Frequently Asked Questions Counterbalance Cartridge Valves

Transcription

1 Frequently Asked Questions Counterbalance Cartridge Valves You say your valves leak 5 drops/min. How can I keep my load from drifting? The 5 drops/min. are at the point of reseat which is 85% of cracking pressure. If the valve is set correctly, it will never see a load that is higher than 77% of cracking (1/1.3). At this pressure in a typical system, the counterbalance can be considered a zero leak device. Redundant circuits add complexity and have their own problems. Using counterbalance valves to prevent drift is an accepted and common practice on almost all man lifts and small cranes. If a load drifts because of the counterbalance valve, the seat has probably been damaged and the valve should be replaced. How does Sun test its counterbalance valves for leakage? Leakage is tested at the point of reseat with extremely clean oil. If a valve leaks in an application, the seat has probably been damaged by contamination and the valve should be replaced. Before removing any valve, ensure machines and loads are mechanically held in position and that the valve is not under pressure at the time of its removal. Can I adjust counterbalance valves in the field? It is always recommended that a counterbalance valve be set before it is installed on the application. Consider the adjust screw to be a manual override. What do you mean by the setting of a counterbalance valve and what should it be? The setting of a counterbalance valve is the pressure it opens as a relief valve. This is the highest braking pressure it can use to stop an actuator. The valve should be set at least 1.3 times the maximum expected load-induced pressure at port 1. In many cases the maximum load-induced pressure is the system relief setting. How do I determine the setting of a counterbalance valve when it is in a circuit? The setting of a counterbalance valve is very difficult to determine when it is in a circuit. This is due to cross-piloting, load-induced pressure, and cylinder ratio. The best way to check the setting is to remove the valve from the circuit. Before removing any valve, ensure machines and loads are mechanically held in position and the valve is not under pressure at the time of its removal. Screw the valve into a simple, single cavity, line mounted body. Port 1 should be connected to a pressure source with gauge and ports 2 and 3 should be open. Increase the pressure on port 1 until the valve just starts to open and note the pressure indicated on the gauge. Repeat several times to ensure consistency. How much pressure will it take to lower a load? Two areas work together to open the counterbalance valve: the relief area and the pilot assist area. Because of this, the pilot pressure required is inversely proportional to the load pressure. The pressure Sun Hydraulics Corporation FAQs: Counterbalance 8/20/01 1

2 required to move the actuator is less than the setting would indicate. Equations shown below demonstrate this. These equations were published in the # catalogue on page L = Load induced pressure P = Pilot pressure required to open valve P R = Pilot ratio (e.g. 3:1=3) S = Setting of Valve Bore Dia 2 C R = Cylinder area ratio = ( Bore Dia2 - Rod Dia 2 ) With a counterbalance valve on the blind end of the cylinder with a load retracting the cylinder rod. With a counterbalance valve on the rod end of the cylinder with a load extending the cylinder rod. With a counterbalance valve on a motor or equal area actuator with an overrunning load. Pilot pressure needed to open the valve is the result of several equations with several variables. Don't try to predict the result of a specific pilot pressure. Note: The equations above are idealized and do not consider flow and valve gain. Note: Backpressure at port 2 (inlet) of a standard (nonvented) counterbalance valve increases the effective setting at a rate of 1 plus the pilot ratio times the backpressure. Sun Hydraulics Corporation FAQs: Counterbalance 8/20/01 2

