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Tightening Technique
During tightening, a torque is applied on the nut or the screw to achieve the desired clamp force that holds the clamped component together. To proper control this process, a clear understanding between the torque and turn relationship is required.
The first zone is called rundown and refers to the time passed until the fastener head or nut contacts the bearing surface. The second zone is called the alignment or snugging zone and occurs while the fastener and joint mating surface are closing together to achieve a ¨snug¨ condition. The third zone is called the elastic clamp range, where the torque-angle curve is constant. The fourth zone is called post-yield zone and begins at the end of the elastic zone.
Moreover, industrialized power tools have been developed and are being used to control the tightening characteristics more effectively. The capability of these tools to apply torque to the fasteners in a controlled fashion made them necessary while developing new friction testing machine for fasteners. Currently industry practices involve using air motors and DC electric nutrunners for tightening applications. The tightening process and the control factor of the tightening influence the quality of the bolted joint. For example, a joint tightened by a hand behaves differently than on tightened by using a power tool and a joint that is tightened having the angle as control factor, behaves differently from one having the torque. The most common ways to control and monitor the nutrunners are presented below.
Torque-controlled Tightening
Due to the cost-effective tools and easy handling, this method is widely used in most of the industrial applications. Both manual wrenches and advanced nutrunner can be used. With nutrunners the torque is usually measured and controlled through a torque transducer that is either built-in the system or attached on the head. This technique is more accurate for low to medium speed tightening scenarios using lightly lubricated bolts within the elastic region of the bolts elongation.
Angle-controlled tightening
This process requires first to tighten the threaded fastener with a relative low torque enough to ensure full contact between the surface and then to rotate it to a preset number of degrees. In that way a more accurate clamp force can be achieved. In addition, it achieves even higher clamp force by deforming plastically and in a more reliable way.
Elongation-controlled Tightening
When it is important to know the exact clamp force in a bolted joint, the elongation-controlled method is used. The bolt head is firstly pulled with a force equal to the desired clamp force and then the bolt is being slowly turned until the head of the bolt touches the bearing surface. This technique is extremely time consuming, and is used high-performance products (nuclear reactors and wind turbines).
Joint Types
Screw joints vary not only in size but also in type, which changes the characteristics of the joints. The most important factor influencing the quality of the joint is the “stiffness”. During the tightening, when all the parts are in contact and tight to each other (snug point), the hardness can be measured by measuring the torque rate. The torque rate is the tightening angle necessary to achieve the recommended torque of the screw dimension and quality in question. [1]
For the same screw diameter the torque rate can vary considerably depending on the length. In a plain metal joint, a short screw reaches the rated torque in only a fraction of a turn of the screw. This type of joint is defined as a hard joint. A joint with a long screw that has to compress soft components such as gaskets or spring washers requires a much wider angle, possibly even several turns of the screw or nut to reach the rated torque. This type of joint is described as a soft joint. [1] Obviously the joint hardness varies in respect to the material and characteristics of the parts contributing. For this research the hard joint is defined a tightening that the screw turns 0-30 degrees after reaching the snug point. When it turns more than 30 up to 120 degrees the joint is considered medium-soft.
Methods of Force and Torque Measurements
Another factor that describes the quality of the joint is the force that keeps the jointed parts together. [1] In this friction test rig, clamp-force is the control parameter while tightening. The system configuration is modified in a way that the output force signal is used as an input control parameter in the nutrunner. Using this configuration increases the reliability of the experiments and allows the user to have accurate force measurements taken from the appropriate measuring instrument.
These instruments are called load cells or force transducers. They are devices that receive a physical signal and change it into a measurable physical quantity through a known relationship. The signal is received using operating principles such as piezo-resistive, piezo-electric, strain gauge, capacitive, non-contact-displacement and magnetoelastic sensors.
In this application the force is applied gradually through the tightening procedure without any build-up of energy. The bolt is forced to elongate, and the desired compression of the joint is achieved. It is considered as a static load that goes between the nut and the bolt since there is no rotation of these parts. There is an angular micro-movement, but there is no dynamic loading or severe impact loading.
