How does the NP231 pump work?

Okay, the enginerd in me couldn't stop thinking how this little bastard works. But I think I figured it out.

The pump is a simple internal gear pump, As fluid is pulled into the pump, it's forced out between the pump body and the pump gear. The oil is pumped up the splines of the output shaft directly to the planetary gears where it's needed. The chain and outer sprockets are lubricated by the oil bath method in the lower part of the transfer case housing. This is a super simplification of how I believe this thing works.

If anybody else with more experience can chime in to back up or correct my information, please do so.

It’s a gerotor pump. The cavities in the rotor and the housing it spins in creates pressure that pulls the fluid in and then gets dispersed out. The pump has a seal on it that seals to the main shaft to keep the pressure in the pump. Without the seal I don’t believe the pump would pick up any fluid.

MountaineerTom said:

I installed my SYE a year ago. I took a few pics, but that was the best one I got of the pump. Looking at the pic, I couldn't figure out where the fluid would get dispersed out. It didn't seem to have any other holes for that. I never thought about it going up the splines of the output shaft. Also, thinking of the chain and the bottom of the gears slinging fluid around got me to thinking, why is the pump even there?

Thanks for helping me understand it better.

That's the first pic I've seen of the inside of the pump.

Click to expand...

There should be a hole in the main shaft that sits inside the pump and the pump pushes fluid out that hole. Then the fluid comes out of the other holes into the bearings. That is actually the purpose of the seal from the pump to the shaft, if that wasn’t there the pump wouldn’t push fluid through the hole and it would just leak back out of the pump into the case.

The chain does a bit of splash lube, but the fluid slung by the chain wouldn’t reach the planetary or the input bearing simply from chain splash lube. So the pump is needed. I think pretty much every chain driven case has a pump, or at least every one I’ve seen from Aisin, Borg Warner, and New Process/New Venture do.

Gear driven cases I believe are all splash lube because the gears do a good job of distributing oil to where it needs to go, like in a manual transmission.

Shaft for reference...take out after installing my SYE.



-Mac

I believe the pump seals on this smooth area and then also the rotor goes on the splines. I think the oil pumps into that hole right at the end of the splines.