Lattice Tower Spares Ltd.
Lattice Tower Spares Ltd.
Wire rope information page
1. Types of wire rope
The most common type of wire rope is called a galvanised steel 6/19 FC wire rope - the 6 represents the number of outer strands each made up of thin wires - the 19 tells you the number of thin wires in each strand.
FC means 'fibre core' - in the early days this was a sisal string but in 2011 it is a plastic string - often blue or green in colour.
2. Testing of wire ropes
This provides an interesting dilemma - if you fit eyelets to a length of wire rope, then subject it to a tensometer test to confirm that this wire rope with eyelets fitted is sufficiently strong, you are effectively damaging this wire rope (by stretching) and it can no longer be described as 'new'.
For this reason, the Certificates of Compliance which we supply (if requested) with every rope certify that the wire rope used conforms to international standards and that the eyelets are fitted using equipment and techniques which comply with international standards.
3. Breaking strength of wire ropes and Safe Working Load
There is a lot of information about this on the internet - the convention is to 'de-rate' the wire rope to 20% of its ultimate breaking strength. For example, a galvanised steel 6mm diameter wire rope has a breaking strength of just over 2000kg - this translates to a 'Safe Working Limit (or Load)' of 400kg (= 20% of 2000kg).
The Safe Working Load (S.W.L.) and/or the year of manufacture is usually stamped on the ferrules at one end of every rope, along with a unique rope identification number.
More information can be found here : http://www.engineeringtoolbox.com/wire-rope-strength-d_1518.html
4. Why not stainless steel wire ropes ?
Stainless steel wire ropes look great - shiny and 'new' for most of their life. There lies the danger - stainless steel is prone to 'work hardening' and breaking without warning. In many other applications the stainless steel rope does not flex regularly and is therefore perfectly safe, but when used on crank-up towers the wire ropes flex a lot when wound onto and off winch drums, around pulleys etc, making stainless steel an unsuitable material for the purpose.
There have been numerous documented cases recently of stainless steel wire ropes on crank-up towers breaking without warning, often where an electric winch system is used to frequently raise and lower the tower.
5. Installation
Follow the installation instructions carefully - wire ropes will quickly wear and could break if they rub excessively against metal parts of the tower. For example on Versatowers, the design is such that the wire ropes rub against parts of the tower sections as they are raised or lowered - keep these areas well greased.
In theory, different pulleys (sheaves) should be used for each different diameter of wire rope 5mm, 6mm, 8mm etc - in practice most common pulleys will be suitable for several different wire diameters.
6. Maintenance
Wire ropes should be kept well greased at all times. Use plenty grease and rub it into the ropes. I have seen a set of wire ropes where a cheap grease had actually caused the galvanised steel wires to rust rapidly (presumably this grease contained some corrosive substance) - ordinary high-melting point grease is suitable but beware of cheap varieties. Spray grease can be useful for reaching inaccessible parts of the wire-rope after installation. Make sure *every* part of the wire rope is greased.
7. Inspection and replacement
A well-maintained set of wire ropes should last at least 5 years or more (depending on correct installation, correct loading for the wire diameter, frequency of use, local weather conditions, etc) - the most common indicator of when a wire rope needs replacement is when one or more of the thin outer wires break - often these small sharp wire ends are difficult to see (especially with a grease covering) so the only way to detect them may be to run your hand along the rope and feel for them (caution - can/will be painful !)
Don't hesitate - if you feel even 1 small outer wire broken, the rope is potentially dangerous and should be replaced immediately.
Another area where wire ropes fray and show signs of wear is just beside the eyelets/ferrules at the ends. Again look/feel for small broken wires.
_____________________________________________________
Extra information :
http://www.unirope.com/inspmanual/inspman_03.shtml
http://www.deltawirerope.com/productsindex.html
_____________________________________________________