This is a general question about hydraulics. The specific piece of equipment I’m working on is a Dynalift D4P80. I had new seals put in one of the leveling cylinders on the machine. I also replaced a few hydraulic hoses as well. Put everything back together. I let the machine warm up, then ran the leveling cylinders, and opened the bleeder valve at the top of both leveling cylinders until air stopped coming out. Almost zero movement from cylinders. So then I cracked all the hydraulic lines and let air out. I’m still not getting any motion on the cylinder. Could this be air still in the system or is there something else I should be looking at? How many times does the cylinder need to be worked to get the air out? Other hydraulic functions are working.
You shouldn't have to bleed air. Sometimes it takes a while to fill the cylinders and build pressure. Are there lock valves? Might have to work the control valve both ways for several seconds to get everything filled with oil.
Gonna step on your toes here. YES, yes you do have to bleed and I'll tell you why. If you trap air in a cylinder and build pressure with that air oil, it can and often does diesel. The oil air mix foams and ignites from the pressure. With fuel, you only need about 275 psi of cylinder pressure to ignite. Now think the average working pressure is between 45 and 5200 psi. You can see how things can get ugly. Usually, in most instances, you can exercise the function slowly, never reaching end of travel in either direction. This gently pushes the air out. You brought up a very good point, holding valves or lock valves or in some instances logic valves. Looking at the second pic, it appears you have a load holding valve. It requires oil from the B side to shift a spool to allow oil to enter A and vice versa. You should be able to loosen the holding valve from the block and slowly work it to push oil through without building pressure. Good luck.
You also need a lot of heat and compression.
It is still air trapped in the system. The valves that are precluding operation require fluid pressure to allow fluid to flow into the main cylinder. These are what are called "counterbalance" valves and keep a cylinder from moving should a hose or severe compromise to the hydraulic system occur. Same scenario if a crane boom or manlift is elevated and a hose blows. You don't want the hydraulic cylinder moving causing damage or injury by the boom falling. Usually with cylinders having an air bleed valve you open this valve, and leave the interconnecting hose to the counterbalance valve loose. Then short cycle the hydraulic function several times till you have fluid flow to the counterbalance valve. Tighten the hose connection and allow fluid pressure unlocking the counterbalance valve allowing cylinder to fill. Once oil starts to flow from the bleed valve, close it and stroke the cylinder full stroke several times both direction to purge air in that operational system. Basically you need fluid pressure to both unlock the valve, and stroke the cylinder.
The compression yields the heat
This makes perfect sense from a safety perspective. I’ll try that procedure and see what happens.
Yes, if the system has bleed screws they are there for a reason. Generally with hydraulics you hook them up and there aren't issues. When you replace a bad hose it's pretty straight forward. If dieseling was that common there would be all kinds of warnings and cautions at every place that sells hoses. If there's no response after 3 or 4 seconds stop and loosen fittings and/or investigate why there's no reaction. Don't hold the valve any longer than it might take to fill the hoses or actuator. Systems with load checks or counter balance valves may need oil or some pressure on both sides in order to operate.
Yeah but all the hydraulic fluid does a lot of cooling.
So I have cracked all the hydraulic lines and bleeder valves and I do not see air working out of the system. I’ve read about check valves and counterbalance valves but am not 100% on how this system is working. The fact that there is hydraulic oil exiting the bleeder valve tells me there is at least some pressure building in the cylinder. These cylinders took quite a bit of RPM to get them to function before I ever replaced anything, not sure if there could be another issue. I snapped a couple of pictures of the attached block to the cylinder. Are the nuts on this block just access to valves or do they adjust in any manner? The closest hydraulic hose in the first picture is the incoming line. Directly across the block is a line that jumps to the opposing cylinder (seen in the 2nd photo). Where do I go from here? Update: The shop that put the new seals on the cylinder said I should pull those holding valves from the block attached to the cylinder. Said they could be faulty. I will try that next.
I don't know your machine so only speaking generally. If your hoses are plumbed correctly and you take a lot of engine rpm to stroke the cylinder, this indicates a worn pump. Those caps cover cartridge valves and some are adjustable, some not. I would not advise you touch them without proper testing equipment given their function in the system. With indications of a worn pump or low system operating pressure I would "tee" into the main pressure line and ensure the hydraulic pump is meeting performance specifications. Counterbalance valves are set to operate at lower pressure than the system operates at. Their function is to ensure you have to "drive" the hydraulic function and it cannot operate by itself.
