# Hydraulic engine/pump sizing?



## Sandhill Crane (Mar 13, 2017)

First here's what I'm thinking, then the questions.
6Kproducts.com sells hydraulic saw set ups for processing firewood. They have several hydraulic motor sizes: 11 gpm, 14, 17, and 30 gpm.

Using a "rule of thumb" formula of GPM x PSI x .0007 = required horse power.

Now the questions start...based on engine and motor comparison. In my case electric motors are not an option.
Question: How do these three power units compare: gas, diesel, and electric? I seem to remember electric hp ratings being very different, much lower in comparison, to other fuels. 

Even gas/diesel hp is not a even comparison, but I don't know how to compare them regarding hp. Other than that, HP seems a rather ambiguous, almost useless, number. 

Also, when checking a Surplus Center catalog, pumps other than two stage pumps seem to be listed primarily in cu. in. capacity more often than GPM. Womack's (on-line, under Hydraulic Pump Calculations), formula for Pump Displacement Needed for GPM of Output Flow: 231 x GPM / RPM = cubic inches per revolution.

So how do I use this? 
For example: Using the 17 gpm cut-off saw.
Cu. In. pump size? and HP needed to run that size pump?
Pump size depends on RPM, therefor type of engine. 3,600 rpm for gas; 1,800 rpm for diesel.

HP needed... 17gpm x 2,500psi x .0007 = 29.75 hp (really?)
17gpm x 3,000psi x .0007 = 35.70 hp

Okay so getting more realistic and affordable.
HP needed... 11gpm x 3,000psi x .0007 = 23.10 hp

23 hp gas, diesel or electric?


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## ChoppyChoppy (Mar 13, 2017)

Where does that math equation come from?

I have a 30gpm pump on my processor for the splitter. Set to 3000 psi. Run by a 35hp Kubota. No issues. The math says I need 63hp?!
When I upgraded to it, Permco said the numbers worked to about 40hp needed. I figured average split is maybe 1500 psi so worst case I could trade off power for speed and lower the relief if it stalled the engine. Never had to fuss with it.

The engine is also running a conveyor all the time too, plus other stuff (though generally just 2 functions at once).
Conveyor runs at around 500psi, relief I think is around 700. I forget the gpm, maybe 5.

My engine runs at 2900rpm BTW. (governed speed). It burns just over 1 gallon an hour. (I get about 13-14 hours to a 15 gallon tank)
1800 rpm engines aren't too common in the smaller hp stuff. Some gen sets, about it really.


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## kevin j (Mar 13, 2017)

Your math looks correct, just by quick review.

Pumps move oil by cubic inches per revolution. There is no real magic '11 gpm' pump unless the rpm is specified.Most assume 3500-3600 engine rpm for the advertised number.

psi is determined by the load. Whatever is required to move the load, or the cylinder, or motor creates resistance that the pump shaft has to twist to overcome and move the load. If a log only needs 500 psi, that is all the gauge will read. The relief valve limits the maximum psi, much like a fuse. It does not 'create' 3000 psi, it will just bypass flow at 3000 to limit the maximum pressure.

gpm = in^3/rev x rpm / 231 in3 per gallon

Torque is really what the pump needs, but lets do hp.
gpm x psi / 1714 is hp. With efficiencies, use gpm x psi / 1500. also easier to remember.

Hp is hp, gas or electric. The 'electric is 2 hp' adage comes from the fact that the electric motor has a maximum breakdown torque ( at a much lower rpm) of about 2x its rated torque at full rpm. A gas or diesel engine only has about a 10-15% torque rise from full load to max torque. So it seems like the electric has more lugging torque, which it does.

Two stage pump is say 11 gpm to the maximum pressure the engine can turn until the large gear section unloads, say 600 or 900 or 1000 psi. That is one torque and hp point. Then the small gear section times 2500 or 3000 psi is the second hp or torque condition. Thus, small engine can turn large flow up to a limited pressure, then a smaller pump section to full pressure.


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## ChoppyChoppy (Mar 13, 2017)

How am I getting by with nearly 1/2 the horsepower? The engine doesn't bog to the point it'd stall or anything like that.

My other splitter has a 11hp Briggs and 16gpm pump. It handles 3000psi too. According to the math I'd need 30hp.


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## Sandhill Crane (Mar 13, 2017)

I found the equation on-line at womackmachine.com; click Engineering Toolbox at top of page; then at side menu click Hydraulic Pump Calculations.

