Evaluating Shrinkage of Wood Propellers in a High-temperature Environment
Evaluating Shrinkage of Wood Propellers in a High-temperature Environment

Author: Richard D. Bergman

Publisher:

Published: 2008

Total Pages: 12

ISBN-13:

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Minimizing wood shrinkage is a priority for many wood products in use, particularly engineered products manufactured to close tolerances, such as wood propellers for unmanned surveillance aircraft used in military operations. Those currently in service in the Middle East are experiencing performance problems as a consequence of wood shrinking during long-term storage at low equilibrium moisture content conditions prior to installation. To evaluate the extent of shrinkage, seven sugar maple (Acer saccharum) veneer propellers were dried from 11% to 3% moisture content in a controlled environment of 150°F (65°C) for 3 days. Two of these wood propellers were encased in polyethylene bags. Results showed 5 to 20 times more shrinkage for the thickness of the propeller hub and the hub face perpendicular to the propeller blades (across the grain), respectively, compared with the hub face parallel to the blades (along the grain). Two hubs, coated with aluminum oxide paint, showed dimensional changes similar to those observed for uncoated hubs. For the two wood propellers encased in polyethylene bags, moisture loss was slowed during the course of the experiment by roughly 46%. Wrapping the wood propellers prior to shipping would slow moisture desorption, thereby minimizing shrinkage during short-term storage. Processing the propellers at a lower equilibrium moisture content would minimize shrinkage during long-term storage.

Operation and Cost of a Small Dehumidification Dry Kiln
Operation and Cost of a Small Dehumidification Dry Kiln

Author: Richard D. Bergman

Publisher:

Published: 2008

Total Pages: 24

ISBN-13:

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Obtaining small quantities of custom kiln-dried lumber can be an expensive process for an individual woodworker. Building and operating a small kiln capable of drying custom cuts of lumber (such as slabs, bowl blanks) gives woodworkers another option. Our approach was to build and operate a small dehumidification dry kiln. The four charges of lumber ranged from 600 to 700 board feet (bf), and a woodworker with no dry-kiln experience operated the kiln. The first charge of mixed air-dried 4/4 hardwoods and softwoods was kiln-dried from 18.4% to 7.3% moisture content (MC) in 15 days with no casehardening. The second charge of 5/4 black cherry lumber was kiln-dried from 47.5% to 6.8% MC in 27 days with no casehardening, and the third charge of 4/4 northern red oak lumber was kiln-dried from 82.9% to 6.1% MC in 45 days and to 5.2% MC in 50 days with severe casehardening relieved by conditioning. A fourth charge of 300 bf 4/4 northern red oak, 200 bf of 4/4 shagbark hickory, and 100 bf of mixed 4/4 box elder and 4/4 black cherry was kiln-dried from 69.7% to 8.5% MC in 29 days with casehardening only in the northern red oak, and that was relieved by conditioning. We found that greater control of the dehumidifier operation earlier in the kiln schedule to maintain the safe drying rate would decrease the severity of casehardening when drying green northern red oak. New building materials, which are most of the initial cost, could be replaced by reused lumber. Drying small amounts of lumber using a dehumidification kiln is a suitable option for woodworkers even with limited knowledge of kilns.