Cylinder speed problem, 4-5 times faster on extend than retract

[ROAMER_AUS]

Hi all!!!
My first post, so please be gentle

I do auto electrics mostly but putting my foot into hydraulics as well since more of my customers keep asking me to soolve those types of problems.

I have a problem with cylinder that puts boom lift basket on an angle. When cylinder is retracting it goes normal speed, but when extending it moves 4-5 times faster and basically slams the basket back to start position.

It has directional control valve XD3C01N2G and hydrostat AM3H3VP18 made by aron / brevini. I have checked solenoids and the valve, it operates fine. Cylinder checked OK. Only thing I can't check is the hydrostat, I pulled it apart and can't find anything wrong.

Anybody has any ideas?

Thanks!!!

 

[hydtools]

A schematic would help.
Perhaps on the extend cycle flow is entering both head and rod ends of the cylinder resulting in the high extend rate.

Ted

 

[ROAMER_AUS]

Thanks for the tip Ted.
I have attached what I have. It is basically as drawn.
P is pump
T is return
A and B are connected to the cylinder.
Valve solenoids are connected strainght to 12Volts supply switches. No valve position control or anything like that.

 

[PNachtwey]

There is an equation called the VCCM equation that can be used to calculate/estimate the velocity in each direction. If the supply pressure is kept relatively constant it is common for extend speeds to be much higher than retract speeds. Much depends on the load and the relative size of each side of the piston area.

Peter Nachtwey
Delta Computer Systems

http://www.deltamotion.com

http://forum.deltamotion.com/

 

[IRstuff]

Offhand, one would think that the pressure would need to be no more than 14.5 psi above atmospheric pressure, since that's roughly the maximum pressure that could be achieved by the air pushing the cylinder back in.

TTFN (ta ta for now)
I can do absolutely anything. I'm an expert!

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[ROAMER_AUS]

@ PNachtwey
I taught it was opposite? Retract speeds are higher for the same pressure because area to be filled with fluid is smaller when retracting due to rod occupying the space as well. I even read about it a while ago.
But this has got to be some fault in the system.
I tried yesterday swapping hoses on the cylinder, and cylinder slams in the opposite direction, which means it's got nothing to do with the cylinder it self.
I'm suspecting the hydrostat, it wirks on one direction but not the opposite, but can't test it.
Does anybody know what happens when you swap P and T hoses on the input of the hydrostat? Can this be the problem? I don't want to try without previous knowledge so I don't damage something.

Thanks!!!

 

[HPost]

The device that you are calling a hydrostat is a actually just a pressure reducer that is controlled by the pressures in the A or B line to the cylinder. Therefore, the pressure at the cylinder will change the pressure in the P line.

The KV (flow coefficient) of the directional control valve is fixed. It is labelled as being proportional, but the solenoids are drawn as On/Off. Assuming that this system was working OK at some point, and that the problem has recently occurred, it can be expected that this valve is OK.

Unless the cylinder bore to rod area ratio is crazy and having a massive bore to a really skinny rod, the cylinder volume ration cannot be 4 or 5 to 1. It is unlikely to be the cylinder.

The problem is very much likely to be in the pressure reducing valve. If the valve cannot reduce the pressure in the P line, the delta P across the spool in the directional control valve will be high and the therefore the flow will be much higher.

If you can, change it. If not, then either take another look inside and see if the spool is stuck or getting jammed under pressure. Can you remove it and run the cylinder without the pressure/ flow control?

You can’t revere P and T, the oil flowing back from the cylinder will not be able to get through the reducer. Your pump pressure will just go to max when you try to move the cylinder.

 

[MikeHalloran]

The valve schematic suggests that flow from B to T is internally restricted,
while flow from A to T is not. That could explain your symptom. I have no idea what the restriction might look like, or how or if it is removable.

The remainder of the schematic does not show us how the cylinder is connected. Some common connections could explain your symptom.

The schematic does not show us how the cylinder connects to the load, nor how the load behaves wrt to gravity. Again, undesirable behaviors sometimes result from factors external to the hydraulics, and your problem statement provides too little information for us to develop a proper mental picture.

Perhaps some photos of the apparatus might help.


Mike Halloran
Pembroke Pines, FL, USA

 

[ROAMER_AUS]

@ HPost
I strongly believe hydrostat to be the problem also, I will take it apart and have a look inside, maybe I'll find something wrong.

