Advice on Hydrostatic Transmission vs Gears

[hayden]

Good advice for someone who does not fully understand the issue. It will not solve the problem I stated. The issue, mainly, is that the HST - at relief - will dump all energy stored in the rotating mass of the engine, and the energy it is producing while running, into the hyd sump. The relief is absolutely necessary due to the zero to max speed control. Right there at 0+ you have infinite "mechanical advantage" to provide very low hyd flow at "infinite" pressure, enabling infinite torque - even with a small engine. You wouldnt even know it til it broke. This situation cannot be allowed.
A gear trans positively dumps its energy to the wheels, or the clutch slips, or it breaks. The mechanical advantage is always finite and constant in accordance with the gear ratio. HIGH overload transients can be supported because a good safety factor must be built in to survive even normal use in such a "direct" drive system. All bets are off with creeper gear ranges tho. Mechanical advantage there is astronomical.

I guess I'm not seeing what you describe in all my years running various HSTs. In my experience, if the tires don't spin, the engine stalls. There is no relief valve function like in lifting hydraulics. Now I haven't looked closely at my HST design to see what actual relief valves exist to protect the internals, but functionally it works just the same as a gear transmission in that it either spins the tires are stalls the engine depending on the selected gear and load. I think if you are hitting some sort of relief valve, you are just operating in too high a gear. I run into that all the time with loader work where I can't push hard enough into a pile of material. Selecting a lower gear lets me push until the wheels spin, which is my cue to stop pushing.

 

[hayden]

eek. That sounds like a like a quick fluid fry, circulating the oil straight back to the pump. The energy dump is across the relief valve. The fluid going back to the pump is then re energized as it is pulled through and forced out to whatever will take it. When the wheels wont turn a small amt leaks past the working elements in the hyd motor that drives the wheels leaving the remainder deadheaded and relieved continually at the relief. That isolated circuit will heat very quickly.

OK, but so what? This is why HSTs have oil coolers. I guess I don't see what teh big deal is, other than an inconsequential downside. HSTs consume more energy - that's well know. In their typical applications, that's a welcome tradeoff for the operational convenience and performance.

 

[hayden]

1st range of course, at or near full rpm. 4wd. If wheels spin I lock the diff. If wheels wont turn I push the pedal a little farther til it spins, moves, or the engine bogs w/o wheels turning. Theres the force limit.

- - What do you mean "shouldnt do that"? - I explained the inherent absolute necessity of a relief valve earlier in post #19. The variable displacement pump, controlled with the pedal, is capable of producing system destructive pressures without enough engine load apparent to cue the operator of the strain developing.

Sounds like the gearing in the NH may not go low enough. I've never run one, so no personal experience with them.

 

[walnutman]

Hydrostatics are extremely controllable, but hugely force limited compared to gear tractors. In my experience based on more than 10 gear or hydro tractors, you will always encounter situations where you cannot apply enough "go" in any range to spin the wheels on an HST. Up to that point you are happy. After it you feel betrayed -- and maybe stuck.

There's a new kid on the block in the Yanmar IHMT hybrid hydrostat transmission. It basically switches between hydo and full coupling depending on usage. Trust me, my YT359C with 59hp has no problem grunting into a pile with the loader and spinning tires... in 4x4.
and with 59hp the trans put 52 into the PTO. That's efficient.

Read up Yanmar YT3 series (YT347/YT359) IHMT/I-HMT Transmission workings

 

[SPYDERLK]

There's a new kid on the block in the Yanmar IHMT hybrid hydrostat transmission. It basically switches between hydo and full coupling depending on usage. Trust me, my YT359C with 59hp has no problem grunting into a pile with the loader and spinning tires... in 4x4.
and with 59hp the trans put 52 into the PTO. That's efficient.

Read up Yanmar YT3 series (YT347/YT359) IHMT/I-HMT Transmission workings

Thats an interesting "new kid". If I were in the market ...

My Boomer 35 has no trouble with pile work either. I often use 2nd range to minimize accidental spinning divots. Its easy to over force the situation in lo range.

 

[Little Red Tractor]

It's more a matter of application and preference. For some jobs, gears are likely the best; for others, hydro gives a lot of advantages. I've used both, as well as power shift types, and for my application--mostly mowing--the hydrostatic is the best option because of the maneuverability. It also is hand with FEL use.

Another comment on cabs: small tractors=small (often cramped) cabs. Bigger tractors give you a lot more creature comfort in the cabs.

 

[fidowanttobe]

For the op's stated concerns, the choice is HST.

Yup.

 

[Fxfymn413]

Had both and my vote (and my last 3 tractors after I had a gear tractor) is HST. Easy to operate and very reliable.

Going away the best HST I have ever owned is my Grand L 4060 HSTC with the high range/low range shift on the fly capability that gives you 6 speeds.

My experience is that the HST will quickly slow when going over the road as you climb hills. The range shift allows you to drop a gear with the range lever without stopping so you do not bog the machine down. It is also handy when you need to operate at very slow speeds such as when you are rototilling.

You do not need to keep the pedal pressed when using cruise mode which I do when moving over the road or doing tasks that require a constant speed such as tilling.