An underhung (or underslung) crane runs on runway beams that are hung from the building's roof steel instead of standing on floor-mounted columns. The trolley rides on the bottom flange of the beam, so the whole crane lives in the space above the working floor. That single difference explains most of its appeal.

 

Because there are no columns and no runway girders on the floor, the area underneath stays open — production lines, storage racks, assembly bays and vehicles can use every square metre of the floor. That is why underhung cranes are so common in workshops, warehouses and process plants where floor space is money.

 

 

 

They win on headroom and on floor freedom. They are also relatively easy to retrofit into an existing building, because the runway attaches to the structure already there. If the roof steel has the capacity, a building that never had a crane can get one without pouring new foundations.

 

 

The roof structure takes the whole load, so the existing steel must be checked — and often reinforced — by an engineer before installation. Runway alignment and deflection matter: a sagging or misaligned runway makes the crane drift and the trolley bind. And because the crane is up in the roof, maintenance access needs planning rather than luck.

 

 

The second meaning of “suspended crane” is the one that matters on high-rise buildings: a permanent crane-like structure on the roof that carries a working cradle down the face of the facade. The industry calls the whole system a building maintenance unit (BMU), and it is what keeps a tower's glass clean, its seals inspected and its panels replaceable without renting scaffolding or rappelling.

 

 

A rail track BMU runs on rails laid along the parapet or roof edge, with a trolley that travels the length of the facade and a cradle that lowers over the edge. It is the right answer for long curtain walls and for buildings that need the whole face covered. Powerston's PB100–PB103 series covers this family; the PB101, for example, offers a maximum lifting speed of 0–12 m/min and a maximum lifting height of 450 m.

 

 

Some roofs do not have room for a full rail run, or the facade steps in and out. A telescopic jib machine mounts on a short rail or a fixed point and extends its jib over the parapet to reach the working position — less travel, but far more flexible about roof geometry.

 

 

For shorter buildings or spot access — a few windows, an atrium, a feature wall — a traversing trolley or a davit arm gives a simple, economical suspension point for a cradle without a full BMU installation. These are the light end of the suspended crane family, and often the right call when a permanent machine would be over-engineering.

 

 

 

Whatever the configuration, the roof structure has to hold the machine — plus the load, plus the wind — without moving. That means either engineered counterweights, sized and secured so they cannot be shifted, or anchor bolts cast and tested into the structure. The design calculation behind that is exactly what EN 1808:2015 (for suspended access equipment) and GB/T 19154-2017 (for building maintenance units in China) demand, and it is not a number you should take on trust: ask to see the calculation for your building.

 

 

 

The decision is mostly about frequency and geometry. A building that needs its facade cleaned and inspected regularly — a glass office tower, a hospital, a hotel — pays for a permanent BMU over the years, because the alternative is renting access repeatedly and losing days to setup. A one-off job, by contrast, is usually better served by a temporary suspended platform or rented access.

 

Geometry decides the machine type: measure the parapet, the setbacks, the roof plant and the height of the facade, and the field of suitable machines narrows quickly. Get those numbers right and the rest of the project is straightforward.

 

 

 

Is an underhung crane the same as a BMU rail?

No. An underhung crane is an industrial overhead crane for lifting loads inside a building. A BMU roof rail is a suspension system that carries a cradle down the outside of a facade. They share the word “suspended” and not much else.

 

Can a roof rail be added to an existing building?

Usually yes, if the roof structure can take the loads. The rail, the trolley and the cradle are engineered around the actual roof — which is why the design calculation and the structural check have to come before the installation, not after.

 

What is the maximum height a roof-mounted BMU can serve?

Practical systems reach several hundred metres. Powerston's PB101 roof track BMU, for example, is rated to a maximum lifting height of 450 m at a maximum lifting speed of 0–12 m/min — enough for nearly any high-rise maintenance regime.

 

How often does a BMU need inspection?

Daily checks before use, plus periodic thorough examinations on a documented schedule per the manufacturer's manual and the applicable standard. For a machine that hangs people over the street, a skipped inspection is not a cost saving.

 

 

 

Whether you need a full rail track BMU, a telescopic jib machine or a simple traversing trolley, Powerston builds the whole family and backs it with engineering support and CE/EAC certification. Send the roof drawings and facade dimensions through powerston.com, and the team will recommend a configuration — and show you the calculation behind it.