Machinery's Handbook, 31st Edition
BALL AND ACME LEADSCREWS 2641 General Leadscrew Design Considerations.— The following design criteria should be considered when selecting leadscrews. Load: The load is the weight “as seen by the screw.” For vertical applications such as lift ing or jacking the load equals the weight of the load being moved. For horizontal applica tions the “load seen” equals the weight of the load times the coefficient of friction of the supporting media. Another way to determine the load in an actual application is to attach a scale to the load and pull it. Base the load on the moving force required, not on the higher starting (breakaway) force. Tension and Compression Load: For reduced screw thread sizes, design for tension loads whenever possible. Compression load designs must factor in column strength to prevent buckling. Back-drive: When a ball screw or acme nut is forced to move linearly along the shaft by an applied force, or gravity in vertical orientation, it forces or back-drives the shaft making it rotate. With their higher efficiency and lower initial start up (breakway) friction value, ball screw assemblies in vertical orientations may back-drive their shafts. Driving Torque: The driving torque ( T d ) is the amount of force required to rotate the screw and move a load. T e P L 2 d # = π where T d = driving torque (lb-in) P = load (lbs) L = lead of screw (in/revolution) e = ball bearing screw efficiency (approximately 0.90 for ball screws, 0.40 for acme screws with plastic nuts, and 0.25 for acme screws with bronze nuts.) Accuracy: Several factors affect accuracy. One factor is the lead deviation in inches per foot. Typical values can range from 0.005 in/ft to 0.001 in/ft. Backlash may also be a consideration in applications that require precise positioning in both directions and in CNC applications. A final consideration is accuracy versus time degradation due to screw wear and ball bearing fatigue. This is especially important in systems that require precise positioning but do not have closed-loop feedback controls. Ball screws have an advantage in long term accuracy since the useful life of ball screws is determined by metal fatigue rather than the ordinary wear characteristics of a conventional screw. Due to roll- ing contact, very little dimensional change occurs over the life of a ball bearing screw, thus eliminating the need for frequent compensating adjustments. Lead: The distance the nut travels with one rotation of the screw. Speed: When rapid travel is required, design for screws with a high lead to reduce shaft speed to avoid rotational and vibrational side effects. Life: The recommended operating loads for ball screw assemblies are based on a pre dicted life of one million inches of travel, providing the assembly is lubricated. However, it is possible to operate at greater loads if a reduced life is acceptable. The load should not exceed the maximum static load shown in manufacturer design tables. Backlash: Backlash, end play, positional inaccuracy, or lost motion between the nut and the screw can be eliminated by utilizing a preloaded nut assembly. Backlash can also be reduced, at increased cost, in ball screws by changing the conformity ratio (ball track ra- dius/ball diameter) as well as using ogival (gothic) ball tracks. Fig. 3 illustrates the contact points between a preloaded ball screw assembly and the shaft.
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