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Improper ball screw bearing preload will result in either excessive backlash in the mill if it is too loose, or rapid wear and excessive friction if it is too tight. It should be noted that if your ball screw, ball nut, or angular contact bearings are worn, or if your gibs are adjusted too tight, you will not achieve appropriate lost motion values.
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The figure to the right below shows how the force of preload is transmitted through the bearings, from the inner race to the outer race. In a preload pair, this force is then transmitted back to the inner race by an opposed bearing. It should be apparent that the correct orientation of the angular contact bearing is critical to the operation.
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To adjust the angular contact bearing pair preload on all 770 and 1100 machines, the AC bearing service tool kit is required (part number 35355). To adjust the angular contact bearing pair preload on the PCNC 440, a pin spanner wrench is required (part number 32655).
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To properly estimate the torque needed to overcome angular contact bearing friction, the bearings must be isolated from the motor detent torque. The following procedure illustrates the X axes; other axes will follow suit:
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