Slowing Down Hydraulic Cylinder Movement
Slowing Down Hydraulic Cylinder Movement
(OP)
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.
Thank You.





RE: Slowing Down Hydraulic Cylinder Movement
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
RE: Slowing Down Hydraulic Cylinder Movement
Thanks.
RE: Slowing Down Hydraulic Cylinder Movement
We will comtinue to try.
RE: Slowing Down Hydraulic Cylinder Movement
Mike Halloran
Pembroke Pines, FL, USA
RE: Slowing Down Hydraulic Cylinder Movement
RE: Slowing Down Hydraulic Cylinder Movement
>>>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
RE: Slowing Down Hydraulic Cylinder Movement
Ted
RE: Slowing Down Hydraulic Cylinder Movement
Ted
RE: Slowing Down Hydraulic Cylinder Movement
Peter Nachtwey
Delta Computer Systems
http://www.deltamotion.com
RE: Slowing Down Hydraulic Cylinder Movement
RE: Slowing Down Hydraulic Cylinder Movement
RE: Slowing Down Hydraulic Cylinder Movement
You need to connect a variable speed pump to port P, or use a smaller or slower pump.
Mike Halloran
Pembroke Pines, FL, USA
RE: Slowing Down Hydraulic Cylinder Movement
Ted
RE: Slowing Down Hydraulic Cylinder Movement
RE: Slowing Down Hydraulic Cylinder Movement
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.
RE: Slowing Down Hydraulic Cylinder Movement
Ted
RE: Slowing Down Hydraulic Cylinder Movement
Mike Halloran
Pembroke Pines, FL, USA
RE: Slowing Down Hydraulic Cylinder Movement
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
RE: Slowing Down Hydraulic Cylinder Movement
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
RE: Slowing Down Hydraulic Cylinder Movement
I hope this helps.
jritz
RE: Slowing Down Hydraulic Cylinder Movement
Basic pressure compensated flow controls also have a nasty characteristic where the cylinder always starts with a jerk. Explanation: while the cylinder is depressurised the compensator in the flow control valve opens fully, when you switch your DCV to pressurise the circuit the compensator needs a finite amount of time to get back to the proper regulating position – during this period the flow is too high hence the jerk. The jerkiness of the initial movement might spoil your test results.
You could go quite high tech and fit some proportional (or servo) valves, then use a stroke transducer on the cylinder and a closed loop speed control system (analogue or digital). But you might find that a simpler (yet still adequate) solution would be to fit a basic electronic speed controller to the electric motor driving your pump.
RE: Slowing Down Hydraulic Cylinder Movement
Mike Halloran
Pembroke Pines, FL, USA
RE: Slowing Down Hydraulic Cylinder Movement
Thank you everyone for your help on this!!!!
jritz
RE: Slowing Down Hydraulic Cylinder Movement
How slow do you need?
Ted