Bolted Joint Preload Calibration Using Tightening and Removal Torque
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Solution Overview
Problem
Existing methods for achieving preload in bolted joints using tightening torque are uncertain, with uncertainties ranging from +/−35% for unlubricated fasteners and +/−25% for lubricated fasteners, due to variations in fasteners, interface conditions, tools, technique, and technician, requiring additional and expensive equipment for reduced uncertainty.
Innovation Solution
Determine preload using thread pitch and the difference between tightening and removal torques, eliminating the need for previous torque-tension data and reducing uncertainty by measuring these torques with common tools at the point of installation.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of operation
If tightening torque control is used to achieve preload, then the method is easy to implement, but the uncertainty in achieving target preload is significant (+/−35% for unlubricated fasteners, +/−25% for lubricated fasteners)
Solution Approach 1:
The patent applies feedback by measuring the actual preload after tightening and using this measurement to determine a correction factor. This correction factor is then applied to adjust the tightening torque for subsequent fasteners, creating a closed-loop system that continuously improves preload accuracy while maintaining ease of implementation through simple iterative adjustment
Solution Approach 2:
The patent changes the parameter being controlled from torque alone to a corrected torque value that incorporates actual preload measurements. By modifying the torque parameter based on measured preload and correction factors, the system achieves both ease of operation and improved preload accuracy
2Measurement precision
If torque-tension test data is used for specific fasteners, joints, and conditions, then preload prediction is improved, but uncertainty remains significant due to differences in actual and test fasteners including geometrical and frictional variations
Solution Approach 1:
The patent employs self-service by having the system determine its own correction factors through actual measurements on the specific fastener being installed. Rather than relying on pre-established test data that may not account for individual variations, each fastener essentially calibrates itself by providing measurement data that reflects its unique geometrical and frictional characteristics
Solution Approach 2:
The patent performs preliminary measurements and calculations to establish correction factors before final tightening. By measuring preload after initial tightening and calculating correction factors in advance, the system prepares adjusted torque values that account for specific fastener characteristics before committing to the final preload state
3Measurement precision
If additional equipment is used to reduce preload uncertainty, then preload accuracy is improved, but device complexity and cost increase
Solution Approach 1:
The patent achieves multi-functionality by using a single preload measurement device to both measure preload and generate correction factors for torque adjustment. This one tool performs multiple functions: measuring actual preload, calculating correction factors, and enabling adjusted torque application, thereby improving accuracy without requiring multiple specialized devices
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
Achieves preload with reduced uncertainty, eliminating the need for additional tools, training, and previous data, and accommodating variations in fastener conditions and installation techniques.
Implementation Method 1
The preload generated in the fastener elements is a function of the thread pitch, the tightening torque, and the removal torque
Implementation Method 2
The preload generated in the fastener elements is a function of the thread pitch, the tightening torque, and the removal torque
Data Source
AI summary
In one embodiment, a method for achieving a target joint preload includes identifying a thread pitch of first fastener element to be used to secure two components together, passing the first fastener element through the two components, threading a second fastener element onto the first fastener element, applying tightening torque to the second fastener element and measuring that torque as the trial tightening torque, applying a loosening torque to the second fastener element and measuring that torque as the trial removal torque, and calculating a required tightening torque that is required to be applied to the second fastener element to achieve the target joint preload using only the thread pitch, the trial tightening torque, the trial removal torque, and the target joint preload.