Written by Calvin on November 20, 2013/Calvin's Corner
Thermodynamics: What can it do for you?
We live in a world of hot and cold, so we might as well put them to use. When you get out of your comfort zone, say, for example, into deep space, you might truly get to experience extreme temperature ranges. Cold so cold that gases become liquids, and hots so hot that metals run like water, or so it is said. Here, closer to home, the temperatures are actually moderate by comparison. Take for example, of course, Minnesota. We get below zero Fahrenheit, like maybe -20 °F (about -30 °C for my metric thinking friends). In the summer, we might hit 100 °F (38 °C). While I know these have an effect on me, what does temperature change do to our work?
This article will deal with the effects of temperature changes in our mechanical work. I will focus on expansion/contraction, ductile changes, and softening of non-metal items. A heat gun will be used and hence, this needs a statement. A HEAT GUN IS NOT A TOY BUT A SERIOUS TOOL. IT CAN BURN YOU AND CAUSE A FIRE SHOULD YOU NOT TAKE PRECAUTIONS.
Heat can come from several sources. Cold can be had from refrigerators, freezers, sticking things outside in the cold, and even with ice. Want it colder? Try dry ice, taking appropriate measure to protect your skin. Need something hot? For most mechanical work we do, a heat gun is plenty. Open flame and torches are not a good idea. Repeat: DO NOT USE OPEN FLAME ON YOUR BIKE OR BIKE STUFF. Even a household hair dryer can provide enough heat to make a difference.
With heat, you will need to take care. Heating tools are potentially dangerous to you and the equipment you are working on. Too hot, and you may end up doing some damage. This can be cosmetic, such as blistered paint, or deformed plastic. It can also be structural, such as weakening and softening the material.
A heat gun uses elements powered by electricity, with an air fan blowing out the heat air. Many guns are adjustable, and can reach temperatures of several hundred degrees.
If water is sprinkled on a surface, and it sizzles and dances, the temperature is probably well above 220 °F (100 °C). You may recall those days when you went to grab a wrench heated by the the sun on a hot day, and you dropped in like the proverbial “hot potato.” You look at your hand and there are no blisters, you were just shocked and surprised. That is about 150 °F (67 °C).
Materials tend to expand when warmed. This can be useful especially when there is a press fit that is too tight or is just very difficult to un-press. Heat the material gradually, it does not take burning hot temperatures to make a difference.
Materials may also get soft when heated, or harden and become more brittle when chilled. The softening can be useful when dealing with adhesives. Most adhesives become softer when heated, such as sticker glue. Retaining compounds in pressed fits and thread lockers inside threads also become softer. This can make removing the part easier.
The term “ductile” refers to a materials’ ability to bend or to be formed. Good ductile properties allow the material to be deformed, but without losing toughness. If a material has become brittle, it will break rather then bend. We might think at first the aluminum, for example, will become brittle and hard when cold. This idea is tested in the two video below. The first is bending a derailleur hanger when cold. The second video bend the same brand on hanger at room temperature. To learn the results, watch them both.