Filters
Definition : Filters are the mediums in oil hydraulic system used to arrest contaminants (impurities) and purify the oil to improve the operational efficiency of hydraulic components.
Need/function of the filter :
● The hydraulic oil may contain various contaminants which may result in wear, corrosion, poor performance and ultimately failure of the equipment.
● Hence it is necessary to use filters to remove contaminants and supply pure oil to the system. The function of the filter is to separate out contaminants from the oil and clean it.
● The various contaminants present in the oil are :
(i) Welding scales, metal chips, tapping compound
(ii) Sand particles, bits of seal materials, wear particles
(iii) Sludge, rust, water, dirt, biological materials, etc.
Principle of filter:
● Oil filter is a hydraulic element which removes impurities from the oil and supply clean oil to the system.
● Most of the filters work on the principle of trapping the particles in small holes or pores. The fluid may pass through filter element and particles above a certain size become trapped.
Effects of Contaminant in the oil :
1. It will cause chocking or blocking of components
2. It will cause friction/ wear of components
3. Poor performance of system
4. Reduces life of components
Classification of Filters :
Oil filters are classified on the basis of:
(a) According to construction of filter medium
i) Surface filter
ii) Depth filter
(b) According to quantity of oil to be filtered
i) Full flow filter
ii) By-pass filter
(a) According to construction of filter medium:
(i) Surface filter : It is a thin screen of filter material.
(ii) Depth filter: It is a thick layer of material.
(b) According to quantity of oil to be filtered:
i) Full flow filter: The entire oil flow is filtered
ii) By-pass filter : Only a portion of oil is filtered.
(a) According to construction of filter medium
Surface filter :
● Surface filters are normally constructed from thin sheets of material folded in many sections and turned into multi pointed star shape as shown.
● There is large surface area available in a small space. The filter is fitted inside a bowl.
● The thin sheets full of pores/holes traps the solid particles as the oil passes through them.
● The material used for surface filters are cellulose,woven, steel fibers, woven nylon fibers.
ii) Depth filter:
● Depth filters are constructed with a thick layer of material with small passages through which the fluid must pass.
● The passages may be formed from granules compacted into thick cylindrical layer or fibers compacted into a tube.
● The particles become trapped in the passages and oil is filtered out.
● The capacity of depth filter is more than the surface filter as well as it also arrest finer materials which can not separated by surface filters.
● The filter materials used in depth filter are cellulose, synthetic fiber, metal fibers, glass fiber, sintered metal granules, etc.
(b) According to quantity of oil to be filtered:
Full flow filter:
● As name indicates, full flow filter is used to filter out the entire flow of oil from inlet to the outlet of filter.
● Full flow filter which consists of thick depth filter element formed into cylindrical shape and it is placed in a bowl.
● The filter has inlet for impure oil and outlet for clean oil.
● The size is such that it can allow entire flow of oil for filtration.
By pass filter :
● When only a portion of the oil is passed through the filter element and main portion of oil is directly passed without filtration. It is called as by pass filter.
● In this filter, due to pressure difference between the inlet and restricted passage, a portion of oil is drawn towards the filter cartridge for removing contaminants.
● By pass filter mounted with a by-pass valve. The valve opens to bypass the large quantity of oil and only small portions is allowed for filtrations.
Advantages of by pass filter:
1. Filtering is independent of system in operation.
2. Easy replacement of filter element without shut down the main system.
3. Less down time, less maintenance.
4. Cheaper.
5. High dirt retention capacity.
Magnetic Filter:
● Magnetic filter uses a permanent magnet to attract ferrous metal particles in the oil. They are in the form of a simple plug to long magnetic pole suspended inside the reservoir.
● They are normally fitted in tanks to conventional filters and attract ferrous metal particles such as wear particles, metal chips, etc.
● They are used as a secondary filters.
Filter Materials:
● Filter materials are fixed pore type on non fixed pore type. Fixed pore materials are preferable to non fixed type.
Location of Filter:
1. Intake or suction filter : A full flow filter may be placed before the pump so that, it can restrict the contaminants before entering in the pump through suction line.
