hyraulics question

[MattJ]

I am trying to troubleshoot a HST problem on a bx1830 and looking through the flow diagrams in the WSM the one part I can't figure out is that it shows the transmission oil comes charged at high pressure from the hydraulic pump to the flow priority valve, which then sends it to the power steering (or to the implements). The flow path then shows it passes through the power steering and on to the HST at low pressure (i.e. charge pressure spec is 40-70psi), where it is then pumped back up to high pressure by the HST pump.

So how did the power steering reduce the pressure from just under 2000psi to 40-70 psi? Did I miss something?

A diagram that is identical to the BX1830 is here http://www.tractorbynet.com/forums/...mouse-nibbled-kryptonite-bx23-hst-diagram.jpg


Thanks

Matt

 

[Stonehaller]

There is a charge relief valve, item 8 in the schematic that I would assumes maintains the charge pressure. The oil coming out of the power steering will not be at 2200 psi as there should not be enough restriction to build that much pressure. The PTO relief will maintain a much lower pressure, sufficient to keep the PTO clutch engaged, maybe 150-200 psi. The drain out of the PTO relief feeds the charge relief and most of the oil flows over the charge relief to tank. If you do not have enough charge pressure, you could have a problem with the charge relief or you could have a leak in the PTO circuit.

 

[EE_Bota]

I agree.
We sent an associate to hydraulics school a couple of weeks ago, so to judge the quality of the course, I asked him "what does a pump do?" His reply was "a pump generates flow." So I asked "where do we get pressure?" He said "Pressure is resistance to flow."

The power steering likely will never see 2000 unless you turn the wheel all the way to the physical stop, at which point the power steering will relieve...but what it relieves past the relief is just flow, not pressure. The pressure was on the power steering side of the relief. So flow goes on to the PTO...and the PTO selectes it's pressure with it's own relief...and that flow (not pressure) goes to the HST, and it selects its desired pressure with its own relief.

The HST relief just mainly tries to make up for leakage if I understand correctly. The pressure is quite low. It tries to form its own local sump...so that that the HST pump has fluid to pickup and pump to the motor side of the HST.

Out of curiosity, what problem are you having?

 

[MattJ]

Its a problem with an bx1830 (500hrs on the clock, garage kept and babied) that I posted about a few weeks back. I don't have power under load and get a chattering that sounds like the HST is starved.

I first checked the transmission oil level, it was fine, went ahead and changed the transmission oil, filter and cleaned the screen as it was due. They all looked fine and I used super UDT2 as directed. This didn't give any positive result.

Next I checked the pump pressure at the front end loader, it was actually about 100psi high so I removed a shim and got it within spec (about 1800 psi). I also left the tester on and drove the tractor under load and then actuated the loader control during the chatter and got a reading of just over 1800 psi, so it seems the pump is keeping up under load.

I then got the parts to check the charge pressure and the forward motion circuit pressure. The charge pressure was at the low end of the range around 45 psi (range per WSM is 42-71) at a standstill, and when I drove the tractor up a hill to get the chatter going the charge pressure dropped to around 20psi or less.

I also checked the forward pressure under load (fyi, chatter is in forward or reverse), and got a max of 1100 psi. The high pressure relief per the WSM is 1991 psi.

So it seemed I wasn't getting oil to the HST in my very amateur opinion. I wondered about the flow priority valve as it splits between the implements in one route, and the power steering/HST in the other route. I took it apart and cleaned it, replaced it, and it didn't help. I also cleaned and inspected the charge relief and PTO relief although they looked fine.

My thought now was that the priority valve may be acting up. If you look at the design its spring loaded with the idea that if the pressure drops it blocks the flow to the implements. On the flip side under high pressure I wondered if it would push the plunger far enough (i.e. with a weak spring), to block the flow to the HST and give it all to the implements (under load chattering the FEL has tons of power).

As an aside the HST, engine, drive shaft all sound fine crusing along under low load. Under heavier load, i.e. a hill or pushing into a mulch pile with the loader, the HST chatters away and has no power while the engine doesn't bog one bit.

Right now I ordered a new priority relief valve and a new spring set for the charge relief valve and will give it a whirl.