@ MikeHalloran
Basket sits vertically up and it's tilted by this cylinder apprx. 30 degrees, gravity plays very little or no role in the setup. It is ordinary double acting cylinder, one port to A, other to B. When tilting one way it takes about 5-6 seconds to tilt whole way, but opposite direction, it basically slams it whole way back within 1 second or so.

I have realised that I attached wrong type of setup previously, wrong hydrostat. I attached correct one here.

 

[hydtools]

A whole circuit schematic would be helpful. There was a question about was the system working and now is not or did it ever work. Not answered. If the system was working and now is not what changed? Maintenance? Repair? Customer adjusted something?
What provides the flow? Is it controlled, constant pressure, constant flow, variable flow? Has something about the pump changed? Why feel so strongly that there is a problem with the valve?

All we know is that the cylinder extends faster than desired and possibly out of control. To do that there must be excess flow to the head end and/or less flow restriction to the outflow from the rod end. Are there flow restrictions built in to the valve as shown, and shown inconsistently.

Ted

 

[ROAMER_AUS]

Sorry, I skipped some of that.

There was a question about was the system working and now is not or did it ever work.- Yes it did work properly and just started happening one day. Not randomly, just started happening all the time. Customer didn't touch anything.

What provides the flow? - Pump provides the flow, driven from truck's engine. One pump for whole system.

Is it controlled, constant pressure, constant flow, variable flow? - Honestly, I don't know.

Why feel so strongly that there is a problem with the valve? - I'm feel strongly for the hydrostat first then valve. Because everything else on the whole system works fine. Boom raises / lowers OK, extends / retracts OK, swings around OK. It's just basket tilt that is the problem. I swapped hoses to the cylinder and then it starts slamming the basket to the opposite side, this rules out cylinder. I swapped around coils and actuatros on the valve, problem stays the same. Measured plungers movement on the valve, same on both sides. This pretty much rules out the vavle. But not 100%. Now hydrostat I can't test. When I look at the symbol and read the description of what it does, or supposed to do, It makes me think that it is not regulating 4 Bar pressure when pressure side is on port B in my case. That's why I strongly think it might be hydrostat.

Are there flow restrictions built in to the valve as shown. - This I didn't see because I didn't take apart whole valve. Just coils and plungers. It's a CTOP system so valve and hydrostat are bolted to eachother. To be honest, I don't know if these restrictors are integral part of the valve or something that can be taken out.

I hope it makes clarer picture now.

As I mentioned, I'm just starting with hydraulics, so please be patient with my begginer's mistakes

Thanks!!!

 

[hydtools]

Maybe the shuttle valve is not working properly and not allowing a pressure pilot signal to the hydrostat.

Ted

 

[ROAMER_AUS]

From the drawing that I have, pilot pressure is taken internally in hydrostat. Only P T A B ports are between valve and hydrostat.

 

[hydtools]

Your schematic shows a shuttle valve inside, apparently, the hydrostat.

Ted

 

[PNachtwey]

It appears you found the problem but about this

I taught it was opposite? Retract speeds are higher for the same pressure because area to be filled with fluid is smaller when retracting due to rod occupying the space as well. I even read about it a while ago.

You are thinking in terms of "flow makes it go". This is a mantra hydraulic technicians are taught.
This thinking is wrong unless the pump can't maintaining pressure while extending.
Newton didn't include flow in his laws of motion.
The piston will accelerate until the sum of force acting on it is zero. The piston will accelerate faster because there is more area to push against to accelerate and less area opposing the the motion. When retracting there is less area to push on to accelerate and the opposing force is much higher due to having to push out all the oil.
See this thread.

https://forum.deltamotion.com/viewtopic.php?f=18&t=497

Peter Nachtwey
Delta Computer Systems

http://www.deltamotion.com

http://forum.deltamotion.com/

 

[ROAMER_AUS]

Hi All!!!!
Sorry I didn't post the update sooner.
It turns out that customer wasn't saying the truth about the basket working OK before. It always worked like I described, and he wanted me to make it go slower and even both ways. Which is good in a way, because I started to doubt even this little bit of knowlege I have with hydraulics
At this stage I have to asume that all parts are working fine but they are just not right for the job, or not set properly.
How would any of you fix this if it lands in your hands?

Thanks,

Dragan