Crane Shock Absorption

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:

\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.

Energy balance crane shock absorber

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.

Dynamic amplification vs speed and stroke curves using crane shock absorber

Types of shock absorbers

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

  1. 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.
  2. Lightweight shock absorbers, which is a patented product only supplied by Norwegian Dynamics, which effectively removes the gas spring and uses dynamic damping that increases efficiency of the shock absorber. The main benefits for the customer are lower cost, lighter weight, smaller footprint and faster setup.