| $ $ | bolt | $\mathrm{ }$ |
| $A_b$ | cross‐sectional area of the bolts | $\mathrm{mm^2}$ |
| $C_1$ | coefficient $ C_1 $ | $\mathrm{}$ |
| $C_2$ | coefficient $ C_2 $ | $\mathrm{}$ |
| $C_3$ | coefficient $ C_3 $ | $\mathrm{}$ |
| $C_4$ | coefficient $ C_4 $ | $\mathrm{}$ |
| $C_c$ | coefficient according to load | $\mathrm{ }$ |
| $D$ | diameter of the shaft | $\mathrm{mm}$ |
| $D$ | basic major diameter of internal thread (nominal diameter) | $\mathrm{mm}$ |
| $D$ | diameter, larger diameter | $\mathrm{mm}$ |
| $D_1$ | basic minor diameter of internal thread | $\mathrm{mm}$ |
| $D_2$ | basic pitch diameter of internal thread | $\mathrm{mm}$ |
| $H$ | height of fundamental triangle | $\mathrm{mm}$ |
| $H$ | width, larger width | $\mathrm{mm}$ |
| $H$ | geopotential altitude | $\mathrm{m}$ |
| $H_b$ | lower limit geopotential altitude | $\mathrm{m}$ |
| $H_p$ | pressure scale height | $\mathrm{m}$ |
| $K_t$ | stress concentration factor | $\mathrm{ }$ |
| $K_{tg}$ | stress concentration factor with the nominal stress based on gross area | $\mathrm{ }$ |
| $K_{tn}$ | stress concentration factor with the nominal stress based on net area | $\mathrm{ }$ |
| $K_{ts}$ | stress concentration factor for shear stress | $\mathrm{ }$ |
| $M$ | air molar mass at sea level | $\mathrm{kg\cdot kmol^{-1}}$ |
| $M$ | moment | $\mathrm{Nm}$ |
| $N_A$ | Avogadro constant | $\mathrm{kmol^{-1}}$ |
| $P$ | pitch | $\mathrm{mm}$ |
| $P$ | load, force | $\mathrm{N}$ |
| $P_{all-B}$ | allowable bearing stress | $\mathrm{MPa}$ |
| $R$ | specific gas constant | $\mathrm{J\cdot K^{-1}\cdot kg^{-1}}$ |
| $R^*$ | universal gas constant | $\mathrm{J\cdot K^{-1}\cdot kmol^{-1}}$ |
| $S$ | Sutherland's empirical coefficients $ S $ | $\mathrm{K}$ |
| $S_F$ | safety factor | $\mathrm{ }$ |
| $S_{y}$ | the minimum yield strength | $\mathrm{MPa}$ |
| $T$ | temperature $ T $ | $\mathrm{K}$ |
| $T_b$ | lower limit temperature | $\mathrm{K}$ |
| $a$ | speed of Sound | $\mathrm{m/s}$ |
| $b$ | width key | $\mathrm{mm}$ |
| $d$ | basic major diameter of external thread (nominal diameter) | $\mathrm{mm}$ |
| $d$ | diameter | $\mathrm{mm}$ |
| $d$ | width | $\mathrm{mm}$ |
| $d_1$ | basic minor diameter of external thread | $\mathrm{mm}$ |
| $d_2$ | basic pitch diameter of external thread | $\mathrm{mm}$ |
| $d_3$ | basic smaller diameter of external thread | $\mathrm{mm}$ |
| $g$ | gravitational acceleration | $\mathrm{m/s^2}$ |
| $h$ | height key | $\mathrm{mm}$ |
| $h$ | thickness | $\mathrm{mm}$ |
| $h$ | height above sea level | $\mathrm{m}$ |
| $l$ | mean free path of air particles | $\mathrm{m}$ |
| $l$ | length key | $\mathrm{mm}$ |
| $l_t$ | tolerance of key length | $\mathrm{mm}$ |
| $n$ | air number density | $\mathrm{m^{-3}}$ |
| $p_b$ | lower limit pressure | $\mathrm{Pa}$ |
| $p_{air}$ | atmospheric pressure air | $\mathrm{Pa}$ |
| $r$ | nominal earth's radius | $\mathrm{m}$ |
| $r$ | radius of curvature of hole, arc, notch | $\mathrm{mm}$ |
| $r_1$ | max. rounding the key | $\mathrm{mm}$ |
| $r_2$ | groove rounding | $\mathrm{mm}$ |
| $t$ | depth in the shaft | $\mathrm{mm}$ |
| $t$ | depth of groove, notch | $\mathrm{mm}$ |
| $t_1$ | depth in the hub | $\mathrm{mm}$ |
| $t_t$ | tolerance of the groove height in the shaft | $\mathrm{mm}$ |
| $t_{1t}$ | tolerance of the groove height in the hub | $\mathrm{mm}$ |
| $v̄$ | mean air-particle speed | $\mathrm{m/s}$ |
| $β$ | temperature gradient $ β $ | $\mathrm{K\cdot m^{-1}}$ |
| $β_s$ | Sutherland's empirical coefficients $ β_s $ | $\mathrm{kg\cdot m^{-1}\cdot s^{-1}\cdot K^{-1/2}}$ |
| $γ_{air}$ | specific weight air | $\mathrm{kg\cdot m^{-2}\cdot s^{-2}}$ |
| $κ$ | adiabatic index | $\mathrm{ }$ |
| $λ$ | thermal conductivity | $\mathrm{W\cdot m^{-1}\cdot K^{-1}}$ |
| $μ$ | dynamic viscosity | $\mathrm{kg\cdot m^{-1}\cdot s^{-1}}$ |
| $ν$ | kinematic viscosity | $\mathrm{m^2\cdot s^{-1}}$ |
| $ρ_{air}$ | density air | $\mathrm{kg/m^3}$ |
| $σ$ | effective collision diameter of an air molecule | $\mathrm{m}$ |
| $σ$ | normal stress | $\mathrm{MPa}$ |
| $σ_{all-A}$ | allowable axial stress | $\mathrm{MPa}$ |
| $σ_{all-B}$ | allowable bending stress | $\mathrm{MPa}$ |
| $σ_{all-C}$ | allowable combined stress | $\mathrm{MPa}$ |
| $σ_{max}$ | maximum normal stress | $\mathrm{MPa}$ |
| $σ_{nom}$ | nominal or reference normal stress | $\mathrm{MPa}$ |
| $τ_{all-S}$ | allowable shear stress | $\mathrm{MPa}$ |
| $τ_{max}$ | maximum shear stress | $\mathrm{MPa}$ |
| $τ_{nom}$ | nominal or reference shear stress | $\mathrm{MPa}$ |
| $φ$ | latitude | $\mathrm{°}$ |
| $ω$ | air-particle collision frequency | $\mathrm{Hz}$ |
