What Is Breakaway Torque?
Try turning a rusty bolt with a wrench. Nothing happens at first, then suddenly it cracks loose and turns easily. That first stubborn moment is breakaway torque — the minimum twisting force needed to start a screw rotating from rest. It exists because static friction between the mating threads is stronger than kinetic friction, the friction you fight once everything is already sliding.
Every threaded assembly has one: lead screws in machine tools, ball screws in 3D printers, valve stems, jack screws, and everyday bolts. Knowing the breakaway value tells you whether a hand ratchet will do the job or whether you need a breaker bar, and it tells designers how much motor torque a linear actuator needs just to get going.
The Breakaway Torque Formula
The idea comes straight from energy conservation. Turn a screw one full revolution and it advances by one lead (the distance traveled per turn), so the applied force does work (F \times L). The same turn sweeps through (2\pi) radians, so the torque does work (T \times 2\pi). Setting them equal:
[
F \times L = T \times 2\pi
]
Solve for torque:
[
T = \frac{F \times L}{2\pi}
]
Where:
- T is the breakaway torque.
- F is the breakaway force — the axial force needed to overcome static friction.
- L is the screw lead — the linear distance advanced per complete revolution.
- 2π converts one revolution of rotation into radians.
Because force times length appears on both sides, the units simply combine: pounds-force with inches gives lbf-in, Newtons with millimeters gives N·mm.
Worked Example
A machine's lead screw needs a measured pull of 400 lbf before its nut will budge, and the screw has a lead of 3 inches:
[
T = \frac{400 \text{ lbf} \times 3 \text{ in}}{2\pi} = \frac{1200}{6.2832} \approx 190.99 \text{ lbf-in}
]
So the ideal breakaway torque is about 190.99 lbf-in, or roughly 15.92 lbf-ft — more than most small ratchets deliver comfortably, so you would reach for a breaker bar or a half-inch-drive wrench.
Check it in metric: 400 lbf is 1779.3 N and 3 inches is 76.2 mm.
[
T = \frac{1779.3 \text{ N} \times 76.2 \text{ mm}}{6.2832} \approx 21{,}591 \text{ N·mm} \approx 21.59 \text{ N·m}
]
Both routes land on the same physical answer, since 190.99 lbf-in × 0.11298 N·m per lbf-in ≈ 21.59 N·m. Try those numbers in the calculator above to confirm them yourself.
What the Result Actually Means
The formula gives the ideal, frictionless minimum — a useful floor, not a guarantee. Real breakaway torque depends on conditions:
| Condition | Effect on real breakaway torque |
|---|---|
| Clean, dry threads | Roughly 2–3× the ideal value once thread friction is included |
| Lubricated threads | 30–50% lower than the same dry joint |
| Medium-strength threadlocker | Adds roughly 50–100% above the installation torque |
| Corroded outdoor steel bolts | Can reach 200–400% of the original value |
| Stainless galling | May seize completely — threads cold-weld together |
Two rules of thumb follow. First, size your tool for at least 1.25× the expected breakaway torque so you are never working at its limit. Second, if removal matters later, prefer medium-strength threadlocker over high-strength, because heat (around 250 °C) softens cured adhesive and cuts breakaway torque dramatically.
Quick Recap
- Breakaway torque is the torque needed to start rotation; it beats static friction, so it exceeds running torque.
- Formula: (T = \frac{F \times L}{2\pi}) — force times lead, divided by 2π.
- It is the ideal minimum: friction, threadlocker and corrosion all push the real value higher.
- Use the calculator above for the floor value, then add margin when choosing tools.
Once the screw is moving, the next question is how much steady torque your application needs — the required torque calculator picks up right where breakaway leaves off.