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The trolley which travels along the screw is notorious for having broken teeth. Normally, the length of the tooth bar on the trolley is long enough to span the gap at the screw joint. With broken teeth, however,it can essentiall " stall : at that point. The trolley is intentionally made of soft metal to protect the screw, but as a result is prone to these problems. Fortunately, the trolley is inexpensive and easily replaced.
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