HST trans -- power relative to HP or hydraulic output?

[Redneck in training]

Ok, I guess that all makes sense. I suppose a better way to have asked my question would have been to say, does HP mean as much to a hydro as it does to a gear when it comes to putting power to the ground.

It also makes me wonder if there's much difference between manufacturers when it comes to this same topic. for example, when I was looking up specifications, while shopping for different cab tractors in the 50 HP range, I noticed that the Massey tractors had much better hydraulic flow ratings than most of the other tractors in the group. Made me wonder, is the 47 HP Iseki able to put more power to the ground than the 47 HP Mitsubishi that's in my tractor, both HST, considering that the Massey hydraulic system runs at about 20 percent higher flow rate than my LS.

The power loss of modern HST is about 20% including mechanical losses (I think). What you can put to the ground depends on traction. My experience with my HST is that I run out of traction before I run out of power even in four wheel drive. My tractor has insudstrial tires so it might be different story with ag tires that have better traction.
Hydrostatic transmissions | Hydraulics content from Hydraulics & Pneumatics Here is a link to good article describibg HST. Scroll to the bottom to find links to associated articles.

 

[milkman636]

My experience with HST (B26 Kubota and NH 2030) is that the engine will stall before the pump goes to relief in most cases or if in L range, the wheels will spin before the pump goes to relief. Having more HP at the engine (via highly tuned /turbocharged etc) even if everything else is the same will allow you to go a bit higher on the torque prior to the engine stalling.
Just my observations.

I think Gary has it right. When plowing my HST has bogged the engine way down at WOT without going into relief. The 50 hp version of my tractor uses the same HST drive unit, and I'd love to try one just to see if the engine could drive the HST into relief. The engine is direct coupled to the hydrostatic pump input shaft, so I have to believe that the additional torque applied to the input shaft would translate to higher pump output by whatever efficiency factor the HST pump has.

I've also seen where construction equipment manufacturers publish torque outputs of the hydraulic wheel motors. It would be really nice if tractor manufacturers would do the same.

 

[Fallon]

I hear that HST trannys are about 15% less efficient than a gear tranny. Personally I think the utility of the HST is definitely worth it. They don't seem to make HST trannys past 50hp or so. I'm pretty sure this is because at that range you are out of utility tractors and into ag tractors. Efficiency when doing ground engagement all day is more important than the utility of the HST.

 

[Gary Fowler]

I hear that HST trannys are about 15% less efficient than a gear tranny. Personally I think the utility of the HST is definitely worth it. They don't seem to make HST trannys past 50hp or so. I'm pretty sure this is because at that range you are out of utility tractors and into ag tractors. Efficiency when doing ground engagement all day is more important than the utility of the HST.

When you get to large ag tractors, a powershift transmission is much more energy efficient than an HST and gives you the best of both worlds, gear drive with almost infinite speed changes (at least as much as one would need for ground engagement work) I had a Yanmar with powershift. It had 4 ranges and each of the ranges had a powershift from R, N 1,2,3 that needed no manual clutching. It wasn't as good as HST for loader work but was great for bush hogging and disking as I could select the range that I could pull in 3 and then have ability to shift down 2 gears when hitting tough areas and even to N and R. The R range sometimes got a little too fast when in range 3 and 4 but otherwise it was very well balanced for speed and power. I looked at the MF QPS system and liked that but the tractor just didn't fit me and the price for a 50 HP was equivalent to my 70 HP LS at the time so I went with a manual shuttle shift and got 20 more HP to boot. It is OK for the Utility tractor but I wouldn't want it in a smaller CUT that did a lot of FEL work with. I can get weary on the clutch leg when you spend all day shifting when working the FEL.