Torque vs HP

[Blade_Runner]

G'day! I'm new to the forum (been reading for a few days) and new to grader driving, having just done my first week on a grader.... so I have all of about 30 hrs of actual grader driving experience!

My job involves maintaining about 2000km of gravel roads within our shire (council, county) in Western Australia. I drive a Deere 670D (~8800 hrs) with rear mounted ripper and roller attachments.

I have been instructed to drive in 2nd or 3rd gear while grading and (max) 4th gear while rolling/compacting only.... and spend most of my time cruising along at about 1900 rpm in 3rd (grading).

I was reading a John Deere brosure (https://www.deere.com/en_US/docs/construction/non_current_products/motorgraders/DKADGDRTD.pdf ) and noticed the 670D puts out more torque/HP in 4th gear, than it does in 1 - 3rd gears.... (611 ft/lbs vs 554lb.-ft max @ 1000 rpm). This got me thinking.....

Because a grader is all about moving dirt efficiently (not quickly), would it make more sense to sit say on 1000 rpm in 4th, rather than 1800 rpm in 3rd, or whatever the equivalent speed would be, given that the engine is producing more net torque at 1000rpm in 4th? Or is the lower rpm going to be harder on the engine in the long term?

Obviously, the lower RPM range is quieter, with less vibration (important when doing 10 hr shifts)

What rev ranges do you guys use on long runs?

 

[Cmark]

I'm not familiar with Deere graders, but this is obviously the same thing as Cat's VHP feature.

IC engines don't produce their torque in a flat line from low to high idle. To assume that the engine will produce more torque at 1000rpm in its high power mode is incorrect. Not saying that it won't, just that you shouldn't assume that it will.

To give a definitive answer, you would need to get graphs of the torque curves for both power modes and compare them side by side.

 

[lantraxco]

My opinion for whatever it's worth is you should run it wherever it feels good to you. If it will do the job in 4th and it doesn't feel like it's straining, then do that but keep the speed the same. Grading too fast will burn up the blades. I think the lower rpm will give you longer engine life over the long term. Interesting that they give you power increases as you go up, i guess they're protecting the drive train and all by limiting torque in the lower gears.

I'm no grader operator, just an old mechanic. :my2c

 

[biggrader]

Its all a matter of how much work you want to get done. Obviously it takes more more power to move the same amount of dirt at say 6 or 7 mph than it does at 4mph. That is why manufacturers limit power in the lower gears ( to prevent wheel spin) and slowly bump it up in each of the higher gears. We could get into the horsepower/torque debate.... throw in tractive effort, tire efficiency, final gear ratio and whole lot of other factors but will keep it simple...... unless you REALLY want to get into it.....? But to answer the OP question, yes you can run a higher gear/lower rpms as long as you are not excessively straining the powertrain and as a bonus, obtain better gph numbers. I run a little faster than you (1800-1900 4th gear) for cutting and usually feather in 5th gear/1275-1300. both essentially the same speed. I use either a 870d or a 772d, so have a few extra ponies to work with.

 

[Blade_Runner]

... To assume that the engine will produce more torque at 1000rpm in its high power mode is incorrect. Not saying that it won't, just that you shouldn't assume that it will....

Cmark, thanks for your responce. I hope I am not assuming anything. I included a brief intro, stating that I am new to grader driving, for that very reason. As far as I know the 670D does not have a "power mode", at least not one that is switchable by the operator. But according to the specs I referenced in my post, the machine has been configured to make more power/torque available in 4th - 6th gears (and more again in the top two gears).

So I was asking for feedback on wether traveling in a higher gear @ lower rpm (at the same speed), conditions permitting, would have any adverse effect?

As I am only doing basic road maintence and (most of the time) am not working the machine hard, it would seem more 'efficient' to run the machine in a higher gear, while maintaining the same speed... but as I am new to this, I figured I would ask if there was any reason why I shouldn't do it that way.

 

[Blade_Runner]

My opinion for whatever it's worth is you should run it wherever it feels good to you. If it will do the job in 4th and it doesn't feel like it's straining, then do that but keep the speed the same. Grading too fast will burn up the blades. I think the lower rpm will give you longer engine life over the long term. Interesting that they give you power increases as you go up, i guess they're protecting the drive train and all by limiting torque in the lower gears.

I'm no grader operator, just an old mechanic. :my2c

Thanks lantraxco. Your feedback is appreciated and valued (more than 2 cents worth!). No better place to get good advise than from "an old mechanic"... and trust me, at half a century, I am no spring chicken either!

 

[Blade_Runner]

... it takes more more power to move the same amount of dirt at say 6 or 7 mph than it does at 4mph....

Thanks for your reply biggrader. I see what you are saying about going faster but my question was about using a higher gear at the same speed (using lower rpms), not about moving mre dirt more quickly.

