Hydraulics Question(s)?

[hfdfailures]

I just purchased a Kubota BX25 for doing small jobs around my yard. In my "feeling out" of the equipment, I have noticed that there appears to be a power deficiency in the backhoe implement. Hopefully, someone will shed some light on this for me.

When I am operating the backhoe, I notice that if I try to extend/retract the arm or swing the arm left/right (using the left handle), I seem to have the full power of the hydraulics. The same can be said when I try to curl the bucket or curl the arm (using the right handle). Full power and quick movement.

However, when I attempt to operate the backhoe with both handles, I end up noticing a severe "drop off" in the speed of the right handles operations. An perfect example:

I was digging a hole in the yard and was swinging the arm back and forth between the hole and the pile of dirt from the hole. I was trying to extend the arm and the bucket in one smooth motion, but was unable to do so. It seems as though the hydraulic power for both motions is not equally divided. Has anyone ever experienced this phenomena?
Is this something that I should be expecting given the small size of the tractor I am working with?
Is this a hydraulics deficiency? I have checked the fluid levels and it appears to be adequately filled. Perhaps there is something wrong with the cylinder? or the handle?

Any help with this would be greatly appreciated.

 

[AKKAMAAN]

The backhoe attachment is power by an open center constant flow system....pump is only about 6gpm...so you cant expect high speed on many cylinders at the same time....
You use control valve to build pressure by restricting the the open center, and diverting part of or full flow to cylinders....
If there is not enough pressure in the control valve, flow will go to cylinder with lowest load pressure first....like swing before lifting the boom...You have to compensate that manually with the control valve for the swing....swing have to go slower to make enough pressure to lift boom.....

Use foot throttle to increase rpm (pump flow) when you demand more flow for simultaneous operation of two or more cylinders.....

 

[J_J]

Why should it be equally divided? Each spool is getting a certain amount, depending on lever movement. If you are feathering one lever and full lever on the other. If using half lever on one spool, and half lever on another spool. they should be about equal, depending on cyl size . A small bore cyl will be faster than a large cyl.

My Case BH has restrictors in some circuits to slow the action.

 

[AKKAMAAN]

Load pressure in cylinders is what prioritize where flow goes....port to a low load pressure cylinder needs more restriction, to make flow go to higher loaded cylinder....

Why should it be equally divided?

btw .....who are you arguing with J_J?

If using half lever on one spool, and half lever on another spool. they should be about equal, depending on cyl size . A small bore cyl will be faster than a large cyl.

It still depends on the load pressure....you have the feather spool that control the lower load pressure....thats why you use restrictors on the easier loaded cylinders...

At the same load force on a small and a large cylinder, the larger one will move faster if both spool valves are feathered the same "distance"...(of course they need to have identical fitting and hosing setup)...Why???
Because the smaller cylinder will induce a higher load pressure....

I am sure this is what you tried to say also, I just explain it another way....:thumbsup::laughing:

 

[J_J]

No argument, hfdfailures mentioned something about unequal. and therefore I mentioned the circumstance that would cause that. He may even have restrictors in his circuits.

 

[hfdfailures]

Thanks for the assistance. I went and asked a mechanical engineer and he gave me an explanation concerning Pascal's Law. I think you both ended up explaining it in layman's terms so that I could understand it rather than trying to digest a lecture of fluid mechanics.

 

[J_J]

Load pressure in cylinders is what prioritize where flow goes....port to a low load pressure cylinder needs more restriction, to make flow go to higher loaded cylinder....



btw .....who are you arguing with J_J?


It still depends on the load pressure....you have the feather spool that control the lower load pressure....thats why you use restrictors on the easier loaded cylinders...

At the same load force on a small and a large cylinder, the larger one will move faster if both spool valves are feathered the same "distance"...(of course they need to have identical fitting and hosing setup)...Why???
Because the smaller cylinder will induce a higher load pressure....

I am sure this is what you tried to say also, I just explain it another way....:thumbsup::laughing:

Perhaps my common sense isn't working here, so maybe you can explain this. One log splitters with a 3 in cyl, and another with a 4 in cyl, both using the same pump size, and same valve, both levers are fully engaged, which one will have the fastest cycle time?

 

[oldnslo]

Perhaps my common sense isn't working here, so maybe you can explain this. One log splitters with a 3 in cyl, and another with a 4 in cyl, both using the same pump size, and same valve, both levers are fully engaged, which one will have the fastest cycle time?

J.J

are these cylinders tied common to the same pump and do they both have an equal load?

If the answer is yes to the above questions then the 4" cylinder will cycle quicker since it can do more work with less pressure.

Roy

 

[AKKAMAAN]

....an explanation concerning Pascal's Law. I think you both ended up explaining it in layman's terms so that I could understand it rather than trying to digest a lecture of fluid mechanics.

Your "mechanical engineer" proofed that he went through the basic hydraulic class, but he did not get into the "advanced" second stage....LOL...Hydraulics is a mechanical sub science, but takes some extra classes beyond basics to fully be able to explain everything...

When fluid is in motion, one have to pay attention about the Bernoulli's Principle, more than Pascal's Law

Pascals law only applies on confined fluids in NO motion/flow....as soon as fluid start to flow, the internal friction in fluid will come into play and cause what we call "pressure drop" or pressure differential....this pressure drop increases when velocity of fluid increases....

Note that velocity is not the same thing as flow.....velocity is a function of flow and the cross section area of the actual part of the valve/orifice/hose/tube etc...

Pressure is only equalized in the entire fluid confinement when flow is zero.

 

[AKKAMAAN]

Perhaps my common sense isn't working here, so maybe you can explain this. One log splitters with a 3 in cyl, and another with a 4 in cyl, both using the same pump size, and same valve, both levers are fully engaged, which one will have the fastest cycle time?

Many times what we think is common sense is just personal sense....:laughing:...

common sense is the sense that the community, "common-unity", agree on....

"common sense" can be determined with a poll.

But since polls are not allowed at this forum or "community", it will make common sense that we can not use common sense here....

...makes common sense??

I think we could make a new thread about "common sense"...but it would probably not make sense....