The effects of shock loading
Wednesday, 28 August 2019
What is shock loading?
The application of a load at a very high rate. More precisely, it is the application of load at a higher rate than has been accommodated in the design of the equipment.
All lifting involves acceleration. We start with something down low and raise it up off the ground by accelerating it with a crane. The payload itself goes from a speed of zero to whatever the hoist speed of the crane ends up being. It usually does this within a time frame we can measure in seconds (or fractions of seconds).
Well, we know that we accelerate our payloads when we use our crane. This acceleration is added to the acceleration of gravity that is already there. This gives us dynamic amplification of the forces in our slings.
How much is this amplification? Well, this can vary. But for standard cranes in a standard lifting environment we’ll expect something in the range of 1.1 times or 1.5 times the force due to gravity. This varies depending upon the crane type, the mass of the payload, and how the two are operated.
Those numbers are ‘dynamic’ factors. They quantify the increase in the force experienced by our sling.
So - if someone lifts a 1000kg payload with a WLL 1000kg sling, we aren’t at all surprised to measure a peak force of more than 1000kgf. Maybe this could be 1000 x 1.5 = 1500kg.
We know that in a standard lifting environment the dynamic factor is seldom more than 1.5 and never more than 2 – why? Because a 2 times WLL proof load is such a common requirement in lifting tackle standards. More than that and things tend to bend or break.
Standards compliant, standards tested lifting gear for a standard lifting environment – remember that!
Stick within the WLL too! The dynamic factor is probably eating away all of that ‘safety margin’ your ‘old mate’ imagined was there…
There also are special lifting environments. Lifting at sea is perhaps the best example. Whilst on the water the heave of a ship can add yet more dynamic acceleration to a crane. That increases the dynamic factor and the lifting gear needs to be made stronger to cope.
Done properly, it is better than standard lifting gear that is required and this must be compliant for a special lifting environment – another thing to remember!
Is this shock loading?
Not really – a special lifting environment is a predictable scenario and should be accounted for… but if overloading occurs sometimes the after effects can look pretty similar. There are lots of reasons for shock loading, but there are basically two types of shock loading. These are:
- Rapid load application
- Rapid load release.
How does shock loading occur?
There are various scenarios. These include:
- The load being lifted is dropped a distance before the sling becomes tight.
- The load is jerked as a result of a crane being raised before the sling is tight.
- The hook is not properly seated before lifting commences causing the load to move in the hook.
- Shuddering of the crane during moving, lifting or lowering.
- Something breaks – shock loading the rest.
- A collision occurs.
- Actually, there are lots of other scenarios – too many to list here.
In every situation, the dynamic loading and acceleration exceeds the norm, and exceeds the designed in strength that the standards specify.
The classic cases are when something breaks, or when a payload falls and is ‘caught’ by a sling.
In these cases the change in speed is almost instant, thus the acceleration is enormous and the loads can be extreme.
Why is shock loading dangerous?
The obvious answer is that it breaks things, and if it doesn’t break them it might inflict such severe damage that they break later on.
There are two basic damage mechanisms. The simplest of these is overloading. At some point the ‘shock’ exerts a peak force that is sufficient to damage something.
The second is less obvious and is due to a high rate of change of load. This could be the load being applied too fast, or released too quickly. This can be a complex phenomenon, and we only have space here to mention a couple:
- If load is applied too quickly to a synthetic device (e.g. sling), heat due to friction can be enough to melt or degrade some of the material.
- A jib arm might be built to resist downward loads very well, and although it should be built very stiff it will still deflect under load. This means that if the load is released rapidly, there is stored energy in the jib and it can flick back up – loading it in reverse!
- A lifting system might have different parts which share the load forces, but which react to those forces at different rates. A classic example being a rotation resistant rope. If the load share on the rope core is applied or released too fast then the core no longer shares load with the strands and the resultant imbalance can cause the rope to have a hernia – which we call a ‘birdcage’.
There are many different phenomena. Rapid load release of course can accompany rapid load application (because something broke). This means that various symptoms and causes throughout a complex system such as a crane and its rigging can be disastrous in terms of rectification cost and manifold risks to personnel.
Avoiding all un-planned shock loads (including release of loads) cannot be overemphasized.
So – what can be done to avoid shock loading.
- Know the load. What does it weigh? Don’t know? Then use a loadcell.
- Know the rigging. What is the WLL? Has it been inspected? Has it been maintained? Is it good quality? Can you trust it? If there’s no WLL and no trust – then don’t lift.
- Know the stability. Where’s the centre of gravity? Will this be controlled by the rigging arrangement? – Don’t know? Do a lift study?
- Know the lift path. Where’s it going? How does it get there? Will it crash, snag or rest against anything? Will it get bumped? Is the landing area safe? Who will direct the crane? How will the payload be steered past obstacles? Don’t know? Prepare a lift plan.
- Always gently take up the slack in slings before lifting off. Check everything is secure and in place.
- Always gently lift the payload just off the ground and pause before taking it any further – ensure it is secure, stable and level before proceeding further.
- Always take enough time, and enough care to do the job safely every time.
The right rigging gear, and the right advice is the starting point for safe lifting and avoiding many things which can lead to shock loading. Nobles can supply you with the right lifting gear, the best inspection and maintenance services, and the right advice. We also offer loadcells and engineering services to analyse the lifting gear design, the lift study, and (should the worst occur) the aftermath – with scientific investigation of shock load damage.
To speak to one of our lifting and rigging specialists, please call 1300 711 559 or send them an email at email@example.com.
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