Cylinder Drift on a FEL

npalen · Dec 19, 2012

I recently installed a seal kit in the BF350 FEL on by Kubota B9200 to fix an external leak past the gland or "packing" letting fluid past the rod on one of the lift cylinders. I was surprised by a couple issues with one being the fact that there was no Oring to stop fluid getting past the interface between the piston and the rod. I did notice that the piston ID and the rod OD were machined to a very close fit. Is this supposed to provide zero leakage?
The other issue was no "no leak" seal on the OD of the piston where it contacts the cylinder bore. The seal at this point consists of two rectangular cross section rings with one fitting over the outside of the other. I would normally expect to see a "polypak" type seal here where the pressure expands the seal preventing leakage. There is a polypak type seal inside the gland bearing against the cylinder rod.
My question is this: Does Kubota and perhaps other loader manufactures design their FEL's to not have zero leakage? I could see this as perhaps a product liability safety feature to allow an FEL to very slowly lower itself if inadvertantly parked with the loader in the raised position.

kennyd · Dec 19, 2012

I can't say they are "designed" that way, but lucky are the few that have "zero" leakdown on their FEL's, no matter what brand. As far as liability, I believe most owners manuals will say to lower all implements to the ground when parking for safety reasons, and that's good advice to follow.

art · Dec 19, 2012

Just thinking of designs and times that at the time the 9200 and the BF loaders were made a lot has changed in that industry for the packing configurations.

EE_Bota · Dec 19, 2012

I watched our forklift mechanic rebuild a cylinder the other day, and the piston seal was right located midway between the two piston faces, and it was a rectangular seal on top of another underneath. It is important that the metal underneath the inner seal be clean with no contaminants. Also, he replaced the o-ring where the piston joins the rod.

That cylinder was not bad to leak down at all until the seal failed. It failed due to simple wear against the cylinder bore. It was easy to tell the difference between the old worn seal and the new seal.

I think that sealing system would be virtually leakless so long as the seal is in great shape. I never assume a spool will be leakless, but one can get close if there are cross-port checks in the valve system.

npalen · Dec 20, 2012

I do notice that the loader settles much slower now that before the seal kit was installed even though I "resealed" the one lift cylinder.
I get a little confused about what exactly happens when, for example, one of the two lift cylinders has a small leak bypass on the piston OD and the other cylinder has "zero" leakage.
The fluid going past the piston has to occupy a smaller volume on the rod side of the piston and consequently there should be a pressure increase on the rod side of both cylinders (since they're plumbed in parallel). Am I correct in assuming this excess fluid has to leak back thru the directional valve otherwise there would be an "infinite" pressure increase on the rod side? This is assuming that the load continues to settle.

SSdoxie · Dec 21, 2012

npalen said:
I do notice that the loader settles much slower now that before the seal kit was installed even though I "resealed" the one lift cylinder.
I get a little confused about what exactly happens when, for example, one of the two lift cylinders has a small leak bypass on the piston OD and the other cylinder has "zero" leakage.
The fluid going past the piston has to occupy a smaller volume on the rod side of the piston and consequently there should be a pressure increase on the rod side of both cylinders (since they're plumbed in parallel). Am I correct in assuming this excess fluid has to leak back thru the directional valve otherwise there would be an "infinite" pressure increase on the rod side? This is assuming that the load continues to settle.

Pressure is equal throughout the system ( where the oil can flow), so a leak in 1 seal will allow oil to pass to rod side in 1 cylinder but the pressure goes with it so the pressure in rod & base side are the same in both cylinders. If the oil cannot escape to outside or leak past the control valve spool, the cylinder will not lead down because with equal pressure on both side, the base end has more surface area at the piston(s) giving it more push force to hold the load in place.
If the weight on the cylinder is trying to pull out, in same type of seal failure, then the rod can extend because the base end of the rod has more push force - pushing the rod out.
When testing for leak down on a lift cylinder you cannot plug both ports or cylinder will not leak, even without any seal in it - must leave rod end port open. When testing on bucket curl cylinder, you can plug both ports and a bad cylinder will still leak down.

