Hydraulic System - Raise / Lower Times

[sajego]

I am trying to remotely troubleshoot a situation that has come up with a hydraulic raise/lower system.

For safety purposes we require the raise and lower times to be within certain specs (Raise: 5.3-5.5 sec, Lower: 6.0-7.5 sec).

The latest times are too fast in both directions (4.5 raise, 5.2 lower).

The last time raise/lower times were measured was March 2002. Raise times were a little fast and lower times were barely within specs (4.9 raise, 6.0 lower)

I'm told that the only thing that has changed since March of 2002 was replacing a single plate orifice in the hydraulic line with a bi-directional multi orifice. My department was not involved in the design of this new "Bi Morf" orifice, but we are told that it did not change the raise/lower times when tested.

I think it is possible that this new orifice was installed backwards, or in the wrong location, but I'm told that isn't the case.

The explanation that I'm given for the change in times is that the cylinders have aged, and its a progressive thing that will get worse gradually. They plan to replace the cylinders ASAP, but can't right now and want me to waive the raise/lower requirements.

So my question, apart from asking for general comments on what I should do, is what generally happens to the performance a hydraulic cylinder as it ages?

 

[unclesyd]

Never heard of such.
If the volume of the pump doesn't change and produces enough pressure to lift the load the cylinder will go up and down at the same rate as before. There might be a little slowdown if an O-ring is swelling and increasing the friction on the piston or seal. If this is happening you would notice a rise in pressure in the system.
If you have a worn piston seal this could increase the rise time and increase the down time depending on the valve type.

You could have trouble with the directional control valve or the bypass arraignment in the system.

Come back with a little more information about the system.
What type pump and the control system for the cylinder?

 

[automatic2]

Look at it in this context, if a small bored cylinder travels a couple of inches in 5 seconds, and has synthetic seals, it could last forever. If a larger bored cylinder is traveling 20 feet in the same 5 seconds and has cast rings, you'll notice a growing bypass over a period of time. If this same cylinder has synthetic seals, all could be well until one day it's dumping fluid.

Is your orifice metering?, then consider pressure differentials at the actuator. Your system pressure regulator likely has the same wear ability as any cylinder, as does your pump.

My answer is this, if you can safely managed the deviance in timing, negotiate a time frame for repair. If the situation cannont be safely managed, shut down for repair.

 

[dwedel]

All wear that I know of in a hydraulic system would cause the raise time to increase and the lower time to decrease. It might possibly cause both times to increase in you required pressure to move the cylinder down. (ie a special load check valve)

Beyond this I can't say too much as I don't know the system.

With my limited knowledge of people, espcially the one typing this reply, I would be suspicious of modifications that don't have adaquete test data. Especially if it is desireable to go faster. However, the person typing this reply is normally suspicious, and should probably do some digging before making offhanded acusations.

 

[JohnGP]

Agree with above posts. I'm no hydraulic expert, but I have the feeling that replacing the cylinders is not going to solve the problem. I would be viewing the pressure/flow control device(s) with suspicion - if the orifice is there for flow regulation then any wear will allow increased flow and hence shorter raise/lower times (for example).

It seems that the cylinders are "seeing" a higher fluid flow in both directions, and you need to find out why.

 

[abeltio]

sajego,
i am not a hydraulic system design expert... but paid my dues as troubleshooter (-maker according to some versions...)
first thing i would do is, production/service conditions permitting:

with the bi-directional orifices do a controlled test to confirm the reported error.

find the original orifices, put them back and check whether it makes a difference or not.

then you can rule out, or not, the orifices as the source of variation.
if the orifice is the source of the variation... the answer is trivial... put a smaller orifice.

if the orifice is NOT the source... some other things may have changed... oil temperature? flow? etc...


Anyway, it does sound a bit strange to me that a 0.9sec variance from the test results to the center of the specifications is so alarming...
based in your operational experience... is it not possible to relax the specs a bit?

Finally... the time honored principle of operation weenies applies:
"the last one that messed is to blame."

HTH




saludos.
a.

 

[sajego]

Thanks for your help everyone.

I don't like talking about specifics on this board as I'm not sure how my superiors would feel this falls under our 'need to know' policy. But even so, I find your answers invaluable. I'm a relatively new/young engineer so the more people and ideas I can talk to and understand the more I learn.

The hydraulic supply is shared from another system, I will definitely check the pressure readings and gage calibrations.

JohnGP - I agree that it's a problem of too much fluid flow. We had this problem once before and they simply swapped a slightly smaller orifice.

In this case we can't go back to the original orifice very easily. The piping was cut to fit the new bi-morf one in. I am seriously considering making a note that they must take times before doing the installation so we would at least have current data to compare with.

I'm going to have them do some timing studies on another system that has newer cylinders with buffering to see how much time change there is by adjusting the cylinder buffer. I don't expect it to be a lot, but as abeltio pointed out, we are concerned over less than 1 second, so even half a second could make a difference.

As for safety I am going to have them bring the panels to halfway up/down and have them held there. This is a safety feature of the system, to remain where they stop even if mid-way through the stroke. That test would at least help prove the integrity of the cylinders and system. And if there doesn't appear to be any leaking and there is no grinding or slamming during operation, then it is probably OK for now.

This new bi-morf orifice is being installed on four systems. So far I have times from 2 and it has affected the lowering times of both by about 1 second. I just got the second times today and they are within the specs, but 1 second faster than the test done 6 years ago (6 years! i'm not sure that's very helpful! who knows what has changed since then!).

Anyway, thanks for your help everyone. You've given a lot of good advice.

 

[EdDanzer]

The problem should be from the new valve assembly. Small orifices with out pressure compensation can give wide flow differences. The best method to do this is to use 2 pressure compensated flow controls that have reverse free low checks, one for up and one for down, and be sure they meter the fluid coming out of the cylinder, not going in. Metering the oil going in will give less consistent speeds. Sun makes some cartridge types that give excellent metering.

 

[kcj]

I would agree with eddanzer-I would put in adjustable, pressure compensated orifices instead of fixed orifice plates. Pressure comp will handle the load variations better. Time your new circuit and adjust xlightly to get what you need for speeds.
If the flow is very large, there are various DIN style cartridge valves that can be operated with a pilot control circuit.

Definitely should be in meter out direction. You want the control to be creating 'compression' in the oil column. Meter in control only works when it is guaranteed, 100%, always a resistive load. If the load begins to run away, the oil column becomes essentially 'tension' and no control.

I am partial to Sun also, have used hundreds. good
valves, and good support from their people.

kcj CFPE