Screen systems have become increasingly important for the cultivation of all kinds of crops and are therefore more often used. The use of screen installations already changed with the use of diaphragm screens and now with conditioned cultivation, air mixers, PAR perfect and 'The New Cultivation'.
Willem Vriezen and Dave Boer
All these new developments mean that ascreen is being used much more frequently and intensively. Dave Boer of Huisman Screen Systems: "Where the screen used to open and close a few times a day we now see dozens of movements a day and sometimes an hour. In existing tensile wire installations we see faster degradation of the tensile wires as a result. In many cases this requires an improvement of the standard pull wire."
Huisman has set itself the goal of developing a tensile cable suitable for this increasingly intensive use. "The pulling cable must be well resistant and flexible at the winding bushings and turning wheels, and in addition, the cable must have minimal stretch at the long distances."
In addition to the intensive use, greenhouses are getting larger, and the number of screen layers is increasing. The lower screen may be closed all day, thus obscuring the view of the installations above it. It is often impossible to "see" the entire screen installation each day and thus discover if any pull wires are broken. Dave: "This intensive use of the screen installation and the changing reliability requirements place different demands on the steel wire rope. That is why we were looking for a steel cable with a longer lifespan and that also meets the wishes and requirements that are currently placed on a screen installation."
Huisman has been testing steel wire ropes on its test bench for years. This test bench is specially built where they can test several cables simultaneously for lifespan and wear and tear. Due to a sophisticated design, the wires are loaded on tensile force, have to wind up and unwind over and over again and rotate over the same reversing wheels. Just like in practice.
Dave Boer
Dave: "To get a good insight, we started to extensively test the regular steel cables. We compared the resulting data with the results and with data from the field and practice. We received data from several growers about the average number of screen movements per day and the use in general. After testing the standard cable, we started testing other existing cables. Second came a steel wire with rope core on the test bench. This cable is also used in plastic cable drums and turning wheels. As a third cable, we tested the plastic cable that is regularly used in film greenhouses."
With the requirements package and the results of all the tests, they sat down with the manufacturer. From this, new designs emerged. We started testing these cables. Some cables were dropped and others performed better.
Eventually, they were able to design a cable. The cable has a new construction of yarns and consists of a mixture of different thinner and thicker yarns. Combining the yarns and the producer's high quality made them decide to test this further. The yarn was loaded with 250 kg. That is a significantly higher load than in practice. Dave: "Compared to standard steel cables and also steel cables with rope core, we see that the Dynamic cable can withstand as much as 7 to 10 times more bending moments."
For more information:
Huisman Screen Systems 
Leehove 39
2678 MA De Lier
Tel: +31 (0)6 549 17 411
[email protected]
www.huismanscreensystems.com