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Metric Knuckle Screw Threads (Rd) per DIN 20400

DIN 20400 Metric Knuckle Screw Threads (Rd)

The Rd-series is a bit confusing because the Rd designation is used by both DIN 405 and DIN 20400, but the screw threads are not identical in size or screw thread form. It appears that DIN 405 is the older standard, and was being used in underground mining operations but was the cause of mechanical failures in hoists (that could hurt). In 1951, to solve the mechanical failures, a new screw thread design: DIN 20400 was developed. Knowing this history, and if you are in the design process of a new Rd screw thread application, I would suggest that you lean toward the DIN 20400 screw thread if at all possible.

Quick summary of the differences between DIN 405 and DIN 20400:
- DIN 405 uses metric nominal major diameter with threads-per-inch as the pitch designation. DIN 20400 thread uses metric nominal diameter with metric pitch. This is the easiest way to differentiate between the two standards.
- DIN 405 has 63 designated screw thread sizes (8mm to 200mm) split between the favored Series 1 (40 sizes) and discouraged Series 2 (23 sizes). DIN 20400 has 47 designated screw thread sizes (10mm to 300mm).
- DIN 405 has a limited range of pitch from 2.54mm (10-TPI) to 6.35mm (4-TPI). DIN 20400 increases the pitch as the nominal diameter increases using a range of pitch from 3mm to 16mm.
- DIN 405 and DIN 20400 both employ a screw thread flank angle of 30°.
- DIN 405 screw thread root and crest have a large full radius. DIN 20400 screw thread root has a large full radius, but the screw thread root has two smaller radii which leaves a flat at the bottom of the screw thread root.

Class-of-Fit
From the copy of DIN 20400 that I purchased from Germany, it appears that DIN 20400 does not offer variations in class of fit. Instead all screw thread sizes are designated by chart in the standard. While this may relate to some class-of-fit paradigm, it is not stated in the standard. If you have documented data on this topic, I would be glad to communicate with you.

Parts of DIN 20400
While it typical for DIN standards to consist of multiple parts, DIN 20400 was not made available to me in that form. It appears to have only one part. If you have documented data on this topic, I would be glad to communicate with you.

Disclaimer
This data is provided as a brief overview of the standard and for general information only. The intention is to provide accurate information; regardless; errors may exist in the supplied information. The actual standard contains much additional information. If accuracy is critical, base your final decisions on the data provided in the root document: DIN 20400 (this data was taken from DIN 20400:1990); which is a copyrighted document. To purchase a copy visit an Authorized Reseller.

Original Posting: 3/23/2011
Last Revision:
Error corrections in, or comments about, the above data can be sent to: gageguy@gagecrib.com

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