Hydraulic Pressure Problem 11

[dik]

We have a large hydraulic system that is running at 1550 psi. When the hydraulic ram is extended, there is a significant vibration at about 4-5 cycles per second. The system is powered by electrical motor and pump and it doesn't seem that there is any noise.

Is it possible that air in the system is causing this? or perhaps some form of 'water hammer'? The system also has a pressure relief valve; is it possible that this is causing the vibration?

Dik

 

[BrianPetersen]

It is quite possible that the relief valve is cyclically opening and shutting in sympathy with something resonating in the system. Even if you don't have air in the lines, there is always some flexibility in the hoses and lines and the fluid itself.

 

[dik]

The lines are long (in excess of 100')and we think that the elasticity of the system renders the fluid as nearly incompressible.

Any suggestions on determining the source? Do you know of a method? Swap pressure relief valves?

Dik

 

[hydtools]

Temporarily change the relief valve setting and note any change in vibration. This will determine whether or not the relief valve is the likely problem.

What type is the relief valve? Direct acting ball or poppet, differential piston?

Is the relief valve at its correct setting? Was it adjusted for some reason resulting in the current vibration?

If the vibration has been going on for some time and the relief valve is the cause, the valve seat may have been ruined and has changed the valve setting. Replace the valve.

Ted

 

[EdDanzer]

It is possible the cylinder seals or bearing area of the piston or rod could generate the vibration. The orientation of the movement and condition of the end connections can exaggerate internal clearance issues.

Ed Danzer

http://www.danzcoinc.com

http://www.dehyds.com

 

[budt]

Without seeing a Schematic of the circuit an answer would be pure speculation (Guess) The schematic needs to show part numbers and manufacturers name. However, if the press is old a schematic often does not show any changes or upgrades and are often almost useless or even mis-leading. On a Trouble Shooting call I always check the physical components against the schematic to see if everything is as shown. I find anything over years old often times has been modified and the schematic never updated.

Can you show a schematic of the circuit?

Does it show the cylinder orientation?

Is "extended" Rod Up or Rod Down?

Is the cylinder Double acting or a Single Acting Ram?

What does the pressure gauge read while extending and making the vibration?

Is this a new system or one that has run successfully for some time?

What is the condition and type of fluid used?

Trouble Shooting at a distance is difficult at best and almost impossible sometime. As mentioned above, Air is one major cause for vibration or oscilation. Another I have seen is seals that can't get lubricationand cause a Slip-Stick motion. Some oil squirted on the Rod will ofteh=n alleviate that type of bad motion.

That's all I can think of until I see a schematic and know what the circuit setup is.


Bud Trinkel, Fluid Power Consultant
HYDRA-PNEU CONSULTING

 

[ScottyUK]

What is actually vibrating? The extended rod, or the supply and / or return piping?

If the RV is lifting then look for the reason it is lifting: pump system not regulating correctly? Setpoint too high, or RV set too low? Do you have pressure gauges at the pump and at the point of use? What are they doing? If there's an accumulator is it charged to design pressure?


----------------------------------

If we learn from our mistakes I'm getting a great education!

 

[IceStationZebra]

Contaminated or worn-out oil can cause a slip stick problem with the seals on the rod.

ISZ

 

[dik]

Additional information based on responses:

The hydraulic lines consist of 2-1/2” diameter Schedule 80 welded stainless steel pipe.

The hydraulic ram is 12” diameter and the cylinder is 22” diameter and the operating pressure is 1550 psi. The stroke is about 40’. The ram was constructed about 40 years ago and the hydraulic fluid is of the same age and we understand that it is ‘conventional’ hydraulic oil. The ram is vertical and the cylinder is at the bottom; the vibration occurs when the ram is extended and lowered. It also vibrates under ‘free-fall’ with no hydraulic pressure.

It’s single acting, but there is oil and seals on the ram side.

The pressure relief valve is at the proper setting and this has not been altered. I will try to determine the type of pressure relief valve. It is possible to alter the setting, but is necessary to dismantle a portion of it.

It is possible that the cylinder seals and bearing area are causing the vibration; we are currently looking into this. The ram has some minor corrosion and the condition of the seals is being reviewed.

The ram is used infrequently, about an hour a month and the vibration started approximately five months ago.

We are not sure what is vibrating, but the vibration is transmitted throughout. There are pressure gauges at both the pump and the the high pressure and low pressure point of use; these do not fluctuate. I don’t know if the vibration can be caused by transient pressures and not show up on the gauges.

Dik

 

[MintJulep]

Water.

Over the years water has leaked in and there is now a bunch of water on top of the piston.

