258 Каталог GEDORE 2014 Профессиональный слесарный инструмент и оборудование Стр.256

Каталог GEDORE 2014 Слесарный инструмент Стр. 256 0258 ENG Lab2u

GEDORE Torques and forces There are different methods to tighten a screw connection. The mechanic works manually and intuitively when using open-ended spanners or ring spanners. The mechanic evaluates if the screw connection fits securely i.e. tightly according to the resistance at the spanner. Seems logical however this process is not reliable. Only modern processes can be taken into account when tightening important screw connections with guaranteed pre-loads as e.g. tightening with torque wrenches (with or without pivoting angle) motor-driven tightening processes tightening with ultrasound (to determine pre-load) or tightening with yield stress determination. Experience has shown that the use of torque tools is sensible. Torque wrenches are a must where controlled screw tightening is required. The following explanatory remarks and terminology explanations are intended for giving you a rough insight into the world of controlled screw tightening. What is tightening torque The tightening torque is the force specified in Newton metres (N-m) which is generated at the end of a lever and creates a corresponding pre-tension force on a screw connection. That might be e.g. the square drive of a torque wrench or as shown in the diagramm the open end of a spanner. The screw-tightening torque consists of the thread tightening torque and the seat frictional torque (screw head or nut seat). In the process the seat's frictional torque does not contribute to an increase in the pre-tension force. a Working principle The diagramm shows how two metal plates are joined together (pressed together) in a plug-type screw connection by the tightening of one nut. The angle of pitch of the thread is responsible for the resulting tensile force produced in the screw. A wind-up force is caused by a tensile force. This pre-tension force is decisive for the optimum screw connection. Why An optimally tightened screw connection develops sufficient resistance against being loosened. If the pre-tension force is too weak the screw connection might vibrate or loosen. If the pre-tension is too great the danger exists that the screw connection might fracture. You can achieve the optimum pretension force with the correct tightening torque. Every screw connection has a certain tightening torque for various fastening requirements. Only if these values are taken into consideration it is possible to tighten a screw connection to a certain pre-tension force in a manner which is safe works properly and is cost-effective. Thread with friction Tensile strength of screw Joint between screw components 256