Is tandem lifting dangerous? Master its unsuspected dangers!

Tandem lifting is a technique used in a number of industries to meet specific needs, as mentioned in the article: “What are the advantages of modern tandem lifting?”. The text mentions that this type of lifting involves lifting a load using 2 devices simultaneously, with either 2 hoists/carts or 2 overhead cranes. And it’s precisely this word « simultaneously » that’s at the heart of this article. But why? Because imperfectly synchronized equipment can lead to overloading, resulting in costly damage or even an accident.

With almost half of our projects involving tandem solutions, we’ve come to the conclusion that some of the risks associated with this lifting technique are often unrecognized, even by experienced overhead crane operators. In this article, we explore these hazards and the best measures to avoid them, to ensure safety and peace of mind.

The unsuspected and overlooked dangers of twin lifts

There are two main hazards that are harder to spot when maneuvering two pieces of equipment. Let’s look at them in detail.

Danger 1 : damage or accident due to latent capacity overload

The first risk lies in capacity overload, which is ironically one of the main reasons some operators use this technique! Contradictory? Not necessarily. This danger actually has two distinct origins: a poorly distributed load, or imperfect synchronism during hoisting.

First origin: a poorly distributed load. If the object is not symmetrical, the load may be heavier on one side and exceed the rated capacity of the hoist on that side. For a tandem lift to be performed properly, the weight must be well proportioned relative to the rated capacity of each piece of equipment involved. To avoid this trap, it is essential to have the drawings of the part, which will provide the weight and, ideally, the centre of gravity.

Second origin: imperfect synchronism during hoisting. If the two hoists do not raise the load simultaneously, the entire weight can momentarily end up on a single piece of equipment. Take an example: a 10-tonne part must be moved using 2 wire rope hoists of 5 tonnes each. Used together, these hoists are indeed rated to lift the part. But if synchronism is off and one of the two pieces of equipment bears a load of 8 tonnes, even briefly, it will have taken a shock at 160% of its rated capacity. Even if this situation lasts only a second, this excessive weight can damage the equipment, or even cause an accident.

This danger is even greater when setting the load down. By lowering the load unevenly, the weight can likewise transfer onto the other piece of equipment for an instant. But unlike during hoisting, no safety mechanism can step in: the load limiter, for example, interrupts hoisting to prevent an overload only while raising, not while lowering.

In all of these cases, the overload is latent: it goes unnoticed unless you pay attention, but it is no less dangerous. The solution? Good preparation. Before performing the tandem lift or setting the load down, it is essential to carry out checks that will ensure the part moves in a synchronized manner: sling tension and positioning, as well as hoist synchronism. The operator can then proceed, at slow speed, to raise or lower the load, all while carefully verifying that the part stays parallel to the ground.

Danger 2 : risk of accident due to out-of-sync movement

The second danger is desynchronized horizontal travel, in other words movement along the runway. When the two pieces of equipment no longer move simultaneously, the risk of an incident is significant. Before the operator realizes what is happening, the cranes unintentionally drift apart or closer together. In these circumstances, at least one of the hoists ends up at an angle and destabilizes the part or the lifting accessories. Ultimately, a sling can break or slip and cause an accident, a falling load for example.

Another risk comes from systems put in place precisely to ensure safety: limit switches and anti-collision devices (e.g. laser). A limit switch is designed to automatically shift a crane into slow speed when triggered, or to stop it. So when the first piece of equipment makes contact with the limit switch, it slows down or stops, while the other keeps travelling without any change! The anti-collision system, for its part, is meant to prevent two overhead cranes from colliding: a laser detects the presence of the other crane at a preset distance, then slows or stops the crane. However, if this system is active at the start of a tandem move and the cranes are already within the detection zone, one of the two may detect the other and refuse to move. In both cases, the travel becomes desynchronized and the potential for an accident becomes high.

How can we control the dangers of tandem lifting?

After such an analysis, it is appropriate to ask how these pitfalls can be avoided. The previous examples are not exceptions. They are common situations, and experience in the field shows that the loads being lifted and circumstances described above are often underestimated. But rest assured: there are solutions!

Solution 1 – Operator training : a good prevention practice

First and foremost, training operators to handle tandem lifts and informing them about the hazards associated with this type of lifting can make a significant difference. The workers involved must be thoroughly familiar with the specific method adopted by your company, ensure the load is properly balanced, and understand its center of gravity. Furthermore, Section 7.6 of CSA B167-26 states that these operations must be subject to specific planning, in accordance with ASME P30.1. In practice, this type of operation must therefore be documented. 

Solution 2 – Interconnection of controls : using remote controls in “master/slave” mode, a protection

An optimal solution is to use remote controls in master/slave mode. When the main (master) transmitter is activated, the hoists automatically switch to tandem mode, i.e. they are operated simultaneously by a single transmitter. This also has the effect of deactivating the other “slave” transmitters. This means that hoists can be operated in tandem with a high level of safety.

Solution 3 – Synchronization : Technologies adapted for tandem operation

Synchronism means that both overhead cranes perform the same movement, at the same speed, at the same instant. This is what prevents the load from tilting at an angle or shifting entirely onto a single piece of equipment.

To achieve this, a state communication module is installed between the two overhead cranes. As soon as a movement changes state on the first crane (hoisting, trolley travel, or bridge travel switching to slow speed, fast speed, or stop), a signal is immediately sent to the second crane so that it makes the same change. This mechanism is what solves the limit-switch and anti-collision issues mentioned earlier: when one crane slows down or stops, the other follows at the same instant. Without such a module, you must at minimum provide an automatic bypass system for the safety devices during tandem operations.

The module, however, synchronizes the commands, not the mechanical speeds. If the two cranes do not run at the same speed, they drift between commands, and the gap widens over long distances. The safest solution therefore remains to use two identical cranes (same model, same speeds), which eliminates any difference in behaviour between the two cranes from the outset.

Solution 4 – The load display : to keep control and avoid overload

Another option is the load display. This will indicate the actual weight lifted by each device to detect any overloading. Although this accessory only indicates the actual weight, it still helps raise operator awareness of any potential load imbalance.

Solution 5 – Indicator lights : to inform and raise operator awareness

And as a final option, warning lights can be a welcome addition to safety in this type of operation. What are warning lights? These are lights installed under each piece of equipment, which light up to indicate which overhead cranes or hoists are in operation (not to be confused with a flashing beacon, which lights up when a piece of equipment moves).

Safety: a matter that should not be neglected

In conclusion, you should always be familiar with your lifting parts, i.e. their weight (or even their weight distribution), their center of gravity, their anchoring points, and so on. In general, a lifting plan should be put in place and followed closely in every situation. When it comes to the equipment, the ideal solution is to acquire identical lifting equipment fitted with the appropriate protection, or to modernize existing equipment. And, of course, it’s always important to be well prepared and careful when handling heavy loads, whether conventionally or in tandem.

Now you’re fully equipped to ensure complete safety during your tandem maneuvers. But are there any other safety features related to your overhead cranes that deserve your attention? If so, take a look at our article The top 10 safety options on an overhead crane.