How much do you know about the hidden dangers of tandem lifting?
Tandem lifting is a technique used across industries for specific purposes. When it comes to overhead cranes, tandem work is when a load is lifted using two devices simultaneously whether two hoists/lifts or two overhead cranes. And this word “simultaneously” is the key. Knowing that certain risks are concealed in this relatively standard concept is essential. Let’s start by understanding why tandem lifting is used.
Why use two pieces of equipment to lift a single load?
There are two major reasons to opt for tandem lifting. The first, simply, is that the lifted object is too heavy for a single piece of equipment. Using two overhead cranes distributes the weight safely. The second reason is that the part is too long or too wide to be lifted by a single lifting device. In this case, the load swings and might tip because of the lack of balance, making it almost impossible to handle. With two pieces of equipment, there are two anchor points on the part, which stabilizes the load.
Although this technique seems simple and logical, it in fact hides dangers that are unexpected to many people, even to experienced overhead crane operators. Let’s take a look at some of them.
Image 1 – Tandem using two overhead cranes
Image 2 – Tandem with two hoists
Unexpected and unrecognized dangers
The first risk is capacity overload… which is the very reason this technique is used! Isn’t this a contradiction? Not necessarily.
If the object is not symmetrical, the load may be greater on one side and exceed the nominal capacity of the hoist on this side. For tandem lifting to conform to standard practices, the weight must be proportional to the nominal capacity of each piece of equipment involved. To avoid asymmetrical weight distribution, it is essential to have the drawings of the part that will provide the weight, and possibly also the centre of gravity.
In other cases, the danger occurs when the two hoists are not in perfect sync. If the two hoists do not lift the load simultaneously, all the weight could be temporarily placed on a single piece of equipment. Let’s consider an example: suppose that a ten ton part must be moved using two wire rope hoists of five tons each. This lifting is performed by two lifting devices that are able to lift this part when used together. But if they are not in sync and one of the two pieces of equipment is subject to a load of eight tons, even briefly, it will absorb a shock at 160% of its nominal capacity. Even if this situation lasts only a second, this overload could damage the equipment or even cause an accident.
Moreover, the danger of capacity overload is even greater when the part is being put down. Why is that? When the part is lowered unequally, the weight can also be transferred to one piece of equipment for an instant. But where this differs from lifting is that there is no safety mechanism that will intervene to prevent overload, such as a load limiter, for example. Load limiters interrupt lifting for an overload when lifting only, not during descent.
So, what is the solution? It’s as simple as being properly prepared! Before performing the lifting or putting down in tandem, it is essential to perform the checks that will ensure synchronized movement of the part. For example, sling tension and positioning must be checked, as well as the synchronism of the hoists. Then, the operator may proceed, slowly, with lifting or putting down the load, while carefully checking that the part is parallel with the ground.
The second danger is desynchronized translational motion. When two pieces of equipment are not moving horizontally simultaneously, the risks of an incident are significant. Before the operator even realizes, the devices involuntarily move closer or further away from each other. In these circumstances, at least one of the hoists ends up at an angle, destabilizing the part or the lifting attachments. Ultimately, a sling could break or slip and cause an accident such as dropping the load.
Another risk comes from the systems that are put in place to ensure safety such as limit switches and anti-collision devices (e.g., lasers). How is this possible? When activated, limit switches automatically put a device in first gear or stop it altogether. This means that when the first piece of equipment reaches its limit, it shifts to low speed or stops, while the other piece of equipment continues its course unaltered! The role of an anti-collision system is to prevent two overhead cranes from coming into contact. A laser detects the presence of the other crane at a certain pre-established distance and stops or slows down the crane. However, if this system is activated during a tandem movement, and the equipment is within the detection zone, one of the two pieces of equipment could detect the presence of the other and stop moving. These two situations result in the desynchronization of the translational motion and an elevated risk of accidents.
Knowing the dangers—and avoiding them!
Now that the dangers have been identified, it is time to examine how to avoid these hazards. The examples provided are not out of the ordinary. They are routine situations, and experience in the field shows us that lifted loads and the circumstances described above are often underestimated. But rest assured: Groupe Industriel Premium can help you by offering… a world of solutions!
An optimal solution consists in using remote controls in master/slave mode. When the master transmitter is activated, the hoists automatically move into tandem mode, which means they operate simultaneously from a single transmitter. This also has the effect of deactivating the other transmitters known as slaves. It is therefore possible to use tandem lifting devices with a high level of safety.
A second option consists in installing technologies adapted to tandem lifting. For example, a shared switch limit system makes it possible to send a signal to the second piece of equipment when the first reaches its limit. The two lifting devices will shift to low speed or stop in a synchronized manner. If not, it is as simple as installing an automatic security device bypass system when tandem lifting is performed.
Another option: load display. This will show the real weight lifted by each device to detect if there is an overload. Although these attachments are used only to provide information on the actual weight, they nonetheless make operators more aware.
In conclusion, we must always fully understand lifting parts, i.e., their weight (even their weight distribution), their centre of gravity, their anchor points, etc. In general, a lifting plan should be implemented and closely followed in each situation. With regard to equipment, it is ideal to acquire identical lifting devices equipped with the appropriate protection or to modernize existing equipment. And of course, a critical factor is always proper preparation and great care when manipulating heavy loads whether conventionally or in tandem.
So, let’s work in tandem together! Contact us to explore the possibilities we offer! Our experts will be happy to guide you to a safe use of your lifting equipment.