hst surges under load

[rglaser]

First of all this is my first tractor so it is possible I am noticing something that is completely normal. I have noticed that the hst surges a bit when under load. Basically going along under a light load (no ground engagement / level ground) then add a load (lower box blade or start going uphill) hst bogs momentarily then surges through the load. It feels almost like an automatic transmission downshifting in a truck or car. I mostly use medium range so that is where I have noticed it most but I have observed the same thing in low at least once. My tractor is a B2630.

My questions: Has anyone else noticed this behavior? Is it normal? What causes it?

Thanks fellas!

-Rob Glaser

 

[kuboman]

First of all this is my first tractor so it is possible I am noticing something that is completely normal. I have noticed that the hst surges a bit when under load. Basically going along under a light load (no ground engagement / level ground) then add a load (lower box blade or start going uphill) hst bogs momentarily then surges through the load. It feels almost like an automatic transmission downshifting in a truck or car. I mostly use medium range so that is where I have noticed it most but I have observed the same thing in low at least once. My tractor is a B2630.

My questions: Has anyone else noticed this behavior? Is it normal? What causes it?

Thanks fellas!

-Rob Glaser

My b3200 does the same thing but my b2620 did not. An annoying quirk I believe.

 

[rjkobbeman]

My b3200 does the same thing but my b2620 did not. An annoying quirk I believe.

Not familiar with your model, but it may be behaving similarly to the Grand L HST Plus tranny. Load sensing will "downshift" (if that is the correct term) so you don't bog-down/kill the engine. Allows you to "plow on through" without having to manually downshift.

 

[MarkV]

Do keep in mind that with a hydrostatic transmission,HST, the less peddle you apply the more torque you have. It is second nature for many of us to use more peddle for more power. Just the opposite with a hydrostatic tranny. Pretty common, for most it is part of the learning curve.

Hope you are enjoying the new tractor.

MarkV

 

[EE_Bota]

Do keep in mind that with a hydrostatic transmission,HST, the less peddle you apply the more torque you have. It is second nature for many of us to use more peddle for more power. Just the opposite with a hydrostatic tranny. Pretty common, for most it is part of the learning curve.

Hope you are enjoying the new tractor.

MarkV

I figure the maximum torque available to the hydraulic motor is limited by the HST pressure relief, and the maximum speed is controlled by the engine rpm, and the pedal displacement, so for max power, I go to increased pedal position. For max torque, I push the pedal only a little, and I do not push it further if the HST is in pressure relief, because it will not help.

 

[perdurabo]

If you get power surges and choppy performance with your hydraulics, first check that your hydraulic fluid level is ok and then check that the hydraulic vacuum return hose (usuaully just forward of the brakes on the right) doesnt have a crack in it. These hoses have been known to crack quite often and cause all kids of weird hydro behavior.

 

[638]

Rob, that is a normal occurance. Engine RPM drops slightly due to the added load. Governer opens up and adds more fuel. Engine picks up RPM. Tractor surges forward. HST has nothing to due with it.

 

[Toolguy]

My B3030 will surge sometimes, usually when I am mowing on level ground and after a turn. It will also surge after going downhill and then going on the level. It feels like it has to catch up and pull after holding the tractor back or just coasting. I don't think it has anything to do with the governor when mine does it. When the surge happens the tractor is under very little load. The engine tone does not change. I assume it is the HST reacting to a load after having no load. Just something to get used to with a HST.

 

[Wheatland KB]

I've noticed the same on my L3400HST. I just verified that the fluid level was proper and kept on tractorin'

 

[firefighter jim]

Believe this is normal. My 2 Kubota HST's do the same thing. Very noticeable in high range.

 

[red4now]

Believe this is normal. My 2 Kubota HST's do the same thing. Very noticeable in high range.

What he said re: high range - same here.

 

[rglaser]

Thanks to all for the input. I don't think this has anything to do with the engine because sound and tachometer indicate no increase (maybe even a slight decrease) in engine rpm during this surging. My best guess after considering the responses so far and a little thinking is that under a light load, the hst is in some type of bybass mode and when it senses a load it comes out of the bypass mode and applys more power. Just my guess.

-Rob