Q: What is the formula for tightening torque?

Tightening torque is T = F·(d₂/2)·(μπd₂ + l·cosα)/(πd₂·cosα − μl), where d₂ is the pitch diameter, l is the lead, α is the half-flank angle (30° for metric threads), μ is the friction coefficient, and F is the axial force.

Q: How should I choose the friction coefficient μ?

For common steel fasteners: μ ≈ 0.15–0.20 without lubrication, μ ≈ 0.08–0.12 with lubrication, and μ ≈ 0.12–0.18 for zinc-plated surfaces. For high-accuracy torque control, use the torque method, angle method, or bolt elongation measurement.

Q: What is screw efficiency η?

Screw efficiency η = F·l / (2πT) represents the conversion efficiency from input torque to axial force. For self-locking screws η < 50%; the higher the efficiency, the more prone the screw is to loosening. Typical metric screws (μ = 0.1) have η ≈ 30–40%.

Question 1

What is the self-locking condition of a screw?

Accepted Answer

When the lead angle λ is smaller than the friction angle φ (φ = arctan(μ)), i.e. tan λ < μ, the screw will not loosen without an external force — this is the self-locking (self-holding) condition. For typical metric screws with μ ≈ 0.1, self-locking is usually achieved.

Question 2

What is the formula for tightening torque?

Accepted Answer

Tightening torque is T = F·(d₂/2)·(μπd₂ + l·cosα)/(πd₂·cosα − μl), where d₂ is the pitch diameter, l is the lead, α is the half-flank angle (30° for metric threads), μ is the friction coefficient, and F is the axial force.

Question 3

How should I choose the friction coefficient μ?

Accepted Answer

For common steel fasteners: μ ≈ 0.15–0.20 without lubrication, μ ≈ 0.08–0.12 with lubrication, and μ ≈ 0.12–0.18 for zinc-plated surfaces. For high-accuracy torque control, use the torque method, angle method, or bolt elongation measurement.

Question 4

What is screw efficiency η?

Accepted Answer

Screw efficiency η = F·l / (2πT) represents the conversion efficiency from input torque to axial force. For self-locking screws η < 50%; the higher the efficiency, the more prone the screw is to loosening. Typical metric screws (μ = 0.1) have η ≈ 30–40%.

Screw / Bolt Tightening Torque Calculator

Theory