Brendan's Hydraulic System tips for everyday use About the author
CONTENTS
When Your Hydraulics Go Bang
Hydraulic Troubleshooting
Here are some tips in which I'm going to explode five popular myths. Now, you may believe one or more of these to be true. So what I have to say will likely challenge your thinking on these issues. But all I ask is you keep an open mind.
Myth #1. Hydraulic pump inlet lines must have a strainer. Myth #2. Creep in a double-acting cylinder is caused by a leaking piston seal.
Myth #2. Creep in a double-acting cylinder is caused by a leaking piston seal.
Myth #3. New hydraulic oil is clean hydraulic oil.
Myth #4. Because oil circulates through hydraulic components in operation, no special attention is required during installation beyond bolting the component on and connecting its hoses.
Myth #5. All oil returning to the hydraulic reservoir should be filtered.
A major cause of rod-seal failure in hydraulic cylinders.
The Value of Hydraulic Symbols
Today I want to switch gears and talk about one ofmy favorite subjects - hydraulic troubleshooting. As I explain in my book 'Insider Secrets to Hydraulics',hydraulic troubleshooting involves a lot of science and a bit of art. While there's no substitute for knowledge, the right tools and experience, you can approach any troubleshooting situation like a pro - just by starting with this one step: Check and eliminate the easy things first. Now, in case you're thinking this advice is too obvious to be useful, consider this troubleshooting situation I was involved in recently: The machine in question had a complex hydraulic system, the heart of which comprised two engines driving ten hydraulic pumps. Six of the pumps were variable displacement and four of these had electronic horsepower control. The symptoms of the problem were slow cycle times in combination with lug-down of the engines (loss of engine rpm). The machine had just been fitted with a new set of pumps. The diagnosis of the mechanic in charge was that the hydraulic system was tuned above the power curve of the engines, that is the hydraulics were demanding more power than the engines could produce, resulting in lug-down and therefore, slow cycle times. The other possible explanation of course, was that the engines were not producing their rated horsepower. Due to the complexity of the hydraulic system, I knew that it would take around four hours to run a complete system check and tune-up. So in order to eliminate the easy things first, when I arrived on site I inquired about the condition of the engines and their service history. The mechanic in charge not only assured me that the engines were in top shape, he was adamant that this was a "hydraulic" problem. Four hours later, after running a complete check of the hydraulic system without finding anything significant, I was not totally surprised that the problem remained unchanged. After a lengthy discussion, I managed to convince the mechanic to change the fuel filters and air cleaner elements on both engines. This fixed the problem. It turned out that a bad batch of fuel had caused premature clogging of the engine fuel filters, which were preventing the engines from developing their rated horsepower. Had the relatively simple task of changing the engine fuel filters had been carried out when the problem was first noticed, an expensive service call and four hours of downtime could have been avoided. The moral of this story and troubleshooting lesson 101 is: ALWAYS check and eliminate the easy things FIRST.
TOP
Myth #1. Hydraulic pump inlet lines must have a strainer. A pump inlet or suction strainer is a 140 micron, mesh screen which is screwed onto the pump intake penetration inside the hydraulic reservoir. These strainers increase the chances of cavitation occurring in the intake line and subsequent damage to, and failure of the hydraulic pump. Piston-type pumps are particularly vulnerable. If the reservoir starts out clean and all fluid returning to the reservoir is filtered, inlet strainers are not required since the hydraulic fluid will not contain particles large enough to be captured by a coarse mesh screen. The main argument for istalling suction strainers is to protect the pump from debris that enter the reservoir as a result of careless maintenance practices. Fact is, nuts, bolts, tools and similar debris pose minimal threat to the pump in a properly designed reservoir, where the pump intake is located a minimum of four inches off the bottom. When you consider the damage that vacuum-induced cavitation can cause to a hydraulic pump, NOT installing a suction strainer is definitely the lesser of two evils.
I generally recommend removing and discarding all filters fitted to pump intake lines. But you don't have to take my word for it. If in doubt, consult the hydraulic pump manufacturer.
