Crane Shock Absorption

Q: How do you size a crane shock absorber’s stroke?

Screen the energy first: S ≥ m_eff v_rel² / (2 μ η W_ref (ψ_allow − 1)), with μ the usable share of the stroke and η the share its force–stroke curve fills. Then verify the actual peak with the configured force–stroke curve and the stiffness of the crane, wire and rigging.

A crane shock absorber sits in line below the hook and turns a sudden load, such as a running pile or a snatch at lift-off, into a controlled force over its stroke. How it works, how to size its stroke, and what it did in a 1,500 t pile run.

By Tord Martinsen, CEO · October 2025 · Updated September 2026 · 9 min read

POLARIS shock absorber with the connected pin, shackle and slack arrest slings.

In brief

A crane shock absorber is a hydraulic cylinder with a gas spring, fitted in line between hook and load. When the load is jerked, falls or runs, it strokes and holds the line force near a set level instead of letting it spike.
The energy it has to take is the moving load’s kinetic energy; the force it may use is the allowed peak minus the static load. Its stroke turns one into the other.
A first screen: with 90 % of the stroke usable, a force–stroke factor of 0.5 and an allowed DAF of 1.3, a load arriving at 3.13 m/s needs about 3.7 m of stroke.
In a matched CONSTELLATION study of a 1,500 t pile run at 5.0 m/s, POLARIS held the hook at DAF 1.43 against 2.28 for a rigid link, using 4.98 m of its 6.0 m stroke.

Heave compensators deal with the repeating motion of the waves. A shock absorber deals with one event: a pile that runs through a soft layer while the crane holds its hammer, a load snatched off a supply vessel’s deck as the vessel drops, a sling that goes slack and snaps taut. Without one, the event is stopped by the stretch of the wire and rigging alone. That stretch is short, so the force is high.

What a shock absorber does

While the lift is steady, the absorber carries the static load like a link in the rigging. When the load is jerked, falls or runs, the unit strokes. Its gas spring takes up the first part of the event, then a valve opens at a set pressure, so the line force rises towards a set level and holds near it while the moving load is brought to rest. The event’s energy goes into heating the oil instead of into a force spike in the crane, the wire and the rigging.

Figure 1. What the absorber does with the event’s energy. The line force rises from the static load, takes part of the stroke to build up, and eases as the load slows; at the screen’s design point the load stops at the end of the usable stroke. The plain shaded area above the static load is the energy taken; the dotted rectangle is the most the usable stroke could take at the allowed peak, and the hatched part is what the curve leaves. The share taken is the factor η in the sizing screen. Schematic.

The shape of that force–stroke curve decides how much of the stroke is put to work. A curve that climbs quickly to the allowed peak and holds it uses the stroke well; a slow ramp wastes the first part of it. That share is the factor η in the sizing screen below.

Sizing the stroke

The event brings kinetic energy: the effective moving mass $m_{\text{eff}}$ at the relative engagement speed $v_{\text{rel}}$ . It is often easier to state as the equivalent free-fall distance $h$ :

E_{\text{event}} = \tfrac{1}{2}\, m_{\text{eff}}\, v_{\text{rel}}^{2} = m_{\text{eff}}\, g\, h

The absorber may use the force above the static load, up to the allowed peak, over its usable stroke. With $W_{\text{ref}}$ the static load the DAF refers to, $\psi_{\text{allow}}$ the allowed DAF, $S$ the stroke, $\mu$ its usable share and $\eta$ the share of that rectangle, up to where the load stops, that the force–stroke curve fills (Figure 1), the energy it can take is:

E_{\text{available}} = \mu\, \eta\, S\, W_{\text{ref}} \left( \psi_{\text{allow}} - 1 \right)

Setting the two equal gives the stroke a first screen asks for:

S \ge \frac{m_{\text{eff}}\, v_{\text{rel}}^{2}}{2\, \mu\, \eta\, W_{\text{ref}} \left( \psi_{\text{allow}} - 1 \right)}

This page uses $\mu = 0.9$ , which leaves a margin at the end stops, and $\eta = 0.5$ throughout, in the worked example and in Figure 2 alike. Real curves fill about half of the rectangle: the force takes part of the stroke to build up and eases as the load slows. In the pile-run study below, the curve filled about 0.53 of it over the 4.98 m it used, so 0.5 is close and errs on the safe side.

Worked example

The event. A load engages at the speed of a free fall from $h = 0.50\ \text{m}$ : $v_{\text{rel}} = \sqrt{2 g h} \approx 3.13\ \text{m/s}$ .
The limits. Allowed DAF 1.3, effective mass equal to the reference mass, $\mu = 0.9$ and $\eta = 0.5$ .
The stroke. With equal masses the screen reduces to
$S \ge \frac{h}{\mu\, \eta\, (\psi_{\text{allow}} - 1)}$
so
$S \ge \frac{0.50}{0.9 \cdot 0.5 \cdot 0.3} \approx 3.70\ \text{m}$
A 4 m unit passes.

The screen checks energy only. The actual peak comes from the configured force–stroke curve together with the stiffness of the crane, wire and rigging, and a transient analysis confirms that it stays below the allowed DAF with stroke to spare.

Figure 2. The fastest engagement a stroke can take at each allowed DAF, on the same basis as the worked example: 90 % usable stroke, η = 0.5, effective mass equal to the reference mass. Read up from your stroke to your allowed DAF. The stroke needed grows with the square of the speed.

