Buttress and Special Load-Bearing Threads

Buttress & Other Special Threads: The Complete Engineering Reference Guide

The 45-Degree Angle That Changed Everything

the practitioner stared at the wreckage of a 14-inch hydraulic press column scattered across the factory floor.

Three months of production — gone. The column's Acme threads had been handling 200,000 units of axial force for years. But when the press was upgraded to handle 350,000 units of unidirectional thrust, nobody stopped to ask the most dangerous question in engineering:

"Is this the right thread form for this load?"

The answer was no. And the radial component of thrust generated by the 29-degree Acme thread profile had slowly expanded the tubular column housing like a balloon — until it split.

the practitioner's competitor across town, the practitioner, ran the same press upgrade six months earlier. Her threading engineer had specified a buttress thread — a profile where the load flank sits nearly perpendicular to the axis, reducing radial thrust to almost nothing.

the practitioner's press is still running.

This is the guide that separates engineers who understand special thread forms from those who learn about them the hard way.

What You Will Master in This Guide

• The 10-Degree Modified Square Thread — the practical equivalent of a square thread that you can actually manufacture economically
• Buttress Thread Forms — from the classic 45-degree to the ANSI B1.9 standard 7°/45° profile
• The German Sägengewinde (Saw-Tooth Thread) — Europe's answer to asymmetric loading
• American National Standard Buttress Threads — complete with every formula, tolerance table, and designation rule from ANSI B1.9-1973 (R1992)
• British Standard Buttress Threads — BS 1657:1950 and how they differ from the American standard
• The Löwenherz Thread — the precision instrument thread born from the metric system
• The International Metric Thread (S.I. System) — the foundation that shaped European threading for over a century
• Three-Wire Measurement Methods for buttress threads
• Complete worked examples with every dimension calculated step-by-step

The 10-Degree Modified Square Thread

The Problem with True Square Threads

A true square thread — with flanks perpendicular to the axis — is the theoretical ideal for power transmission. Zero radial thrust. Maximum axial efficiency.

But try manufacturing one.

True square threads cannot be cut with standard milling cutters. They cannot be ground efficiently. They cannot be produced with taps in any practical sense. The perpendicular flanks create cutter interference problems that make economical production nearly impossible.

The 10-Degree Solution

The 10-Degree Modified Square Thread solves this problem elegantly. By introducing a mere 10-degree included angle between the thread flanks (5 degrees per side), the thread becomes the practical equivalent of a square thread while remaining fully manufacturable by milling, grinding, and other standard processes.

Critical Manufacturing Note: Multiple thread milling cutters and ground thread taps should not be specified for modified square threads at larger lead angles without first consulting the cutting tool manufacturer.

Master Formula Card — 10-Degree Modified Square Thread

Important: A clearance must be added to depth h to avoid interference with threads of mating parts at minor or major diameters. This is the single most commonly overlooked detail in modified square thread design.

Worked Example — 10-Degree Modified Square Thread

Given: A 3-inch diameter lead screw with 4 threads per inch and a clearance of 0.010 inch.

Total thread depth with clearance = 0.1250 + 0.010 = 0.1350 in

Threads of Buttress Form — The Asymmetric Power Thread

Why Buttress Threads Exist

Go back to the practitioner's factory floor.

The fundamental problem was radial thrust. Every time a symmetric thread form (like Acme, V-thread, or even the modified square thread) carries a heavy axial load, a portion of that load gets converted into a radial force that tries to expand the nut or split the housing.

The buttress thread eliminates this problem by making the load-carrying flank nearly perpendicular to the thread axis. The result: axial force travels straight down the thread with minimal radial spreading.