3 What do you mean by hysteresis of a counterbalance valve? Hysteresis is the difference between the pressure at which the valve will crack open and then reseat closed. Sun's counterbalance valves' hysteresis is typically less than 15% which means the valve will have reseated at 85% of its cracking pressure. Where should I mount the counterbalance valve in my system? Generally, counterbalance valves should be mounted as close to the actuator as possible to provide maximum protection in the event of hydraulic hose or line failure. Sun offers many mounting configurations that will accomplish this close positioning. Additionally, these cartridges can be installed directly in the actuator by machining a cavity in the actuator housing. What happens to the counterbalance valve if I have backpressure in my system? Backpressure at port 2 (inlet) of a standard (non-vented) counterbalance valve increases the effective relief setting at a rate of 1 plus the pilot ratio times the backpressure (1 + pilot ratio x backpressure). 500 psi (35 bar) of backpressure will increase the effective relief setting of a 3:1 valve (CBCA-LHN) by 2000 psi (140 bar). If you depend on the counterbalance valve to be a dynamic relief this may be excessive. If you place these new numbers into the "Pressure Required to Lower Load" equations, you will find that P (pressure to lower load) is only increased a small amount more than the backpressure. Sun's counterbalance valves are direct-acting relief valves. Backpressure can make direct-acting relief valves go unstable. Vented counterbalance valves are unaffected by backpressure. How do I size a counterbalance valve? Counterbalance valves are pressure dependent devices, not flow dependent. In order for them to control a load they must create pressure drop. It is therefore important not to oversize them. A higher pressure drop will generate greater system stiffness and help improve stability. How do I decide which pilot ratio to select? The vast majority of counterbalance applications are satisfied with a 3:1 pilot ratio. Lower pilot ratios will increase system stability and higher ratios will be more efficient. 10:1 pilot ratio valves typically should be restricted to use in stable motor applications. Should I replace the counterbalance valve if my cylinder is drifting or moving? While cylinder drifting is often attributed to a leaking or damaged counterbalance valve, it can also be caused by cylinder seal leakage or changes in oil temperature. If you believe the seat of the counterbalance valve has been damaged, which can be caused by shock or contamination, it is advisable to replace the valve with a new factory set valve. Always follow the manufacturers recommendations for servicing of hydraulically actuated machinery and ensure all loads are mechanically supported and cartridges are not under pressure when removed. Why do you offer two check valve cracking pressures? Generally, the 25 psi (2 bar) check spring is recommended for most applications as it is more robust and insensitive to rapid flow reversals. The 4 psi (0.3 bar) check cracking pressure should be used if there is a need to pull in make-up oil. Sun Hydraulics Corporation FAQs: Counterbalance 8/20/01 3

4 When should I use a vented counterbalance valve? Vented counterbalance valves should be used if you have backpressure in port 2 (inlet). Typical applications are regeneration circuits, master slave circuits and servo/proportional valve circuits. Where should I vent a vented load holding valve? The best place to vent a vented load holding valve is immediately downstream of the reason you need a vented valve. For instance, if you need a vented valve because you have a meter-out flow control, connect the vent just down stream of the flow control. This will lessen the possibility of false pilot signals. My machine is: Hunting, Bouncing, Ratcheting, Jerking, etc.? The control of a distributed mass, multi-spring system is a very complex problem. Different masses and compliance, combined with interactive coupling, leads to multiple natural frequencies and modes of instability. Most counterbalance applications fit this description. It doesn't matter how you are trying to control the load -- it can be a difficult and frustrating task. Whether you use restrictors, pressure compensated flow controls, counterbalance valves, pressure controls, proportional valves or servo valves, it doesn't matter; your new design wants to jump around the yard like a pogo stick. What to do? There have been many text books and papers written about load control and dynamic stability, mainly about applying to servo valves. System dynamic instability depends primarily on system stiffness and the amount of damping. Stability of a system is typically measured and evaluated using Bode plots, Routh criterion, and Nyquist diagrams. However, these procedures are complex, and rarely is the required data available and the trial and error method prevails. When controlling loads with pilot assisted, load reactive counterbalance valves, one very influential factor is counterbalance pilot gain. Pilot gain is the change in flow verses a change in pilot pressure. Lower pilot gain means a smaller change in flow for a given change in pilot pressure. This can be achieved by lowering the overall gain characteristic (capacity) of the valve or by lowering the pilot assist ratio or both. If a system is unstable, lowering the pilot gain of a counterbalance valve will tend to make it more stable. Carried to extreme, this may mean eliminating pilot assist altogether. Sun has created a table of counterbalance valves that qualitatively ranks Sun's counterbalance valves for stability by pilot gain. This table is an aid for selecting the right valve for specific application. There is a lot of trial and error in selecting the right counterbalance valve for an application. Swapping valves until you achieve a stable machine may seem haphazard, but at this time it is probably the most efficient and inexpensive route. If stability is not achieved with this method, then other system Sun Hydraulics Corporation FAQs: Counterbalance 8/20/01 4

