10 Things to Consider When Buying Magnetic Particle Testing Equipment

Yokes are some of the most versatile and trusted pieces of MPI equipment when you are out inspecting in the field. They also come in a variety of shapes and sizes, which means some yokes are more suited to your needs than others.  

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Choosing the right yoke makes a tremendous difference to the inspector who is going to be using it over 8 hours a day under difficult conditions. Here are five things to consider when choosing the right yoke for the job. 

 

1. Weight 

How heavy does the yoke feel in your hand? Try holding it for more than a few minutes and ask yourself the same question. 

Not all yokes are weighted equal; some yokes are almost twice as heavy as others. When you are using a yoke all day, a heavier yoke can slow you down by 15% and cause fatigue. This is especially true if you are inspecting surfaces overhead or in difficult to reach areas.  

Another factor to check is if the weight is equally distributed across the yoke. Is the yoke bottom heavy? That could make it difficult to inspect sideways. Some yokes can cause wrist strain when used at awkward angles. Others may require the use of two hands to hold the yoke steady &#; effectively needing a second operator to disperse the powder. 

Figure: Differences between a bottom-heavy yoke vs a balanced yoke.  

A bottom-heavy yoke essentially becomes cantilevered when holding sideways, which requires more force at the handle to stabilize. A lightweight, balanced yoke is the best choice if you are inspecting for longer periods, and for overhead or hard-to reach areas. 

 

2. Ergonomics 

How does the yoke feel in your hand? Is it too big? Too small? Are there sharp edges that could get annoying over time? How does the trigger feel? Is it positioned in the right place?  

These are all questions that address how comfortable your work experience is going to be with the yoke in your hand.  

The ideal yoke allows the inspector to comfortably wield it with one hand, with a body that allows the fingers to grip it tightly and a trigger that is correctly positioned for use in all orientations.  

Figure: How the shape of the yoke affects the grip. 

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3. Durability 

How long does the yoke need to last? Is it just for one job? Or do you need a yoke that will last a few years?  

Choosing a yoke that can survive the harshness of various tough environments  can be the difference between a smooth job and a frustrating experience.  

If you want to keep the yoke for a while, choose a yoke that you can easily service yourself. Be mindful of the most common points of failure for yokes &#; the cord and the trigger. Make sure the yoke you are investing in has a cord and trigger you can replace or repair yourself. 

 

4. Power 

How easily does the yoke pass the 10-lb test? Does it pass the same test after a day of work? What about after a year? 

Most new yokes today meet ASTM requirements for magnetizing power. However, some yokes can hold their magnetizing ability for longer than other yokes.  

Before any job, ensure the yoke you are using can still lift a 10-pound weight. Check for and replace rounded feet on the yoke to make sure you will always get good contact. Dual core yokes with coils above the yoke legs tend to be stronger than single core yokes with the coil in the handle. 

 

5. Desired Indications 

What type of indications do you need to find? Do you want to see sub-surface indications? Or only surface indications?  

It is easy to overlook this small detail but adding DC power to a yoke means adding substantially more bulk and weight. If all you really require is to see surface indications, opt for an AC yoke instead. 

 

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Summary

There are a lot more options today in the world of yokes than there used to be. Gone are the days when operators had to live with a yoke that was uncomfortable - and sometimes dangerous - to use.  

When choosing your next yoke, consider the factors you learned today. Pick up a few different ones to see how each feel in your hand, and then imagine how they will feel after a full day's work. Think about which features you really need, and what you can live without. Consider how long you would like the yoke to last. With these factors in mind, you will pick the yoke that is best for you. 

 

 

Published June 4,  

Since its origin in the late 19th century when a magnetic compass was used to detect defects in magnetized cannon barrels, the selection of the right magnetic particle inspection system could be challenging. With the implementation of magnetic particles in the s, the means to locate discontinuities greatly improved.

This article will give you the criteria to use in choosing the right system for your inspection. First, the part has to have magnetic properties (ferromagnetic) to be tested by magnetic particle inspection.

The following are the steps you should use in choosing the right equipment.

