Answers to Frequently Asked Questions
Q: What information do I need before calling Technical Support?
A: You will need model number and serial number of the component, system and assembly and a determination of system the problem.
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Q: I have a servohydraulic machine or component that is over 10 years old. Will ServoCon ALPHA be able to continue to support the products on it?
A: Yes, in most cases, very seldom are we unable to renew systems to a like new condition.
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Q: What oil shall I use in my HPU System?
A: If the machine does not have fire retardant requirements one can use Mobile DTE 24 or 25 Extra Heavy. This oil is 600 SSU at 100 degrees F. If you tell your Account Specialists this information and they can not find Mobile DTE Extra Heavy, they should be able to find a substitute. If it has fire retardant requirements you can use Fyrquel EHC which is a fire retardant phosphate ester based synthetic fluid. We keep them in stock for you and for our own testing purposes.
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Q: The 3-phase motor on a hydraulic testing frame will not operate is there anything I can check?
A: The motor has an overload protection devise on the 3 phase line starter in the electrical box. Turn off the main power and open the box. At the bottom of the reversing line starter is a reset button, it must be pushed in to reset the devise. Most of them have the overload in about the same place.
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Q: What are some of the reasons for incorrect hardness readings?
A: You should check the following:
- Replace wrong or damaged penetrators.
- Replace worn or damaged anvil.
- Remove dirt and oil from under the anvil or penetrator.
- Change the rate at which the major load is being applied.
- Replace the test block with a new one.
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Q: Why are my valves sticking when this is a new system?
A: When starting up a new facility or a large addition, there can be and usually is an abundance of foreign material circulating throughout the system. Dirt from the pipe, metal shavings from drilled holes and weld slag are just a few. All of the strainers use a screen to trap most material that would affect the performance of the control. Several manufacturers use a 60 Mesh screen that will stop anything larger that .009 from passing through it. Using a screen with a smaller opening would catch smaller particles, but would produce a very high-pressure drop and require frequent servicing.
Because this is generally a one-time startup issue, ServoCon ALPHA offers a filter cart. The cart comes with a custom-made 110 VAC filtration system on two wheels. This cart is designed to catch almost anything circulating through a system that doesn't belong there. For that reason, the spin on filters will fill up rapidly. As they do, the pressure drop across them will rise (Monitoring the pressure drop is essential when using any type filtration system). After the pressure drop has risen a few pounds or you are unable to maintain system conditions, the filters should be replaced. This is done until the pressure drop is minimized and the system is cleaned of debris. See our fluid contamination filtration sections for more information.
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Q: Every so often I have a problem with valves sticking. When I remove the top cover I find the piston stuck down about 30% of its stroke. I remove the piston for inspection and really don't find anything that should cause the valve to hang-up. Before I put the valve back together, I go over the bore with a polishing pad to remove anything that might cause the piston to hang. When the valve is put back in service it seems to work fine for a couple of days, then it hangs up again! Why?
A: This problem can occur in systems that are very dry (no oil) or they are varnished. This is relatively easy to fix sometimes if the parts are just dry. Remove the top cover and pull the piston from the bore (you need to be careful not to damage the piston seal ring during this procedure). After you have removed the piston, wet a clean rag with refrigeration oil and wipe the bore and the edge of the piston including the ring (never polish the bore or piston with an abrasive). You should see a black film on the rag after completing this step. Reassemble the valve and its ready to be put back in service. However, the discoloration of the oil could mean that there is silt in the system or varnish which can only be removed with special processes.
If the above suggestions fail then the valve should be serviced at ServoCon ALPHA: We have the proper stripping and polishing equipment which operates in a controlled environment. We also have the proper flows stands and techniques to restore assemblies to their original operating condition.
