Choosing bearings, gears, and fasteners — and beating bearing failure and joint loosening
How to choose the mechanical parts that carry load and motion — bearings, gears, fasteners — with the reasoning, the risks, and a confidence level. Source components from the Mechanical Components supplier directory, and see the full engineering guides index.
Use deep-groove ball bearings for general light-to-moderate radial (and some axial) load at high speed and low cost. Use cylindrical roller bearings for heavy radial load. Use tapered roller bearings for combined radial and thrust load (wheels, gearboxes). Use plain / sleeve bushings for low speed, high load, shock, or where cost, quietness, or contamination tolerance matter more than friction.
Bearing choice follows the load direction and magnitude. A ball's point contact is low-friction and fast but carries less load than a roller's line contact; rollers carry more radial load but a straight cylindrical roller takes essentially no thrust. Tapered rollers are angled specifically to react both radial and axial load together, which is why they're mounted in opposing pairs. Plain bearings have no rolling elements to brinell under shock and tolerate dirt, at the cost of higher friction and needed lubrication.
A deep-groove ball bearing under heavy radial load is short-lived. A cylindrical roller asked to take thrust skids and fails. Tapered rollers mounted without the right preload run rough or seize. Match the bearing to the load type first, then speed and life.
Size a rolling bearing from its rated dynamic load (C) versus the actual equivalent load (P) using the L10 life equation: L10 (millions of revolutions) = (C/P)p, where p = 3 for ball bearings and 10/3 for roller bearings. Pick a bearing whose C gives the required hours at the operating speed, with margin for shock, temperature, and lubrication.
Rolling bearings fail by fatigue, so life is statistical: L10 is the life 90% of bearings will reach. Because life scales with the load ratio raised to the third power (or 10/3), a modest increase in load slashes life dramatically — doubling the load on a ball bearing cuts life to about one-eighth. That steep exponent is why bearings are sized on the load ratio, not just "will it fit," and why shock and misalignment (which raise effective load) matter so much.
Sizing on static fit alone ignores fatigue and gives unpredictable life. Underestimating dynamic/shock load (the cube law) makes a bearing that "looked fine" fail early. Ignoring speed, temperature, and lubrication — which modify the basic L10 — over-predicts life.
Use spur gears for simple, efficient parallel-shaft drives where noise isn't critical. Use helical gears for quieter, higher-load parallel-shaft drives (accepting axial thrust). Use bevel gears to transmit power between intersecting (usually right-angle) shafts. Use a worm gear for a high reduction ratio in one stage, right-angle layout, and (often) self-locking — accepting lower efficiency.
Gear type follows shaft geometry, ratio, noise, and efficiency. Spur teeth engage all at once (efficient but noisier); helical teeth engage gradually along an angle, so they're quieter and stronger but generate axial thrust that the bearings must react. Bevels turn the axis. Worms achieve very large ratios compactly and can self-lock (the output can't back-drive the input), which is useful for holding loads — but sliding contact makes them less efficient and heat-prone.
Spur gears where noise matters are loud. Helical gears without thrust-capable bearings wander axially. A worm drive where efficiency matters wastes power as heat. Choosing self-locking when back-driving was actually needed traps the mechanism.
Choose the property class/grade for the load (e.g. metric 8.8 general, 10.9/12.9 high-strength; SAE Grade 5/8), then tighten to a target preload — typically around 75% of the bolt's proof load for reusable joints — using torque, angle, or (best) direct tension. A properly preloaded joint carries external load mostly in the clamped members, not by cycling the bolt.
A bolted joint works by clamping: high preload keeps the joint faces compressed so external loads relieve clamp force rather than stretching the bolt, which is what gives fatigue resistance and stops loosening. Torque is only a proxy for preload — most of the torque fights friction under the head and in the threads, so preload from torque scatters widely (±25% or more) unless friction is controlled. That's why critical joints use angle or tension control.
Too little preload lets the joint cycle and loosen or fatigue. Too much yields the bolt or crushes the members. Trusting torque without accounting for friction (dry vs lubricated) can double or halve the actual preload. Mixing a soft grade into a high-load joint strips or breaks it.
Noise, vibration, heat, and roughness; on teardown, flaking/pitting of the races (spalling), dents at ball spacing (brinelling), discoloration from overheating, or a seized bearing. Most bearings fail well before their fatigue life for a preventable reason.
Contamination (dirt/water in the lubricant), inadequate or wrong lubrication, misalignment, overload or shock (brinelling from static impact), and improper mounting (press force through the wrong ring). Lubrication and contamination together account for the majority of premature failures.
1) Seal and keep the lubricant clean and correct — the single biggest lever. 2) Maintain re-lubrication intervals; don't over- or under-grease. 3) Align shafts and housings and mount by pressing on the correct (fitted) ring. 4) Size for real shock/overload, not just steady load. 5) Read the failure pattern — the wear signature points to the cause (contamination vs misalignment vs overload).
Fasteners back off over time, joints develop play or leak, or bolts fail suddenly by fatigue (a clean crack across the first engaged thread). Often appears under vibration or cyclic load.
Insufficient preload (the root cause of most loosening), vibration causing transverse slip (self-loosening), embedment/gasket creep relaxing the clamp, and a joint designed so the bolt carries the cyclic load directly instead of the clamped members.
1) Achieve and verify adequate preload — most loosening is under-tension, not "needs threadlocker." 2) Design a stiff joint so external cyclic load relieves clamp force rather than stretching the bolt. 3) Control embedment/creep — avoid soft gaskets under the head, re-torque after seating if needed. 4) Add locking features (prevailing-torque nuts, wedge-lock washers, adhesive) for genuine vibration. 5) Use angle/tension control on critical joints to cut preload scatter.