when cross-over relief?

[subsearobot]

Hello
Are there any rules-of thumb for requiring cross-over relief valves at a ram?

In my present application, I am cycling a ram with relatively high fluid line speeds (~20-40 fps, TDB). distance between control valves and system relief will be about 15', most of which will be coiled hose.

the piston will reverse after traveling full stroke (piston will collide with endcap then reverse).

there will be no inertial load attached to the piston when it hits the endstops, either end. There is however the fluid inertia. might this require relieving at the ram to spare the ram from pressure spikes?

this is a "mobile" application- space is definitely a premium, and I am trying to be minimalist. hours of operation will not very high.


cheers

 

[MikeHalloran]

My Dad installed a snowplow on a Jeep FC truck.
He complained to me that it kept blowing seals out of the double rod cylinder that yawed the plow, and they always blew when the plow hit a mound of snow or a speed bump at an oblique angle.

I sketched the schematic of a crossport relief valve and explained how it worked, then I sketched a typical actual outline and suggested he go buy one. He said "we got one with the plow, but nobody could figure out what it did, so we left it out". I said, "Now, you know what it was supposed to do."

That's one circumstance where it's required.

Here's another: If you're running a single rod cylinder with a directional valve that doesn't have much internal leakage, you need a crossport relief valve to prevent the cylinder from acting as its own intensifier (no external load or inertia is required for this to happen) and blowing out the rod seal.



Mike Halloran
Pembroke Pines, FL, USA

 

[kiran21]

@subsearobot: there is no thumbrule for require of cross relief valve. If it's cylinder then double counterbalance valve or double pilot operated check valve will work out.

cross-port relief valves are for the motors, rotory actuators, winches, as it prevetnts any intensification on motors when you have load on the motor and you centralize the control valve the load wants to run away and becomes a pump and cross line relief valve prevets motor from being overload.

 

[hydtools]

A ram is generally a one-way hydraulic cylinder, powered extension and gravity or spring return. Is this what you have?
What happens when the ram collides with the end cap to make the ram return?
If you are concerned about a pressure spike at the ram input, put a relief valve there.
A cross-relief presumes two hydraulic lines to the cylinder. Is that what you have?

Ted

 

[hydtools]

An accumulator would also absorb the spike.

Ted

 

[subsearobot]

Ted,
Sorry for the confusion, I have a dual acting hydraulic cylinder. I initially called it a piston, but then that sounded confusing... I did not realize 'ram' specified single acting... confusing. is there a standard name in the industry?

I am wondering about a pressure spike caused by the fluid inertia in the supply line when the piston impacts the endcap at the end of stroke.

Not sure I need to worry- it does not seem like much energy, but I am doing some initial diligence.

thanks all

 

[hydtools]

Here is a link to cylinder descriptions:

http://www.hydraulicspneumatics.com...ors/Article/True/6423/MotorsActuators#scene_1

What is the inside diameter of the 15 feet of hose? This will lead to knowing how much fluid is moving and the energy therein.

How fast will the piston be moving when it collides with the end cap?

Ted

 

[subsearobot]

Ted,
hose is 3/8" ID, which gives about 3/4# water moving at 17 fps. (working fluid is a water/glycerol mix)

piston speed will be about 15"/s.
cylinder is 1.5" ID with a 1/2" rod.

I may be using a rudimentary regeneration circuit, but have not assessed its impact on the system yet. speeds and pressures listed above do not reflect regen. this is why I stated 20-40 fps line speeds in the OP.


cheers,
seton