hp vs psi vs flow

[husky362]

i understand a 2 stage pump works good on small motors and the 22 to 28 gal pumps work best on 12 to 16 hp but what if you have 20 hp would a <1.22 or 1.52 >17 and 22gal single stage pump out perform a 2 stage both single stage pumps should build 2250 psi at 10 to 12 gal min at 2000 rpm under load

I'm thinking the extra gpm at load should make a faster splitter
if a 5"cylinder at 1500 psi has close the same force as a 4" cylinder at 2500 psi would more gpm just make it faster not stronger until psi is bumped up..... but keeping max psi at around 12 gpm under load for good balance with 20 hp

most cheap cylinders that i can buy local come with 1/2"npt ports will matching 3/4 inch lines and fittings so they all have about 1/2" id work well with 17 to 22 gal ss pump with a 20 hp engine

with 20hp i should get a single stage to keep up with a 2 stage at a cheaper cost with more speed under load

just some thougts before i buy anything thank you for you input

 

[triptester]

An engine that is rated at 20 hp. only produces that amount of hp. at near maximum rpms. As rpms decrease so does the hp.. While some single stage pumps may cost less than a 2-stage most are equal or greater in cost. When the total cost of of each combo is compared the smaller engine with a 2-stage pump is the most cost effective and efficient.

I have a splitter with a 5" bore and have found that it seldom requires the 2-stage pump to bypass to the low flow/high pressure section only and when it does it is only for a second or two.

I am not a engineer so I can't offer a real good technical reason why some combos work better than others. I can only look at what combos both the top and bottom line splitter manufacturers have spent years putting together to produce their most cost efficient products.

Only in a very few cases is a single stage pump used.

 

[husky362]

the 20hp moter im using came off a ice storm damaged lawn mower and was free so im trying to build on the cheap but want to have the power with some speed to run a four way wedge on beam. ..a 28 gpm pump is $400 that most say hardly shift down with 5" cylinder why spend the money if a singel stage pump will keep up with it.... if it has to shift down your only getting 7 gpm at full rpm so if a ss pump does not shift down and the hp can keep the flow and psi up ..... would that not make a faster splitter under load if it only needs the psi for a few seconds to split a 5" at 1500psi is making around 30,000 pounds of force .....i under stand all parts have to work together

 

[cuttinscott]

What is the engine you want to use?????? Alot of hp #'s are not up to par. I had a 20hp Onan Performer on a 16GPM single stage running a 5x36 cylinder then swappped it out to a 27 hp L/C Kawasaki and a 20GPM single stage



Scott

 

[kevin j]

You are on the right track.
First, hp is (gpm ) x ( psi ) / 1714 for unit conversions

Use gpm x psi /1500 for real world. It is easier to remember, accounts for some pump mechanical inefficiency, and for the engine always less then advertised hp.

Hp is torque x rpm with some unit conversions thrown in.
Torque in ft-lbs x rpm / 5252 = hp

Advertised hp is usually 3600 rpm, although larger ones are sometimes 3000 rpm or less.
Thus, at 2000 rpm, you will have similar (or slightly more) maximum torque, but only 2000/3600 or only 55% of its ‘rated’ hp at 3600 rpm. So your 20 hp engine is down to 11 hp or so.

Pump torque to turn is proportional to size of pump and pressure. Same torque (almost) to turn it 2000 rpm or 3600 rpm. Hp is of course less, but torque is the same.

Pump flow (fixed displacement) is proportional to speed, so at 2000 rpm, the 1.22 and 1.51 in3/rev pumps you listed are indeed about 10 and 12 gpm as you calculated, instead of 17 and 22.

Given the pumps and pressures, assuming you are happy with the speed the cylinder gives you, a two stage IMO is not worth it. With enough hp, a single stage pump makes good sense. You can drive a single stage to the maximum speed you want. A two stage, with two or three times the flow at light load, may be way too fast for what you want.

A two stage with 2:1 ratio instead of 4:1 ratio would be great for bigger engines. It can be done with two section gear pump and separate external unloading valve, but that is beyond the skills of most home builders. I have not found a retail source for a built up two stage pump with a 2:1 ratio though.


The biggest reason for two stage is that most splitters can’t afford a 20 hp engine to turn the 20 gpm pump at 11 gpm output.

