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 technique Adjusting technique Remarks 
1.1 to 1.2 ±​5% to ±​9% Elongation-controlled tightening with ultrasound Echo time
  • Calibrating values necessary
  • Allow for progressive increase in errors at lK /d < 2
  • Smaller errors with direct mechanical coupling, larger with indirect coupling
1.1 to 1.3 ±​5% to ±​13% Mechanical elongation measurement via pressure screws arranged in the nut or bolt head Preset screw length, adjusted via setting the torque of the pressure screw
  • Hardened washer to support the pressure screws
  • from ca. M24
1.2 to 1.5 ±​9% to ±​20% Mechanical elongation measurement via multi-part nut with threaded bushing Torque of the tightening tool
  • Largely torsion-free tightening
  • from ca. M30
1.1 to 1.5 ±​5% to ±​20% Tightening with mechanical elongation measurement or control

Direct method: adjustment via longitudinal measurement

Indirect method: reduced axial clearance to control pin
  • Exact determination of the axial elastic resilience of the bolt is necessary
  • Scatter depends substantially on the accuracy of the measuring technique
  • Calibration required for low values
  • Allow for progressive increase in errors at lK /d < 2
1.1 to 1.4 ±​5% to ±​17% Hydraulic frictionless and torsion-free tightening Adjustment via pressure or elongation measurement or further rotation angle of the nut
  • Smaller values can be achieved with lK /d ≥ 5, α​A = 1.05 possible for mechanically machined bolts and plates
  • For standard bolts and nuts α​A ≥ 1.2
  • Low clamp length ratios lead to greater α​A values.
  • Spring-back losses occur, which are not considered in the tightening factor.
  • Applicable from M20
1.2 to 2.0 ±​9% to ±​33% Impact wrench with hydraulic air motor, torque and/or angle-controlled Adjustment via angle or further torque
  • Small values only with presetting the screw joint via angle, compressed air servo valve, and pulse counting
  • Assembly beyond the elastic limit in some cases
1.2 to 1.4 ±​9% to ±​17% Yield-controlled tightening, motor or manually operated Input of the relative torque/rotation-angle coefficient Scatter in preload substantially determined by the scatter in the bolt yield point. Here, the bolts are dimensioned for FMmin; design of the bolts for FMmax with the tightening factor αA therefore does not apply to these tightening techniques.
1.2 to 1.4 ±​9% to ±​17% Angle-controlled tightening, motor or manually operated Experimental determination of pre-tightening torque and angle of rotation (steps)
1.4 to 1.6 ±​17% to ±​23% Torque-controlled tightening with hydraulic tool Adjustment via pressure measurement From ca. M30
1.4 to 1.6 ±​17% to ±​23% Torque-controlled tightening with torque wrench, indicating wrench, or precision tightening spindle with dynamic torque measurement Experimental determination of required tightening torques on the original bolting part, e.g. by measuring bolt elongation Lower values: large number of calibration or check tests (e.g. 20) required; low scatter of the transmitted torque (e.g. ±​5 %) necessary Lower values for:
  • Small angles of rotation, i.e. relatively stiff joints
  • Relatively soft mating surface
  • Mating surfaces which are not inclined to "seize,“ e.g. phosphated or with sufficient lubrication
Higher values for:
  • Large angles of rotation, i.e. relatively resilient joints and fine threads
  • High mating surface hardness combined with a rough surface
1.6 to 2.0 (friction coefficient class B) ±​23% to ±​33% Torque-controlled tightening with torque wrench, indicating wrench, or precision tightening spindle with dynamic torque measurement Determination of the required tightening torque by estimating the friction coefficient (surface and lubrication conditions have a significant influence)

Lower values for: measuring torque wrenches with steady tightening and for precision tightening spindles

Higher values for: signaling or automatic tripping torque wrenches
1.7 to 2.5 (friction coefficient class A) ±​26% to ±​43%
2.5 to 4.0 ±​43% to ±​60% Tightening with an impact wrench or impact wrench with momentum control; tightening by hand Calibration of the bolt by means of re-tightening torque, made up of the required tightening torque (for the estimated friction coefficient) and an additional factor; hand-tightening according to subjective measurement Lower values for:
  • Large number of calibration tests (re-tightening torque)
  • On the horizontal segment of the bolt characteristic
  • Moment transfer free from play
Technique only suitable for pre-tightening; when tightening by hand, risk of over overstretching with M10 and lower

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