Bearing Selection Basics for Projects

Almost every rotating mechanism in a student project — a robot arm joint, a conveyor roller, a lead screw, the wheels of an in-pipe inspection robot — needs something that lets a shaft spin inside a housing without grinding itself to dust. That something is a bearing, and choosing wrong shows up as wobble, noise, excess motor current, or a shaft that seizes halfway through the demo.

Bearing selection looks intimidating because catalogues list thousands of variants. In practice, student projects need about six bearing types, one life formula, and a short list of mounting rules. This guide covers exactly that: how loads work, which bearing to pick, how to size it with the L10 life equation (worked example included), what fits to use on the shaft and housing, and the mounting mistakes that destroy bearings before they ever rotate under load.

First, understand your loads

Bearings are classified by the direction of the load they carry relative to the shaft axis:

Radial load — pushing sideways on the shaft (perpendicular to the axis). A conveyor roller carrying a belt's weight, a wheel axle, a pulley with belt tension: overwhelmingly radial.
Axial (thrust) load — pushing along the shaft axis. A vertical lead screw carrying a platform's weight, a propeller pushing a shaft forward.
Combined load — both at once, which is the common real case. A robot arm joint sees radial load from the arm's weight plus axial load from the direction the arm points.

Also distinguish static from dynamic: a bearing sitting still under load (a jack holding a load, a parked robot arm) needs static capacity; a bearing rotating under load needs dynamic capacity and life calculation. Students usually need the dynamic case.

The 80/20 observation: most student mechanisms are light, slow, and radial-loaded. A deep-groove ball bearing, correctly sized and mounted, handles the overwhelming majority of cases. Exotic bearing types solve problems most student projects do not have.

The bearing types you will actually meet

Bearing type	Looks like	Carries	Speed	Typical student use
Deep-groove ball bearing (6000/6200/6300 series)	The classic: balls in a grooved race	Radial + moderate axial	High	The default — robot joints, rollers, small gearboxes, lead screws
Angular-contact ball bearing (7200 series)	Races angled to take thrust	Radial + high axial (one direction)	High	Precision spindles, applications with real thrust
Thrust ball bearing (51000 series)	Flat washers with balls between	Axial only (no radial)	Low–medium	Vertical screw jacks, turntables
Tapered roller bearing (32000 series)	Conical rollers	Heavy radial + axial	Medium	Heavier shafts, gearboxes with real loads
Needle roller bearing	Long thin rollers, tiny cross-section	High radial in tight spaces	Medium	Compact pivots, rocker arms
Bush / sleeve bearing (bronze, PTFE-lined)	A plain cylinder — no rolling elements	Radial, low speed	Low	Slow oscillating joints, linkages, dirty environments
Linear ball bushing (LM8UU, LM12UU)	Balls recirculating around a shaft	Radial on a sliding shaft	— (linear)	3D-printer-style linear motion, sliding carriages
Flanged / pillow-block units (F204, P204)	Bearing pre-mounted in a cast housing	Depends on insert	Medium	The student shortcut — bolt-down bearing mounts, no precision bore needed

Reading a deep-groove designation: 6204 = type 62 (medium series), bore 04 × 5 = 20 mm. The last two digits times five give the bore in mm (for bores 20 mm and up). So 6205 = 25 mm bore, 6306 = 30 mm bore, 608 = 8 mm bore (the skate-bearing size — special small-bearing numbering). Width and outside diameter grow with the series: 6300 is chunkier than 6200 is chunkier than 6000, for the same bore.

The 608 reality check: 608 skate bearings (8×22×7 mm) are dirt cheap and everywhere, which is why student robots are full of them. They are fine for light, slow duty — small wheels, idlers, light linkages. They are not fine as the main pivot of a loaded arm: their dynamic capacity is small, and the cheap ones have sloppy clearances and steel shields that drag. Use 608s where loads are genuinely light; step up to the 6200 series where they are not.

Sizing: the L10 life formula with a worked example

Bearing catalogues give two capacity numbers:

C — dynamic load rating (kN): the load at which 90% of a bearing population survives 1 million revolutions.
C₀ — static load rating (kN): the load causing a defined permanent deformation; do not exceed it even stationary.

The L10 life (million revolutions) for a ball bearing:

L10 = (C / P)³

where P is the equivalent dynamic load (kN). For roller bearings the exponent is 10/3. Convert to hours:

L10h = (10⁶ / (60 × n)) × L10, with n in rpm.

