Mechanisms: bearings and conversion of motion

Reciprocating is straight-line back and forth (a piston, a jigsaw). Oscillating is back and forth on a pivot, an arc not a straight line (a pendulum). Rotary is continuous turning; linear is straight, one way. Candidates call a swinging part reciprocating, or a piston rotary. Test: straight line = reciprocating; on a pivot = oscillating.

First, candidates control the wrong load: a bearing for a radial load (across the shaft) when it has a thrust (axial) load along it, or vice versa. Identify the direction first (radial across, thrust along the shaft), then pick a bearing to suit. Second, on why nylon suits a plain bearing, the wanted property is self-lubricating, low friction.

Two cam errors. First, candidates omit the spring that keeps the follower pressed on the cam; without it it only falls under gravity and at speed loses contact and bounces, so its motion no longer matches the profile. Second, dwell is misused: it is where the follower stays still because the cam edge is a constant radius, not the point it is highest.

Candidates mix up a ratchet and pawl with a rack and pinion, claiming it turns rotary into linear motion. It does not. A ratchet is a toothed wheel with a pivoted pawl so it turns one way only, locking against turning back. A rack and pinion changes rotary into linear motion. Rack and pinion changes the motion type; ratchet and pawl controls the direction.

For a crank and slider the slider stroke = 2 x throw, because the pin swings from one side of the shaft centre to the other -- twice the radius to the pin. Candidates set stroke equal to throw and lose half, or forget to halve and give twice the true throw. Always double the throw for the stroke, halve the stroke for the throw.

A screw's velocity ratio is the effort distance round the handle divided by the pitch: VR = effort circumference / pitch. Two slips: putting the pitch on top (pitch / circumference), which inverts it; or forgetting the handle sweeps a circle, so the effort distance is the circumference (2 x pi x handle length), not the bare length. Small pitch means a large VR.

A wrong belief: that a coarse thread (large pitch) makes a screw more powerful. A screw's velocity ratio is the effort circumference divided by the pitch, so a larger pitch makes the VR smaller, giving less force. A fine thread (small pitch) gives a large VR and more force -- which is why a vice or screw jack uses a fine thread.