Today I want to return to the gear pump topic, and use this Parker PGP 365 pump to showcase what can go wrong when you assemble such a unit, demonstrate that sometimes an assembly step can be less intuitive, even for a seasoned mechanic, and also present an example of how an overhaul can be ruined even when you have a "perfectly original" service manual on hand and follow it to the letter.
"Dude, gear pups are easy, why are you wasting time writing about them?!” I agree, they are, especially for people who do them every day. But most service manuals show you how things should be done. I, on the other hand, want to show things that should not be done! You learn best from mistakes, so allow me to show you some – it’s good education! And even though I am using a specific Commercial model, the bullet-points are generic.
A quick aside - I still call these pumps "Commercial" pumps, but they are not. The name appeared in 1988 when the "Commercial Shearing" was renamed to “Commercial Intertech”, but it was acquired by Parker in 2000, so it's all Parker now. I must confess that whenever I have to deal with the Parker gear pump division – my head begins to ache because I can't remember who's who anymore... An Ultra? Commercial? A PGP what? That’s way too many pumps with the PGP prefix, if you ask me. Oh yes, and then you have the Permco, the Metaris, and whoever else that makes or sells these copies in your region…
Anyways, it doesn't matter, you get a pump - you look it up. And I should get back to my list of things that can go south when you assemble a gear pump "of this caliber".
You will use Vaseline or grease to "glue-in" the pressure plate seals. Special attention must be paid to make sure the grease is clean because if you take it from that open can that has been sitting on your bench for the last couple of months, something like this can easily happen. As you can see - a random piece of plastic got "caught" in the Vaseline and then deposited under the seal during the assembly.
This is why it is imperative to always turn the shaft when you are tightening the housing bolts - an abnormally high torque will be an immediate indicator that there's something fishy going on with the gear group and you must check for misplaced and/or extruded seals or contamination. With this particular series, I like to run a pointy tool along the edges of the pressure plate seals to make sure they are inserted all the way into the grooves.
Another classic mistake concerns multiple gear units and lies in forgetting to install the connecting shaft. Very silly, and yet, it can happen and does happen. So remember - if your pump has more then one gear set - there should be a shaft in between.
Classic mistake number three - the position of the lubrication grooves. The manual clearly states where the lubrication grooves of the bushings should point and yet some mechanics still place them incorrectly. More details and examples of this mistake can be found here.
This, once again, only concerns multiple units. And this mistake is sneaky. If you look at the middle section - you will see that is houses the suction port that supplies both sides of the pump, and a pressure port that, naturally, connects to one side only, which means that this side has a pressure connection, and this side does not, so if you flip the mid section - you are essentially plugging the pressure outlet of the front pump!
I had a client who did this and then wondered why suddenly the engine would not start or even turn. I am so glad they didn't manage to start it before figuring it out!
This is the counter-intuitive part that I was promising in the beginning. Normally, when you install a shaft seal - you seat it in its place till it's inserted all the way. In this particular model - the shaft seal has do be secured with a non-hardening sealant flush with the recess surface. This is the seal in the correct position, and here's the seal pushed in. Even though in both positions the seal lip works on the prepared part of the shaft , I believe the flush position is recommended because the bottom of the seal housing can be not straight, and the seal may end up not perpendicular in relation to the shaft if you push it all the way in.
This is another thing that I have seen go wrong more than once. With this particular model this failure is a classic example of how following instructions "blindly" can do more harm than good.
Of course, every mechanic knows that tightening bolts to the required specs is very important, but the situation with the PGP 300 series is... how do I put it?... interesting. If you look it up in the service manual, you will see that it clearly states to use 450 ft. lbs. of torque on PGP/PGM365 units.
I can only say one thing: Good luck using that torque on 5/8'' rods! On high strength 3/4'' - maybe, but definitely on the 5/8''s. You will probably realize that something doesn't feel right after a couple of "extra" turns - but it will be too late at that point.
If you use the old P300 manual - you will only read 200 ft. lbs, which is about right. My advice with bolt torque is to always confront what service manuals state with a generic tightening torque table you trust. Typos and mistakes do happen, you know...
This mistake has nothing to do with the assembly process, but it is connected with the overhaul. When these pumps get new bodies and gear sets - running them in is very important. I've seen them fail after clients put them in full pressure service immediately after the overhaul.
The best way is, of course, running them in on a test bench, but if you don't have one - it can be done on the actual machine as well. Gradually increasing the speed and pressure over the course of several minutes usually works just fine.
So, them gear pumps are easy, right?