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EG 269 Design of Machine ElementsOctober 26, 2016Swansea UniversityCollage of EngineeringEG 269 Design of machine elementsProject – Guided Gear Design for an EscalatorBackgroundIt is necessary to for you to design a gearbox system for a passenger escalator.For design purposes, the escalator may be considered as steps mounted on twoendless chains, where each chain passes over two 250 mm pitch-circle-diameter(PCD) sprockets with a centre distance of 11 m, and a difference in height of 6.5metres, as shown in Figure 1.The escalator has 28 steps, each 800 mm wide, which are pin jointed to thechains and carried on fixed slide ways coincident with the full length of bothchains. The weight of each step and its portion of the chain is 27 kg. The escalatormust be capable of moving fully laden with passengers up the incline atapproximately 0.8 ms-1, continuously for 10 hours per day, for 300 days peryear. It can be assumed that, when fully laden, each step is supporting twopeople each with a weight of 75 kg. The coefficient of friction between the stepsand the slide is 0.1.The drive for the escalator is provided by a geared motor with an output speed of200 rpm through a single reduction helical gearbox. Flexible couplings are to beused between the geared motor and gearbox, and between the gearbox andescalator. These may be assumed to be capable of transmitting torque, but notbending moments. In designing the gearbox, it is advisable to make an allowancefor a 50% overload. A schematic diagram of the drive system is given in Figure 2.Prior KnowledgeA colleague has provided a preliminary estimate for the design of the gears andhas suggested a module of 4 might be appropriate. This colleague also pointedout that in order to avoid the need for undercutting of the gear teeth, a minimumnumber of 18 teeth would be required on either gear. However, these arepreliminary estimates, and they will need to be checked in your analysis.Design ConstraintsThe gears are to be manufactured from EN27 steel forgings and will have astandard involute profile, with a 20pressure angle and a helix angle of 30.Assume 95% efficiency of gears. The shafts are to be machined either fromcarbon steel or from alloy steel. For carbon steel, a yield stress of 280 MPa maybe assumed. For alloy steel, a yield stress of 460 MPa may be assumed.
EG 269 Design of Machine ElementsOctober 26, 2016Description of TaskAs part of your design, you will need to undertake the following tasks:Estimate the loads imposed on the escalator;Calculate the torque required from the gearbox;Calculate the gear forces;Determine the face width of the gears;Assess whether the preliminary estimates for the gear size aresatisfactory;Suggest any modifications to the gear design if necessary;List all assumptions.Provide concluding remarks for the selected designYou can use and ISO and BS Specification documents (provided) and a data table(provided) as a guide to help you design of the gears (e.g. face width, module,