HP rule(s) of thumb for BH & Log splitter

[NearBeer]

Thank you for your time in looking at my question. Are there any quick rules one could use in selecting engine HP for a given flow rate/desired psi of a hyd pump to be used on either a DIY backhoe or log splitter? I am looking to make my own and haven't even bought the first cylinder for either one...I would like to make a splitter to split logs no longer than 24" and/or use a DIY back hoe to dig out my crawl space; backhoe would not be used for rock removal, stump removal, etc...just scooping away dirt/clay here in the upstate of SC. If you need more info on what I am looking to do, please flame away!

I am seeing some deals on engines and want to make sure I get a strong enough engine to do either project.

In search of my next cold one,

NearBeer

 

[LD1]

(GPM x PSI)/(1714 X Efficcency of pump)

example... 10gpm pump at 2500psi and 85% efficcency
(10 x 2500)/(1714 x .85)
(25000)/(1456.90)=17.1HP required

If you go with a 2 stage pump...say a 16gpm, you don't use 16gpm x 2500 because 16gpm is only up to 650 psi.
Or you could use the second stage # which is like 3.5gpm @ 2500psi.

So a 16 gpm pump will require 7.1 hp to kick into second stage and 6 hp to build the 2500psi.

always round up. In the examples above for #1 use 18 or 20 hp motor.
In example 2, you would use an 8 hp motor.

Size accordingly. Going more hp only wastes fuel as it won't go any faster or make any more pressure.

 

[dfkrug]

I am seeing some deals on engines and want to make sure I get a strong enough engine to do either project.

I use "free" engines salvaged from m/cs, generators, lawn tractors, etc.

My home built hoe specced at least 8hp to run the pump. I started with
a Yamaha 750cc m/c engine....very smooth, powerful, and quiet. I switched
to a Tecumseh 10hp from a generator....super loud with high vibration.
Keeping a m/c air-cooled engine cool is tough. The ideal engine is going
to have more hp than you need so it does not have to run a full speed. That
keeps the noise and vibration down. I have a 16hp twin B&S from a
mower that is smooth and will prob put out 10hp or so at 2/3 max RPM.
Another freebie. If you can find a water-cooled m/c engine, those are
about the best, IMO.

 

[LD1]

The ideal engine is going to have more hp than you need so it does not have to run a full speed. That
keeps the noise and vibration down. I have a 16hp twin B&S from a
mower that is smooth and will prob put out 10hp or so at 2/3 max RPM.
Another freebie. If you can find a water-cooled m/c engine, those are
about the best, IMO.

By only running at 2/3 speed you may make 10 hp and be able to operate the pump to max psi but the slower you turn a pump, the fewer gpm you will make. So if you have a 16 gpm pump but you only run it at 2/3 speed, it will be like an 11 gpm pump and your cycle times will be 2/3 slower.

The water cooled engines are nice to think about but they are not practical. More maintenance, heavier, you have to find a place to mount a radiator and fan, etc. Air cooled motors are simple and easier to build a project with.

 

[LD1]

Oh...I forgot to add that motorcycle engines are even more difficult because they don't have a shaft to connect a pump to. They have a transmission, which shares the same oil as the engine (most of the time), and if you couple to the driveshaft/sprocket out of the transmission, now you have to mount the clutch so you can shift into igear once the motor is running.

 

[dfkrug]

By only running at 2/3 speed you may make 10 hp and be able to operate the pump to max psi but the slower you turn a pump, the fewer gpm you will make. So if you have a 16 gpm pump but you only run it at 2/3 speed, it will be like an 11 gpm pump and your cycle times will be 2/3 slower.

The water cooled engines are nice to think about but they are not practical. More maintenance, heavier, you have to find a place to mount a radiator and fan, etc. Air cooled motors are simple and easier to build a project with.

The OP has not purchased a pump yet, I assume. You buy the pump with
the displacement needed to give the flow you want at the RPM you plan to
run the engine and drivetrain. There is no cost difference between these
low-displacement pumps designed for fairly high speed.

As for water-cooled engines from m/cs, I will use as example a Honda
CX500 twin I have on my engine stand. The fan runs off the water pump
shaft and the radiator bolts right to the block. Very easy and integrated.
It has shaft drive and 5 speeds, allowing for starting in gear or neutral and
shifting up to 5th, without using the clutch. The Yamaha I mentioned
also had shaft drive and 5sp, but I used the drive shaft to run a chain
sprocket. I could have gone direct drive of the pump, but I had already
bought it and it needed higher speed.

I operated my hoe for many hrs using both engines I mentioned. Vibration
and noise was a big negative for the B&S single. The only reason I gave up
on the m/c engine is that it got too hot since it was air-cooled and not
moving. Clearly a turnkey engine off the shelf is going to be easier to
set up, but hardly the ideal choice, and, of course, not free.

 

[LD1]

There is usually a significant difference in the cost of the pumps.

If he wants 16 gpm flow he has two options. Buy a 16gpm pump and run it 3600rpm, or spend almost double to get a 22gpm pump, and run it at 2/3 speed to obtain the 16gpm. I just doesn't make any sense not to run it to it's full potential.

As for most people, we dont have access to a bunch of free motorcycle motors, and more than likely he will be purchasing a motor for his project. And the cheapest and easiest way is an air cooled motor.

 

[sizzami]

If I had it to do over I would build a hydraulic power pac that had the ability to run my hoe splitter, and anything else I dreamed up .
a 13gpm two stage haldex pump w/ a 8 hp ac engine is a good compact powerplant and not for nothing if you have a lot of vibration sumpin aint right
I could set my coffee on the beam of my splitter at 3600 rpm well for a minute.