High Strength Friction Grip Assemblies (HSFG)
High Strength Friction Grip bolting is specified in BS 4395:1969: Parts 1 & 2. Part 1 specifies general grade, (similar to grade 8.8) and Part 2 specifies higher grade, (similar to grade 10.9). The standard specifies bolts, nuts and through hardened washers. The use of HSFG is specified in BS 4604:1970: Parts 1 & 2.
HSFG bolting provides a simple method of transmitting loads through a structural joint by frictional resistance on the contact face of the mating plies. Friction is developed by pre-loading the bolt in considerable tension, which clamps the plies together. The bolts are fitted in clearance holes and, since the joint is designed to resist slip, they are not subject to bearing or shear forces and therefore remain in virtually static tension throughout their working life with fluctuating external loads having very little effect, if any, on bolt stress.
The main benefits of this form of bolting are the elimination of slip, resistance to vibration and fatigue and resistance to slip when load reversal occurs in moving structures such as cranes.
In order to ensure that the required bolt loads are achieved three bolt tightening procedures are specified in BS specifications. Two are defined in BS 4604, the Part-Turn Method and the Torque Tightening Method, the third option; the use of a Direct Tension Indicator (Load Indicating Washer), is specified in BS 7644: Parts 1 & 2: 1993.
During 2007 the defining specification for HSFG, BS 4395, is being replaced with a series of new European Standards under the general heading BS EN 14399. Initially six parts will be published referring to the individual assembly components i.e. Bolts, Nuts, Washers and methods of testing etc. During the next 2 to 3 years further issues will cover Countersunk Bolts and Direct Tension Indicators.
McLean Buchanan & Wilson have many years experience in advising on procedures for the tightening of these fasteners; please contact us to discuss a tailor-made seminar to meet your company’s specific requirements.