2. Return line filter : When oil is circulated through various components and may contain wear particles, hence filter is located in the return line to separate them before going to the reservoir.
3. Pressure filter: A pressure filter is used at the pump outlet to prevent entry of contaminants generated in the pump, into other components such as valves, actuators, etc.
4. Off line filter : Sometimes a separate pump is used solely for the purpose of circulating the fluid through a filter with little pressure. This filter independent of the main system is called off line filter.
Seals and Gaskets :
Definition: Seals and gaskets are the fluid power Elements which are used to reduce leakage effectively at tactical dynamic portions of hydraulic components.
Importance of Seals and Gaskets:
● Leakage is a common problem which is to be prevented to keep the system at optimum efficiency.
● It is very difficult to design an absolutely leak-proof system due to limitations of fluid power elements.
● Wear and tear of moving components increases leakage chances.
● Leakage in a hydraulic system may be internal/external and tiny seal plays important role to prevent leakage effectively.
● Seals are used in both static (stationary parts) and dynamic (moving components) portions of hydraulic components.
● Static seals are used for stationary mating parts between valve bodies, pumps, reservoirs, pipes, couplings, etc.
● Similarly dynamic parts such as pump, hydromotor shafts, bearings, cylinder piston and wall, valve spools, having motion are protected by dynamic seals.
Functions of Seals :
Seals are basically used to :
(a) Prevent both internal and external leakage of fluid.
(b) Prevent entry of dirt, dust and other contaminants in the system
(c) Improve efficiency of the system.
(d) Maintain oil pressure by minimising pressure drop due to leakage.
(e) Prevent loss of fluid from the system.
(f) Enhance working life of hydraulic components and functional reliability over a longer period.
Classification of Seals:
1. As per method of sealing:
(a) Positive Seals : The seals prevent even a minute amount of fluid leakage. E.g. Piston seals.
(b) Non positive seal : It may allow a small amount of internal leakage for lubrication purpose. E.g. spool movement in valve body.
2. As per area of application :
(a) Static seals : They are used between stationary parts.
(b) Dynamic seals : They are used between the surfaces of moving hydraulic parts.
3. As per shape of the seal :
(a) O-ring
(b) T-ring
(c) V-ring
(d) U-cup
(e) Quad-ring
(f) Flat ring
4. As per seal material :
(a) Leather seal
(b) Metal seal
(c) Polymer seal
(d) Plastic seal
(e) Elastomer seal
(f) Asbestos seal
(g) Nylon seal, etc.
1. Static Seals:
Definition : When seals are placed between surfaces of stationary (non moving) components, it is known as static seal.
● The seal is made of soft material is usually compressed between the surfaces and force the seal material into the small imperfection in the surfaces, with the help of fasteners.
● Static seals are in the form of gaskets cut out of thin sheets of material and placed between mating surfaces which are then squeezed together by screws or bolts.
● In static seals no relative movement of the mating components is allowed. A perfect static seal is difficult to achieve perfectly over a period of time due to vibrations, shocks, mechanical load, change in pressure, etc.
● The static seal is in the form of flat gasket, O-ring, U ring or washers. Ring used in pressure cooker is also a example of static seal.
2. Dynamic Seals:
Definition: When seals are used between the surfaces of hydraulic parts where motion occurs and controls both leakage and lubrication are known as dynamic seals.
The dynamic seals are divided into two types
(i) Sliding seals:
● Sliding seals are mainly used with cylinders to prevent fluid escaping around a piston rod or from passing from one side of a piston to the other.
● Generally sliding seals are ring type may be solid ring like O-ring or rings with rectangular sections. U-ring, cup are also used.
● The examples of sliding seals are:
(a) U-ring seal for pistons : It prevents fluid passing from top to the bottom of the piston.
(b) Cup seals : They are suitable for simple single acting cylinders with low pressure.
(c) Wiper ring for removal of dirt : They are used to wipe out oily dirt from the rod during movement from the cylinder.
(ii) Rotary seals :
● They are used on pumps and motors to prevent fluid leakage out through the gap between the shaft and shaft bearing.
● They are designed with a spring loaded lip which presses to the shaft.
Effect of Leakage on Hydraulic System :
(1) Loss in volumetric efficiency.