A two stage pump does not so much ‘shift down’ and waste any unused engine hp. The small section is sized to the maximum pressure (say 2700 or 3000 psi) that the engine can turn. Say a 5 hp engine can turn about 2 or 3 gpm. A single stage pump would then be always at this 2 or 3 gpm flow, even when moving at 500 psi and requiring 1 or 2 hp. SLOW most of the time.

The two stage adds in a larger section when pressure is low. Thus, the 5 hp can move 11 gpm at a load up to about 700 psi, then unload down to 2 or 3 gpm when the load requires more pressure. Think of it as though the system really ‘speeds up’ from the rated hp slow speed condition to move faster when the load is less.

kcj

 

[husky362]

What is the engine you want to use?????? Alot of hp #'s are not up to par. I had a 20hp Onan Performer on a 16GPM single stage running a 5x36 cylinder then swappped it out to a 27 hp L/C Kawasaki and a 20GPM single stage



Scott

i have a choice of low hr Tecumseh 20hp ohv or a low hr 18hp b/s op/cyl im getting for free........... i honesly never seen a tecumseh ohv v twin so dont know how they hold up but it is fairly new the tractor got crushed by big oak limb

how well did the 16 gpm single stage pump work do you feel a 22 gpm max at 3600 rpm would strain 20 hp to much ....what where you cycle times with a 5" cylinder ,,,its the cylinder at work where im looking to gain speed

i know where 2 low low hr vert shaft 27hp liquid cooled kawi's are.. i pulled them off a couple of hustler mower's for gearbox issues he just wont turn them loose for cheap or free like the other's

 

[Fronty Owner]

Here are a couple options.

Use a compensated pump. This pump will only push the volume required to remain at max pressure (the pump itself limits pressure, not a relief valve) so your putting less heat into your oil and the engine runs under less of a load except when there really is a load (cylinder moving). Compensated pumps can be pricey, but are really designed for sustained cylinder loads. In operation, the cylinder will run out to the wood, slow down until it pushes thru, then speed up as it can.

Run a tandem pump system. run the cylinder out under say 15 gpm at 1000 psi, and another pump at 5 GPM at 3000 PSI. Until the pressure in the cylinder reaches 1000 PSI, you got 20 GPM, as pressure passes 1000 PSI, you drop to 5 GPM with upto 3000 PSI of force. a tandem pump is two single stage pumps piggy backed. This will require two or three relief valves along with some an extra check valve to prevent back feeding the high pressure thru the low pressure. This may be cheaper, but will put 10 HP of heat into your oil and tank when your at high pressures. You may (read will) need a cooler or a really large tank.

Granted, most of my work is with electric motors.

 

[drmiller100]

compensated pumps are for closed center systems. really a bad idea to use a compensated pump on open center like log splitters are.

 

[drmiller100]

go do the math, but a 20 horse motor will only push about 10 gpm at 3000 psi.

so, if you use a 20 gpm pump on a 20 horse engine, about 1500 psi, which is half the splitting pressure most splitters run.

however, if you take your 20 horse engine, and buy the 30 gpm 2 stage pump, you will get two or 3 times the speed of most everyone else.

 

[Fronty Owner]

compensated pumps are for closed center systems. really a bad idea to use a compensated pump on open center like log splitters are.

true, you need to use the appropriate valve. All ports blocked being the most common, but becareful with cylinder mounted vertical, they can bleed off if you dont also have a pilot to open check valve plumbed in.

 

[husky362]

go do the math, but a 20 horse motor will only push about 10 gpm at 3000 psi.

so, if you use a 20 gpm pump on a 20 horse engine, about 1500 psi, which is half the splitting pressure most splitters run.

however, if you take your 20 horse engine, and buy the 30 gpm 2 stage pump, you will get two or 3 times the speed of most everyone else.

remember a 28 gpm 2 stage pump will only hold 900 psi with would yield 11000 #force with a 4" cyl then goes to high psi and only pumps 7 gpm@3600 now take and idle it back around 2000 rpm it may have hi flow till it hits a load then drops to 5 gpm at 2500 psi

now if a 20 hp with a [1.52ci] 20 gpm single stage pump will hold 1500 psi at 3600 would yield 18000#force with a 4"cyl and can push 12gpm at 2250 psi or 10gpm at 2900 psi at 2000 rpm i realize variable wood loads at different rpms has a lot to do with it but the math shows a 20 gpm single stage should make respectable cycle time with good power i understand 20 hp will not keep 20 gpm at 3000 psi as i use gpm x psi x .0007

 

[kevin j]

I wasn't clear in my post. I think a single stage pump (but smaller than 20 gpm) would be appropriate in your engine only if running at higher speed, to use the full engine hp but get up to at least 2500 psi. If you run that 20 pump at 2000 rpm, 1500 psi, and are engine limited to only about 10 hp, I think you will be disappointed. Then I would go two stage.

