UHMW-PE friction coefficients compared
We often get asked why QuayQuip only uses virgin grade UHMW-PE when other companies propose regenerated and double sintered grades. So we asked external experts to look at the properties and performance of both virgin and recycled grades. The results confirmed why virgin UHMW-PE is the only grade that QuayQuip recommends as safe for marine fenders.
There are two classes of UHMW-PE for fenders – first quality virgin grade and ‘everything else’. First quality virgin grade UHMW-PE contains no scrap materials, bulk fillers or anything that detracts from ultimate performance.
‘Everything else’ includes:
– Recycled, regenerated and double sintered UHMW-PE (different names for the same thing)
– Blended UHMW-PE which, as the name implies, is ‘cut’ with scrap and low-grade polyethylene to cut costs at the expense of performance and life expectancy.
UHMW-PE face pads are largely installed to reduce friction. There are two main cases: dynamic and static friction. When two surfaces in contact slide, friction is slightly lower than when the two faces are static and begin to move. Friction also reduces with an increase in contact pressure.
A variety of virgin and recycled materials were investigated and the results are obvious. Virgin UHMW-PE has about half the friction coefficient of regenerated, recycled and double sintered grades. That means about half the shear force into the structure and half the tension in the chains.
QuayQuip discovered that some manufacturers were misleading their customers (and themselves) by claiming low friction coefficients for inferior materials, their values being based on tests of virgin grade UHMW-PE. At best this shows poor engineering competence, at worst some questionable motives.
QuayQuip can proudly state that we have only ever used first grade UHMW-PE. Perhaps this is one reason why none of our fenders has ever lost a face pad in normal service – a claim we know our competitors cannot match!
Ships moor against fenders for long periods. They can have rough hulls, beltings and all manner of protrusions. UHMW-PE facings need to work for many years without maintenance, so wear resistance properties are vital.
QuayQuip’s investigation also compared the wear rate of virgin UHMW-PE with recycled grades. We found that virgin lasted up to five times longer under the same conditions. Not only that, some of the cheapest blended grades were supplied in such thin sections they would need changing every year or two – a huge burden on maintenance budgets.
QuayQuip remains the only manufacturer to use virgin UHMW-PE exclusively because it lasts longer and costs our customers much less over the lifetime of their investment. As the old saying goes, ‘Don’t spoil the ship for a ha’porth of tar’.