Hydraulic Data

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clawmute

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Here is some handy hydraulic data which I often use. These are "rules of thumb, etc.

1. a gallon is equal to 231 cubic inches. Sometimes it is handy to convert flows/volumes into cubic inches.

2. 1hp input is required for each 1gpm @1500psi. This is good for rough estimating the approx. hp rating of an engine or motor for driving a pump.
for example, a 5gpm pump operating at 1500 psi needs a 5hp motor, or at 300psi a 10hp motor.

3. It takes about 5% of the full rated hp to idle an unloaded pump.

4. The secret to a long lived hydraulic system is clean oil. Keeping the filter(s) changed will add life to the system.

5. One of the most common problems encountered is cavitation of a hydraulic pump inlet due to dirt buildup on the suction strainer. Higher pump noise & system heat output are usually indicators that something is not right.

6. Increasing a cylinder size using the same pump will result in slower extend/retract cycle times.

7. Decreasing a cylinder size using the same pump will result in faster extend/retract cycle times.

8. Hydraulic cylinder speed; S = CIM/A where S = speed in inches per minute,
CIM is oil flow into the cylinder in cu. in./min., A is the piston area in square inches.


9. Hydraulic cylinder retract speed is similar only the rod dia. must be subtracted from the piston area; RS = CIM/(A-ra) where RS is retract speed in inches/min., CIM is oil flow into the cylinder in cu. in/min., A is the piston area in sq. inches, and ra is the area of the rod cross section in square inches.


Misc. input horse powers @ given gpm

GPM________1000psi_______1500psi_______2000psi_______3000psi

5-------------3.43 --------- 5.15 ---------- 6.86----------10.3

10 -----------6.86----------10.3------------13.7-----------20.6

15------------10.3----------15.4------------20.6-----------30.9

20------------13.7----------20.6 -----------27.5-----------41.2

25------------17.2----------25.7------------34.3-----------51.5



Hydraulic Cylinder speeds (inches per minute)
These are the "extend" speeds. The retract speeds will be greater since you have the same oil flow against a smaller area (piston area less rod dia. area)

Piston Dia.______ 5gpm_______8gpm______12gpm______20gpm

3.25"---------------139---------223----------332---------556

4" ------------------92----------147---------220---------368

5" ------------------59---------- 94----------140---------235

6" ------------------41----------65----------- 96---------163


If you are thinking about building a hydraulic splitter, you might consider having a cylinder made to suit your purposes. There is a small hydraulics mfgr in our area that can make any cylinder you want with mounting and rod end conditions to suit your setup. Most likely there is probably one near you, or you can even pick up a used one from a scrap equipment dealer and refurbish it with new seals/packing.

You can have a hefty oversize rod made since the retract power required will be considerably less that the extend.

Below is a picture of 5 press roll machines I've been designing that are used to crush wood. Each uses 2 - 8" dia hydraulic cylinders and is lifting a vaned roll assembly that weighs about 6tons. The rods on these cylinders are 4" in diameter. Both rolls are also driveshaft driven with a 50hp motor on each. Since there are two cylinders per roll, a pair of linear transducers are used to keep the roll level and to control the flow of oil into and out of the cylinders.

DSC01840.jpg
 
so....when i see manufactured log splitters with 5 or 6 hp engines on them, based on the info above, the engines are too small for the application?
 
so....when i see manufactured log splitters with 5 or 6 hp engines on them, based on the info above, the engines are too small for the application?


I can't speak for all of them but they probably are. Being production items they are designed right down to the gnat's hair. Performance charts are mostly too enthusiastic and based on ideals - reality of operating conditions is often totally ignored..... Working in the forest product industry full time, we design equipment to be tough and for the long haul.

Frank
 
That sounds like data for single stage pumps. If using that data for a 2-stage pump, wouldn't one use the GPM rating for when it has shifted into high pressure/low volume mode? As I understand it, that's how they are able to get the high GPM ratings with lower hp rated engines. I would think the GPM rating of the high pressure/low volume mode would be far more relevant in figuring out the required engine size. Odd that figure is far less publicized. If I remember correctly, it's about a third of the low pressure/high volume mode.
 
Note: The crushing equipment is for producing a new structural wood product that is made by crushing/scrimming southern pine, resinating it and pressing into beams. This is a gif I created to show part of the process.

CRUSHING2.gif
 
Here's another gif

ROLLINE1.gif


Two stage pumps on splitters use a high pressure lower volume phase on the extend/split cycle and a high volume low pressure flow on the retract.
 
Last edited:
Two stage pumps on splitters use a high pressure lower volume phase on the extend/split cycle and a high volume low pressure flow on the retract.

It was my understanding that 2 stage pumps stayed in low pressure/high volume mode until pressure increased to a predetermined point, then shifted into high pressure/low volume. The direction of the cylinder (extend vs. retract) is controlled by the valve, and as such, the pump doesn't know or care which direction the cylinder is going and isn't going to change modes based on whether it is extending or retracting.
 
It was my understanding that 2 stage pumps stayed in low pressure/high volume mode until pressure increased to a predetermined point, then shifted into high pressure/low volume. The direction of the cylinder (extend vs. retract) is controlled by the valve, and as such, the pump doesn't know or care which direction the cylinder is going and isn't going to change modes based on whether it is extending or retracting.