Your 40 year old seals are getting a bit crusty, and as the cylinder moves some water is burping through. The change in friction from oil lube to water lube at the seal causes a slip-stick like condition.

 

[automatic2]

is your cylinder motion jerky in relationship to the 'vibration'? Suspect mechanical binding at piston or rod seal/bearing.

 

[dik]

Automatic2:
The vibration can be felt by placing a hand on almost any part of the steel. In addition, there is a noticeable vibration at a few cycles per second and a loud 'banging' sound accompanying it. I've been unable to isolate the noise.

The reason for the concern is related to cutting some of the large steel beams to effect maintenance; I've designed the fasteners as slip critical, not being advised of the vibration.

Dik

 

[hydtools]

Perhaps the piston has come loose from the rod. The piston tilts slightly causing a piston bind/unbind or piston leak/no-leak condition that causes the vibration.
Is something in the external structure connected to the rod stick-slipping?

Ted

 

[hydromech]

If the ram is single acting and the vibration is seen on the down stroke, then it would seem to suggest that the fault is more mechanical than hydraulic.

I have seen this before. I that case the piston had become loose on the rod. When the cylinder moved it would resonate. It would only resonate when moving as oil was moving over piston.

The piston seal was effectively useless and was allowing oil to pass accross the piston freely. The oil flow between the pison and the inner wall of the cylinder made the piston vibrate.

Question...If you extend the piston and stop it, what makes it stay extended? Is it mechanically loacked or does it tend to creep back down.

Adrian

 

[gerhardl]

OK.Summary:

This is a large and old system, and vibration started some months ago. Something is not as before.

Theoretically all the speculations on fault sources above could be correct, alone or in combination.

In addition comes possibly dirt and grit in the system, mechanical wear, loose parts of old sealings in the systems, wear on mechanical fastening and support of the system, and any faulty check valve (or valve in general) in addition to the PRV.

Obviously the logical step is to dismantle the whole thing, clean and change oil and all necessary parts. Of cost reasons this will probably be tried avoided.

Alternatives to complete overhaul:

a) New rig.
b) Change oil and flush and clean system before new oil, in addition to changing the most 'suspect' parts.
c) Let the old rig live 'as is' as long as safety makes this possible.

a) is the long-term solution- (?) Why not check if there is some sales value in the old rig in a none-competetive market location? Some cheap version of b) is probably worth to be tried, but you could also possibly risk with b) to 'stir up old dirt' and come out worse.

 

[dik]

We've switched the pressure relief valve and the problem persists... leaning more towards binding (stiction) or differing fluid as being the cause.

Dik

 

[limey1]

Q. What controls the ram fwd and return? Open/close or proportional valves? Is the ram single or double acting? Is it a hydraulic system or water hydraulic? Need to answer these questions before changing things out.

 

[IceStationZebra]

As far as the fluid goes:

- One quick thing would be get the oil sampled. This will tell you if water is the problem. The system should be up to temp and have the oil circulating for some time to get a good representative sample. You didn't say where you are, but many of the CAT dealers provide this service.

- Did someone add the wrong kind of oil? I had an experience many years ago where our oil supplier changed the formulation to save money but didn't tell us. We wouldn't have noticed except our cylinders started to "squeal". We tried changing the glands, seals, etc. with little inprovement -- until we actuated the cylinders with an old machine with the old oil. We finally got the supplier to admit the change took place and they changed the friction modifier. (unfortunately this one came out of suspension in the presence of too much water, plugging our return filters, necessitating another formulation change!) Our short term solution was an Anti-Chatter additive, with one sorce being CAT.

ISZ

 

[dik]

Forgot to add that the system is double acting.

 

[dik]

hydtools:
They are currently checking to determine if the piston has become loose from the rod.

hydromech:
hydraulic pressure keeps it extended. When the pressure is shut off, the ram collapses an inch or so.

gerhardl:
the ram cannot be disassembled. It is a large system, carries a large liability and is a 'one-off' type of thing.

limey1:
The pump and proportioning valve are used for two cylinders and the problem is only with one. It is an oil hydraulic system and each ram uses approximately 20 barrels of oil. There does not appear to be an emulsification and I don't know if this can be a local thing.

ISZ:
Oil was not added to the system at the time the problem developed and no hydraulic fluid has been added since. I don't know what the frequency of 'topping' up the system is.

Anti chatter additive is a lower friction fluid? something like a teflon synthetic. It would be a matter of determining if this is compatible (I don't know what the current hydraulic oil is).

Does anyone know if it's possible to 'isolate' the piston to test if the problem is with the piston?

I'll see if I can 'cobble' together a schematic of the system.

Dik