Due to the enormous power of winding engines and total dynamics of the winding plant these stresses are not noticed and could only be picked up during periodic inspections such as annual NDT’s. This is what Gert van der Walt, Technical Director at Quest Technical Services (Pty) informs mining companies.
About 470 000 South African mine workers are transported on a daily basis to their work sites to depths of up to three kilometres underground. This calls for a well-controlled and regulated transport infrastructure. Mine hoists, or Winding engines (Winders) are used for this purpose and vary in size as per requirement. Power output can be as much as 8 MW.
A winder consists of various components that ultimately qualify it to be safe and effective transport. Various NDT inspection methods are used to determine the integrity of winder components such as the drum shaft, brake components, winder ropes, conveyance attachments, sheave wheels, gearboxes, anchoring bolts etc. For the purpose of this article we will focus on the main shaft that houses the drums on which the conveyance ropes are coiled.
Winder drum shaft
The winder component that has failed most in the past is the winder drum shaft. These failures have often been a single catastrophic event and according to witness reports, have often been accompanied by a loud bang as the shaft finally broke. Drum shaft dimensions vary between 250 mm to 1.1 meters and can be as long as 18 meters on BMR winders.
Why would a component with a good design safety factor, suddenly fail? The answer is in the fact that all drum shaft failures had been accompanied by fatigue cracking, often as a result of induced localised stresses caused by shaft misalignment, as well as heat induction caused by failed bearings and drum bushes.
There are normally 2 drums on a winder, referred to as the overlay and underlay. This means that the winder rope is coiled in such a way that the one conveyance ascends while the other descends, assisting in creating a counter balance.
The drum shaft is normally supported by 3 bearings, one on each side of the drums and one in the centre between the drums. Through years of continuous working, movement in the foundation and failed foundation bolts may cause the shaft to become misaligned. Due to the enormous power of winding engines and total dynamics of the winding plant these stresses are not noticed and could only be picked up during periodic inspections such as annual NDT’s.
- Drum shaft alignment
A fully assembled winder offers a challenge when the shaft alignment needs to be determined. The relative proximity of certain positions of the shaft such as the drive end bearing, centre bearing and non-drive end bearing is vectored to permanently fitted plates on the winder house floor. This is done using sophisticated survey equipment that gives accurate horizontal and vertical alignment results. These tests are done and reported in terms of the requirements of the OEM’s.
- Drum shaft NDT
ToFD (Time of Flight Diffraction), Pulsed echo, Permanent fitted transducers and Dynamic effect are all Ultrasonic based methods used to determine the integrity of a drum shaft. As many as possible techniques are used per drum shaft but the fact that about 30% of shafts are NOT hollow bore eliminates the ToFD test on those.
- Time of Flight Diffraction (ToFD)
ToFD is an ultrasonic pitch/catch technique, done from the bore inner surface where we rely on diffracted signals from crack edges to reveal the presence of a fatigue crack. This method is also sensitive to sound reflection from geometric features, thus enabling us to determine the scanning head position and helping us to determine the source of a reflection. This means that a good assessment of the shaft condition can be made without having the shaft drawing at hand.
- Ultrasonic scan through length of shaft
The end face of a winder drum shaft is used for scanning the entire length in an axial and radial pattern. Due to sound behaviour in solids the result is not easy to interpret. Therefore images obtained are saved at set sensitivity levels and fixed positions to be used in a fingerprinting program and compared with previous results. Indications are also compared to the shaft drawing when available.
- Permanent fitted transducers
A set of 4 transducers are permanently glued to the face of the shaft in predetermined positions. Indications obtained from these probes are also stored in terms of the fingerprinting scheme but has the added benefit that the image is always taken in the same position and at the same probe pressure.
The added advantage of this method is that the ultrasonic signature of a shaft can be studied whilst rotating the shaft through 360°. In the presence of a crack, as illustrated in the images below, the energy reflected from the crack face will vary as the shaft turns. This is because the crack face pressure changes during the rotation as a function of the ultrasonic reflection factor.
A comprehensive review of the winder drum testing using NDT is available on the website https://ambriefonline.com/