The four types of motion
Four words cover every movement in a mechanism. ROTARY turns round in a circle (a wheel, a gear). LINEAR travels in a straight line ONE way (a lift, a conveyor). RECIPROCATING moves back and forth in a STRAIGHT line (a piston, a saw). OSCILLATING swings to and fro along a CURVED arc about a pivot (a pendulum). The two muddled most are reciprocating and oscillating — both go to-and-fro, but reciprocating stays STRAIGHT and oscillating follows a CURVE.
Choosing spur gears
Spur gears have straight teeth cut parallel to the shaft and drive PARALLEL shafts. Choose on: (1) VELOCITY RATIO — small driving large cuts speed but raises torque; (2) DIRECTION — meshing gears turn OPPOSITE ways, so an idler keeps the same direction; (3) SPACE and module — gears must fit and share the same module to mesh (angled shafts need bevel or worm gears); (4) LOAD, material and NOISE — steel for high load, nylon for light, quiet running with no oil.
Reducing friction — design and lubrication
Friction wastes energy as heat and wears surfaces, so cut it two ways. BY DESIGN: fit BEARINGS (rolling instead of sliding, or a low-friction bush), keep surfaces SMOOTH with the right clearance, and choose SELF-LUBRICATING materials (nylon, acetal, PTFE). BY LUBRICATION: OIL suits fast parts but drains away and needs a seal; GREASE stays put, for slow or hard-to-reach parts; DRY lubricants (graphite) suit dusty or hot places where oil would attract grit.
Drawn from real examiner reports.
Reciprocating vs oscillating motion
The definition trap here. RECIPROCATING and OSCILLATING both go to-and-fro, so they get swapped. Reciprocating moves back and forth along a STRAIGHT line (a piston); oscillating swings along a CURVED arc about a pivot (a pendulum). Also, not everything that turns is 'rotary': a crankshaft rotates but its piston reciprocates, so one mechanism shows both.
s23 P42 Q5; s23 P43 Q2b; w23 P42/P43 Q6 — reciprocating, oscillating and rotary were confused; the springboard diver reciprocates, and the piston reciprocates while the crankshaft rotates.
Gear ratio written the wrong way round
For a spur-gear pair the VELOCITY RATIO = teeth on the DRIVEN gear divided by teeth on the DRIVER gear. Candidates invert it (driver over driven) or quote it backwards, 1:5 instead of 5:1. A quick check: a SMALL driver turning a LARGE driven gear must slow the output, so the ratio is greater than 1 (a reduction) — a figure below 1 means you divided the wrong way round.
s22 Q11a(iii); w22 Q11c(iii) — the gear formula was inverted and ratios quoted the wrong way round (use driven/driver for velocity ratio).
Why nylon suits a bearing
Asked why nylon (or acetal) suits a small bearing or low-load gear, many answer 'it is cheap' or 'it is light' — true, but not the marking point. Examiners want SELF-LUBRICATING: a naturally low coefficient of friction lets it slide against metal with no oil, so the bearing runs quietly and needs no maintenance. Heavily loaded, fast gears still need steel.
w23 P42/P43 Q10a(iii) — nylon suits bearings because it is self-lubricating; the reason was under-known.
Spur gears need parallel shafts
Spur gears have straight teeth and can only connect PARALLEL shafts. Candidates try to use them for shafts meeting at an angle, which spur gears cannot do — that needs BEVEL gears (shafts at a right angle) or a WORM and wheel. Before choosing spur gears, check the layout: parallel shafts suit them, angled shafts do not.
Meshing spur gears reverse direction
When two spur gears mesh they turn in OPPOSITE directions — if the driver turns clockwise, the driven gear turns anticlockwise. Candidates forget this and assume both turn the same way. To make the output turn the SAME way as the input, add a third gear (an idler) between them, which reverses the direction again without changing the speed ratio.
Matching the lubricant to the part
Choosing the wrong lubricant loses marks. OIL is a thin liquid for FAST-moving parts (engines, gearboxes), but it drains away and needs a seal. GREASE is a thick semi-solid that STAYS PUT, so it suits SLOW-moving or hard-to-reach parts that are rarely serviced. A DRY lubricant (graphite) suits places where oil would attract dust or the temperature is too high.
A pendulum shows reciprocating motion
A testable wrong belief: that a pendulum, because it goes back and forth, is reciprocating. It is not — a pendulum swings along a CURVED path (an arc) about a fixed pivot, which is OSCILLATING motion. Reciprocating motion is back and forth along a STRAIGHT line (a piston, a saw). The give-away is the pivot: anything swinging about a pivot is oscillating.
Show working and state the unit
Systems & Control calculations are marked for METHOD, not just the answer. On a gear question write the FORMULA, name driver and driven, substitute, then give the answer with its UNIT (rev/min for a speed). Working scores at each stage even if the arithmetic slips.
Name a motion with two questions
To name a motion reliably, ask two questions. First: does it go ALL THE WAY ROUND (rotary), straight one way (linear), or back-and-forth? Second, if back-and-forth: is the path STRAIGHT (reciprocating, a piston) or CURVED about a pivot (oscillating, a pendulum)?
Label driver and driven, then check
Before calculating a gear ratio, label which gear is the DRIVER (turned by the motor) and which is the DRIVEN (output). Then sanity-check the direction: a small gear driving a large one must SLOW the output, so the velocity ratio should be greater than 1 (a reduction).
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