasions. Every once in a while I catch one of these out of place. A good example would be the hidden orifice in the newer A10s, and today I want to show you another one.

I finally received the parts for the Parker PV140 that I described in my "Longevity Hack" article, and as I was cleaning the pump case - I turned it upside down and heard a faint "clink" as if something tiny and metallic fell over my bench.

Obviously, one should never disregard a sound of a part falling, no matter how tiny it sounds, and as soon as I lifted the case to investigate - I saw a small grub screw. It was somewhat deformed - a clear indicator that it fell inside the pump when it was working, and got into a couple of "close shaves" with rotating components, before finding a safe spot somewhere inside the casing. Good for him!

But where did it come from? It was pretty easy to find out in this case because I had a complete set of new parts in my hands, and as soon as I looked at the new swash-plate kit - I saw a bag with three set screws (two plugs and an orifice) - and I immediately knew it had to be one of these.

This was easy. But I confess that if I were to re-use the swash-plate and if I hadn't heard the tiny screw fall - I would most likely have missed the missing plug and let the swash-plate leave the shop without it. Now - in that case, it was the suction side, so it's not a great deal, and the 14000 hours of this particular unit kind of prove it, although it is impossible to tell when the plug slipped out.

Now that I think about it... The fact that the unplugged orifice was venting the pressurized oil cushion from the slipper chambers - maybe this is why there was a distinct second circular wear pattern on the swash plate face (from the Longevity Hack article - look at the picture of the swashplate)?.. In any case - with normal speed and no piston load on the suction side the dynamic lubrication is more than enough, but had it been the plug from the other side - the pump most likely wouldn't work because the swash-plate would lose "hydraulic suspension" and get stuck.

My point is - set screw plugs and orifices can escape their places even when glued in place, and I've seen this many times, so when you work on a hydraulic component that uses them, make sure that:

a) you know where the set screw plugs and orifices should be (check them parts lists and exploded views)

b) you check if they are still there

c) you use an Allen key and check if they are still tight.

This will save you from nasty surprises, which, in the case of hydraulic machinery, are both nasty and expensive.

Oh yes - another very important thing. Knowing where such screws and orifices are placed is also important because whenever you scrap used parts - you should remove and save them "for emergencies". You wouldn't believe how many times a tiny box with an assortment of used grub screw orifices and plugs has saved my day! I am definitely keeping these: