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Machines and Devices Todor Kableshkov Higher School of Transport
Purpose of subject training • Common machine culture - for students; • Common machine culture and design abilities – for students on not Machine construction specialties; • Preparation for specific training – for Machine Engineering training.
Student’s background (for professional training) Students should pass successful subjects like: • Mathematics, Chemistry, Physics; • Mechanics, Material Science, Machine Drawing, Material Strength.
Forms of training • Lectures; • Workshops; • Training projects/assignments; • Seminar lessons.
Pedagogical tools used for training • Traditional: classrooms, books, ICT etc.; • Methods for training projects, assignments, catalogs, reference books; • Special stands for workshops and methods for them;
Specific features of the module presented • Joints: not detachable (riveted, welded, solder and agglutinate) and detachable (tread, cotter, groove); • Elements of rotation – shafts, axes, sliding supports, bearings, gear (driving, pulley) wheels etc; • Clutches: not movable (bush, flange) and movable (cardan, friction, pin, centrifugal, cog-wheel, elastic) • Gear drives: gear, friction (variation), belt, chain.
Main objectives (specific technical problems studied) • Criteria for workability; • Design, technical notes; • Models for constructing; • Technical approximations; • Sequence for constructing; • Investigation (workshop); • Application of materials studied for training projects development
Lessons • Lectures: 30 – 60 hours; (the exactly number depends on the level of training); • Workshops: 15 – 30 hours; • Seminars: 15 hours (if they are available); • Knowledge realization in practice (for training purpose).
Typical module on Machine Elements THREADED FASTENERS The module introduces the basic concepts for constructing of threads such as: • - the power screws such as a lathe leadscrew or the screw in a car lifting jack which transforms rotary motion into substantial linear motion (or vice versa in certain applications), and • - the threaded fasteners similar to a nut and bolt which joins a number of components together again by transforming rotary motion into linear motion, though in this case the translation is small
THREADED FASTENERS module consists of: • Introduction to threaded fasteners - types, nomenclature and the basic geometry of ISO Metric and some power screw threads • The mechanics of screw threads illustrating the dominant effect of friction • Strength classes and static failure of steel screws • The dependence of bolt loading on the indeterminate assembly - preload and calculation of component stiffness • Tightening - preload and its control by torque measurement, nut rotation and other methods • The role of component stiffness in fluid pressurised joints, and the concept of joint factor • Bolt fatigue • The behaviour of non-uniformly loaded bolt groups - torsion and bending.
Typical features of constructions is investigated, like: • hexagon headed bolt and nut; • socket headed setscrew; • diameter of the bolt shank; • radiused fillet at the junction of shank and head; • shank diameter of a waisted bolt; • assembly incorporated a washer under the nut; • bolt; • stress area as a function of thread;
Thread geometry A thread 'system' is a set of basic thread proportions which is scaled to different screw sizes to define the thread geometry. Whitworth, Sellers, British Standard Pipe (BSP) are just three of the many systems which proliferated before the adoption of the ISO Metric thread system.
Screw thread mechanics • There are always three major components in practical applications of the screw thread mechanism : • the screw - a generic name applied to a setscrew, leadscrew, bolt, stud or other component equipped with an external thread, • the nut - refers to any component whose internal thread engages the screw, such as the nut of a nut & bolt or a large stationary casting with a tapped hole into which a stud is screwed, and • the thrust bearing - that is the contact surface between two components which rotate with respect to one another. Examples of thrust bearings include : • the under-surface of a screw head which is being tightened by a spanner; • the spherical seating of a G-clamp screw in the stationary self-aligning anvil.
Static failure Here the concentration is on steel fasteners, which, because of their strength and cheapness, constitute over 90% of all fasteners used. Steels for commercial fasteners are graded into property classes as in Table 1. Each class number consists of two figures: • the first figure is Su /100 where Su is the steel's nominal tensile ultimate (MPa) • the second figure is the ratio Sy /Su where Sy is the nominal 0.2% offset yield strength.
Bolt fatigue The external load on a joint assembly may vary over many cycles during the joint's life - the cylinder head joint of an i.c. engine is a typical case. The bolts of such an assembly undergo fluctuating tension and so may fail in fatigue. Failure is most likely in the exposed threads close to the nut face where a stress concentration factor models the damaging effect of the thread root notch. A rolled thread is less prone than a cut thread to fatigue damage, due to better grain orientation and surface work hardening conferred by manufacture.
Non-uniformly loaded bolt groups A single bolted lap joint in which the bolt is more or less centrally located in the clearance holes in the two components is investigated. Provided it is not too large, the external load is transmitted by friction due to pressure between the components, caused in turn by the bolt's initial tightening load. If the load is such as to cause the components to slip then the components bear against the bolt shank. Further increase of the external load could lead to shear failure across the bolt shank, however failure in other modes might have occurred before P reached this level.