Stregthen your machines' muscles
You can't overemphasize the importance of hydraulics in the proper functioning of today's equipment. Hydraulics are its muscles. They aid equipment in lifting, steering and driving systems. If you want your equipment--with its expensive initial cost and expensive potential repair bills--to last, it's important that you service these systems regularly. Doing so, you'll avoid early wear and breakdown.
Servicing your hydraulic system Your first step in servicing a hydraulic system is to look for the service-maintenance schedule in the owner's manual. Then follow these procedures while keeping good records. Typically, one of the first service procedures emphasized is maintaining the correct type, condition and level of hydraulic fluid. Use the fluid recommended for your system by the equipment manufacturer. The recommendation of what type of fluid to use is based on the system component that has the most severe tolerances.
Next, change the fluid and filter at the recommended interval. You sometimes need to shorten these intervals if your operating conditions are severe, such as in high dust or moisture. Some machinery owners and operators mistakenly believe that if the system is tight, then the fluid will stay good indefinitely. Wrong. Hydraulic fluid and its additives wear out over time and need replacement. Also, sources for contaminants aren't just from the outside environment. Some are "built in" during the engine's manufacture. In addition, the normal operation of components creates them.
As a result, the most important fluid and filter change is the first one. You should do it within the first 50 hours of operation to remove all debris from the original manufacture of the system and from early component wear.
Another form of contamination is condensation. This occurs when you put away the machinery while it is still warm after operation. Air within the hydraulic system holds moisture. When it cools down, that moisture condenses into the hydraulic fluid. The water in the system reduces the fluid's lubricating qualities. You can detect water by looking at the fluid. It usually has a milky color. Another quick test for water--the "Snap, pop, crackle" test--is to place a little of the fluid in a can and heat the can with a propane torch. If it makes a snap, crackle or popping sound, the fluid has water in it. (Note that, sometimes, milky looking oil can result from aeration, not water. So, before performing the heating test, let the fluid sit for 5 minutes. If the fluid has air in it, it will turn back into its normal color. Aerated oil can cause jerky and slow operation. The source for air entering the system is usually a leak in the inlet line to the pump. This leak must be found and sealed.)
When changing the filter, always use the manufacturer's recommended filter. Filters are designed to remove the correct sizes of particles and hold the correct amount of contaminants between changes. Some filters have an integral bypass valve. This valve is designed to open at the appropriate pressure and is large enough to let out the appropriate amount of fluid.
Another condition to check is the fluid's temperature. If the fluid gets too hot for extended periods, it can break down. Fluid that runs at too high of a temperature also can't lubricate as well. Therefore, a simple test you can perform to check whether your oil has gotten too hot is to smell it. If it smells burnt, you've got a problem. The solution usually lies in checking the hydraulic oil cooler. These are usually air-to-oil systems and consist of a radiator-type cooler or fins that surround the main reservoir housing on the hydrostatic drives. In either case, you must keep the fins or cooler clean. Wash them periodically. On some radiator-type coolers, a screen protects the system. If your system has such a screen, clean it daily.
Other components that need periodic service include hoses, lines, fittings and quick-disconnect couplers. Hoses can heat oil by restricting its flow if they are kinked or pinched. Other problems occur when hoses are routed improperly, causing them to chafe or rub on other engine components (see Figure 1, above). Damage also can occur from stretching, which eventually also can cause leaks. When a hose does fail, bring it with you to the repair shop. That way you can be sure you replace it with the correct length, type and strength of hose.
Leaking fittings and hoses don't just hurt your equipment engines, they can damage turf as well. For example, when pressure and return lines have problems, they can leak hot oil, which kills any plants with which it comes into contact. Tighten all fittings appropriately (see Figure 2, page G 40).
Kinked or plugged intake lines and hoses cause additional problems. They restrict oil flow into the pump and can cause cavitation. Intake-line holes or leaks also allow air into the system, causing additional aeration, which can destroy a pump.
A major source of hydraulic system contamination is through the remote quick-disconnect couplers. Most users don't keep them covered. Thus, after being connected, they are full of dirt. Proper procedure is to cover, plug and wipe these clean before connecting. Another solution is to replace them with a new style coupler, such as a flush-face coupler (see Figure 3, page G 40). These newer couplers do not leak as much when disconnected and have no place in which dirt can lodge.
One final component to which you should give periodic maintenance is the cylinder shafts. To keep them in proper operating condition, grease any of these that are exposed for any length of time.
Troubleshooting your hydraulics When troubleshooting, you must keep in mind that problems having to do with slow hydraulics are related directly to loss of flow and problems related to weak hydraulics are directly related to loss of pressure.
The first step in troubleshooting is to know where in the system oil flows and what each component's function is. The next step is to visually examine the system and check the obvious:
* Oil level
* Oil condition (its color, smell or obvious dirt particles)
* Filter condition
* Discolored paint (from components getting too hot)
* Linkage (loose, bent or misadjusted)
* Drive (disconnected)
* Engine (improper rpms, etc.).
The next step--if you weren't operating the machine when the problem occurred--is to operate the system and observe it yourself. Of course, only do so once you've determined from your visual inspection that it is safe to do so.
While performing these tests, consider other reasons for your problem. Move systematically through the hydraulic system, from component to component, following the oil's flow. Figure 4 (page G 40) lists potential problems that can happen between components (other than problems between the valve and the cylinder after the dashed line). Each of these problems affects only one function. For example, a leaking circuit relief valve on the lift or boom only affects that circuit. (Many technical manuals offer similar illustrations to help you through step-by-step troubleshooting procedures.)
