L785 Hyd Pump Seal Leak

[robbp]

There is a hyd leak from around the shaft on the back pump which operates the left drive side of the skidsteer. It only leaks when the forward/reverse control is engaged to move or steer the machine. The schematic only shows one seal on the shaft at the back where the leak is coming from, which sits behind a snap ring. The service manual only gives a procedure that explains to press the shaft out upon disassembly in a press. I pulled the Hyd Pumps out and got a look at the leaking seal. Before removal you could tell it had a rough spot at one place on the shaft. The inside of the seal showed major rubbing damage, with a damaged round seal spring and the washer next to had been rubbing against it. The shaft had about a 1/8 inch back and forth movement prior to removal. I put a little pulling pressure on the shaft after removal of the seal and to my surprise it pulled out. The 3rd snap ring on the shaft, the one on the inside of the thrust bearing was broken in 3 pieces. The shaft looks good and looking in the pump shaft hole nothing else looks worn or damaged. So now I am confused. What would break that snap ring in 3 places while it was in place, and what is suppose to keep this shaft from moving in and out. Would the outside snap ring, seal, washer and 2nd snap ring keep if from destroying the seal. The coupler on the end of the shaft was a taperd fit with washer and bolt in the end. I did have to hit the end of the shaft while blocking behind the coupler to get it off, could this have broker the snap ring? Do you think replacing these damaged parts will cure the leak or should I disassemble the whole pump? thanks, Robb

 

[Tazza]

Its possible the shock broke the snap ring. As you did say the wear from the seal was in the same spot and not a big wear mark from the seal. There should be no back and forth play or any load when its running so a snap ring is all thats needed. The sauer sunstrand/danfos pumps that run in the later Bobcat machines use a small snap ring to hold the bearings and shafts in place. Changing the snap ring and seal should fix the problem, but ensure you get the rite seal! non genuine ones can fail.
Good luck

 

[robbp]

Its possible the shock broke the snap ring. As you did say the wear from the seal was in the same spot and not a big wear mark from the seal. There should be no back and forth play or any load when its running so a snap ring is all thats needed. The sauer sunstrand/danfos pumps that run in the later Bobcat machines use a small snap ring to hold the bearings and shafts in place. Changing the snap ring and seal should fix the problem, but ensure you get the rite seal! non genuine ones can fail.
Good luck

I am in the process of putting the new seal in and wanted to double check the way it should face. The spring side of the seal should face the oil under pressure right? That is the way the old one came out, but it seems that the seal is tapered the other way. Also, the book says the two directional relief values on the pump limit the maximum pressure to 5000 PSI, but there is no way to test them. It says they either work or not. What would the symptons be of them not working? Would the pump building too much pressure possibly cause the broken snap ring I found, since that is apparently what let the shaft move against the seal. thanks, robb

 

[Tazza]

I am in the process of putting the new seal in and wanted to double check the way it should face. The spring side of the seal should face the oil under pressure right? That is the way the old one came out, but it seems that the seal is tapered the other way. Also, the book says the two directional relief values on the pump limit the maximum pressure to 5000 PSI, but there is no way to test them. It says they either work or not. What would the symptons be of them not working? Would the pump building too much pressure possibly cause the broken snap ring I found, since that is apparently what let the shaft move against the seal. thanks, robb

With a seal, you need the lips to face the pressure side. The spring you speak of put the lips under like a pre-load to hold the oil back, so yes you have it the rite way.
If your relief valves weren't working you would loose drive in one direction on one side. They will NOT build up pressure in other parts of the pump. It will allow for the oil to go back through the system without going through a motor. It will take the path of least resistance.

 

[robbp]

With a seal, you need the lips to face the pressure side. The spring you speak of put the lips under like a pre-load to hold the oil back, so yes you have it the rite way.
If your relief valves weren't working you would loose drive in one direction on one side. They will NOT build up pressure in other parts of the pump. It will allow for the oil to go back through the system without going through a motor. It will take the path of least resistance.

I replaced the seal and broken snap ring by just pulling out the shaft w/o having to take the pumps apart. I got everything back together and reinstalled the pumps in the machine. I ran the machine some this afternoon and did not have any leaks. Thanks for your advice and help. Robb

 

[Tazza]

I replaced the seal and broken snap ring by just pulling out the shaft w/o having to take the pumps apart. I got everything back together and reinstalled the pumps in the machine. I ran the machine some this afternoon and did not have any leaks. Thanks for your advice and help. Robb

*fingers crossed* hope it holds up for you.