Screw Threads - CourseWiki

A fastener holds parts together; a power screw moves a load

A thread is a ridge wrapped around a cylinder. A matching internal thread in a nut follows that ridge. Relative rotation can produce straight-line travel along the screw's axis.

A machine screw usually holds parts together. A lead screw uses the thread to position or drive a load: a slide, a vise jaw, or a linear actuator. Belts, chains and gears commonly transfer rotation between shafts; a lead screw converts rotation into linear motion. See Lead Screws for the calculations.

Keep four descriptions separate: form is the thread's cross-sectional shape; series selects diameter and pitch combinations; hand determines direction; starts count the separate helices. A helix is the spiral path around the cylinder.

Match the thread form to fastening, repeated travel, or one-way thrust

Four schematic axial profiles: square has parallel sides; V has steeply inclined sides; ACME and metric trapezoidal have flat crests and sloping sides; buttress has an asymmetric load flank and supporting flank. — Profiles cut through the screw axis. These drawings compare shapes, not manufacturing dimensions. The load-bearing side of a thread is its **flank**.

Thread-form trade-offs for sliding screw-and-nut contact.
Form and use	What it offers	What it costs
Square: power screws and jacks	High efficiency relative to angled flanks under otherwise comparable conditions.	Difficult, expensive machining; wear adjustment is difficult.
V / machine-screw thread: bolts and fastening screws	Widely available fasteners; mass production by rolling makes standard parts economical.	Flank wedging increases friction, so it is a poor choice for efficient repeated power transmission.
ACME / metric trapezoidal: vises, clamps and linear actuators	Easier machining than square; a broader thread root provides a sturdier profile than a comparable square thread. Split-nut (an adjustable, divided nut) arrangements can take up clearance.	Flank wedging increases friction relative to square. A precision drive screw can cost more than an ordinary stock fastener.
Buttress: heavy axial thrust (push along the screw) predominantly in one axial direction, such as a screw press	A nearly perpendicular working flank limits wedging in the intended load direction; the sloping back supports the thread.	Its strength and efficiency advantages are directional. A special asymmetric profile and matching nut affect manufacturing and cost.

No thread form wins every comparison. Actual strength also depends on diameter, pitch, engaged length, material and the failure being checked. ACME is not automatically stronger than every V thread. Nor does its name guarantee a larger lead. ACME and metric trapezoidal are related forms, not interchangeable parts.

Why the efficiency and strength comparisons need conditions

The IIT Kharagpur lesson Power Screw Drives and Their Efficiency (sections 6.1.1–6.1.2, hosted by IDC) explains square-thread machining cost, trapezoidal sturdiness, and added friction from inclined flanks. Its force model supports a conditional comparison: hold lead angle, contact friction and other losses comparable. It does not establish a universal ranking against V-thread fasteners.

Pratt & Whitney's ACME explanation describes the manufacturing and split-nut advantages. IIT Kharagpur's thread manufacturing lesson explains fastener rolling and other production routes. For thread cutting on a machine tool, see Lathe Operations.

The buttress working flank makes efficient thrust transmission possible in its intended direction; that is a geometry-based explanation, not a promise of the same measured efficiency as square. Reverse travel is possible, but heavy loading on the opposite flank does not retain the same advantages. Compare actual screw-and-nut ratings, not shape alone.

No radial wedging (outward force on the nut) does not mean no wear

In the ideal square-thread geometry, axial load does not create the outward wedging associated with angled flanks. That reduces a source of radial loading on the nut. The loaded surfaces still slide against each other. Friction, lubrication, materials, load and speed still affect wear.

Wear can increase backlash: clearance that must be taken up after reversing direction before the load moves. An adjustable nut can reduce that clearance; adjustment does not restore lost material or guarantee longer life. Do not infer nut life from the thread profile alone.

Where wear and operating limits enter the selection

Thomson: Lead Screws Q&A — Application Considerations describes sliding-contact heat and wear and the relationship between load and speed. Its discussion also distinguishes the generic lead screw from the ACME form. The absence of radial wedging does not remove contact pressure on the working flank.

Inch callouts count threads; metric callouts state pitch; Practice

Inch callouts count threads; metric callouts state pitch

Pitch is the axial distance between adjacent thread crests. Inch fastener callouts commonly give threads per inch (TPI), so pitch = 1 / TPI in inches. Metric callouts give pitch directly in millimetres.

Selected callouts transcribed from checked public standard sources; pitches in the last column are calculated.
Callout	Read it as	Pitch
`1/4-20 UNC`	Nominal diameter 1/4 in; 20 threads per inch; Unified coarse series	1/20 = 0.050 in
`1/4-28 UNF`	Nominal diameter 1/4 in; 28 threads per inch; Unified fine series	1/28 ≈ 0.03571 in
`M6 × 1.0`	Metric thread; nominal diameter 6 mm; pitch 1.0 mm	1.0 mm

Fine means a smaller pitch at the same nominal diameter. It does not mean a better-quality thread. Metric threads also have coarse and fine pitch choices. The 6 in M6 is a diameter, not a pitch; the 20 in 1/4-20 is a count per inch, not a lead.

These are basic size-and-pitch callouts, not complete manufacturing specifications. Fit and tolerance information may follow them. A fastener-series label does not tell you the efficiency of a lead screw; for a multiple-start drive, obtain both pitch and starts, or the lead directly.

The exact standard entries used here

NBS Circular 479, Unified Screw Thread Standards, tables 4 and 6, prints the designation entries 1/4, 20, UNC-1A and 1/4, 28, UNF-1A. The examples above omit the class suffix to teach size and pitch. This historical public standard verifies those combinations; it is not a source of current production acceptance limits. The current Unified inch thread standard is ASME B1.1-2024.

NAAMS Metric Fastener Standards, archived sheet F–2.1, dated 05/29/15, lists M6 x1.0 under screw size and M6 x 1.0 under nominal thread diameter and pitch. Only that designation is used here. ISO 261 identifies the general-purpose metric M-thread family; its catalog entry is not being used as a dimension table.

Practice reading an inch callout

Right-hand and left-hand threads advance in opposite directions

Fix the nut and look along the screw from the end you turn. Clockwise turns a right-hand screw away from you into the nut. The same clockwise rotation moves a left-hand screw toward you. Reverse the rotation and the travel reverses.

With the nut fixed, clockwise rotation viewed from the driving end moves the right-hand screw away from the viewer and the left-hand screw toward the viewer. — Same observer, same rotating member, same clockwise rotation. State all three before assigning a travel direction.

If the screw is held against axial motion and the nut travels instead, the nut moves opposite to the screw travel just described. Handedness changes the direction, not the magnitude of travel per revolution.

Direction reference

IIT Kharagpur: Friction in Screws, opening page, defines right-handed advance using clockwise rotation away from the person turning the screw. This observer convention avoids ambiguous arrows.

Next, distinguish pitch from lead in Lead Screws.