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Section 6.7 / 6.8

Springs & Bearings

Spring types and spring rate; plain and rolling-element bearings, load types, fit and failure modes.

Notes

Springs & bearings, summary notes

Main ideas
  • Springs store and release energy and apply a controlled force. Types: helical compression and tension (extension), torsion, leaf, spiral and Belleville (disc) springs for high load in a small deflection. Materials include spring steel and phosphor bronze (non-magnetic, corrosion-resistant).
  • Spring rate (stiffness) is the force needed per unit of deflection, F = k·x (Hooke's law), a 'stiffer' spring has a higher rate k. Springs fail by fatigue, permanent 'set' (loss of free length) and corrosion.
  • Bearings reduce friction between moving parts. Plain (journal/bush) bearings slide, white metal, bronze or PTFE; rolling-element bearings use balls or rollers on inner and outer races with a cage separating them.
  • Ball bearings mainly take radial load (plus a little thrust); roller bearings take higher radial loads; TAPER-roller bearings carry combined radial AND axial (thrust) loads; dedicated thrust bearings take pure axial load.
  • Bearing care: correct fit (interference on the rotating race), set pre-load/end-float, keep them clean and lubricated. Failure modes include contamination, spalling and brinelling (indentation of the races from shock or static overload).
  • ⚠ Exam trap: a taper-roller bearing takes combined radial and thrust load, whereas a plain ball bearing is essentially radial. Spring rate is force ÷ deflection, do not confuse a stiff (high-rate) spring with a strong one.
Key formulas
Spring rate (Hooke)
F = k · x (k = force per unit deflection)
Solved examples

  1. A wheel hub must support both the aircraft's weight (radial) and the side loads of cornering/turning (axial). Which bearing type suits it and why?

    Taper-roller bearings, usually fitted in opposed pairs. Their conical rollers and races react radial load and axial (thrust) load at the same time, and the pair can be pre-loaded to set the correct end-float, ideal for a wheel that carries weight and side loads together.

Mind map

Springs & bearings concept map

Springs & Bearings

Quiz

Springs & bearings quiz

Springs & Bearings, quiz

Ref 6.7 / 6.88 of 14Pass 75%

  1. 1. A spring of stiffness 25 N/mm is compressed by 12 mm. The force it exerts is:

    300 N
    2.1 N
    37 N

  2. 2. A spring deflects 20 mm under a load of 200 N. Its stiffness (spring rate) is:

    10 N/mm
    0.1 N/mm
    4000 N/mm

  3. 3. Spring rate (stiffness) is defined as:

    Force per unit deflection (F = k·x)
    Deflection per unit time
    Force times deflection

  4. 4. A Belleville spring is a:

    Disc (washer) spring for high load, small deflection
    Long helical tension spring
    Leaf spring

  5. 5. Permanent loss of a spring's free length is called:

    Set
    Creep rate
    Backlash

  6. 6. A plain (journal) bearing works by:

    Sliding contact on a low-friction surface
    Rolling balls
    Magnetic levitation

  7. 7. Which bearing carries combined radial AND axial (thrust) loads?

    Taper-roller bearing
    Plain ball bearing
    Needle bearing (pure radial)

  8. 8. A ball bearing is best suited to:

    Mainly radial loads with some thrust
    Pure heavy thrust only
    No load at all

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