Presentation on theme: "Lubrication Nizwa College of Technology. Friction Friction is the force resisting the relative motion of surfaces or material elements in contact.force."— Presentation transcript:
Friction Friction is the force resisting the relative motion of surfaces or material elements in contact.force The ratio of the force of friction between two bodies and the force pressing them together is known as co-efficient of friction (μ) F f/ N = constant (μ) F f = μ N
Reducing frictional resistance Friction between moving parts in contact is unavoidable -but it is desirable to keep this to as low as possible Provide a fine or smooth surface finish. Replacing the one of the material having lower coefficient of friction. Introducing a fluid film which separates the parts slightly thus reducing the friction (Lubrication) Installing balls, rollers or needles between the sliding surfaces which replace the sliding movement by rolling action (Bearings)
Lubrication Lubrication is the process, or technique employed to reduce wear on surfaces in close proximity, and moving relative to each another Lubrication is effected by introducing a substance called lubricant. The lubricant film can be a solid, (graphite), a liquid, a semi solid (grease) or exceptionally a gas
Objectives of Lubrication To reduce frictional resistance To reduce wear on the bearing surfaces To protect the bearing surfaces from corrosion To carry away heat from the bearing To reduce noise from the moving components of the machine
Different Lubricating conditions Dry Lubrication- No lubricant present between the surfaces Boundary lubrication (Thin film) -This condition is found on machine slide ways (Lathe cross slide, milling machine table). No oil film Thin oil film
Full film lubrication- The surfaces are separated by thick film of lubricant and there will not be any metal-to-metal contact Mixed lubrication- Boundary and full lubricating conditions(Hydro dynamic) Thick oil film
Hydrodynamic lubrication (Lubricant is dynamic) At Rest Starts to Rotate (Mixed Lubrication) At Running (Full Film Lubrication)
Properties of Lubricant Viscosity - Resistance of a liquid to flow Oiliness- Ability of the lubricant to stay in place between the bearing surfaces Stability –Retention of properties as long as possible (Oxidization, Biological degradation, Dilution)
Types of Lubricant Animal oils - From animal fats Vegetable oils - From plant seeds Mineral oils - Hydrocarbons obtained from minerals Synthetic oils - From various chemicals Grease - Semi-fluid lubricant Dry - Molybdenum disulphide, Graphite
Additives The properties of a lubricant can be improved by adding certain chemicals Anti oxidants Foam depressants Corrosion inhibitors Detergent Dispersers Oiliness improvers
Selection of Lubricant The selection of lubricant depend on: Type of mechanism being lubricated Type of lubricating system Rate of heat removal Cost
Selection of Lubricant -typical examples FeatureLubricant Lubricating system Heat removal Maintenance cost Comments Plain bearing OilHandLowHighLight duty CirculatingHighLowContinuous PorousLow Low speeds GreaseHandNilHighLight duty Rolling bearingOilOil mistlowLowHigh cost Oil bathlowLowChurning SplashModerateLow Used in gear box GreasePackedNilLowLong life CentralNilLowConvenient GearsOilBathModerateLow Simple gear box Circulating HighLowHi-duty gears GreaseHandNilHighHi-duty gears Housing filledNilLowLow speeds
Lubricating Methods Oil can Oil gun Grease packing Wick type Bath type (Splash) Circulation (pressure)
Grease packing & Wick Type Grease Packing Wick Type Lubrication Bearing Shaft OilWick Housing Grease Ball bearings Grease Cap
Circulation (pressure) Oil Pump Relief valve To various Moving parts Oil filter Coarse Filter Sump Oil Filter Oil Pump Relief Valve
Gaskets& Seals Sealing is the process of preventing gases, liquids and solids escaping from containers or mechanisms. Seals can be broadly divided into two categories: Static seals- where there is no movement at the contact points. Dynamic or Mechanical seal - where there is movement between the contact points.
Selection of seal The choice of seal to be used on a particular application is determined by the following factors: Property of the material being contained Amount and speed of any movement Operating pressure Life expectancy Operating temperature
Seal Materials Non-metallic materials: Felt, rubber, asbestos, cork, plastics, P.V.C, paper, cloth etc., Metallic materials: Copper, lead, tin, plated steel, beryllium, and often a combination of several materials is used to make a particular seal or gasket.
Gaskets Gaskets are static seals used to prevent fluids or gases passing across mating surfaces, where there is no movement, on a mechanical assembly Gasket materials- cork, rubber, asbestos, plastic and metallic materials.
Fitting gaskets Important rules to follow when fitting a gasket Conformation The sealing pressure should be sufficiently high to make the gasket conform to the joint surfaces. Too little pressure can result in voids in the joint where leakage could occur. Too much pressure can result in distortion of both gasket and joint surfaces. Condition of joint surface The mating surfaces of the joint should be:- Free from burrs and indentations. ' Reasonably flat and mate together. Clean and free from oil, grease and other materials.
Reasons for leakage in the joint Wrong gasket material Incorrect fitting gasket Poor Joint surfaces No conformation Distortion of surfaces Inaccurate tensioning
Dynamic or Mechanical Seals Mechanical seals are used where there is movement between the parts being sealed Types Radial lip seal Exclusion seal Clearance seal Ring seal Face seal Diaphragm seal Packing
Radial lip seals Parts of Lip Seal Frame - stainless steel, Monel metal, titanium, bronze,hastalloy. Lip - rubber, nitrile, chloroprene, ethylene, propylene. Spring -stainless steel, monel, titanium hastallloy. Features Take up Little space Easy to install Low cost Accommodate slight misalignment. One of the most widely used.
Fitting mechanical seals Guide lines to fitting seals Make sure that the correct type of seal is used, particularly when replacing a seal Ensure that the housing shaft and seal is in good condition and free from dirt and other foreign matter Lubricate the surfaces of seal, shaft and housing if necessary Use a suitable assembly tool if needed Use a cone or sleeve over any sharp corners of the shaft Check seal for correct operation before applying power to drive After a short period of use inspect seal for leaks or other failures