Many types of wire ropes are now nearly as much plastic as they are steel, by volume. For at least 20 years, wire rope manufacturers have been adding a layer of plastic, or filling void spaces with plastic inside the rope. Although some wire ropes can come completely covered with plastic, these ropes are almost never used in lifting because of difficulties with inspection. This article covers only ropes with a layer of plastic inside the rope, primarily surrounding the rope core.
According to Bridon crane technical manager David Hewitt, there are four principal advantages of plastic-impregnated wire ropes.
First, plastic acts as a cushion, softening the pressure of contact of wires inside the rope. As crane ropes take a load, they stretch. As they roll over a sheave in the boom, they bend. These forces tend to cause the strands and wires inside them to move. Wire ropes are constantly moving, and as they move they rub against each other.
Cushioning is particularly important in new rotation-resistant ropes, which have layers of strands wrapped in opposite directions to cancel out internal twists. As a consequence, there is a cross-over layer where the wire rope strands rub against each other violently. Robert Traxl, head of research and development at Austrian rope manufacturer Teufelberger, says that this cross-over zone is a major reason why there is a plastic cover between the outer layer and the core in rotation-resistant ropes. “Ropes like a 17×7 without plastic are prone to wire breaks between contact points. In the past it was always important to consider whether wires were breaking first inside rotation-resistant ropes before they broke on the outside.” Now, he says, partly because of plastic, the core is usually in better shape than outer layers.
Second, plastic prevents water penetrating into the rope, which reduces the likelihood of internal corrosion. Both of these effects tend to prolong the wire rope’s usable life. Traxl points out that water exclusion depends on the type of plastic used; he recommends against using polyamide (nylon), because it tends to absorb water.
Plastic also helps to stabilise the rope and make it more difficult to alter it, Hewitt says. Wire ropes filled with plastic cannot stretch, because their void spaces are filled with plastic.
When it is under tension, wire rope becomes longer and thinner; plastic stops this from happening. Traxl explains. “A spiral of a strand would like to decrease its diameter under tension. If you look at the cord of your telephone, you see a spiral; if you pull on it, the diameter decreases.”
Traxl says that one side-effect of more plastic in the rope is increased internal pressure. ‘If you put your finger inside the spiral, it creates pressure inside.” He says that rope makers have to balance the increase of pressure against the reduced internal rubbing. “There is an optimum amount of plastic in a rope.”
Bridon’s Bristar rope has an extruded tube around the inner wire rope core that has flukes to hold the six or eight outer strands into position. The rope was designed to keep its diameter, and this characteristic improves rope spooling on the drum.
Fourth, Hewitt at Bridon says, plastic tends to stick the rope together. Because wire ropes are spun closed, ropes that are rotated might tend to twist apart. Plastic prevents this by sticking the wires together.
This last benefit is particularly important for one type of modern rotation-resistant wire ropes, says Traxl. “If you twist a rotation-resistant rope in one direction, you open the outer layer but close the inside, or vice versa. The plastic coating of the core helps to reduce deformations like birdcaging very much.” Michael Gehring, managing director of German manufacturer Diepa, adds that plastic helps to make ropes more resilient to shock loading.
Not good enough
Although plastic-filled ropes may perform better than non-filled ropes across industry, peculiarities of the crane and lifting industry tend to cancel out these benefits. There are three prime disadvantages of plastic ropes.
First, plastic impregnation tends to make ropes stiffer and less easy to bend. Teufelberger measures the stiffness of a rope in a metric called reeving efficiency. Its newest crane rope is the Evolution range, which comes with plastic impregnation (TK 16), and without (TK 17). The TK 17 rope has a slightly better (0.2%) reeving efficiency than the TK 16 rope. But this efficiency is multiplied by the number of falls of rope, so if the crane is operating with 10 parts of line, the effect is magnified 10 times.
In fact, the ability of a wire rope to bend is one of the most important properties for large crawler cranes and large all-terrain cranes, says Traxl. These cranes make a rope’s job hard for three reasons: the wire rope has to bend tightly to wind around a small drum, compared with its own width; it terminates in a relatively light hook block; and it has to wrap around the drum in many layers.
Teufelberger does not recommend plastic-filled ropes on large tower cranes either.
