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Unit Thirteen Hydraulic Fluid Conditioning. The illustration is that of a “power unit” which includes the reservoir. Hydraulic Fluid Conditioning The Importance of Filtration.
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Unit Thirteen Hydraulic Fluid Conditioning The illustration is that of a “power unit” which includes the reservoir.
Hydraulic Fluid Conditioning The Importance of Filtration Maintenance of hydraulic oil is critical to the service life of a machine. Clearly, over 80% of all machine failures can be traced back to contamination of the hydraulic oil. Because of the costs associated with the loss of production time, proper maintenance of the hydraulic and lubricating systems becomes as critical an element in design as the selection of the components that make up a system.
The Purpose of Hydraulic Fluid Transmit Power Dissipate Heat Lubricate Moving Parts Seal Clearances between moving parts
Hydraulic Reservoirs Reservoirs should be fabricated in accordance to NFPA standards and include all the individual components above.
Sizing a Hydraulic Reservoir First of all, heat from a hydraulic system is generated when energy is consumed and no work is being done. Relief valves, flow controls, and pressure reducing valves are examples of heat generating devices but there are more. In sizing a reservoir it is generally a good idea to oversize slightly to ensure adequate heat dissipation. The general rule is that minimum reservoir capacity should be 3 times the rated flow of the pump plus 10 percent extra for air space. The extra 10 percent is to ensure room for expansion of fluid level and escape of air. For more information see: “Design Engineer’s Handbook” by Parker, page H-3
Basic Parts and Operation • Reservoir Capacity: Three times rated flow of pump. • 2. Mounting Plate: Facilitates a secure surface for mounting the prime mover.
Basic Parts and Operation 3. Flexible Motor Coupling: Connects prime mover to hydraulic pump. Allows for variances in alignment. 4. Filler Breather Cap: Allows for adding of fluid, escape or entry of air, and filtration of air. 5. Baffle: Forces return fluid to travel further before suction to ensure greater fall out of contamination and escape of air.
Basic Parts and Operation 6. Reservoir Legs: Allow for mounting and circulation of air. 7. Additional Return and Drain Lines: Allow for machine modifications. 8. Oil Level Indicator: To allow monitoring of fluid level.
Basic Parts and Operation 9. Cleanout Covers: Allows easy access for removal of contamination. 10. Drain Plug: Allows for draining prior to cover removal. 11. Reservoir Tank: Holds fluid and acts as a conduit by which heat escapes to surrounding air. Material is usually SAE1020 but aluminum is becoming popular due to its unique heat transfer characteristics.
Reservoir Types Conventional Overhead L-Shaped Conventional reservoirs, also known as “flat tops,” are by far the most common designed for motors of 7.5 to 40 horsepower. The overhead design allows for positive head pressure while being able to accommodate more than one pump. The L-shape design offers positive head pressure and accommodates motors of 40 to 50 horsepower.
Coolers Basic Symbol Air Cooler Water Cooler The primary function of an auxiliary cooler is to dissipate the heat that is not removed by the reservoir alone. Coolers are easy to add to a system after initial installation and can greatly reduce system temperatures.
Affect of Dirt 1. Interferes with lubrication. 2. Interferes with heat transfer. 3. Interferes with tiny openings and clearances.
Micrometer Scale A micrometer is one-millionth of a meter. Twenty five micrometers is equal to .001 inch. With good vision, the human eye can see down to 40 micrometers. A grain of salt is about 100 micrometers in diameter. A human hair measures about 70 micrometers in diameter.
Fluid Cleanliness The “cleanliness” of oil is a specific term, measurable, and is defined by ISO 4406. Generally speaking, the human eye cannot be relied upon for determining the cleanliness of hydraulic oil. Automatic particle counting is the most effective means of determining the amount, average size, and type of contamination in oil.
Filter Elements Filter materials used for cleaning hydraulic oil are usually of the absorbent type. They are further divided into two categories: surface and depth. Surface type elements, composed of stainless wire mesh, are used primarily for coarse filtration such as suction strainers. Depth type elements are usually used for fine filtration and are composed of a thickness of material in which dirt particles become trapped as they try to weave around the media.
Filter/Strainer Symbol The schematic symbol for a filter is the same as a strainer and is drawn the same way in pneumatics as well as in hydraulics.
Nominal and Absolute Rating Nominal and Absolute are terms used to describe the way a filter is constructed rather than how it filters. A nominal rated filter simply indicates an average “pore” size and that some particles bigger than the pore size will probably get through but certainly smaller ones will. An absolute rating indicates the exact pore size of an element such as a wire screen and that a certain diameter particle will be stopped. The problem with this rating is that it doesn’t take into account a particle that is long and narrow that may get through.
Beta Ratio The beta ratio is a filter efficiency rating that was taken from the “multi-pass” test. In the multi-pass test, fine dust of a certain micrometer size is sent through a filter element several times. After several passes, a comparison would be made between upstream and down stream contamination with the difference being the amount of contamination trapped by the filter. The two were divided(upstream divided by downstream) and the result was a percentage of particles trapped. The greater the percentage of a known size, the greater the filters efficiency at that value. Example: If 100 particles of a known size are circulated through a filter and 75 of them are trapped, the beta is 4 or 100 divided by 25, the amount left uncaught, equals 4 or 75% efficiency.
Filter Ratings in Practice Filter ratings can be manipulated or misunderstood very easily so the rating itself is no guarantee of how well a filter can remove a certain size particle. It is recommended that the component manufacturer or a reputable filter manufacturer be consulted in determining which type of filter is best suited to a particular application.
Sources of Dirt Dirt is any insoluble material circulating in a hydraulic or pneumatic system There are three main sources of dirt: 1. Built in- during manufacture or assembly. 2. Ingressed or added by dirty makeup fluid or environmental defective seals. 3. Self-generated by failing components such as bearings or other wear surfaces.
Proportional Flow Filter Examine the filter and its connection to tank in the above illustration. Notice that part of the flow can go through the filter and part would go over the relief valve. The amount of oil filtered would depend on system pressure and flow. Cylinder speed would be affected.
Full Flow Filtration A full flow filtration filter is one in which all the system’s oil must pass through the filter element as it travels through the system. This type of filter is by far the most common in use today.
Parallel Flow Filtration In this method of filtering, the machine runs continuously while oil is being cleaned. Even when the machine is shut down for maintenance or repair, oil is being filtered. Notice that the filtering system incorporates its own pump.
Location of Filter and Pressure Location and type of filtration is determined by demand. Oil coming from the reservoir should have almost no restriction so filtration will be minimal. Oil returning to the reservoir will be warm and contain particles picked up from the system components. Sensitive components on the high pressure side of the system will generally use a filter with no by-pass check valve.
Filter Bypass Valve Most filters, except high pressure, use a by-pass valve to protect the system from running dry in the event of filter blockage. The thought here is that dirty oil is better than no oil.
Filter Maintenance Although filters may come with a “by-pass” indicator, this is not a preferred method of monitoring filters because if the indicator is showing by-pass then the damage has already been done. Some types of indicators include a needle, warning light, and sound alarm. When servicing filters, it is best to consult the manufacturer’s recommendations but in most cases the hours of operation should dictate the replacement as this is the only reliable means of ensuring peak system performance.