Do all other functions operate? Have you verified the oil level in the hydraulic tank? I know, silly question, but we've all been there.
I did check the oil previously. There was around a half tank. Needs to be filled but was planning on draining and replacing, so I had not added any. All the other hydraulic functions are working. I double checked all my markings to make sure all the hoses were connected properly. I pulled the caps that cover the safety mechanism previously referred to as a counterbalance valve. It was a simple spring and round bearing that looked to function just as 1693TA described. I pulled those out and tested figuring it would allow oil to bypass them if they weren’t functioning properly. No luck there. Might just have the shop that rebuilt the cylinder take a look at the machine and see if they can come up with some answers. Thank you guys.
If you are certain you rehosed the circuit matching and routing them correctly and it worked prior to cylinder and hose repair work, it should work now. Remove your hoses from their mounts at the cylinder and ensure you have oil flow to that cylinder when commanded. May require a helper to stroke the control while you watch for discharge into a bucket or something along those lines. This is where a simple gauge tee'd into the line with a ball valve afterward makes easy work to ensure what pressure is reaching the cylinder.
Two guys from the business where the cylinder was rebuilt came by my shop today. After breaking into the block where the counterbalance valves are located, we made a discovery. There was a “valve seat” missing from the assembly on the cylinder that they rebuilt (that’s what he called it.). He said it could have been removed by the previous owner. They took the one from the opposing side and found one at their shop that matched very closely. When they put them back in the cylinders, the machine immediately functioned again. However, there is now chatter in the machine when leveling the machine. It is actually a smooth motion on the way “back up” but as soon as the machine levels off and begins the arch downward again, it will jump and move roughly until it reaches complete motion to that side. Then smooth operation on the way back up and repeat. Any thoughts? Could this thing have functioned without this part beforehand, but not after it got new seals? *Also previously when I open the block and removed what I thought was the internals, I had only pulled out the first 3 items from the cap. So the last thing I pulled out was the round sealing bearing. (I thought that round ball sat against a machined hole in the block like I’d seen in a check valve on my Harley.) I never made it to those last two items.
If I'm u understand g correctly, either the "shuttle piston" or it's seat was missing? This assembly only serves to keep the cylinder from moving without hydraulic pressure applied. If this part is not installed I don't really think the cylinder could stroke as the ports for extension, and retraction would both be paralleled precluding any pressure build. I can't say for certain but it seems the repair shop neglected to reinstall that part, (to me).
The shuttle piston (marked in my drawing) was missing in the cylinder that was reworked. If the cylinder for sure can’t function without it, I’m guessing the shop missed it on reassembly, but who knows. Maybe the piston can function without it and it was removed by previous owner to fix chattering issue?? I’m thinking it’s possible they didn’t get into that block because the O-ring on that cap was pretty messed up when I took it apart the first time when trying to diagnose. I would think if they had been in there, they would have replaced that. They said they are going to speak to one of their really experienced guys to see if he has an idea about the rough action. *They also mentioned the shuttle piston they found at their shop was “very close” to the other side, so maybe it was missing, and the one they put in there isn’t exact and is causing the rough motion.
Rough action in a hydraulic circuit is usually traced to cavitation in the pump. Restricted filters, thick or improper oil, etc. Are suspect here. If a vane type hydraulic pump, possibly broken, or shattered vanes. Regardless you have erratic pressure/flow with "chatter". I can't see that cylinder working without that piston myself and we all make mistakes from time to time.
Depending on the design and mounting location on the machine it may be difficult to get all the air out of the cylinder and that air could cause a chatter at the cylinder lock check valve.
That is accurate and usually compensated for with a cylinder air bleed valve. Not always a dead giveaway, but a very good indicator when they are present that they need used to purge the system of air.
I had to put an anti-chatter valve on the spool for the boom lift on my skid steer. Not all machines required it but I got it on warranty. At certain travel of the lift spool the hydraulics would start to shake almost violently. If I recall it happened when trying to slowly position the boom.