Specifying Torque Values for Fasteners

Specifying Torque Values for Fasteners

Specifying Torque Values for Fasteners is required for…..

• Reliability of the joint is dependent upon the bolt’s ability to clamp the parts together
• Preload generated by a bolt can be indirectly controlled by regulating the applied torque
• Typically, only 10% to 15% of the torque is used to stretch the bolt.
• Of the remaining torque, typically 30% is dissipated in the threads and 50% to 55% under the nut face.
• Variations in friction have a significant influence on the bolt’s preload.
• Different bolt surface finishes have different friction values.
• Torque required for larger bearing face of standard hexagon bolt is higher in comparison to that for a socket headed screw because more torque is being dissipated between the nut face and the joint surface.
• Stresses induced into a bolt.
• When a bolt is tightened the shank and thread sustain a direct (tensile) stress due to it being stretched.
• To effectively utilize the strength of the bolt, an equivalent stress of 90% of the yield stress is commonly used.
• Caution should be exercised when using theoretical values as  preload and torque is dependent upon friction values

Calculation Procedure:

• Formulae applicable to metric and uinified thread forms which have a thread flank angle of 60°.
• The calculation procedure distinguishes between thread and under-head friction as well as differences which can be caused by bearing face diameter variations.
• Fastener Details – Refer Table in the folder – Dimensions and strength grades are specified in various standards
• Table 1- Mechanical properties of fasteners; the most common grade for metric fasteners is grade 8.8. Estimating the appropriate friction coefficient can be problematical.

• Tables 2 – Thread coefficient of friction µT and 3 Under-head coefficient of friction µT may be used as a guide when other information is not available.
• Tables 4 ISO Metric coarse threads and 5 ISO Metric fine threads provide relevant information relating to thread dimensions.

Terms used in calculation   

T = Tightening torque to be applied to the fastener.

F = The preload (or clamp force) in the fastener.

σE = Equivalent stress (combined tensile and torsional stress) in the bolt thread. A figure of 90% of the yield or proof stress of the fastener is usual.

σT = Tensile stress in the fastener.

d2 = Pitch diameter of the thread.

d3 = Minor (or root) diameter of the thread.

P = Pitch of the thread.

μT = Thread friction coefficient.

μH = Under-head coefficient of friction.

Df = The effective friction diameter of the bolt head or nut.

Do = Outside diameter of the nut bearing surface.

Di = Inside diameter of the nut bearing surface.

Establish the preload         

▪ The preload F is related to the direct tensile stress σT by: F = AS × σT

▪ The stress area of the thread AS represents the effective section of the thread. It is based upon the mean of the thread pitch and minor diameters. It can be obtained from tables or calculated using the formula:

Tensile stress in the fastener       

▪ Establish what proportion of the fasteners yield strength you wish to use. Normally a figure of 90% of the yield strength is acceptable but may be varied to suit the application.

▪ The following formula can be used to determine the tensile stress in the fastener.

Tightening torque T           

▪ As can be seen from tables 2 and 3, upper and lower limits to friction values are stated. Traditionally a mean value of friction is used when calculating the tightening torque and preload value.

▪ Be aware however, that for other conditions remaining constant, the higher the value of friction – higher is the required tightening torque and lower is the resulting preload.

▪ Determine the tightening torque. The relationship between tightening torque T and bolt preload F is:

Effective friction diameter

▪ If units of Newtons and millimeters are being used, T will be in N.mm. To convert to N.m, divide the value by 1000.The effective friction diameter Df can be determined using the following formula:

Example       

▪ Preload and tightening torque for a grade 8.8 M16 hexagon headed bolt.

▪ Step 1.

▪ Establishing the dimensions and friction conditions.

▪ d2 = 14.701mm, d3 = 13.546mm, P = 2mm

▪ μT Taken as 0.11 μH Taken as 0.16

▪ Taking the stress area as 157mm2, gives the bolt preload F to be 77087N.

Calculating the tensile stress in the fastener

▪ Using 90% of 640N/mm2 gives σE=576N/mm2, substituting values into the formula gives σT = 491N/mm2

Determination of the tightening torque T

▪ The effective friction diameter.

▪ Taking Do = 24mm and Di = 17.27mm gives Df = 20.6mm.

▪ Using the values calculated gives a tightening torque T of 350437Nmm, that is 350Nm.