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Q: Replacing an old worn valve rather than rebuilding is always my first choice, but in today's economy, more and more customers opt for rebuilding. In the area of rebuilding, I've had mixed results. Valves with identical service life on similar applications have yielded very different results when rebuilt! Example: I recently rebuilt a couple of valves used for head pressure control. Both valves had pistons, modulating plugs and springs, diaphragms and all gaskets replaced. The valves were back in service for no more than 10 minutes before one of the two caused a high head pressure alarm to sound. I opened the valve using the manual stem and the head pressure dropped like a rock! The valve was disassembled and inspected for anything that could have caused the failure, I found nothing. The valve was reassembled and again head pressure skyrocketed! I opened the valve with the manual opening stem and ordered a new replacement. I replaced all of the internal components, yet only one of the two valves worked afterward! What am I missing?
A:The first common mistake is to assume that any two valves regardless of how identical the application is will experience the same type of wear over the same period of time. Variables like the amount of oil that circulates through the valve, system load changes and frequency including pressure/temperature, and basic material design and manufacturing tolerances that are part of most everything we buy effect the life of the valve.
Let me briefly go through some of the mechanics of how our valves work and the reason rebuilding is not always successful. Unless a valve is mechanically opened and closed (motorized, etc.) pressure acting on a piston is used to control flow through the main valve orifice (port). In the case of a solenoid valves, inlet pressure is fed through a long drilled hole in the body up to the needle and seat pilot. When the coil is energized, the needle is pulled from the seat allowing flow to the top of the piston. The piston in turn pushes downward forcing the modulating plug off its seat and the valve opens. Valves are machined using very tight tolerances between the piston and the valve bore. The total tolerance or air gap cannot exceed .0025 - .003 of an inch. Exceeding this gap produces a valve that requires more pressure drop too fully open. The larger the gap, the larger the pressure drop until the bleed past the piston exceeds the supply through the pilot orifice. At this point, the valve will not fully open and pressure control will become impossible.
ServoCon ALPHA specializes in the determination of whether a valve has reached the end of useful life. This is decided by how much wear there is to bodies, bushings and spools and whether it is economical to repair again. This requires accurate and calibrated measuring instrumentation.
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Q: I have several old valves installed in a system that date as far back as 20+ years. My problem is parts or finding a valve that is by the manufacturer called "obsolete". One example is the packing, glands and seals look very different today than they did 20 years ago. Do I need to know how old they are when ordering parts? Does the current style packing fit in valves made 20 years ago? Do I need to replace the whole valve, or can I replace the assembly with the newer model.
A: Prior to ServoCon ALPHA's existence, there has been at least three different companies who have owned every valve type. Unfortunately, the current designs are not backward compatible with previous models. The good news is that ServoCon ALPHA has longevity in repair services and knows where to find or can manufacture every assembly or part needed for valves of every type.
Note: Manufacturers change model numbers and designs to force the purchasing of new products to avoid competition in the marketplace. This is where we have become strong by continuing to offer products that have otherwise become obsolete. Many times only the names, model numbers or the way they look have been changed. We know the difference from our service experience of over 300 varieties of hydraulic control valves.
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Q: Do you have new parts for repairing servovalves and components?
A: Yes, we purchase them directly from the out source suppliers of the valve and component manufacturers when possible. Otherwise, we manufacture them using parametric modeling techniques in our automated machine shop where we have been fitting spools and producing other wear related parts for many valve types and other components since 1980.
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Q: How long have you been in the servocontrols business?
A: ServoCon ALPHA has been supplying servocontrol assembly solutions since 1979.
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Q: Are you certified to service and rebuild servovalves?
A: Yes, we are the only Abex Servovalve-certified rebuild/ repair facility in North America trained at Oxnard, CA: We have an ISO 9001-2000 compliant Quality System. We have been trained at many other major servovalve manufacturing plants such as Atchley, Dowty, Ultra and the head of our engineering department worked at Moog in their Aerospace and Industrial plants in East Aurora.
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Q: What is the experience level of your servocontrol engineers and technicians?
