One of the more common things we see with customer furnaces is the use of double shank holders to hold the heating elements in the furnace. As shown in the picture above, the double shank holder is the white ceramic clamp that holds the two legs (shanks) of the element, keeping it from falling into the furnace.
It’s not that double shank holders are inherently bad – in fact, our element manufacturer recommends them for a variety of applications. The challenge is that double shank holders require significant design considerations and strict adherence to clearances and tolerances on the part of the furnace builder. Often times, whether by furnace usage, mass manufacture, or otherwise, these clearances are not quite what they should be, and as a result, we see commonalities in element failures that are likely related to the issue of element mounting.
Specifically, we hear often that the heating elements are breaking – either in the bend, or near the terminals. The remedy is to switch to using single shank holders. Let’s explore why.
I promise I won’t bore you too much with the physics, but a quick explanation (followed by a more easily relatable graph) will adequately convey the point.
If you Google, “forces from anti-parallel currents,” you will see a bunch of web pages related to Physics education. What these pages are explaining is the fundamental relationship between magnetism and electricity. That is, where there is electricity, there is magnetism (and vice versa).
All you need to remember is that magnets have force. Applying that knowledge, whenever there is electricity, a magnetic field is formed, and that field has real life force (just like a magnet).
Since MoSi2 elements are powered by large currents, they have a measurable force. And, since the elements are U-shaped, there is a magnetic field around each leg. Similar to two magnets repelling each other, each leg has the same magnetic pole (more or less), and as a result, the legs wind up pushing each other apart.
Okay, what does this mean in practice? It means this:
Most of the elements used in dental sintering furnaces are going to be 100A or below. That gives approximately a 1.15 ratio between dimensions A and a – which, for a standard 25mm a element, the deformation would grow the element to 28.75mm (25*1.15), a difference of 3.75mm!
This may not seem like much, but considering that these elements are ceramic, that growth is tremendous. Thankfully, the elements are elastic when they are at temperature, so this deformation is not a problem. That said, if the elements are constrained by, say, a dual shank holder, and are not able to move, good things are not going to happen.
If you are seeing element failures due to breaks near the bend, taper, or anywhere else on the skinny portion of the MoSi2 element, it is a good bet that changing to a single shank holder will improve your heating element life.
Single shank holders are a one time purchase that should last the life of the furnace (barring some catastrophic failure). From a ROI standpoint – they are a great deal. Based on the price of MoSi2 elements, even extending life a handful of months will pay for the cost of the single shank holders in no time at all.
Thank you for reading, and as always, don’t hesitate to drop us a line if you have any questions!
Dental Furnace Elements LLC
17621 Narragansett Avenue
Lakewood, OH 44107