As to diesel 1,800 rpm, that was an assumption on my part for small diesels. I have a Cat 3126 in my truck an rarely run the tach past 2,500 to 2,700 when shifting. Still, way over my 1,800 rpm assumption.


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## muddstopper (Mar 13, 2017)

Sand, when your on the surplus center site, go to their technical section. They have all the calculators there so you can just punch in the numbers and get the answers. 

Sounds like you are wanting to build a processor saw. You may already know this, but there is more to it than just rpms of the saw. Chain speed is a factor of rpms and rim tooth count and size of chain, and is measured in feet or inches per minute I would guess that the 6kproducts saws have already done the math to size their motors to the saw. They should have a recommendation as to pump flow and pressure to match. Of course the bigger the saw motor, the more power it will have and the more oil it will take to run it at the recommended chain speed. 

Your 11gpm @ 3000psi should take a 21hp electric motor. It is recommended that you double that number for a gas engine. While that might be true, I have never found it to be so. There are many factory processors running 25hp engines. One thing to consider is that the pump only needs that kind of hp if it is producing the full 11gpm @ 3000psi on a continuous basis. A saw motor doesnt take continuous torque and full pressure when it is sawing. On large dia wood, you might see the pump load up and bogg the engine, but only if your wood took the full bar length to reach thru. The pump and motor combination should maintain speed all the way until the pressure reaches 3000psi. A quick look at 6kproducts website, (danzco),it seems he is recommending 2000psi for his 14gpm cutoff saw. I didnt see what he suggest for his 11gpm saws. I also didnt see anything listing saw motor size sprocket size or rpms speed. I have talked to him on the phone and he wont give you those numbers unless you buy his saw. I can say from looking at his saw motors, they seem to be plain old gear motors and I am just going to guess that they are around 1cuin, or possibly less to get the speed and torque needed to pull a chainsaw chain. If you knew what cuin his saw motors are, its just a matter of doing the math to calculate chain speed, hp and torque the saw will produce. then you could compare this number to one of your gas chainsaws ratings to get some ideal of how fast or slow the saw will cut.


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## Sandhill Crane (Mar 13, 2017)

Sandhill Crane said:


> Using a "rule of thumb" formula of GPM x PSI x .0007 = required horse power.





ValleyFirewood said:


> How am I getting by with nearly 1/2 the horsepower? The engine doesn't bog to the point it'd stall or anything like that.



35hp = 30gpm x Xpsi x .0007
35hp = .02Xpsi
1,750 = psi you can push using a 35 hp engine and 30gpm pump.

My question is does it make a difference if it is gas or diesel?
keven j mentioned torque. How does that factor in?

EDIT: Just saw muddstopper's post. Have to read it and catch up.


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## muddstopper (Mar 13, 2017)

ValleyFirewood said:


> How am I getting by with nearly 1/2 the horsepower? The engine doesn't bog to the point it'd stall or anything like that.
> 
> My other splitter has a 11hp Briggs and 16gpm pump. It handles 3000psi too. According to the math I'd need 30hp.


Becasue your 16gpm pump is 2 stage and only producing about 950psi at full flow and only needs about 9hp to pull it. When it kicks into high pressure mode it is only pumping about 4-5gpm at 3000 psi and only needs about 7hp to pull it. Sandhill is looking at using a single stage pump


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## muddstopper (Mar 13, 2017)

went back and read the manufactors page again. It seems his 11gpm saw is supposed to run between 4000 and 5500 rpms. This would mean the saw motor is around .5 to .65 cuin. I dont think you are going to like it, it would be a pretty week chainsaw.


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## Sandhill Crane (Mar 13, 2017)

Okay. Just looked at 6kproducts.com again with a little different eye.
The connect and cut saw, under instruction manual, says 2,000 psi for each of the different ones I looked at.
Doing the math...
11gpm would require 15.4 hp; 14gpm, 19.6 hp; 17gpm, 23.8 hp; 20 gpm, 28 hp all at 2,000 psi. 
30gpm: 30 gpm x 2,000psi x .0007 = 42 hp


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## ChoppyChoppy (Mar 14, 2017)

muddstopper said:


> Becasue your 16gpm pump is 2 stage and only producing about 950psi at full flow and only needs about 9hp to pull it. When it kicks into high pressure mode it is only pumping about 4-5gpm at 3000 psi and only needs about 7hp to pull it. Sandhill is looking at using a single stage pump



It's a single stage pump.