A: The technical staff at ServoCon ALPHA has over 200 years of combined experience in the repair and rebuild of servovalves and servocontrols several of the engineers associated with ServoCon ALPHA worked for major valve manufacturers for over 25 years. Several of our technicians have over 15 years experience at ServoCon ALPHA.
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Q: What is your warranty on new and used servovalves and servocontrols?
A: Our rebuilt servovalves and servocontrols carry a one-year warranty on parts. See extended warranty for longer periods.
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Q: What condition are the used servovalves and servocontrols that you sell?
A: Rather than simply ultrasonically cleaning, inspecting and remanufacturing our used servovalves and servocontrols, we submit each used unit through four critical levels of assessment and care before allowing it off the floor: validation of operation, inspection, recondition, and final testing with service reporting.
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Q: Do you do all of the work on your servovalves and servocontrols in-house?
A: Yes, we maintain our own state-of-the-art parts cleaning equipment, wash booths, servolab, controlled paint environment and machine shop inside our 25,000-square-foot facility.
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Q: Can you provide service to maintain our servovalves and servocontrols and evaluate them regularly?
A: Yes, we have a comprehensive preventive maintenance program to ensure that your used servovalves and servocontrols continue to perform flawlessly.
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Q: Do you provide a servovalve and servocontrols recovery program?
A: Yes, our unique exchange program that keeps your business up and running by allowing servovalves and servocontrols to swap the identical servovalves and servocontrols to the one that has failed.
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Q: Do you offer emergency servovalves and servocontrols repair services?
A: Where available, we have personnel who can be dispatched in emergencies to repair your servovalves and servocontrols or we can work with you to get a replacement servovalve and servocontrol installed.
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Q: Can you provide training on the servovalves and servocontrols and systems that you sell?
A: We offer an extensive training to our customers in the areas of system operations, system maintenance, and mechanical servicing. We provide our training expertise either at your location or at our facility. Please call for pricing and scheduling.
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Q: How can I get a quick overview of the products, services and capabilities offered by ServoCon ALPHA?
A: We have one of the most capable facilities in the world due to our state of the art equipment, service technicians with years of knowledge, and inside sales people who are ready to assist you with any fluid power product and service need 24 hours a day, 7 days a week. See our Company Profile.
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Q: How can we be sure ServoCon ALPHA is qualified and capable to handle our fluid power product needs?
A: ServoCon ALPHA is a company that has been operating since 1979 in fluid power product and service fields. We have had to deal with all kinds of hydraulic and pneumatic applications that require expert knowledge and years of experience. With this experience, we have gained what today’s modern technologies expect out of a company in this field, like 24 hour on site expedite service, repairing and testing any kind of hydraulic and pneumatic item including pumps, motors, valves, cylinders, electronic valves, boards, and many more. We can also build custom power units to your specification.
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Q: Industrial Repair Emergencies do occur. How fast is your typical response time?
A: We can turn your emergency repair around typically in 24 hours if we have parts in stock, if parts are not in stock, we can manufacture them or have them the next day and get you up and running in a short time in most instances.
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Q: Does your company have insurance to work on-site in our plant?
A: We have a million dollar liability insurance to cover any kind of accident that may occur on your plant and are fully insured for all our employees and trucks.
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Q: Do you charge anything to pick up and deliver?
A: We offer a pick up and delivery service to all our customers. We will pick up your repair and deliver it back to you at a price and schedule that is better than any trucking service, many times same-day delivery.
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Q: Do you offer any kind of classes on servocontrol hydraulics?
A: Two or three times a year, we can offer, based on demand, a 3-day servocontrol hydraulics class on the basic components, where you can learn about how a servovalve works, what a hydraulic directional valve is, how a hydraulic cylinder works, etc. This class gets into a little of every thing to get some knowledge about hydraulics and how they function. We will set a special class for your specific needs.
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Q: How and when can I place an order?
A: Order any of the following 3 ways.
- Fax a Purchase Order anytime to us at 540-337-4901.
