Cylinder drift......

[AKKAMAAN]

I think there is some different opinions about what causes cylinder drift/drop.....

It is usually gravity loads that makes cylinder drift....some times even inertial loads can make cylinders drift....while loads from friction never do....

A double action cylinder can hold a load on each side of the piston.....the capped side have a larger piston area than the rod side of the piston....the rod cross area make the net piston area that smaller on the rod side....

Let's make an example...we put a load on a double action cylinder capped side, and lets also say the hoses are disconnected and plugged with QD's.

If a certain load, lets say 1000 lbs, is held up by the pressure on the capped side, that pressure will sooner or later equalize both sides of the piston. If the cross area of the bore is 20 square inch, the pressure will initially be 1000/20=50psi. Due to inevitable piston natural seal leakage, pressure will equalize, and we will have the same pressure on both sides of piston. But now it is not 20 sqin that hold the load, it will be the net area difference between the capped side and the rod side, which is the area of the piston rod, lets say 5 sqin. 1000/5=200psi.
OK, piston seal have leaked a little but the cylinder WILL NOT DRIFT, unless there is an external leakage. Cylinder will actually not drift until something bursts.
If this cylinder would be connected to a control valve, it will be the control valve only that can make cylinder drift. PERIOD!! Either through work port valves or through the spool "tolerances".....
Spool leakage would be either natural from design, a few c/c per minute, if more it it would be due to MANY years of wear on the spools. It is unheard of, that spools can wear so much so a bucket can just starting dropping down fast during "a lunch break".
So, if the cylinder is suddenly dropping down "fast" it can only be the work port valve, or another valve connected to the hydraulic lines.

Next example will be when the load is held up by the rod side of the double action cylinder....

If we use the same cylinder and load as the above example, the initial pressure will be 1000/(20-5)=1000/15=67psi. At this setup the piston rod is being pulled out of the cylinder and will create a void on the capped side. Then this pressure will never equalize the sides of the piston, and the result will be that cylinder will drift if piston seal is leaking. Still, leakage through the control valve is possible. This can be tested by disconnecting and plugging the rod side hose. If we still are drifting it will be from a bad piston seal.

Now I go camping over the weekend and will be back on Sunday!

 

[mmurphy]

Hope you have a good time camping, here in this area skeeters would have you for lunch!!!
I would sure like to see one of your animated graphics done on this example.:thumbsup: (couldnt find one that begs:laughing

 

[AKKAMAAN]

Hope you have a good time camping, here in this area skeeters would have you for lunch!!!
I would sure like to see one of your animated graphics done on this example.:thumbsup: (couldnt find one that begs:laughing

Oki...I accept the challenge....need some time though....
Peace
Per A

 

[c23380]

Had to think about it but I think I would have to agree. The way the first illustration is set up, when the fluid leaked from the lower side of the cylinder toward the top side then it would first equalize the presure from the different sides but no more due to the amount of fluid on the top. the top side, containing a rod, has less cubic inches than he bottom side, so in theory, then all the fluid in the bottom could not move to the top, because there is less space. Very nice illustration.

 

[mmurphy]

Oki...I accept the challenge....need some time though....
Peace
Per A

I did not mean to ruin your camping!!!!!!!!
We will patiently wait.

 

[wdchyd]

AKKAMAN,

I see where you coming from, but something was missed when you said "cylinder WILL NOT DRIFT, unless there is external leakage"....in your first illustration you show a rod size of 5 sq in rod dia & 20 sq in bore....basically 4:1 ratio.....

if a higher load were introduced (say lifting a typical load developing 1000psi in the base end of cyl) then if the piston seals "equalizes" it shows up as a potential intensified load of 4000 psi(displacement of the rod is only holding up load)....If WPR is set at say 2000 psi then WPR would let down load resulting in cyl drift/drop.....of course the valve will never see the 4000 psi but the load will drop accordingly......

