The main function of Ball and Acme (Lead) Screw Assemblies is to provide a drive mechanism, usually within a positioning table, to move a load. The drive mechanism is an element that contributes to position accuracy, repeatability, speed, and mechanical system resolution.
Ball Screws are the screw of choice for high duty cycle, high speed, and long life applications. The ball screw nut uses one or more circuits of recirculating steel balls which roll between the nut and ball screw grooves, providing an efficient low friction mechanical drive system. The nut, which is attached to the table carriage, moves back and forth in parallel with the linear bearing system as the ball screw rotates. Using a higher lead ball screw (for example a 0.500 inch lead instead of a 0.200 inch lead) will offer greater carriage speed for applications requiring rapid traverse, or fast, short incremental moves. Key features of a ball screw system are low wear and long life.
A Rolled Ball Screw system utilizes a tapped nut with a standard accuracy grade rolled screw. A Precision Rolled Ball Screw system utilizes a ground nut with a higher accuracy grade rolled screw. A Ground Ball Screw system utilizes a ground nut with a high accuracy precision ground screw, and provides the highest performance and accuracy.
Accuracy grades of acme screws and ball screws are typically reflected by their “lead error” specification, which is generally given in a “in/ft” (microns/300 mm) rating. This “lead error” is the primary element in determining the position accuracy of a positioning table. The lower the “lead error” (better position accuracy), the more costly the drive screw assembly.
Acme Screws typically use a turcite (polymer) or bronze nut. The nut, which is attached to the table carriage, moves back and forth in parallel with the linear bearing system as the acme screw rotates. The threads of the plastic or bronze nut ride in the matching acme screw threads, much like an ordinary nut and bolt system. This produces a higher friction (thus lower efficiency) system than a ball screw assembly, since there are no rolling elements between the nut and the acme screw threads. Acme screws work fine for applications requiring low speeds, low noise and low duty cycles. Acme screws are also a good choice for most vertical applications as they typically prevent back-driving of the attached load. However, in some applications the friction of the acme screw can affect low speed smoothness and its long term life is significantly lower than a ball screw.
A turcite nut option is available with rolled ball screws and operates very similar to an acme screw. However, the polymer nut contacts the ball screw grooves differently than an acme screw assembly producing a drive mechanism that has less friction than an acme screw thereby allowing for potential higher speeds and less input torque required from a coupled motor. This option provides smooth motion, low audible noise, and is ideal for vertical applications. Also, using the turcite nut with high lead ball screws provides for faster linear speeds.
The chart below compares the primary differences between Ball and Acme Screw drive systems.
More information on the Ball Screw Assembly Products from Electromate can be viewed at-
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Tags: Ball Screw, ACME Screw, Ball Screw Assembly, Precision Ground Ball Screw, Roll Thread Ball Screw, Precision Roll Thread Screw