- If you'd prefer to talk to a real live person you have the option
to call us on our toll-free order line at 800-447-7747 during regular
business hours and ask your Account Specialist
for complete instructions. Our regular business hours are typically
8:00 A.M. – 4:30 P.M. Eastern Standard Time.
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Q: Where do I call for answers to my non-order questions?
A: If you have non-order questions please call our corporate office at 800-447-7747. Office hours are typically 8:00 A.M. to 4:30 P.M. Eastern Standard Time. If we're not here to take your call please leave a brief message with the receptionist or on the answering machine and we'll get back to you promptly.
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Q: What payment options are available?
A: You may pay for any products you order by credit card—Visa and Mastercard are accepted; by company check, money order, or cashier's check in U.S. Dollars; or by electronic wire transfer.
There is an additional $20 charge on payments via wire transfers due to bank transfer fees. We also accept company purchase orders from U.S. and Canadian companies that have established an account with us.
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Q: How do I establish an account?
A: If you're a U.S. or Canadian company that would like to place orders with a purchase order please fax or mail us your company's standard trade reference information. Our fax number is 540-337-4901 and mailing address is ServoCon ALPHA, Expo Industrial Park, 167 Expo Road, Fishersville, Virginia 22939.
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Q: I'm not in the United States. Can I order from your company?
A: Yes, you can order products from anywhere in the world; However, International orders must be prepaid via credit card, bank check in U.S. Dollars, or via wire transfer.
Note: For certain countries we are unable to accept payment via credit card. If you are located in one of these countries you will be notified via email when you place an order what payment options are available to you.
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Q: How will my order be shipped?
A: If you live in the United States your order will be shipped by UPS unless otherwise specified. Ground, 2nd day, and next day service are available. In the continental United States shipping costs may be charged to your account, or prepaid and added to your invoice. You make your choice when you place your order. Here is a partial list of our other carriers: UPS, Fed-EX, Averitt, Consolidated Freight, Central Transport, Emery, RPS, Roadway and Yellow Freight.
International orders are shipped via UPS International Service. UPS service typically takes 3-5 business days from the date of shipment to arrive in your country.
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Q: How long will it take to receive my order?
A: If an item is in stock your order will generally ship no later than the next business day. We typically stock most products. Because we offer over five thousand different products we are not able to stock everything available on our site. Non-stock items typically take from 7-10 days to arrive at your location.
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Q: Is expedited shipping available?
A: Yes for the continental United States. Naturally this costs a bit more. Next day service within the continental United States is charged by weight and destination rates at the time of order based on your need. We can provide a delivery truck to deliver in the continental US.
For our customers in Alaska or Hawaii and for our international customers shipping and handling charges will be calculated based on the weight and destination of the shipment. You will be notified by email or FAX for approval of the shipping costs before we will proceed with your order.
International customers may have a duty/tariff charged by their country's customs office based on the total value of the items being imported. These fees are your responsibility and are not included in the cost of the item or shipping charges.
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Q: Do you accept job postings from recruiters?
A: Not at this time.
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Q: How do I get a password if I do not already have a password?
A: In the “Secure Member Login” on the website there is a link titled “Become a Member Now.” You can send an e-mail to servoconalpha@servoconalpha.com and we will take your registration request that way. After we register you we will send you an email with your “administrative” username and password.
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Q: How can I track my order after it leaves ServoCon ALPHA?
A: If you have or can obtain (by email from your account specialist) your UPS tracking number, go to the UPS website. Furthermore, the tracking number is actually a “hot link” to the UPS website. By clicking on this link, not only do we automatically transfer you to the UPS site, we actually “plug-in” the number for you. At this point you can view the package progress from hub to hub, and even who signed for it at your facility.
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Q: Can I see what I have ordered in the future after I have ordered?
A: You can ask your account specialist for a report with your P.O. number, all open orders, or view all orders. Our order status goes back three years and even further depending upon the customer.
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Q: What about viewing past purchase history?
