Tightening factor

Symbol αA
Unit -
Attribute-ID vdi_2230_tightening_factor
Numeric ID 12103
Value Type PMDouble

The tightening factor αA takes into account the scatter of the assembly preload between FMmin and FMmax. It is dependent on the tightening and adjusting techniques and if need be the friction coefficient class.

αA = FMmax/FMmin

The following options are available:

  1. User defined
    Direct specification of the number
  2. Hydraulic, frictionless, and torsion-free
    αA=1.3
  3. Yield-controlled, angle-controlled
    αA=1.4
  4. Torque-controlled
    αA=1.6
  5. Torque-controlled (friction coefficient class B)
    αA=2.0
  6. Torque-controlled (friction coefficient class A)
    αA=2.5
  7. Impact wrench or by hand
    αA=4.0
Tightening factor
α​A		 Scatter  Tightening method Setting method Comments 
1.1 to 1.2 ±​5% to ±​9%

T

ightening with elongation control or monitoring by ultrasound		 Sound travel time
  • Calibrating values required
  • When l K /d < 2 progressive increase in error to be taken into account
  • Smaller errors with direct mechanical coupling, larger with indirect coupling
1.1 to 1.3 ±​5% to ±​13% Mechanical elongation measurement via pressure screws located in the nut or bolt head Prespecified elongation of the bolt, setting via forcing torque of the pressure screws
  • Hardened washer for supporting the pressure screws
  • from approx. M24
1.2 to 1.5 ±​9% to ±​20% Mechanical elongation by means of multipartite nuts with threaded bushing Torque of the tightening tool
  • Largely torsion-free tightening
  • from approx. M30
1.1 to 1.5 ±​5% to ±​20% Tightening with mechanical elongation measurement or monitoring

Direct method: setting via elongation measurement

Indirect method: axial play at monitoring pin used up
  • Required: precise determination of the proportional axial elastic resiliences of the bolt
  • The scatter depends to a considerable extent on the accuracy of the measurement method
  • For low values, calibration is necessary
  • When lK /d < 2 progressive increase in error to be taken into account
1.1 to 1.4 ±​5% to ±​17% Hydraulic frictionless and torsion-free tightening Setting via pressure or length measurement or further rotation angle of the nut
  • When lK /d ≥ 5 lower values achievable, with mechanically machined bolts and plates α​A = 1.05 is possible
  • With standard bolts and nuts α​A ≥ 1.2
  • Smaller clamping length values result in higher α​A values.
  • Recovery losses occur which are not taken into account in the tightening factor.
  • Application from M20 upwards
1.2 to 2.0 ±​9% to ±​33% Impulse driver with hydraulic impulse generator, torque- and/or rotation-angle-controlled Setting via angle of rotation or further torque
  • Small values only in the case of presetting to the bolting case via rotation angle, compressed air servo valve and impulse counting
  • In special cases, even assembly up to the yield strength point is possible.
1.2 to 1.4 ±​9% to ±​17% Yield-point controlled tightening, motorized or manual Presetting of the relative torque-rotation angle coefficient The preload scatter is determined to a considerable extent by the scatter of the yield point in the installed bolt batch. Here the bolts are dimensioned for FMmin; a design of the bolts for FMmax with the tightening factor αA does not therefore apply with these tightening methods.
1.2 to 1.4 ±​9% to ±​17% Rotation-angle controlled tightening, motorized or manual Experimental determination of preliminary tightening moment and rotation angle (stages)
1.4 to 1.6 ±​17% to ±​23% Torque-controlled tightening with hydraulic tool Setting via pressure measurement From approx. M30
1.4 to 1.6 ±​17% to ±​23% Torque-controlled tightening with torque wrench, signalling wrench or motorized nut-runner with dynamic torque measurement Experimental determination of the setpoint torques at the original joint member, for example, by elongation measurement of the bolt Low values: large number of setting or monitoring attempts required (20, for example); low scatter of the output moment (for example, ±5 %) required Low values for:
  • Small rotation angles, in other words, relatively stiff joints
  • Relatively low hardness of the countersurface
  • Countersurfaces which do not have a “galling” tendency, for example, phosphated or adequately lubricated
High values for:
  • Large rotation angle, in other words, relatively resilient joints as well as fine thread
  • Great hardness of the countersurface, combined with a rough surface
1.6 to 2.0 (friction coefficient class B) ±​23% to ±​33% Torque-controlled tightening with torque wrench, signalling wrench or motorized nut-runner with dynamic torque measurement Determination of the setpoint tightening moment by estimating the coefficient of friction (surface and lubrication conditions are a great influence)

Low values for: measuring torque wrenches with even tightening and for precision nut runners

High values for: signalling or buckling torque wrenches
1.7 to 2.5 (friction coefficient class A) ±​26% to ±​43%
2.5 to 4.0 ±​43% to ±​60% Tightening with impact wrench, “stalling driver” or impulse driver; tightening by hand Setting the driver via retightening moment, which is formed from the required tightening moment (for the estimated coefficient of friction) plus a supplement; manual tightening based on subjective assessment Low values for:
  • Large number of setting attempts (retightening moment)
  • On horizontal branch of the driver characteristic
  • Backlash-free impulse transmission
Method only suitable for preliminary tightening, in the case of tightening by hand risk of overstretching with M10 and smaller

Table from: VDI 2230 Part 1, VDI Verlag Düsseldorf, 2015, p. 118, "Table A8. Guide Values for the Tightening Factor αA"