That is why manufacturers limit power in the lower gears ( to prevent wheel spin) and slowly bump it up in each of the higher gears.

This actually answers my question to some degree... as it helps me understand why the power/torque numbers are higher in higher gears... Cool!

We could get into the horsepower/torque debate.... throw in tractive effort, tire efficiency, final gear ratio and whole lot of other factors but will keep it simple...... unless you REALLY want to get into it.....?

No need. I was asking a simple question. Not wanting to start a war with my first post on this forum! ;P

But to answer the OP question, yes you can run a higher gear/lower rpms as long as you are not excessively straining the powertrain and as a bonus, obtain better gph numbers. I run a little faster than you (1800-1900 4th gear) for cutting and usually feather in 5th gear/1275-1300. both essentially the same speed. I use either a 870d or a 772d, so have a few extra ponies to work with.

See, I never thought about running a little faster while feathering... as I said, I'm very new to this and am just doing what the previous operator told me to do (I had a couple of hours 'training' before being told: "you'll learn as you go".).

Very useful reply. Thanks.

 

[Nige]

I'm not familiar with Deere graders, but this is obviously the same thing as Cat's VHP feature.

IC engines don't produce their torque in a flat line from low to high idle. To assume that the engine will produce more torque at 1000rpm in its high power mode is incorrect. Not saying that it won't, just that you shouldn't assume that it will.

To give a definitive answer, you would need to get graphs of the torque curves for both power modes and compare them side by side.

:iagree

I will throw in this missive of torque vs horsepower though ..........

it seems as though torque is a subject that carries a bit of mystique. Ask an average person to define exactly what it is - and its relationship to horsepower - and often the room falls suddenly silent.

Sometimes the mark of a true genius is his ability to explain a tricky subject in a way we can all understand. The late John Robinson - he of PB Magazine fame - was, in my humble opinion, a genius. Here's how he defined the difference between torque and horsepower;

"Suppose some miscreant were to chuck a 20 pound labrador off a cliff - only a puppy, really. You scramble down the cliff, discover the animal is fine, affix a rope to its collar then cast about for ways and means of raising it to the top of the cliff.

Any engine making 20 ft/lb of torque, and attached to a 1ft radius winch drum, would be able to lift the labrador. An engine which gave this torque at 4,000rpm would be able to lift the dog twice as fast as one that did the business at 2,000rpm.

And that's the difference between torque and horsepower. The 4,000rpm engine would get your dog back twice as quickly - be twice as powerful - as the 2,000rpm engine."

 

[Cmark]

As far as I know the 670D does not have a "power mode", at least not one that is switchable by the operator. But according to the specs I referenced in my post, the machine has been configured to make more power/torque available in 4th - 6th gears (and more again in the top two gears).

But surely you've just contradicted yourself. If the engine makes more power in 4th than 3rd and the operator is able to select 4th or 3rd then, ipso facto, the operator is also able to select either power setting.

But don't get too hung up on this. I'm just playing the devil's advocate.

So I was asking for feedback on wether traveling in a higher gear @ lower rpm (at the same speed), conditions permitting, would have any adverse effect?

Industrial diesel engines are happiest running at their rated RPM which by definition is their maximum torque/HP. However, in the real world, I would agree with the general thrust of the previous replies; run it at whatever you think seems right and sensible. In my experience an operator is far more likely to damage a machine by not checking the fluid levels every day than by driving it incorrectly.

 

[biggrader]

Let me throw this out there also. IF you are chugging along at say 1200 rpm, sitting back listening to the tunes, watching the neighbor girls sun bathing in the front yard and the engine suddenly stalls. you were probably lugging it to hard.
Remember that at lower rpms it will affect other things also. Throttle response and to some degree hydraulic response. Also your engine cooling fan speed is reduced and you may have some overheating problems.
Keep asking questions, I will be happy to give you my 2cents. Just dont expect the million dollar answer, thats over my head.

 

[Delmer]

"lugging it too hard" and torque rise are the problems I'd see. Two different things though. You could be running along at 1,000 without lugging it, but it won't have any torque rise (reserve power) if you hit a hard/soft/wet/dry spot and it stalls or nearly stalls. If you had been at 1,900 the torque would rise as the engine slows down so the engine will be perfectly happy pulling through. The link claims 46% torque rise (from either max speed, or max power which is usually a little less than max speed).

Lugging an engine is one of the worst things you can do for it's health. 1,000 RPM is pretty slow IMHO, I'm guessing that engine has a max speed of 2,200 RPM or so? and at full load it is happiest at 1,600-2,000, that is where the engine would produce the most HP hours before destruction on a test stand. Best HP hours/gal would be at full load at something like 1,400 RPM. If we're talking about less than 1/4 of max HP and what speed will be most economical for fuel and engine life, I'll say closer to 1,400 than 1,000 RPM?

