KNOWLEDGE HUB / DISCRETE SHOCK EVENTS

Crane Shock Absorption

Screen the energy. Then verify the peak.

By Tord Martinsen, CEO · October 2025 · Reviewed August 2026 · 8 min read

REVISED ILLUSTRATIVE EXAMPLE / 50 CM EVENT

Passing the energy screen is one result.

The example compares the event demand with a simplified energy allowance. The actual transient peak still needs a separate verification.

EQUIVALENT EVENT DISTANCE50 cm

What the event brings.

Relative engagement speed of approximately 3.13 m/s corresponds to approximately 0.500 m of equivalent free-fall/run distance.

EQUIVALENT ENERGY ALLOWANCE54 cm

What this screen allows.

For the declared assumptions, μ × η × S × (ψallow − 1) gives 0.540 m. This is not the physical stroke of the absorber.

Worked-example assumptions

Effective mass equals reference mass · physical stroke S = 4 m · usable-stroke fraction μ = 0.9 · energy factor η = 0.5 · allowed DAF ψallow = 1.3.

Required physical stroke from this screen: 0.50 ÷ [0.9 × 0.5 × (1.3 − 1)] ≈ 3.70 m. This example uses 4 m.

EventEquivalent distance

50 cm

AllowanceFor this model basis

54 cm

0204060 cm

The bars compare equivalent event distances under the equal-mass assumption. They do not represent physical stroke, a measured force–stroke curve or verified peak loads.

Energy screen: passes under these assumptions.

50 cm is below 54 cm. That does not establish that the transient peak stays below the allowed DAF of 1.3. The configured force–stroke response and coupled crane, wire, rigging and payload system must verify the actual peak.

Read the energy equations and revised calculation ↓

Revised worked example and assumptions

Worked screening example: from relative velocity to stroke

The event. Use an illustrative equivalent free-fall/run distance of 50 cm. This corresponds to a relative engagement speed of approximately 3.13 m/s.
The stated assumptions. Let m_eff = m_ref, physical stroke S = 4 m, usable-stroke fraction μ = 0.9, energy factor η = 0.5 and allowed DAF ψ_allow = 1.3. These are example inputs, not transferable design values.
The screen. Required physical stroke is about 3.70 m. Using 4 m gives h_cap = μ η S (ψ_allow − 1) = 54 cm. Since 50 cm ≤ 54 cm, the example passes the energy screen. A transient check must still verify the actual peak and working-stroke margin.

The chart in the chart-assumptions section is a separate illustrative screen and uses the assumptions stated in its caption.

TWO SEPARATE ILLUSTRATIVE BASES

Do not read the chart as the worked example.

The original chart and worked example use different energy factors. Keep their assumptions attached to their results.

Worked exampleη = 0.5

With μ = 0.9, S = 4 m, equal effective/reference mass and allowed DAF 1.3, the example gives a 54 cm equivalent allowance.

Original chartη = 0.9

The chart uses μ = 0.9, equal effective/reference mass and g = 9.81 m/s². It varies physical stroke and allowed DAF to plot maximum relative engagement speed.

Neither factor is a universal product value. Use the case-specific usable-stroke fraction and verified force–stroke response. A plotted curve is an illustrative screen, not an equipment performance certificate.

Maximum payload engagement velocity against shock-absorber stroke for allowed dynamic amplification factors from 1.1 to 2.0 (energy balance, illustrative) — Original illustrative chart: μ = 0.9, η = 0.9, g = 9.81 m/s² and effective mass equal to reference mass. The worked example uses η = 0.5 instead.

BEYOND THE ENERGY SCREEN

Keep three checks in the selection.

Reference and demand

Define effective moving mass, relative engagement speed or equivalent event distance, allowed peak and the static load used as its DAF reference.

Transient response

Verify the configured force–stroke curve, system stiffness and preload, actual peak and the margin within usable travel.

Repeated duty

Check recurrence, reset time, thermal duty, environment, orientation and the nominated project/class basis.

Use the equipment configuration and project event to establish the assessment. See the inputs to send with a case →

Energy balance

A crane shock absorber reduces a payload’s relative engagement speed over a controlled stroke. For a first-pass screen, define the effective mass participating in the event, $m_{eff}$ , and either the relative engagement speed, $v_{rel}$ , or its equivalent free-fall/run distance, $h$ :

Practical application: this screen is one input to a case-specific selection; see POLARIS crane shock absorber and engineering studies and analysis.

E_{event} = \frac{1}{2} m_{eff} v_{rel}^{2} = m_{eff} g h

The simplified absorber-side energy allowance used on this page is:

E_{available} \approx \mu \eta S W_{ref} (\psi_{allow} – 1)

Here $\mu$ is the case-specific usable fraction of the physical stroke, $\eta$ represents the verified force–stroke response used for the screen, $S$ is physical stroke, $W_{ref}$ is the stated static load used as the DAF reference, and $\psi_{allow}$ is the permitted peak divided by that reference. The resulting first-pass stroke is:

S_{req} \ge \frac{m_{eff} v_{rel}^{2}}{2 \mu \eta W_{ref} (\psi_{allow} – 1)}

This is a screening relation, not final peak-load verification. Final sizing checks the configured force–stroke curve, crane/wire/rigging stiffness and preload, stroke margin, event recurrence, reset and thermal duty, environment, orientation and the project/class basis.

