LIT-AT100 AIMCO Master Catalog rev 09-14-21 low res - Catalog - Page 79
Torque Measurement: Overview
MEASURING QUALITY — PROCESS CAPABILITY
Statistical Analysis Is An Important Step In Any Quality Control Process
One of the most widely accepted statistical indicators of process quality, and therefore product quality, is Cpk, or the
process capability for a centered process. This value indicates how capable a process is and whether the results of
that process are properly centered near a specific target. A capable process is one that approaches, as a limit, 100%
conformance to specifications.
Cpk is a statistical value that indicates how tightly grouped a series of samples is around the target value. Cpk is a
function of the Upper Specification Limit (USL), the Lower Specification Limit (LSL), the mean of the samples and the
standard deviation (σ) of the samples.
Cpk = either
(USL - Mean)
(3 x σ)
(Mean - LSL), whichever is smaller.
(3 x σ)
An acceptable, or capable, process will normally have a Cpk value of at least 1.33.
MEASURING QUALITY — CLAMP LOAD
The purpose of threaded fastening is to produce the correct amount of clamp load within the joint.
Due to the cost and difficulty of measuring clamp load during the actual
assembly process, torque is used as the control parameter during tightening.
Many manufacturers use torque values as the primary indicator of threaded
assembly quality. However, achieving repeatable clamp load is a better
indicator of joint quality.
Studies have shown that discontinuous drive tools can produce equal or
better clamp load results when compared to continuous drive tools.
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