Boom Cylinder Pressure Intensification

[Chris5500]

Boom Cylinder Pressure Intensification.

Ok, for 5 points can anyone explain to me what it is, how it can happen and how to identify it?

And I'm not talking about when grampa Joe's Halla 220 LC freezes up in the winter!

I'm talking 400 tonne plus, and why it's not an issue with the stick, bucket and clam cylinders.

 

[cps]

Well for some points its something to do with pressure on the rod side of a ram being restricted and the pitson side of the ram overcoming it I.E something gota give!

As to the cause, i would guess a valve sticking or some thing!

Am i on the same page as you Chris?

 

[OCR]

Boom Cylinder Pressure Intensification:

Boom Cylinder Pressure Intensification. can anyone explain to me what it is

This guy does pretty good....

http://www.insidersecretstohydraulics.com/cylinder-intensification.html

And the main site....

http://www.hydraulicsupermarket.com/technical.html



OCR

 

[John C.]

I can't see how that would happen in a properly designed and operating hydraulic excavator. First of all the control valve handles both sides of the circuit. The lands in the spool are cut for different flows to ensure that oil can come out of the cylinders as fast as needed. I suppose if someone removed the spool and turned it end for end you might have a problem. I also suppose it might be possible in a huge excavator boom circuit as the weight of the front is being handled on two cylinders. It might be possible that you could drop the boom faster than the oil could pass out of the rod end of the cylinder. Again for that to happen the engineers would have screwed up the design of the machine.

There is a circuit relief in both the rod and piston sides of the cylinder that should relieve any time the over pressure could be felt. That should take care of the spike in pressure from a dropping boom.

I have heard of pressure intensification on two cylinders set up in series. I've only seen that done once and it wasn't real successful.

 

[rockmuncher]

Did you get anywhere with this Chris?


Shane

 

[Komatsu 150]

I can see this happening in an excavator because the rod side piping is restricted, generally because of an unhappy meeting with a tree, boulder, take you pick, but the boom circuit is pretty protected. I think it can happen with the stick and bucket, it's just the the boom cylinders are bigger and the pressure would be higher on the rod side.

 

[MrKomatsu]

pressure compensators.........thank you Lord

 

[John C.]

Stick circuits most all have regeneration now days. Bucket cylinders are generally only fed half the volume of the stick or boom. In both cases they are protected by circuit reliefs.

Pressure compensators on the control valves enable two circuits of different pressures to keep the same flow priority. Say you are picking up the boom and dropping the stick at the same time. The boom circuit is at high pressure while the stick circuit is at low pressure. In the days of yesteryear the boom would stop until the stick pressure was the same or more than the boom. With a pressure compensator system, pressure from the boom would apply to the back of the stick compensator. This means the boom could still raise instead of stopping until the stick stopped moving.

Pressure compensators in pumps change the flow of the pump as pressure increases beyond a set point. This is the starting point of load sensing systems.

This is an interesting mind game. Has anyone got any other ideas? Why did this question come up?

 

[excavator]

Like John C said, if the system is designed properly this shouldn't be a problem. However, like was said in the "insidersecrets" story, if someone wanted to slow down a circuit and put a restriction in the rod side of the circuit, there's a problem. Also, if a flow meter is installed in the rod side of a cylinder circuit, there can be pressure intensification enough that even a high pressure flow meter will blow up. I've read about it, never done it but I do know someone who did.

 

[phamthebac]

thanks for posting. you can refer here

__________________
eagle air conditioning compressor
Fire Truck

 

[John C.]

What I am reading is gobbledegook.

What they are describing is blocking the rod side of the cylinder and determining the pressure applied to that side with a specific amount of pressure on the piston side.

Basically they are saying that a higher pressure will be attained on the rod side because of a smaller surface area to bear against that on the piston side which has a larger area to bear against.

From my point of view then, this is only a mind experiment and not a concern in modern excavators. Basically it can't happen in a properly designed and operating system.

 

[MrKomatsu]

it sounds like an engineering boo-boo

 

[excavator]

"It can't happen in a properly designed and operating system." Think about this. A guy builds a log splitter but the cylinder moves to fast. The line going to to the piston end of the cylinder has a bracket in the way so he installs a flow control valve in the rod end instead. Crazy you say, but for all the mechanics out there who work on hydraulics I would dare say there are few who would know about the possible results from doing this. I work on many different hydraulic systems but do not consider myself to be an expert in any way. But I have repaired many jobs that were done by hydraulic shops and even dealers. There is a sad lack of hydraulic knowledge out there amoung many people in the business.

 

[Komatsu 150]

"it can't happen in a properly designed and operating system. "

IF all the safety systems and pressure reliefs work properly. Rare, no doubt but everything fails at one time or other. A lot of times a catastrophic failure is the result of multiple problems. A good operator could run a machine with a bad relief for years and not even know it because he would back off when the hydraulics complained.