Typical applications include:

• Breech assemblies of large guns — where enormous forces act in one direction during firing
• Airplane propeller hubs — where centrifugal forces create massive unidirectional axial loads
• Columns for hydraulic presses — where tubular members must resist splitting under extreme thrust
• Vise screws and jack screws — where power transmission is overwhelmingly in one direction

The Three Major Buttress Thread Variants

Variant 1A: The Classic 45-Degree Buttress

The simplest form. The front (load-resisting) face is perpendicular to the axis and the clearance flank is inclined at 45 degrees.

Pitch Rule: P = 2 × screw diameter ÷ 15

Variant 1B: The Inclined Load Flank (1° to 5°)

In practice, the load-resisting flank is often inclined 1 to 5 degrees from perpendicular. This prevents cutter interference when milling the thread.

With a 5-degree front face angle and 50-degree included angle:

Variant 1C: The Sägengewinde (German Saw-Tooth Thread)

Known as the "Sägengewinde" in Germany and "Fillettatura a dente di Sega" in Italy, this is Europe's standardized buttress form.

Buttress Thread Variants — Quick Comparison

American National Standard Buttress Inch Screw Threads — ANSI B1.9-1973 (R1992)

This Is the Standard That Matters

When the practitioner's threading engineer specified "buttress thread" for that hydraulic press column, this is the standard they referenced. ANSI B1.9-1973 (R1992) is the definitive American standard for 7°/45° buttress inch screw threads.

Form of Thread — Characteristics

The ANSI standard buttress thread has four defining characteristics:

A) A load flank angle of 7 degrees from the normal to the axis (measured in the axial plane)

B) A clearance flank angle of 45 degrees from the normal to the axis (measured in the axial plane)

C) Equal truncations at the crests of both external and internal threads, producing a basic height of thread engagement of 0.6p (assuming no allowance)

D) Equal radii at the roots of external and internal basic thread forms, tangential to both the load flank and clearance flank. When specified, equal flat roots may be supplied instead.

What Makes a Buttress Thread "Standard"

A buttress thread is considered standard when ALL four conditions are met:

1. Opposite flank angles are 7 degrees and 45 degrees
2. Basic thread height is 0.6p
3. Tolerances and allowances conform to ANSI Tables 4 through 6
4. Length of engagement is 10p or less

Master Formula Card — ANSI 7°/45° Buttress Thread

Thread Diameter Formulas — Complete Symbol Reference

Basic Dimensions Table — ANSI B1.9-1973 (R1992)

All dimensions in inches.

Preferred Diameter–Pitch Combinations — ANSI B1.9-1973 (R1992)

Root Form Options

The ANSI standard provides two root form options for external threads:

Round Root (Default): The rounded root form shall be a continuous, smoothly blended curve within the zone defined by:

• Maximum radius: r = 0.0714p
• Minimum radius: r = 0.0357p (one-half of maximum)

The resulting curve shall have no reversals or sudden angular variations and shall be tangent to the flanks of the thread.

Reality check: The standard acknowledges that "there is, in practice, almost no chance that the rounded thread form will be achieved strictly as basically specified, that is, as a true radius." This is engineering honesty at its finest.

Flat Root (Specified with FL designation):

• Maximum flat width (S): S = 0.0928p
• Minimum flat width: 0.0464p

Buttress Thread Tolerances — The Numbers That Matter

Pitch Diameter Tolerance Formulas

Class 2 (Standard Grade):

$$PD\ Tolerance = 0.002\sqrt[3]{D} + 0.00278 \times L_e + 0.00854\sqrt{p}$$

When the length of engagement is taken as 10p, this simplifies to:

$$PD\ Tolerance = 0.002\sqrt[3]{D} + 0.0173\sqrt{p}$$

Class 3 (Precision Grade): Two-thirds of Class 2 PD tolerances.

Class 2 (Standard Grade) Tolerance Table

Tolerances on Major Diameter of External Thread, Pitch Diameter of External and Internal Threads, and Minor Diameter of Internal Thread (inches):

Class 3 (Precision Grade) Tolerance Table

Taper and Roundness Rules

Class 2: No requirements for taper and roundness.

