Help Understanding Hydraulic Pumps

[fhon]

I'm currently looking into building a log splitter. I've been doing a lot of research but I'm left with a few questions I would just like to get cleared up so I can be sure i'm not pushing anything past its limits.

My main question is how the pump works when it reaches it a point where i can no longer move the cylinder. Lets say I hit a nasty knot and the cylinder stops. The pressure builds inside the cylinder until ______? If the motor keeps running the pump keeps running, so how am I not hurting the pump and what stops it from just building pressure until it explodes?

Bonus question: What are the pressures you operating at vs the max pressure of the cylinder? For example if a hydraulic cylinder is rated up to 3000 psi what is the maximum pressure you're actually operating it at?

 

[peakbagger]

I'm currently looking into building a log splitter. I've been doing a lot of research but I'm left with a few questions I would just like to get cleared up so I can be sure i'm not pushing anything past its limits.

My main question is how the pump works when it reaches it a point where i can no longer move the cylinder. Lets say I hit a nasty knot and the cylinder stops. The pressure builds inside the cylinder until ______? If the motor keeps running the pump keeps running, so how am I not hurting the pump and what stops it from just building pressure until it explodes?

Bonus question: What are the pressures you operating at vs the max pressure of the cylinder? For example if a hydraulic cylinder is rated up to 3000 psi what is the maximum pressure you're actually operating it at?

Hydraulic fluid is essentially incompressible, if someone were to build a system with no pressure reliefs or internal bypasses, the pressure will rise until something breaks or you run out of horsepower from the engine and it stalls. Many but not all hydraulic pumps will have an internal bypass that is set to keep the pump from physically failing (usually the casting splits or mechanical seal lets go) by leaking bypassing fluid from the discharge back to the inlet. This is more of safety device than a control that should be routinely used as the fluid and pump can heat up quickly when running bypassed. The piping from the pump usually runs to a splitter control valve, it has three connections, one is the supply to the hydraulic cylinder in forward, one is to the cylinder in reverse and one is a return to the tank. Built into the control valve is an adjustable bypass usually hidden under a screw off cap. If the cylinder stops traveling, pressure builds up until the internal bypass valve opens at a set pressure and dumps fluid to the tank return line. When this happens, you can usually hear a distinct change in sound from the splitter. If there is an internal bypass in the pump, the relief valve in the control is set to a lower pressure. By the way, many but not all splitter control valves have a small threaded port that you can screw a pressure gauge into to check the pressure. The pressure can be changed by putting in a chunk of wood sideways and intentionally stalling the cylinder, the cap is removed and the bypass valve can be screwed in or out to adjust the pressure to the lowest rated component. Alternatively someone could just intentionally size the motor so the pump stalls before the maximum pressure but I do not think a reputable manufacturer would sell one that way.

Generally the maximum pressure rating is set by the weakest link and usually that is the hose rating. The scariest failure is usually a pinhole leak sometimes from casting or a fitting. The hydraulic fluid can jet out with a lot of pressure and the oil can get injected directly into the skin. Take look at this link (broken link removed to http://www.constructionequipment.com/injection). Very scary, I met someone who did it to his arm and he was out of work for a year.

Folks can get in a lot of trouble if they do not understand what they are doing substituting hardware store threaded fittings for rated fittings. Homebrew splitters can be the worst, folks pick up parts from various sources and dont understand how to design them so they are safe. Rarely do they explode but they sure can make a heck of mess when something fails. The other thing many people dont understand is the relationship of cylinder diameter and total force applied to the wedge. A big diameter cylinder puts out a lot more force than a smaller one at the same pressure. Its directly related to the area of a cylinder bore which goes up by the square of the radius. A bigger cylinder requires more GPM. Put a small volume pump on a big diameter cylinder and the travel speed will be slow, put one with too much GPM and the cylinder will move too fast. Many splitters have two stage pumps, they start out at fast RPM to retract the cylinder and move it up to the log but the second the pressure increases the pump goes into "low" gear and pumps less fluid at a higher potential pressure.

