Properties of Dowels

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Properties of Dowels
Dowel pins help buses to run, though not necessarily on time. (Medioimages/Photodisc/Photodisc/Getty Images)

Steel dowel pins, which are solid, cylindrical, alloy rods, are used as mating devices in machinery, where one surface has to sit precisely on another. They are also employed as roller bearings in the wheel housings of buses and trucks. Because their strength is critical, dowel pin specifications are covered by standards set by ASME, The American Society of Mechanical Engineers.

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Material and Manufacture

Dowel pins are made of alloy steel with a maximum sulphur content of 0.05 per cent, and a maximum phosphorus content of 0.04 per cent. The pins are heat-treated to harden them, using a process called austenitization, according to metallurgist Charlie R. Brooks. This consists of quenching the heated pins into oil to temper them, thereby changing the crystal structure of the iron to austenite, also known as gamma-iron.

Hardness

The alloy must be such that the pin achieves a specified hardness for case, or outer surface, and core, or interior. Both are measured on the Rockwell C scale. According to the Materials Information Society, the Rockwell C scale is based on how far a steel is indented by a pointed diamond cone pressed into it. Harder steels are indented less than softer steels. The numbers in the scale are comparative, with harder steels achieving higher numbers, representing less indentation. A dowel pin's core hardness is 47 to 58 on the Rockwell C scale. The case hardness is a minimum of 60.

Chamfered End

The chamfer on a dowel pin is the sloping end. The angle of slope must be between 4 and 16 degrees. This creates an end face referred to as a "point." The minimum and maximum point diameters are specified. For example, for a 1/8-inch-diameter dowel pin, the minimum point diameter is 0.110 inch and the maximum point diameter is 0.120 inch.

Crowned End

The crowned end is a rounded end formed by a process called radiusing. The crown height is the distance between the point along the pin's length where the curve of the crown begins, and the end of the pin formed by the crown's centre point. For a 1/8-inch-diameter dowel pin, the crown height is 0.041 inch. The crown radius is the distance between the crown centre point and any point on the crown's edge. For a 1/8-inch-diameter dowel pin, the crown radius is 0.016 inch.

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