Thanks to both of you for your help and I'll let you know how it turns out. The kubota dealer has been very helpful and said they would have done what I did, and are stumped at this point as well. My question about the pressure coming out of the power steering was I looked at the hydraulic schematic and it shows high pressure to the power steering, and low coming out. Perhaps because its dumping to the chamber around the HST and then the charge pressure relief dumps the flow to maintain lower psi?

 

[EE_Bota]

Its a problem with an bx1830 (500hrs on the clock, garage kept and babied) that I posted about a few weeks back. I don't have power under load and get a chattering that sounds like the HST is starved.

I first checked the transmission oil level, it was fine, went ahead and changed the transmission oil, filter and cleaned the screen as it was due. They all looked fine and I used super UDT2 as directed. This didn't give any positive result.

Next I checked the pump pressure at the front end loader, it was actually about 100psi high so I removed a shim and got it within spec (about 1800 psi). I also left the tester on and drove the tractor under load and then actuated the loader control during the chatter and got a reading of just over 1800 psi, so it seems the pump is keeping up under load.

I then got the parts to check the charge pressure and the forward motion circuit pressure. The charge pressure was at the low end of the range around 45 psi (range per WSM is 42-71) at a standstill, and when I drove the tractor up a hill to get the chatter going the charge pressure dropped to around 20psi or less.

I also checked the forward pressure under load (fyi, chatter is in forward or reverse), and got a max of 1100 psi. The high pressure relief per the WSM is 1991 psi.

So it seemed I wasn't getting oil to the HST in my very amateur opinion. I wondered about the flow priority valve as it splits between the implements in one route, and the power steering/HST in the other route. I took it apart and cleaned it, replaced it, and it didn't help. I also cleaned and inspected the charge relief and PTO relief although they looked fine.

My thought now was that the priority valve may be acting up. If you look at the design its spring loaded with the idea that if the pressure drops it blocks the flow to the implements. On the flip side under high pressure I wondered if it would push the plunger far enough (i.e. with a weak spring), to block the flow to the HST and give it all to the implements (under load chattering the FEL has tons of power).

As an aside the HST, engine, drive shaft all sound fine crusing along under low load. Under heavier load, i.e. a hill or pushing into a mulch pile with the loader, the HST chatters away and has no power while the engine doesn't bog one bit.

Right now I ordered a new priority relief valve and a new spring set for the charge relief valve and will give it a whirl.

Thanks to both of you for your help and I'll let you know how it turns out. The kubota dealer has been very helpful and said they would have done what I did, and are stumped at this point as well. My question about the pressure coming out of the power steering was I looked at the hydraulic schematic and it shows high pressure to the power steering, and low coming out. Perhaps because its dumping to the chamber around the HST and then the charge pressure relief dumps the flow to maintain lower psi?

Where you say "high pressure" in the bolded place above, I would substitute "potential for high pressure" since resistance to flow causes pressure. So, driving straight down the road, high pressure will not be true. Turned hard against the steering stops, high pressure could be true, with the relief setting the limit.

Always remember that all pressure drops with flow not doing useful work are producing heat. If that area had a continuous pressure drop, your tractor would get really hot, and not last so long.

But I think I am telling you things you already know, since what you write sounds like you really know your stuff. But, others read this stuff to, and maybe it will help them.

I am a bit worried about your tractor, since you are getting deeper and deeper, and finding more of the easy fixes don't need fixing. I will definitely watch this thread. I am at a bit of a loss to understand how to differentiate the axial piston motor from the axial piston pump. If it gets to that, which is likely a problem? The pump is more complex, what with the yoke and the variable swashplate, etc.

I need to review the earlier thread sometime, to see what you guys said about linkages from the pedals, ect, as a possible cause.

 

[MattJ]

Thanks for all the great advice. I am trying to learn how these systems work while I also try to get this fixed. Its been kind of frustrating but being an engineer by training I don't want to run up the white flag just yet! Your explanations have really helped a lot and I appreciate that.

 

[MattJ]

I thought I would post an update. I picked up the flow priority valve yesterday and once I installed it seemed to clear 90% of my problems with loss of power and vibration with my BX1830. Before it didn't even have power to drive in and scoop up a pile of light mulch. Now it can easily drive into heavy topsoil and doesn't seem to have any power problem.