The demands of multiple-layer spooling have become more and more important on construction cranes. As cranes have gotten smaller, so have their rope drums, says Hewitt. At the same time, telescopic booms and work below grade have tended to increase the length of crane rope reeved on a drum. The result is that cranes coil rope on multiple layers, and because the ropes rub against each other in the drum, they tend to grind against each other until they fray.
Gehring says that the advantages of plastic are especially significant when the rope is coiled into a single-layer drum – which hardly any cranes use anymore. “In multi-layer spooling drums not all of these advantages are relevant. Other properties such as resistance to abrasion, low reduction of the actual rope diameter under load, less tendency to ovality and so on play more importance and could be negatively affected by the polymer insert.”
He adds that despite the benefits of plastic in stabilising the core and reducing internal wear, Diepa’s rotation resistant ropes do not need plastic, and generally do not include it, except by special request.
David Hewitt feels even more strongly. Plastic-impregnated ropes are simply more expensive, he says, and that cost will need to be justified.
“For many years I have maintained that 95% of all crane ropes are removed because of mechanical damage. If that is the truth, what is the point of extending the rope life from the point of view of fatigue, corrosion and wear, when mechanical damage will destroy the rope anyway?
“You’ve got to find the few applications where plastic can offer benefits that customer can use. If we talk about ore unloader ropes, which are on a single layer coiling, it is very expensive to change the ropes out,” he says, so every week of rope life matters. Not so on mobile cranes, where it only takes one man-hour to change out a worn mobile crane wire rope.
Hewitt says: “You need to look at the needs of the customer, or the application, and select the particular method of putting plastic in to give the benefit. It is not a one-fix wonder for all applications.”
Although plastic-filled ropes may perform better than non-filled ropes across industry, peculiarities of the crane and lifting industry tend to cancel out these benefits. There are three prime disadvantages of plastic ropes.
First, plastic impregnation tends to make ropes stiffer and less easy to bend. Teufelberger measures the stiffness of a rope in a metric called reeving efficiency. Its newest crane rope is the Evolution range, which comes with plastic impregnation (TK 16), and without (TK 17). The TK 17 rope has a slightly better (0.2%) reeving efficiency than the TK 16 rope. But this efficiency is multiplied by the number of falls of rope, so if the crane is operating with 10 parts of line, the effect is magnified 10 times.
In fact, the ability of a wire rope to bend is one of the most important properties for large crawler cranes and large all-terrain cranes, says Traxl. On these cranes, the wire rope rope spools These cranes make a rope’s job hard for three reasons: the wire rope has to bend tightly to wind around a small drum, compared with its own width; it terminates in a relatively light hook block; and it has to wrap around the drum in many layers.
Teufelberger does not recommend plastic-filled ropes on large tower cranes either.
The demands of multiple-layer spooling have become more and more important on construction cranes. As cranes have gotten smaller, so have their rope drums, says Hewitt. At the same time, telescopic booms and work below grade have tended to increase the length of crane rope reeved on a drum. The result is that cranes coil rope on multiple layers, and because the ropes rub against each other in the drum, they tend to grind against each other until they fray.
Gehring says that the advantages of plastic are especially significant when the rope is coiled into a single-layer drum – which hardly any cranes use anymore. “In multi-layer spooling drums not all of these advantages are relevant. Other properties such as resistance to abrasion, low reduction of the actual rope diameter under load, less tendency to ovality and so on play more importance and could be negatively affected by the polymer insert.”
He adds that despite the benefits of plastic in stabilising the core and reducing internal wear, Diepa’s rotation resistant ropes do not need plastic, and generally do not include it, except by special request.
David Hewitt feels even more strongly. Plastic-impregnated ropes are simply more expensive, he says, and that cost will need to be justified.
“For many years I have maintained that 95% of all crane ropes are removed because of mechanical damage. If that is the truth, what is the point of extending the rope life from the point of view of fatigue, corrosion and wear, when mechanical damage will destroy the rope anyway?
“You’ve got to find the few applications where plastic can offer benefits that customer can use. If we talk about ore unloader ropes, which are on a single layer coiling, it is very expensive to change the ropes out,” he says, so every week of rope life matters. Not so on mobile cranes, where it only takes one man-hour to change out a worn mobile crane wire rope.
Hewitt says: “You need to look at the needs of the customer, or the application, and select the particular method of putting plastic in to give the benefit. It is not a one-fix wonder for all applications.”
