|
In engineering and machining, an allowance is a planned deviation between an actual dimension and a nominal or theoretical dimension, or between an intermediate-stage dimension and an intended final dimension. The unifying abstract concept is that a certain amount of difference ''allows for'' some known factor of compensation or interference. For example, an area of excess metal may be left because it is needed to complete subsequent machining. Common cases are listed below. An ''allowance,'' which is a ''planned'' deviation from an ideal, is contrasted with a ''tolerance,'' which accounts for expected but unplanned deviations. Allowance is basically the size difference between components that work together. Allowance between parts that are assembled is very important. For example, the axel of a car has to be supported in a bearing otherwise it will fall to the ground. If there was no gap between the axel and the bearing then there would be a lot of friction and it would be difficult to get the car to move. If there was too much of a gap then the axel would be jumping around in the bearing. It is important to get the Allowance between the axel and the bearing correct so that the axel rotates smoothly and easily without judder. ==Examples of engineering and machining allowances== * Outer dimensions (such as the length of a bar) may be cut intentionally oversize, or inner dimensions (such as the diameter of a hole) may be cut intentionally undersize, to allow for a predictable dimensional change following future cutting, grinding, or heat-treating operations. For example: * * the outer diameter of a pin may be ground to oversize because it is known that subsequent heat-treatment of the pin is going to cause it to shrink by . * * A hole may be drilled undersize to allow for the material that will be removed by subsequent reaming. * Outer dimensions (such as the diameter of a railroad car's axle) may be cut intentionally oversize, or inner dimensions (such as the diameter of the railroad car's wheel hub) may be cut intentionally undersize, to allow for an interference fit (press fit). * A part may be cast intentionally too big when it is desired to later machine the surface. This ensures that the roughness that the casting process leaves is removed, and a smooth machined surface is produced. This machining allowance may be e.g. 1mm, but this depends on the size of the part and the accuracy of the casting process. * A chain segment may be oversized so that at the end of its useful service life (around 20 years), the corroded chain is still above the minimum diameter required to meet the minimum break strength. This is called a corrosion allowance and accounts for the steel molecules lost through oxidation, erosion and wear, and Microbially Influenced Corrosion. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Allowance (engineering)」の詳細全文を読む スポンサード リンク
|