The life span of mechanical drive systems for screens mostly depends on the frequency of use, the amount of load, the technical implementation, the quality of the materials used and maintenance. In a well-laid and maintained screen system it’s the frequency of use and the load that are the two most important factors.
A long life span for a mechanical drive system cannot be taken for granted. An installation with perfect materials that has been sloppily laid and is not maintained can quickly wear out. But also a perfectly laid installation can be disappointing if the quality of the materials is poor. The same applies if a system has been properly installed but the load is too heavy or it is used too often. On the other hand, if a system is not used very often the engineer can make some concessions. Then, for example, corrosion prevention will be an important factor for increasing the life span.
Open and close 3,000 times
The development of screen systems to save energy started to be taken seriously 38 years ago. During that initial period some people were already discussing the lifespan of the system in relation to the materials used, screen material, pull wires, cable pulleys and motors.
In an attempt to create a standard it was sometimes assumed that a screen system would open and close 300 times per year over a period of ten years. And thus the number 3,000 appeared. When this figure was chosen there was no thought that the energy screen could be used to control the climate.
Open and close 300,000 times
A screening system used to control the amount of (sun) light or remove moisture by means of a small gap far exceeds the figure of 3,000. It no longer is about how often a screen opens and closes, but how often the screen in a particular position moves back and forth over a small distance (figure 1).
Here is a numerical example: We know a frequency of 10 times per hour is quite possible. If we assume that this intensive movement occurs on average 12 hours per day for 250 days a year, then the number of (short) movements amounts to 300,000 in ten years.
Therefore it is clear that intensive use of the screen for climate control means that the number of movements is many times greater than when energy screens were initially being developed and adopted. Compare this for example with the lifespan of the camshaft (timing belt) in your car. If it needs to be replaced after 100,000 km, for one person this occurs after one year, for another it is perhaps after seven years. To know how often the screen is used an automatic registration in the climate computer of the total number of open and close commands would yield useful information.
Load on drive
The forces acting on the screen installation are very variable and occur during three separate situations: When the screen closes; when the screen is compressed into its package; and during the back and forth movement of the screen, also to create moisture removal gaps.
The largest forces occur when the screen is pushed into a packet and during closing. The force needed to properly compress the screen into the packet is 18 N (1.8 kgf) per meter of profile. If the pull wires or the pull-push pipes are, for example, 4 m apart this implies a force of 72 N (ca. 7.2 kgf) per attachment point. Knowing this it is simple to calculate the total force per pull wire or rack and pinion. The forces during the back and forth movement are much smaller. Therefore it is not necessarily so that the highest load occurs with the highest frequency of use. However, it is always the case if the screen fully closes at every adjustment cycle.
Stress and breaking strength
Cables shouldn’t be loaded by more than 1/5 to 1/4 of the breaking strength. The normal stainless steel cables used in screen installations have a breaking strength of about 470 – 520 kgf. The pull force on screen installation cables often has a value of 100 – 165 kgf. Therefore in practise they are highly stressed; exactly by how much can only be assessed by measuring the force. That applies to both fixed and spring blocks as well as slip blocks.
Slip blocks only offer the security of limiting the pull force if the slip force of the element taken with it is accurately measured. Checking the slip force within an installation is not very difficult and sometimes leads to surprising results. For example, there can be a relatively large difference in slip strength in a situation where a cable doesn’t quite slip and when it does slip when the screen closes or is folded into a packet.
Expansion of gutter
The greenhouse construction standard NEN 3859:2012 gives temperatures changes per 24 hours. It mentions 20ºC to 40ºC for light-coloured gutters and similarly specifies 20ºC to 40ºC for construction parts in the greenhouse. An example: The gutter and cable measure 100m; the aluminium gutter has a cross section surface area of 1,200 mm2; the cable is type 7×7, diameter 3 mm, material stainless steel. Based on normal temperature differences a situation can arise in which the stainless steel pull cable remains at 20ºC and the temperature of the gutter rises to 40ºC. Then the length of the gutter compared with the cable can increase by 46 mm.
