RPM's, HP and Torque, Oh MY!!!

[mjrwood]

Ok, first sorry for the length of this. I’ve tried 3 times to write this and make it shorter have failed. So much appreciation and kudos to anyone who hangs with me on this.

I’ve been thinking about some things (dangerous for me to do that), and I have a question about the practical application of torque, hp, and rpm’s. I understand (I think, someone might chime in and tell me I actually don’t) the basics of what torque and hp are, and I understand the equation for torque and hp. The old adage I’ve heard is that torque is the amount of capability of work, hp is how fast you can get it done. That makes sense to me.

But when I think about how this applies I tend to find myself in a loop of circular logic. This stems from a discussion with a friend of mine about my tractor. I’ve got an L2501, gear driven. Engine hp is ~25, pto hp is ~20, right? Ok, so it’s a 540 pto. It’s got a 100 cubic inch engine, and has more torque, and achieves the 540 pto rpm at a much lower engine rpm.

Ok, so if you’re still with me, here’s where I’m getting fuzzy. Again mine is a gear driven tractor, and I’ve put my tractor through some pretty tough tasks of bush hogging and tilling, through some really rough brush, really high weeds, really hard ground, and on some pretty decent inclines. I have yet to find a situation where, if the load gets heavy on the tractor, I cannot simply give it a little foot throttle to keep my engine rpm’s at around 2000 and breeze right through it.

So to get to my question, say you did a side by side with an L3400. It’s my understanding that the L2501 engine is close to the one that the L3400 had, governed and geared so that the rated hp is ~25 to put 540 at the pto. Ok, if that is correct, then the disadvantage of the 2501 is that at load where pto is running 540, you’re at less hp and so the tractor is going slower. Is that right? Because a pto running at 540 rpm is a pto running at 540 rpm, in my mind it shouldn’t matter if tractor running the implement is at 40 hp or at 30. Is my thinking straight on that?

Given that assumption is correct, then if your max engine rpm was such that it allowed you to grab a higher gear and give more throttle so that your engine rpm was where it needed to be to spin the pto at 540, what disadvantage are you at with the L2501? If this is true then hp for the 2501 is less a disadvantage than the widely spaced gear speeds, and things like the non-live pto for the DT become more important a consideration than the hp.

I don’t know, maybe I’ve got all this wrong, or maybe one fundamental point wrong that is causing everything else to derail. Any thoughts or input or discussion is welcomed, and if I’m flat wrong about everything and you want to call me a dummy I’m fine with that too.

 

[SDT]

Ok, first sorry for the length of this. I’ve tried 3 times to write this and make it shorter have failed. So much appreciation and kudos to anyone who hangs with me on this.

I’ve been thinking about some things (dangerous for me to do that), and I have a question about the practical application of torque, hp, and rpm’s. I understand (I think, someone might chime in and tell me I actually don’t) the basics of what torque and hp are, and I understand the equation for torque and hp. The old adage I’ve heard is that torque is the amount of capability of work, hp is how fast you can get it done. That makes sense to me.

But when I think about how this applies I tend to find myself in a loop of circular logic. This stems from a discussion with a friend of mine about my tractor. I’ve got an L2501, gear driven. Engine hp is ~25, pto hp is ~20, right? Ok, so it’s a 540 pto. It’s got a 100 cubic inch engine, and has more torque, and achieves the 540 pto rpm at a much lower engine rpm.

Ok, so if you’re still with me, here’s where I’m getting fuzzy. Again mine is a gear driven tractor, and I’ve put my tractor through some pretty tough tasks of bush hogging and tilling, through some really rough brush, really high weeds, really hard ground, and on some pretty decent inclines. I have yet to find a situation where, if the load gets heavy on the tractor, I cannot simply give it a little foot throttle to keep my engine rpm’s at around 2000 and breeze right through it.

So to get to my question, say you did a side by side with an L3400. It’s my understanding that the L2501 engine is close to the one that the L3400 had, governed and geared so that the rated hp is ~25 to put 540 at the pto. Ok, if that is correct, then the disadvantage of the 2501 is that at load where pto is running 540, you’re at less hp and so the tractor is going slower. Is that right? Because a pto running at 540 rpm is a pto running at 540 rpm, in my mind it shouldn’t matter if tractor running the implement is at 40 hp or at 30. Is my thinking straight on that?