Class 3:

• Major and minor diameters shall not taper nor be out of round beyond specified limits
• Pitch diameter taper and out-of-roundness shall not exceed 50% of the pitch-diameter tolerances

Lead and Flank Angle Deviation Rules

Class 2: Deviations in lead and flank angles may consume the entire tolerance zone between maximum and minimum material product limits.

Class 3: Combined diameter equivalents of variations in lead (including helix deviations) and flank angle shall not exceed 50% of the pitch diameter tolerances.

Functional Size

Deviations in lead and flank angle increase the functional size of an external thread and decrease the functional size of an internal thread by the cumulative effect of the diameter equivalents of these deviations. The functional size of all buttress product threads shall not exceed the maximum-material limit.

Allowances for Easy Assembly

The Rule

An allowance (clearance) shall be provided on all external threads to secure easy assembly. The amount of the allowance is deducted from the nominal major, pitch, and minor diameters of the external thread when determining the maximum material condition.

The minimum internal thread is basic — no allowance is applied to internal threads.

The allowance amount is the same for both Class 2 and Class 3 and equals the Class 3 pitch-diameter tolerance.

Complete Worked Example — Every Dimension Calculated

This is where the practitioner's engineer earned their paycheck.

Given Parameters

• Diameter: 2 inches
• Threads per inch: 4
• Thread class: Class 2
• Flank angles: 7° and 45° (standard)

Step 1: Extract Basic Values from Tables

Step 2: Internal Thread Dimensions

Step 3: External Thread Dimensions

Step 4: Summary — Internal vs. External Thread Limits

Buttress Thread Designations — How to Read and Write Them

Pull Type vs. Push Type

BUTT = Pull type buttress (external thread pulls). The clearance flank leads, and the 7-degree pressure flank follows.

PUSH-BUTT = Push type buttress (external thread pushes). The 7-degree load flank leads, and the 45-degree clearance flank follows.

Best Practice: Whenever possible, confirm the designation with a simplified view showing thread angles on the product drawing.

Designation Abbreviations

Designation Sequence — Single-Start Threads

Order: Nominal Size → TPI → PUSH (if applicable) → BUTT → Class → A or B → LH (if applicable) → FL (if applicable)

Example 1: 2.5-8 BUTT-2A

• 2.5-inch diameter
• 8 threads per inch
• Buttress thread, pull type
• Class 2, external
• Right-hand
• Radiused root

Example 2: 2.5-8 PUSH-BUTT-2A-LH-FL

• 2.5-inch diameter
• 8 threads per inch
• Push type buttress
• Class 2, external
• Left-hand
• Flat root

Designation Sequence — Multiple-Start Threads

For multiple-start threads, pitch is given instead of TPI, followed by lead, and the number of starts appears in parentheses after the thread class.

Example: 10-0.25P-0.5L-BUTT-3B (2 start)

• 10-inch diameter
• 4 threads per inch (0.25 pitch)
• 0.5-inch lead
• Buttress, pull type
• Class 3, internal
• 2 starts
• Radiused root

Three-Wire Measurement of Buttress Threads

Why Measurement Is Different for Buttress Threads

Unlike symmetric threads where wires contact both flanks equally, the asymmetric profile of buttress threads requires specialized formulas that account for the different flank angles.

General Formula for Any Buttress Thread Angles

$$M = E - \frac{P}{\tan A + \tan(A - a)} + W\left(1 + \csc\frac{A}{2} - a\right) \times \cos\frac{A}{2}$$

Where:

• M = measurement over wires when pitch diameter E is correct
• A = included angle of thread and thread groove
• a = angle of front face (load-resisting side), measured from perpendicular
• P = pitch of thread
• W = wire diameter

Wire Diameter Formula (Pitch-Line Contact at Back of Thread)

$$W = P \times \frac{\cos a}{1 + \cos A}$$

Simplified Formulas for Common Buttress Forms

Degree Buttress (Front Face Perpendicular)

$$M = E - P + W(3.4142)$$

Wire diameter for pitch-line contact: W = 0.586 × pitch

Degree Buttress (5° Front Face Inclination)

Wire diameter for pitch-line contact: W = 0.606 × pitch

If flat at crest and root = ⅛ × pitch, then depth = 0.69 × pitch.

