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 is capacity overload… which is one of the main reasons why some people use this technique! Contradictory? Not necessarily.
If the object is not symmetrical, the load may be higher than the hoist’s rated capacity on one side. For a tandem operation to work properly, the weight must be in proportion to the rated capacity of each piece of equipment involved. To avoid this pitfall, it’s essential to have drawings of the object to provide the weight, and perhaps also the center of gravity.
In other cases, the danger lies in imperfect synchronization between the two hoists. If the two hoists don’t lift the load simultaneously, all the weight can momentarily fall on one piece of equipment. Let’s take an example: let’s assume that a 10-ton part has to be moved, and that we have to use 2 wire rope hoists each with a 5-ton capacity. The lift is indeed performed by 2 hoists which, used together, are suitable for lifting this part. But if the two hoists are not synchronized, and one of them is subjected to an 8-ton load, even if only briefly, it will have taken a shock at 160% of its rated capacity. Even if this situation only lasts a second, this excessive weight can damage the equipment or even cause an accident.
However, the danger of capacity overload is even greater when the part is lowered to the ground. How can this happen? By lowering the part unevenly, the weight can also be transferred to the other equipment for a moment. But unlike during lifting, there are no safety mechanisms to prevent overloading, such as a load limiter. The latter’s function is to interrupt lifting to prevent overloading during lifting only, not lowering.
As the previous examples show, these are situations where the overload is latent, meaning that it may not be noticed if you don’t pay attention. But it’s no less dangerous! So, what’s the solution to this type of problem? Simply good preparation! Before performing the tandem lifting or lowering, it’s essential to carry out a number of checks to ensure synchronized movement of the part. For example, you’ll need to check that the slings are correctly tensioned and positioned, and that the hoists are synchronized. The operator can then proceed, at low speed, to lift or lower the load, while carefully checking that the part is parallel to the ground.
Danger 2 : risk of accident due to out-of-sync movement
The second danger is out-of-sync translational movement. When two pieces of equipment are no longer moving horizontally at the same time, the risk of an incident is high. Before the operator is aware of the situation, the hoists unintentionally move towards or away from each other. In such circumstances, at least one of the hoists is at an angle, destabilizing the part or the lifting accessories. Ultimately, a sling may break or slip, resulting in an accident, such as a load fall.
Another risk comes from systems that are put in place precisely to ensure safety! These include limit switches and anti-collision systems (e.g. lasers). But how? The function of a limit switch is to automatically put a device into first gear when it is activated, or to stop it. This means that when the first piece of equipment comes into contact with the limit switch, it will switch to low speed or stop, while the other piece of equipment will continue its run unchanged! As for the anti-collision system, its role is to prevent two overhead cranes from coming into contact. A laser will detect the presence of the other crane at a preset distance and will stop or slow down. However, if this system is activated at the start of a tandem movement, and the equipment is within the detection zone, one of the two pieces of equipment may detect the presence of the other and not move. Both of these situations will result in out-of-sync travel and a high accident potential.
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, the American standard (ASME P30.1, Section 2-1) recommends a lift plan for operations involving multiple lifting points.
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
Another important factor in managing the risks associated with tandem lifting is ensuring the synchronization of the overhead cranes, particularly by checking their condition and comparing their operating speeds. For example, a status communication module can send a signal to the second crane when the first one reaches a limit switch. This allows both lifting devices to switch to low speed or stop simultaneously. Alternatively, an automatic bypass system for safety devices can be installed when tandem lifting is performed.
It is also important to ensure that the lifting and travel speeds are similar on both devices used in tandem. The greater the distance to be covered, the more a slight speed difference can lead to misalignment between the equipment. For instance, over a distance of 100 feet, even a small speed variation can become significant and compromise the safety of the operation. Conversely, during a shorter lift, such as 10 feet, the impact of this variation will be less noticeable.
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.
