Springs, Bearings and Transmissions
Springs
Core concepts
Spring types are helical compression, helical extension, torsion, leaf, Belleville (disc), wave and spiral. The controlling parameters are spring rate (load per unit deflection), preload or installed length, solid height for compression springs, and stress at the working deflection. Hooke's law applies within the elastic range: deflection is proportional to load, so doubling the load doubles the deflection until solid height or yield.
Manufacture and treatment matter for questions: springs are stress-relieved after coiling, and shot peening raises fatigue life by putting the surface into compression. Extension spring hooks and the inside of coils are stress raisers where cracks start. Springs are identified by part number and sometimes by colour coding, but colour alone means nothing without the approved chart.
Traps in questions
Never heat a spring to straighten or re-temper it, and never stretch an extension spring to increase its free length - both destroy the heat treatment. Electroplating can cause hydrogen embrittlement, so plated springs need a post-plating bake. A spring that has taken a set or lost tension is replaced, not adjusted. Do not cut coils to shorten a spring.
Rule of thumb
Rate k = load divided by deflection. Two identical springs in parallel give twice the rate; in series they give half the rate.
Worked example
An extension spring of rate 10 N/mm is installed with a 20 mm preload stretch. It carries 200 N working load: total extension = (200/10) = 20 mm plus the 20 mm preload = 40 mm, and the hook stress must be checked at that length.
Bearings and Transmissions
Core concepts
Bearings are plain (bush), rolling (ball, cylindrical roller, needle, tapered roller, spherical) or thrust types. Designation and clearance codes tell you the fit and internal clearance. The interference fit goes on the rotating race - normally the inner race on a rotating shaft - so the race cannot creep. Fit bearings by pressing on the race being fitted, or by controlled heating (typically not above about 120 C), never by hammering on the cage or balls. Sealed and shielded bearings differ: shields retain grease and keep out dirt but are not watertight.
Transmissions: spur gears are simple and noisy, helical gears run quietly with axial thrust, bevel gears turn the drive through an angle, worm drives give high reduction, and epicyclic (planetary) sets give large ratios in a small space. Backlash is the clearance between meshing teeth and must be within limits - too little causes overheating and seizure, too much causes impact loading.
Traps in questions
False brinelling arises from vibration in transit or storage, not from overload; true brinelling is from excessive static load. Pressing on the wrong race damages the bearing. Tapered roller bearings are adjusted by preload, not left slack.
Rule of thumb
Gear ratio = driven teeth divided by driving teeth; torque is multiplied by the same ratio while speed is divided by it.