Ball screws convert rotary motion to linear motion, or torque to thrust, and vice versa. They are primarily a power screw with a train of ball bearings

riding between the screw and the nut in a recirculating track. The screw has a rounded shape to conform to the balls. Ball screws have a predictable service life and low wear rate. ACME screws and other lead screws transmit torque into linear motion through direct sliding friction. A lead screw device is very similar to a regular nut and bolt combination. Typically, the assembly comes complete with a screw, nut, and bearing support. For low speed and high accuracy applications acme screws are cost effective.

Manufacturing Process

Choices for manufacturing process for ball screws, lead screws, and ACME screws include:

• Rolled screws are cold rolled from a blank. Typically, this produces a screw with a .004 inch per foot accuracy.

• Milled screws are made on a milling machine using a cutter with an Acme form. Typically, this produces a screw with a .002 inch per foot accuracy.

• Ground screws are made on a grinding wheel with an Acme form. Typically, this produces a screw with .0005 inch per foot lead accuracy.

Physical Specifications

Important physical specifications to consider when searching for ball screws, lead screws, and ACME screws include: 

• The design arrangement can be English or metric. Some suppliers specify both.
• The length of the screw is the most important dimension to consider.
• The outer screw diameter is the largest diameter value of the screw.
• Screw lead is the axial distance a screw travels during one revolution, measured in inch/turn. Screw lead accuracy is represented as a percentage.
• The dynamic load rating is the load that can be sustained while the assembly is moving.
• The maximum static capacity is the load that can be sustained, measured while the load is stationary. 

Design Features

Important design features to consider when specifying ball screws, lead screws, and ACME screws include zero backlash design, provisions for screw lubrication, right hand thread, left hand thread, and twin leads.

• Backlash is the axial free motion between the nut and screw. It is a measure of the stiffness of the assembly.
• Right hand thread specifies that the direction of the threads on the screw shaft causing the ball nut to travel away from the end viewed when rotated is clockwise.
• Left hand thread specifies that the direction of the threads on the screw shaft causing the ball nut to travel away from the end viewed when rotated is counter-clockwise.
• Lead is the axial distance a screw travels during one revolution.
• Twin leads are useful in opposing dual motion using a single drive system. This design features both the left and right hand threads.
• Common nut materials for ACME screws include plastic and bronze. 

Standards

ISO 3408 establishes the vocabulary for ball screws and specifies their designation.

BS 6101-2 applies to re-circulating ball screw assemblies having the shaft and nuts manufactured from hardened steel.

ISO 3408-2 specifies the nominal diameters and nominal leads metric series for ball screws. 

References

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Joyce | Lee Controls | Thomson Industries