1. Part size, weight and geometry

In most cases, you will have a drawing of the part you will inspect. This will give you the size, weight and the geometry of the part to be inspected. It could also identify the industrial specification which provides the type of inspection required for the part. If a drawing is not available you will of course need a sample of the part that will be tested to determine the best method to inspect the part.

The size and weight of the part determines if the operator can place the part into the machine by hand or if special material handling is required. Some parts may weigh hundreds to thousands of pounds and may be too large to be put on a machine. The need for a crane, forklift or some other lifting device to move the part may be necessary. You may need a custom fixture or mobile power pack to magnetize parts such as large valve bodies.

The geometry of the part determines the type of equipment and process you will need. For example, an automotive steering knuckle by design has protrusions sticking out and difficult places to generate a proper field for the inspection. This will require a special fixture and multiple mag shots.

2. Type of current required to find the discontinuity type (surface or subsurface)

For this determination you can reference the appropriate specification for inspecting the part. Also determine if you are inspecting a manufactured/processed part (OEM) or a service part. Let&#;s use a crankshaft as an example. To inspect a crankshaft at the point of manufacturing, finding subsurface indications is most important. The best means to accomplish this is to use DC (direct current). For overhaul shops inspecting crankshafts finding surface indications with AC (alternating current) is critical since stress cracks start at the surface while in service.

3. Volume of parts to be inspected

With smaller parts you may be concerned with production rates or whether or not the part will be inspected manually or by automation. A common mistake made in determining a production rate is to omit the time an inspector takes to inspect the part. A unit can be designed to process at a given rate but the number of inspectors needed to match the process rate must be determined. Obviously if the process rate is higher than the inspection rate you will have a bottleneck in your inspection department.

4. Strength and direction of the magnetic field

It is important in magnetic particle inspection to ensure the adequacy of existing fields and their direction to be adequate in relation to the size and direction of the discontinuity so that a good indication can be produced at field leakage points. Using too much current in the creation of a proper magnetic field could create too much background thus masking an indication or damage parts due to arcing. It is recommended to start low and slowly increase power to meet the requirement for inspection. To help you determine the correct field strength and direction there are a variety of test pieces you can use such as QQI&#;s (Quantitative Quality Indicator) Pie Gauge, actual parts with known discontinuities, or using formulas to determine proper field strength.

5. Budget available

Like any purchase, the budget available to you must be considered in choosing the type of equipment you will require. Though you can purchase equipment to perform magnetic particle inspection for less than $1,000 such as a yoke, your application may require a unit that costs in the tens of thousands of dollars range. This may require you to create a justification document for your facility to move forward.

6. Personnel dedicated to the inspection process

Personnel consideration goes back to the inspection rate and budget available. There is an ongoing budget after the purchase of the equipment such as installation and training. Another requirement that is sometimes overlooked is the ongoing maintenance and upkeep of the equipment. Mag benches are very durable but if not maintained properly the lifespan will be significantly decreased.If proper preventative maintenance such as calibration checks, cleaning of the tank and the bath are not changed on a scheduled basis there will be an increase in failed pumps, hoses and other mechanical components.

7. Requirement for demagnetization

There are several reasons to demagnetize an inspected part: if the part is to be plated or painted, if the part will be a moving part, subsequently machined or electric welded or residual magnetic field will affect instruments sensitive to magnetic fields, i.e. aircraft parts. This goes back to the part size, weight and geometry. You can demagnetize a part using the magnetic particle equipment but for higher production rates the demagnetizing process may be accomplished using a separate demagnetizing coil that can be integrated into the inspection system.

8. Other considerations

You should consider the electrical limitations of the facility in which the equipment will be used. Like any industrial piece of equipment, safety concerns pertaining to your facility should be addressed since there are dangers associated with the usage of magnetic particle inspection equipment. What type of inspection booth/hood will be required, or will the unit be placed in a darkened room?

 In summary, the selection of the right equipment that will meet your requirement for magnetic particle inspection involves a step by step evaluation. The characteristics of the part, the type of discontinuity and available budget must all be taken into consideration when selecting the best fit of an inspection system.  

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