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Ball Screws vs. Lead Screws |
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Which should I use on my stage? |
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Before being able to make a decision of which screw type to use in a linear motion stage, one must understand “what is” a ball screw and lead screw. A ball screw runs on ball bearings; more specifically, a screw and nut that have matching helical grooves that allow ball bearings to recirculate in those grooves. The grooves, or races, are typically semi-circular in shape to accept the spherical ball bearing. A leadscrew uses deeper helical recesses in a form of ACME, triangular or similar-type thread. The mating nut does not have internal moving components; instead it is a single part most commonly fabricated from a polymer composite or brass. |
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Leadscrews were once considered a cheap substitute for ball screws to be used in less demanding applications. Today, both screw types offer distinct advantages. Leadscrews are typically less costly than ball screws and offer a greater variation of available screw leads. Additionally, with a polymer-based nut, it is easily modifiable if the need arises. Ball screws, on the other hand, have a tremendous advantage with efficiency and load carrying capacity. Ball nuts are rarely fabricated with any material outside of the steel family, making them more rigid and less viable for modification. Before any decisions are able to be made between the two types of technology, accuracy must be discussed. Accuracy grades are most commonly referred to between C0 and C10. Dependant upon the source at which the screw / data is being presented and which specification(s) are being referenced, G0 to G10 and T0 to T10 are also seen. Be advised that there may be minor differences between the C, G and T designations. Standard accuracy for a rolled ball screw is C7 (lead variation of ±50 µm / 300 mm length) while C10 is standard for a lead screw (lead variation of ±210 µm / 300 mm length). Higher accuracy leadscrews can achieve an equivalent C7 rating, but at a higher cost; while anything tighter than C7 is not feasible. As the accuracies tighten, ball screws transition from being thread-rolled to thread-ground allowing for lead variation as small as C0 (±3.5 µm / 300 mm length). Other than material composition, assembly type and accuracy there are other variables that exist and must be factored into specific applications; for instance, wobble error, noise, maintenance, efficiency and longevity to just name a few. Those items are outside of the scope of this article. Generally speaking, verifying design elements of load, accuracy and lead will most frequently answer the question of ball screw versus leadscrew. If the loading is too high for a leadscrew, you must use a ball screw. If either screw type can handle the load, verify if C10 accuracy is acceptable for the application. An application requiring more accuracy than C7 requires the use of a ball screw. If multiple leads, especially high thread pitches, is a main design factor, leadscrews are most appropriate. Do not be surprised that many applications can utilize either screw type and still perform flawlessly. |
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