Plowing & pushing HST to the limit

[jinman]

Got an "update" for you on the reaction of the HST to a heavy pull. A couple of weekends ago my neighbor who has a Kubota 3410 with 4wd, ag tires. GST and I with my TC45D4, R4s decided to tackle a brush pile which had been pushed up by a D8 Cat. Well, we both ended up with "egg" on our faces. We wrapped a chain around the pile and pulled for all it was worth. The Kubota spun its wheels and the front wheels bounced off the ground a little. All the while, my boomer dug in and pulled so hard it stalled the engine with minimum tire slip. [sad] This with the rpm above 2,700. Both of us had box blades of similar weight with welded hooks on them to pull the chain. I put my box almost on the ground an he kept his higher. My front wheels never left the ground and of couse, I had the differential lock engaged. I guess the results are predictable. On the second attemp, I started pulling him backwards by the chain around the brush pile, and then the 3/8" chain broke. /w3tcompact/icons/shocked.gif We looked at each other and said, "Loaders!"
We dug into/split the pile and found that the center was all dirt hidden by brush around the outside. We just used our loaders and boxblades to move most of the dirt/rocks about 200' to a deep gully. With all the dirt gone, the brush was easy to move.

I guess the moral to the story is "Brush pile built by dozer takes dozer to push." /w3tcompact/icons/blush.gif

What this proves is that you can stall a TC45D without spinning the tires.


JimI

 

[RPM]

On the tractors with hydrostats that I know you won't stall the engine with a heavy load - you'll either spin the wheels or pop the pressure relief valve. Of course, this is limited to my experience!!! You'll stall the engine on a gear transmission though - but these are vastly different mechanisms.

 

[DVerbarg]

RPM,

What tractors w/hydros are you familiar with? I've stalled my TC40D numerous times without spinning the wheels. At first it was probably due to lack of experience as I was trying to dig into a huge pile of hardened clay (with the FEL) while in a higher range. I can still stall the engine now (150 hours seat time later) with everything in low ranges with minimal tire slippage (guess - less than 15%), if I use the loader lift, breakout, and push into the pile all at the same time.

Just like to understand and learn from others experiences as to engineering differences.

DaveV

 

[Bird]

If you come down on the hydro pedal too fast, you'll definitely stall the engine, but if you ease down on it, you probably won't stall it; it'll either spin the wheels or go nowhere.

Bird

 

[djradz]

OK, I've got to give my opinion/observation, again. It appears to me, that it all comes down to whether you apply enough pressure to the hydro unit to make the relief valve pop or not. In other words, in the higher ranges, the force the transmission can place against the engine is high enough that the engine cannot overcome it and thus stalls, before the relief pressure is reached. However, in the lower ranges, the engine can exert enought force to succeed in taking the transmission to its limit, and thus poping the relief. Does this agree with your observations???

 

[DVerbarg]

Ok, this pressure relief valve has me thinking. How can you tell the pressure relief valve has relieved the pressure? I've assumed that there would be a sudden drop in torque applied to the wheels and that it would be felt as though the transmission were put in neutral. Is this correct? I've stalled the tractor before in low ranges (both on the TC40D) and was not "punching" the hydro pedal but very slowly increasing the pedal pressure. Maybe my machine is not functioning as it should. I do not have any other significant hydro experience to compare to (my 16hp lawn tractor does not count in my book as a relative comparison).

DaveV

 

[paccorti]

DaveV,

There is one additional piece of information to consider. If my tractor is running at max HP rated RPM then I cannot stall it (due to hydro) under low or high range. Less than full power and I can stall my tractor. When my machine goes for its new gears I think I'll have the dealer check the hydro and hydraulic relief valves. I just want to be sure that they are open when they should (and not too early). You are exactly right, when the HST relief valve "relieves" the pressure the tractor just stops moving but the engine still sounds like it is working.

Peter

 

[jinman]

There are at least two types of pressure relief valves. On aircraft, often a relief valve is used to maintain a constant pressure (like a regulating relief valve). If the valve is under constant pressure from a spring, the pressure it takes to lift the spring is maintained (like a zener diode sets voltage levels, for you electrical types). A pop-off type pressure relief valve (like on an air compressor) will unseat at a given pressure and the pressure has to be reduced to a reseating pressure before it will again quit relieving the pressure.

I believe our tractors are the first type of valve (pressure maintaining/limiting) since there seems to be some torque still getting to the wheels when the relief valve is unset. Of course, there may be a "bump" just above the relief set point and then the release. I think there is a percieved pop-off effect because the wheels stop turning and we think the tractor has stopped pulling due to feedback through the seat of our pants /w3tcompact/icons/shocked.gif.
I've also observed that under heavy load, there is a significant delay between the time I remove my foot from the pedal and the nulling of the hydro pedals. As the engine approaches stall, the rpm suddenly decreases and the hydro pump puts out less fluid, etc., etc., and it seems the pedal automatically trys to maintain the instantaneous torque which was previously applied (in other words, the pedal goes forward by itself and the tractor stalls /w3tcompact/icons/blush.gif. That's what happened to me with the brush pile. Whether this is "normal" behaviour for a NHTC45D I do not know. I just know it did it and the engine stalled and died./w3tcompact/icons/frown.gif
What have others experienced? I'm not talking theory here; I mean actual experience.


JimI

 

[DVerbarg]

JimI,

Your experience as described is exactly my experience ("As the engine approaches stall, the rpm suddenly decreases and the hydro pump puts out less fluid, etc., etc., and it seems the pedal automatically trys to maintain the instantaneous torque which was previously applied (in other words, the pedal goes forward by itself and the tractor stalls)").

This has happened repeatedly to me while digging clay and am getting adept at listening/feeling when I am nearing that point and backing off just before. I never thought that anything was wrong until these discussions about the pressure relief valve. Sounds as if everything is working correctly or at least is working consistently (based on two people's experiences /w3tcompact/icons/wink.gif).

DaveV

 

[jinman]

Dave, thanks for the info. Like you, I have wondered if everthing is "adjusted" properly in the hydro. I think it is fine, because I had plenty of down-pressure on my wheels when the tractor stalled and since I had the drivetrain locked in 4WD, there was nothing that could slip. As the tractors get older and broken-in more, the pedal action may not be so severe. I've noticed already that in normal use, my pedals return to null much quicker than they did why my tractor was new. We will see...but, it's nice to know someone else has experienced the same action-reaction with our hydros./w3tcompact/icons/smile.gif


JimI