My only reservation to a large two stage is that if running at higher speeds, you might be too fast on cylinder stroke. I have one in process, but sort of on hold, designed for 3 to 4 seconds out IIRC. That is 18 hp, 2500 engine rpm, 28 gpm (at 3600) two stage and 4 or 4.5 inch bore cylinder.

Pressure comp piston pumps can run an open center valve but it is not recommended for a homeowner project. Piston pumps need to work against some minimum load pressure to keep the pistons and slippers held down on the swash plate. If running against less than maybe 200-400 psi. the pump can have a quite short life.

The reason for using a more expensive variable pump usually is because of many cylinders in parallel control, or for holding cylinders clamped or applying force continuously. For example, all the cylinders and clamps on a processor. There is less heat than using a fixed pump and relief valve. However, when moving motor loads with continuous flow at variable pressure, the PC pump can create MORE heat than a fixed pump. More than I should address here.

However, the peak hp on the engine is still the same with a variable PC pump. Maximum gpm and maximum psi are the same corner, or peak, hp unless a much more elaborate constant hp controller is used. Those are often used with engine driven equipment but not something for a home project to deal with in this small engine hp range.

Two gear pumps with control of one section is essentially the principle of a two stage pump. The second stage must have an unloading valve (back to tank at almost 0 pressure) NOT a relief valve. If going across relief, the same maximum hp is needed (pressure times gallons/1500) and the engine would still be overloaded, plus overheating the system.

Many ways to accomplish any given hydr circuit goal, but log splitters are so simple and so cookbook that the cheapest and most efficient solution almost always circles back to two stage pump on a small engine.

Bigger engine, bigger pump, more features, now the creativity and fun starts.

k

 

[Fronty Owner]

I wasn't clear in my post. I think a single stage pump (but smaller than 20 gpm) would be appropriate in your engine only if running at higher speed, to use the full engine hp but get up to at least 2500 psi. If you run that 20 pump at 2000 rpm, 1500 psi, and are engine limited to only about 10 hp, I think you will be disappointed. Then I would go two stage.

My only reservation to a large two stage is that if running at higher speeds, you might be too fast on cylinder stroke. I have one in process, but sort of on hold, designed for 3 to 4 seconds out IIRC. That is 18 hp, 2500 engine rpm, 28 gpm (at 3600) two stage and 4 or 4.5 inch bore cylinder.

Pressure comp piston pumps can run an open center valve but it is not recommended for a homeowner project. Piston pumps need to work against some minimum load pressure to keep the pistons and slippers held down on the swash plate. If running against less than maybe 200-400 psi. the pump can have a quite short life.

The reason for using a more expensive variable pump usually is because of many cylinders in parallel control, or for holding cylinders clamped or applying force continuously. For example, all the cylinders and clamps on a processor. There is less heat than using a fixed pump and relief valve. However, when moving motor loads with continuous flow at variable pressure, the PC pump can create MORE heat than a fixed pump. More than I should address here.

However, the peak hp on the engine is still the same with a variable PC pump. Maximum gpm and maximum psi are the same corner, or peak, hp unless a much more elaborate constant hp controller is used. Those are often used with engine driven equipment but not something for a home project to deal with in this small engine hp range.

Two gear pumps with control of one section is essentially the principle of a two stage pump. The second stage must have an unloading valve (back to tank at almost 0 pressure) NOT a relief valve. If going across relief, the same maximum hp is needed (pressure times gallons/1500) and the engine would still be overloaded, plus overheating the system.

Many ways to accomplish any given hydr circuit goal, but log splitters are so simple and so cookbook that the cheapest and most efficient solution almost always circles back to two stage pump on a small engine.

Bigger engine, bigger pump, more features, now the creativity and fun starts.

k

4" cylinder with a 24" stroke is about a gallon
5" cylinder with a 24" stroke is about 2 gallons
Im thinking with something more than 10 gpm, your gonna be watching some fingers and might want to add an accumulator to take some of the shock loads.