The only reason I won't call it 100% is I still have some minor vibration when I was really pushing it and normally the high pressure relief trips on the HST.

I am thinking those valves might need to be checked as I was pretty certain in the past when I got to the point where the HST just simply hit its pushing limit there was a more smooth relief feeling, and usually a bogging down of the engine. Anybody else want to comment on how there's feels? In a few weeks I may get the ambition to check the forward/reverse HST pressure and dig into the relief valves but for the time being I'm going to mow some grass!

I just wanted to let you know the flow priority valve does seem to be the main culprit for my BX's loss of power, and in the slight chance any of you see a BX with the same issues you might want to look there.

Thanks again for everyone's help

- Matt

 

[EE_Bota]

I thought I would post an update. I picked up the flow priority valve yesterday and once I installed it seemed to clear 90% of my problems with loss of power and vibration with my BX1830. Before it didn't even have power to drive in and scoop up a pile of light mulch. Now it can easily drive into heavy topsoil and doesn't seem to have any power problem.

The only reason I won't call it 100% is I still have some minor vibration when I was really pushing it and normally the high pressure relief trips on the HST.

I am thinking those valves might need to be checked as I was pretty certain in the past when I got to the point where the HST just simply hit its pushing limit there was a more smooth relief feeling, and usually a bogging down of the engine. Anybody else want to comment on how there's feels? In a few weeks I may get the ambition to check the forward/reverse HST pressure and dig into the relief valves but for the time being I'm going to mow some grass!

I just wanted to let you know the flow priority valve does seem to be the main culprit for my BX's loss of power, and in the slight chance any of you see a BX with the same issues you might want to look there.

Thanks again for everyone's help

- Matt

I have turf tires, Matt, so what do you have?
I can get into a relief situation in rabbit only usually, since in turtle, my wheels almost always break traction before relief can act.

Regardless, any time I am in relief, it sounds very little different than FEL or 3pt relief. But usually, I break traction before relief, and on dry dirt, my wheels will hop and jump....grab,break,grab,break. That can seem like a serious vibration.

I believe you about your vibration, even though mine is a grab break. But it is hard to get a look at all four wheels of my BX while in the line of fire, since my rear fenders hide the rear tires pretty good.

I am glad you got some improvement, but I am curious, what did the priority valve set you back? Was it a shot in the dark, or did you have indication, and if so, what was your best indication?

Did you recheck the charge relief pressure to see if it was up more now? I guess I am curious if your charge flow could have been low, and your charge relief pressure low as well, and now you may have your flow back up, but your charge relief could be to the low side of spec still. Just a though...not a fact.

 

[MattJ]

My thought on the flow priority valve was based off the fact I had about 45 or so psi charge pressure at a standstill (just within spec), and then it dropped like a rock under load up a hill to sub 20psi, while the pressure to the loader was OK. It shouldn't be this way, the valve should shut off flow to the loader/3pt and give priority to the HST/power steering per the manual.

Looking at the design (attached pic) if the spring assembly was shot it could push the plunger at high pressure back and block the HST port and open the accessories port. Holding the valve in my hand it seemed like it was a delicate balance.

The gamble was $140, since kubota doesn't sell the spring and other essential parts a la carte. As for the result after install the charge pressure stayed around 55 psi at rest and under load.

The vibration I am seeing is a bit atypical. I do have turf tires and they do break free but I get a high frequency vibration that is not the tires just grabbing. You can feel the feed back all the way through the rocker peddle. It only happens when I was really pushing hard with the loader, or when I chained the tractor to a tree to force the wheel spin!

So my thoughts on why this happened. The only idea I have so far is about 50 hours back I checked the pressure at the loader as it seemed weaker than it was new, it was below the lower end per the manual, so I shimmed the pressure relief valve on the assembly shown in my picture. When I rechecked, I was perhaps 50 psi over the limit, but thought there was little harm that could be done (per the tons of posts on TBN related to this) and left it. It worked great until the vibration suddenly occurred. Right now the shims are back out and I am running at the low end of the range.

Thanks again for all your help. It was great for helping me understand.

 

[leonz]

Hello Matt,

Over time the springs do do loose thier
tensile strength and this affects the ability of the
control valve to work properly.


The centering springs in the directional valve may be
weak as well and this may be something to look at as well.:thumbsup: