Loose Electrical Connections Can Wreak Havoc in Conveyor Ovens By Daniel Pierre III
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conveyor oven is not a simple machine. It has high amperage, operates at high temperatures, and has some very fast rotating components. Some oven parts must endure more than one of these things, too. So, placing an oven into production is not a “once and done” task. This article focuses on why vibrations, a sudden in-rush of power and heat/cooling cycles affect threaded studs and cause nuts to loosen, resulting in a burst of energy via an electrical arc (i.e. a mini explosion). There are two areas in an oven with threaded studs that need constant checking and should be part of a regular maintenance program. When a fastener—usually a nut—loosens on
a stud, it creates a small air pocket where electricity can arc and cause an explosion. Such explosions can affect other nearby components, too. What is important to note is, it is not always the outer nut that loosens. A quick twist with a nut driver is not the correct preventative maintenance action, as detailed later in this article.
Studs on Circuit Breakers Usually there are six studs, and they are all close together, making it hard to reach the inner nut. JN’s internal policy is to make sure that inner nut is tight against the wall of the circuit breaker, and it is a specific line item in our testing checklist. However, once testing is over and test power wires
From left to right: a component, a washer or ceramic collar, an inner nut, a washer, a ring terminal, a washer and an outer nut.
This outer nut will not tighten any further, but a gap to the left of the inner nut still exists.
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are removed, there is no way to tell how much vibration the stud endures while in transit to a customer’s plant. It is expected that a licensed electrician will install an oven, but if not, it is imperative that these inner nuts are tightly on the stud before connecting power wires to the oven. The next photo shows how the nuts can look and feel tight, but the inner nut is not against the back wall, as evidenced by the washer sitting at an angle.
Inner Nut Issues on the Tip of an Element There are many ways to terminate a heating element, and each supplier seems to have their preferred method. JN likes a ceramic collar, as the material is immune to temperature extremes and high voltage. Ceramics introduces a brittleness factor, but a controlled torque solves this potential issue. Note: If you crack your ceramic collar, it is a sign that you are relying only on torqueing the outer nut, and that is not good for the element or your oven. It is another line item quality check for JN to ensure the inner nut is properly tightened. So, elements installed in a JN oven will have the inner nut in a safe and tight position. The same cannot be said for replacement elements. While the inner nut is tightened prior to shipment, JN can’t control what happens in shipment or during installation. No matter how carefully one tightens the outer nut, if the inner nut is not hard against the collar (or the end of the element sheath), then after several heat/cool cycles, a gap will probably form and eventually arc and cause an explosion.