Checked against a pile run

In our matched CONSTELLATION study CS-2026-005, a 1,500 t pile and hammer runs at 5.0 m/s. The same model was run twice from one parameter set: once with a rigid link, once with a 2,000 t POLARIS with 6.0 m of stroke.

Result	Rigid link	With POLARIS
Peak line load	37.7 MN	23.6 MN
Hook DAF, on the 16,553 kN static load	2.28	1.43
Full-slack rebounds after the arrest	2	0
Stroke used	–	4.98 m of 6.0 m

The screen agrees. The 1,500 t moving mass at 5.0 m/s brings

\tfrac{1}{2} \cdot 1{,}500\ \text{t} \cdot (5.0\ \text{m/s})^{2} = 18.75\ \text{MJ}

On the 16,553 kN static hook load, at the DAF the study reached and with $\mu\, \eta = 0.9 \cdot 0.5 = 0.45$ , each metre of stroke can take

0.45 \cdot 16.553\ \text{MN} \cdot 0.43 \approx 3.203\ \text{MJ/m}

so the screen asks for $18.75 / 3.203 \approx 5.85\ \text{m}$ of stroke. The 6.0 m unit passes, and the simulation used 4.98 m. Read the pile-run arrest study →

Shock absorber or heave compensator?

A discrete event points to a shock absorber; repeated wave-cycle motion or a continuous tension duty points to heave compensation. Pick-up from a supply vessel shows the boundary. The snap happens once per lift, when the line takes the load as the vessel drops, so a shock absorber such as POLARIS takes it. A lift that stays connected to the moving deck is a heave compensator’s job. For the cases in between, how often the event recurs, how quickly the unit must reset, the heat from repeated events and the stroke available decide.

Two ways to build one

Conventional shock absorbers put the gas spring in external accumulators and control the flow with a restriction, which may or may not be adjustable. POLARIS, our lightweight design, keeps a small gas volume inside one hydraulic cylinder and controls the flow with CFD-designed valves.

	Accumulator-based	POLARIS
Working principle	Gas spring in external accumulators, with a hydraulic restriction	Small internal gas volume and CFD-designed valves in one cylinder; no external accumulators or needle valves
Adjustment	The restriction may or may not be adjustable	Valve and preset chosen for the stated event
Weight and footprint	External accumulators add mass and deck space	Fewer parts, so lighter and smaller for a comparable duty
Setup	Accumulator filling and valve setting per configuration	No external accumulator filling; preset per configuration
Supply	Several vendors	Patented (Norwegian patent NO 349575); supplied only by Norwegian Dynamics

Frequently asked questions

What does a crane shock absorber do?

It sits in line below the hook and turns a sudden load into a controlled force over its stroke. When the load is jerked, falls or runs, the unit strokes and holds the line force near a set level while the load is brought to rest, so the crane, wire and rigging do not see the spike a rigid connection would.

How do you size a crane shock absorber’s stroke?

Screen the energy first: $S \ge m_{\text{eff}}\, v_{\text{rel}}^{2} / \left( 2 \mu \eta W_{\text{ref}} (\psi_{\text{allow}} - 1) \right)$ , with $\mu$ the usable share of the stroke and $\eta$ the share its force–stroke curve fills. Then verify the actual peak with the configured force–stroke curve and the stiffness of the crane, wire and rigging.

Do I need a shock absorber or a heave compensator?

A shock absorber is for a discrete event: a pile run, a snatch at lift-off, a sling that snaps taut. Heave compensation is for repeated wave motion and continuous tension duty. How often the event recurs, how quickly the unit must reset, heat and the stroke available decide the cases in between.

Can a shock absorber take the snap at supply-vessel pick-up?

Yes. When a load is lifted off a supply vessel’s deck and the vessel drops, the line can snap taut; an in-line shock absorber such as POLARIS limits that peak. For lifts that stay connected to the moving deck, a heave compensator is the better fit.

What did POLARIS do in the pile-run study?

In our matched CONSTELLATION study CS-2026-005, a 1,500 t pile and hammer running at 5.0 m/s reached a hook DAF of 2.28 with a rigid link and 1.43 with a 2,000 t POLARIS. The peak line load fell from 37.7 MN to 23.6 MN, the unit used 4.98 m of its 6.0 m stroke, and no full-slack rebound followed the arrest (two with a rigid link).

How does a shock absorber protect crane capacity?

By keeping the peak line load below what the crane chart allows for the lift. A lower verified peak or DAF can preserve margin in the applicable chart cell; check the chart’s load definition, radius and configuration, the reference load and the project criteria.

Sources and assumptions

The screen is an energy balance on the effective moving mass, with $\mu = 0.9$ and $\eta = 0.5$ throughout. η counts only the force above the static load, up to where the load stops; at the screen’s design point that is the usable stroke. Back-calculated from CS-2026-005 (1,500 t at 5.0 m/s, 16,553 kN static load, DAF 1.43, 4.98 m used) it is about 0.53. The energy-absorption efficiency on the POLARIS page is a different ratio (all of the force over the peak force), which is why it is higher.
Pile-run results are from our published CONSTELLATION study CS-2026-005, a modelled study rather than a certified project analysis.
Final sizing checks the configured force–stroke curve, the stiffness and preload of the crane, wire and rigging, the stroke margin, event recurrence, reset and heat, the environment and the project or class basis.