True enough, but for all practical purposes its high vol/low press on the reduced work pull out & reversed on the high force requirement split. That predetermined pressure is - or can be - settable on some configurations where you can adjust the "kickout" range.
 
So then when it goes to high pressure/low volume, does the low volume component of that equation decrease the required engine hp?
 
I have the Northern make your own splitter manual that I bought years ago.It basically shows the dimensions of their splitters,hydralic tables and parts lists.
 
True enough, but for all practical purposes its high vol/low press on the reduced work pull out & reversed on the high force requirement split. That predetermined pressure is - or can be - settable on some configurations where you can adjust the "kickout" range.

I've never had a 2 stage pump apart or found a working diagram. I always figured there were 2 pumps in there. One being a vane pump for high vol/low pressure the other being a gear pump or other positive displacement pump to provide the high pressure under load. When you say it 'kicks down' does it actually shift gears or are there two positive displacement pumps that work against each other. Under load does the the high volume pump stop and cause the low pressure pump to engage or possibly engage faster ? Without seeing the inner working that is all I am coming up with.

I might never need to know but understanding the inner working might help at some point in the future.

Theorectically the 'lumber' from the process you are working on could be extruded into nearly any working form or shape for strength, weight or durability. Even hollow or foam filled if a customer desired. Interesting.
 
two stage pump: there were earlier posts in more detail.
I can't paste the schematic here.

There are two gear sections in the same housing with same drive shaft but separated by seals and bearings. Small one goes to the system out all the time. Larger section goes through a check valve then adds to the outlet flow. Between the larger gearset and the check valve is an unloading valve back to the suction side. Pressure signal from the small stage/system outlet operates the unloading valve. When at low pressure load, both gear sections add flow out for high flow. When load increases to say 600 psi, the unloading valve opens and vents the large gear set back to suction side. Check valve prevents small gear set from backflowing to unloading valve. Note, unloading valve is NOT a relief valve. it is not relieving at 600 psi because that would still create the high hp load.

At that point, small gear set is low flow, high pressure. Larger gear set is at maybe 100 psi to unload and is very low hp.

Total hp needed is the low flow say 3 gpm at 2500 psi, plus the 10 or 15 gpm section at 50-100 psi. Most of the time the large section hp needed is ignored, but since these engines are so small, I include it in calcs.

Barnes haldex site had a good schematic and cutaway.

k
 
so....when i see manufactured log splitters with 5 or 6 hp engines on them, based on the info above, the engines are too small for the application?

I agree with Clawmute, but I also say there are two different types of hydraulic operation. His wood crusher have to work with continious max pressure to crush the wood. A woodsplitter only need max pressure during a portion of a second. The engine will use it's momentum (fly wheel) to overcome this short peak of pressure.

It's rare to utilize both that high flow(max rpm) and max pressureat the same time. That only happens when oil by-pass to tank thru the relief valve. Let's say an average log take 5tons/800psi with a 4" cylinder to split. That splitting force is just needed initially to make the crack. That will take just a small part of a second. When the crack is made force/pressure will drop to almost nothing. How does the engine react to this "part of a second" peak pressure?....Can you hear it slowdown a little? maybe....

Have you ever "killed" the engine on your splitter? Max power take out comes on max rpm's and when it's so much splitting resistans so piston dont move (all oil goes over relief by pass). If this kill the engine, you can think about getting more horsepower. But a good operator can probobly handle this with a good "hands on feel" on the valve....listen to your engine...

An important rule to remember when analizing and trouble shooting hydraulic systems is that the PUMP DON'T MAKE THE PRESSURE. THE PUMP MAKE THE FLOW. PRESSURE COMES FROM THE RESISTANS. NO RESISTANS NO PRESSURE.

Always close center fully on the operating valve. (lever max to end) to get max pressure and avoid over heating

How much HYDRAULIC power are we using?
Put a pressure gauge 0-3000psi on the line between pump and valve.
Figure out the exact engine rpm. There are rpm meters to buy cheap.http://www.northerntool.com/webapp/wcs/stores/servlet/product_6970_524744_524744
Figure out the exact displacement size of your pump in cui per rev. This can be the tough part. Splitter retailer dont want to show this number because they cant lie about it. They want to talk about the "15gpm pump", but they dont tell AT WHAT RPM........
Flow (gpm) is displacement times speed (rpm)
Do the math...Power in hp is pressure (psi) times flow (gpm) divided with 1714 P=p x Q / 1714

example: Hydraulic Power= 800psi x (0.61 cui/rev x 3000rpm)/1714=800x7.9 gpm/1714=3.7 Hp

DONT BUY A PUMP or a SPLITTER WITHOUT KNOWING THE PUMP DISPLACEMENT in cui/rev and MAX RECOMMENDED PUMP SPEED in rpm..and MAX RECOMMENDED PRESSURE in psi.....

this data is what manufacturers and wholesale's can show you
http://www.northerntool.com/webapp/wcs/stores/servlet/product_6970_200330127_200330127
 
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