ANSI B1.9 Buttress (7°/45°, 52° Included Angle)

Recommended wire diameter: W = 0.54147 × P

The wire angle correction factor c is less than 0.0004 inch for recommended combinations of thread diameters and pitches and may be neglected.

British Standard Buttress Threads — BS 1657:1950

How the British Standard Differs

the practitioner's shop sometimes receives British-manufactured components. Knowing the differences between American and British buttress standards can prevent costly mismatches.

Key Differences from American Standard (ANSI B1.9)

Critical Warning: The basic depth difference (0.6p vs. 0.4p) means American and British buttress threads are NOT interchangeable, even at the same nominal size and TPI. A British buttress nut on an American buttress screw will have inadequate thread engagement.

When Smaller Tolerances Apply (BS 1657)

The British standard provides for smaller major and minor diameter tolerances in two cases:

1. When crest surfaces of screws or nuts are used as datum surfaces
2. When the resulting reduction in depth of engagement must be limited

The Löwenherz Thread — Precision Instrument Threading

The Story Behind the Name

While the practitioner and the practitioner were dealing with heavy-duty power transmission, a third engineer — the technical practitioner — was wrestling with an entirely different threading challenge in her optics laboratory.

She needed to mount precision lenses with micrometer-level positioning. Standard 60-degree threads had too much play. The backlash in V-threads made fine adjustments unreliable.

Then she discovered the Löwenherz thread — a German thread form specifically designed for measuring instruments.

Löwenherz Thread Specifications

Three-Wire Measurement Formula for Löwenherz Threads

When measurement M is known:

$$E = M + P - 3.2359W$$

When pitch diameter E is used:

$$M = E - P + 3.2359W$$

Why 53° 8′?

The seemingly arbitrary angle of 53 degrees and 8 minutes is not arbitrary at all. This angle produces a thread profile that offers an optimal balance between:

• Axial positioning accuracy — the shallower angle provides finer control than 60-degree threads
• Manufacturing practicality — the flat crests and roots allow reliable production on precision lathes
• Resistance to radial play — the wider flat at the root provides a more stable seating surface

For the technical practitioner's lens mounts, the Löwenherz thread delivered positioning repeatability that V-threads simply could not match.

International Metric Thread System (Système Internationale)

The Thread That United Europe

The Système Internationale (S.I.) thread was adopted at the International Congress for the Standardization of Screw Threads held in Zurich in 1898. It formed the basis of the normal metric series for many European countries for over a century.

Thread Form Characteristics

Complete S.I. Thread Series Table

Three-Wire Measurement for S.I. Threads

The International Metric Thread uses the same formula as the American National Standard Unified thread:

When measurement M is known:

$$E = M + 0.86603P - 3W$$

When pitch diameter E is used:

$$M = E - 0.86603P + 3W$$

Master Decision Matrix — Which Special Thread Do You Need?

the practitioner eventually became a threading consultant. Here is the decision framework she uses with every client:

Quick Identification Guide

Your Next Step

Pull up the last three thread specifications you signed off on. For each one, ask yourself:

"Is the axial load unidirectional? If so, what percentage of the total thrust is being wasted as radial force?"

If you cannot answer that question with a number, you now have the formulas, tables, and decision framework to find out.

The thread you don't question is the one that fails at 2 AM on a Saturday.

Reference Standard: ANSI B1.9-1973 (R1992) — American National Standard Buttress Inch Screw Threads. British Standard: BS 1657:1950. International Standard: Système Internationale, Zurich Congress, 1898. All dimensions in inches unless otherwise noted.