We’ve previously discussed a variety of methods for applying secondary (dressing) lubricants to wire rope and while several options exist, it’s also important to consider the end application of your rope and the features you desire from your lubricant.
We can consider the performance of a lubricant in terms of core functionality (corrosion, wear protection, EP performance, heat protection, etc…), secondary features such as, environmental performance, water wash-off, fling-off, UV Resistance and tertiary features like the application process.
Core functionality – what to look for:
Internal wear and/or bend fatigue is a typical failure mode for many ropes. Cyclic tension holds outer strands back while the rope core extends, abrasion across sheaves & drums and spooling with neighbouring rope lengths all lead to wire-on-wire contact, wear, and potential failure.
Unless the rope is failing prior to fatigue from other obvious means i.e., heat exposure, we should look for a lubricant with good 4-Ball Wear Scar (IP 239) to limit wire-on-wire contact. As we’re measuring wear scar here, the lower the figure, the better protection offered.
Shock loading can be problematic for all ropes engaged in lifting and especially so if slewing or travelling under tension. Wire rope can experience a force double the static load through shock loading if any interruptions are experienced, likewise, cranes are designed to take a load gradually and steadily but due to increasing demands and human errors this is not always achievable.
4-Ball Weld Load (IP 239) provides an indication of the lubricant’s performance against extreme pressures and hence, shock load. As we’re performing this test under increasing loads, the higher the weight reported, the better performance and protection offered.
Corrosion is another significant failure mode ropes experience that lubricant can play an important part in preventing. Not only does the quantity and active corrosion inhibitor present play a role but also the technology housing the inhibitor, (while wax-based technology can house corrosion inhibitors, their stiffness and susceptibility to cracking leaves large veins of wire unprotected).
Heat exposure is a very specific and readily identifiable failure mode which can affect us in two ways. In addition to the obvious high temperatures, we can also suffer from a rapid loss in lubricant and subsequent failure from one of the modes outlined above. Rope lubricants specifically designed with heat resistance are available and should be sought for these specific applications, however, you should also be mindful of how the lubricant is being applied as typically it’s introduced as a secondary lubricant and it’s likely that a wax (melting point <120oC) is present in the rope core. Fortunately, a thoroughly applied dressing lubricant, followed up with a second application relatively soon after will overcome the use of wax and prolong rope life for much longer (after this second addition further applications can revert to a standard redressing period).
While not directly affecting rope life, secondary features are an important consideration given the overall effects on our operation. Features like water wash-off, fling-off and UV resistance can all impact lubricant lifecycle and subsequent protection provided.
Consider the end application and form a list of potential exposures to identify subsequent requirements from our lubricant, for instance, a rope operating at sea, on deck, will require excellent water wash-off, low fling-off and high UV resistance while a permanent subsea rope may only benefit from water wash-off properties and both applications may require biodegradability. Alternatively, a rope operating in a factory will likely be less exposed to the elements, but dust contamination could be a concern, and a non-toxic, non-hazardous lubricant will be easier to store and safer to handle.
Fling-off is a major consideration for cranes, especially those operating in public areas where safety concerns and contamination needs to be avoided, while mining ropes operate in particularly demanding conditions, where corrosive dust and high temperatures can be commonplace and results in ropes with a significantly shorter lifespan.
Temperature stability is a major concern and one that can impact ropes across multiple sectors and multiple locations. Temperature can be generated from the environment or from the rope operating at speed. It is important to understand that many ropes are built with a wax-based coating, which are known to ‘thin’ at relatively mild temperatures, i.e., 50oC. Waxes are also susceptible to ‘shear-thinning’ so even despite relatively cool environmental conditions they will still become thinner while in operation and can ultimately drain away.
Also be mindful that cold temperatures can cause problems with stiff waxes becoming hard and cracking, which inevitably leads to both water ingress (corrosion) and ultimately breaking off.
In summary, you should think of your rope lubricant performance in terms of core functionality and secondary performance properties. Not all lubricants and coatings will be suitable for all applications and locations, pull together a list and work with a rope lubricant supplier to identify the likely exposure and benefits to include in your lubricant (remember a rope distributors lubricant selection may not always align with the end users’ requirements). A well lubricated rope can last >4 times than that of an unprotected rope. Differing lubricants can offer a large variety of benefits and some knowledge of these benefits will aid your selection. The ability to apply a secondary/dressing lubricant also gives us the opportunity to introduce protection tailored to our needs and replenish lost performance as our existing lubricant dwindles.