HST pedal creeping - how does this system work?!

[4lane]

Do you have some knowledge in how Kioti HST works?

My Kioti CK30 HST creeps forward or backward when you let off the rocker pedal. There are threads littered with diagnoses, none of which seem to clearly fix the issue. Some blame the HST, others point to lubricating where the the pedal mech enters the transmission, others claim damper replacement... I did some trials myself to see if I can isolate the issue. There are three components I played with:

a. HST
b. Damper (orange in image below) - this controls the rate of pedal return from fwd/reverse
c. Centering spring (blue in image below) - this centers the pedal to neutral when you step off the pedal



1. Normal operating mode, with orange damper and blue centering spring working together
On a flat if you let off the pedal it just snaps back to neutral. But when the transmission is under load, such as going up a hill, the pedal is more firmly held (I call it "loaded) in the fully extended position and requires a tap on the pedal the opposite way to get it to neutral, but it will still creep. You have to keep tapping it gently fwrd/reverse to find neutral. It's dangerous, so let's see if I can narrow down the problem...

2. Removed both the blue centering spring and orange damper:
Some have said that the HST has a role in centering to neutral so I wanted to see if this was the case by removing everything connected to the pedal. Press forward and it just stays there. Press reverse and it stays there there. It just flops back and forth between positions - there is NO resistance, and therefore no feeling the HST has any responsibility for returning to center. Consistent with #1, when going up a hill the pedal is "loaded" ()))more firmly held in the fwd position) but it takes just a tap harder to push it back to neutral than when unloaded, as in on a flat.

3. Removed Blue centering spring but kept the orange damper attached:
If you put the pedal to reverse and release, the damper pushes the pedal to the forward position. This seems odd. Since the pedal just flops fwd/reverse when there is no damper I would expect the damper to just keep the pedal at whatever position you put it in and it just gives the pedal some resistance when doing so. I found this odd and potentially wrong.

4. Removed the orange damper, but kept the blue centering spring attached:
Snaps back to neutral on flat ground. On a hill, it takes a few seconds for it to pull it out of it "loaded" state, as the pedal is slowly pulled from this position to center. If you take your foot off from full fwrd, you can watch the pedal slowly return to center. It does not do this quick enough (4sec), but once it does, it's in neutral, and does not creep. This seemed like a good sign.


My takeaways from all of this is that the creeping is not an issue with the HST but is an issue with the orange damper/blue centering spring. But I don't see how the damper/centering spring work together to have more force to pull the pedal back when it's "loaded" and then less force when it's not. I would say the blue centering spring needs to be stronger to pull the pedal back when it's "loaded" but that strength would need to be less when it's not "loaded" (i.e. on flat ground)

If anyone has some experience in how this system works, I'd love to hear your input.

 

[xcgreene]

I'll lend a bit of info on the pump itself if it helps your troubleshooting,

The HST pump/motor itself is nothing special. Its an axial piston pump. (two of them) The foot pedal actuates the swash plate, which stokes the axial pistons to increase fluid flow at the same pressure. swash plate at center is zero flow. Search axial piston pump on the youtubes and it will become clear. These types of pumps are used widely around the world in many different industries. Constant pressure, variable flow. very useful for ring line, and site wide systems, utilizing 1 HPU to service all equipment.

The swash plate may not always want to return to neutral on its own, depending on what the tractor is doing, and how the tractor is pushing back on the hydraulic fluid, which must be why they have the spring and damper. I agree, this seems like a spring damper issue.

 

[4lane]

I'll lend a bit of info on the pump itself if it helps your troubleshooting,

The HST pump/motor itself is nothing special. Its an axial piston pump. (two of them) The foot pedal actuates the swash plate, which stokes the axial pistons to increase fluid flow at the same pressure. swash plate at center is zero flow. Search axial piston pump on the youtubes and it will become clear. These types of pumps are used widely around the world in many different industries. Constant pressure, variable flow. very useful for ring line, and site wide systems, utilizing 1 HPU to service all equipment.

The swash plate may not always want to return to neutral on its own, depending on what the tractor is doing, and how the tractor is pushing back on the hydraulic fluid, which must be why they have the spring and damper. I agree, this seems like a spring damper issue.

Thank you for this. I watched some of those videos - they were helpful. On this Kioti, by feel at least, it seems the swash plate is not intended to return to neutral on its own and instead relies on the damper/centering spring to handle this.

I would still like to know why, the higher the load on the transmission (i.e. going up a hill), the more the pedal is "loaded". I don't understand this piece yet and want to eliminate this as a potential problem so I know its a damper/centering spring issue.

 

[xcgreene]

Thank you for this. I watched some of those videos - they were helpful. On this Kioti, by feel at least, it seems the swash plate is not intended to return to neutral on its own and instead relies on the damper/centering spring to handle this.

