Hoist drives rise to the challenge

As tall building design increasingly becomes more architecturally innovative, providers of systems for maintenance access are challenged to keep pace, with novel approaches required for every development. In one of its latest projects, Integral Cradles Ltd designed and installed two Building Maintenance Units or BMUs for the iconic 52-floor Leadenhall Building in the City of London.

Also known as "The Cheesegrater', the 225m skyscraper features a distinctive tapered shape with one faade angled at 10 degrees. The custom-tailored BMUs consist of traversing cranes, cradles, and support equipment permanently mounted on the roof. Importantly, a telescopic jib provides them with a reach of over 20m - thereby they can get to all parts of the building's exterior. The machinery lifts glass panels with a combined weight of up to 1000kg. It also assisted with the dismantling of the tower cranes by lifting parts up to 2000kg. With safety and reliability clearly major design considerations, the installation includes several helical bevel geared motors and NORDAC PRO series inverters supplied by Nord Drivesystems to precisely position the cradles over and around the entire tower.

From the project outset, Integral Cradles worked with the architects and the construction company, developing the BMUs using a "virtual construction' approach called Building Information Modeling (BIM). This allowed all parties to visualise the access system concurrently as the building work progressed, taking into account even minute design changes that might influence the faade access system and maintenance regime. Integral Cradles developed the Leadenhall Building BMU design in collaboration with its Spanish supply partner G³ndolas in Design, S.L. (GinD), who manufactured and sourced major component parts for the project. A longstanding Nord Drivesystems customer, Madrid-based GinD has supplied BMUs for several prestigious installations worldwide that also use NORD drives. Quality, service, and global support are key requirements of Integral Cradles' and GinD's design process. Therefore, it was crucial that all of the critical specifications could be studied and each drive axis optimally sized by NORD engineers in Germany, with local sales support from NORD Gear Ltd - its UK subsidiary and one of 36 that operate across all major manufacturing countries worldwide.

Hoist drive functions on board

Each BMU comprises four inverter-driven axes: a traversing axis runs the counterbalanced crane on a rail system over each half-length of the roof; a slewing axis rotates the crane; a telescopic axis extends the jib; and a winch drive feeds cable to elevate the cradle over the building faade. The luffing axis that positions the jib angle is powered with a hydraulic actuator. The application requires smooth acceleration and deceleration. These functions are controlled by NORDAC PRO series cabinet frequency inverters. The user-friendly NORD CON software allows for a quick and safe inverter parameterisation. In addition to acceleration and deceleration ramps to maintain the soft starts and stops, a comprehensive list of parameters relevant to the applications includes a range of operating speeds, brake control, and safety-relevant conditions. By means of NORD's optional ParameterBox, the customer can simply adjust certain parameters on-site or copy an entire parameter set to another inverter.

Featuring precise vector control and a high overload capacity, the drive electronics enables safe operation of geared motors in this application with high torque requirements. With each NORDAC PRO including a complement of I/Os, the customer's PLC control takes care of synchronisation across all axes. The inverters are mounted in a weatherproof control cabinet close to the BMU. The NORDAC PRO series offers a wide choice of configuration levels. The inverters can handle sophisticated control functions based on the standard integrated PLC functionality, POSICON positioning control, safe stop (STO), encoder feedback, fieldbus communications, and much more.

All drive components manufactured in-house

The geared motors driving the BMU axes were also all manufactured and assembled at NORD. The energy-efficient IE3 motors are rated up to 5.5kW, some featuring safety brakes with a manual brake release. All vertically mounted motors include a rain canopy on the top of the fan cowl to prevent debris and water entering the back end of the motor where the brake and fan are located. All of the motorised axes benefit from NORD's Unicase high-efficiency helical bevel gear units where rigid one-piece cast iron machined housings resist deflection under the heaviest load conditions. With all bearings and seal seats contained within the single casting, splits or bolt-on carriers that can weaken the housing or allow oil leakage are eliminated. The use of larger bearing journal and gear diameters results in larger torque capacity, improved life and reliability over competitive gear units.

Since commissioning, Nord Drivesystems has conducted inverter product and maintenance training on location - at 225m high, with fantastic views of London. Looking forward into the future, the manufacturer expects repeat business. "The Cheesegrater' is only one in a long list of highly impressive installations by Integral Cradles and GinD working with NORD. Architecture buffs are already poised for "The Scalpel' at 52 Lime Street in London, currently under construction, which will be outfitted with two complex BMUs designed by Integral Cradles.

For more information please visit www.nord.com.