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General Purpose Knuckle Screw Threads (Rd) per DIN 405

DIN 405 General Purpose 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
The tolerance paradigm for DIN 405 is somewhat complicated in that it gives class-of-fit several options for various diameters and is related to the length of engagement for the product threads. This structure allows for engineering flexibility; however; according to DIN 405-2 paragraph 4.1: … Where the tolerance class is not identified, it shall be taken to be 7H6H for internal threads and 7h6h for external threads. While the official standard allows for the DIN 405 Rd-series class-of-fit to be left off of engineering drawings; IMHO; it is a very simple task to include it in all references to the thread. By not reporting the class-of-fit on the drawing questions can arise which cost time and delay in manufacturing; which reduces profits. For the cost of the small amount of time and ink used to report the full thread requirements, many man-hours may be saved by an uncounted number of people over time.

Parts of DIN 405
DIN 405-2 General-purpose knuckle threads Part 1: Profiles and nominal sizes
DIN 405-2 General-purpose knuckle threads Part 2: Tolerances
DIN 405-2 General-purpose knuckle threads Part 3: Gauges for external and internal threads – Types, profiles and tolerances

Disclaimer:
This data is provided for general information only. The intention is to provide accurate information; regardless; errors may exist in the supplied information. If accuracy is critical, base your final decisions on the data provided in the root document: DIN 405; which is a copyrighted document. To purchase copies visit an Authorized Reseller.

Comments:
Original Posting: 3/23/2011
Last Revision: 4/23/2013
Error corrections in, or comments about, the above data can be sent to: gageguy@gagecrib.com

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