A common problem: internal leakage A common problem on lift and steering systems is internal leakage. Leakage can lead to the oil overheating and losing pressure. Sometimes, you can detect this problem by operating the circuit you think is leaking. Test your theory by extending the cylinder all the way and holding it in that position. Doing so sends oil through the relief valve. This will heat up the oil and--if the circuit is leaking--the hot oil moving past the leaking valve or piston seal will therefore heat up that component. You then can detect the leak by feeling the outside of these components to check whether they are warmer than other hydraulic components in the circuit.
If this doesn't work, you can use another step-by-step procedure to determine internal leakage in a cylinder.
* To begin, operate the system and fully extend or retract the cylinder. Make sure you're not placing any load on the cylinders while you do so. This may determine whether you extend or retract the cylinder--either way is acceptable as long as the rod is at the end of its range of movement (fully extended or retracted). For example, a load would result from extending a cylinder to lift a loader's bucket. Instead, you would make sure the cylinders are fully retracted (for example, the bucket down) or, if you wanted them extended, removed from the loader so that no load is bearing on them.
* Shut off the engine and remove all pressure from the line by moving the valve (control) lever back and forth. (Make sure that no accumulator--a device that maintains pressure when the engine is shut off--is hooked into the line maintaining pressure.)
* Remove the return line from the cylinder and plug the holes from the valve. (Note that, on some systems, you should plug the hose from the valve because oil still may be available at the return line.)
* Start up the engine and continue to move the valve in the same direction as in Step 1. In other words, if the cylinder is fully retracted, operate the valve lever or control as if you wanted to retract it even farther (even though it can't actually move any more).
* Observe the cylinder for leakage from where you removed the return line. If you see any, the cylinder wall is scored, or the piston seals are leaking or both, allowing oil to flow past.
If components operate slowly, your pump may be worn out. If this is the case, oil slippage within the pump usually is extreme. A simple way to test this is to time how fast a cylinder extends or retracts. You can compare that number to the factory-specified time it should take a cylinder to retract or extend (as listed in your technical manual). Or, if your manual does not list such specifications, you can use the formula:
Cylinder volume (cubic inches) x length of stroke (inches) / time (seconds) x 0.26 = gpm
to determine the rate of movement (actually the flow rate, in gpm) for the cylinder. Figure 5 (table at left) helps you determine the cylinder's volume as long as you know the cylinder's bore and stroke. Compare this number with the flow rate (in gpm) your system should have if it were functioning properly
It is best to time the cylinder both with and without a load. Then compare these two times or flow rates. You shouldn't have any significant difference as long as the load doesn't cause the relief valve to open. A good rule of thumb is that the flow rate (time) should be at least 80 percent of the specifications.
Troubleshooting your hydraulic drive is similar to your lift and power steering. The main difference is that you actuate the motor rather than a cylinder. A motor with excessive leakage runs hot, slow and doesn't develop as much torque. You can test for internal leakage similar to testing a cylinder. For example, on a direct-drive reel motor, place a block of wood in the reel to jam the reel from moving. Be careful not to bend the blades. Take off the drain line and pressurize the motor. Keep track of the amount of leakage for 15 seconds and multiply this by four to get the amount in 1 minute.
You also can perform this test with belt-drive motors equipped with a drain line. Disconnect the return line and plug or cap it off while applying pressure to the other line. On hydraulic equipment where a hydraulic motor directly drives the wheels, you can accomplish this test by applying the brake while supplying pressure to the motor. If the drive motors have no drain lines, remove the opposite line that is being powered (return line) and take note of the amount of leakage. Most manufacturers indicate how much oil slippage or leakage is allowed before you must replace the motor (see Figure 6, above).
The hydrostatic-drive difference Hydrostatic-drive systems are different from hydraulic drives in the sense that the pump and motor are connected in a loop (sometimes referred to as a closed loop). The oil is sent to the motor and then back to the pump. The system allows some oil to leave the circuit due to slippage in the pump and motor. A set of replenishing valves (charge-check valves) replaces this with new oil (see Figure 6).
When a loss of drive happens in just one direction, the pump and motor are probably not in danger. Instead, a particular component is probably either leaking (such as the high-pressure relief valves or replenishing valves) or the shuttle valve is stuck in one direction. You can solve this problem by switching the high-pressure relief and replenishing valves one at a time and checking whether the problem follows the valves. In other words, if your test causes you to drive in the opposite direction, then whatever valve you switched is the problem.
Sometimes a leaking seal also can cause a loss of drive in one direction. If your equipment experiences a loss of drive or the drive is slow and weak in both directions, it's likely that both the motor and the pump are worn.
Another problem that occurs frequently in hydrostatic drives is creeping in neutral. To solve this problem, remove the drive linkage and adjust the hydrostat for true neutral. Then replace the linkage to maintain that neutral. In some older hydrostat models, however, it is virtually impossible to remove all the creeping.
These troubleshooting techniques don't cover every problem that can occur in a hydraulic system. And each system's uniqueness dictates the exact procedures you'll need to follow to solve your problems. Nevertheless, always refer to your technical manual before performing any testing. And remember always to test the simple things first.
Dr. Michael McCaskey is a professor in the Agricultural Engineering Department at the State University of New York--Cobleskill.
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