Given that assumption is correct, then if your max engine rpm was such that it allowed you to grab a higher gear and give more throttle so that your engine rpm was where it needed to be to spin the pto at 540, what disadvantage are you at with the L2501? If this is true then hp for the 2501 is less a disadvantage than the widely spaced gear speeds, and things like the non-live pto for the DT become more important a consideration than the hp.

I don’t know, maybe I’ve got all this wrong, or maybe one fundamental point wrong that is causing everything else to derail. Any thoughts or input or discussion is welcomed, and if I’m flat wrong about everything and you want to call me a dummy I’m fine with that too.

I didn't understand your reasoning in the 5th paragraph but, there is no performance "disadvantage" to a gear type transmission vis a vis a HST. Indeed, the HST suffers the performance disadvantage.

HST transmissions provide infinitely variable speed but do so at a cost. HST transmissions are very inefficient because pumping all of that oil around turns a considerable amount of power into heat which is wasted energy.

A gear type transmission turns MUCH less power into heat. Accordingly, a gear type transmission is considerably more efficient than a HST. This is why large, high HP AG tractors are NOT HST. Yes, other than HST, there are other ways to achieve infinitely variable speed control but most such systems are complex and expensive.

The advantage of a HST transmission is operator convenience and, usually, time savings.

Anecdotal case closely on point: I own an M9960, 90 PTO HP tractor. This tractor is equipped with a dual range 6 speed synchronized gear type transmission with a hydraulic shuttle (reverser). I also own a Grand L6060, 53 PTO HP tractor equipped with a HST. I use both tractors almost exclusively for mowing but with different mowers and in different areas where they are most well suited.

Occasionally, for whatever reason, I have used both tractors to mow the same parcels. Result: The L6060 HST will use nearly twice as much fuel to mow the same parcels (with a 6' mower) than will the M9960 )(with a 7' mower).

Caveat: Unlike JD with their 4066R, Kubota does not offer EPTO on the L6060. My M9960 is equipped with EPTO. EPTO allows 540 PTO RPM at considerably lower engine RPM when conditions permit. In my case, my M9960 will run my 7' Woods BB840X at about 1,700 engine RPM if conditions are not too heavy. Correspondingly, to achieve 540 PTO RPM with my L6060 (EPTO not available), I must operate the engine at approximately 2,400 RPM whether conditions require the HP or not. If my M9960 was not equipped with EPTO, it would be necessary to operate the engine at considerably higher RPM to achieve 540 PTO RPM. This would result in higher fuel usage but not so much as the HST L6060.

Would be nice if Kubota offered the L6060 with EPTO for those light load conditions, as does JD with their competitive 4066R.

SDT

 

[rademamj]

No such thing as bad questions. That's how we all learn. Ok. No such thing as a 540 PTO is a 540 PTO is a 540 PTO for different tractors with horsepower differences. Real case: Imagine a 50hp tractor and 25hp tractor running the same model rotary cutter into 4 feet high grass at same 540 PTO. The higher torque 50hp cutter runs without slowing down.....while the 25hp cutter nearly stalls. The higher 50hp torque at the 540 PTO is much desired for this task. The higher torque at the 540 PTO allows more work to be completed more efficiently. That is why we always say ".....buy the largest horsepower tractor for your work needs". It could also be said to buy the highest torque tractor for your needs.

Sent from my SM-G930V using Tapatalk

 

[mjrwood]

No such thing as bad questions. That's how we all learn. Ok. No such thing as a 540 PTO is a 540 PTO is a 540 PTO for different tractors with horsepower differences. Real case: Imagine a 50hp tractor and 25hp tractor running the same model rotary cutter into 4 feet high grass at same 540 PTO. The higher torque 50hp cutter runs without slowing down.....while the 25hp cutter nearly stalls. The higher 50hp torque at the 540 PTO is much desired for this task. The higher torque at the 540 PTO allows more work to be completed more efficiently. That is why we always say ".....buy the largest horsepower tractor for your work needs". It could also be said to buy the highest torque tractor for your needs.

Sent from my SM-G930V using Tapatalk

Thanks for the responses guys.

rademamj: your post makes good sense to me. It makes me realize that in my scenario I as not factoring in the role torque plays at the pto, that was kind of a missing piece in my thinking.

SDT: thanks for your post as well, and it does make sense to me. I wasn't so much thinking about the difference between gear driven and hst, but the points you make fit into the picture nicely.

So basically when I said earlier a 540 pto is a 540 pto, the problem there is that the rpm's are only the starting point, and the torque is the capacity to remain at that rpm without bogging down, does that sound more like it?