A: You can view every single item you have purchased from ServoCon ALPHA back to year 2000 with a report from your account specialist. If you have assigned your own part number to these items that will be reflected too.
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Q: How does a nuclear power plant work?
A: The fuel in a nuclear power plant is uranium, which is a natural ore found in the ground. Unlike coal, gas and oil, plants don't burn uranium in a nuclear power plant. A plant gets the heat needed to create steam from a process known as fission, or the splitting of atoms—the smallest parts of any element. Like all pressurized water reactors, nuclear plants have distinct loops of water that do not intermix. A closed loop of water is heated by fission in the reactor. A plant then transfers this heat to a second loop of water. The steam drives the turbine-generator that produces electricity.
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Q: Where does a power plant get the water used in their operations?
A: Water used in the reactor and most other plant systems is piped in from the municipal water supply. The plant then purifies the water (at the plant) to reduce mineral deposits that could otherwise reduce equipment efficiency. The water quality is routinely tested in the plant's on-site laboratory. A separate supply of water that cools the turbine steam supply for reuse comes from a unique, closed system of interconnected canals or lake. The lake or canals act like a giant radiator to cool the water in a several-day, multi-mile journey before it is circulated back to the condenser for reuse.
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Q: How safe are nuclear plants?
A: Nuclear power plants are very safe. Power generating plants maintain very high operating standards and meet or surpass regulatory requirements. Employee and public health and safety are of highest priority in everything that is done. Plant workers are thoroughly trained and are experts in their fields. To further protect the public, nuclear units are designed to use automatic safety systems and multiple safety barriers as part of their "safety first" policies.
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Q: Who regulates or oversees the operation of nuclear power plants to make they are safe?
A: First, their employees are our most expert safety guardians. Each employee understands that nuclear safety is his or her primary responsibility. A: A: Power plants are also monitored closely by the Nuclear Regulatory Commission, a federal government agency that oversees all U.S. nuclear power plants. Daily and special inspections—some unannounced—are conducted on site at the plants to make sure they meet federal regulations that protect public health, safety and the environment.
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Q: What kind of maintenance is done on plant equipment?
A: Plants have rigorous maintenance programs to make sure that the plants runs reliably. Employees are skilled in using preventive maintenance techniques to test and repair or replace equipment on regular schedules. As a result of this ongoing maintenance, plants can operate safely and reliably now and in the future.
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Q: What kind of security is in place to protect the plants?
A: Nuclear plants have always had a comprehensive security program, and extensive measures are in place to ensure the security of their facilities. Their security plan is very detailed in how the security force is organized and what actions would be taken to protect the plant. Although they are not allowed to discuss the specifics of their plan, their comprehensive security measures at all nuclear plants are designed to protect public health and safety.
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Q: Do power plants communicate with other law enforcement agencies?
A: Nuclear security programs and personnel are part of a larger local and national security network and are tested routinely to ensure the safety of their employees, their customers and the communities in which the plants operate. Plant security personnel coordinate with local law enforcement agencies, which also participate in onsite security exercises. They also keep in close communication with the FBI, Coast Guard and other government agencies relating to security.
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Q: How do nuclear power plants test their security programs?
A: Each nuclear power plant conducts tactical security drills periodically, and the Nuclear Regulatory Commission evaluates these drills. Drills are quite rigorous, and plants successfully complete drills regularly.
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Q: What physical barriers and emergency equipment protect the nuclear reactors?
A: The nuclear reactors are housed in massive steel-reinforced concrete reactor containment buildings that are designed to withstand earthquakes, floods and extreme weather conditions, such as hurricanes and tornadoes.
The reactor vessel itself is made of solid steel and is housed within the containment building. Additionally, the uranium fuel pellets inside the reactor are all encased in zircaloy, an extremely strong metal alloy. Redundant safety shutdown systems are also available to respond in the event of an emergency.
