METRIC TRAPEZOIDAL THREAD DIMENSIONS

ISO metric trapezoidal thread profile is a thread form with 30° angle of thread which is used for lead screws (power screws). Dimensions and tolerances for metric trapezoidal screw threads are defined in   ISO 2901:1993, ISO 2902:1977, ISO 2903:1993 and ISO 2904:1977 standards. Gauges and gauging for ISO metric trapezoidal screw thread are defined in the DIN 103-9:1985 standard.

The calculator given below can be used for the dimensional calculations of ISO metric single and multiple start trapezoidal threaded rods and nuts. The calculator was developed by using the data tables given in the ISO standards. Calculation can be done for different pitch diameter tolerance classes of rods and nuts. Recommended tolerance classes for trapezoidal threaded rods and nut are given in the "Definitions" section.

Schematic representation of the thread form of metric trapezoidal screw thread is given in the “Definitions” section.



Calculator:

INPUT PARAMETERS
Choice of Nominal Diameter
Nominal Diameter
Choice of Pitch1
Rod Thread Pitch Tolerance Class2
Nut Thread Pitch Tolerance Class2
Number of Start 3

Note 1: See the "Results" section for preferred pitch value for selected nominal diameter.
Note 2: See the "Definitions" section for recommended tolerance classes.
Note 3: The tolerances for multiple start threads are same with single start threads except pitch tolerances. The tolerance values of Td2 and TD2 for single start are multiplied by following factors; Number of start - Factor : 2 - 1.12 , 3 - 1.25 , 4 - 1.4 , 5 - 1.6

 


RESULTS
Parameter Symbol Value Unit
Designation -- --- ---
Nominal Diameter d --- mm
Selected Pitch P ---
Preferred Pitch for Selected Dia.1 Ppref 1.5
Lead L ---
Normal Length of Engagement2 --- >
--- ---
LIMITS OF SIZES FOR TRAPEZOIDAL THREADED ROD (EXTERNAL)
Major Diameter Tolerance Class --- 4h ---
Maximum Major Diameter dmax --- mm
Minimum Major Diameter dmin ---
Pitch Diameter Tolerance Class --- --- ---
Maximum Pitch Diameter d2max --- mm
Minimum Pitch Diameter d2min ---
Minor Diameter Tolerance Class --- --- ---
Maximum Minor Diameter d3max --- mm
Minimum Minor Diameter d3min ---
LIMITS OF SIZES FOR TRAPEZOIDAL THREADED NUT (INTERNAL)
Minimum Major Diameter D4min --- mm
Pitch Diameter Tolerance Class --- --- ---
Maximum Pitch Diameter D2max --- mm
Minimum Pitch Diameter D2min ---
Minor Diameter Tolerance Class --- 4H ---
Maximum Minor Diameter D1max --- mm
Minimum Minor Diameter D1min ---
BASIC DIMENSIONS
Major Diameter for External Threads  d --- mm
Pitch Diameter for External Threads d2 ---
Minor Diameter for External Threads d3 ---
Height of the Overlapping H1 ---
Height of Internal Threads H4 ---
Height of External Threads h3 ---
Major Diameter for Internal Threads D4 ---
Pitch Diameter for Internal Threads D2 ---
Minor Diameter for Internal Threads D1 ---
Clearance on the Crest ac ---
Max. Radius-1 R1max ---
Max. Radius-2 R2max ---

Note 1: Preferred pitch value for selected nominal diameter of trapezoidal metric thread.
Note 2: If the length of engagement is larger than this range, length of engagement is classified as long. See the "Definitions" section for preferred pitch diameter tolerances according to length of engagement.

Definitions:

Basic profile: The theoretical profile of a screw thread in an axial plane defined by theoretical dimensions and angles common to internal and external threads

ISO Metric Trapezoidal Thread Form
Metric Trapezoidal Thread Form

Crest: The surface at the top of the ridge connecting its two flanks.

Gauges and Gauging: Details for the manufacture and use of gauges for checking ISO metric trapezoidal screw threads are specified in the DIN 103-9:1985 standard. It specifies the features of the types of gauges (thread plug and ring gauges, plain plug and ring gauges, wear check plug and ring gauges etc.) which are recommended for checking trapezoidal threaded rods and nuts.

Lead: The distance that nut advances in one revolution of the trapezoidal threaded rod.

Major Diameter: The maximum diameter of a thread which is diameter of the crest of a male thread or root of a female thread.

Minor Diameter: The minimum diameter of a thread which is diameter of the root of a male thread or crest of a female thread.

Recommended tolerance classes for Pitch Diameter: For general engineering use, select medium tolerance quality. For cases where manufacturing difficulties exist, coarse tolerance quality can be selected. If the actual length of thread engagement is unknown, use "Normal" length of engagement .

Tolerance Quality Tolerances for the pitch diameter
Normal Long
Recommended tolerance classes for threaded nuts
Medium 7H 8H
Coarse 8H 9H
Recommended tolerance classes for threaded rods
Medium 7e 8e
Coarse 8c 9c

Tolerances for Major and Minor Diameter: The tolerance position for the minor diameter D, and the major diameter D4 of the nut threads is always H, i.e. with zero fundamental deviation. The tolerance position for the major diameter d and minor diameter d3 of the threaded rod is in all cases h, i.e. with zero fundamental deviation, and it is independent of the tolerance position of the pitch diameter.

Tolerances Grades: The following tolerance grades are specified for the features in the ISO standard.

Feature Tolerance Grade
Minor diameter of nut threads [D1] 4    
Major diameter of rod threads [d] 4    
Pitch diameter of nut threads [D2] 7 8 9
Pitch diameter of rod threads [d2] 7 8 9
Minor diameter of rod threads [d3] 7 8 9

The tolerance grade for the minor diameter (d3) of the threaded rod is always the same as for the pitch diameter (d2).

Reference:
  • ISO 2901 : 1993, ISO Metric Trapezoidal Screw Threads - Basic Profile and Maximum Material Profile
  • ISO 2902 : 1977, ISO Metric Trapezoidal Screw Threads - General Plan
  • ISO 2903 : 1993, ISO Metric Trapezoidal Screw Threads - Tolerances
  • ISO 2904 : 1977, ISO Metric Trapezoidal Screw Threads - Basic dimensions