By: Wayne Meulendyk
Buttress Screw Threads area special thread type which is used for unidirectional force screw thread applications. They are very good at applying pressure in one direction, but very bad at applying pressure in the opposite direction.
The Buttress screw thread design contains a steep leading flank angle, known as the pressure angle. The standard UN-series or M-series flank angle is 30°. The American and/or British Buttress screw thread has a pressure angle of 7°. The S-series (ISO) Buttress screw thread has a pressure angle of 3°. I think you can understand how this steep angle would be better at applying pressure in the one direction.
The Buttress screw thread design contains a shallow trailing flank angle, known as the relief angle or back angle. The standard UN-series or M-series screw threads are bi-directional, so their trailing flank angle is the same as the leading flank angle of 30°. The American and/or British Buttress screw thread has a relief angle of 45°. The S-series (ISO) Buttress screw thread has a pressure angle of 30°. I think you see can how this trailing flank angle is not good for applying pressure.
Unfortunately, there are many engineers which have made-up their own form of Buttress screw thread, ignoring published industrial specifications for unknown reasons. Sometimes they opt for a 0° pressure angle. The 0° pressure flank angle sounds like it would be the best, but unfortunately a 0° pressure angle incurs some unintended manufacturing difficulties.
Regardless of the details of your Buttress Screw Thread; Contact Gage Crib Worldwide Inc. so we can get the best inspection tools possible for your specific requirement.
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