Key issues in diaphragm coupling maintenance and installation not to be overlooked

Diaphragm couplings utilize elastic deformation of diaphragms to compensate for relative shaft displacements. Resistant to temperature, oil contamination, acids, alkalis, and corrosion, they are ideal for high-temperature, high-speed shaft transmissions in corrosive environments, making them a prioritized coupling solution.

1.Problems and suggestions regarding maintenance and installation

Current alignment standards specify radial runout ≤0.15mm and axial runout ≤0.10mm for diaphragm couplings per GB50231-1998, with similar requirements in chemical industry standards. However, these tolerances may not apply universally during routine maintenance and installation of all diaphragm couplings.

2.Diaphragm couplings primarily withstand four types of forces

2.1 Membrane stress induced by torque. 

The torque-applied force on one side distributes evenly across four spaced bolt holes. On a quarter-section of the diaphragm, this force acts circumferentially at the mid-section of intermediate bolt holes, with fixed radial and axial displacements.

2.2 Centrifugal stress caused by inertial forces. 

Centrifugal forces in high-speed machinery significantly affect structural stress calculations. These forces can be modeled as radial body force per unit volume: f = 2rρ(2πn/60)2, directed radially outward (where n=speed, r=radius, ρ=density). Radial, circumferential, and axial displacements at intermediate bolt holes are fixed, with no external loads on the periphery.

2.3 Bending stress due to axial misalignment.

Actual installation errors along the shaft axis cause bending deformation of the diaphragm in the axial direction. This displacement is applied axially at the intermediate bolt holes, while radial and axial displacements are constrained.

2.4 Bending stress from angular misalignment (periodic stress)

Actual angular misalignment between shaft axes causes periodic bending deformation in the diaphragm, a key factor in diaphragm fatigue life. Additional stress depends not only on relative angular/axial/radial displacements but also on bolt quantity, diaphragm thickness, and bolt distribution radius. Alignment tolerances should follow equipment design specifications rather than universal radial/axial limits (≤0.15mm radial runout, ≤0.10mm axial runout).

3.Face Distance Deviation in Diaphragm Couplings

Tolerances for face distance deviation are based on formulas for additional stress caused by relative shaft displacements. Axial installation deviations generate diaphragm forces during bolt tightening. Combined angular/radial deviations produce superimposed stresses during assembly. Operational axial displacements further accelerate stress accumulation, increasing diaphragm fatigue risk.

4.Key Issues to Consider During Installation

4.1 Dynamic Balancing

As machinery speed increases, eccentricity between the inertial principal axis and rotational axis of rotating components generates unbalanced inertial forces. These forces induce shaft vibration and component failure. Flexible couplings with movable parts are particularly prone to imbalance. Many high-speed diaphragm couplings undergo pre-balancing, and their installation on shafts requires secondary balancing to ensure system stability. While balanced installation is prioritized for critical equipment, maintenance crews often overlook coupling balancing for high-speed pumps. Coupling mass relative to the pump rotor becomes significant at high speeds, producing substantial imbalance. This imbalance disrupts bearings and gears in the complex-force environment of pump input shafts, leading to operational abnormalities. Strict adherence to design specifications during diaphragm coupling installation is essential for reliable operation.

4.2 Half-Coupling Deformation

Improper usage or installation can deform half-couplings, inducing abnormal bending stresses in diaphragms during bolt tightening. 

Unlike axial-misalignment-induced stresses, these bending stresses are asymmetric, localized in deformed 

areas, and influenced by deformation severity, diaphragm thickness, bolt count, and bolt radius. 

Deformation causes vibration, noise, and premature diaphragm cracking.

Current maintenance practices rarely include half-coupling deformation checks. Industry standards specify:

Runout ≤0.05mm for coupling outer diameter/end faces

Runout ≤0.08mm for diameters >250mm or tapered bore fits

Recommendation: Establish systematic inspection protocols during installation to verify deformation

limits and ensure operational reliability.

4.3 Minor Critical Details

Key installation specifications:

Dedicated Fasteners: Use coupling-specific bolts (never standard substitutes) tightened to specified torque values.

Bolt Orientation: Ensure directional correctness during insertion.

Diaphragm Preload: For diaphragms requiring pre-tension/compression:

Lock prime mover and equipment in initial alignment before adjusting shims.

Tapered Bore Validation: Verify ≥75% contact area ratio for couplings with tapered shaft fits.