Worked example: a robot arm elbow joint uses a 6204 bearing (C = 12.8 kN from the catalogue). The joint sees a radial load of 300 N (0.3 kN) — mostly the arm's weight — at an average speed of 30 rpm during operation.

P = 0.3 kN (pure radial, so equivalent load equals radial load)
L10 = (12.8 / 0.3)³ = (42.7)³ ≈ 77,700 million revolutions
L10h = 10⁶ / (60 × 30) × 77,700 ≈ 43 million hours

That number is absurdly large — which is the point. For light student loads, fatigue life is essentially infinite; the bearing will die of contamination, corrosion, or bad mounting long before fatigue. The formula still matters because it tells you when you are under-sized: if L10h comes out under a few thousand hours for your duty, step up a size. And for a second scenario — say a bearing fault simulator rig deliberately running a small bearing hard at 1.5 kN and 1400 rpm: L10 = (12.8/1.5)³ ≈ 621 million revs → L10h ≈ 7,390 hours. Still comfortable, but now you are in a range where the math means something.

What the formula assumes: clean conditions, correct mounting, adequate lubrication. In student reality, a correctly mounted, sealed, greased bearing outlives the project; a mis-mounted or unsealed one fails in weeks. Mounting quality dominates the equation for light loads.

Fits: how the bearing meets the shaft and housing

This is where bearing selection meets the ISO 286 tolerances-and-fits system (hole-basis H7/k6-style designations — the language of shaft and bore limits):

Rotating inner ring (shaft rotates, e.g. a motor shaft): the inner ring must grip the shaft — use a transition-to-interference fit: k6 or m6 on the shaft. If the inner ring is loose, it creeps on the shaft and frets both parts.
Stationary outer ring (housing fixed): the outer ring can be a clearance-to-transition fit: H7 or J7 in the housing — snug enough not to spin, loose enough to remove.
The reverse case (housing rotates, shaft stationary — rare in student work): flip the logic.

Practical student translation: turn or buy your shaft to k6 (a few microns over nominal), and bore or ream the housing to H7. A 20 mm shaft for a 6204: Ø20 k6 = 20.002–20.015 mm — a light press or firm push fit by hand with a mallet. The housing bore: Ø47 H7 = 47.000–47.025 mm (6204's outside diameter is 47 mm).

The mounting commandments (violating these kills more student bearings than load ever does):

Press on the ring being fitted. Pressing a bearing onto a shaft: push the inner ring. Pressing into a housing: push the outer ring. Force through the rolling elements brinells the races — tiny dents that become noise and early failure. Use a socket or tube that contacts only the correct ring.
Never hammer a bearing directly. A soft drift on the correct ring, or better, a vice/arbor press. Hammer blows on the rings crack them.
Keep it clean and keep the seals on. Remove seals only if you have a reason; an open bearing in a dusty workshop collects grinding dust and dies.
Support the shaft close to the bearing. A long overhung load bends the shaft and edge-loads the bearing. Two bearings spaced apart beat one heroic bearing.
Do not overtighten locknuts and retaining arrangements. Axial preload is a precision technique, not a "tighter is better" situation — excess preload overheats the bearing.

Sealing: ZZ vs 2RS vs open

The suffix after the bearing number specifies sealing, and for student environments it matters more than the bearing series:

Suffix	Meaning	Seal type	Use
(none) / OPEN	No seals	—	Clean, oil-bath environments; instrument spindles. Not for dusty workshops
ZZ	Metal shields both sides	Non-contact shield	Keeps large debris out; low drag; the general-purpose student choice
2RS / LLU	Rubber seals both sides	Contact seal	Keeps dust and moisture out; slightly more drag; the right choice for dirty/mobile robots
Z / RS	Shield/seal one side	—	One side exposed to clean conditions, other to dirt

For a mobile robot or any mechanism operating outside a clean lab, 2RS sealed bearings are worth the small premium and the tiny extra drag. An open or ZZ bearing in a dusty environment ingests abrasive particles and gets noisy within weeks; a 2RS bearing shrugs it off. The in-pipe inspection robot is the extreme case — dust, moisture, no maintenance access — where sealing is not optional.