The Electric Power Research Institute, an independent, non-profit energy research group, also recently analyzed aircraft impacts on nuclear plant structures that house reactor fuel. The study results determined that the structures would not be breached, the fuel would be protected and there would be no radiation release. In addition to the containment building, other protective barriers include concrete shields within the building and the 5- to 9-inch-thick steel reactor vessel.
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Q: What is the definition of hydraulics?
A: If energy transfer is in the form of pressurized liquid flow then it is called hydraulics. The oil is kept in a reservoir and the pump draws it in and pushes it into the system. Because the oils can't escape, pressure builds up and the energy stored in the oil is then used to operate machinery, using high pressure hoses, valves and actuators.
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Q: What is a hydraulic-mechanical press brake?
A: A mechanical press brake that uses hydraulic cylinders attached to mechanical linkages to power the ram through its working stroke.
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Q: What is the torque valve on the bolts to mount a 15 GPM servovalve?
A: 120 in-pounds. See Mechanical Null Adjustment in downloads.
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Q: What is a Closed-Loop Control System?
A: An automatic control system in which the system input is compared with a measurement of the system output, and the resultant error signal used to drive the system towards the desired output.
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A: A continuously acting, bidirectional closed-loop control system.
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A: A device used to produce hydraulic control in a servomechanism.
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Q: What is an Electrohydraulic Servovalve?
A: A servovalve which produces hydraulic control in response to electrical signal inputs; sometimes called a transfer valve.
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Q: What is an Electrohydraulic Flow Control Servovalve?
A: A servovalve designed to produce hydraulic flow output proportional to electrical current input.
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Q: What is a Hydraulic Amplifier?
A: A fluid valving device which acts as a power amplifier, such as a sliding spool, or a nozzle flapper, or a jet pipe with receivers.
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A: The portion of a servovalve which includes a hydraulic amplifier. Servovalves may be single stage, two stage, three stage, etc.
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A: The final stage of hydraulic amplification used in a servovalve, usually a sliding spool.
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A: A fluid connection to the servovalve; for example, a supply port, a return port, or control port (sometimes called load port, strut port or output port).
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A: A multi-orifice fluid control element with supply, return and one control port arranged so that valve action in one direction opens the control port to supply and reversed valve action opens the control port to return.
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A: A multi-orifice fluid control element with supply, return and two control ports arranged so that valve action in one direction simultaneously opens control port #1 to supply and control port #2 to return. Reverted valve action opens control port #1 to return and control port #2 to supply.
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Q: What is Flow Force Compensation?
A: A design technique for shaping the fluid passages about a variable orifice so as to reduce steady state flow forces.
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A: A valve design condition which yields relatively constant flow gain over a wide range of load pressure drop; sometimes called gain compensation.
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A: The electromechanical transducer commonly used with the input stage of a servovalve. Displacement of the armature of the torque motor is generally limited to a few thousandths of an inch.
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A: The current which is required for control of the valve, expressed in mA: For three and four lead coils, input current is generally the differential coil current, expressed in ma.
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A: The specified input current of either polarity to produce rated flow, excluding any null bias current, expressed in mA: For three and four lead coils, rated current must be associated with a specific coil connection (series, differential. or parallel).
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A: The dc current present in each coil of a differential coil connection, the two coil currents having opposing polarities such that no electrical control power exists.
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Q: What is Electrical Control Power?
A: The electrical power dissipated in the valve coils which is required for control of the valve, expressed in mw. For differential coil connection, the total electrical input power supplied to the servovalve is the sum of the electrical control power and the power due to quiescent current.
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Q: What is the Electrical Null?
A: The zero input current condition.
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A: The complex ratio of coil voltage to coil current. It is important to note that coil impedance may vary with signal frequency, amplitude and other operating conditions due to back emf generated by the moving armature. Therefore, coil impedance should be measured with explicit operating conditions.
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A: The dc resistance of each torque motor coil, expressed in ohms.
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A: The apparent inductive component of the coil impedance, expressed in henrys. For a valve torque motor having more than one coil, the total coil inductance will include mutual coupling effects.
