Safety Factor for Caster Load: Dynamic vs Static Ratings and What to Write in the RFQ

Why the Static Rating Cannot Be the Sizing Basis

A caster's static rating describes the weight it holds while the equipment stands still. The dynamic rating describes what it carries while rolling, and that is the number a moving cart must be sized against 2 , 6 , 23 .

Rolling adds acceleration, deceleration, turning and rolling resistance that a parked load never applies, which is why the two figures are not interchangeable 6 , 23 . The gap between them is real but not fixed.

One source reports dynamic capacity at 60-75% of static 2 ; a manufacturer's polyurethane wheel is listed at 3086 lb static and 1543 lb dynamic at speeds up to 1.86 mph, roughly half 22 .

Because the ratio moves with the product and the operating condition, you cannot convert a static figure into a dynamic one by rule of thumb. Ask the supplier for the dynamic rating at your reference speed instead 2 , 22 .

A caster bought on its static number is undersized the moment the cart moves. If your equipment is genuinely stationary — a machine base or fixed stand — the static rating is the correct basis, and only then 2 , 6 .

The Load per Caster You Put in the RFQ

The method is the same across the sources: add equipment weight to maximum payload, divide by the number of casters, then apply a safety factor 1 , 2 , 4 , 12 , 23 , 28 , 29 . The disagreement is only about the divisor, and the conservative choice is to divide by one fewer caster than you have — three for a four-caster cart 2 , 4 , 15 , 23 , 29 .

The reason is mechanical rather than cautious bookkeeping. On an uneven or worn floor, frame racking or floor slope lifts one caster clear, and the remaining three absorb the whole load 2 , 15 .

Dividing by four assumes an even distribution that a real concrete floor does not deliver, and the casters then run 25-33% over their rating 4 .

So the figure you write into the RFQ is the required dynamic capacity per caster, derived from the three-caster assumption and then multiplied by the factor you choose in the next step.

State the divisor and the factor explicitly in the RFQ, because a supplier quoting against a four-caster division is quoting a different, smaller caster.

Choosing the Factor from the Published Range

Published factors rise with operating severity, because rough floors, obstacles, speed, shock and impact all consume capacity that the same load does not consume in a clean aisle 1–5 .

The table below collects the factors the sources actually publish; treat it as a menu to match against your duty, not as a ladder where the highest number is always right.

Derating for Speed and Shock Before You Fix the Factor

A published dynamic rating is established at a reference speed, commonly 2-3 mph walking pace, and it does not hold above that 2 , 15 . Bearing friction, heat build-up in the tread and centrifugal force on the swivel all rise with speed.

So a caster rated 1,000 lb at 3 mph is not a 1,000 lb caster at 6 mph 15 . One source quantifies the loss for standard polyurethane at roughly 25% of dynamic capacity per mph above 3 mph 2 .

Another advises derating 20-30% for powered towing at 5 mph or more, or moving to a higher-capacity unit 15 . If your fleet runs above walking pace, ask the supplier for a rating at your actual speed rather than derating a 3 mph table yourself.

Shock is a separate rating again. Dock drops, pallet-jack transitions and expansion joints produce momentary peaks of 2-5x static 4 , and one source puts the shock rating at roughly 3x static with repeated shock cycling at about 1.5x static 5 .

Those peaks are brief, but they load the kingpin and raceway in a way steady rolling does not 4 . Where your route crosses a dock plate or a joint, specify a shock-rated caster in addition to the static and dynamic figures, and do not expect the safety factor alone to cover the peak 4 , 5 .

Whether the Higher Factor Is Worth Paying For

A higher factor pushes you into a larger, heavier caster, and that shows up as an upfront price difference. One source puts the delta between a 2x and a 4x selection at 20-30% up front, and claims the 4x choice pays back 3-5x in avoided replacement labour 5 .

The same source describes 2x as a marketing minimum that fails within 6-12 months under real duty cycles, and 4x as the industrial standard for 3-5 year service life 5 . Those are that source's positions, not a settled industry benchmark.

The trade only makes sense where replacement is expensive. If a failed caster stops a production line or an AGV route, the labour and downtime behind the 20-30% premium is the number that decides it 5 .

If the cart is a tool crib unit that moves for thirty minutes a day, the same premium buys life you may never consume.

Weigh the premium against your own downtime cost per replacement, and note that the sources publish no fatigue curves or bearing-life data that would let you predict the service life of a given factor 5 .

What to Demand as Evidence Before the PO

A compliance statement is not the same as a tested rating. ANSI MH28.3 sets a test methodology and pass/fail criteria, but two casters that both pass at the same rated capacity can still differ in bearing quality, wheel compound consistency and swivel durability 29 .

Name the test and the acceptance figure in the RFQ rather than accepting a compliance claim.

• ✓Ask for the dynamic rating at your operating speed, not the headline static figure.
• ✓Require the test standard cited by name — ANSI MH28.3 / ICWM or the applicable EN part — with the report, not just a compliance statement.
• ✓Ask whether the static test was run at 150% of rated load held 24 hours.
• ✓Ask whether the dynamic test ran 100,000+ cycles at 5 km/h, and request the report.
• ✓Confirm the factor you specified is the one the supplier sized against, and that the divisor was three casters for a four-caster cart.
• ✓For dock or transition routes, require a shock rating in addition to static and dynamic.
• ✓For 24/7 AGV duty, require fatigue-test data, not a general-purpose rating.

Where the Sources Disagree

Two numbers you will need for the RFQ are not settled by the sources: the factor itself and the static-to-dynamic ratio. Both tables below keep the competing positions visible so you can decide which side to verify with your supplier.

What the sources do not establish

• No manufacturer's published safety factor for a specific caster series is provided; every value in this article is generic guidance.
• No fatigue curves or bearing-life data link a chosen safety factor to a predicted service life.
• No volume thresholds for custom ratings, and no lead-time impact data.
• No definitive statement on whether ICWM/ANSI MH28.3 or EN 12530 compliance is mandatory or advisory in any market.
• No acceptance criteria thresholds for inspection beyond pass/fail deformation checks.
• No single universal safety factor is established; the sources do not resolve which factor applies to a given application.