If your pump is volumetrically challenged, you might consider a regenerative circuit. its going to cut your effective force down to the area across your rod. add a check valve and a relief back to tank and you can help that out some.

 

[trialanderror]

it's a variable.

basically for HP, it basically boils down to this:

to have high GPM and high pressure, you need massive HP.
1 gpm at 3000psi, a briggs is plenty.
35gpm at 3000psi, that'll gag a 6 cylinder cummins no problem.



Also understand PSI, pressure per square INCH

a 4" bore cylinder with 3000PSI is comparable with a 6" bore and 1500PSI; in reguards, takes less HP because of the drop in PSI.
maybe not that compariable, but my point is your surface area is tremendously different and difficult to comprehend unless you do the math.

some of the ugly nasty pieces of red oak i was splitting would bring my 4" bore cylinder to a halt at 3200PSI. This 6" bore at 1500PSI walks through like a hot knife though butter....

PI plays a big part.


i have a 5 horse briggs that drives a pair of 9" bore for our tin crusher. 200ton when they're up to 3500PSI. Mind you the pump is only 2Gpm, ultra slow, but it will crush ALOT, and runs cheap. Just depends how you want it to run and function.


http://www.surpluscenter.com/Hydraulic.htm

bookmark it. i use it alot.

 

[drmiller100]

you can for sure use a relief valve to make a 2 stage pump.

Take a dual pump. make one your high speed circuit, the other the pressure side.
on the high speed side, use a relief valve to dump back to the tank.

the relief is used so little it won't heat anything significant.

I'm doing 30 gpm through a half inch fitting on the ram. i had worries before i ran it, and in a perfect world it would have 3/4 or 1 inch fittings, but it works fine and doesn't build any heat

for 18 inch stroke in and out, it is about 7 seconds, which is quick, but by no means dangerous.

 

[kevin j]

you can for sure use a relief valve to make a 2 stage pump.

Take a dual pump. make one your high speed circuit, the other the pressure side.
on the high speed side, use a relief valve to dump back to the tank.

the relief is used so little it won't heat anything significant.


-------

Good point, you can make sort of a two stage if the large section relief is set very low, say 200 psi. That may work well to get fast advance to and from the wood and would be a good time saver.

I am assuming this is on your processor. If you have plenty of engine power this would work great. If the engine hp is maxed out (as most small consumer machines are) there isn't enough extra power to handle the waste across the RV.

However, it is not really a two stage. The hp required in that section is still used pushing oil across relief valve to tank at the RV setting, it doesn't unload to tank at low pressure.
So doing it this way, the engine has to be large enough to run small section to 2500 psi, and the large section to 200 or 500 or 900 psi, whatever the RV is set to.

The heat generated in each cycle would be large section flow across the relief valve pressure setting.

It would be a trade off: the higher the large section RV setting, the more engine hp required and more heat generated, but more splitting force at high speed before it goes across the relief.

Either way, a check valve is still required to isolate the small section from backfeeding through the large section RV. Without it, the whole system would be limited to 200 psi (or whatever). The isolation check prevents that.

Since the RV and the check are already required, I would still make it into a true unloading circuit. The only difference is that the pressure signal that opens the RV spool is watching the output of the small high pressure and system load on the other side of the isolation check valve. When system load is high, large pump section is unloaded to tank. With the valve used as a normal RV, the signal comes from the inlet, which in this case is the pressure out of the big section. Doesn't unload at low pressure, just lets oil through to maintain a constant differential pressure (200 or 500 or whatever).

Sometimes the RV will have external ports for configuration in several different ways and can be easily made into an unloader. A cheap three ported inline spool RV might be able to be modified if it is a spool type by some drilling and tapping. A cheap inline spring and ball RV can't be modified into an unloading valve.

So bottom line, yes the two section pump can be made into a pretty good approximation of a two stage pump if there is enough engine and cooling for the hp loss, but they are indeed quite different circuits.

kcj

 

[George G]

If I were to use two pumps, I would do it like this.

 

[kevin j]

thanks george for the schematic.

That is exactly an unloading circuit. The pilot line into the unloading valve is what I talked about. When it senses on other side of the check, it is unloading the large pump essentially to tank at low pressure when load signal is above (usually) 600 to 900 psi. When it senses from its own inlet, which is the outlet of the large pump, it is relief valve and will keep the large pump working against the setting of the relief. That is the wasted hp and heat and extra engine load.

