Pressure-control valves are found in virtually every hydraulic system, and they assist in a variety of functions, from keeping system pressures safely below a desired upper limit to maintaining a set pressure in part of a circuit.
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Relief Valves
Most fluid power systems are designed to operate within a preset pressure range. This range is a function of the forces the actuators in the system must generate to do the required work. Without controlling or limiting these forces, the fluid power components (and expensive equipment) could be damaged. Relief valves avoid this hazard. They are the safeguards which limit maximum pressure in a system by diverting excess oil when pressures get too high.
Cracking pressure and pressure override. The pressure at which a relief valve first opens to allow fluid to flow through is known as cracking pressure. When the valve is bypassing its full rated flow, it is in a state of full-flow pressure. The difference between full-flow and cracking pressure is sometimes known as pressure differential, also known as pressure override.
Direct-acting relief valves. A direct-acting valve may consist of a poppet or ball, held exposed to system pressure on one side and opposed by a spring of preset force on the other. In an adjustable, normally closed relief valve (Fig. 1), the force exerted by the compression spring exceeds the force exerted by system pressure acting on the ball or poppet. The spring holds the ball or poppet tightly seated. A reservoir port on the spring side of the valve returns leakage fluid to tank.
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Valve Type for controlling a Hydraulic Ram
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(Marine/Ocean)
(OP)
4 Jan 21 21:12Good day all
This is not my specialty (Hydraulics) and I am finding it very challenging.
What I am trying to do is to pressurize a hydraulic cylinder in the pull direction (ram is always under tension) at a very slow speed from a electric powerpac . I need to control the speed and the max pressure which has not been determined as yet so that along with the flow rate will need to be adjustable . If i was to do it all with separate valves this is how it would go . first a inline check valve , next a adjustable throttle valve , next a adjustable pressure relief valve to the tank , then a 12v dc controlled pressure relief valve hosed to the tank. My question is can I get a manifold block with any of these components incorporated into one unit to simplify installation.
Any help would be appreciated
Regards John.
Replies continue below
(Marine/Ocean)
4 Jan 21 23:03Have you selected your pump yet? A pressure compensated variable displacement pump does everything you need without any additional valving.
(Mechanical)
5 Jan 21 09:03Good day!I think this product catalog might help you. Look at page 33
(Marine/Ocean)
(OP)
5 Jan 21 14:32Thanks for the reply'sYes I already have the power pack installed its a 12v watt unit. It is also used in the system to power two reversible winches . This is itI will be taking the power off the bottom of the manifold block which I have mounted separately to power these two rams. I was thinking of using a poppet valve like this https://www.hydraforce.com/products/valves/solenoi... and when powered it would dump the pressure into the tank instead of just back to the pump side so it would have to have a 3 way block .Regards John
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(Mechanical)
5 Jan 21 22:29That is a fixed-displacement (although with two possible choices) gear pump.
Does the required speed of your cylinder motion coincide with the displacement of the pump (in either the high-volume-low-pressure regime, or the low-volume-high-pressure regime)?
Do you need "precision" motion control? (difficult to achieve with hydraulics in general)
Hydraulics with a fixed-displacement pump want to be "bang-bang". Either moving full speed one direction with all pumped fluid directed into one side of the cylinder and let the pressure float with whatever it wants to be in order to push the load, or stopped with the pumped fluid being bypassed to tank at minimal back pressure and the fluid in the cylinders trapped by closed-off valve ports, or moving the other direction full speed. They're unhappy with trying to modulate the flow or trying to split it partially bypassed and partially applying load. Possible, yes - but unhappy.
IMO wrong pump for the job. Should really be a variable-displacement pressure-compensated pump with an adjustable pressure setting. There are a multitude of control strategies for the displacement control.
'Course, the right pump is going to cost a lot more than that one ... (C$400-ish for a new one)
You may want to check the spec sheet at Princess Auto. There's a duty-cycle limitation, which isn't very long. One of the bad things about fixed-displacement pumps with a pressure regulator, is that they're really good at converting full motor and pump load into lots of heat and noise, and not much work.
