How do you size a crane shock absorber?

From the energy balance: the absorber takes E = μ·η·S·mg·(ψ−1), with μ = 0.9 as a stroke-reserve safety factor, η the efficiency and S the stroke. Worked example: a 1 m stroke at 50% efficiency limited to 30% dynamic amplification absorbs the equivalent of a 13.5 cm free fall.

Do I need a shock absorber or a heave compensator?

A shock absorber protects against a single event — deck pick-up, lift-off, snag or overload. Where slack-and-snap cycles repeat through the wave zone, heave compensation is the right tool, because it keeps the wire tensioned continuously.

What types of crane shock absorbers exist?

Conventional units combine a gas spring with a flow restriction, much like a basic passive heave compensator. Lightweight shock absorbers — a patented Norwegian Dynamics product — remove the gas spring and use dynamic damping instead, which lowers cost, weight, footprint and setup time.

How does a shock absorber protect crane capacity?

Offshore load charts are derated by the dynamic factor. Keeping the snap peak near the minimum factor instead of letting it spike means the same chart cell keeps most of its capacity — the mechanics are on the crane load chart page.

Crane shock absorption

One bad second decides the peak load — a stroke and a damper turn the spike into a push.

Crane Shock Absorption

Q: What does a crane shock absorber do?

It brings a payload from an initial relative velocity to zero over a controlled stroke, absorbing the energy of a snap or slam event so the peak hook load stays near the static weight instead of spiking to a multiple of it.

Energy balance

Shock absorbers are used to reduce speed of a payload from an initial relative velocity between the shock absorber and the payload to zero. To specify the correct shock absorber settings we must provide either a free fall distance or a relative velocity. The relationship between them is found in the energy balance:

Practical application: For practical application of this topic, see POLARIS crane shock absorber and engineering studies and analysis.

\frac{1}{2} m v^2 = m g h

Where $v$ is the relative velocity, $g$ is gravity and $h$ is the free fall distance.

To learn more about calculating the relative velocity from vessel motion check out the section about load charts.

Shock absorbers absorb energy according to the following equation:

E_{SA} = \mu \eta S m g (\psi – 1)

Where $\mu$ is a safety factor set to 0.9 (to avoid fully using all stroke length), $\eta$ is the shock absorber efficiency, $S$ is the shock absorber stroke length and $\psi$ is the dynamic factor (static weight =1).

A visual representation of the energy absorbed by the shock absorber is shown in the below figure.

As a simple example let us say we have a shock absorber with stroke length 1 meter and efficiency 50 %. What is the maximum free fall the payload can have if allowed dynamic amplification is 30 %?

We then simple use the energy balance to estimate:

m g h = \mu \eta S m g (\psi – 1)

Which simplifies to:

h = \mu \eta S (\psi – 1) = 0.9 \cdot 0.5 \cdot 1 \cdot (1.3-1) = 13.5\ \text{cm}

Finding correct stroke length

It is relatively simple to find the correct stroke length based on the above energy balances, which are illustrated in the (simplified) chart below which gives the maximum allowed relative velocity based on dynamic factor (DAF, $\psi$ and shock absorber stroke.

Worked example: from relative velocity to a stroke that holds

The event. Take the deck-lift example from the crane load chart page: relative velocity v_r = 1.25 m/s at pick-up. Equivalent free fall: h = v²/2g ≈ 8 cm.
The allowance. A 1 m stroke at 50% efficiency, with μ = 0.9 stroke reserve and the dynamic peak limited to ψ = 1.3, absorbs the equivalent of h = 0.9 × 0.5 × 1 × 0.3 = 13.5 cm of free fall.
The check. 8 cm ≤ 13.5 cm — the unit holds the peak below 1.3× static weight, with margin. Faster events need more stroke or higher efficiency: S = h / (μ η (ψ−1)).

Same energy balance as above, run in reverse — the chart in the previous section plots exactly this trade of velocity against stroke and allowed dynamic factor.

Types of shock absorbers

There are two main types of shock absorbers in the market:

Conventional shock absorbers, which do not differ substantially from basic PHCs, that utilize a gas spring combined with a flow restriction (which may differ in extension and retraction direction) that may or may not be adjustable.
Lightweight shock absorbers, which is a patented product only supplied by Norwegian Dynamics, which keeps the gas spring internal and replaces external accumulators and needle valves with CFD-designed valves, raising efficiency of the shock absorber. The main benefits for the customer are lower cost, lighter weight, smaller footprint and faster setup.

	Conventional shock absorber	Lightweight shock absorber
Working principle	Gas spring + flow restriction (like a basic PHC)	Internal gas spring + CFD-designed valves — no external accumulators or needle valves
Adjustability	Restriction may or may not be adjustable	Damping designed to the event
Weight & footprint	Heavier, larger	Lighter, smaller
Cost & setup	Higher cost, slower setup	Lower cost, faster setup
Supply	Several vendors	Patented — supplied by Norwegian Dynamics (POLARIS)

Single event vs repeated cycles is the real dividing line: a shock absorber handles the one-off spike; where slack–snap repeats through the wave zone, use heave compensation.

Crane shock absorption — frequently asked

What does a crane shock absorber do?

It brings the payload from an initial relative velocity to zero over a controlled stroke, absorbing the snap or slam energy so the peak hook load stays near static weight instead of spiking to a multiple of it.

How is one sized?

From the energy balance on this page: E = μ η S mg (ψ−1), with μ = 0.9 stroke reserve. The worked example: 1 m stroke at 50% efficiency limited to ψ = 1.3 absorbs a 13.5 cm free-fall equivalent.

Shock absorber or heave compensator?

Absorber for a single event — deck pick-up, lift-off, snag, overload. Heave compensation for repeated wave-cycle motion, because it keeps the wire tensioned continuously rather than resetting after one hit.

What types exist?

Conventional (gas spring with external accumulators and needle valves) and lightweight (internal gas spring with CFD-designed valves) — the lightweight architecture is a patented Norwegian Dynamics product, lower in cost, weight, footprint and setup time.

How does it protect crane capacity?

Offshore load charts derate by the dynamic factor. Capping the snap peak near the minimum factor keeps the chart cell close to full capacity — the arithmetic is on the crane load chart page.

Related on Norwegian Dynamics

Damping — How orifice tuning sets PHC efficiency across wave periods.
Crane Load Chart — Velocity formula, DAF derating, worked example.
Dynamic Amplification Factor (DAF) — Formula, worked example and DNV context.

Working on a lift that needs this?

POLARIS is purpose-built for crane shock absorption. Send the shock case (load, velocity, height) and we'll size it.

See it in action

Passive heave compensator used for shock absorption

ND SolutionPOLARIS →Crane shock absorber that takes the impact into stroke instead of the crane boom and rigging.