How does an HST compare in pulling power with the same tractor and a gear trans?

[super55]

I have two tractors JD 4310 power-reverser (32hp) and JD 4400 (35hp) that is hydro that are for the most part identical except a couple hp and the transmissions. The 4310 will out-pull the hydro 4400 all day long. Biggest difference is with heavy loads a gear transmission will start to lug the engine whereas with a hydro the transmission will go into relief if loaded to heavily. That flywheel and pistons cycling are creating inertia to keep momentum. With the hydro the pump goes into relief and pulling capabilities decrease the warmer the fluid gets.

They all have their place and benefits which everyone in previous posts has already discussed. Years ago tractors were also spec'd at drawbar power but they don't do that so much anymore. 95% of the time when I have ground engaging work or heavy pulling I use my old Oliver Super 55. It's just a 35hp gasser but the gearing, weight and even though its 2wd with R1s, it will out-pull the other ones by far.

 

[wolc123]

My field tractors (John Deere, Ford, and IH) are all gear. They do ground engaging work and occasional heavy pto work.

For lawn mowing (I only cut 1 acre) I have (2) riders that are each about 15 hp gas powered. One is gear and the other is hydro. The vast majority of the time, I use the hydro because I can knock the job out a lot faster with that, always perfectly matching my ground speed to the conditions. I hate to spend more than an hour cutting the lawn.

The only exceptions are in the spring and early fall, when we get tons of rain, and the grass grows real thick. In those times, I can get the grass cut faster with the gear mower, because it puts a greater percentage of its available horsepower into cutting the grass, with very little of it wasted on a much less efficient hydro transmission.

 

[Helogabals]

I have Yanmar SA424, which is HST. Great tractor, love it. It is very often mentioned, that Yanmar has built entire tractor, from motor till frame, hydraulics included, all systems live and work in perfect harmony. At least they say so. I am not an expert to confirm this
Recently I went to help neighbor to pull out few bushes and small trees. 98 % easy job, has done that on idle speed. We pulled with 800 kg rated strap, so I thought - if anything goes wrong that strap should break first. But one bush was rather big. With full rpm there was just all wheel spinning, even though I had 300 kg ballast on 3p
I have "kinetic energy recovery rope" and I took that. Tied everything according feng-shui. Run tractor with full speed, rope stretched and ... engine died. Repeated. The same. With third pull bush gave up.
Tractor & motor is fine

I was surprised that it's possible to kill engine, which is attached to HST powertrain. I thought, it have some kind of pressure relief valve or something, so transmission should start to slip at one point.

 

[SPYDERLK]

I have Yanmar SA424, which is HST. Great tractor, love it. It is very often mentioned, that Yanmar has built entire tractor, from motor till frame, hydraulics included, all systems live and work in perfect harmony. At least they say so. I am not an expert to confirm this
Recently I went to help neighbor to pull out few bushes and small trees. 98 % easy job, has done that on idle speed. We pulled with 800 kg rated strap, so I thought - if anything goes wrong that strap should break first. But one bush was rather big. With full rpm there was just all wheel spinning, even though I had 300 kg ballast on 3p
I have "kinetic energy recovery rope" and I took that. Tied everything according feng-shui. Run tractor with full speed, rope stretched and ... engine died. Repeated. The same. With third pull bush gave up.
Tractor & motor is fine

I was surprised that it's possible to kill engine, which is attached to HST powertrain. I thought, it have some kind of pressure relief valve or something, so transmission should start to slip at one point.

YES. Sounds like you are using full pedal. The relief is in effect as the engine dies. Think of how much hydraulic power you are dumping back into the sump. Then consider feathering the go pedal. Regardless of range, if you have enough traction the hyd pressure will exceed relief pressure and the engine will give little or no indication. Even though the hyd pressure is high there is only a little fluid being pumped - all still going to waste if youre not moving. Push the pedal further and the pressure and motive force stays the same but more power is dumped to sump and the engine lugs down and dies.

I think your relief pressure is probably set appropriately.