(Marine/Ocean)
5 Jan 21 23:33I'm thinking, what needs to be done here, is to put your control valve downstream of the cylinder kind of like a diesel fuel injector. A valve down stream would close to divert flow through a needle valve to control the extension speed. A regulator between the needle valve and cylinder with a return to tank would control max force. In order to actuate two cylinders independently, a selector valve upstream of the cylinder would divert flow to one cylinder or the other. A check valve on each cylinder outlet would allow them to be combined so each doesn't require a needle valve and regulator.
(Marine/Ocean)
5 Jan 21 23:50This is probably an ideal pump for your application, a Metaris MHPVB5. It's 0.64 in^3/rev which should get you down into the ~ 2 horsepower range. Metaris makes cheap knock-offs of the brand names.
(Electrical)
6 Jan 21 02:00So if the pressure goes away the rod will be pulled out?Quote:
What I am trying to do is to pressurize a hydraulic cylinder in the pull direction (ram is always under tension) at a very slow speed from a electric powerpac .
What are you really trying to do? Pulling requires force. That depends on pressure AND area. Do you know the pressure on the cap or blind side of the piston?Quote:
I need to control the speed and the max pressure which has not been determined as yet so that along with the flow rate will need to be adjustable .
This assumes the pressure on the cap side is 0 psia.Quote:
If i was to do it all with separate valves this is how it would go . first a inline check valve , next a adjustable throttle valve , next a adjustable pressure relief valve to the tank , then a 12v dc controlled pressure relief valve hosed to the tank.
So if the pressure goes away the rod will be pulled out?If not how do you get the rod extended?What are you really trying to do? Pulling requires force. That depends on pressure AND area. Do you know the pressure on the cap or blind side of the piston?If the cap or blind side is vented to the tank then no problem. If not the pulling force will beForce = Pressure_rod_side*area_rod_side-pressure_cap_side*area_cap_side;This assumes the pressure on the cap side is 0 psia.Relief valves are not control vales. They are safety valves.Controlling speed will be a problem without position or speed feedback.
Peter Nachtwey
Delta Computer Systems
http://www.deltamotion.com
http://forum.deltamotion.com/
IFPE Hall of Fame Member
(Marine/Ocean)
(OP)
6 Jan 21 20:20This is what I am movingThey are charged with nitrogen as the reverse force and holds up the boom.As for duty cycle on the pump the most it will run is one minute .I figure in order of operation the pump would first see an adjustable pressure relief valve then a check check next there would be a tee there that has a poppet valve going back the tank (for releasing pressure) then the last thing would be a adjustable needle valve (for very slow movement) then the cylinder .What do you think ?Thanks for all the help guys
(Mechanical)
6 Jan 21 21:40That's a picture of the cylinder. What about a picture of what you are actually trying to do with that cylinder?
Probably someone (maybe even myself) has experience with trying to move something similar to what you're trying to do?
(Mechanical)
6 Jan 21 21:46Is this the application? Boom vang?
Ted
(Marine/Ocean)
(OP)
6 Jan 21 23:10Yes sorry , that&#;s the right application . Forgot you guys are not all sailors.
There is always a compression force on the cylinder. It uses a nitrogen charge inside the cylinder to counteract the compression and hold up the load . When pressurized with hydraulic pressure the cylinder is pulled down against the nitrogen counter force and moves the boom downward .
Regards John
(Marine/Ocean)
6 Jan 21 23:14J Tully, the reason I suggest putting all control valves between cylinder and tank is that the cylinder itself will act as a bit of an accumulator when the system is operated. This will eliminate some of the jerking associated with the bang bang operating of fixed displacement systems.
If you put the reliefs and needle valves before there will be a large shock as pressure instantly builds relief. It will also always run under relief when the isn't moving. If you put the controls downstream there will be minimal loss until the system is moving or has stalled.
I'll try to draw it shortly.
(Mechanical)
6 Jan 21 23:51Pick a pump that is small enough that its full displacement moves the cylinder at the rate that you want it to move. If you use that dual-displacement pump and are able to control which mode it is in, you can have a fast and slow speed. Then you need only a three-position center-open (pump shorted to tank, A port blocked, B port blocked) directional valve and NO flow controls, and the pump will not be operating on the pressure regulator.
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