Lead screws and ACME screws are used to drive a nut in linear motion via direct contact between the screw and the nut. Lead screws usually incorporate proprietary screw and nut geometries and materials, and can be configured in zero-backlash arrangements. ACME screws are square-topped and have a specific thread-geometry. By directly contacting the nut, ACME screws provide linear motion through direct sliding friction between the nut and the screw. For both lead screws and ACME screws, choices for component type are nut only, screw only, and nut and screw assembly. Typically, the nut is made of plastic or bronze.
Different Kinds of Threads
Lead screws and ACME screws may have rolled, milled, or ground threads. Rolled screws are cold-rolled from a blank. Typically, this manufacturing process creates lead screws and ACME screws with a .004 inch-per-foot lead accuracy. Milled screws are made on a milling machine using a cutter with an ACME form. Most milled screws have a .002 inch-per-foot lead accuracy. Ground screws also have an ACME form, but are made on a grinding wheel. These lead screws and ACME screws usually have a .0005 inch-per-foot lead accuracy.
Specifications for lead screws and ACME screws include length, screw outer diameter (OD), screw lead, screw lead accuracy, dynamic load rating, and maximum static load capacity. Screw OD is the largest diameter of the lead screw or ACME screw. Screw lead is the axial distance a screw travels during one revolution, as measured in inch/turn or mm/turn. Dynamic load rating is the load that can be sustained while the assembly is moving. Maximum static load capacity is the load that can be sustained while the load is stationary.
Lead screws and ACME screws differ in terms of features such as zero-backlash designs and twin leads. Backlash, the axial free motion between the nut and screw, is a measure of the stiffness of the assembly. 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 left-hand and right-hand threads. With left-hand threads, the ball nut travels away from the end that is viewed when the screw is rotated counter-clockwise. With right-hand threads, the ball nut travels away from the end that is viewed when the screw is rotated clockwise.
BS 4185-10 - Specification for machine tool components
ASME B1.8 -Stub ACME screw threads
BS 1104 - Specification for general purpose ACME screw threads