12-Aug-2010 Source: Zephyr International
Zephyr International LLC offers a Non Destructive Inspection (NDT) system for rescue hoist wire ropes. The MagSens TM system provides the capability to measure and record defects in the rescue hoist cable and to provide a permanent record of the cableâ€™s structural integrity.
Rescue hoist cable is technically wire rope, but we use the terms wire rope and cable interchangeably in the following discussion.
Wire rope NDT has been in use for many years for mine hoist inspections but has not been incorporated into rescue hoist inspection until now. Safe use of the rescue hoist requires knowing the condition of all the wires that make up the cableâ€™s structural integrity. Due to the dynamic loads that a rescue hoist cable can see in service it is critical that the cable be in perfect condition before the start of the next mission.
Rescue hoist cables are made of stainless steel cable that is strong and paramagnetic as a result of cold forming. The type of construction is a 19 x 7 design, which means there are 19 strands of 7 wires each. The wire rope is spin resistant and uses two layers of strands to support the load. The inner strands are left hand Lang lay and the outer strands are right hand regular lay. As the rope is spooled onto the rescue hoist drum it is stored in multiple layers on the hoist drum. This type of storage tends to cause crushing and abrasion of the wire rope that begins from the inside and progresses to the outside of the cable. 19 x 7 cables have a nasty tendency to fail from the inside out.
Rescue hoist wire rope inspections are required every thirty days, or during a post flight inspection if the wire rope was used during the mission. Rescue hoist wire rope rejection criteria are the most stringent wire rope rejection criteria in the wire rope applications world. A single broken wire is cause for rejection.
Prior to the introduction of the MagSensTM system the inspection of a rescue hoist cable required a tedious and subjective visual and tactile examination of the wire rope over its entire length. The inspector could only examine the external wires and due to the small size of the wires the inspection usually involved pulling a rag over the surface of the cable in the hope that if a wire was broken it would snag the rag. The inspection had no documentation requirements and if it was performed, there was no evidence of the results. The inspection also required many people to hold the cable off the ground or it required paying the cable into a drum and taking the chance of kinking the cable or catching ones hands in the cable twists as it exited the drum. Because of the way the wire rope is fabricated it is not possible to inspect the internal wires visually or by feel. In other words there are 133 individual wires in a rescue hoist cable and 49 are internal wires, therefore 37% of the wires can not be inspected with visual methods at all, and visual methods are time consuming and not objective.
With the MagSensTM system the inspection can be carried out in accordance with current wire rope industry standards applied to the results of the inspection and the results can be documented. The MagSensTM system uses the magnetic flux leakage inspection technique. The results are stored on a computer and can be used to compare the cables condition to its original condition when it was first installed.
Two signals are measured and recorded by the MagSensTM system. The first is the LMA signal which stands for Loss of Metallic Area. The cableâ€™s strength is directly proportional to its cross sectional metallic area. LMA defects include internal and external abrasion, and corrosion. The second signal is the LF signal which stands for local fault. LF indications can be broken wires, broken strands, wire welds, external welds due to static discharge, minor shock load damage etc.
The entire length of cable can be inspected and the results reviewed in less then 15 minutes and by one individual. The results are objective and the defects location is recorded so that it can be located easily if it is external and to identify the type of defect that it is if it is internal.
The MagSensTM system is easy to apply and the results are then used to determine the type of defect and the pass fail determination for the cable.
The MagSensTM system provides:
â€¢ Scientific and objective method to inspect the most critical part of the rescue hoist system that has before now lacked any effective inspection methodology
â€¢ Detects damage that may occur as a result of a mission in a minimum amount of time
â€¢ Virtually eliminates unnecessary replacement of the wire rope
â€¢ Tracks the airworthiness of the cable over its installed life
With the MagSens TM system 100 percent of the wires are checked with an objective system that provides repeatable results in a very short period of time. The MagSens TM inspection system should be an essential part of any rescue hoist maintenance program.