Bolted Flange Coupling Torque Capacity
Bolted Flange Coupling Torque Capacity
A bolted flange coupling joins two shaft ends by bolting together matching flanges keyed or shrink-fitted to each shaft, transmitting torque through shear in the bolts arranged around a bolt circle. Because each bolt acts at the same radius from the shaft centerline, the coupling's total torque capacity is simply the shear capacity of one bolt, multiplied by the number of bolts and by that common radius.Flange couplings are chosen over simpler couplings (like a single key and sleeve) specifically because distributing the torque over multiple bolts on a bolt circle lets the connection scale to much higher torque without a single, hard-to-manufacture oversized fastener — but that also means every bolt in the pattern needs to be properly torqued and maintained, since a single loose or failed bolt shifts more load onto its neighbors.
The bolted flange coupling torque capacity is T_allow = n_c·F_b_allow·R_bc, the number of bolts times the allowable shear force per bolt times the bolt circle radius. where n_c is the number of bolts in the coupling, F_b_allow is the allowable shear force per bolt, R_bc is the bolt circle radius, and T_allow is the resulting torque capacity of the coupling.
Inputs
n_c := 6=6
F_b_allow := 15 kN=15000 N
R_bc := 0.12 m=120 mm
Each bolt contributes a shear force at the same radius from the shaft center, so multiplying the per-bolt allowable force by the bolt count and the bolt circle radius gives the total torque the coupling can transmit before any bolt reaches its shear limit.
T_allow := n_c·F_b_allow·R_bc=10800 Nm
Results
A torque capacity of 10.8 kN·m for this six-bolt coupling shows how effectively multiple bolts on a bolt circle can share a large torque demand — a single bolt at the same radius could only manage a sixth of that capacity. Because torque capacity scales directly with bolt circle radius, moving the bolts out to a slightly larger circle (within the flange's available diameter) is often a more material-efficient way to raise capacity than adding more bolts or upsizing bolt diameter, since it doesn't require redesigning the bolt pattern spacing. This calculation assumes the bolts share load equally, which depends on all bolts being properly and evenly torqued during assembly — an unevenly tightened flange coupling can load some bolts well beyond their proportional share long before the calculated capacity is reached.
Assumptions & limitations
- Bolts assumed to share the transmitted torque equally, which requires uniform, correctly specified bolt preload.
- Only bolt shear capacity is checked; flange plate bending and the shaft-to-flange connection are separate checks.
- No coexisting bending moment or axial thrust across the coupling is included, only pure torque transmission.
- Bolt circle is assumed to have bolts evenly spaced so each acts at the same effective radius.