Double flanged steel wheels are a small but integral component to many lumber-drying kiln operations. Wet, or "green," lumber has a much higher mass than dry lumber, meaning loads can be incredibly taxing on transfer carts.
For kilns using metal rails to move lumber in and out of drying facilities, it's common for these rails to lose their shape with repeated use. Unfortunately, bent or misshapen rails can be major hazards when transporting drying stock. Safety is the biggest concern, but even slight shifts in stacked lumber can affect drying times and overall productivity.
Double flanged cart wheels are ideal for situations where rails may be bent or untrue. Wheels can be installed so that they float, within limits, across an axle. As a wheel encounters rail that bends in or out, the flange on either side catches the rail, pushing the wheel along the axle, preventing it from running off the tracks.
Why dry lumber?
Drying lumber, also known as "seasoning," is an important process in improving the quality and usability of wood for any number of uses. The goal is to achieve an equilibrium moisture content where the moisture in the wood is the same as its surrounding environment. Once this threshold has been achieved, the moisture content can change with shifts in ambient humidity without compromising the wood's strength or integrity.
Some key benefits to drying wood include:
- Reducing weight and transportation costs. Trees and fresh-cut logs contain significant amounts of water—often up to 50 percent of the actual wood's weight. Transporting all that extra water weight can be expensive to transport.
- Protecting from microorganisms. Insects, fungal growth and other microorganisms can eat away and compromise the strength and integrity of wood structures. Using high kiln temperatures to dry wood kills most unwanted organisms.
- Increasing strength. Wood is prized for its use in construction. Drying lumber ensures the best strength and consistency necessary for builders.
- Preparation for other finishing or preservation treatments. Wood materials can undergo a range of processing treatments that can be compromised by moisture. Drying ensures wood is ready for any additional treatment.
Air drying versus kiln drying
Air drying is the most natural method for drying wood. It is a more passive method that relies on ambient conditions of the surrounding environment to extract moisture from wood. Air drying can be a much cheaper process, but it requires a significant amount of time—typically several months, if not years.
Kiln drying is an accelerated process that relies on the use of isolated chambers, called "kilns," to extract moisture quicker. Most commercial lumber sold in hardware stores has been kiln dried. Key components of a wood-drying kiln include:
- Insulated construction. Insulated construction allows temperature and humidity to be controlled according to pre-determined drying schedules. Brick masonry or concrete are common building materials, but insulated aluminum is also common.
- Heating. Heat is typically provided by coils of steam-filled pipe, which can also be a source for regulating humidity.
- Air circulation. Airflow is necessary for circulating heat and removing moisture from lumber. Air is typically circulated by internal fans or blowers mounted on the outside.
Kiln drying dries wood more evenly than air drying, which is important for preserving the quality of wood structures. Case hardening can be a problem when wood is dried unevenly. When this happens, outside surfaces dry and stretch around a damp core. Then, when the core begins to dry, it is prevented from shrinking by the stretched and hardened surface. The tension between the competing shrink stages can cause the wood to warp or split. Wood that has split internally can be difficult to detect and is irreversible. Kiln drying also offers better control over the final temperature and humidity, which can be especially important when drying different species of wood.
Kiln drying preparation
Uncut logs can be dried in kilns, but it is more common for them to be sawn into boards and planks first. This has a few benefits. Sawing lumber increases surface area and decreases thickness, which improves drying rates. It also ensures a more uniform length and shape that helps individual pieces dry at a similar rate. At a minimum, removing bark surfaces prevents moisture isolation, which can slow drying, and avoids fungal decay. Sawed wood is then stacked with spacers, called "stickers," and raised above ground to promote airflow.
Once stacked and placed into a drying kiln, the kiln environment is manipulated according to strict drying schedules. These schedules are designed to dry woods as fast as possible without compromising quality. They also vary according to the species of wood, its thickness and its intended use. Most drying schedules begin at a higher humidity with lower heat and gradually reverse these conditions. Existing moisture content and material density can also affect drying times and processing techniques.
Steel wheel applications
Steel is an exceptional material for rail applications. Compared to other materials, it offers an ideal balance of strength and hardness to prevent early wear and potential failure. This minimizes maintenance time and keeps operations running with maximum output and profitability. Double flange wheels are ideal for lumber kiln operations, as they can be installed to float slightly to accommodate rails that have been misshapen from heavy-duty applications.
- Meier, Eric. "Drying Wood at Home." The Wood Database.
- Prestemon, Dean R. "Forestry Extension Notes: Kiln Drying Lumber." Forestry Extension. 1999.
- Reeb, James E. "Drying Wood." University of Kentucky.
- Tsoumis, George Thomas. "Wood." Encyclopedia Britannica.