A powered metal-working crank press (a generic press is shown at the end) has been designed to be capable of punching blanks from mild steel sheet up to 3 mm thick.
1. the ultimate shear strength of the mild steel (260 MPa), and
2. the eccentricity of the press crank and the length of its connecting rod (25 mm and 450 mm, respectively),
The press has a flywheel, of 750mm outer diameter and 185mm width, which mounts onto one end of its crankshaft. When the press is not operating, the flywheel turns freely on the crankshaft. To operate the press, the flywheel is connected to the crankshaft by a sliding-key type of clutch located in the hub of the flywheel. The flywheel is driven by an AC electric induction motor with two magnetic pole pairs via a V-belt drive from the motor to an intermediate shaft and by a chain drive from the intermediate shaft to the flywheel. When the press is operating continuously, the flywheel is to have an average speed of 180 RPM and an average torque of 62.3Nm. The press is to operate 16 hours/day, 6 days/week, 48 weeks/year.
(a) calculate the power required to drive the press in continuous operation, when it is punching at its maximum designed capacity;
(b) calculate the input power to the chain drive and the belt drive, assuming a power loss of 5% from belt drive, 10% from chain drive, 20% for the rest of the machine;
(c) select a suitable motor;
(d) determine the overall speed reduction ratio from the motor to the crankshaft and decide how this is to be shared between the belt and chain drives;
(e) select tooth numbers for the chain drive;
(f) determine the actual belt ratio required;
(g) select a V-belt section and pulley sizes;
(h) determine the design power for the belt drive;
(i) select a suitable centre distance for the selected pulleys;
(j) calculate a provisional pitch length of belt based on the selected pulley sizes and centre distance;
(k) select a suitable belt length;
(l) calculate the actual centre distance of the belt drive, based on the selected pulley sizes and belt length;
(m) calculate the arc of contact of the belt on the small pulley;
(n) determine the belt length correction factor and the arc of contact correction factor;
(o) calculate the belt speed;
(p) determine the basic power per belt and the required number of belts;
(q) select an appropriate chain drive, and specify the chain size, the centre distance and the number of links in the chain;
(r) define the relative positions of the three horizontal shaft axes of the motor, intermediate shaft and the crank (side view), considering compact spacing and low support reactions on the intermediate shaft.
The intermediate shaft is to be supported by two rolling-contact bearings with an estimated span of 300mm, with the chain drive overhung at one end and the belt drive overhung at the opposite end. The overhangs are better to be kept minimum. The two bearings are to be located in an oil-bath type of housing which will mount to the press frame. Each bearing will require one associated oil seal.
(s) provide a preliminary layout of the intermediate shaft showing the mounted elements, i.e. large pulley, small sprocket and two bearings, and the various steps and shoulders in the shaft which will allow the various components to be appropriately positioned, and fastened;
(t) estimate the weight of the large pulley and of the belt pull force that will act on the pulley and hence on the shaft;
(u) construct Free Body Diagrams for the intermediate shaft, ignoring the weight of the shaft and of the small sprocket but including the effects of the weight of the large pulley and of the chain pull force and belt forces; and then calculate the reactions at the supports;
(v) select suitable rolling contact bearings which will provide a service life of 5 years, and verify whether the static loading capacity Co is also sufficient,
(w) select appropriate lip seal sizes for each of the corresponding bearings.
Note: The design should be practically considered, meet required functions with minimum cost. The above steps are suggested approaches for the design sequences only, a logical design report with proper report format is expected, with justifications for all the design decisions. As most of the items are off-the-shelf parts, no engineering drawings are expected.