| 摘要 |
A dam has a plurality of substantially parallel ribs extending from a base surface. At least one of the ribs is formed of a body of metal having a coating as its exterior surface, which has a low coefficient of friction over which sheets of a media move. The body has a longitudinal slot in its exterior surface. An insert, which has a high coefficient of friction with sheets of a media and is preferably polyurethane, is supported within the body. The insert is preloaded so that a projection extends a predetermined distance through the slot in the body for engagement with each advancing sheet. When the sheet is stiff, the projection is pushed interior of the body so that the low coefficient of friction surface, which has a much larger area for engaging the sheet than the insert, of the body is engaged by the advancing sheet. When the sheet is flexible, the projection of the insert engages the advancing edge of the sheet so that only the high coefficient of friction of the insert is initially engaging the advancing sheet to cause it to buckle or become corrugated so that portions of the sheet engage the low coefficient of friction of the exterior surface of the body to separate the sheet from the next adjacent sheet in the stack.
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