Step 1 — the load math every source agrees on
Published sizing guidance converges on one procedure regardless of class: add the equipment's own weight to the heaviest expected payload, then divide by one fewer than the number of casters. A four-caster cart is divided by three, not four, because on uneven floors a four-caster cart rocks and three wheels carry the load while the fourth lifts clear; three-caster and six-wheel rocker layouts follow the same principle with their own counts. Industry guidance is explicit that this is not theoretical — assuming all four casters share equally is the most commonly cited selection mistake.
Then add margin. The safety factors published guidance reports differ — roughly 1.25–1.5× the calculated per-caster load, or 25–30%, or a stated 20% minimum, with one publisher recommending doubling for towed equipment on inclines — and they disagree by design: the right margin depends on speed, floor condition, how often the unit moves and shock exposure, which is why no single factor is correct for every application. Impact spikes in high-shock environments can reach 2–3× the static load for milliseconds — brief enough to feel nothing, hard enough to dent bearing races, crack treads and shear kingpins — so one guide suggests an explicit 1.5× shock factor for high-shock duty on top of the calculated load.
Worked example from published guidance: a cart and payload totalling 6,000 lb on four casters divides by three to 2,000 lb per caster; with a 1.5× margin the specification target is a 3,000 lb-rated caster — and guidance adds that if the unit moves multiple times per day, the total load rating may be doubled again rather than specified at the bare figure.
Step 2 — read the class bands as references, not ratings
With a per-caster target in hand, you can use duty classes for what they are good at: narrowing the assortment fast. But treat every published band as that publisher's own lineup description. Side by side, the disagreement is obvious:
| Class name | Publisher A (product-line FAQ bands) | Publisher B (shopping-category bands) | Publisher C (metric comparison table) |
|---|---|---|---|
| Light / general duty | General to 250 lb; light-medium to 350 lb | Light duty up to 350 lb | Light duty 10–140 kg (22–310 lb) |
| Medium duty | Up to 1,250 lb (kingpinless 1,800 lb) | Medium duty up to 7,000 lb | 100–420 kg (220–925 lb) |
| Heavy duty | Heavy and extra-heavy lines to 2,500 lb | Heavy duty up to 16,000 lb; extreme duty to 40,000 lb | 350–1,000+ kg (770–2,200+ lb) |
Three publishers, three non-overlapping meanings of "medium duty". A 1,250 lb medium-duty caster from one line and a 7,000 lb medium-duty caster from another are both correctly labelled. This is why sizing guidance converges on a different rule: calculate your maximum load, divide it by one fewer than the wheel count, and choose a caster category that comfortably exceeds the per-wheel figure — with one publisher advising that if your requirement sits near the top of a class band, move up a class rather than trusting the label's headroom.
Step 3 — duty is more than a number
The same per-caster load supports very different service depending on how the equipment works, and the published condition lists cluster into four checks:
- Movement frequency. A load that moves multiple times per day is dynamic duty; published guidance sizes it above a load that sits mostly parked — one source doubles the total load rating for daily-moved 15,000 lb equipment.
- Static dwell. Static load is the weight supported while parked; dynamic load is what rolling, turning and shock add. Parked-loaded time also feeds the tread-material choice (polyurethane flat-spots under long dwell, metals do not) — see the flat-spot guide for that failure mode.
- Shock and towing. Repetitive impact pushes selection toward kingpinless swivel rigs, which published heavy-duty guidance contrasts with kingpin designs on mounting integrity under shock; towed equipment on inclines earns the doubled safety factor.
- Floor and environment. Special floor conditions — tracks, grates, thresholds, seams, dock plates — concentrate load on small contact patches; washdown or corrosive areas push toward stainless rig construction regardless of class.
What the class name does not encode
Even within one publisher's line, two casters labelled 1,500 lb are not necessarily the same product — rated capacity follows a test protocol (published as roughly 3 mph continuous rolling under rated load, on the order of 10 miles, with bearing seizure, wheel deformation, kingpin failure or raceway brinelling as failure criteria), and ratings mean comparable numbers only when manufacturers follow the same protocol. Class names add another layer of looseness on top of that. Construction details the class name also omits: rig type (swivel vs rigid, and the common two-rigid/two-swivel four-wheel layout), kingpin vs kingpinless, wheel material and diameter, bearing type, and plate size.
What the sources do not settle
No source in the located set provides quantified service life, replacement frequency or maintenance cost by duty class — the class bands describe capacity, not how long a medium-duty caster lasts versus a heavy-duty one under the same load. If total cost of ownership matters to the decision, ask for the vendor's own service-life or warranty terms for the specific line at quotation.
Where this leaves the three classes
Use the math to size, the bands to shop, and the quotation to confirm. Our lines carry the same published classes: light duty for loads that stay light and move rarely, medium duty for everyday industrial rolling, and heavy duty — plus kingpinless and stainless constructions — for impact, towing and washdown. If your per-caster figure lands near a band's top, the published advice and ours agree: move up a class.
What to confirm at quotation
- The per-caster dynamic rating under a stated test protocol — not the class name
- Your own math: total working load ÷ (caster count − 1), with your chosen margin and its stated basis (speed, floor, frequency, shock)
- Static dwell pattern (hours parked loaded) — it sizes both the rating margin and the tread compound
- Shock and towing conditions — kingpinless rig, shock-absorbing design, or a higher factor if applicable
- Wheel material and bearing matched to floor condition and environment, not inherited from the class label
- Service-life or warranty terms for the specific line, since classes carry no published life data
The load-capacity calculation page walks the arithmetic step by step with more worked examples; the load-rating standards page explains what a rating number legally and technically means; and the wheel material comparison covers the tread choice this guide deliberately leaves open.