5 components, such as directional control valves, supply capacity, and frictional stick-slip are causing the dynamic instability. What about Noise? When we talk about valve instability, or noise, we are referring to audible sound with a frequency probably higher than 100 Hertz. There are a number of factors that apply in most, if not all, cases. Backpressure. Backpressure is a factor in almost all of the cases. Pressures as low as 45 psi (3,0 bar) can be a problem, with psi (14-15 bar) being the worst. Valve Gain. We offer three different gains in our non-vented valves: standard, semi-restrictive, and restrictive. As the gain goes down, the resistance to noise goes up. Many times, switching from standard to semi-restrictive gain will solve the problem. The restrictive gain valves very seldom have noise problems, but not all applications will accommodate the flow capacity. Cylinder seals. We like to say that our counterbalance valves don't like to sing solos. The sound frequency that seals generate is close to the natural frequency of our valves and tends to excite them. Sometimes the cylinder seals are solely responsible. Internal Geometry. The geometry of the working parts of Sun's standard counterbalance valves constitutes a fluidic oscillator. Our vented valves have a completely different geometry than the standard valves and do not suffer from noise problems. Temperature. Higher temperatures exacerbate the noise problem. This may be due to the change in fluid dynamics in the fluidic oscillator, but is more likely due to the cylinder seals. High temperatures increase the seal loading due to thermal expansion and reduced lubricity of the fluid. Why can't I block the pilot vent port? That area is positively sealed, isn't it? Yes it is sealed, however every time the valve is cycled a small amount of oil passes into the spring chamber, about 1 drop for every 4000 cycles. If the vent is blocked, the spring chamber will fill with oil, building pressure until the valve won't open. I don't need pilot assist, can I just block port 3? We don't recommend it. There is a possibility that the pilot area could get filled with oil and slow down or prevent the closing of the valve. A correct solution is to connect port 3 (pilot port), to port 2 (inlet port). Can I use a three port vented counterbalance valve instead of the four port version to save on plumbing? 3 port vented or atmospherically referenced, counterbalance valves are considered problem solvers for existing circuits that used a 3 port, non-vented valve. If a vented valve is required in a new design, 4 port valves are recommended. Atmospherically referenced valves will leak externally or, over time, allow moisture into the spring chamber (resulting in corrosion) even though the spring chamber is sealed. Why can't I use a counterbalance valve in a closed loop transmission circuit? A closed loop transmission is meant to dissipate the heat from dynamic braking through the prime Sun Hydraulics Corporation FAQs: Counterbalance 8/20/01 5

6 mover. A counterbalance valve dissipates the heat into the small amount of oil contained in the loop. Backpressure caused by dynamic braking from the pump and charge pressure complicate matters. "Putting a vented counterbalance in a closed loop transmission is a crime, putting a non-vented valve in is a felony, and putting a 10:1 valve in is a capital offense!" -Jeff Baker, Sun Hydraulics I have a pilot-to-open check on a large cylinder and the decompression is violent, can I use a counterbalance instead? No. The counterbalance will be less violent than the check valve but the flow dynamics create backpressure spikes that can damage the valve. The circuit needs to be designed to dissipate the energy in a controlled fashion. Why is it that Sun valves have pilot ratios below 10:1 and other manufacturers start at 7:1 and go up? There are two styles of load-reactive counterbalance cartridge valves. The first was designed by Racine and has the pilot in port 1, or the nose of the cartridge. The second style that Sun adapted was conceived by Fluid Controls and has the pilot in port 3. John Allen, retired Sun Hydraulics engineer, said that a counterbalance was a valve designed around a spring. For a given size of cartridge, you design a spring that has the most force and rate that can be achieved and then design the rest of the valve. The spring force dictates the relief area needed to achieve the desired setting. The Racine design has two diameters. There is a pilot diameter and a larger diameter that creates the annular relief area. In order to maintain capacity, the larger diameter has to be as large as possible. In order to increase pressure settings, the annular relief area needs to be reduced. Because the big diameter is fixed and the differential annular area is being reduced, the result is the pilot diameter increases, hence the high pilot ratios. The Fluid Controls design has three diameters, the small one and the middle one defining the relief area, and the middle one and the large one defining the pilot area. This design gives Sun the freedom to create pilot ratios that fit the application. Sun Hydraulics Corporation FAQs: Counterbalance 8/20/01 6