So basically I'm agreeing in principle with your instructions, but then I don't know how much load you're putting on the engine at the lower RPM, any chance any of the electronics will give you this info? I'm a little surprised that the lower rpm is smoother than 1,900, does that sound right to the experts?

Keeping the driveline from grenading in low gears plays a big part in derating the engine, I suspect. It may be that the computer also protects you from damaging the engine from lugging, I don't know?

 

[biggrader]

I agree with Delmer here that if you are running at a lower rpm to the point where stepping on the throttle does not provide immediate increase in RPMs, than you are lugging it to hard. I should have stated in my earlier post that in the use of the higher gear/lower rpm is only if the conditions permit. Do I use that situation all the times, heck no.
If the 670D has autoshift enabled , that feature will protect from excessive lugging. If the load becomes too great it will downshift the transmission.

 

[John C.]

Nice discussion. I think everything stated above is great information.

More than likely the number of hours of experience running the machine is the reason for the instruction of using specified gear and engine speed range. Generally I find that new operators should ask questions but should also be careful of getting too technical before they have the necessary skills to operate the machine efficiently.

 

[JDOFMEMI]

Well said John

When I teach new operators I stress that it is best to learn slow, and get smooth. Once you are smooth at what you are doing, find the speed that you are comfortable at.

Smooth first, fast will happen once smooth is mastered.

As for using a higher gear, once you have learned to be smooth in the gear you are in, work up. Remember, smooth is not just the finished product, but how you treat the machine. If the machine is struggling, lugging, spinning tires, bouncing, rough shifts, etc...., you need to slow down and work on being a smooth operator.

New operators now have a great resource in HEF, and can learn from many old hands willing to share their knowledge. The older generation did not always have a willing teacher at hand.

 

[Buckethead]

Smooth first, fast will happen once smooth is mastered.

Excellent advice, That's what I was taught and it's served me well some 20 years! :drinkup

 

[Blade_Runner]

Thanks for all of the great replies! The members of this forum certainly have a very deep and wide range of knowledge/experience in this field. The responses are extremely helpful, respectful and well thought out.

As noted in my original post, I'm new to this job... having made a significant career change a couple of years ago, first driving trucks (general freight & dump trucks) and now on a grader. Until a week ago, I had NEVER even been in the cab of a grader! I worked in I.T. for many years and have spent a lot of time online, using various forums, but have never found one as helpful as this. And now I'm starting to sound like a stary eyed teenager, which at 50 I certainly am not.

Unlike some others I know, at my age, I am still very keen to learn from those who have more skill and experience than I do. I want to become the best (or "smoothest", thanks JDOFMEMI) 'operator' I can be... not just a driver or "steering wheel attendant"... I want my boss to go: "Wow, how did you pick that up so quick"?

Thanks again for the time and effort taken to answer my questions...

My next problem (for another thread) is what do I ask next? You know, when you don't know something, you don't know what to ask for, because you don't know what you don't know.... And, yes I have read all 19 pages of the 'Motor Grading Techniques' thread (https://www.heavyequipmentforums.com/showthread.php?11428-Motor-Grading-Techniques) in case you were going to point me in that direction...

:drinkup

 

[Fatgraderman]

The older mechanical injection graders got ran at higher RPM. Cummins 8.3's in Champions ran hard, but they needed a little revs. Especially snowplowing. The newer electronic IC engines pull real hard at lower RPM's. When the "October 02 Emissions" come out we had to change how we ran things a bit. Honestly though, the guy that owns the grader and signs the cheques has to have some say in it- run her his way. Ultimately, HP and Torque are tied together. Torque x RPM / 5252 = HP- So if an engine makes 500 ft lbs at 2000 rpm as another that makes 1000 ft lbs at 1000 rpm, their horsepower is identical. You end up doing a gearing change to match the speed. Use whichever "gearing change" you don't bounce in

 

[JDOFMEMI]

Without trying to sound like a smart **s, I would suggest reading over each thread in the motor grader section, and then keep putting what you learn there to use.

Another thing I teach people just learning is that they first have to learn enough to realize what the next question should be. When you get to the "end" of what you know, at least you should have an understanding of what comes next.

There are also many publications available to teach road maintenance.

One I have found useful is "Gravel Roads Maintenance and design manual"

It is too big to post, but a quick google search should locate it.

If not, PM me your email and I will send you a copy.

 

[Blade_Runner]

...

There are also many publications available to teach road maintenance.

One I have found useful is "Gravel Roads Maintenance and design manual" ...
.

Thanks Jerry. Found it and have downloaded it.

 

[JDOFMEMI]

Glad to be of help.

Keep us posted as you learn.