Myth #2. Creep in a double-acting cylinder is caused by a leaking piston seal. A popular misbelief about hydraulic cylinders is that if the piston seal is leaking, the cylinder can creep down. Fact is, if the piston seal is completely removed from a double-acting cylinder, the cylinder is completely filled with oil and the ports are plugged, the cylinder will hold its load indefinitely - unless the rod-seal leaks. What happens under these conditions - due to the unequal volume either side of the piston, is fluid pressure equalizes and the cylinder becomes hydraulically locked. Once this occurs, the only way the cylinder can move is if fluid escapes from the cylinder via the rod seal or its ports. If you grasp the theory at work here, you'll probably realize there are a couple of exceptions. The first is a double-rod cylinder - where volume is equal on both sides of the piston. And the second is when a load is hanging on a double-acting cylinder. In this arrangement, the volume of pressurized fluid on the rod side can be accommodated on the piston side. In this case a vacuum will develop on the piston side and depending on the weight of the load, this may eventually result in equilibrium that arrests further creep.
Myth #3. New hydraulic oil is clean hydraulic oil. New hydraulic oil straight from the drum, has a typical cleanliness level of ISO 4406 23/21/18. Now that number may not mean a lot to you, but it's four cleanliness code levels below that considered ideal for a high pressure, high performance hydraulic system. Looking at it another way, a 25 GPM pump operating continuously in hydraulic oil at 23/21/18 will circulate 3,500 pounds of dirt to the hydraulic system's components each year. To add hydraulic oil, and not the dirt, always filter new oil prior to use in a hydraulic system. This can be accomplished by pumping the oil into the hydraulic reservoir through the system's return filter. The easiest way to do this is to install a tee in the return line and attach a quick-connector to the branch of this tee. Attach the other half of the quick-connector to the discharge hose of a drum pump. When hydraulic oil needs to be added to the reservoir, the drum pump is coupled to the return line and the oil is pumped into the reservoir through the return filter. As well as filtering the oil, spills are avoided and the ingress of external contamination is prevented. Myth#4 Because oil circulates through hydraulic components in operation, no special attention is required during installation beyond bolting the component on and connecting its hoses. Nothing could be further from the truth, as this example illustrates: I recently conducted failure analysis on a hydraulic motor that was the subject of a warranty claim. The motor had failed after only 500 hours in service, some 7,000 hours short of its expected service life. Inspection revealed that the motor's bearings had failed through inadequate lubrication, as a result of the hydraulic motor being started with insufficient oil in its case (housing). After this motor was installed, its case should have been filled with clean hydraulic oil prior to start-up. Starting a piston-type motor or pump without doing so, is similar to starting an internal combustion engine with no oil in the crankcase - premature failure is pretty much guaranteed. Myth #5. All oil returning to the hydraulic reservoir should be filtered. True. With one VERY important exception: The case drains of hydraulic piston pumps and motors. Connecting case drain lines to return filters can cause excessive case pressure, which has a number of damaging effects. High case pressure results in excessive load on the lip of the shaft seal. This causes the seal lip to wear a groove in the shaft, which eventually results in a leaking shaft seal. The effect of high case pressure on in-line piston pumps is the same as excessive vacuum at the pump inlet. Both conditions put the piston ball and slipper-pad socket in tension during intake. In severe cases this can result in buckling of the piston retaining plate and/or separation of the bronze slipper from the piston, causing major failure. Under certain conditions, high case pressure can cause the pistons of radial piston motors to be lifted off the cam during outlet. When this happens, the pistons are hammered back onto the cam during inlet, destroying the motor.
A major cause of rod-seal failure in hydraulic cylinders. As a product group, hydraulic cylinders are almost as common as pumps and motors combined. They are less complicated than other types of hydraulic components and are therefore relatively easy to repair. As a result, many hydraulic equipment owners or their maintenance personnel repair hydraulic cylinders themselves. An important step in the repair process that is often overlooked by do-it-yourself repairers, is checking rod straightness. Bent rods place load on the rod seal causing distortion, and ultimately premature failure of the seal. So rod straightness should always be checked when hydraulic cylinders are being re-sealed or repaired. In most cases, bent rods can be straightened in a press. It is sometimes possible to straighten them without damaging the hard-chrome plating, however if the chrome is damaged, the rod must be either re-chromed or replaced. BUT a word of CAUTION before I go. Attempting to straighten induction-hardened rods can cause the hardened case to shatter - with the potential for serious personal injury and/or property damage. For this reason, before attempting to straighten any cylinder rod - make sure it hasn't been induction-hardened first.