I've seen this happen in perfectly good WPR and Counterbalance valves (cartridge valves used in other construction/utility equip)....because of the unwanted high pressures they let themselves down predictably to the preset pressures due to piston seal bypass....http://www.insidersecretstohydraulics.com/hydraulic-cylinder-drift.html

If no WPR or CBV then you have intensified overpressure and it should hold a load at 4000 psi (assuming nothing else blows apart...ie: cyl gland, hose, ect)

In a perfect world none of this would happen, but it's not....WPR's & CBV's are there for protection but add to your Cyl Drift in this scenario....Seen this happen on outriggers and boom cyls on different types of mobil equip....

but then again were only talking tractors right??? and they do have WPR's right????......yes/no???

 

[wdchyd]

Here's another along this subject....Hydraulic Troubleshooting Test

 

[dcyrilc]

AKKAMAN,

I see where you coming from, but something was missed when you said "cylinder WILL NOT DRIFT, unless there is external leakage"....in your first illustration you show a rod size of 5 sq in rod dia & 20 sq in bore....basically 4:1 ratio.....

if a higher load were introduced (say lifting a typical load developing 1000psi in the base end of cyl) then if the piston seals "equalizes" it shows up as a potential intensified load of 4000 psi(displacement of the rod is only holding up load)....If WPR is set at say 2000 psi then WPR would let down load resulting in cyl drift/drop.....of course the valve will never see the 4000 psi but the load will drop accordingly......

I've seen this happen in perfectly good WPR and Counterbalance valves (cartridge valves used in other construction/utility equip)....because of the unwanted high pressures they let themselves down predictably to the preset pressures due to piston seal bypass....Troubleshooting hydraulic cylinder drift

If no WPR or CBV then you have intensified overpressure and it should hold a load at 4000 psi (assuming nothing else blows apart...ie: cyl gland, hose, ect)

In a perfect world none of this would happen, but it's not....WPR's & CBV's are there for protection but add to your Cyl Drift in this scenario....Seen this happen on outriggers and boom cyls on different types of mobil equip....

but then again were only talking tractors right??? and they do have WPR's right????......yes/no???

I see your point here, but remember for his example the hoses are disconnected and plugged.

Let's make an example...we put a load on a double action cylinder capped side, and lets also say the hoses are disconnected and plugged with QD's.

 

[wdchyd]

If this cylinder would be connected to a control valve, it will be the control valve only that can make cylinder drift. PERIOD!! Either through work port valves or through the spool "tolerances".....

True with QC's, but here it's connected to control valve.....

 

[dcyrilc]

True with QC's, but here it's connected to control valve.....

Got ya.:thumbsup: Missed that.

 

[AKKAMAAN]

if a higher load were introduced (say lifting a typical load developing 1000psi in the base end of cyl) then if the piston seals "equalizes" it shows up as a potential intensified load of 4000 psi(displacement of the rod is only holding up load)....If WPR is set at say 2000 psi then WPR would let down load resulting in cyl drift/drop.....of course the valve will never see the 4000 psi but the load will drop accordingly...

Absolutely RIGHT.....I did not want to bring that into the first post because I did not have enough time at that moment....

We have had a few cylinder drift threads lately, and all of them have been "low pressure drift", at little or now load at all....

If heavy load (high pressure on capped end of cylinder) is causing drift, AND the piston seal is leaking MORE than the natural spool leak 2-3c/c per min, pressure will equalize and pressure will multiply with the bore/rod diameter ration, (in my example 4:1).......a 2000psi pressure will become 8000psi.....and if neither of the bucket cylinder port have a WPR, something will burst, and we'll get our leakage, by a cracked CV housing, burst a hose or the cylinder...
Even if the load pressure is 500psi, and the piston seal is leaking, the steady state pressure will equalize and become 2000psi, but with FEL lifted up and travelling, hitting some bumps on the road, inertial force might create pressure PEAK's at 5000-10000psi, if we don't have a WPR, at high loads DRIFTING IS NECESSARY to protect the equipment structure from over load damages
This whole thing, cylinder drift from load, is a reason why some GREAT hydraulic designers, sometimes choose a WPR pressure setting on an application like this, a little HIGHER than the regular system pump pressure.
A knuckle boom crane, or an excavator, lifting a heavy load with the boom, and later reaching out further with the stick (longer leverage arm), would drop the boom from the increased pressure in the lift cylinder

(This scenario can NOT happen with load pressure on the rod side of the piston)

I've seen this happen in perfectly good WPR and Counterbalance valves (cartridge valves used in other construction/utility equip)....

