HST Power Consumption

[jenkinsph]

A clutch has some limit of torque that it will transmit before slipping, and a hydraulic drive system has some limit of torque it can transmit before relieving. Although the component costs and efficiency are different, either system could be designed to transmit just as much as the other before giving up the ghost.

There is no inherent reason an hst must give up sooner than a clutch as was previously implied by Spiderlk. It may be the case in his BX and some other machines, but it is not an inherent quality.

xtn

I agree with this ^^^^

If you are running a tractor engine hard enough to make it stall you are overworking it and abusing the engine. I would prefer to go in relief than to stall the engine.

 

[RaydaKub]

Exactly. I run into that sometimes pulling a heavy load with the BX - not stuck, just wont go up the hill or spin ... all power thru the relief into the sump. Frustrating.
larry

My BX may or may not go up my hills. I normally don't open the throttle wide open because it just seems way too fast. I usually run 3/4 to 7/8 throttle, no tach on mine so this is just by ear/feel. I usually have to use low gear to get up the hill.

I'm going on vacation and taught a neighbor kid how to mow my lawn last night. I had him throttle up for the mower and he just yanked it wide open. I had warned him it might not go up the hill in high gear, but it went up just fine.

So...is the relief valve variable with the pressure applied? It would have had max pressure applied last night with max rpm. Yet it didn't open. At slower rpm it opens and the tractor stalls out (engine doesn't pull down at all). What's happening?

 

[MHarryE]

My Kubota Grand L brochure says that for every model that offers both HST & gear transmissions (3240, 3540, 3940, 4240 & 4740), the PTO power is 1.5 HP less in the HST model than the gear model.

So, if I'm interpreting it correctly, for Kubota Grand L's it appears that the HST power consumption is an additional flat 1.5 HP more than the gear model, rather than a percentage.

(The 3 other Grand L's offer: 5040 = GST only, 5240 & 5740 = HST only)

You are not reading it wrong but you are interpreting it wrong. PTO HP is determined with the tractor sitting still running a dynomometer off the PTO shaft. No power is transmitted through the transmission - except for the step down gear ratio that brings it down from engine speed to 540 rpm PTO. The difference you see between bare engine power and PTO power goes to parasitic loads. The hydro requires an additional hydraulic pump for its controls and it is running anytime the engine is running. In my Kubota there is one way to stop this pump - step on the clutch which does stop the charge pump. Only problem is if you step on the clutch you also stop the PTO.

 

[xtn]

Which should be well above the HP the engine can produce.
A properly working clutch should never slip before the engine runs out of power.

If your statement is intended as some kind of blanket, universal truth, then I disagree. Kind of depends on the design goals, doesn't it? Some clutches in a drive line might be designed to slip before the engine stalls. We design/adjust clutches depending on our usage intent. The one you might find near a tiller gearbox is an example of one you might want to slip before the engine stalls.

The same thing can be said of an hst unit. It could be - and is on some machines - designed to never relieve before the engine runs out of power. Now maybe the machines that won't do it were designed by companies that didn't think the extra cost of managing higher fluid pressures/volumes was worth it for a particular model; I don't know, but it's certainly not because it's impossible.

Look I'm not debating that hst units are less efficient. I'm just correcting the idea put forth by Spyderlk that somehow hst units inherently have some limitation that prevents them from putting as much power down as a clutch can. They don't have any such limitation. If he can design a clutch/engine combo that will put 1000hp to the ground, then so can I design an hst/engine combo that will put 1000hp to the ground. Due to efficiency differences I will need more engine power to do it, and in addition to the component costs of the hst system my design will ultimately cost more and burn more fuel, but it will do the same work his clutch unit will do.

So the net result is simply that to do a given amount of work, an hst unit trades extra cost for better usability in some usage situations. If you do enough of the type of work that people seem to think hst units are better at, then maybe it's worth it to you to pay extra for the convenience. If you don't, then maybe it isn't. Either way, don't go around supporting your preference by generally claiming that one type can just never turn the wheels as strongly as the other type. It just isn't true.

xtn

 

[DK35vince]

If your statement is intended as some kind of blanket, universal truth, then I disagree. Kind of depends on the design goals, doesn't it? Some clutches in a drive line might be designed to slip before the engine stalls. We design/adjust clutches depending on our usage intent. The one you might find near a tiller gearbox is an example of one you might want to slip before the engine stalls.