A schematic diagram is a 'road map' of the hydraulic system and to a technician skilled in reading and interpreting hydraulic symbols, is a valuable aid in identifying possible causes of a problem. This can save a lot of time and money when troubleshooting hydraulic problems. If a schematic diagram is not available, the technician must trace the hydraulic circuit and identify its components in order to isolate possible causes of the problem. This can be a time-consuming process, depending on the complexity of the system. Worse still, if the circuit contains a valve manifold, the manifold may have to be removed and dismantled - just to establish what it's supposed to do. Reason being, if the function of a component within a hydraulic system is not known, it can be difficult to discount it as a possible cause of the problem. The humble hydraulic symbol eliminates the need to 'reverse engineer' the hydraulic circuit. As most hydraulic technicians know, there's usually a better than even chance that a schematic diagram will not be available for the machine they've been called in to troubleshoot. This is unlikely to bother the technician because it is the machine owner who pays for its absence. Where do all the hydraulic schematic diagrams go? They get lost or misplaced, they don't get transferred to the new owner when a machine is bought second and and in some cases they may not be issued to the machine owner at all. Why? Because generally speaking, hydraulic equipment owners don't place a lot of value on them. So if you're responsible for hydraulic equipment and you don't have schematic diagrams for your existing machines, try to obtain them - before you need them. And ensure that you are issued with schematic diagrams for any additional hydraulic machines you acquire.
And if you'd like to know how to read schematics like a pro, watch this video: http://www.hydraulicsupermarket.com/symbols
An intermittent and problematic source of noise in hydraulic systems - decompression. This problem arises because hydraulic oil is NOT incompressible.The ratio of a fluid's decrease in volume as a result of increase in pressure is given by its bulk modulus of elasticity. The bulk modulus for hydrocarbon-based hydraulic fluids is approximately 250,000 PSI, (17,240 bar) which results in a volume change of around 0.4% per 1,000 PSI (70 bar). When the change in volume exceeds 10 cubic inches (160 cubic centimeters) decompression must be controlled. The compression of hydraulic fluid results in storage of energy, similar to the potential energy stored in a compressed spring. Like a compressed spring, compressed fluid has the ability to do work. If decompression is not controlled, the stored energy dissipates instantaneously. This sudden release of energy accelerates the fluid, which does work on anything in its path. Uncontrolled decompression stresses hydraulic hose, pipe and fittings, creates noise and can cause pressure transients which can damage hydraulic components. Decompression is an inherent problem in hydraulic presses for example, due to the large volume cylinders operating at high pressures. Although hydrocarbon-based hydraulic fluids compress 0.4% - 0.5% by volume per 1,000 PSI, in actual application it is wise to calculate compression at 1% per 1,000 PSI. This compensates for the elasticity of the cylinder and conductors and a possible increase in the volume of air entrained in the fluid.
For example, if the combined captive volume of the hydraulic cylinder and conductors on a press was 10 gallons and operating pressure was 5,000 PSI, the volume of compressed fluid would be0.5 gallons (10 x 0.01 x 5). This equates to potential energy of around 33,000 watt-seconds. If the release of this amount of energy is not controlled, you can expect to hear a bang! Decompression is controlled by converting the potential energy of the compressed fluid into heat. This is achieved by metering the compressed volume of fluid across an orifice. Three different circuits for controlling decompression are illustrated and explained on page 130 of 'Industrial Hydraulic Control'. http://www.industrialhydrauliccontrol.com
About the Author: Brendan Casey has more than 20 years experience in the maintenance, repair and overhaul of mobile and industrial hydraulic equipment. For more information on reducing the operating cost and increasing the uptime of your hydraulic equipment, visit his web site: http://www.HydraulicSupermarket.com