5.2 Causes of Crush and Shear Hazards

Causes of crush and shear hazards

Domain 1L asks you to name why a playground amputates or crushes a body part, not merely to wave a dowel. CPSC 2025 §3.1 Crush and Shearing Points states that anything that could crush or shear limbs should not be accessible to children on a playground. Crush and shear points are caused by parts moving relative to each other, or to a fixed part, during a normal use cycle — the handbook’s own example is a seesaw. To decide whether a point is a hazard, CPSC tells you to consider two facts together: the likelihood a child could get a body part inside the point, and the closing force around the point. Testing criteria live in ASTM F1487; equipment-specific notes live in CPSC §5.3. Domain 1M will put the rod in the gap. This section is the cause.

Keep the vocabulary separate from the last two chapters. Entrapment is a head or neck stuck in an opening. Entanglement is clothing or a cord catching and cinching. Protrusion is impact on a projection. Crush/shear is two surfaces that close on tissue while the equipment moves. A merry-go-round undercarriage can be all four on the same visit; write the finding that matches the mechanism.

What “two parts moving” looks like in the field

The injury is mechanical. A finger, hand, arm, foot, or head occupies a gap. The gap gets smaller as the ride cycles. Tissue is crushed (compressed between two surfaces) or sheared (cut as one surface slides past the other). Fingers are the most common occupant of a sub-inch gap; a limb or a head occupies the larger under-platform and under-seat spaces. Closing force on public-use moving equipment is more than enough to amputate. That is why CPSC §3.1 treats an accessible crush/shear point as a condition that must not be accessible, and why CPSI course ranking treats crush/shear on moving equipment as a Priority 1 / imminent finding.

CPSC §3.1 then sends you to §5.3 for equipment that has already produced this injury pattern. Memorize the classic locations. Each one is a pair of surfaces plus a foreseeable motion.

• Merry-go-round / spinning-platform to ground (or to a skirt). The platform rotates over the surfacing. A child lying, sitting, or reaching under the deck puts a finger or a limb into the closing gap. CPSC §5.3.4 requires the undercarriage to be free of accessible shearing or crushing mechanisms, forbids up-and-down (oscillatory) motion that would pump the gap, and sets flush-platform and raised-platform clearance rules described below.
• Seesaw fulcrum. The board rotates on a center pin. CPSC §5.3.5.1 states that the fulcrum should not present a crush hazard, and that partial car tires or other shock-absorbing material should sit under the seats (or on the underside of the seats) so a limb is not crushed between seat and ground.
• Track-ride / trolley under-seat clearance. The trolley rolls along a track or cable. A hanging foot or a child standing under the path occupies the space beneath the seat. The NRPA Table (F1487 8.13.2.3) sets a 12 in minimum under-seat clearance for seated trolleys — that dimension is a crush/clearance cause, not a decoration.
• Swing hanger joints. The hanger pivots as the occupant swings. A finger in the joint is sheared as the bushing cycles. Connecting-device wear that opens a moving pinch is a crush/shear cause even if the same hardware is also an entanglement S-hook.
• Spring-rocker coil springs. Adjacent coils open and close as the rider bounces. A finger between coils is the textbook shear.
• Roller-slide rollers. Adjacent rollers, and the ends of the rollers against the stationary rail, create a series of in-running nips. CPSC §5.3.6.3.2 requires those spaces to be less than 3/16 in.
• Gate hinges on play enclosures. A play-area gate that can close on a finger at the hinge knuckle is the same two-surface mechanism on a vertical pin.

Equipment-specific probe diameters are part of the cause

F1487 does not use one rod for every machine. The diameter of the probe is the diameter of the body part the standard is protecting at that equipment. Know the three published figures before you walk onto a moving piece.

• Common crush/shear probe: 0.62 in (15.7 mm) neoprene rod (NRPA Table, F1487 6.5). This is the default finger-sized rod for seesaws, spring rockers, swing hangers, gates, and other general moving gaps.
• Merry-go-rounds: 0.30 in (5/16 in) (NRPA Table, CPSC §5.3.4). CPSC §5.3.4 also says that a platform flush to the protective surfacing may have no gap greater than 5/16 in measured in any direction. The NRPA Table prints that flush maximum as 0.312 in (F1487 8.8.1.5). A spinning deck that is supposed to sit on the surfacing cannot leave a finger-wide slot.
• Roller slides: 0.19 in (3/16 in) (NRPA Table, F1487 8.9.2.1; CPSC §5.3.6.3.2). Adjacent rollers and roller-to-rail gaps that admit that rod are an in-running nip.

Rotating-equipment clearance bands — the 1/2-inch versus 4-inch scenario

Raised spinning platforms have a second crush family: the under-deck clearance that can take a limb or a head, not just a finger. CPSC §5.3.4 (spinning equipment at least 20 in in diameter) says the clearance between the lowest point of the underside — including a protective skirt, if provided — and the protective surfacing should be greater than 9 in or less than 3.5 in, so long as a space less than 3.5 in does not itself introduce an accessible crushing or shearing point. The NRPA Table (F1487 8.8.1.6) tightens the “small” band for vertical rotating equipment with a protective skirt, and for rigid platforms that are not flush, to 2.38–3.5 in when the clearance is not already greater than 9 in. Those two numbers are the published skirt range. A gap that sits between 3.5 in and 9 in is the body-part zone: large enough for a limb or a head, small enough that the rotating deck can still close on it.

Work the exam scenario. Station A under a spinning platform is a 1/2-inch (0.50 in) gap. That gap admits the 0.30 in merry-go-round crush/shear probe and exceeds the 0.312 in (5/16 in) flush-platform maximum. It is an accessible finger crush/shear. Fail it. Station B is a 4-inch gap. Four inches is larger than 3.5 in and smaller than 9 in. It is outside the 2.38–3.5 in protective-skirt range and it does not earn the 9-inch large-clearance relief. A child’s arm or head can occupy that space while the deck turns. Fail it. Do not pass a 4-inch under-deck gap because “it is bigger than a finger.” Do not pass a 1/2-inch gap because “it is smaller than the 0.62 in common rod” — this is rotating equipment, so the published rod is 0.30 in, not 0.62 in.

The owner still chooses repair (skirt to 2.38–3.5 in, raise the underside above 9 in without abrupt rigid members, or close a flush deck to ≤5/16 in) versus removal. You name the cause, cite CPSC §3.1 / §5.3.4 / §5.3.5 / §5.3.6.3.2 or the matching F1487 section, and rank an accessible moving crush/shear as Priority 1.