642 Каталог GEDORE 2014 Профессиональный слесарный инструмент и оборудование Стр.640

Каталог GEDORE 2014 Слесарный инструмент Стр. 640 0642 ENG Lab2u

GEDORE Guideline values for the coefficient of friction p Screw thread Steel Nut thread blackened or Zinc-phosphate pressed rolled sharpened rolled cut cadmium-plated zinc-plated 6 n 6 n ground 014 010 016 010 010 sharpened 016 010 016 010 010 ground sharpened Zinc- phosphate Slightly oiled 014 010 & turned sharpened 010 turned sharpened 010 010 010 010 cadmium-plated 014 zinc-plated - 010 cadmium-plated 010 010 014 zinc-plated 010 010 014 Choosing the right friction value Guideline values for the coefficient of friction p In order to exactly define the pre-tension force and the tightening torque it is essential to know the coefficient friction. However it would seem almost impossible to specify definite values for the coefficients of friction for the large variety of possible surface and lubrication conditions and above all for their variance. Added to this are the variances of the various different tightening methods which also constitute a greater or lesser factor of uncertainty. For this reason it is only possible to make recommendations on the choise of the coefficient of friction. 80 % of the tightening-torque values apply for countersunk head screws on account of the remaining base thickness. In order o exactly define the pre-tension force and the tightening torque it is essential to know the coefficient of friction. However it would seem almost impossible to specify definite values for the coefficients of friction for the large variety of possible surface and lubrication conditions and above all for their variance. The following circumstances influence the friction value The surfaces and the nature of the materials being screwed the method of lubrication the sliding path due to the flexibility and the tightening method i.e. the number and the speed of the tightening cycles and finally the tightening path - the so-called hard or soft screw case. The sum total of these items represents a greater or lesser factor of uncertainty. Even DIN-equivalent screws con differ considerably in their friction value because of being delivered by different suppliers depending on the screw lot and depending on their storage and in particalar on the oiling or greasing performed in the course of installation. Please note that around 80 to 90% of the tightening torque in most tightening procedures is required for overcoming the friction in the screw. Important remark For this reason it is only possible to give recommendations on the choice of the friction value. We point out explicitly that the following tables only contain guideline values. In all cases a detailed screw calculation is more reliable than these tables That applies particularly for parts which are relevant to safety are subject to official regulations or perform sealing functions. The tables should only be utilised where the manufacturer of the screws or elements being connected has made no specifications on the required tightening torques. ii f hr i