NOTE The Rs-series per BS 21 is an obsolete thread series and should not be used in new designs; however; you may find a requirement for Rs-series screw threads on an older product/part design drawing. Similarities The G-series and the Rs-series British Standard Screw Threads have much in common: a. Both are British Standard Pipe Parallel. b. Both have the Whitworth 55° included thread angle form with Full Form rounded root and crest of thread. c. In all situations the G-series can be used when the Rs-series is specified, but the reverse is not true. Differences
Comparison of Pitch Diameters I think a basic understanding of the two screw threads is embodied in a comparison of their effective pitch diameters. Above is a chart displaying the G1” effective pitch diameters compared with the Rp1” effective pitch diameters: Conclusion Notice how the G-series will always be within the specified parameters for the Rs-series, but the Rs-series may be smaller than the G-series. Based on this the G-series is always useable when a Rs-series is requested, but an Rs-series may not work when a G-series is requested.The reason for this is that both the G-series screw thread and the Rs-series screw thread have the same Basic Effective Pitch Diameter. The Internal G-series screw thread then has an amount of tolerance applied to the minus and none applied to the positive of the Basic Effective Pitch Diameter. The Rs-series has the same amount of tolerance added to the positive side of the Basic Effective Pitch Diameter AND an equal amount applied as a minus from the Basic Effective Pitch Diameter. A word on Gages We reccommend that you use gages which are defined in a specification. In this case: - BS-21 Rs external screw threads should be checked with BS-21 R-series Tapered Screw Thread Ring Gage. - ISO-228 G-series external screw threads should be checked with G-series Screw Thread Ring Gauges. Disclaimer This information given is my opinion from my study of the standards. I have no authority and my opinion thus has no value.
1 Comment
9/14/2022 07:24:15 am
I'm glad that you could understand how the function works with two screw threads. A couple of days ago, my friend mentioned that he and his business associate were hoping to find British thread gages that could provide easy thread carriage for more efficient production, and he asked if I had any thoughts on what would be the best option to consider. I love this enlightening article, I'll tell him that it will be much better if they consult a trusted British thread gages company as they can provide details.
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AuthorWith unceasing appetite for knowledge - Karthik Kumarasamy Archives
August 2015
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