Saw motor on my processor runs at around 800psi if that helps you. Cylinder for up/down at 500.
How it works is clamp needs to squeeze to 500psi, then the bar cylinder gets fluid.


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## Sandhill Crane (Mar 14, 2017)

That is very helpful. I'm assuming I can run a bigger pump with a smaller engine. For example:
30gpm x 1,000 psi x .0007 = 21 hp
I would have to run a log clamp, and a log deck, and log feed as well.
I think a 17gpm pump would be a better choice, or a larger engine.
At this point I do not have any components. Over all price is a factor. 
Basically what I would like to end up with is something like Built-Rite's Block Buster cut off saw, for less money of course.
The other option is wait, and buy an inexpensive light weight processor for starters.


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## muddstopper (Mar 14, 2017)

Just a observation I have noticed about cheaper model processor saws. Most all manufacturers will use some sort of gear motor. The rpms that they turn these motors can only be achieved by over flowing the oil to the motor. My meaning is that they take a motor rated for x amount of oil flow and throw xx amount of oil to it. You can get by doing this as long as you keep the pressures on the low side. 

Maxing out pressure and over flowing will usually result in a short motor life. Not all gear motors are rated for 5500 rpms as Jrproducts suggests as the max recommended speed of their saw motor. Most hyd motors are bushing types and they wont hold up to the high rpms under full pressure. Some manufacturers will have there motors spec'ed to use roller bearing instead of bushings. 

Some also use gerotor motors, these usually will come with roller bearings and are designed for higher rpms. This is an example of a gerotor motor, http://www.surpluscenter.com/Hydrau...in-PARKER-MGG20025-BB1A3-HYD-MOTOR-9-8502.axd, notice the 5000rpms max speed and the 2000psi pressure rating. I know some who have tried this motor and its bigger brother, the .070, and did not like the performance, but they did work. One fellow I know of that tried the smaller .58 motor and then switched to the larger .70 motor and decided to just live with the results after that. Kind of a expensive lesson of trial by error. 

Then you have the bent axis, f11 f12 motors like you would find on old harvesters. Very expensive motors and require lots of oil at high pressures (5000psi) to get max performance. Not only is the hyd motor high dollar, but you have to upgrade engine hp and hyd pumps to get the full benefit of owning one, but they sure do cut fast.

I have considered a lot of different hyd motors for my hyd saw and I decided to use a axial piston motor like this one, http://www.poclain-hydraulics.com/en/products/motors/m-mv/m1. I have 2 of these motors setting on the shelf. My thought process is that I can get the rpms and the torque needed to run a large pin sprocket, that allows me to get the ftpermin of chain speed, without having to use high pressures and big hp to get there. Notice the 4641ftlbs of torque at 1000psi. It only takes about 20 gpm and 13hp to get there, but you can run the pressure up to over 4600psi if you need to. Of course if you start cranking up the pressure, you will have to crank up the engine hp to get there. You can also over flow the hyd to get more speed out of them, as long as you keep the pressures low. I talked to a rep at poclain about this and he claims i can run it about 4600rpms as long as I dont run the pressures over about 2000psi. Theres a pretty big safety factor for long liveity in industrial applications.


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## Sandhill Crane (Mar 14, 2017)

muddstopper said:


> Kind of a expensive lesson of trial by error.



This is one of the reasons to not build something. When I converted the hand crank lift to a hydraulic cylinder on my conveyor this winter I spent a good deal more than anticipated. Much of this could possibly have been avoided if I knew if I had a power beyond valve to start with. Not being familiar with hydraulics, a bought a new two spool valve. I also bought a cylinder from the conveyor builder, having been told by them it would bolt in. It did not and I paid for some fab work. For a larger project I would buy a welder. This tread is the planning stage of what 'could be' next winters project. I have several ideas including a live bottom hopper, or possibly a merry-go round, splitter feeder for rounds, or split wood for bundling.


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## T. Mainus (Mar 14, 2017)

https://milwaukee.craigslist.org/grd/5980984912.html

or if you really want to blow your wad

https://milwaukee.craigslist.org/hvo/6028459330.html


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## ChoppyChoppy (Mar 14, 2017)

The 1610 was about 22k new in 2008. I have the receipt for the one the shop had, found it yesterday in a pile of random papers actually.

It's "meh" uses chainsaw chain and the splitter isn't strong.

The CRD is a good buy.


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