Power at Work: Cleanliness is essential when opening newer hydraulic systems for repair
Friday, February 29, 2008
With today's more sophisticated hydraulic systems and tolerances measured in microns, rigorous attention to cleanliness is the order of the day
by RALPH WINFIELD
Some of us started our "mechanics training" in the 1950s, when tolerances between metal components were not as close nor as critical as they are today. Neither diesel tractors nor hydrostatic drive units entered the small village garage of that era.
I learned to do valve jobs, piston ring and bearing replacements - and even repair tires. Clean up involved a putty knife, a can of gasoline, a brush and an old sheet or shirt for wiping.
Do remember that in the 1950s, if an engine had an oil filter, it was probably of the alternate-flow type.
If the filter became plugged, the engine oil bypassed it. We bought a nearly new 1955 Pontiac Laurentian car with the small V-8 engine. That engine did not have an oil filter! After 90,000 miles, with the oil changed regularly, that car simply got a rebuilt engine - as was common practice at that time.
In the late 1950s and early '60s I worked with my brother repairing heavy construction equipment and particularly industrial diesel engines. Cleanliness took on a whole new meaning. If a diesel injection pump, or injectors, required servicing they were taken to a service provider who maintained a "clean room." All component fitting and assembly took place under clean diesel fuel. The air in that room was also filtered to eliminate airborne contaminants.
The diesel fuel provided lubrication and, more importantly, ensured that the two pieces being fitted together were at exactly the same temperature. Tolerances (clearances) between metal components were measured in microns, not the thousands of an inch that we used in the 1940s and '50s. Please remember that 1/1000 of an inch is equal to 25.4 microns.
By the 1970s, we started to use much more sophisticated hydraulic systems. We adopted - or should that be demanded? - power steering, hydraulic motors, flow dividers and priority valves on tractors and combines.
Along with that came the hydrostatic drive systems for most combines, some tractors and swathers. Again, clearances or tolerances went way down, thus requiring much cleaner hydraulic fluids. We had to start filtering hydraulic fluids with five- or 10-micron filters. Ideally, hydraulic fluids should be filtered to even lower particle sizes (e.g. two microns), but we do have to be careful that we do not remove essential additives that are put in hydraulic (hydrostat) fluids.
Pistons in hydrostat pumps and motors are commonly fitted with about two microns of clearance. If that clearance gets into the 12-micron range, the pump or motor must be replaced because the fluid bypass rate (leakage to case) will be too great for good performance. An early sign of excess leakage to case is often overheating of the hydrostat fluid. Please note that 12 microns of clearance is only about one half of one thousandth of an inch (or 0.0005 inches)!
The need for cleanliness. During initial assembly of present-day hydraulic systems, and when opening the systems for repairs, great care must be exercised to eliminate contamination of the components and the fluids.
When you buy hoses or fittings for newer, more sophisticated hydraulic (hydrostatic) systems, you will note that they are capped on both ends. Leave those caps on until connection is imminent. If caps are removed beforehand, dust particles in the air will contaminate the components and eventually the system, causing sticking control valves and excess wear of close fitting pistons in hydrostatic drive pumps and motors.
I am assuming that each of you have seen those small dust particles floating in the air when the early morning sun enters through an east window into a relatively dark space, such as a basement. It really does make you appreciate your nose hairs that prefilter that dusty air!
In the 1990s, I had the privilege of teaching a series of hydraulics courses to factory assemblers of hydrostatic drive equipment. That experience was great for both parties. Many of the assemblers had not previously been shown the inside parts of the sophisticated components they were connecting. Cleanliness took on an entirely new meaning.
Most farm equipment dealer shops have a high-pressure wash area. When a tractor or combine comes in for repair, its first visit is to that wash area. This is good, but only if the pressure washer is used correctly. It is absolutely critical that a high-pressure water stream not be directed at shaft seals or hydraulic cylinder wipers. Water can enter the gear oil, or hydraulic fluid.
Many drive systems and hydraulic systems are compromised by water entry during normal clean-up activities on the farm or by machine operators of such things as custom spray rigs. High-pressure washers are great devices, but only when used with discretion!
On many occasions, field repairs are essential if the machine cannot be moved readily. I was a party to many field repairs of final drives and transmissions in my early years. In most cases, mechanical-type repairs could be made successfully in the field. Some care and discretion had to be exercised in an attempt to keep dirt out of opened components.
I recently witnessed the field repair of a mechanical gear-box which powered all the hydraulic pumps of a fully hydrostatic field-going harvester. The mechanics had taken reasonable care to plug (not cap, unfortunately) all of the opened hydraulic connectors. However, no attempt had been made to clean the gear-box, pumps and hydraulic connections prior to disassembly. This could have been achieved with an engine-powered pressure washer, which is readily available from rental companies.
A short time ago, I purchased a new subcompact tractor with a hydrostatic drive system and hydraulics powered by the gear type charge pump of the hydrostat system. The hydraulics did not work well from day one.
After some serious encouragement, the dealer made significant component replacements in the hydraulic system. Based on comments and observation, I am convinced that foreign, abrasive materials were in the hydraulic system when assembled. Unfortunately, this does happen. The good news is that the tractor is working fine now.The need for care. If you must open a hydraulic or hydrostat system, even if it is just to replace a hose that has been physically damaged, do take care. Clean around all connectors. Have clean plastic caps available to cap or plug all opened fittings. Make sure the components you buy are clean and capped. Even most responsible hose suppliers who crimp ends onto bulk hose now have cleaning systems available.
After the hydraulic system has been repaired and test run, change the system filter(s). Filters are cheap! If you don't think so, just price the components you are protecting by changing the filters.
By the way, if you have a number of hydrostatic drive systems in your equipment fleet, please invest in apump and filter unit so that new fluid can be filtered into the system. The filter should be of the same type and size as the one in the machine system. New fluid is never too clean and the system fluid does not get filtered until it is returning to the tank or reservoir.
A good filter, if located between the reservoir and the pump, is likely to cause cavitation to occur in the pump when the filter becomes clogged or the system is operated in cold weather. Cavitation causes catastrophic system failures! When the pump fails, metallic particles will contaminate the entire hydraulic system. Relatively course strainers (not filters) are often located between the pump and reservoir. They only remove large contaminants the size you see.
Last but not least, please buy lint-free shop clothes. Old bed sheets and shirts can also be a source of hydraulic system contamination. We did use them as wipers back when systems were much more tolerant. BF
Agricultural engineer Ralph Winfield farms at Belmont in Elgin County.