Why torque control decides ring-joint performance
A shield-tunnel lining is held together by the clamping force of its segment bolts. Too little torque and the joint opens under TBM thrust or groundwater pressure; too much and the bolt is driven close to yield, risking delayed fracture or concrete damage around the handhole. Torque values therefore have to be calculated, tested and recorded — not copied from a generic table.
Huilang supplies curved and straight segment bolts in M24–M36, grade 8.8 and 10.9, in hot-dip galvanized or Dacromet/zinc-flake coating. Because those coatings have very different friction behaviour, the same torque can produce preload differences of more than 30%.
The torque–preload relationship
The standard engineering simplification is:
T = K × F × d
- T: tightening torque (N·m)
- K: torque coefficient (nut factor), covering thread and bearing friction
- F: target preload (N)
- d: nominal bolt diameter (m)
Preload is normally a fraction of the bolt's yield capacity:
F ≈ 0.6–0.7 × Rₚ₀.₂ × Aₛ
- Rₚ₀.₂: 0.2% proof strength (about 640 MPa for 8.8, 940 MPa for 10.9)
- Aₛ: tensile stress area (M24 ≈ 353 mm², M30 ≈ 561 mm², M36 ≈ 817 mm²)
Reference tightening tables
Reference values only. Production torque must be based on the project specification and a batch torque-coefficient test. K = 0.20 represents a Dacromet/lubricated condition; K = 0.28 represents dry hot-dip galvanized.
| Size | Grade | Aₛ (mm²) | Preload F (kN) | Torque, K=0.20 (N·m) | Torque, K=0.28 (N·m) |
|---|---|---|---|---|---|
| M24 | 8.8 | 353 | ≈158 | ≈760 | ≈1,060 |
| M30 | 8.8 | 561 | ≈251 | ≈1,510 | ≈2,110 |
| M36 | 8.8 | 817 | ≈366 | ≈2,640 | ≈3,690 |
| M24 | 10.9 | 353 | ≈232 | ≈1,110 | ≈1,560 |
| M30 | 10.9 | 561 | ≈369 | ≈2,210 | ≈3,100 |
| M36 | 10.9 | 817 | ≈537 | ≈3,870 | ≈5,420 |
The preload column uses 0.7 × Rₚ₀.₂ × Aₛ and is rounded. Treat the table as a starting point for discussion, sizing and tool selection.
Choosing the K-factor
K depends on both thread friction and bearing friction:
- Coating: hot-dip galvanized bolts are thicker and rougher, typically K = 0.26–0.32 dry; Dacromet/zinc-flake is thinner and more uniform, typically K = 0.18–0.22; black oxide or phosphated with lubricant can be 0.15–0.18.
- Lubrication: whether and what lubricant is applied changes K significantly. Specify "same batch, same lubricant" for consistency.
- Thread tolerance and coating thickness: coating must not prevent go-gauge acceptance; thick zinc changes friction.
- Bearing surface: spherical washers, handhole inserts and concrete bearing faces all change friction.
- Tightening speed and temperature: fast tightening and low temperature shift K.
The practical answer is a batch torque-coefficient test (per GB/T 16823.3 or the project specification) to establish the construction torque.
Tightening and re-tightening sequence
- Initial tightening: 50–70% of the design torque, in a symmetrical, staged pattern so the joint seats evenly.
- Final tightening: full calculated torque, symmetrical sequence, with a calibrated torque wrench (calibrate per shift).
- Re-tightening: after several rings of advance and again before settlement stabilises, record each pass.
- Sampling: torque audit per batch; ultrasonic preload measurement where required.
- Records: torque value, wrench ID, ambient temperature, operator and re-tightening date — traceable to the ring.
FAQ
Q1: Is higher torque always better?
No. Above the design preload the bolt approaches yield, increasing the risk of delayed fracture and segment cracking. Follow the calculated value.
Q2: Can we use Dacromet torque values for hot-dip galvanized bolts?
Not advisable. Dry hot-dip galvanized bolts have higher friction, so the same torque gives lower preload. Recalculate using the measured K-factor.
Q3: What torque is typical for an M24 grade 8.8 segment bolt?
About 760 N·m at K = 0.20 up to 1,060 N·m at K = 0.28 — always confirm with the project specification and torque-coefficient test.
Q4: How do you verify torque on site?
Calibrated torque wrench audits, preload spot checks (ultrasonic or strain gauge), and re-tightening records, supported by third-party test reports.
Need segment bolts or torque support?
Send the segment drawing, bolt size (M24–M36), grade (8.8/10.9), coating (hot-dip galvanized or Dacromet) and quantity. We can advise on material, supply samples, and provide torque-coefficient and coating test reports.
- Segment bolts → /en/segment-bolt.html
- Standards and grades cross-reference → /en/standards-grades.html
- Surface treatment options → /en/surface-treatment.html
- Metro and shield tunnel applications → /en/applications/metro-shield-tunnel.html
*Reference data for engineering discussion only. Follow the project specification, GB/T 1228–1231 (segment bolts), GB/T 16823.3 (torque-coefficient testing) and ISO 898-1.*