Illustrative screen. Assume $m_{eff}=m_{ref}$ , a 4 m physical stroke, a case-specific usable-stroke fraction $\mu=0.9$ , $\eta=0.5$ , and an allowed peak $\psi_{allow}=1.3$ . The equivalent event distance that fits the simplified energy allowance is:

h_{cap} = \mu \eta S (\psi_{allow}-1) = 0.9 \cdot 0.5 \cdot 4 \cdot 0.3 = 0.540\ \text{m}

For $v_{rel}=\sqrt{2g\cdot0.50}\approx3.13\ \text{m/s}$ , $h_{event}=v_{rel}^{2}/(2g)=0.500\ \text{m}$ . Because 0.500 m is below 0.540 m, this case passes the simplified energy screen under the stated assumptions. That result does not by itself prove that the transient peak stays below 1.3; the coupled system and configured force–stroke response must verify it.

Finding the correct stroke length

The simplified chart in the chart-assumptions section above illustrates maximum relative engagement speed against physical stroke and allowed DAF. The plotted values correspond to $\mu=0.9$ , $\eta=0.9$ , $g=9.81\ \text{m/s}^{2}$ and $m_{eff}=m_{ref}$ . Those are chart assumptions, not universal product values. Use a project-specific usable-stroke fraction, verified force–stroke response, effective mass and stated DAF reference before selecting a unit.

Types of shock absorbers

This page compares two common arrangements:

Conventional accumulator-based shock absorbers commonly combine a gas spring in external accumulators with a hydraulic flow restriction. The restriction may differ by stroke direction and may or may not be adjustable.
POLARIS, Norwegian Dynamics’ patented lightweight architecture, uses a small internal gas volume as its gas spring and CFD-designed valves within one hydraulic cylinder—with no external accumulators or needle valves. Fewer external components make the package lighter, smaller and more economical for suitable topside shock duties; final performance and setup remain configuration- and case-specific.

Conventional and lightweight crane shock absorbers compared — working principle, adjustability, weight and footprint, cost and setup, and supply.
	Accumulator-based arrangement	POLARIS architecture
Working principle	Gas spring in external accumulators + hydraulic flow restriction	Small internal gas volume + CFD-designed valves in one cylinder — no external accumulators or needle valves
Adjustment	Restriction may or may not be adjustable	Valve and preset selected for the stated event and configuration
Weight & footprint	External components add mass and deck space	Fewer external components; lighter and smaller for a comparable suitable duty
Cost & setup	Configuration-dependent	Integrated layout can be more economical and quicker to set up for suitable duties
Supply	Several vendors	Patented — supplied by Norwegian Dynamics (POLARIS)

A useful first split: a discrete impact or overload event points to a shock absorber; repeated wave-cycle motion and continuous tension duty point to heave compensation. Event recurrence, reset time, thermal duty and available stroke decide the boundary cases.

Crane shock absorption — frequently asked

What does a crane shock absorber do?

It reduces relative engagement speed over a controlled stroke and limits the peak to the case-specific force or DAF used for sizing. It does not inherently hold the load near static weight; the permitted peak depends on the crane, rigging, payload, load reference and project criteria.

How do you size a crane shock absorber?

A first-pass screen compares ½ m_eff v_rel² with μ η S W_ref(ψ_allow − 1). Define the effective moving mass, relative speed or equivalent fall/run distance, usable stroke, verified force–stroke response, allowable peak/DAF and its reference load. The μ = 0.9 and η values on this page are illustrative, not universal. Final sizing also checks system stiffness, margin, recurrence, reset and thermal duty, environment and the project/class basis.

Do I need a shock absorber or a heave compensator?

A shock absorber is normally for a discrete impact or overload event. Heave compensation is for continuous or repeated cyclic motion and tension control. Event recurrence, reset time, thermal duty, stroke and environment determine boundary cases.

What types of crane shock absorbers exist?

Conventional arrangements often use external gas accumulators and adjustable restrictions. POLARIS uses a small internal gas volume plus CFD-designed valves within one cylinder, with no external accumulators or needle valves. Fewer external components make the package lighter, smaller and more economical for suitable topside shock duties.

How does a shock absorber protect crane capacity?

Reducing a verified peak force or DAF may preserve margin in the applicable crane-chart cell. It does not automatically keep full capacity: check the chart load definition, radius and configuration, the force reference and the project criteria.

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.

Adding shock absorption below the hook?

Send the effective moving mass and load definition, relative engagement speed or equivalent fall/run distance, allowable peak or DAF and its reference load, available stroke/headroom, event recurrence/reset duty, environment and project/class basis. We’ll return a case-specific POLARIS screening and the inputs needed for final verification.

See it in action

Passive heave compensator used for shock absorption

Crane Shock Absorption