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