I Just Tightened It… So Why Is It Loose?
I recently noticed that the glass on my oven door was hazy and needed to be cleaned. However, after cleaning both sides of the door, the haze was still there. Upon closer examination, it became apparent that the haze was located on the inside surface of the glass sheet that was a part of the multi-layer door. Unfortunately, I needed to disassemble the oven door before I could clean the haze from the glass.
The multiple layers of metal and glass that the door was made with were held together by four screws (one screw at each corner) that were accessible from the inside surface of the door. Two lower screws attached the door to the hinges, while the upper two screws projected through the two-inch thick door and threaded into the ends of the door handle.
After supporting the door and removing the four screws, the layers of the door separated, thereby providing access to clean the inside surface of the glass. Once I was confident that the glass was clean and no streaks or fingerprints were left behind, I reassembled the door. Given there were only four screws, the reassembly process was straightforward. However, noting that the ends of the door’s handle were made from a non-metallic material, I was careful to not apply excessive torque to the top screws. In other words, I did not want to strip or otherwise damage the internal threads on the handle and create more of a headache for myself.
While using the oven later that evening, I opened the oven door and noticed that the door handle was a bit loose. After letting the oven cool down, I grabbed my tools and went back to tighten the screws. However, to my surprise, the door handle was tight. Given that my wife and children act as if they would rather visit the dentist than embark on a “home project,” I was confident that the handle was not tight because someone happened to retrieve a T25 Torx screwdriver from my toolbox to tighten the screws.
The oven was at room temperature when I disassembled, cleaned, and reassembled the door. However, when the oven was heated to 450 degrees Fahrenheit, the screws expanded in length ever so slightly, thereby allowing the handle to loosen (note that the heads of the screws are located on the inside of the oven door and are exposed to elevated oven temperatures). A quick calculation revealed that when heated the screws grew in length by approximately 0.005 inches (i.e., about the thickness of a sheet of paper). Regardless, this change in the screw’s length was enough to cause the door handle to feel tight at room temperature but loose when the oven was hot.
When screws or other fasteners are torqued properly they stretch by an unperceivable amount. In the case of the oven door handle, this stretch was necessary to counteract the thermal growth of the screws and keep the handle tight at both room temperature and oven temperatures. After applying the correct torque to the screws, the door handle remained tight regardless of the temperature of the oven.
What’s the concern?
Serious issues and damage can result when the thermal expansion of materials is not accounted for. For example, most pieces of large equipment/machinery – such as pressure vessels, engines, etc. – are anchored to a concrete or steel foundation that may or may not thermally expand at the same rate as the equipment. For this reason, slotted anchor holes or guides are often used to allow for differential thermal expansion of hotter equipment compared to a cooler foundation.
Damage can occur if, for example, an end-user installs a piece of equipment on an aluminum I-beam foundation rather than the specified steel I-beam foundation. This is due to the different rates of thermal expansion of aluminum compared to steel. For example, consider two bars that are dimensionally identical at room temperature; however, one bar is made from aluminum and the other bar is made from steel. If both bars are heated to the same temperature, the aluminum bar will experience a change in length that roughly doubles the change in length of the steel bar.
An awareness and understanding of thermal expansion is a critical part of the design, installation, and maintenance process. Regardless of whether it is an oven door handle or a large foundation for a pressure vessel, significant issues and damage can result if a designer, installer, maintenance technician, and/or end-user does not consider the effects that temperature has on a specific material.
About the Author
Jeffrey A. Groves, P.E. is a Consulting Engineer in our Mid-Atlantic (Cherry Hill, NJ) Office. Mr. Groves provides consultation in root cause analysis, scope of damage, and value of loss consultation for industrial, commercial, and residential incidents involving equipment, machinery, and systems. You may contact him for your forensic engineering needs at firstname.lastname@example.org or (856) 662-0070.