Slowing Down Hydraulic Cylinder Movement 3

[jritz]

We have a hydaulic circuit with a single acting hydraulic cylinder (spring return) that moves against some resistance for a few inches - then is allowed to return via the spring. We want to slow down the outward movement of the cylinder when under pressure. We put a needle valve into the pressure circuit - but even when nearly closed , it does not slow the advancement of the cylinder. What do we need to do to make the cylinder movement slower and be able to vary the speed of the cylinder???

Thank You.

 

[Oldhydroman]

To be honest, you should attach a circuit diagram so that we can see more of the picture, but here's a simplistic explanation based on the presumption of a very simple circuit.

I guess there is some sort of relief valve in the system which opens at a particular pressure and spills some of the pump flow back to tank. The pressure at the outlet of the pump will be the pressure needed to move the load plus the sum of all the pressure drops across all of the valves, pipes, hoses, filters etc. in that part of the circuit. The needle valve you mention can be thought of as a variable resistance. As the needle valve opening is adjusted to become smaller and smaller then the pressure drop across it (for any particular flow rate) will get bigger and bigger.

When the pump outlet pressure is below the critical value (the relief valve setting) then all of the pump flow will be going into the cylinder and it is this flow rate which is determining the rod extension speed.

As you start to close your needle valve the resistance to flow increases a little and the pump pressure rises a fraction but all of the flow will still be going to the cylinder and, at first, you won't see any change in cylinder speed. But, when you have eventually closed the needle valve enough for the pump pressure to rise to the critical value only then will some of the pump flow spill to tank over the relief valve and the flow available to the cylinder will be reduced... and now the cylinder rod extension will slow down.

Actually you probably need a throttle-check valve rather than a simple needle valve so that the adjustable restriction is only effective for flow entering the cylinder and not for flow coming out of the cylinder. Alternatively you might try connecting the needle valve between the cylinder port and tank so that you can deliberately spill some of the pump flow.

It is possible that these suggestions are nonsensical if your actual circuit is different from the one I'm imagining. You obviously didn't realise that the needle valve would control the speed of the cylinder by virtue of that valve's interaction with other components in your circuit. Without having the complete circuit to view no-one here will be able to give you any reliable suggestions. If someone does give you a definitive answer without seeing the complete circuit then don't trust it to be a safe solution.

DOL

 

[jritz]

I will try to get the Shop Manager to draw up a cicuit diagram.......then try to figure out how to attach it to one of the treads!!!! It may take a while, but I will get it posted......

Thanks.

 

[jritz]

We have a circuit diagram ready, but cannot attach it at this time.
We will comtinue to try.

 

[MikeHalloran]

You need to slow down the pump, or dump some of the flow coming out of the pump back to the tank.


Mike Halloran
Pembroke Pines, FL, USA

 

[Compositepro]

If the needle valve is restricting flow, the cylinder speed must slow down. I suspect that you are using a standard speed control valve on the cylinder. These consist of a needle valve with a built in check valve that bypasses the needle valve in one direction. The flow restriction should almost always be on the outlet flow of a cylinder, not the inlet. If you restrict the inlet flow the piston can cavitate under load and you cannot control speed. A single acting cylinder would be an exception. If you installed the valve in the standard direction, flow into the cylinder is bypassing the needle valve through the check valve. Reverse the speed control valve. To control speed in both directions independently you need to use two speed control valves in series, but pointed in opposite directions.

 

[MikeHalloran]

CP, the OP said single acting.

>>>If the needle valve is restricting flow, the cylinder speed must slow down.<<<
That is usually true for pneumatic systems, but not guaranteed for hydraulic systems.
It sounds a little like the subject system lacks a relief valve, so I am curious to see the diagram.





Mike Halloran
Pembroke Pines, FL, USA

 

[hydtools]

Get a smaller pump.

Ted

 

[hydtools]

Or a three-port bypassing flow control to meter flow to the cylinder and bypass the excess.

Ted

 

[PNachtwey]

jritz, does the speed or pressure control need to be precise?




Peter Nachtwey
Delta Computer Systems

http://www.deltamotion.com

 

[jritz]

I am trying to get someone to figure out how to post the circuit diagram. The speed does not have to be precise - just reasonably variable.

 

[jritz]

The URL for the image of the hydrualic circuit is attached.

 

[MikeHalloran]

There is nothing in the circuit shown that can control the speed of the cylinders, given a fixed flow pump connected to port P.

You need to connect a variable speed pump to port P, or use a smaller or slower pump.



Mike Halloran
Pembroke Pines, FL, USA

 

[hydtools]

Replace the Snap Tite inline flow controls with something like the Brand pressure-compensated three-port bypassing flow controls:

http://www.brand-hyd.com/fc/fc.htm

Ted

 

[jritz]

Is there a bypassing flow control available that can take 5000 psi ot greater???

 

[Oldhydroman]

Well there's the answer!

Your pump flow rate is 60 cubic inches per minute - that's less than 0.26 USGPM. Your flow control valve is actually a 3/8" NPTF pressure compensated flow control valve with reverse flow check and a quick check of the valve's datasheet shows that the MINIMUM setting of the valve is 1.0 US GPM. Even when you've wound the setting as small as it can go the full pump flow can still get through it.

Don't believe some of the other answers - a restrictive flow control valve where you have shown it WILL be able to control the speed of the cylinder... but just not the particular valve you have chosen. You need a much smaller valve and, at such low flow rates, I would question whether or not you really needed a pressure compensated unit.

Also be aware that the other components in your circuit (Enerpac) are rated for 10,000 psi but those Snap-tite valves are only rated for 5,000 PSI. It may well be that your pump relief valve is set to 5,000 psi (or less) but I wouldn't want anyone to come to any harm with this. The Brand flow control valves are only rated to 3000 PSI. You might want to try a Sun hydraulics FDBA-LAN-GCB/S (0.1-6 US GPM, 5075 PSI).

It would be more efficient to use a bypass style flow control valve, but it would be even more efficient (and might be easier) to use a smaller pump or run the pump at a lower speed. The problem with poor efficiency isn't your electricity bill, it's how much heat you generate in the hydraulic fluid - you might need to fork out for a cooler.

 

[hydtools]

If both cylinders are to operate at the same speed, you may want to consider a smaller adjustable variable displacement pump. If the cylinders are to operate at different speeds, then use Oldhydroman's Sun valve recommendation.

Ted

 

[MikeHalloran]

Yeah, there's a variable bypass, in the form of an adjustable relief valve, within the directional valve. If it's not too physically difficult to adjust, it can be used as a sort of flow control. Open it up far enough and all the pump flow gets recirculated. Close it, i.e., raise the pressure setting, and the piston should extend, until the load on the piston balances the pressure setting. It will probably be twitchy to use, but it's already there.





Mike Halloran
Pembroke Pines, FL, USA

 

[Oldhydroman]

Hi Mike

The thing looks like a press of some sort. It is possible that using the existing relief valve as a bypass flow control will prevent the cylinder from acheiving the full application force.

DOL

 

[MikeHalloran]

Well, yeah. Start with the press open and the relief valve open, and the press won't move. Close the relief valve a little and the press will start moving, at whatever the current speed is. Open the relief valve a little and the press will stop closing. At that point, you have speed control, sort of, wiggling the relief valve back and forth between just enough to overcome friction and not quite enough. Eventually the press will close, but then you have to crank the relief valve more closed to make the press exert any force, and more to bring the piston pressure up, as seen on the gages.

I didn't say it was a _good_ way to do it.

However, we're lacking quite a bit of info on what the desired behavior might actually be.






Mike Halloran
Pembroke Pines, FL, USA