What are the common Types of Thread

Threaded joints and power screws carry clamping load and transmit motion in almost every machine, and the thread profile - the axial cross-section of the thread - sets the strength, friction, efficiency and manufacturing cost of the connection. Choosing a profile is therefore a reliability and life-cycle cost decision, not a detail. On bolted joints the same choice also decides how evenly preload reaches the flange gaskets sealing the two faces.

What the Thread Profile Controls

A thread profile is the shape of the axial section through the thread: flank angle, pitch, tooth thickness, crest and root geometry. Those few dimensions decide how much of the bolt load becomes axial thrust, how much is lost to friction, how the root handles stress concentration and how the thread can be produced.

• Flank angle drives wedging action and efficiency: a large angle gives strong self-locking but low efficiency, while near-vertical flanks convert almost all force into axial thrust.
• Metric trapezoidal threads (designation Tr) use a 30 degree included angle between the flanks; the inch-based Acme thread uses 29 degrees.
• V-threads come in two main families: the international Metric thread with a 60 degree thread angle and the Whitworth thread with a 55 degree angle.
• Root geometry sets fatigue and shear capacity: a thick root raises strength margin, a thin root raises efficiency.

V-Thread: The Standard Fastening Profile

The V-profile is the most common thread by a wide margin, used mainly for fastening. Its triangular section appears in the 60 degree Metric and 55 degree Whitworth forms, and almost every bolt, nut and screw is a V-thread - including the studs and bolts that assemble pipe flanges and pressure equipment.

• Good self-locking: the large flank angle creates strong wedging and friction, so joints resist loosening under vibration, although locking devices remain common practice.
• High root strength: the 60 or 55 degree section leaves a relatively thick root and keeps connection strength reliable.
• Mature manufacturing: rolling, thread forming and cutting are all well established and cost-effective across a wide size range.
• Efficiency penalty: the same wedging action that gives self-locking makes the V-thread the least efficient profile for transmitting motion.

Power-Transmission Profiles: Square, Trapezoidal and Buttress

Where a screw converts rotation into linear motion, three profiles dominate, trading efficiency against manufacturability and wear behavior.

• Square thread - perpendicular flanks (0 degrees) and a square or rectangular section with tooth thickness equal to half the pitch. It gives the highest mechanical efficiency and high load capacity, but it is hard to cut with standard dies or taps, and wear play cannot be removed by axial nut adjustment.
• Trapezoidal thread - an isosceles trapezoid with flat crests and roots: 30 degrees included angle for Tr, 29 degrees for Acme. The flank slope of 15 degrees from vertical allows split nuts that take up wear play, and the flat surfaces retain lubricant and dissipate heat.
• Buttress (sawtooth) thread - an asymmetric trapezoid with a nearly vertical load flank (3 to 7 degrees) and a steeper non-load flank (30 to 45 degrees). It combines square-thread efficiency with V-thread root strength for extreme one-direction axial loads.

Standardization matters as much as geometry: trapezoidal threads have international series sizes and tolerance classes, which simplifies design and procurement, while square threads are usually single-point machined and specified case by case.

Table: typical thread profiles, their main trade-offs and usual duties.

How to Select a Thread Profile

Strength alone never decides the profile. Work through three questions before specifying.

• Load: is it bidirectional or unidirectional, and is vibration present? Vibration pushes toward V-threads with locking, while one-direction peaks favor buttress threads.
• Motion: static fastening or continuous travel? For leadscrews and valve stems, efficiency and positioning accuracy dominate.
• Manufacturing and maintenance: is interchangeability with standard parts required, and how will wear be compensated over service life?

Practical rule: fastening means V-threads, general power transmission means trapezoidal threads, extreme one-direction axial load means buttress threads, and square threads only where their efficiency justifies the manufacturing cost.

Frequently Asked Questions

Which thread profile has the highest mechanical efficiency?

The square thread. Its flanks are perpendicular (0 degrees), so almost no radial force acts on the nut and most of the applied force becomes axial thrust, giving the highest efficiency of any profile.

Why is a trapezoidal thread preferred over a square thread for leadscrews?

The 15 degree flank slope allows split nuts that take up wear play, the profile is far easier to manufacture, and international Tr series sizes and tolerance classes simplify design and sourcing.

What flank angles does a buttress thread use?

The load-bearing flank is nearly vertical, typically 3 to 7 degrees, while the non-load-bearing flank is steeper, typically 30 to 45 degrees. That layout keeps efficiency high and the root strong under one-direction axial force.

When should a buttress thread replace a V-thread?

When the load is a large axial force in one direction only, such as in heavy jacks, presses and screw-down mechanisms. For fastening under vibration, the self-locking V-thread remains the better choice.