However, it is also good to realise that at night the temperature of the gutter could be 10ºC or lower (closed screen) and the stainless steel cable is 20ºC. During the days the temperature of the gutter could rise to 40ºC and the cable to 25ºC for example. In the latter case, the difference between the gutter and the cable can amount to 61 mm. Because the gutter is longer and the ends of the pull wire drive system (figure 2) coil around cable pulleys that are attached to the greenhouse construction, the cables stretch approximately 61 mm.
Effect on the load
In this example of 61 mm, the pull force on the cable will rise by about 300 N (30 kgf). That sounds reasonable, but it can still mean an increase of 20 to 25%. The extent to which it is damaged will vary from case to case. The actual increase in tension will depend on the original situation: At what temperature were the greenhouse and screens built and fitted? The amount of tension on the pull wires when the screen was fitted mostly determines the load on the cable.
The expansion difference also play a role in push-pull systems (figure 3) but here the situation is different. On one hand the push-pull tubes contains much more material than a pull wire and, on the other hand, the drive is mounted approximately in the middle of the push-pull tube. In addition the drive system can only be fitted when it can be attached to the greenhouse construction. In this example you should then work with lengths of 50 m instead of 100 m.
The parts making up the cable drive systems have improved in recent years: The diameter of the cable pulleys has increased and the grooves in the cable pulleys are better suited for the normal cables. Also a number of suppliers use other cable types for the reverse wheels and axles.
Another important improvement is the use of grooved axle boxes (cable drums). As a result, the diameter has increased relative to the 60.3 mm shaft (2 “axle) and the cable is well supported and led. In some cases better quality cables are also used. A problem with judging the quality of a cable is that the standard specifications don’t indicate its resistance to wear and tear. However, manufacturers can carry out a number of measures to ensure the cable can better withstand intensive use.
Grooved cable pulley/drum
To optimally support the cable in the reverse wheel and on the axel, the diameter of the groove in the cable pulley or cable drum has to be at least 5% and maximum 10% larger than the diameter of the cable (figure 4). For a cable of 3 mm, this means that the diameter of the groove has to be at least 3.15 mm and not more than 3.3 mm. If the groove is too narrow or too wide it reduces the life span of the cable. If the cable pulley and cable drum are too small that also reduces the life span.
The neatness and accuracy with which the installation is carried out remains essential. A slanting revolving cable in the cable pulley is very damaging to both cable and pulley. For that reason expansion possibilities are applied to the gable ends and screen separations.
Diameter of groove and cable
For the most common types of cable, recommendations exist for the minimum diameter of the corresponding cable pulleys and cable drums. For type 7×7 with a diameter of 3 mm the absolute minimum diameter of the groove is 90 mm. For type 7×19 with a diameter of 3 mm the diameter of the groove has to be at least 60 mm. But it is recommended to have a groove diameter of 120 mm for type 7×7 and at least 90mm for type 7×19. However, different suppliers and manuals give different values. Overall, larger diameters extend the life span. Only if the ratio of the diameter of the groove and cable is greater than a factor 50 to 60, does it have little effect on lifespan.
Diameter cable drums
Further increasing the diameter of the cable drums (axle boxes) does have direct consequences on the axels and certainly on the motors of traditional cable drives. The use of axle boxes and/or grooved plastic drums is certainly an improvement compared with the direct winding of the cable onto the axle with a diameter of 60.3 mm (2”axle).
The situation is easier for the reverse wheels. A reversing wheel with a large diameter could be mounted horizontally instead of the usual vertical position. This was sometimes done in the past.
In principle, there are opportunities to vary the composition, structure and method of manufacture of cables. That is even possible with the smaller cable diameters in screen installations. This, however, requires further research and with the collaboration of cable manufacturers.
Text and images: Harald Vahl