I would still like to know why, the higher the load on the transmission (i.e. going up a hill), the more the pedal is "loaded". I don't understand this piece yet and want to eliminate this as a potential problem so I know its a damper/centering spring issue.

While most all axial piston pumps are similar, the actuation of the swashplate varies a lot, especially when it comes to tractors, some are direct linkages others use pilot systems, and again still there is electronic actuation. From what I am seeing here, it looks like your CK30 has a few things to consider.

I think this diagram and description will be most helpful to you:

View attachment Pages from 04 CK25, 30 Transmi.pdf


If that PDF does not load up properly, here is a lower quality screenshot

 

[Sawworcs]

Talk to your Kioti dealer. Kioti made an updated damper that helps pull the HST back to neutral.

 

[4lane]

Yes I saw this too, but while it describes the various components in play, it's still not clear where the issue resides. I did email with a Kioti dealer and got a good response (see below). Still nothing to direct me toward but helpful in eliminating the HST from the equation. That leaves me back to my test where I was able to stop the creep: by removing the damper but keeping the centering spring attached. It's like the centering spring is not strong enough to overcome the damper. I would jump at rebuilding the centering spring by replacing the springs but you can't just buy the springs, unlike what the parts breakdown suggests. So...still grasping at straws here.


<---- Kioti dealer ---->

"As hst tractors get older this Creep becomes something that most customers live with. We find a laundry list of things that can contribute to creep. Most likely is not the hst system as it is a swash plate and does not have effect on centering. Most often we find linkage issues, like worn parts and bushings are the cause. Dempers and spring issues are not unheard of and the hst lever bolt on connection at the swash plate shaft is also common. It could be spring tension related also. Lastly yes the resistance of the pistons in the swash plate area (or lack of ) in an older unit could in theory contribute to a limited amount of stiffness to this system."

 

[4lane]

Talk to your Kioti dealer. Kioti made an updated damper that helps pull the HST back to neutral.

Ah! the Kioti dealer never mentioned this to me. By damper do you mean the orange damper or blue centering spring (which they also call a damper):

 

[SSdoxie]

The blue centering spring assembly was changed. If yours has 2 small Phillips screws in the case, 1 on top & 1 on the side, it is NOT the latest version. The change did not affect creep was only to assist in return to neutral but new ones still do not snap pedal back fast. If the rear mounting hole or the ball joint on the end show wear or if the shaft as any free play this would allow creep. The orange damper is to provide consistent amount of pressure to move pedal to aid with smooth control much like a shock absorber. When you have it removed and move it by hand it should require same effort throughout the stroke. The centering/return spring and damper are actually working against each other when you take foot off the pedal.

Looking at listed #4. If it does not creep with damper removed but does when connected I would be addressing the damper.

 

[4lane]

The blue centering spring assembly was changed. If yours has 2 small Phillips screws in the case, 1 on top & 1 on the side, it is NOT the latest version. The change did not affect creep was only to assist in return to neutral but new ones still do not snap pedal back fast. If the rear mounting hole or the ball joint on the end show wear or if the shaft as any free play this would allow creep. The orange damper is to provide consistent amount of pressure to move pedal to aid with smooth control much like a shock absorber. When you have it removed and move it by hand it should require same effort throughout the stroke. The centering/return spring and damper are actually working against each other when you take foot off the pedal.

Looking at listed #4. If it does not creep with damper removed but does when connected I would be addressing the damper.

This is great info, thank you! I have not seen any obvious wear on the rear mounting hole or ball joint but I will take a more thorough look at all the connection points - the Kioti dealer suggested this as well.

The orange damper has resistance both ways, but it does make a bit of an oil gurgling sound which I don't hear on new dampers of similar type. The blue centering spring (I have the older one) feels good with no slop at the center point.

Regarding my #4, with the orange damper removed and only the blue centering spring attached, the tractor stops without creeping but is slow to come to a stop when in full fwd position. I'm actually now thinking the issue is the blue centering spring being too weak to pull the pedal back from full fwd AND to counteract the force of the damper. Because, when the orange damper is added back into the system it is possibly dominating the centering spring's strength and therefore the orange damper is keeping the pedal in neutral position (but only because the blue centering spring is too weak). It's too bad the springs can't just be replaced to make it stronger...

Does that make sense?

 

[dfkrug]

Interesting problem.

My CK30 is only the 2nd year of this model, and the only times I get slow return-to-neutral is when the tractor (and fluid) is cold. It is worse if the engine speed is low. I use UDT-equivalent THF, not a cheap hyd fluid. Fluid temp and type could be an issue beyond the more obvious damper and spring. I am at just over 800h.