(2) Loss in system power.
(3) Lack of control and loss of sensitivity and accuracy.
(4) Generation of undesirable heat.
(5) Resultant system malfunctioning.
Factors for Seal Selection :
(1) Working pressure and pressure range.
(2) Environmental condition (Wet, Dry, Hot).
(3) Fluid medium.
(4) Type of application (static or dynamic)
(5) Temperature of the system.
(6 Functional reliability ad expected life.
Failure of Seal:
Seal failure is progressive in nature and it has following causes for its failure :
(a) Excessive clearance: Excessive clearance between moving parts causes extrusion of the O-ring and other rings and reduces the performance of the seal.
(b) Fluid temperature : High working temperature of fluid may cause seal abrasion.
(c) Damaged or worn out parts : Causes improper fitting of seal.
(d) Incompatibility of the seal material and oil : When oil is improperly selected with respect to type of oil.
(e) Excessive side loads/overloading : It causes wobbling of piston rod and dislocate the seals.
(f) Vibrations : Caused due to bent shaft, broken vane, misaligned or damaged bearing, defective coupling, etc.
(g) Low speed : It causes spiral failure of seals.
(h) Lack of lubrication : It increases heat and reduces life of seals.
Intensifier (Pressure Booster)
● Hydraulic intensifier or pressure booster is a device which is used to increase the intensity of pressure of any hydraulic fluid or water, with the help of the hydraulic energy available from a huge quantity water or hydraulic fluid at a low pressure.
● Some hydraulic machines require high pressure for working but this high pressure can't be obtained by using pump. Some of these hydraulic machines are hydraulic press, hydraulic ram and hydraulic lift etc. These machines require high pressure for this operation to obtain the required amount of pressure.
The Need for Hydraulic Intensifiers (or Boosters)
● In most of the hydraulic machinery used, the usual pressure of 80 to 100-psi may not be sufficient to operate certain spool valves and other mechanisms. To meet the need for a high pressure requirement for a comparatively short period of time, pumps and accessories are definitely not able to generate high pressure.
● But it is possible with hydraulic intensifiers which can increase the pressure from 100 psi to 40,000 psi, using small volumes of fluid.
● A hydraulic intensifier is mounted in between the pump and the working machine. It has two types one is single acting and other is double acting.
Construction and Working :
It consists of three main parts
1. Fixed ram :
● A hydraulic intensifier consists of fixed ram through which the water, under a high pressure, flows to the hydraulic machine.
● It is the inner most and smallest part of the hydraulic intensifier. It is surrounded by a sliding cylinder. The high pressure liquid is supplied to the machine through this fixed ram.
2. Hollow inverted sliding cylinder:
● A hollow inverted sliding cylinder, containing water under high pressure, is mounted over the fixed ram.
● It is the middle part of hydraulic intensifier i.e. it slides in between the fixed ram and fixed cylinder.
● This is the only moving part of this device. It slides under the action of hydraulic force. Sliding cylinder contains high pressure liquid which is supplied to it through the fixed ram.
3. Fixed inverted cylinder:
● The inverted sliding cylinder is surrounded by another inverted fixed cylinder which contains water from the main supply at a lower pressure.
● A large quantity of water at a low pressure from the supply enters the inverted fixed cylinder. The weight of this water presses the sliding cylinder in the downward direction.
● The water inside the inverted sliding cylinder gets compressed due to the downward movement of the sliding cylinder and its pressure thus increases.
● This high pressure water is forced out of the sliding cylinder through the fixed ram, to the hydraulic machine.
Valves :
● Hydraulic intensifier consists of four valves named A, B, C and D. 'A' and D' allows low pressure liquid from the supply into the device.
● The liquid comes in from valve 'D' goes into the fixed cylinder and the liquid goes in through the valve 'A' goes into the sliding cylinder. Valve is for exhaust purpose.
● The outlet valve 'B' is used to supply high pressure liquid to the outlet of intensifier which is attached with hydraulic machines.
Applications :
It is most commonly used where high intensity of pressure is required for lift the loads in hydraulic press, hydraulic ram, hydraulic cranes and hydraulic lifts etc.