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A: A low amplitude, relatively high frequency periodic electrical signal sometimes superimposed on the servo-valve input to improve system resolution. Dither is expressed by the dither frequency (Hz) and the peak-to-peak dither current amplitude (ma).
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Q: What is Control Flow, also called Load Flow * or Flow Output*?
A: The fluid flow passing through the valve control ports, expressed in cis or gpm. In testing a four-way servovalve, flow passing out one control port is assumed equal to the flow passing in the other, This assumption is valid for no-load valve testing with a symmetrical load (e.g., with an equal area piston having insignificant friction and mass) and for static testing with loaded flow.
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A: The specified control flow corresponding to rated current and specified load pressure drop, expressed it -cit or gpm. Rated flow is normally specified at the no-load flow.
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A: The servovalve control flow with zero load pressure drop, expressed in cis or gpm.
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A: The null region associated with a spool overlap condition.
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A: The input current required to bring the valve to null under any specified set of operating conditions, excluding the effects of valve hysteresis, expressed as percent of rated current. Hysteresis effects may be discounted by taking the arithmetic average of the null bias currents measured on a symmetrical hysteresis loop.
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A: The change in null bias required as a result of a change in operating conditions or environment, expressed as percent of rated current, Null shift, sometimes called centershift*, may occur with changes in supply pressure, temperature, and other operating conditions.
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A: The pressure existing at both control ports at null, expressed in psi; sometimes called centering pressure*.
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Q: What is Null Pressure Gain?
A: The slope of the pressure gain characteristic at null, expressed in psi/ma.
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A: The slope of the control flow versus input current relationship at null, expressed in cis/mA: Null flow gain may be between 0 to 200% of the nominal flow gain due to the lap condition.
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A: The total valve internal leakage flow at null, expressed in cis.
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Q: What is Frequency Response?
A: The complex ratio of control flow to input current as the current is varied sinusoidally over a range of frequencies. Frequency response is normally measured with constant input current amplitude and zero load pressure drop, and is expressed by the amplitude ratio and phase angle. Valve frequency response may vary with the input current amplitude, temperature, supply pressure, and other operating conditions.
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A: The ratio of the control flow amplitude to a sinusoidal input current amplitude at a particular frequency divided by the same ratio at a specified low frequency (usually 5 or 10 Hz). Amplitude ratio may be expressed in decibels where db= 20log10AR.
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A: The time separation between a sinusoidal input current and the corresponding variation of control flow, measured at a specified frequency and expressed in degrees (deg = time separation, sec x frequency, Hz x 360).
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Q: What is considered a standard design configuration of a servovalve?
A: The manufacturer should make available a written description of the design configuration of the servovalve, together with a brief description of the operating principles. Designs for flow control servovalves vary widely, reflecting the preferences and experiences of both manufacturers and users. A basic distinction is the number of stages of hydraulic amplification utilized within the servovalve. In a single stage valve, the torque motor is coupled directly to the output stage which is usually a sliding spool. In two stage servovalves, an additional hydraulic amplifier is interposed between the motor and the output stage. A moving flapper together with one or two nozzles, a jet pipe with receivers, or a small sliding spool are commonly utilized as the intermediate hydraulic amplifier. Some valves utilize three stages of hydraulic amplification to achieve high capacity flow control.
Single stage servovalves often use flow force compensation to reduce the required electrical driving forces. Most multistage flow control servovalves utilize some form of internal feedback for stabilization of the intermediate hydraulic amplifiers. Both hydraulic feedback end mechanical feedback are commonly used. Occasionally internal electrical transducers are included to provide a signal proportional to valve output. These valves require an external servoamplifier to achieve stable flow control. Flow control servovalves are available with both three-way and four-way output stages. Three-way valves are normally used in conjunction with single-ended pistons of 2:1 area ratio. Four-way valves are considerably more common and are usually used for double-sided control of pistons or hydraulic motors.