Slip clutchs on attachments are designed to slip before the tractor/engine runnings out of power. Thats what they are there for. To protect the tractor/gearboxs.
A properly working clutch on a car/truck/tractor will not slip before the engine runs out of power.

 

[IndyIan]

Re underpowered cars and towing, I have to say, if its a good transmission cooler in it, I'd rather heat up some transmission fluid than burn up a clutch if there is a lot of starting/stopping to be done. Our Matrix is a 4 banger and first gear is setup for fuel economy, not towing. Trying to start on any kind of grade with a load just roasts the poor clutch. Once moving its fine.

Tires are cheaper than clutches My buddy had a Yaris with a 16' aluminum boat and on the ramp we just revved it to 3k and let the clutch out smoothly. 3 ft of spinning and off we went with no clutch smell. I tow with my Tracker and it does have a short 1st gear so you it starts pretty much like normal. And there's always low range to get to somewhere flat to use high range again.
I'm impressed you 5030 does fine in high range on the hills with a load. I don't think mine would be anywhere near top speed on any type of slope with a hay wagon and in high range.

 

[xtn]

So...is the relief valve variable with the pressure applied? It would have had max pressure applied last night with max rpm. Yet it didn't open. At slower rpm it opens and the tractor stalls out (engine doesn't pull down at all). What's happening?

I'm no hydraulic engineer, but I'll take a guess.

No the relief valve pressure isn't variable on the fly. If your tractor went up the hill just fine at full throttle, then it wasn't seeing max pressure. It wasn't loaded up near it's limit. I expect maybe the increased volume of fluid you were pumping for a given swash-plate angle was doing the work, whereas at lower rpms you need to really put the pedal down increasing the swash-plate angle and increasing the pressure to do the same work.

That's all just the guess of somebody who doesn't really have a deep understanding of the fluid dynamics going on. So it's worth what you paid for it. Maybe somebody else will correct me?

xtn

 

[xtn]

Slip clutchs on attachments are designed to slip before the tractor/engine runnings out of power. Thats what they are there for. To protect the tractor/gearboxs.
A properly working clutch on a car/truck/tractor will not slip before the engine runs out of power.

Did you read the rest of my post? A suitably designed hst system will not relieve before the engine runs out of power. So what differentiation are you trying to make?

xtn

 

[CJONE]

My BX may or may not go up my hills. I normally don't open the throttle wide open because it just seems way too fast. I usually run 3/4 to 7/8 throttle, no tach on mine so this is just by ear/feel. I usually have to use low gear to get up the hill.

I'm going on vacation and taught a neighbor kid how to mow my lawn last night. I had him throttle up for the mower and he just yanked it wide open. I had warned him it might not go up the hill in high gear, but it went up just fine.

So...is the relief valve variable with the pressure applied? It would have had max pressure applied last night with max rpm. Yet it didn't open. At slower rpm it opens and the tractor stalls out (engine doesn't pull down at all). What's happening?

The relief pressure is set that does not change, your pedal application or flow is what changes. Say 1/2 pedal is 10 gpm at 4000psi that is 27 hp, full pedal 20 gpm at 4k is 54 hp. If you are in high range and you hit 4k at half pedal the relief opens. If you back off to 1/4 pedal you drop to 1/4 flow and if the resistance is low enough to keep below 4000psi you move. If not you have to drop down a range, effectivly lowering resistance and pressure. Now if you only have a 30 hp tractor you will only be able to use 1/2 pedal travel at the relief pressure, if you go over that in ANY range at the relief the engine will stall. CJ

 

[DK35vince]

Did you read the rest of my post? A suitably designed hst system will not relieve before the engine runs out of power.

I didn't disagree with that part of your post

 

[TBarD]

My BX may or may not go up my hills. I normally don't open the throttle wide open because it just seems way too fast. I usually run 3/4 to 7/8 throttle, no tach on mine so this is just by ear/feel. I usually have to use low gear to get up the hill.

I'm going on vacation and taught a neighbor kid how to mow my lawn last night. I had him throttle up for the mower and he just yanked it wide open. I had warned him it might not go up the hill in high gear, but it went up just fine.

So...is the relief valve variable with the pressure applied? It would have had max pressure applied last night with max rpm. Yet it didn't open. At slower rpm it opens and the tractor stalls out (engine doesn't pull down at all). What's happening?

