Screwdriving Technology and Quality Assurance
20Pages

J Screwdriving Technology Automation

This special edition is a way to convey our expertise in regards to theoretical basics and to simultaneously offer a practical application guide. Basics of the Screwdriving Technology Page 3 Tightening Process Page 6 Selection of a Suitable Screwdriving System / Application Advice Page 9 Process Reliability Page 9 Accuracy in the Screwdriving Technology Page 14 Measuring Principles Page 15 Calibration of Measuring Instruments Page 17 Standards / Guidelines / Literature Page 18 Screwdriving technology continues to be a key field of technology for many varied assembly tasks. Screw connections...

Basics of the Screwdriving Technology Tension display of a connection under pre-tension FV C Realisation of realignment and disassembly procedures In these cases the described processing dimensions are either a length measurement (screw-in or screw-out depth) or an angle measurement (number of rotations). These dimensions can be recorded directly via a suitable measurement system or indirectly over time. One example is the assembly of terminal connectors: The clamping screw is firstly tightened to a predefined torque and then unscrewed to a certain angle in order to enable easier connection...

Assembly tightening torque and assembly preload force Ma = Assembly tightening torque FM = Assembly preload force p = Thread pitch lG = Thread friction value lK = Head friction value d2 = Standard thread diameter Dkm = Effective diameter for the friction torque in the screw head connecting with the surface

Headless screws with metric fine thread

Tightening Process Torque-Controlled Tightening Within screwdriving technology, torque is still used most frequently as the control parameter. The preload force resulting from the tightening torque is significantly influenced by the fluctuating coefficient of friction as well as the screwdriving device’s torque spread. In particular one must differentiate between head friction and thread friction. The sum of these fluctuating frictional conditions results in fluctuations of the preload force of up to 50 % or more, despite high torque repeatability. This means that the screw connection...

Looking at the stress-strain diagram you can see that the increase is linear at first and levels out upon reaching the limit value. The axial force acts in proportion to the torque - the strain is proportional to the angle. Mathematically, the increase of a curve is defined as the reduction of the function. If the reduction of the torque falls to approximately 50 % of the benchmark, the limit value is reached and the tightening process is ended. It is possible to overlay this process with angle and torque limits for additional safety. This procedure reduces the drawbacks of fluctuating...

Tightening Process Ultrasonic Sensor Reflection Area Length measurement Length Measurement The mathematical context between the tension of the screw and the created preload is much more accurate than the connection between the torque and the preload. A direct tension measurement leads therefore to a much more accurate preload value. This can be accomplished for example by the mechanical measurement of a bore in the screw, which must be deeper than the clamping length of the used screw. However, this method is only suitable for special cases using large screws and it practically does not...

Selection of a Suitable Screwdriving System / Application Advice DEPRAG offers a comprehensive range of screwdriving tools for the most varied of applications. Tools for the assembly of sophisticated products with high requirements in process reliability are dealt with differently to e.g. an impact wrench which can be used in a broad assembly field. Electric Screwdriver with Mechanical Shut-off Clutch: Drive with brushless direct-current motor, shut-off via mechanical shut-off clutch Cordless Screwdriver with Mechanical Shut-off Clutch: Drive with brushless direct-current motor, shut-off...

Process Reliability However as already shown in the preload diagram – image D (under Basics of Screwdriving Technology page 3) there is a settling of the screw joint which in practically all cases reduces the remaining torque or preload force. Seating processes appear during screw assembly of soft materials e.g. screw assemblies of plastics or if sealing elements are placed between two part to be screw assembled. Assembly Pre-Load An example: A silicone seal is assembled between the pump housing and cover using four screws. Even if the tightening torque allows for one hundred percent...

Statistical Basis To retain an equal and permanent production quality, exact production means as well as suitable control-mechanisms are necessary. Through new and stricter regulations in the product liability area, the requirements in regards to quality documentation will continue to rise. Especially the car industry has taken on a forerunner role in this matter and has enforced its numerous suppliers to take similar measures. This documentation measure applies in particular to our screwdriving technology. Besides the need to display a selection of suitable measuring units (torque, angle,...

Statistics Capability Study It is necessary to assign a high importance to the capability study concept, because of the development of the quality-assurance procedures and preventive quality assurances. These statistical methods have a goal, to oversee the manufacturing process during production. An important item of such a study is the determination if the used production machine is even suitable for the production process. Such a study is termed a machine-capability or short-term capability. The used index is designated Cm and Cmk, and it confirms whether the required quality...

Machine Capability Study Example of an assembly task: Tightening torque = 5 Nm ±10 % Cmk >= 1.67 First it is assumed that the average of the measurement series exactly equals the required tightening torque i.e. to determine the max. allowable standard deviation we calculate with Cm = 1.67 instead of Cmk = 1.67. Cm = (upper limit value - lower limit value) / 6 s Cm = T / 6 s s = T / 6 Cm s = 1.0 Nm / 6 * 1.67 s = 0.1 Nm or in percent: s = 2 % Only tools whose standard deviation is less than or equal to 2 % are suitable for this assembly task. In practice the average is usually less than the...