EE_Bota · Dec 21, 2012

npalen said:
I do notice that the loader settles much slower now that before the seal kit was installed even though I "resealed" the one lift cylinder.
I get a little confused about what exactly happens when, for example, one of the two lift cylinders has a small leak bypass on the piston OD and the other cylinder has "zero" leakage.
The fluid going past the piston has to occupy a smaller volume on the rod side of the piston and consequently there should be a pressure increase on the rod side of both cylinders (since they're plumbed in parallel). Am I correct in assuming this excess fluid has to leak back thru the directional valve otherwise there would be an "infinite" pressure increase on the rod side? This is assuming that the load continues to settle.

I like what SSdoxie is saying. Here is my version of the same thing to show how I think about it.

Usually, the piston side is what is holding up the load on a loader. Have you ever seen one of those huge cylinders that only have a connection on the piston side, and the load itself contracts the cylinder when the pressure is released? That is called a "hydraulic ram." A bottle jack is a simple hydraulic ram.

In a situation where a cylinder has a load trying to collapse the cylinder, normally the piston is holding the load. But if the seal leaks, the pressure is equalized on both sides of the piston, but the rod itself extends outside the cylinder proper. So the cylinder gets reduced to a simply hydraulic ram, and the effective piston size is the size of the rod. Therefore, the leakage of the piston seal will not result in much movement at all, but instead will result in a huge increase in pressure. For example, if it took 2500 to raise the load, then when the spool is centered, the pressure may go to 7500. Your leak is converting the cylinder to be just like a bottle jack. The rod now effectively becomes the only remaining piston.

If there are two cylinders in parallel, that piston seal leak may as well be on either cylinder, or both, since both are converted to simple hydraulic rams.

It is possible that the increased pressure is enough to burst a hose.

If the spool leaks, the leakage rate will be increased because the pressure is increased. In the normal case, the spool leakage rate on the piston side is the concern, and at normal pressures. In the bad seal case, both leakage paths (A and B) are the leakage paths, and at much higher pressure. In nice systems, there are cross port checks, so this may be negligible.

I have cases where there are 8 cylinders in parallel. If one develops a bad piston seal, it is difficult to tell which cylinder has the bad seal.

I have other cases were there are many cylinders in parallel, but through a series of flow divider valves, all cylinders should move at the same rate. Sometimes I can tell which cylinder has a bad seal by that cylinder lagging.

Overall, I hate having cylinders in parallel. I think if I ever have to rebuild a cylinder on a loader, I will rebuild the pair.

Here is a great writeup on it, and the video is also worth watching. I would love to have the simulation software he is using.

http://www.insidersecretstohydraulics.com/hydraulic-cylinder-myth.html

npalen · Dec 22, 2012

Interesting stuff, guys! Is adding a dual pilot operated check valve between the valve and cylinders a good solution to loader drift? Is the dual pilot operated check valve the same thing as the "cross port checks" that you refer to? One scenario for which I would like to have zero drift is when using the loader as a scaffold support.

J_J · Dec 22, 2012

If you have down force and a leaking seal, the cyl will drift.

http://www.machinerylubrication.com/Read/1119/hydraulic-cylinder-drift

These mechanical cyl rod locks are about the only positive cyl locking device.

The load checks are for a good cyl.

oldnslo · Dec 22, 2012

npalen said:
Interesting stuff, guys! Is adding a dual pilot operated check valve between the valve and cylinders a good solution to loader drift? Is the dual pilot operated check valve the same thing as the "cross port checks" that you refer to? One scenario for which I would like to have zero drift is when using the loader as a scaffold support.

Some better quality loader valves have mechanical opened checks built into them. These stop or significantly reduce cylinder drift caused by spool leakage as long as the load is trying to retract the cylinder.

If the checks are hydraulically piloted the ywill cause the FEL to chatter or bounce while being lowered under load. Reason is that to open you must build pressure in the rod side of the cylinder. The P.O. check opens the load drops and pressure is lost in the rod end so the check closes until pressure builds, check reopens, FEL drops, check closes, etc.

FYI: To meet safety codes for handling personal the load holding device must be hard plumbed or mounted to the lift cylinder.

Roy