I think this relief valve theory for the HST is bogus. The HST does not have a relief valve in the sense that the hydraulic system of the tractor does for the loader and auxiliary attachments. At low engine RPMs in high range you just weren't generating enough power to drive the tractor up the hill. HST can easily deliver enough power to stall the engine - that is why some tractors have the "stall guard" feature for example, which will back off on forward speed if the engine RPM starts to slow too much due to loading.

To deliver high torque to the drive axles with HST you need to up the engine RPMs and back off on the pedal. It should deliver enough power to creep up the hill even in high range. On the other hand, when flooring the HST foot pedal at low RPMs under load the engine is simply not delivering enough power to move the tractor at the requested speed.

I suspect what is happening is that when one floors the pedal the swash plate on the variable displacement HST drive pump is fully tilted to increase the hydraulic flow to try to increase the output speed, but given limited engine power output and RPM, an increase in flow/displacement in the pump will result in a reduction of hydraulic pressure. The reduced hydraulic pressure is insufficient to drive the output hydraulic motor against the load. The delivered power is not enough to overcome leakage in the variable displacement pump so the output speed drops to zero. Backing off on the pedal will increase the delivered output pressure and you may get some forward motion. Up the engine RPMs and you get more until eventually the engine lugs or stalls due to maximum power being delivered.

The bottom line is, if the HST lugs, back off on the pedal and/or up the RPMs.

 

[Gary Fowler]

I am not sure,but I think that may be what happens to my Kubota RTV. If going up a hill and it starts to lug down with your foot full on the floor, just reduce throttle and it climbs right on up. It is hydrostatic drive but different from a tractor.
Regarding efficiency, when my BIL with his NH 2030 and I were digging out our old pond and moving the dirt several yards to a spoil pile, my gear drive Yanmar with powershift would slightly out work his tractor. He had to shift from Med to Low range to load the bucket and then shift back to make the long trip. With my gear drive but powershift hydraulic clutch, I could down shift to 1st load up, reverse to turn around then shift up to 3rd all in same range and do my round trips slightly faster due the the powershifter. I don't know if I could have done it with manual syncronized shift with reverser or not. I dont think there is enough difference in pulling power between the two on a CUT to really notice the difference in either fuel economy or power to the wheels. No doubt the hydro does use a bit more fuel but in forward reverse work, it may make up for it in time needed to do the task. I guess we could go on and on, but the bottom line is that the best tool for the job is whatever you are comfortable using. I like hydrostatic for lawnmowers vs straight gear drive. I prefer my gear drive when bushhogging open pastures as I can just set the throttle and go. I will let you know how I like my TLB with 3 range hydro whenever I get home to use it. I think I like the powershift best of all as it combines the ease of shifting to vary the speeds and the efficiency of a gear in power to the wheels. Powershift isnt variable to 0 speed but it does slow enough to ease up on a task slowly and then there is alway the foot throttle that you can vary even more with all without loosing any power via a hydrostatic pump.

 

[slowzuki]

We used to tow with a Tracker before the Matrix, big drop in towing ability. The 5030 only does well once you get moving, you can't drive slow in high, and if you stop on a hill with a big load, you are back to medium until you stop on the downhill to shift back to high.

Its really not that bad, its far better than driving the old Ford 5000 at my buddies farm, you have to start in gear, when loaded starting in road gear is nearly impossible. I used to be able to double clutch so you could start in a lower gear it but its a hand throttle and between the legs floor shifter with a weird pattern, hard to do, and hard to recover if you miss a gear. Not really meant to be shifted.

Tires are cheaper than clutches My buddy had a Yaris with a 16' aluminum boat and on the ramp we just revved it to 3k and let the clutch out smoothly. 3 ft of spinning and off we went with no clutch smell. I tow with my Tracker and it does have a short 1st gear so you it starts pretty much like normal. And there's always low range to get to somewhere flat to use high range again.
I'm impressed you 5030 does fine in high range on the hills with a load. I don't think mine would be anywhere near top speed on any type of slope with a hay wagon and in high range.

 

[CJONE]

I think this relief valve theory for the HST is bogus. The HST does not have a relief valve in the sense that the hydraulic system of the tractor does for the loader and auxiliary attachments.

It does have a relief valve just like a standard open system. In fact they have 3. 1 for forward, 1 for reverse and 1 for the charge pressure. Some use a shuttle valve with 1 relief for the HP system but they can be a pain when the shuttle messes up. CJ

 

[TBarD]

I think this relief valve theory for the HST is bogus. The HST does not have a relief valve in the sense that the hydraulic system of the tractor does for the loader and auxiliary attachments.