 

[North Idaho Wolfman]

I think you're under some false assumptions.

Engine HP is not PTO HP
And Torque is a combination of HP, speed and gearing, so speed being the same and gearing being the same, a tractor with more PTO HP will have more torque avalible.

FYI:

L2501
Engine HP 24.8
PTO HP 20.5 @2425 RPM

L3400 Early 1.6L
Engine HP 34.7
PTO HP 29@2425 RPM

L3400 Late 1.8L
Engine HP 35.7
PTO HP 30@2425 RPM

 

[mjrwood]

I think you're under some false assumptions.

Engine HP is not PTO HP
And Torque is a combination of HP, speed and gearing, so speed being the same and gearing being the same, a tractor with more PTO HP will have more torque avalible.

FYI:

L2501
Engine HP 24.8
PTO HP 20.5 @2425 RPM

L3400 Early 1.6L
Engine HP 34.7
PTO HP 29@2425 RPM

L3400 Late 1.8L
Engine HP 35.7
PTO HP 30@2425 RPM

Thanks NIW, I think I probably wasn't articulating my question well enough. I do understand that the engine hp is different than the pto hp, that there is some lost in transfer, I get gross hp vs net, etc.

I'm still probably going to mess up what I was getting at in my original post, but more or less I was thinking about the role that the torque plays in the implement operating efficiently.

For example using the figures you quoted. Maybe those figures are correct and I was wrong (or am misreading the specs), I read that the gear driven 2501 puts 20.5 hp to the pto at 540 at 1910 rpm. If that is correct, then when I look at the numbers my understanding is that 540 pto rpm is met at such low engine rpm because of the large displacement engine giving higher torque to the equation.

So if that were true, although my 2501 is only putting 20.5 hp at the pto, it is doing so at lower engine rpm leaving some top range rpm and available torque allowing me to add throttle in demanding situations so the tractor doesn't bog down. Does that make sense? I feel like I'm butchering my thought process, which is likely all wrong anyway. I'm not defending my thought process, only trying to make sure you guys understand what my question was. Again thanks for the replies, this is an interesting topic to me.

 

[mjrwood]

It might also help to know where I'm coming from to explain how this came about. I had a mx4700 tractor years ago. It was too big to work around my house and really wasn't ideal for cutting my pasture. I rented out my pasture so I thought I'd sell the 4700 and get something smaller, enter the 2501. My thinking was I'd use it for a few years, then when the time came I'd buy a larger tractor, something in the 60-70hp range I could pull a larger cutter with.

So anyway, unexpectedly I've now found myself with another 30 acres I'm going to have to maintain. So it was in my debating with myself whether I wanted to go ahead and get something bigger, or try to tackle it with the 2501. So my thinking was this, I was keeping up 100 acres with the 4700. Honestly my 2501 with a 5ft cutter mows as cleanly and as quickly as my 4700 did with the 6ft cutter, I'm just losing a foot. But as far as ground speed and the quality of cut, I really can't tell a difference. So I figures while not ideal, if I could do 100 with the 4700 might as well see how it works with the 2501 next year, the added 30 acres will put me at 45 that I'm keeping up.

But it was in that debate with myself that I started pondering how such a low hp tractor could work so efficiently as the larger 4700 (granted with a slightly larger cutter).

 

[85Hokie]

This is a great simplistic video of torque vs horsepower for those that wish to understand it better!

https://www.youtube.com/watch?v=u-MH4sf5xkY&t=220s

 

[hagrid]

Torque is an expression of rotating force. Horsepower is an expression of work accomplished per unit time. Both need considered when matching an engine to a particular load.

 

[BruceP]

I have seen what happens when a tractor with TOO MUCH power is driving an implement rated for less power. (Implement Gears shred, PTO driveshafts twist, implement-frame bends.) In this case, it was a brand-new, china-made, rototiller working in dry clay.

Moral of that story: Do not surpass the MAX rated power for your implements.

---------- Just a story this discussion reminded me about ---

Years ago, I just purchased a new motorcycle. One of the first to surpass the elusive 100HP per Liter. (In my case, it was 750cc rated for 85HP.)

I was telling my buddy about my powerful new motorcycle and he responded that his 4x4 John Deere tractor was 'only' 80HP at the PTO. (The tires on the tractor were more than 6 feet tall.... the thing was HUGE)

Obviously, the 80HP tractor was WAAAYYYY more powerful compared to my puny 85HP motorcycle.

