Steel has been used in lifting applications since the 1870’s. Before this time, iron structures were predominantly used. Aluminum, on the other hand is, is relatively new to the industry. Aluminum lifting devices have become prevalent across the industry much more recently.
Both aluminum and steel have their advantages and disadvantages when used for industrial lifting. This article looks specifically at gantry cranes, exploring the differences between the two material choices, and offering advice to users and specifiers.
Aluminum vs steel - strength-to-weight ratio
For gantry cranes with comparable load rating and size, steel can weigh 2.5 to 3 times more than aluminum. This significant difference in strength to weight ratio means aluminum gantries have a lower overall weight than steel and are easier to handle manually.
In a commercial landscape where worker health and safety has never been more important, the ease of assembly, operation and disassembly of tools is paramount. The fact that aluminum gantries are essentially portable means they offer significant economic benefits.
Corrosion resistance
Any coating used to cover a steel surface will eventually peel, chip, crack or erode. This will leave an exposed surface that is prone to rusting. This dramatically decreases the life of the equipment and can cause safety concerns if inspections are not regularly carried out. Aluminum does not have the same issue, especially when anodized (artificially aged).
A report by E. W. Skerrey from the Research Laboratories of British Aluminum showed little change in the surface of anodized aluminum over 30+ years of salt spray testing. The tests highlighted little or no degradation in the mechanical properties of the aluminum alloy. This is particularly helpful in environments such as coastal regions or marine vessels, where the salt spray or high air humidity will cause rusting issues with steel at a much faster rate than normal.
BS EN ISO 12944-2 (British Standard) states that for a very high corrosion area (Category 5 of 6), around 80-200 microns of material thickness can be lost per year in low-carbon steel. With no need to repaint or treat surfaces at regular intervals, the aluminum structure will warrant a lower maintenance cost over the product’s life cycle.
Deflection
Deflection (in this case, curvature of a beam under load) can be quite pronounced in aluminum gantries. The ratio of length to deflection can vary, mainly depending on the material, but also on the design of the structure. This will present itself in a steel beam appearing to stay fairly level under load, while an aluminum beam can bend but then return level once the load is lifted.
What is often not discussed about this difference is how the deflection can serve as a safety mechanism. When the load limit on an aluminum gantry is exceeded, the beam will slowly continue to bend further and further, eventually the material will reach its yield point and this is referred as soft failure or benign failure. The way to identify this failure has occurred, it’s once the load has been removed from the system, permanent deformation can be seen, meaning the beam will not return to its original position.
The bend in the beam can also serve as a visual overload indicator. The operator will have ample time to notice the excess load and release the strain on the system before failure occurs.
The same cannot be said for steel structures. When overloaded, steel will not bend in the same way as aluminum, instead, it will remain relatively stiff and level. Eventually, when the pressure becomes too strong, the structure will snap in what is referred to as “catastrophic failure.” i.e. the load is dropped rather than lowered slowly.
The other side of the deflection argument favours steel systems, however. With a loaded beam remaining relatively flat, the movement of a trolley along the steel beam remains very easy. The same cannot be said for aluminum, especially when dealing with higher loads and longer beams.
The deflection referenced earlier creates a slope on either side of the trolley. Even a small incline of 2 degrees becomes difficult to overcome when the load is 6000lbs. This effect can be mitigated with the use of multiple trollies, or gearing systems that use mechanical advantage to overcome the slope, but the larger functionality issue remains.
Overall material strength
Although aluminum has the upper hand in strength to weight ratio, steel most certainly has the advantage in overall strength. Steel gantries are available in anything from 0.5t to over 100t capacity. The world’s leading aluminum gantries have a maximum capacity of 5t.
Steel gantries are also available in much larger configurations, both in height and beam length. It’s not unusual to find steel gantries with 100ft beams, where the maximum available aluminum beam length is around 30ft.
Conclusion
The most important consideration regarding gantry material selection should be based entirely on the individual scenario or application of the gantry. If you’re looking for a 20t gantry to be permanently installed in a warehouse for a single purpose, then steel is definitely your best option.
If, however, you’re looking for a gantry that you can lift into the back of your truck, take to multiple sites and set up quickly to lift a few tons, then an aluminum gantry crane will be much better suited. There is no clear winner between the two for all situations; instead, there are two options to choose from that give the flexibility to suit all the multitude of applications gantries will be used for.
For more information, or if you’re interested in a range of aluminum gantry systems, check out our PORTA GANTRY and PORTA GANTRY RAPIDE, give us a call on +1-(888)-721-6411 or contact us online.