It does have a relief valve just like a standard open system. In fact they have 3. 1 for forward, 1 for reverse and 1 for the charge pressure. Some use a shuttle valve with 1 relief for the HP system but they can be a pain when the shuttle messes up. CJ

Yes there are relief valves, all I meant was that I doubt they are activating and limiting final output drive power in the case being described. Rather with low engine RPMs and full displacement (pedal down) probably there isn't enough power being delivered to overcome normal losses in the HST pump. There can't be high pressure being delivered to the HST drive motor as otherwise the axles would turn. I admit though I am not sure when a HST relief valve would be activated - does anyone on here know?

 

[TBarD]

I looked into this a bit more thoroughly. Simple enough really. If the tractor stops without stalling out while under heavy load with the pedal down (e.g. hill climbing) then the appropriate forward/reverse relief valve has been activated. Pushing the pedal further merely increases the pump displacement hence flow, diverting more flow through the relief valve and hence delivering less power to the HST motor and the tractor slows. Back off and you are generating maximum torque at maximum efficiency. Up the RPMs and you should be able to deliver a bit more torque out, although inefficiently and with some abuse to the HST if the relief has been activated (but a bit more aggressive ground speed and momentum may help get up that hill). Eventually you have to shift to a lower range which will allow the HST to operate more efficiently at a lower pressure.

So to answer my own question, yes the HST relief valves will kick in if one pushes it hard enough. Could also happen due to shock loading. But it is still possible to stall an HST tractor under heavy load, so the relief pressure must be set near the limit of what the engine can deliver.

 

[Redneck in training]

You are not reading it wrong but you are interpreting it wrong. PTO HP is determined with the tractor sitting still running a dynomometer off the PTO shaft. No power is transmitted through the transmission - except for the step down gear ratio that brings it down from engine speed to 540 rpm PTO. The difference you see between bare engine power and PTO power goes to parasitic loads. The hydro requires an additional hydraulic pump for its controls and it is running anytime the engine is running. In my Kubota there is one way to stop this pump - step on the clutch which does stop the charge pump. Only problem is if you step on the clutch you also stop the PTO.

I think the difference between maximum power and PTO power has lot to do with the power characteristic of the engine. In example maximum power is produced at 3000 rpm but power at PTO speed is produced at 2150 rpm. As a rule PTO speed is set little past the maximum torque the engine produces. reason being maximum torque is a point of the best specific fuel consumption. PTO power is power delivered on the output shaft therefore it is the engine power minus gear loses.

 

[JDgreen227]

I think the difference between maximum power and PTO power has lot to do with the power characteristic of the engine. In example maximum power is produced at 3000 rpm but power at PTO speed is produced at 2150 rpm. As a rule PTO speed is set little past the maximum torque the engine produces. reason being maximum torque is a point of the best specific fuel consumption. PTO power is power delivered on the output shaft therefore it is the engine power minus gear loses.

There are those who argue that maximum torque rpm equals best fuel economy is only the case when an engine is on a dyno and not in a real world situation, and that lower rpm below torque peak will usually result in a fuel savings. To be honest, I have never seen what a torque curve looks like for a small diesel engine like my Yanmar and I am now curious enough to wonder what it's like.

 

[dennis52]

Excellent post and I agree with you, especially the last line. To owners like me, who almost never run at a constant speed, HST is well worth the small amount of extra power consumption. I was out mowing my smallest patch of yard last night, it's the one that has a lot of trees and obstructions so I need to change speed and direction constantly and I HATE the thought of having to do all the shifting and clutching a gear tractor would require.

I have 26hp "Greenfield" lawn tractor that uses a belt/pulley/ friction clutch transmission that is as responsive as a hydrostatic, more efficient and mechanically simple.
I use that for my lawn yards and a geared CUT for my larger horse pastures.

 

[JDgreen227]

I have 26hp "Greenfield" lawn tractor that uses a belt/pulley/ friction clutch transmission that is as responsive as a hydrostatic, more efficient and mechanically simple.
I use that for my lawn yards and a geared CUT for my larger horse pastures.

I have never seen the name "Greenfield" as applied to power lawn equipment. Hmm, maybe somebody needs to start a new thread about the power consumption of a belt/pully friction clutch transmission vs. a hydro....