STROS, the largest manufacturer of construction hoists in the Czech Republic, supplies customers all over the world. Three systems ship per week. Eight of the high-rises at ?Moscow City?, a major construction project in Russia, are being built or have already been completed with STROS hoists. The hoists reach heights of 300 m or more and have to withstand wind speeds of up to 20 m/s. Safety is therefore fundamental. STROS has been working with Nord Drivesystems as its single-source drive supplier for more than 15 years.

STROS specialises in rack-and-pinion mast-climbing equipment which allows for extended service heights and can be quickly installed. The gearbox output shaft is fitted with a gear that meshes with a rack attached to the guide mast. This robust drive method withstands extreme environmental conditions over long time periods. Apart from building equipment hire and construction companies, STROS frequently supplies power plants, industrial plants, and offshore facilities. Completed projects include locations beyond the Arctic Circle and in the tough climatic conditions of the tropics. The NOV series of personnel and material hoists reaches maximum lifting speeds of 100 m/min and load capacities up to 3,200 kg.

Moscow City construction project
The highest hoist to date was erected at the ?Moscow City? construction site. The complex consists of several high-rise buildings, eight of which are equipped with STROS hoists. One of the hoists serves the Mercury City Tower which at 340 m is currently the tallest building in Europe. The NOV 2032 hoist has a 350 m lifting height, a 2 tonne load capacity and a 70 m/min rating. With a control panel that allows the operator to select the floor number, the hoist provides the same level of control comfort as a standard elevator in an apartment block. The drive unit consists of three helical bevel geared motors with external braking resistors. ?The motors are equipped with electromagnetic disc brakes which can be released by hand,? says Zdenek Coubal, Chairman, STROS. ?This is a custom solution from NORD Drivesystems. And it exemplifies our cooperation: whatever new challenges we meet, NORD provides consulting and tests and develops a solution.? A control cabinet inverter with hoist function controls all three motors. It enables soft starts and stops and high leveling precision. Coubal says, ?We are working very closely with the construction companies. They say that our technology saves time and money.?

Patented safety device
The STROS engineering team has designed a mechanism to safely stop hoist cages in the event of a failure. If the nominal lowering speed is exceeded, the safety device will trip and engage its pinion with the rack to gradually stop the hoist cage. The centrifugal mechanism is individually set for each machine model. The STROS safety device is certified by the internationally recognised German technical testing authority, T_V S_D.

Cooperation with NORD
The cooperation with NORD as its sole drive supplier began in 1997. At that time, STROS? predecessor company made the first serious foray into international markets. NORD Poh?necf Technika, s.r.o., the Czech subsidiary of NORD Drivesystems, convinced the management in the course of the first trial projects. NORD supplied the reliable, long-living products STROS needed. But even then, the most important factor for STROS was NORD?s ability to guarantee service even in remote locations. Coubal says, ?NORD is very good as a one-stop supplier. They suggest the best solution for any application that crops up in our everyday business. For example, we are not limited to standard gearboxes. The units are assembled exactly to fit the requirements. And NORD customises drives for us when we need it, supplying reinforced bearings, special gear case materials, or ATEX and NEC-compliant drives for refinery applications.?

Higher requirements
Initially, STROS only used geared motors. As the manufacturer built only relatively slow hoists at that time, start and stop control via the motor switch was fully sufficient at speeds up to 40 m/min. But with the growth of the customer list, the projects grew accordingly. STROS started building hoists for higher and higher buildings. Consequently, the cages had to travel faster to decrease waiting times. New models have rated speeds of 55-100 m/min and are always equipped with variable frequency drives. In addition to geared motors, NORD supplies control cabinet inverters that control motor speed and enable precise braking. Sophisticated positioning and safety functions such as STO and SS1 for safety requirements up to SIL3 are available.

Special and permanent elevators
The 2008 collapse of the construction industry severely affected suppliers such as STROS. To survive, the company sought applications for its technology apart from construction hoists and branched into hoists for permanent use on buildings, chimneys, or technological equipment. This new field makes up a large part of STROS projects today.

Conclusion
If a hoist does not work, the entire construction site will come to a standstill. Therefore, the drive must never break down under any circumstances. The NORD concept for STROS hoisting equipment with up to three autonomously working drives prevents costly downtimes. Looking back on 15 years with NORD as their sole drive supplier, STROS Chairman Coubal says, ?NORD technology is reliable, and the service is excellent.? And how is the outlook? ?According to the trend in the USA, Canada, and Russia, we can expect more demand for greater-performance drives and inverters due to increasingly larger cages, load capacity, and speed. We have our work cut out for us.?