Which last longer, single or double nuts

摘要

Engineers working with ball screws usually use conventional life-expectancy equations for nuts with two-point contact. When it comes to four-point contact nuts, however, many manufacturers that sell them do not discuss how to calculate their operational life. But the difference between single and double-nut life calculations lies in the preload. When applied external loads are low, for example, preload determines as much as 90% of the life. Classic L{sub}10 lifetime equations are based solely on material fatigue. But although material fatigue may be the most important, it is not necessarily the only factor in determining ball-screw life. Adverse conditions such as contamination or lack of lubricant, for example, make other factors more important. When engineers determine the life of a ball screw, they first break down its typical duty cycle into distinct steps, each with specified (external) loads or thrusts, speeds, and travel duration. The goal is to determine how many revolutions the ball screw will make under each load. These forces, designated F{sub}i are converted into an equivalent load F{sub}m that results in the same life expectancy as the actual duty cycle.