One-way speed control for cylinder retract?

[ColonelMonk]

Howdy

Was hoping for some expert advice from you all. I'm an off-site engineer working on a hydraulic system that I didn't design. In addition to some other changes, I'm looking at ways of controlling cylinder speed to slow down retraction.

OK, have a pretty simple hydraulic system. I would attach a schematic but I am still waiting for our hydraulic power pack supplier to send ME a schematic

Two hydraulic cylinders, both double-acting. One lifts a mast, one operates a boom.
Each cylinder has it's own pressure line for extend, but for some reason, they are sharing returns thru a TEE. Yes, seems odd.
Power Unit is 12V type, like those you can buy to run a snowplow, truck boom, or liftgate. There are two solenoids on the pump motor, and 3 wires going into the motor, so I suspect that this is one of those units that is "bi-directional". Pump rotates backward for retraction, and this pressurizes the "return" line to rod end of both cylinders. Then, when the solenoid valves open to P1 or P2, it becomes the return flow path for retraction. The manifold is made by our supplier, and I suspect he's using cartridge valves/solenoids from Parker etc... When I started on this project a short while ago, there was only two valves, P1 and P2. There was some issues in the field, and though I'm not entirely in that loop (I should be) I spied a picture and it looks like they added a valve on the Return port as well. They were having some issues with it settling, and sometimes when the boom was retracted the mast would drop a little before stopping.....

Oh yeah, the controls are simple switches. So that's great, except on retraction you don't have any control over the speed.

I hope that is clear enough. Supplier has been very reluctant to supply me with basic spec information on this system? We are working on that....

Main issue, is that I'd like to investigate some options for slowing down the retraction of the cylinders. We want them to deploy quickly (and it's fine right now) but the retraction is too fast, mostly on the boom which has a linkage that amplifies the speed. Not only that, but the return stroke of the cylinder is already faster due to the reduced volume in the rod end.

Flow controls have been tried on the fluid return lines, but because they are a shared return, it affected both cylinders, on both retract and extend! So no go there.

Is there such a thing as a "one way flow control"? So in the extend direction, I get full speed, like a check valve in the direction of flow - but in the retract, the speed or flow control is active and adjustable.

Thanks!

CM

 

[Gilmiril]

Yes, there certainly are such things! For example, here's the URL for Bifold Fluidpower's webpage on flow-control valves:

http://www.bifold.co.uk/products/fcv_series_flow_controllers.html

That series of valves has built-in check valves so that flow rate is controlled in one direction only. I would think your preferred supplier would have something similar.

 

[ColonelMonk]

Gilmiril

Thanks!

Hah! Preferred Supplier?! You mean, the one that won't give me the flow rate, pressure, or valve schematic?

Well you sir, are onto me big time. If this guy can't see that this is basic information they should supply to all customers we'll be going elsewhere.

I checked out your link and that appears to be just what I'm looking for. Sometimes knowing where to look or what to call it is the battle.

 

[hydtools]

You may want to consider a three-port flow control which bypasses excess flow rather than the two-port control which will force excess flow to the other cylinder causing it to move even faster.

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

Ted

 

[hydtools]

You may, when you are looking for something to read through, look at hydraulics & pneumatics for reference and information.

http://hydraulicspneumatics.com/fluid-power-basics

Ted

 

[ColonelMonk]

hydtools

Thanks so much for that suggestion, I didn't mention it but I was hoping that someone would suggest some reading to get me more up to speed.

Also, thanks for the suggestions on the flow control. "consider a three-port flow control which bypasses excess flow rather than the two-port control which will force excess flow to the other cylinder causing it to move even faster"

What do you mean here? Are you referring specifically to the way our system is plumbed? I'm not sure what you mean that it would force the other cylinder to go faster. Explain that for me if you can.

I think I was clear enough that I don't like how this guy is doing our power pack valving. For the cost? I found a dual double-action pump for 2/3 the cost of what he's charging, and it would have separate pressure and return for each cylinder. Basically, it's a commercially available power pack with control and ports to run two double action cylinders.

I'm looking at replacing an electric linear actuator on the same system with a hydraulic cylinder, so we'd need more valving.

I'm thinking that a cartridge spool valve would be the way to go on this instead of separate poppet valves - does that make sense?

Regards,

CM

 

[hydtools]

You describe the rod ends being connected to a common tee. When flow is directed to retract the cylinders, the cylinder with the least resistance will get the most flow. The flow control will effectively create resistance by restricting flow and the other cylinder will have to accept the excess flow not allowed by the flow control to pass to the controlled cylinder. If you use a three-port flow control in the common line between the pump and the tee, the flow control will reduce the speed of both cylinders and bypass the excess flow back to the reservoir.
You have a fixed speed, fixed displacement pump/motor which delivers a fixed flow rate of fluid. The flow control will reduce the flow delivered to the cylinders, but the excess flow ( pump delivery minus control flow) must go somewhere. A two-port flow control will force the excess to flow through the system relief valve creating heat. A three-port flow control will bypass the excess to the reservoir at the lower operating pressure and create less heat.

Ted