 

[Dirtdozer]

I'm having the complete opposite problem, loss of pressure ( boom and bucket) on a 235c LC Cat. I guessing my problem is in the pump head.

 

[MR. KOBELCO]

boom down (lower) on any excavator is equipped with a regen circuit as is the arm cyl. this is automatically actuated either thru the hyd main control valve, or in todays machines by a proportional solenoid valve, and in some older machines by an internal sub-spool within the boom spool itself.
this regeneration is designed into the machine, for if not there, the boom would drop quickly and run away from the hyd pump flow causing cavitation of the hyd cylinders.
the hyd pump would not be able to keep up.
once the boom is on the ground this regenertion needs to turn itself off or else you will end up with hyd preessure on both sides of the cylinder piston.
because the resultant force (basically) is calculated by the pressure X the area of the rod only. the boom cylinders will not be able to lift the tracks off the ground.
question on your machine, are your boom cylinders upside down compared to a conventional excavator?
meaning does your barrel connect to the boom and the rod ends at the machine structure? if so, you will definitely be running into a pressure intensification on boom lower.
example using the arm cylinder to describe:
when air digging, if your arm cylinder is fully retracted (stick out)
when functioning arm in hard, the regen is applied on hard by hyd control valve as it is designed to allow hyd pump flow to keep up with the cylinder as not to fall away and flop like a limp .... and sway until the hyd oil catches up, for it takes the rod oil exiting the cylinder, restricts the oil path to tank as to enable it to join with the pump flow feeding the piston.
the arm cyl piston end (barrel) sees relief pressure say 5,000 psi, for regeneration to occur, the hyd oil flow exiting the rod end must join up with the piston side and in order to do this the rod end must be at a much higher pressure. oil flows from a high pressure to a low pressure. cannot have it any other way. this is physics.
i have seen 5000 psi on the piston and up to 7000 psi at the rod.
just tee in pressure guages at each end of the cyl to read.
"F" = P x A, 5000 psi X area of piston equals the force on piston bringing the arm inwards, the rod end of which is a smaller area due to the rod taking away ends up
P = "F" in direction of rod moving out divided by remaining Area around the rod will equal your pressure intensification.
the highest pressure point in any excavator is the rod end of the arm cylinder.
common problems of arm cyl regen not working as should,
try strecthing the arm all the way out, turn throttle to low idle and pull the arm in hard, if the arm stops at mid point and sways back and forth until the hyd pump cathes up with it then your arm regen is not getting proper pressure signals by the control valve to work properly.
another common problem is if the arm regen stays on, you will end up stalling the bucket in the ground very easily and have to fight to fill the bucket with material.
when the boom or arm is on/in the ground the regen circuits must turn themselves off to allow the work to be done.

 

[MR. KOBELCO]

if that picture on your profile is your machine.
yes the boom cylinders are backwards. you should check that your hyd system reliefs have not been jacked up over normal specs and the your boom cyl port reliefs have not been turned up either. if either were tweaked up before this can be cause. return both to spec to help cap this pressure.
however, you will need to verify the the boom lower regen turns itself off when the bucket is placed on the ground and you say lift the tracks up.

try to get tee'd into each side of the cylinder. refer to your hyd schematic to see how and what controls the boom regen circuitry.

 

[John C.]

I'm not trying to unravel an explanation here but I don't recall ever working on a hydraulic excavator with a regen circuit on the boom.

You don't need a regeneration circuit when the piston is on the weight side. Basically there is more oil in the piston side than on the rod so it is easy for the pumps to keep up with keeping the rod sides full. If for some reason there should ever be a vacuum on the rod side, the circuit is protected with anti cavitation valves. But the spools are generally made to prevent the possibility of letting oil out of the boom cylinders faster than the pumps can supply it.

Regeneration circuits are put in the stick because the piston side of the cylinder can pull a vacuum when the stick is let free fall. In this case a spool opens and lets oil from the rod side of the cylinder get pulled into the piston side and will actually let the stick free fall. Before the advent of regen valves in the stick circuits they used to have two pump flow to the piston side and only the possibility of one pump flow out the rod side.

Maybe Kobelco has something different. If so I'd like to see a schematic that shows it.

 

[machine]

Hitachi uses boom regen on lowering the boom. Some of their EX machines have a nasty vibration if the hydraul oil is hot, the arm is extending full speed, the bucket is opening and lowering the boom to start the next digging cycle. I have thought it had to be the boom regen poppets but have not been able to solve the problem. Of coarse Hitachi will not recognise it as being a problem.

 

[MR. KOBELCO]

review your hyd schematic,
if the boom circuit uses a T2 spool, second spool to aid in dumping oil to tank for keeping speed cycle times up ensure it is not sticking.
also, ensure your boom cyl main hyd lines are correct and have not been reversed.