Lubrication: what student bearings actually need

Nearly all small ball bearings ship pre-greased and sealed-for-life — you do nothing. This covers every 608, 6200, and 6300 bearing you will buy off the shelf. Relubrication only enters the picture for:

Open bearings in clean mechanisms: a drop of light machine oil or a smear of lithium grease during assembly.
Bush/sleeve bearings: bronze bushes want oil or grease on assembly; PTFE-lined (e.g. Igus-style) bushes run dry by design — do not grease them, grease attracts the dust they are meant to ignore.
Linear bushings (LM8UU): a light oil film on the shaft; keep the shaft clean and smooth (ground shafting, not rough bar).

The two lubrication failures: running a greased-for-life bearing dry because someone washed the grease out with solvent during "cleaning", and over-greasing an open bearing until it churns, overheats, and throws grease everywhere. Sealed bearings: leave them alone.

Bush bearings: when plain beats rolling

Not every joint needs balls. A bush (sleeve) bearing — a cylinder of bronze, brass, or PTFE composite — wins in specific cases:

Slow oscillation (linkages, rocker arms): rolling bearings gain nothing at a few degrees of oscillation; a bush is cheaper and more compact.
Dirty environments: a PTFE-lined bush tolerates contamination that would kill a ball bearing.
Shock loads: bushes distribute load over a large area; no point contact to brinell.
Cost and simplicity: a bronze bush pressed into a reamed H7 hole is a complete bearing solution for under ₹50.

The trade-offs: higher friction (plan for it in motor sizing — a bush joint can need noticeably more torque to start moving), wear over time (bushes are consumables; design them replaceable), and no good at high speed. The hydraulic arm trainer with syringe actuators is typical bush territory: slow, oscillating, lightly loaded joints where a ₹40 bronze bush outperforms a ₹120 ball bearing that would never see enough speed to justify itself.

Sizing a bush: bore to suit the shaft with running clearance (H8/f7 or H9/d9 territory — bushes need more clearance than ball bearings because there is no internal clearance to absorb misalignment), length roughly 1–1.5× the shaft diameter for stability, and always provide a lubrication path or use self-lubricating material.

Pillow blocks and flanged units: the student shortcut

If the thought of boring an H7 housing on a lathe is daunting, pillow-block (P204) and flanged (F204) bearing units are the answer: a standard insert bearing pre-mounted in a cast-iron housing with mounting holes. Bolt it down, slide the shaft in, tighten the grub screws or eccentric collar, done. No precision machining required — which is why half the conveyor and rig projects in student labs run on them.

Limitations to know: the insert is usually a light-duty 6200-series equivalent with grub-screw locking (fine for steady loads, less good under reversing shock), the housings are bulky, and alignment between multiple units depends on your mounting surface being flat. For a bearing fault simulator rig, pillow blocks are actually the natural choice — easy bearing swaps are the whole point of the rig.

Selection walkthrough: a robot arm elbow joint

Putting it together for a concrete case — the elbow joint of a small pick-and-place arm:

Loads: the forearm plus payload weigh ~1.5 kg acting at ~200 mm from the joint → radial load ≈ 15 N plus moment. Axial load is small. Speed: intermittent, under 30 rpm.
Type: deep-groove ball bearing — radial-dominant, light, slow. Two bearings spaced 40 mm apart to handle the moment (a single bearing under moment load edge-loads).
Size: shaft diameter chosen as 12 mm for stiffness → 6201 (12×32×10 mm). C = 6.82 kN vs P ≈ 0.05 kN → L10 life effectively infinite; selection is driven by shaft size, not fatigue.
Sealing: 2RS — the arm operates in a normal lab but dust happens; the drag is irrelevant at these torques.
Fits: shaft 12 k6 (or a ground 12 mm shaft, which is typically h6 — acceptable here with a retaining compound or a light press; for the textbook answer, k6), housing bore 32 H7 reamed.
Mounting: press inner rings onto the shaft with a tube drift; slip outer rings into the housing; retain axially with a circlip or shoulder — bearings must be located axially, not just radially.

Total cost: two 6201-2RS bearings, about ₹150–250. That is the economics of correct bearing selection: the right answer is usually also the cheap answer.

