In screw thread terminology, allowance refers to the intentional difference or clearance between the maximum material condition of the internal thread (like a nut) and the external thread (like a bolt). It is a small, predetermined amount of space designed to ensure that the threads can be assembled easily without excessive tightness, even when they are manufactured to their maximum material limits.
Allowance is particularly important in threaded assemblies where parts are subject to variations in production, wear, or environmental conditions, as it helps prevent issues such as binding or seizing during assembly. By incorporating an allowance, manufacturers can ensure that parts fit together smoothly while still maintaining the strength and integrity of the connection.