 

[NoJacketRequired]

Coming back to the original question... or one of them, at least!
You are right to say that operating your tractor at 540 RPM PTO speed is causing the engine to run at something less than maximum power.

What you may be missing in your thinking is the topic of governor "overhead" or "margin". If your tractor were to make its 540RPM PTO speed at wide open throttle, the governor would have no "overhead" to allow it to add more fuel in the event of an underspeed condition as a result of increasing load on the PTO. This would mean the tractor would lug below 540 RPM PTO speed most of the time since it can only make 540RPM when no load is applied. If this were the case your implements would be operating below their rated speed/capacity most of the time and overall efficiency of the end-to-end system of tractor and implement would suffer horribly.

If you think of it in other terms, maximum fuel delivery by the governor results in maximum RPM at NO LOAD (this last bit is critical to understanding governor "overhead"). No matter how much you load the engine, then unload it, the engine will never rev above the design redline. Over-revving can only happen if the "load" starts to impart energy back into the engine, as would be the case, for instance, driving downhill with a heavy load behind the tractor pushing it downhill - this is a situation where the governor can no longer exercise control over engine speed since it likely has already cut the fuel flow back to almost zero.

By having PTO RPM made at less than full engine RPM the governor has some authority left to add more fuel without risk of an engine overspeed if the PTO load is suddenly decreased. This process of building in some "overhead" capacity is a critical element in designing any system in which a governed speed is desired.

 

[twomany]

I'm sure it's "over the heads" of much of this, but one should familiarize themselves with the term BMEP Brake Mean Effective Pressure.

The action of the combustion process as Pressure, on the top of the piston.

Pressure on the piston crown is the ONLY power producing element of an internal combustion engine. Steam as well, for that matter.

The piston area pushing on the crank shaft via the connecting rod wis were the torque comes from. Pushing on the piston top while the piston is decending at a rapid rate is what the motor guys strive for. That is, Turning force at high revolutions per minute.

The advantage of a smaller piston size, is that it takes less fuel to achieve the same BMEP as would a larger piston area.

There is no replacement for displacement!

 

[SDT]

It might also help to know where I'm coming from to explain how this came about. I had a mx4700 tractor years ago. It was too big to work around my house and really wasn't ideal for cutting my pasture. I rented out my pasture so I thought I'd sell the 4700 and get something smaller, enter the 2501. My thinking was I'd use it for a few years, then when the time came I'd buy a larger tractor, something in the 60-70hp range I could pull a larger cutter with.

So anyway, unexpectedly I've now found myself with another 30 acres I'm going to have to maintain. So it was in my debating with myself whether I wanted to go ahead and get something bigger, or try to tackle it with the 2501. So my thinking was this, I was keeping up 100 acres with the 4700. Honestly my 2501 with a 5ft cutter mows as cleanly and as quickly as my 4700 did with the 6ft cutter, I'm just losing a foot. But as far as ground speed and the quality of cut, I really can't tell a difference. So I figures while not ideal, if I could do 100 with the 4700 might as well see how it works with the 2501 next year, the added 30 acres will put me at 45 that I'm keeping up.

But it was in that debate with myself that I started pondering how such a low hp tractor could work so efficiently as the larger 4700 (granted with a slightly larger cutter).

Keep in mind that a 6' cutter is 20% larger than a 5' cutter. I would consider this something more than "slightly larger."

SDT

 

[mjrwood]

Keep in mind that a 6' cutter is 20% larger than a 5' cutter. I would consider this something more than "slightly larger."

SDT

That's a good point. I was looking at it as just another foot, but yea it's a substantial percentage of the cutting capacity, so point taken.

I appreciate everyone's responses and the points made. It wasn't what the thread was meant to be about when I started it, but now I'm back around to debating with myself what I should do as far as stepping back up to a larger tractor now or trying to make it work for a year. I could handle it with the 2501 but jeez it'll take forever to cut the whole lot. Plus there are some tricky hills out there and I'm not crazy about the thought of tackling them with the L, it feels much more tippy to me than my MX did.

If I go bigger I'd ideally want to go much bigger, like something closer to 70hp. Then I'd need to get new implements, I reckon that'd be too much for my little 5ft stuff. On the other hand I could just go up to something in the 40-50 range and keep my implements, still cut with a 5ft'er but be able to do it faster. Then I end up practically where I started when I had the 4700 only with smaller implements, and I'm not sure I would be able to NOT kick myself where it hurts, lol.

Or I can just hold with the 2501 and see how it goes. I'm not crazy about the prospect of it but it might be the most reasonable route to take at this point.