My friend had one set up for 2500 psi, it could cut logs in half sideways. I am not sure what commercial units run.

 

[fhon]

Thanks alot! This was very helpful!

I have one other question though. What happens when the cylinder changes direction, as far as pressure? If you've got 2500 psi built up in one side and suddenly change directions isn't that enormousness amount of pressure instantly released into the holding tank?

 

[peakbagger]

Not really, remember fluid is not compressible. Air is compressible so when you let go of a ballon full of air the air expands and blows out through the tip of the ballon as the pressure inside the ballon equalizes with the air pressure outside the ballon, Fluid doesn't act that way, its always roughly the same volume so when the pressure is released it doesn't expand. If you take a can of iced tea you can shake it as long as you want to and when you open the tab, nothing sprays out. If you do it with soda, there is compressed carbon dioxide inside the can and its what expands rapidly under lower pressure when you open the tab and it caused the flavored sugar water in the can to spray everywhere as the expanded gas bubble are taking up volume in the can so the fluid has to go somewhere.

The pump only puts out the pressure it needs to split the wood. If you look at pressure gauge when the cylinder touches the split, it starts to climb as the wedge moves forward. as the going get tougher the pressure rises until the log starts splitting, then pressure rapidly drops as the wood fully splits. At the end of the stroke with the log split, the pressure is quite low. There is no spring attached to the cylinder to return it (on most splitters) so when the control valve is pulled to reverse, hydraulic fluid is applied to the back side of the cylinder, there is little of no pressure on the wedge. If the operator isn't paying attention and runs the cylinder all the way back to closed position the pressure will spike and the relief valve in the control may open to limit the pressure.

To complicate things a bit remember that the entire pressure side of the hydraulic system is full of fluid, air is a bad thing as it can compress and cause problems (remember the soda analogy above, the air is acting like the carbon dioxide). The hydraulic cylinder has fluid on both sides of the piston. The control valve is set up so that when one side of the piston is under pressure, the other side is draining to the tank. When the control valve is put in reverse, the hose under pressure has to be switched to the tank and the hose on the drain side of the piston has to be switched to pressure. Usually when you look at buying control valves there will be schematic of what is going on inside the valve.

I have simplified things a bit, fluid does change volume slightly as its heated. Its a very small amount but a completely sealed hydraulic system will have a sealed expansion tank to deal with the slight volume change as the fluid goes from cold to hot.

 

[fhon]

Thank you again

 

[Jags]

Typical operating pressure of store bought units will range from about 2500-2750 psi as a general rule of thumb. System pressure SHOULD NOT exceed the rating of the weakest component. If you have a 2500 psi rated hose in the system - do not exceed 2500 psi on the relief valve.

(Nice posts Peakbagger, well done).

ETA:
Also - outside of the hydraulic side of things - remember...A splitter uses enormous (relatively speaking) pressure. Tons of pressure, literally. Building a piece of equipment to contain that kind of pressure is a bit unique. Not difficult, just something to consider during the design phase. A little mig welder ain't gonna get the job done.

 

[peakbagger]

I am glad that folks found it helpful. The scary thing with hydraulics is the injection injury potential, most folks have no clue how dangerous it is or have even heard of it.

 

[jaoneill]

I am glad that folks found it helpful. The scary thing with hydraulics is the injection injury potential, most folks have no clue how dangerous it is or have even heard of it.

Compressed air can do the same thing.......

 

[jetsam]

I am glad that folks found it helpful. The scary thing with hydraulics is the injection injury potential, most folks have no clue how dangerous it is or have even heard of it.

Anyone who's ever worked on the flight line has heard of it!

Hydraulic fluid injection is no joke. A big leak will deplete the system fast, but a tiny pinhole leak is deadly dangerous, especially if the emergency room has no reason to suspect hydraulic injection.

 

[blades]

Note: Log splitter systems are open to atmosphere not sealed from it. Never seen a sealed one. Could be done just added cost that is unwarranted.

 

[sportbikerider78]

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