Example data table
| Scenario | m (kg/m) | μ | Lh (m) | Lv (m) | Rise (m) | θ (deg) | Final T (kN) | Design T (kN) |
|---|---|---|---|---|---|---|---|---|
| A | 1.20 | 0.25 | 30 | 10 | 6 | 180 | 0.413 | 0.516 |
| B | 2.00 | 0.30 | 60 | 0 | 0 | 90 | 0.566 | 0.735 |
| C | 0.90 | 0.20 | 20 | 15 | 12 | 270 | 0.430 | 0.516 |
| D | 1.50 | 0.35 | 45 | 5 | 8 | 360 | 3.382 | 5.073 |
| E | 3.00 | 0.15 | 80 | 0 | 0 | 45 | 0.397 | 0.496 |
| F | 1.10 | 0.28 | 25 | 25 | 20 | 180 | 0.884 | 1.238 |
Example input sets
Click Load to populate the form with realistic values, then adjust for your route.
| Scenario | m (kg/m) | μ | Lh (m) | Lv (m) | Rise (m) | θ (deg) | R (mm) | SF | Allow T (kN) | Allow SWP (lbf/ft) | Action |
|---|---|---|---|---|---|---|---|---|---|---|---|
| G | 1.40 | 0.25 | 40 | 10 | 5 | 180 | 300 | 1.25 | 12 | 300 | |
| H | 2.20 | 0.32 | 70 | 0 | 0 | 90 | 250 | 1.30 | 15 | 350 | |
| I | 1.00 | 0.20 | 25 | 20 | 15 | 270 | 350 | 1.40 | 10 | 250 | |
| J | 2.80 | 0.18 | 90 | 5 | 2 | 45 | 400 | 1.25 | 20 | 400 |
Formula used
- w = m · g (unit weight in N/m)
- T₁ = T₀ + μ · w · (Lh + Lv) + w · rise
- T₂ = T₁ · e^(μ · θ) (capstan-style bend amplification)
- T_design = T₂ · SF
- SWP ≈ T₁ / R (optional sidewall pressure estimate)
θ is the total bend angle in radians. Rise is positive when pulling upward overall. Downward pulls can reduce required tension, but never below zero.
How to use this calculator
- Choose whether you know cable mass per length or unit weight.
- Enter μ, gravity, and a safety factor suited to your process.
- Enter horizontal length, vertical length, and net rise.
- Sum all bends into one total bend angle, then enter it.
- Optional: add bend radius to estimate sidewall pressure.
- Optional: enter allowable limits to flag potential issues.
- Click Calculate, then export results as CSV or PDF.
Practical notes for cable pulling tension
1) What drives the pull
Cable pulling tension is driven by unit weight, friction, elevation, and bends. This calculator treats the route as straight runs plus a capstan bend multiplier. Start by entering mass per length or unit weight, then set μ for your conduit condition. Typical μ values range from 0.15 lubricated to 0.50 dry, rough installs. Record conditions, then rerun scenarios.
2) Straight runs and elevation
Straight-run tension increases with length. The model uses T₁ = T₀ + μ·w·(Lh+Lv) + w·rise. For a 1.2 kg/m cable (w≈11.77 N/m), μ=0.25, and 40 m of total run, friction adds about 118 N. A 6 m rise adds about 71 N more to T₁. Longer runs and heavier cables raise tension.
3) Why bends matter
Bends amplify tension exponentially, so angle matters. The bend factor is e^(μ·θ) where θ is radians. One 90° sweep equals 1.571 rad; at μ=0.30 the factor is e^0.471≈1.60. Two 90° bends (180°) becomes 3.142 rad and factor e^0.942≈2.57, multiplying whatever T₁ you already have very fast.
4) Summing bend angles correctly
Use total bend angle as the sum of every turn in the pull, including offsets. If you track bends as counts, multiply each bend’s degrees and add them. Keep θ realistic; totals above 360° can explode tension. For μ=0.35 and θ=360° (6.283 rad), the factor is e^2.199≈9.02, magnifying T₁ dramatically overall today.
5) Using safety factor and limits
Safety factor converts calculated pulling tension into a planning target. If the final pulling tension is 4.2 kN and you apply SF=1.25, the design tension becomes 5.25 kN. Choose SF based on variability: lubrication quality, conduit fill, temperature, and crew technique. Use the optional allowable tension field to flag when the design value exceeds your limit.
6) Interpreting sidewall pressure
Sidewall pressure estimates help protect insulation at tight bends. With a minimum bend radius R, this tool reports SWP≈T₁/R. If T₁ is 2,000 N and R is 0.30 m, SWP≈6,667 N/m, about 457 lbf/ft. Compare against your project’s allowable SWP and consider larger radius sweeps or intermediate pulls to reduce stress safely.
FAQs
What friction coefficient should I use?
Use a value that matches conduit condition and lubrication. Many pulls fall between 0.15 and 0.50. Start conservative if you are unsure, then run a second case with better lubrication to see how tension changes.
Why do I enter total bend angle instead of bend count?
Total bend angle is what the capstan factor uses. Add the degrees of every turn along the pull, including offsets. For example, two 90° sweeps equal 180°. The calculator converts degrees to radians internally.
What if my route is mostly vertical?
Enter vertical length for completeness, but net rise controls gravity loading. If you pull upward 12 m overall, set rise to 12. If you rise and drop back down, rise may be smaller than vertical length.
Can the calculated tension go down on a downward pull?
Yes. A negative rise subtracts w·|rise| in the straight-run step. The tool will not show negative tension; it floors at zero because slack cannot “push” the cable through the conduit.
How do I pick a safety factor?
Use higher factors when friction is uncertain, lubrication may fail, or routing is tight. Values like 1.10–1.50 are common for planning. Always compare the design tension against your allowable pull rating.
Is the sidewall pressure number exact?
No. It is a quick estimate based on SWP≈T₁/R using the minimum bend radius you enter. It helps spot risky bends, but manufacturer charts and field conditions should guide your final decision.