Hardness –Ability of a material to resist surface abrasion Difficult to measure Measure the resistance of the surface layer to penetration by an indenter Brinnell Test Vickers diamond test Metals (properties) clean (luster) Solids under ordinary conditions Usually conduct heat and electricity Malleable –rolled or hammered Ductile –drawn into wires Hard/soft Low melting/high melting Polymers –Molecular solid Thermosets usually crosslinked Permanently hard, when heated do not soften but char Superior flexural hardness Thermoplastics Soften when heated their glass temp(tg) –shaped, harden on cooling Superior flexural and impact properties Wood 4
= TBNB NB = NC B C = TDND 1400 x 10 = 700 D 20 700 x 10 = 350 20 Components of a gear box Grub screw –Disconnects to protect the drum Pulleys or belts –Adjusts tension so they can slip or break Pinions/cogs –Made out of polymer Sacrificial Cheap or replacement is cheap –not noisy. Hydraulic systems An industry requires 1) Objects or substances to be moved 2) Force to hold shape or compress products The industrial prime movers use; Electricity (electrical equipments) Hydraulic –liquids Pneumatic –gasses Fundamental principles Pascal’s law states that pressure in an enclosed fluid can be considered uniform throughout the system. P=F/A = Force Area 5kg 1.5m2 8kgf F37500f 2000kgf
Materials of construction. Cast iron properties Resistance to compression Resistance to chemical attack Heat conduction Advantages of cast iron in sammying machine Compression of rollers Squeeze water out Resistance to stress Advantages of cast iron in fleshing machine Resistance to sulfur corrosion attack Resistance to other chemical attack Resistance to stress Machine safety legislation A comprehensive standard of ‘Tannery machines Safety requirements’developed in UK by the European committee for standardization, published as an EN(European Norm) standard making numerous references to EN standards that detail individual and more general safety requirements and systems. EN standards deal with design and function of safety devices, sometimes indicating alternatives. The device adopted must meet what is specified in the standard. Compliance with the requirements is mandatory in the member countries, but the member country machinery manufacturers tend to fit in the safety systems even when exporting their equipments to non member countries. it obviously is always the responsibility of the user to ensure ongoing effectiveness of the safety systems installed on all types of the equipment in operation, a regular check of the safety elements on machines, because of the complexity, should be scheduled. Vital areas to be checked in this way should include: Visual examination of the devices. Verify the interlocks and self checking systems are functional. Verify that safety switches operate correctly. Verify the time taken to stop dangerous motion after tripping device. Check guarding of electrical equipment. Verify that protective devices as such do not represent danger. 36
Shell Water Fire Fire tube tube boiler –Water is contained in small diameter tubes connected to water system and hot gases come into contact with the external surface. Water High heating surface good overall thermal efficiency Efficiency is improved by - Super heater Economizer Air heaters Straight tubes –Good for cleaning and replacement b) Bent tubes –More flexible in heating surface layout. –More difficult to clean and replace c) Forced circulation –Thinner and lighter tubes –Increases efficiency –More compact Hot boilers Heating boiler (sectional boiler) Domestic boiler