Another design classification for flow control servovalves distinguishes between wet motor, stale motor and dry motor valves. Isolation of the torque motor magnetic circuit from fluid filled areas prevents the accumulation of magnetic contaminant within the valve, thereby increasing the reliability of most servo systems. With a stale motor design, fluid is dead-ended in the motor cavity so flow about the motor is considerably reduced. Dry motor valves utilize a sealed flexure member to transmit motor motion through a fluid barrier.
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Q: What is the standard for servovalve construction and workmanship?
A: The mechanical design materials and processing, and general workmanship should be compatible with the intended operational, environmental and life requirements. These requirements vary widely for applications which range from machine tools to manned aircraft, missiles and satellites. The general requirements of MIL-H-8775A, MIL-H-5440 and MIL-H-25475 should govern the design wherever possible. Especially pertinent considerations are the following:
- All threaded parts and fastenings should be lock wired per MS33540 where practicable. Where necessary, other approved locking methods may be used.
- Retaining rings should not be used in any location where failure of the ring could result in loss of function of the unit or in loss of hydraulic fluid.
- If practicable, all screw threads should conform to Mil-S-7742.
- All parts and matched subassemblies subject to replacement should be completely interchangeable with comparable parts of like servovalves.
- Materials and finishes in contact with the fluid should be selected for fluid compatibility.
Workmanship and tolerance control on all parts utilized in production servovalves should insure uniform quality and performance to the extent that all units will meat or exceed pertinent customer and government specifications.
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Q: What are the standards for servovalve installation and envelope details?
A: The installation details and envelope dimensions for flow control servovalves should be furnished by the manufacturer. Information given should include: envelope drawing, mounting details, mating electrical connector, coil connections, lead wire color code, supply pressure, rated flow at rated current and rated pressure drop, nominal coil resistance and resistance tolerance, valve polarity, intended fluid, recommended fluid filtration, operating temperature range, identification of manifold seals, dry weight.
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Q: What are the servovalve standard mounting configurations?
A: Recommended porting and mounting configurations for flow control servovalves may be obtained from our Engineering Department. Three-way valves should use the same porting configurations, except that control port 2 is eliminated. All ports on each servovalve should be legibly identified by markings on the sides of the valve using P for the pressure supply port, R for the return port, 1 and 2 for the control ports.
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Q: What is the Centering Adjustment?
A: If an external centering adjustment is provided to set the valve null, the adjustment should operate smoothly and without difficulty while the valve is operating with rated supply pressure.
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Q: How does a servovalve get lubrication?
A: The fluid being controlled should serve as the sole and sufficient lubricant for the servovalve.
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Q: What types of seals are used in servovalves?
A: Military standard (MS) seals, selected for compatibility with both the fluid and the anticipated temperature operating range, should be used whenever possible within the servovalve. In general, seal sizes and groove dimensions should conform with MIL-P-5514D.
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Q: What is a general life of a servovalve?
A: The useful life of servovalves supplied for most applications should exceed 1000 hours of normal operation with fluid having the recommended filtration. The useful life of a servovalve will be greatly influenced by a number of operating conditions, including environment fluid contamination, number of cycles (including dither if present), frequency and magnitude of electrical and pressure overloads, etc.
Fluid contamination generally causes gradual erosion and wear of metering orifice edges. This wear tends to increase valve internal leakage, to increase the nonlinearity through the null region, and tends to decrease valve pressure gain. Contamination also generally tends to increase friction of sliding spools which may increase valve hysteresis and threshold.
The number of cycles which may be accumulated during 1000 hours of normal operating life may be several million, so the valve mechanism should be designed for essentially infinite fatigue life. A suggested life test for flow control servovalves is one million sinusoidal cycles at ± rated current input followed by one million square wave cycles at ± rated current input, each accumulated at a rate of 10 Hz. Control flow on higher flow servovalves may be restricted by a suitable orifice during the life test.
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See also Glossary.
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