A two stage pump is identical. Just has all the parts built into one housing.
In fact, in this case, the phantom line around it all is an enclosure line, indicating all in one component, i.e. this is a two stage pump schematic symbol. (the newer ANSI standards will use a center line, with only one small dash instead of two, but they mean the same thing).

No big deal, both ways may work, but I just want people to understand the difference and when the differences means it will or will not work. Logsplitters, who cares, but someone building a larger machine may learn something.

kcj

 

[drmiller100]

KevinJ,

This is EXACTLY how all the two stage splitter pumps work.

call it an unloader valve, call it a relief valve, call it whatever you want.

you can maybe run it a little faster in high pressure mode with an unloader valve, but the plumbing is a lot tougher. A relief valve is a lot easier.

You should try runnign a splitter with a 2 stage pump. They spend about 97 percent of the time in high speed mode, with very little time in high pressure mode. just enough time to crack the log, then back to high volume.

 

[kevin j]

I am struggling to explain this without being a lecture in hydraulic theory or boring people to death. Most readers won’t need to understand it, but for the small few that care…..

Not just word game, but want to help people avoid the ‘cut and try and replace’ method of building a hydraulic machine of some sort.

Yes, a two stage pump is an unloading circuit with two pumps in the same body.

No, a relief valve circuit is not the same thing. It is more than just semantics of the name.

By RV circuit I mean this one approximating the unloading function, not the main system high pressure RV (usually located in the manual spool valve for the cylinder). This RV is in addition to that, and set much lower.

An unloading circuit unloads the pump at very low pressure (maybe 100 psi) and drops off the hp draw of the large section. All of the engine hp is available to use on the small high pressure section of the pump.

A relief valve circuit sends the oil back to tank across the relief valve setting. The hp draw of the large section pushing across this RV pressure is still a load on the motor, and is converted to heat because no work is being done with the energy put into the oil by the pump.

Example 1: If the large RV is set low, to conserve engine hp, say 100 or 200 psi, then the circuit goes from high flow to low flow as soon as the load reaches 200 psi. Essentially as soon as it touches the log, it will slow down. If the log requires more than 200 psi to split, it will not use all the engine hp and will split at a slow speed.

If the wood split runs easily, it won’t have to stay on slow speed very long until it picks up again. If it is tough stringy wood like elm, it will stay on low speed the entire stroke of the wood and be very slow. Most of the wood I see is that way. In low speed high pressure much of the time.

Example 2: If the large RV is set high, say 1000 psi for illustration, then it will have good force at high speed and split most wood just like a two stage with the unloading set to 1000 psi. However, if the load rises higher, the cylinder slows down to low speed, but the flow from the large section is not unloaded but is going across the RV at this higher pressure. This is a significant load on the engine, heat, and is wasted power that the engine cannot put to the small section pump.

Best way to understand this is simply do the math. Calculate the hp in each pump section in each example and total up the hp load on the engine.

If the engine is big enough to run both sections, then setting the RV at 1000 psi would behave exactly the same as a two stage that unloaded at 1000 psi. The only difference would be the extra heat and hp wasted, but the engine can turn the pump. drm: I think that is the case you are describing and I agree with you 100%

If the engine is small (sized just for the small section at high speed) then the extra load of the large section going across the RV pressure would stall it. That would be the case with most smaller hp machines. Then the RV has to be set lower and we are back to example 1.

Bottom line again: with enough extra hp, the discussion does not matter. Either way will work, just with some extra heat. If the wood is easy to split even that does not matter. But if the engine is sized as small as possible (or more properly the pump is sized as large as possible for maximum production), or if the wood is tough to split the entire stroke, then the RV circuit has disadvantages and reduced production. Lots of hp covers many problems !


I made a schematic of the RV circuit compared to the unloader circuit george posted above. Shows the difference of where the pilot line connects. Just not sure if I can insert it.

Yes, I have designed, built, operated, rented, owned or otherwise touched a splitter or two. Pretty simple compared to our bigger machines: 1000-1500 hp, 25 or 30 pumps, and 500 gallon oil tanks, but the principles are the same. We just can’t afford to be wrong the first time out.

And now, back to the Sominex sleeping pill commercial…..

kcj