 

[NoJacketRequired]

Time in the tractor seat is time not spent in other, less attractive places. Run the L, get good at it, know its limitations. Eventually you'll not want a bigger machine because the smaller one gets into tighter places etc. As the old shooting saying goes... "Beware the man who owns only one gun, because he likely knows how to use it!"

If you are worried about "tippiness", take an hour or two and change your wheel spacing - I believe most of the B and L series tractor feature the ability to change rear wheel track width fairly easily. I know my three B-series tractors have this feature and one of the three is several inches wider because the rear wheels are set out as far as they will go.

 

[mjrwood]

Time in the tractor seat is time not spent in other, less attractive places. Run the L, get good at it, know its limitations. Eventually you'll not want a bigger machine because the smaller one gets into tighter places etc. As the old shooting saying goes... "Beware the man who owns only one gun, because he likely knows how to use it!"

If you are worried about "tippiness", take an hour or two and change your wheel spacing - I believe most of the B and L series tractor feature the ability to change rear wheel track width fairly easily. I know my three B-series tractors have this feature and one of the three is several inches wider because the rear wheels are set out as far as they will go.

You make great points and I agree with everything you said. If I knew that the added acreage would be it then the option to just roll with my current setup would be pretty locked in. But, within the next 5 years at the most the lease will be up and I was planning on adding the other 100 acres back into the mix and at that point was already planning on getting a larger tractor. Even with that being the case I'm heavily leaning toward trying it for at least a year and seeing how it goes. Like you said, time in the tractor seat is time well spent anyway

 

[rademamj]

Keep in mind that a 6' cutter is 20% larger than a 5' cutter. I would consider this something more than "slightly larger."

SDT

I remember a general rule for rotary cutters; allow 5hp at PTO on the tractor for every 1 feet of rotary cutter. Therefore a 5 foot rotary cutter TYPICALLY requires 25hp at PTO; a 6 foot rotary cutter requires 30hp at the PTO.

Sent from my SM-G930V using Tapatalk

 

[twomany]

I have seen what happens when a tractor with TOO MUCH power is driving an implement rated for less power. (Implement Gears shred, PTO driveshafts twist, implement-frame bends.) In this case, it was a brand-new, china-made, rototiller working in dry clay.

Moral of that story: Do not surpass the MAX rated power for your implements.

---------- Just a story this discussion reminded me about ---

Years ago, I just purchased a new motorcycle. One of the first to surpass the elusive 100HP per Liter. (In my case, it was 750cc rated for 85HP.)

I was telling my buddy about my powerful new motorcycle and he responded that his 4x4 John Deere tractor was 'only' 80HP at the PTO. (The tires on the tractor were more than 6 feet tall.... the thing was HUGE)

Obviously, the 80HP tractor was WAAAYYYY more powerful compared to my puny 85HP motorcycle.

Offer the fellow 100 to 1 on a half mile drag race. then tell us about how POWERFUL his J-D 4X4 was.

You know they are running the quarter mile in under 4 seconds these days. And that with less than 1000 turns of the crankshaft.

Go figure!

 

[SDT]

I have seen what happens when a tractor with TOO MUCH power is driving an implement rated for less power. (Implement Gears shred, PTO driveshafts twist, implement-frame bends.) In this case, it was a brand-new, china-made, rototiller working in dry clay.

Moral of that story: Do not surpass the MAX rated power for your implements.

---------- Just a story this discussion reminded me about ---

Years ago, I just purchased a new motorcycle. One of the first to surpass the elusive 100HP per Liter. (In my case, it was 750cc rated for 85HP.)

I was telling my buddy about my powerful new motorcycle and he responded that his 4x4 John Deere tractor was 'only' 80HP at the PTO. (The tires on the tractor were more than 6 feet tall.... the thing was HUGE)

Obviously, the 80HP tractor was WAAAYYYY more powerful compared to my puny 85HP motorcycle.

HP is HP. So, actually, your motorcycle produced more power than your neighbors JD.

That said, your motorcycle was designed to produce 85 HP for very short periods of time at very high RPM whereas, the JD is designed to produce 80 PTO HP all day, everyday, for years at probably somewhere around 2,200 engine RPM.

SDT

 

[troverman]

I mowed 50 acres, twice a year, with a 4-foot flail mower behind a B2920. It took awhile but I got good at it. Now I use an MX4800 with a 6 foot flail. Much quicker, but the B got it done.