Higjh or low RPM when going down steep grade with HST?

[Sysop]

HSTs like lots of fluid flowing to keep cool. RPMs should be up anytime you're moving. I've never heard of an issue in a HST from running the engine at too high of RPM, they can keep up with whatever the engine puts out. I've heard of tons of HST issues from running the RPMs too low, not providing ample flow and burning up the swash plates/other stuff.

 

[MHarryE]

A properly engineered hydro is not going to melt down due to slow engine speed. Cooling comes from external flow is f cooled oil entering the loop. If the high pressure loop did not have this cooling flow entering on the low pressure side of the loop, yes, it would melt down. If that happens it's due to a design flaw. Hydro design one is to flush heat from the loop under all conditions, but it is high load operation that produces the most heat. Through 49 years of hydrostatic drive engineering I have not had one failure report due to melt down due to low speed operation but I can supply dozens of pictures of high speed operation melt downs where cooling was inadequate. Slow engine speed going sown a slope isn't going to fry the OP's hydro. Just like my Kubota arTV - it's designed for low engine speed downhill - speed control deal controls both hydro swash plate angle and engine speed. Same with Auto-Throttle on my Kubota L5740. Taking my go it off the pedal to slow gown descending a slope also slows the engine.

 

[AxleHub]

I think its important to recognize that tractors vary considerably how pump pressures and engine rpms interact. So blanket rpm numbers is not logical unless only a specific model is considered. My point is tractors have many different tachometers used because tractor rpm ranges are different. I'll use 2 current model scuts as an example.

A kubota bx2370 (23 hp) or bx2670 (25 hp) engine running at 1800 rpm demonstrates extremely slow hydraulic movement of the fel or 3 pt. Meanwhile a Massey GC1705 (23 hp) or gc1715 (25 hp) at the same 1800 rpm moves considerably faster. This is a differene of 1 pump vs 2 pumps and that rpm DOES effect pump flows which effects any part of the hydraulic system including power steering. The bx units even at 2200 or 2400 rpm are still slower and far away from their 3200 rpm operating range while a gc1705 at 2600 rpm is at full load maximum range and a gc1715 at 3000 rpm is at full load maximum range.

My point is that full load isn't an issue for this thread but a workable hydraulic flow level is. On a bx that number might be 2800 rpm and on a gc1700 series it would be 2000 or 2200 depending on model. Myself, I would not want to go down a steep slope on a bx at 1500 or 1800 or 2000 rpm but 1800 or 2000 on a gc1705 might be acceptable.

But a better question is why? What purpose do you serve in running the engine very slow when your hydraulic transmission is the primary speed of movement control. My thinking is I want slow ground movement but good response time to hydraulic control because if I need to adjust fel position or 3 pt ballast position on a steep slope . . I want that control immediately. That doesn't mean maximum rpms but it also doesn't mean very low rpms or slow drawn-out response times.

On my gc1715 I'd be at 2200 but on a bx I'd be at 2800 rpm.

In addition . . Ballast and fel load might require directional considerations. Do I want to drive down the steep slope or back down it? In all cases 4 wd so that front axle dynamic braking is involved. And certainly not a high gear because your sensitivity on the foot pedal is greatly different for ground speed between high gear and low gear.

Lastly . . an empty fel is a different rpm need compared to a loaded fel. My rpms stated refer to an empty fel . . If loaded . . then rpms need to be higher.

All inmy opinion.

 

[furnone]

One thing that is clear to me is that I don't have nearly as good understanding of a tractor's hydraulic system as most of the members of this forum. However, I think that Axelrod's thinking is most in line with my own.

As I understand it, more RPM's equals more hydraulic fluid movement and thus more power of the HST to move or hold back the tractor. So, I think my past practice of 2000-2200 RPM is a good prudent choice. I have been doing that for the past five years successfully, but always, as I started down the hill wondered if I was doing it right.

Thanks for all the input.

 

[RaydaKub]

All in my opinion.

I don't disagree with your reasoning at all, rapid hydraulic response could be critical. It's outside the bounds of the OP's question, but perhaps his question was self-limiting in scope. But inside the scope of his question, it shouldn't matter the rpm of the engine as long as it is enough to stop or even reverse the wheels.

 

[k0ua]

In my opinion, and in my experience, it doesn't make any difference. At least with the last two tractors I have owned that had hydro's.

 

[jenkinsph]

I don't disagree with your reasoning at all, rapid hydraulic response could be critical. It's outside the bounds of the OP's question, but perhaps his question was self-limiting in scope. But inside the scope of his question, it shouldn't matter the rpm of the engine as long as it is enough to stop or even reverse the wheels.

I agree with this, the engine needs to be fast enough to allow me to utilize forward and reverse engagement and be able to adjust the bucket quickly if needed. All without stalling the engine.

 

[brown40]

One thing that is clear to me is that I don't have nearly as good understanding of a tractor's hydraulic system as most of the members of this forum. However, I think that Axelrod's thinking is most in line with my own.

As I understand it, more RPM's equals more hydraulic fluid movement and thus more power of the HST to move or hold back the tractor. So, I think my past practice of 2000-2200 RPM is a good prudent choice. I have been doing that for the past five years successfully, but always, as I started down the hill wondered if I was doing it right.

Thanks for all the input.

The diversity of knowledge and experience on TBN is remarkable. There is a level of like-mindedness for folks that want to get it done themselves, ranging from farmers, doctors, engineers, law enforcement, welders, mechanics, every flavor of 9-5er, shift worker, independent contractors... it is a great resource, and a better community.
I'm no expert on HST's but MHarryE nailed my understanding of the pump and swash function--those who have rpm auto-tied to the transmission have got a really nice set-up in getting more power when you need more, but regardless the plate wants to go to neutral/stop/cease flow at any rpm and in any range when the pedal is released. Every HST I've been on wants to stop (more or less) when the pedal gets released.
On my 3520 the higher ranges "freewheel" and slow slower than low. If I have a big grade to go down working, I don't vary the rpm at all... if its transport then I like it quieter anyway so I get to 1500-1800 like most say. I don't count on the HST for braking, or parking, regardless of rpm, on or off.
There are some great exploded/cutaway HST videos on Youtube to see the basic functions of the system--also CVT and other types.