The type of devices that are applicable for measuring the clamp force of threaded fasteners and the torque are listed below. In modern sensors the force is usually measured by the strain produced in an elastic member when loaded, by balancing the input force with an electromagnetically developed equal one or by converting it to fluid pressure.
As far as the torque measurements is concerned, torque is the tendency of the turning force to twist an object about an axis. There are two kinds of torque measurements, static torque and dynamic torque. The static torque measurements can be either a rotary torque which is subjected to continuous rotation and are connected in line with the rotating part, or a reaction /static torque which involves little (less than 1 revolution) or no rotation of the object being measured. In this project, the static torque measurement systems are used and analyzed.
Strain Gauge Load Cells
Strain gauge load cells are the most common type of force transducers. They are using bending elements and strain gauges to achieve accurate results. Strain gauges are electrical sensors whose resistance is a function of applied strain. When a force is applied in the system, the bending element bends or compresses. The strain gauge is connected to the bending element, gets excited by this displacement and converts it into a change in electrical resistance that can be measured. Usually, Wheatstone bridges are used to eliminate unknown bias with a beam assembly. [2] The cells are elastic elements where several strain gauges are bonded. The geometrical characteristics of this element determines the load capacity of the system. The total amount of force applied is measured by combining all the local strains from each strain gauge. The shape of the elements also determines the dimensional limits, the performance and the cost. The material used is usually steel or aluminum and a heat treatment is performed to achieve high level of repeatability.
Table of contents :
NOMENCLATURE
1 INTRODUCTION
1.1 Background
1.2 Purpose
1.3 Delimitations & Requirements
1.4 Methodology
2 FRAME OF REFERENCE
2.1 Tightening Technique
2.1.1 Torque-controlled Tightening
2.1.2 Angle-controlled tightening
2.1.3 Elongation-controlled Tightening
2.2 Joint Types
2.3 Friction Influence
2.3.1 Under-head Friction Torque
2.3.2 Thread Friction Torque
2.4 Methods of Force and Torque Measurements
2.4.1 Strain Gauge Load Cells
2.4.2 Strain Gauge Torque Cell
2.4.3 Piezoelectric Crystal Force Cells
2.4.4 Piezoelectric Crystal Torque Cells
2.4.5 Hydraulic Load Cell
2.4.6 Pneumatic Load Cell
2.4.7 Magnetoelastic Force Sensor
2.5 Combined Load & Torque Sensors
2.6 Crosstalk
2.7 Bearing Selection
2.7.1 Radial Bearings
2.7.2 Thrust Bearings
2.7.3 Bearing Materials
2.7.4 Bearing Friction
2.7.5 SKF model of bearing friction:
2.8 Tolerances
2.9 Existing Technology
2.9.1 TesT Friction Testing Machine
2.9.2 PCB Piezotronics Testing Machine
2.9.3 BLM Testing Machine
3 CONCEPT DEVELOPMENT
3.1 Nut-adaptor
3.1.1 Conceptual Design
3.1.2 Concept Evaluation
3.1.3 Detailed Design
3.2 Main Device
3.2.1 Conceptual Design
3.2.2 Concept Evaluation
3.2.3 Detailed Design
3.2.4 Working Principle
3.3 Stiffness Variation Device
3.3.1 Conceptual Design
3.3.2 Concept Evaluation
3.3.3 Detailed Design
3.4 Stiffness of the Clamp Joint
3.5 Prototyping
4 EXPERIMENTS & DESIGN VALIDATION
4.1 Validation of the design
4.2 Experimental validation
5 CONCLUSION
6 RESULTS & DISCUSSION
7 RECOMMENDATIONS AND FUTURE WORK
7.1 Accuracy & Tightening Speed
7.2 Stiffer Design
7.3 Thrust bearing
7.4 New torque cell and force cell
7.5 Under-head torque measurement
7.6 The Stiffness Variation Device
REFERENCES
