Hey Hydraulic fans, Adel here from LIFCO. I'm on the content team here and always get to chatting with Fraser about, well, about everything hydraulics. This Cylinder that came was a cool story, so we made a video on it, but also had to clarify a few points. Sharing my convo below for you!
VIDEO LINK: Leaky Cylinder Repair
Adel: Just editing this cylinder video, and came across you mentioning doing a post about counterbalance valves. That big square block thing on the top of the yellow cylinder. We don’t really see many of those around in the shop do we?
Fraser: The block itself is a manifold, which has a bunch of machined passages between components. The counterbalance valve specifically is the cartridge sticking out of the side of the block. We see them every now and then, but they don't all look the same. Some are simple cartridge valves ($150) like this one, while others are large fancy blocks ($2500) like we see on the Linde motors. They both do the same thing but the application can require how fast, accurate, efficient, and consistent they will do their job.
Adel: Now I really want to see one of those fancy $2500 ones! Must be an important piece to the cylinder, right?
Fraser: It is a motion control valve and it makes sure the cylinder moves only when it is told to move. A common example of this would be if gravity is pulling something down, like on a hoist/crane. This one in particular was from a vehicle that gets raised and lowered on railroad tracks. Very important.
Adel: So it stops it from dropping or drifting basically. Still doesn't answer exactly how?
Fraser: There are few things that could make the load drop, which is called an overrunning load. This one just handles the hydraulic aspect of it, meaning controlling the fluid. It does this by controlling the pressure on the outlet. The "pressure" specifically is what makes it great. If you were to just restrict the flow to slowly lower something to the ground then you would have to change the restriction based on how heavy the load is, speed the temperature of the oil, and whatever else is happening in the system. But with using pressure, it becomes very stable motion and efficient control.
It does not stop drifting though. The cylinder and motor have internal leakage, plus the valve itself is not zero leakage. For that you need a mechanical brake. A CB can hold in place though.
Adel: And by pressure you're talking about, hydraulic fluid I think?
Fraser: Yes, specifically hydraulic fluid on the outlet of the cylinder or motor. It will make sure there is a minimum amount of pressure on the output. If the load was dropping too fast then the pressure inside the outlet would drop and the CB valve would close. But does it by "modulating" which is like a fluttering. It opens a bit and closes a bit very quickly and this gives it a smooth motion, as opposed to just completely opening and completely closing.
Could you imagine if a load was dropping too fast, and then everything just closed shut? The pressure would spike and open up relief valves or damage components, then it would attempt to close again. It would be a mess.
Adel: No, couldn’t imagine it… guess that’s why someone invented the counterbalance valve.