I can't more than agree, this will happen at higher pressure levels.....

If no WPR or CBV then you have intensified overpressure and it should hold a load at 4000 psi (assuming nothing else blows apart...ie: cyl gland, hose, ect)

In a perfect world none of this would happen, but it's not....WPR's & CBV's are there for protection but add to your Cyl Drift in this scenario....Seen this happen on outriggers and boom cyls on different types of mobil equip....

but then again were only talking tractors right??? and they do have WPR's right????......yes/no???

If you read my previous post about drifting, you can see that I didn't not exclude the WPR leak option.....I just did not give it a lot of attention, because we had been talking about "low pressure" drift...

I can see here at TBN that most members is carrying around the misunderstanding that, a single rod double action cylinder, leak the same way from both sides of the piston.....when this only happens whit hydraulic motors, cross connected double action cylinder or double rod cylinders.....

I am from the logging world, and equipment wont even get rolled off the trailer if they do not have a proper design....but I can see the "light equipment" manufacturers taking short cuts to save some extra bucks (more profit)...It is also a very competitive market.....but some smaller equipment I learned to know about here at TBN, is definitely pretty well equipped on the hydraulic side....

Before buying equipment like this, buyer need to know what to look for on the hydraulic side.....WPR's and such.....it is a tough world to be a buyer with no hydraulic/electric engineer degree....you are in the hands of a sales man that probably DO NOT KNOW S**T about this, but still promises that hydraulics is properly designed....

 

[wdchyd]

Very well stated as always AKKAMAN!!....Your comments and suggestions (here and HYD&PNEU Forum) make for great reading where theory & mechanical applications collide in this world of "Hydraulic Nightmares"....

I've often said "How can you fix anything properly if you do not understand how it works!"......This holds true in all aspects of troubleshooting (Mechanical, electrical, pneumatics & Hydraulics).....This gives us more and more reasons to learn other things in life to enhance our performance in this world of "Broken Things":laughing:

 

[dcyrilc]

I've often said "How can you fix anything properly if you do not understand how it works!"......This holds true in all aspects of troubleshooting (Mechanical, electrical, pneumatics & Hydraulics).....This gives us more and more reasons to learn other things in life to enhance our performance in this world of "Broken Things":laughing:

Well said.:thumbsup: I've always tried by best to understand any system I need to repair as it makes it much easier to repair something when you understand how it is suppose to work and why it isn't working.

 

[AKKAMAAN]

Well said.:thumbsup: I've always tried by best to understand any system I need to repair as it makes it much easier to repair something when you understand how it is suppose to work and why it isn't working.

AKKAMAAN,
What did your wife say about the picnic? Any chance you guys will be able to make it next weekend? We can accomidate the camper if you need/want.

Wife is positive......please chk PM....

 

[kennyd]

Wife is positive......

Your crazy?:laughing::laughing:

Sorry, I couldn't resist

 

[AKKAMAAN]

Your crazy?:laughing::laughing:

Sorry, I couldn't resist

So wife's cant be positive to a picnic and a weekend camping??
matter of fact , my wife and I love going camping on weekends, just came home from Klawhoya Campground in the Olympic National Park...great place...only 45 min from my house....

Well....at least you made an effort, so I suppose you sort of care a little:laughing:....."ignoring is worse than hate"......:thumbsup:
Peace and "Skål" (cheers in Swedish)

 

[dcyrilc]

So wife's cant be positive to a picnic and a weekend camping??
matter of fact , my wife and I love going camping on weekends, just came home from Klawhoya Campground in the Olympic National Park...great place...only 45 min from my house....

Well....at least you made an effort, so I suppose you sort of care a little:laughing:....."ignoring is worse than hate"......:thumbsup:
Peace and "Sk虱" (cheers in Swedish)

He's just jealous 'cause he's not close enough to be able to come.

 

[wdchyd]

He's just jealous 'cause he's not close enough to be able to come.

I'm jealous too!!:laughing: