Newer hydraulic systems must be kept clean
Monday, January 5, 2015
Today's hydraulic systems are much less tolerant of contaminated hydraulic fluids than their predecessors. Fluid contaminate that cannot be seen with the naked eye can indeed jam and/or score the hydraulic pump cylinders or pistons
by RALPH WINFIELD
In the early days, starting in the late 1930s, tractor and machinery hydraulic systems were relatively crude. However, they sure did provide a very useful service by eliminating mechanical lift systems.
Some of us remember when you pulled a rope to activate a mechanical trip system to lift a trail plow or the grain drill. In the same time period, bulldozer blades were lifted by a cable system that was controlled by a combination winch/brake system on the rear of the dozer.
If you are old enough or go to vintage tractor shows, you might also remember that the J.I. Case Co. invented a mechanical trip system to lift row crop cultivators. Shortly after, International Harvester introduced a rather unique pneumatic "Lift-All" system that used engine exhaust gas pressure to lift the row crop cultivators.
The Ferguson system
In the late 1930s, Harry Ferguson developed the hydraulically-operated "Ferguson System" that lifted the three-point hitch plow. Shortly afterwards, three-point, rear-mounted manure loaders followed. It did not take long before the loader appeared out front, so it could be seen more readily.
Yes, I remember operating one of these. They worked well as they very effectively transferred the weight to the drive wheels, but you sure needed front-end weights on those little Ford tractors so you could keep the front wheels on the ground.
Fast forward into the 1940s. Every tractor manufacturer provided a hydraulic pump mounted externally or internally to operate a remote ram or a hydraulic front-end loader. All of those early hydraulic systems used relatively crude piston or gear-type hydraulic pumps that would tolerate some abrasive materials in the hydraulic fluid. But times have changed!
Volume and pressure. Requirements of both volume in gallons per minute (litres per second) and pressure in pounds per square inch (kilopascals) changed significantly to match the needs of the owners and operators of tractors and combines. We all demanded power steering in addition to the other work functions.
Thus, hydraulic systems became much more sophisticated and much less tolerant of contaminated hydraulic fluids. The allowable clearances in pumps and control units shifted from thousands of an inch to microns. Can you visualize the difference? One inch is 2.54 centimetres or 25.4 millimetres. A micron is one thousands of a millimeter. Thus there are 25.4 microns in one thousands of an inch.
Let's put that into perspective. You can see an item that is effectively one thousandth of an inch, but you certainly cannot see one is that is one twenty fifth of that. Where am I going with this?
Newer variable output piston pumps have piston to body clearances of a few microns. They do not have any other means of controlling bypass flow of the pistons, like the control rings that were on the piston in the old Ford or Ferguson hydraulic systems. Thus, fluid contaminate that cannot be seen with the naked eye can indeed jam and/or score the hydraulic pump cylinders or pistons.
Hydrostatic drives. Most of the early hydrostatic drives on units like combines were self-contained and were not affected by external fluid contaminants coming in from hydraulic cylinders and other exposed components. However, we are now seeing a high number of self-propelled, high-clearance field sprayers using fully hydrostatic drive systems. These systems are much more sophisticated. They have taken us into a whole new level of need for fluid cleanliness.
These hydrostatic drive systems are of the closed-centre type with multiple small pistons in both the pump and the motors for each wheel. Because of the high speed and pressure, the tolerances or clearances of the "selectively fit" pistons are very small – only a few microns. So fluid contaminants invisible to the naked eye can cause serious problems.
High-efficiency fluid filters with a high Beta rating are essential. Beta ratio (x) = particle count in upstream oil/ particle count in downstream oil where x is the given particle size in microns.
To put the importance of the Beta ratio into perspective (and it is usually printed on filter casings), a ratio or rating of two indicates that the efficiency of the filter is only 50 per cent. By contrast, if the rating is 10, the efficiency increases to 90 per cent, or 99 per cent when the ratio is increased to 100. You can see from this example how important it is to buy filters with higher Beta ratios for hydraulic fluids, as well as for engine oils.
But, remember, on cold start-up these highly effective filters will be bypassed through a pressure relief valve in the top of the filter housing. This is essential to prevent burst filters, which would create disastrous results in that most of the collected contaminants would be released back into the hydraulic fluid. This relief pressure is usually preset at about 25 pounds per square inch (psi) or 172.4 kilopascals (kPa).
Some equipment will have a small pressure gauge installed at or near the filter to indicate that the filter is becoming plugged with contaminants. If the set pressure limit is exceeded, the filter will be bypassed continuously, a very bad situation. Some even newer power units have a red light on the dash warning the operator that the filter is being bypassed.
Heating hydraulic fluid. In some equipment that is started frequently in cold conditions, an electric (or fuel-fired) heater can be installed to preheat the hydraulic fluid prior to machine startup, just the same as if you use an engine coolant (block) heater.
A word of caution is required here. If you choose to install a hydraulic or any oil heater do not use immersion-type water-heating elements. Their watt density is too high for oil and will effectively destroy the lubricity of the fluid. Use only low-watt density oil-heating elements, usually available from the equipment manufacturer.
Some early production tractors equipped with power-shift transmissions came with immersion-type electrical heaters in the transmission as well. They should be used whenever the block-heater is used.
CVT transmissions. Many new high-horsepower tractors are coming with Continuously Variable Transmissions (CVT). The principle is great since forward speed can be varied without the step-and-jerk of powershift transmissions.
Please be aware that many of these transmissions have a built-in, closed-centre hydraulic system that requires a very high level of fluid contaminant control.
Pea/bean harvesting units. Many of these harvesting units require infinite speed control to maximize their field efficiency (hourly/daily throughput). Note that, in the drive section of the one pictured here, all of the diesel power input goes directly to a series of hydraulic pumps. A hydraulic activator powers every moving function of these harvesters. Hydraulic activators are primarily motors for rotating components, but cylinders are also used for lifting.
Selective motor control devices are designed to permit very low field speeds and much higher transport (road) speeds for movement between fields.
To sum up, remember that hydraulic fluid cannot be filtered, only screened, before it enters the hydraulic pumps that now contain very close-fitting components. A restrictive filter will cause cavitation that will destroy the pump. Therefore, it is often a wise idea to filter even new hydraulic fluid while it is being added to an equipment reservoir. This action can be made relatively simple by using a manual or an electrically-powered pump to force new fluid through a filter of exactly the same Beta rating as the one on the machine.
While fluid suppliers do make a concerted effort to provide clean fluid, it is often not clean enough for the new and sophisticated hydraulic systems that we are now seeing on today's tractors and other harvesting units. If you have doubts, take a close look in the bottom of the last 10-litre jug of hydraulic fluid you slowly poured. If you can see sediment, some of it will have been added to the reservoir! BF
Agricultural engineer Ralph Winfield farms at Belmont in Elgin County.