Can a plastic toolbox on wheels clear a 20 mm threshold?

Answer: A 100 mm caster with a 90 Shore A tread will, if the handle is tipped to about 30° so the leading wheel lifts 15 mm first.

Should all four casters swivel, or only two?

Answer: Below 1000 mm of aisle width, four swivels save a three-point turn. Above 1200 mm, two fixed plus two swivel track straighter.

How much difference is there between PU and TPR on concrete?

Answer: PU rolls with twenty per cent less resistance; TPR runs five to eight decibels quieter but wears twice as fast with grit.

Can a plastic toolbox on wheels replace a stair trolley?

Answer: No. Standard caster plates are not braced for side loads, and a 150 mm step is enough to start a crack.

Plastic Toolbox on Wheels: Casters Sized by Load and IP Class

Quick answer: A plastic toolbox on wheels rolls cleanly when diameter, tread hardness and the static-to-dynamic split match the route. Two tables map three tiers to four floor types, with ISO 868 and IEC 60529 tied to readings.

Three routes and their caster spec
Wheel diameter tiers and the ground each handles
PU PP and TPR hardness bands
Static and dynamic load plus brakes and swivels
Telescopic handle stages and the staircase limit
FAQ

1. Three routes and their caster spec

An assembly-line shuttle, a move between repair bays and a haul across an outdoor yard ask three different things of one set of casters. Change the wheels and push effort can halve. For a plastic toolbox on wheels, the fitting sheet converts route into diameter, tread and load rating.

Wheel diameter	Usual tread	Route for 60 kg dynamic load	Site check
75 mm	PP or TPR	Bay-to-bay on epoxy, under 40 m	A joint over 20 mm stops it
100 mm	PU	Concrete shop floor with a sill	A 15 mm sill rolls through
125 mm	PU capped	Outdoor yard and gravel, over 200 m	Centre of gravity 10 mm higher

Each diameter step buys tolerance rather than speed: a 75 mm caster is quickest on epoxy, but a 20 mm joint means a hand lift.

2. Wheel diameter tiers and the ground each handles

Ground sets the floor of the range and load the ceiling. On plant floors the answer is 100 mm, and the reason is the threshold.

Plastic toolbox on wheels: dark blue hard case tilted on a dark bench, folding handle on the long side, two wide latches, grey gradient background

Measure the narrowest gap, then the tallest step, then the longest run. One third of the gap is the lower bound for diameter, and 15 mm is the step most plants settle on: a 100 mm caster with a 90 Shore A tread clears it.

Concrete carries expansion joints and grit, epoxy marks under sharp edges, grating gaps run to 30 mm, and yard gravel sits near 10 mm. Epoxy asks least of a caster and grating the most. A plastic toolbox on wheels crossing a grating must come in on the diagonal, or the wheel drops through. Gravel adds about forty per cent to rolling resistance. On wet concrete PU grip falls roughly thirty per cent, so the case is slowed rather than shoved. Tread wear starts as rim fraying, and a frayed wheel slips long before the tread is flat.

3. PU PP and TPR hardness bands

Tread behaviour splits between hardness and rebound. Hardness reads on the Shore A scale under ISO 868, and one wheel measures three to five points apart across temperatures.

PU: 85 to 92 Shore A, abrasion resistant, low rolling resistance, thirty per cent less grip when wet.
PP: near 95 Shore A, hard, cheap, noisy, light loads on flat flooring only.
TPR: about 85 Shore A, soft, quiet, grippy, wears fast, suited to indoor corridors.
Below 80 Shore A a tread flats under standing load; above 95 it feeds vibration into the case.

Cold storage pushes treads harder. At minus 10 °C TPR stiffens five points and rattles, while PU loses rebound and slips at joints.

4. Static and dynamic load plus brakes and swivels

Static and dynamic ratings are two numbers; mixing them under-specifies a caster. Static load means no permanent set after 24 hours standing; dynamic load is the cycling load while rolling, usually half the static figure.

Plastic toolbox on wheels: blue blow moulded case closed on a workshop bench, white screen laptop and dial gauge beside, timber shelving behind

Add four caster ratings and multiply by 0.6 for uneven sharing, and that is the usable total. A plastic toolbox on wheels rated 120 kg static and 60 kg dynamic is near its limit at 70 kg, because weight shifts forward as it rolls.

Item	Value to check	What it means
Static load	120 kg for 24 h	Rim takes no set beyond 1 mm
Dynamic load	60 kg rolling	One wheel survives 10,000 cycles
Brake	Twin wheel brake	Holds on an 8° ramp

Swivel layout sets aisle width: four swivels need 1.5 times the case length to turn, two fixed plus two swivel 1.2. A tread brake on a swivel also blocks rotation. Parked on a ramp, a plastic toolbox on wheels holds when both brakes go down.

5. Telescopic handle stages and the staircase limit

More stages do not make a better handle. Three cover users from 1550 to 1800 mm: 700 mm retracted, 850 mm mid, 1000 mm extended.

Stages: keep clearance within 0.3 mm, since rattle comes from play, not tube gauge.
Tubes: 22 mm outer, 19 mm inner aluminium, wall from 1.2 mm.
Sealing: IP54 under IEC 60529 resists splashes; wash-down needs IP65.
Vibration: bolt retention per MIL-STD-810H Method 514.8, with locking washers.
Salt spray: axles and brake plates run 96 hours of neutral salt spray.

Stairs are the usual misuse. One 150 mm step puts a side load into the caster plate, and ten in a row crack the joint.

Conclusion

Sizing casters means measuring the route into three numbers: the narrowest gap sets the diameter floor, the longest run sets the tread, and the loaded weight sets the static-to-dynamic margin. Get those right and 75 mm feels quick on epoxy; get them wrong and 125 mm still stops at the threshold. Against the floor you actually have, the plastic toolbox on wheels rolls as promised.

This article discusses only the container (case body) side of specifications and design intent, and does not cover any handling, loading or use of the contents.