Life in nylon
15 April 2008
Cast nylon-6 is produced by the chemical engineering plastic in the early 1970s. CastLower raw material cost and improvednylon-6 became a commercialprocess technology made it available to the market. process of anionic polymerization of caprolactame to nylon-6 (see chart 1).
The material offers a high module of elasticity combined with a good impact resistance. Its unique properties helped it into the market and soon it was replacing metals, including steel, stainless steel, cast iron and aluminium.
Most producers today cast stock shapes and then the parts are machined out of these sheets and bars. Only a few companies are casting parts in a near net shape using custom moulds. This process offers a material with a much higher molecular weight, which translates to parts with higher mechanical properties than those machined out of stock shapes. Today cast nylon-6 is used in crane applications for sheaves, rollers, boom slider pads and outrigger floats.
Cast nylon crane sheaves
Since the early 1970s cast nylon-6 has been known for its potential to substitute for metals. Mechanical engineers were first astonished by the performance of this plastic material and started to use it as a substitute for cast iron, steel and aluminium. In the early years the material faced a lot of challenges. The material was too soft or too brittle, which caused problems. Wire ropes would wear into the groove of soft materials and cause rope failure.
The nylon producers were challenged and researched an improved polymerization process. In recent decades the raw material has improved and the process technology has become more stable and controllable. The increase in the module of elasticity was more than 20% and the unpolymerised components in the polymer were reduced from 2% to 0.5% (Chart 2). This dramatically improved the long term performance of the material and offered crane companies higher levels of safety. Sheaves made of cast nylon-6 can be used in the temperature range from -40 C to +60 C.
Sheaves by crane type
Tower cranes have many wheels and sheaves. Due to the static and dynamic loads in most cases these parts are produced out of stock shapes using lower end materials (Chart 3).
In all terrain cranes there are sheaves machined out of stock shapes and also near net shape castings. Normally the specific loads are higher than in tower cranes and a stronger material is requested. The sheaves see more dynamic than static loads and, therefore, the material must be capable of resisting these circumstances.
In some rare cases high, long-static loads and high temperatures have led to sheave failure. The cause for these failures is normally low crystalline material (mostly from stock shape material) which cannot cope with these circumstances. Cast nylon-6 is a thermoplastic material and its mechanical properties decrease with increasing temperature. In the last couple of years crane manufacturers have switched to near-net shape cast sheaves because they offer a higher safety factor.
On crawler cranes normally only near net shape cast sheaves are found. Some crane companies still use sheaves machined out of stock shapes but it is becoming less common. In crawler cranes the sheaves can see a long-time static load under different environmental conditions. Sheaves for crawler cranes have been the subject of recent research. Liebherr and Schwartz Technical Plastic, for example, tested these sheaves in a “worse case scenario” environment. The result was that the near-net shape cast nylon-6 sheave can withstand all possible lifting works encountered during operation (Chart 6).
Challenge from the rope
A challenge in the last 15 years for producers of cast nylon sheaves has been the constant improvement of wire rope and an increase in line pull capability for the same diameter rope (Chart 4). This was resulting in higher specific pressures in the groove of the sheave.
The only way for the sheaves to take the higher loads was to improve the material and its load capabilities. In conjunction with the raw material producers for the cast-nylon process (Bayer, DSM, Brueggemann and others) the sheave producers were able to find ways to improve the process of the anionoic polymerization. A material with higher molecular weight and higher crystallinity was developed.
The use of cast nylon-6 sheaves in cranes offers a wide range of benefits for crane design. Great design benefits include lower weight (1/7th that of steel), it is corrosion free, and wire rope life is extended by up to 4.5 times (Chart 5). The prolonged life of wire rope has also been researched by the University of Dresden and by wire rope manufacturers. The benefit of longer rope life is an advantage but it may be offset by the rope wearing from the inside to the outside. This means that the Ablegereife, or Endurance life cycle, cannot be determined in the usual way, requiring an alternative to be found.