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Linear Motion Systems – Drive Mechanism Load Capacities

January 2, 2020

Linear positioning table drive mechanisms will have static and dynamic load capacity ratings for the acme screw, ball screw, and belt drive assembly. These values are used to help select a correct drive mechanism for a given load/life table application. For most acme screw driven positioning table applications, the screw (and not the linear bearing) is the major factor in determining the life of the table. This is due to the high friction of the nut assembly. For most ball screw driven positioning table applications, the linear bearing system (and not the screw) is the major factor in determining the life of the table. This is due to the high efficiency and high load capacity of the nut. For most belt driven positioning table applications, the linear bearing system (and not the belt) is the major factor in determining the life of the table. This is due in large part to the fact that belt driven tables usually travel lots of inches at high speeds. The use of adequate safety factors is a key element in the selection process of the drive mechanism for a given application. Selecting a system with no safety margin can lead to problems relating to performance and long-term life.

The actual (axial) load a drive mechanism “sees” needs to be determined first. Then the effects of that load on the drive mechanism can be reviewed. For both screw & belt driven positioning tables, the actual load the drive mechanism experiences will vary as the table moves. During acceleration and deceleration intervals of a positioning table, the force exerted upon the drive mechanism changes as the acceleration or deceleration rate varies. In most cases, the extra force acting upon the drive mechanism during the acceleration interval is offset by a reduced force during the deceleration interval. Therefore, using just the forces acting upon the drive mechanism during constant velocity can be used. The applied (axial) load “as seen by the screw nut or belt” depends upon the table orientation. See the equations below.

Drive Capacities

Static Loads can exert an extreme force upon the drive assembly in a non-moving state. For acme screw driven tables, if the static load rating of a particular screw is exceeded, the nut assembly can permanently be deformed, or crack outright. For ball screw driven tables, if the static load rating of a particular screw is exceeded, a localized permanent depression in the screw shaft and ball nut could cause the system to not operate smoothly or fail prematurely. For belt driven tables, if the static load rating (maximum belt tensile force) of a particular belt is exceeded, the belt will permanently stretch, or tear. To ensure proper life, external forces should never come close to the static rating. Repeated forces at or near the maximum rating can fatigue the elements causing premature failure. Some static forces will be known and can be accounted for (i.e. drilling, insertion, stamping, engraving, etc.). Other unexpected forces that are difficult to determine could come from vibrations, impacts, or inertial forces. To ensure proper life, external forces should never come close to the static rating. Repeated forces at or near the maximum rating can fatigue the elements causing premature failure. Thus, a safety factor should be considered to account for these forces. Also, by using a safety factor, extra unforeseen loads that arise within an application sometime in the future, would not affect the positioning table chosen.

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