04-BS-15 · December 2019
Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)
National Exams December 2019 — 04-BS-15, Engineering Graphics and Design Process (3-hour, closed-book, no calculator; 6 questions, 100 marks total; all sketches freehand, no straightedge).
Reference texts: Bertoline & Wiebe, Technical Graphics Communication (4th ed.) — orthographic/isometric/section-view theory; Giesecke et al., Technical Drawing / Engineering Graphics (15th ed.) — ASME Y14.5 dimensioning and glass-box projection theory; Ulrich & Eppinger, Product Design and Development (7th ed.); Hibbeler, Mechanics of Materials (10th ed.).
Question text not reproduced: the examination questions are © Engineers and Geoscientists BC. Open the official past paper (linked at the top of this page) to read the question, then follow the worked solution below.
A “fit” describes the range of tightness or looseness that results when a shaft is assembled into a mating hole, once the manufacturing tolerance bands on each part's diameter are accounted for. ASME/ISO tolerancing (ASME Y14.5, ISO 286) standardizes this by defining, for every nominal size, a basic size plus an upper and lower tolerance limit for both the hole and the shaft; the three families of fit are defined by how those two tolerance bands sit relative to one another.
Clearance fit: the hole's tolerance band lies entirely above (larger than) the shaft's tolerance band, so the shaft is always smaller than the hole and the parts can always be assembled and disassembled by hand, with a positive air gap between them. Applications: rotating or sliding assemblies that need running clearance for lubrication and thermal expansion — journal bearings, sliding pistons, shafts in plain bushings, and any location-only fit where frequent disassembly is expected (e.g., a removable shaft collar).
Transition fit: the hole's and shaft's tolerance bands overlap, so depending on where within their tolerance ranges the two individual parts actually land, the resulting assembly may end up as a slight clearance or a slight interference. Applications: fits that must locate a part accurately and resist rotation under light load but still allow occasional disassembly with hand tools or an arbor press — gears, pulleys and couplings keyed onto a shaft, dowel-pin locating holes, and bearing outer races in a housing.
Interference (force) fit: the shaft's tolerance band lies entirely above (larger than) the hole's tolerance band, so the shaft is always larger than the hole and the parts must be forced together — by press, by shrink-fitting the hole (heating it to expand it) or by expansion-fitting the shaft (cooling it to shrink it) — developing a permanent, self-locking clamping pressure at the interface. Applications: joints that must transmit torque or axial load with zero relative motion and no separate fastener — press-fit bearing inner races onto a shaft, a wheel hub shrink-fit onto an axle, and permanently located bushings.
Industry standardizes these bands with a hole-basis system as the default (the hole's lower deviation is held at zero — i.e., the hole is always cut to its basic size or larger — and the desired fit class is achieved by selecting the shaft's tolerance grade/position) because a hole of a given size is far more expensive to resize than a shaft (holes are bored/reamed with size-specific tooling; shafts are simply turned to any diameter on the same lathe), so standardizing the hole minimizes the number of expensive reamers/gauges a shop must stock. A shaft-basis system (holding the shaft's upper deviation at zero instead) is used only where a common shaft size must mate with several different-fit-class housings, such as standard-diameter commercial shafting.
