Maintaining in Difficult Times
Maintaining Machinery in Tough Times-Part 2: Second in a two-part series of articles about proper preventative maintenance for automated pallet nailing machines; part two focuses on hydraulics.
By Jim Gookin, Viking Engineering
Date Posted: 5/1/2009
This is the second in a series of articles on machine maintenance. In this article I will cover some of the basics related to hydraulic systems. As always, it is important to contact your machinery supplier for schedules and procedures that are specific to your equipment and situation. Remember to lock out and tag it!
The fluid storage tank, when designed properly, is more than a bucket with a top. Whether it is located under, on top, or beside the equipment, simple maintenance will go a long way in protecting your high dollar hydraulic components. Cleanliness is the main task. Cleaning the external breather will help eliminate tank leakage caused by either positive or negative tank pressures. These pressures are a result of fluid temperature fluctuations and fluid movement. Located inside most tanks are a diffuser and a suction strainer. Fluid, entering the tank from the filter assembly, is diffused below the fluid level to reduce aeration and turbulence.
Contaminants either missed by the filter or introduced directly into the tank are trapped by the suction strainer before entering the pump. Each time the fluids are changed, clean and inspect both the suction strainer and diffuser. All too often, pumps are replaced when the real issue was a plugged strainer. A plugged strainer reduces the volume of fluid the pump needs to perform at peak levels. System pressure may not be affected by a partially plugged strainer. Installing a new pump that has a plugged suction strainer will cause pump cavitations, thus destroying a new pump. Fluid selection is also critical to the performance of the entire system. Always use the fluid recommended by the machinery manufacturer.
There are many types of hydraulic pumps used in our industry; the piston pump being the most common. A piston pump utilizes a group of small pistons attached to a swash plate. The swash plate revolves around a center shaft driven by an electric motor. These pistons move fluid under pressure to the pump outlet. In addition to the pump inlet and outlet, a pump will also have a case drain. As the pump spins, some of the fluid is used for lubrication. That fluid then leaves the pump cavity via the case drain hose and goes to the tank.
Most pumps have both volume and pressure adjustments. The volume adjustment limits the movement of the swash plate, changing the pump’s GPM. Adjustment is usually preset at the factory for maximum flow. The pressure adjustment is located on the pump’s compensator. The compensator maintains overall system pressure as the load to the system varies.
Accumulators are used to reduce line shock and assist the pump when going from a low to a full demand situation. Accumulators are pre-charged with dry nitrogen gas to approximately 60% of hydraulic system pressure. Two basic types are used, the bladder and the piston. The bladder style uses a rubber like bag, filled with nitrogen, inserted into an egg shaped container. As fluid enters the chamber, the bag compresses to equalize the pressure. If the hydraulic system drops, the accumulator bag expands and reduces the pressure drop.
The piston style has a piston that moves in a tube like container. On the top side of the piston is the dry nitrogen gas. On the bottom side is the hydraulic fluid. The piston moves up and down reducing pressure fluctuations. Too low or too high of an accumulator pre-charge will cause significant pressure fluctuations.
Control valves are typically classified as either manual, pressure sensing, or electrically activated.
A recirculation valve, or warm-up valve, is manual valve. The most common is a needle valve. The needle valve looks like a water faucet valve. Turning the handle moves the needle in or out changing the flow restriction. We use needle valves to warm the fluid as it passes the restriction. Another common type is the ball valve. As the name implies, a ball with a hole through it is rotated by a lever type handle. When the hole is aligned with the ports, free flow is achieved. Ball valves are usually more reliable than needle valves. Other than seal replacement for leaks, no maintenance is required.
Pressure sensing or relief valves are normally used as an overpressure setting device. When system pressure reaches the relief preset, fluid is “dumped” over the valve directing fluid back into the tank. Setting the pressure relief is easy. Turn the pressure relief adjustment all the way in. Then increase hydraulic system pressure to the correct set point. On Viking Duo-Matics 503 & 504 that setting is 1200psi. Then watch the gauge as you slowly back off on the pressure relief. When the gauge indicates a pressure drop, lock the pressure relief in place. Reset the system pressure back to 850-900 psi. The system pressure will not be allowed to climb past 1200 psi. A pressure relief setting that is too low will cause excessive heat buildup. A setting that is too high places other components at risk for failure.
Electrically activated control valves can be as simple as a directional on/off type valve or as complicated as a proportional valve. When power is applied to a directional on/off type of valve, the coil uses its magnetism to move a pin in the solenoid tube. This pin pushes on the spool, opening up valve ports to both pressure and tank.
A common error is to replace an entire valve when only the pin is damaged. The solenoid pin is designed to be softer than the spool. It is considered a wear item. Pins mushroom out causing the valve to stick. Pins also can wear to the point that the spool will not open the ports sufficiently, causing a slow condition in a device. Note: A quick way to check a coil is to test for magnetism when power is applied. Also most valves can be manually spooled by pressing in on the solenoid tube pin.
Proportional valves use varying voltage to change the flow characteristics of the valve. Unlike on/off valves, varying the control voltage varies the flow. This allows machinery manufacturers the ability to change hydraulic motor speeds without adjusting manual flow controls. Better control and higher machine performance is the result. Specially designed computer output cards send the varying voltages to the valve depending on the machine requirements. Other than replacing pins and cleaning, proportional valves can only be repaired by professionals.
Hydraulic motors convert fluid power into a mechanical force. Viking uses a motor designed specifically for the bed conveyor, board shuttle, and turner. Each motor has different speed and torque specifications matched to the machine operation. They are not interchangeable. Since these motors look identical, check the model numbers to ensure you have the correct motor for the application.
Hydraulic cylinders consist of the barrel, rod, and piston. Repairing external leaks with seal kits are common practice. Checking a cylinder for internal bypass is also very easy for a qualified technician. Apply fluid under pressure to the rod end of the cylinder. Loosen, but do not remove, the blind end connection. Fluid should not bypass the piston seals. If it does, the loosened blind end fitting will continue to leak. The same test can be done on a work bench with compressed air. Make sure the cylinder is emptied out and adequate precautions are taken.
Hoses are usually only replaced when they leak. But another common issue is internal hose failure causing a collapse which restricts flow. Removal and close inspection of the internal cavity of the hose should be part of your preventive maintenance program. Also check for soft spots and bulges that indicate hose failure.
Most systems run efficiently in an operating range of 125-180 degrees Fahrenheit. Heat buildup can be caused by internal bypassing and/or the inability of heat transfer to the environment. Internal bypasses can be checked by looking for hot spots prior to complete system warmup. Keeping tanks, hoses, and hydraulic tubing clean will help dissipate heat. Addition of fans and powered coolers will also keep things cool.
Replacing system fluids and filters are your best defense against contamination. Hydraulic fluids can cost $10.00 or more per gallon, so some manufacturers have begun to offer portable cleaning stations. These stations use highly efficient filtration systems. Easy to hook up, they use their own 110 volt powered pump to filter the fluid in the reservoir tank. This process extends the life of the fluids and components.
The hydraulically powered systems of today will provide long service life with minimal amount of downtime. Excessive heat and contamination are still the main causes of premature system failure.
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