Diagnosis table: bearing problems and causes

Symptom	Likely cause	Check / fix
Roughness or noise when turned by hand	Contamination, brinelled races from bad mounting, counterfeit bearing	Replace; review mounting method; buy from a reputable supplier
Bearing gets hot in operation	Excess preload, over-greasing, misalignment, wrong clearance grade	Check axial retention; verify alignment; confirm the fit is not too tight
Shaft spins inside the inner ring (fretting, black dust)	Fit too loose — inner ring creeping	Refit to k6/m6; in a pinch, retaining compound on a cleaned shaft
Outer ring spins in the housing	Housing bore oversize or out-of-round	Measure the bore; line-bore or sleeve the housing
Premature failure in a dusty environment	Wrong sealing (open/ZZ where 2RS was needed)	Replace with 2RS; add external shields if the environment is severe
Excess play / wobble at the joint	Bearing undersized for the moment load; single bearing where two were needed	Add a second bearing spaced apart; step up the series
Seized after sitting unused	Corrosion (moisture ingress), grease hardened with age	Sealed bearings and dry storage; replace — a seized bearing is not recoverable

Selection checklist

Loads identified: radial, axial, combined; static vs dynamic; worst-case values with margin.
Bearing type matches the load direction (deep-groove for radial-dominant; thrust types only where axial dominates).
Size checked with the L10 formula — or justified by shaft diameter for light duty.
Sealing matches the environment (2RS default for anything dusty or mobile).
Shaft fit k6/m6 for rotating inner rings; housing H7/J7 for stationary outer rings.
Mounting method decided: press on the correct ring, axial location provided (shoulder/circlip).
Lubrication plan exists (usually: sealed-for-life, do nothing — stated explicitly, not forgotten).
Bushes considered where motion is slow and oscillating — and made replaceable if so.

Putting it together

Bearing selection for student projects is a short decision chain: identify the loads, pick the type that carries them, size it (the L10 math will usually tell you that light duty is fine), seal it for the environment, fit it correctly, and mount it without force through the rolling elements. Master that chain and every shaft in every mechanism — robot arms, rigs, conveyors — spins smoothly and quietly, which is exactly what the examiner notices when they put a hand on your project. For the measurement side of getting those k6 shafts and H7 housings right, the companion CAD-to-prototype guide covers taking machined features from model to part.

Buying bearings: reading the box and spotting fakes

A bearing's designation string encodes everything: 6204-2RS = 62 series, 04 bore code (20 mm), 2RS rubber seals both sides. Common extras: C3 = greater-than-normal internal clearance (for high speed or tight fits that squeeze the bearing); TN9/P = polyamide cage. For student work, standard clearance without suffix letters is correct unless you have a specific reason.

The counterfeit problem is real at the cheap end of the market: no-name bearings in copied boxes with rough grinding, uneven seals, and clearances you can feel by wiggling. They are false economy — a ₹60 fake that fails during the demo costs more than the ₹180 branded bearing. Buy from established industrial suppliers or reputable online industrial stores, not the cheapest listing. Quick quality checks on arrival: rotate the bearing slowly by hand (it should feel smooth with slight, even grease drag — not gritty, not notchy), check the seals sit evenly, and compare weight against a known-good sample if you have one.

Stock a small kit if you build mechanisms regularly: 608-2RS, 6201-2RS, 6204-2RS, 6205-2RS, a couple of LM8UU linear bushings, and bronze bush stock in 8/10/12 mm ID. That kit covers an enormous fraction of student mechanisms for a few hundred rupees.

Speed ratings: when rpm actually matters

Catalogues list a limiting speed (thermal/mechanical ceiling) for each bearing. Student mechanisms rarely approach it — a 6204's limiting speed is around 15,000+ rpm with grease, and your rig runs at 1,400. The cases where speed deserves a thought:

High-speed spindles (rotary tools, small CNC spindles above ~10,000 rpm): use standard clearance or C3, light oil or high-speed grease, and balance the rotating assembly. Vibration at speed kills bearings faster than load.
Seal drag at speed: 2RS contact seals add measurable drag above a few thousand rpm. For a high-speed application, ZZ shields or open bearings with external shielding run freer.
Linear bushings have speed ratings in m/s — check them for fast 3D-printer-style axes, though student axes are well inside limits.

For everything else — robot joints, conveyors, lead screws, rigs — speed is not the design driver. Load direction, sealing, fits, and mounting quality are.

Bearing Selection Basics: Ball, Roller and Bush Bearings for Student Mechanisms