Concern in some quarters over the use of polyamide sheaves is likely to be heightened by an advisory notice published by the US Navy warning that 45 defective nylon sheaves found their way onto Terex cranes in the second half of 2001.
Terex has denied a general problem, however, and said that only one crane – owned by the US Navy -had been found to have a problem, and that was caused by side loading.
Advisory CSA-117, which was published in the latest issue of The Crane Corner, an online technical bulletin, is entitled ‘Defective nylon sheaves installed on top and bottom boom head on Terex model RT 160, RT 175, RT 1000, and T 750 cranes manufactured between May and December 2001’.
According to the Navy, a failure occurred on a top boom head fleeting sheave on an RT 175 during 110% load testing.
The Navy advises those with Terex RTs or truck cranes made during this time to verify their build dates with Terex and to replace top and bottom sheaves matching the given description ‘at the next scheduled type B maintenance inspection.’ Type B maintenance inspection should be carried out at least yearly or each 2,000 engine operating hours, according to Navy documentation.
In a written statement to ConnectingCranes, Terex’s director of product safety Norman Hargreaves played down the significance of the failure. ‘It is obviously very important that crane users be informed of any significant product issues,’ he said. ‘However, there are additional facts of which you should be aware: Terex is aware of only one sheave failure (involving the RT 175 crane owned by the Navy that was discussed in The Crane Corner article). The investigation of Quadrant Engineering Plastic Products, the sheave manufacturer, found there had been severe side loading on the Navy RT175 crane involved in the accident.’
The Crane Corner advisory notice says that the Navy sent the sheave back to the manufacturer. ‘Failure of the sheave was attributable to a combination of lower than expected typical tensile strength and the location of the grease fitting near the fill area (gate) of the reaction injection moulding (RIM) processed sheave.’ Quadrant stopped producing sheaves using this method in October 2001 for unrelated reasons, according to the advisory. The Navy investigation found that these 45 sheaves were installed on the four Terex RT models produced during 2001, but could not narrow down possible cranes any more specifically.
The top and bottom sheaves, which have a similar design, are Terex part numbers 3207N4F1 and 3207N4F2, according to the advisory. RIM-processed sheaves can be identified by the marking left by the gate. The gate area is located on the sloped portion of the web towards the outer diameter of the sheave. The gate can be seen as a lighter coloured ring approximately semicircular in shape (i.e., similar to a water glass stain). RIM-processed sheaves can also be identified by a parting line running the circumference of the lightening holes at mid-wall thickness, it said.
Terex is not the only manufacturer impacted by doubts over polyamide sheaves. Engineering contractor Foster Wheeler detected a failure pattern with nylon sheaves used in heavy lift cranes, as detailed in the cover story of the January 2003 issue of Cranes Today. Saudi Aramco has also had problems with a nylon sheave on its Liebherr LTM 1500 breaking. Its discussions with Liebherr continue.
Despite such reports, polyamide sheaves are a significant benefit to crane designers seeking to take weight out of boom systems to increase lifting capacities.