UT & Wet Mag Testing
Ultrasonic testing and Magnetic Particle testing has become a rapidly requested inspection for all of our incoming rolls. You might wonder why you would need this? Well not all cracks are visible to the eye and the last thing you want is a roll to break down in the middle of the day and your company to have to shut down because you couldn’t see what was there. This is why we have started to offer this on all rolls and highly recommend it. Not only will you be saving your company money, you will also make your employees feel more comfortable working around your rolls knowing that something isn’t cracked or broken. Now, what exactly is Ultrasonic Testing (UT) and Magnetic Particle Testing (Wet MAG testing).
Ultrasonic Testing (UT)
Ultrasonic testing is one of the original NDT techniques, dating back to the 1950s. Since then, technology has evolved to the point where the original UT techniques would likely be unrecognizable, but the principle remains the same. In ultrasonic testing, a transducer is manually moved over an object. The transducer introduces high-frequency sound waves into the material and hollow spots—tiny cracks, hairline fissures, microscopic pocks—create an echo. These soundwaves are used to create a full 3D image of the object, allowing the user to objectively see where the flaws are and how deep they might run. There are many types of UT, including phased array, straight beam, angled beam, time of flight diffraction (TOFD), and guided wave. Phased arrays and TOFD are the most common and trusted. The versatility and efficiency of UT have contributed greatly to the growth of Universal Urethane.
Benefits of Ultrasonic Testing (UT)
Speed. Reliability. Versatility. There’s a reason this is one of the most frequently used methods. Unlike some other methods, UT is portable, can go under the surface, penetrating objects dense or large, to find flaws that would be invisible to the naked eye and hidden from other types of testing. It is ideal for volumetric inspections and easily handles complex geometries. Many of today’s phased array UT offerings can provide a true 3D image, created in real-time for immediate results. This allows inspectors to objectively analyze the full thickness of the object by looking at a screen; the image can be downloaded and shared for others to do the same. This allows someone to visualize the object of inspection without having to be physically present. Another benefit of UT is the ability to record inspection results and compare them over time. This allows inspectors to understand how different materials react to different environments and pressures.
Why doesn’t everyone offer Ultrasonic Testing
While UT has a host of benefits, there is one thing to consider: the need for human guidance. The transducer still needs a human to perform. The incredibly accurate results are partly the result of the operator knowing just how to angle the path of the signal. While getting full coverage is easy, this still requires training.
The reliability and ease of this system come from a few pieces of instrumentation, along with the scanners, the software, and the different probes and wedges. Accurate, 3D results require a certain level of complexity
Magnetic Particle Inspection (MAG)
Magnetic Particle Inspection (sometimes referred to as Magnetic Particle Testing) is a very common and easy-to-use NDT technique. It works, as its name applies, by having the tester create a magnetic field around the object being inspected. This is done with a permanent or electromagnet, induced magnetic field (such as from a current-carrying coil) or by a direct electrical current run through the object.
Benefits of Magnetic Particle Inspection (MAG)
While this might seem complicated, it is actually pretty simple and easy-to-use. It is fast, can be inexpensive, and many MPI instruments are portable. MAG is considered much more accurate, effective, and efficient than inspections that use dye penetrants. MAG is excellent at detecting flaws on the surface of objects.
Why can’t everyone Wet MAG test?
Likely the biggest drawback of MAG is that it has a much more limited use for subsurface flaws or cracks. It is not particularly effective when it comes to measuring the height or depth of a surface crack (length and width only). This leads to inspectors receiving only a partial view of the potential issue. Another drawback is that it is, essentially, subjective. It relies on the interpretation of the inspector, leaving room for human error. The readings can also be inaccurate if the magnetic field is not perfectly aligned. While MAG is easy-to-use, it can only be used on ferromagnetic materials—materials that can be magnetized. They have to be clean and unpainted, and then they have to be demagnetized afterward.
What test do I need?
Over the last few years, more and more industries have begun to change their Inspection techniques. For example, austenitic stainless-steel makers have made the shift from radiography to ultrasonic. While many techniques may be able to detect flaws, not all technology is created equal. Like in the case of ultrasonic testing and magnetic particle inspection, one technique may work better than another depending on the application.
When considering which technique best suits your needs, the key is to weigh the advantages and disadvantages, examine your budget, understand the costs of the most accurate tests, and then make a decision. When accuracy matters, Universal Urethane’s inspection testing is more an investment than an expense. It is also a hedge against downtime, non-compliance, and potential safety hazards.
Need to make sure your roll is balanced?
Roll balancing is the manipulation of the center of gravity so that it falls along the axis around which it must revolve. If this condition does not exist, unbalanced results and the axis will tend to rotate about the center of gravity, with the journals tracing some circular orbit. The resultant unbalanced forces are detrimental to the rolls involved and to the associated machine elements. In most rolls, balance exists at some points along the roll face and not at others. If the roll is rigid and short, the unbalanced points will be carried by adjacent balanced or counterbalanced points and smooth operation will result. We offer two different types of balances, these are Static Balancing and Dynamic Balancing.
If the roll supported without friction will remain at rest whenever the rotation is stopped, all the local irregularities are self-compensating and static balance exists. If the roll is not static balanced, sufficient weight is added to the “light” side to bring it into balance. This is usually done by drilling a hole and adding weights prior to covering. Static balance is a simple adjustment and would suffice if forces of rotation did not interfere.
Although the roll as a unit is statically balanced, it is possible that one end of the roll may have an eccentric center of gravity if correction has been applied to the opposite end. This cannot be detected by simple static balance, but it will become noticeable when the roll speed is increased. The resulting oscillation is known as dynamic unbalance or couple or moment unbalance. This produces an end-for-end motion and requires investigation to determine which end, or both, require counterweighting. It is probable that each end will require its own peculiar counterbalance, with the sum of the two again producing static balance. Simple static balance will be satisfactory if the roll is short and operates below 70 percent of resonant speed or 800 surface feet per minute.
Dynamic balance can be accomplished by adjusting the center of gravity of both ends of the roll. The local unbalanced points in the roll are then carried with no external vibration if the roll is a rigid body.