一種管狀無螺紋物件，一端帶或不帶凸緣，設計用於插入孔中以減小孔的內徑，並保護周圍的主體結構免受應力、應變和/或振動造成的損壞。總長度必須超過外徑的25%，或主體週邊平面之間的最短距離。包括分成兩段或多段的物件，且軸承週邊的旋轉角度不超過180度（3.141弧度）。有關橡膠製品，請參閱非金屬襯套。另請參閱軸承，套筒；錐形襯套；墊片，套筒；絕緣體，襯套；以及電導體襯套。不包括：襯套，鑽頭，夾具。
to be used for repairs only supplementary features, bushing-pitch housing, main rotor part name assigned by controlling agency, finish no 34 per epb4-230 except plating thkns to be cl 3 (.00020-.00040) national spec all treatment responses overall surface treatment document and classification, 1520-01-106-9519 helicopter, attack end item identification, 17-4ph cres per ams5643 heat treat to conditionh1025 rc35-42. per h+1-17 national spec all material responses body material document and classification
