SJM Type Double Flexible Diaphragm Coupling

SJM Type Double Flexible Diaphragm Coupling

The SJM Type Double Flexible Diaphragm Coupling uses two stainless steel diaphragm packs separated by a centre spacer to deliver twice the misalignment compensation of the single-pack DJM — including direct radial offset up to 3.0 mm. Spanning 41 sizes from 9.8 N·m to 8,100,000 N·m at up to 20,000 rpm, it is the preferred zero-lubrication solution for long shaft spans, turbomachinery, and drives where multi-directional misalignment cannot be avoided.

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What Is an SJM Double Flexible Diaphragm Coupling?

SJM type double flexible diaphragm coupling dual pack stainless steel key connection

The SJM type double flexible diaphragm coupling builds on the proven single-pack DJM architecture by adding a second stainless steel diaphragm pack on the opposite shaft end, separated from the first by a rigid cylindrical spacer. This dual-pack arrangement provides a step-change in misalignment capacity: axial compensation doubles, angular tolerance increases to 2°, and — critically — direct radial offset becomes possible because the two packs can flex in complementary directions to accommodate shaft centreline offset without bending moments on the bearings.

Like the single-pack DJM, the SJM transmits torque entirely through the elastic deformation of its stainless steel diaphragm packs, without any sliding contact, grease, or wear-limited elastomeric element. The coupling is torsionally stiff, backlash-free, and serviceable at temperatures from -40°C to +280°C — making it the preferred choice for turbomachinery, long-span pump installations, power generation couplings, and any drive where shafts cannot be held within the tighter tolerance required by a single-pack unit.

Dual-Pack Geometry and Why It Matters

In a single-pack coupling, radial shaft offset produces a bending moment at the single diaphragm location. Two packs separated by the spacer C create a mechanical linkage: radial offset is absorbed by equal and opposite angular deflection at each pack, so the bending moments cancel. The result is true radial compensation without transmitting offset-induced forces to the shaft bearings. This property is essential on long shaft spans where thermal growth, soft-foot settlement, or foundation movement routinely produces radial offsets beyond the single-pack's capacity.

Key Differences from the DJM Single-Pack

Criterion SJM Double DJM Single
Diaphragm packs 2 (with spacer) 1
Axial compensation ±1.6–±16.4 mm ±0.8–±8.2 mm
Angular compensation Up to 2° Up to 1°
Direct radial compensation 0.5–3.0 mm Indirect only
Overall length Longer (spacer C) Shorter
Long shaft spans Preferred choice Limited

SJM Coupling Specifications and Dimensions

SJM double diaphragm coupling dimension drawing D L A B C spacer bore specifications
SJM type double flexible diaphragm coupling assembly key connection hub spacer

Materials

Diaphragm packs: stainless steel sheet (two packs per coupling). Spacer and hubs: carbon steel or alloy steel. Bolts: high-tensile alloy steel. Imperial bore machining and corrosion-resistant surface treatments available on request.

Ordering Mark Example

SJM-07   YA55×112 / YA50×112
→ Type SJM double, size 07  |  Drive: Y-bore A-keyway d=55 mm L=112 mm  |  Driven: Y-bore A-keyway d=50 mm L=112 mm

Complete Parameter Table — SJM00 to SJM40

Sizes 00–11: bore length L per Y-type recommendation. Sizes 12–40: hub flange type, J dimension not applicable (–). Spacer C is standard value; custom spacer lengths available. Radial compensation: 0.5 mm (00–04), 0.7–0.8 mm (05–11), 1.2 mm (12–14), 1.3–3.0 mm (15–40).

Size Nom. Torque N·m Max Speed rpm Bore d mm D mm A mm B mm L mm C (spacer) mm Axial mm Angle Radial mm Weight kg
00 9,800 20,000 3–20 57 4.9 20 100 60 ±1.6 2° 0.5 0.7
01 33,000 20,000 5–22 68 6.1 26 141 89 ±1.6 2° 0.5 1.2
02 90,000 20,000 6–32 81 6.6 26 141 89 ±1.6 2° 0.5 1.9
03 173,000 18,000 8–35 93 8.4 29 160 102 ±2.4 2° 0.6 2.9
04 245,000 15,000 10–42 104 11.2 34 195 127 ±2.8 2° 0.7 4.7
05 420,000 13,000 15–50 126 11.7 42 211 127 ±3.2 1°30' 0.7 7.1
06 772,000 12,000 20–60 143 11.7 48 223 127 ±3.6 1°30' 0.8 10.8
07 1,270,000 10,000 25–75 168 16.8 58 243 127 ±4.0 1°30' 0.8 16.3
08 2,080,000 10,000 30–82 194 17.0 64 268 140 ±4.4 1°30' 0.9 24.7
09 3,328,000 9,000 30–95 214 21.6 77 306 152 ±4.8 1°30' 0.9 32.5
10 4,900,000 8,000 40–108 250 23.9 89 356 178 ±5.2 1°30' 1.0 50.0
11 6,368,000 6,300 52–118 276 27.2 102 382 178 ±5.6 1°30' 1.2 75.0
12 8,900,000 6,300 60–110 276 17.5 128 409 153 ±3.6 1° 1.2 72.2
13 15,280,000 5,000 60–135 308 19.0 160 492 172 ±4.0 1° 1.2 120.0
14 25,410,000 4,700 60–155 350 21.5 182 554 190 ±4.0 1° 1.2 175
15 37,130,000 4,300 60–160 375 24.0 198 620 224 ±4.0 1° 1.3 234
16 47,120,000 3,900 70–180 410 29.5 214 682 254 ±4.4 1° 1.3 306
17 57,000,000 3,500 70–190 445 29.5 225 720 270 ±4.4 1° 1.4 369
18 63,186,000 3,500 80–205 470 31.0 248 770 274 ±4.8 1° 1.5 448
19 82,590,000 3,200 90–230 512 32.0 278 843 287 ±4.8 1° 1.6 596
20 102,100,000 2,800 90–255 556 32.5 305 902 292 ±5.2 1° 1.8 763
21 126,070,000 2,450 100–265 588 34.0 318 948 312 ±5.4 1° 1.8 919
22 146,350,000 2,150 100–275 630 34.5 332 1008 344 ±5.6 1° 2.0 1068
23 173,830,000 2,000 100–290 655 35.5 348 1052 356 ±6.0 1° 2.0 1235
24 200,000,000 1,400 210–305 680 44 350 1080 380 ±7.0 30' 2.0 1350
25 250,000,000 1,250 225–340 745 44 350 1100 400 ±8.0 30' 2.0 1580
26 315,000,000 1,200 250–365 785 50 350 1120 420 ±8.4 30' 2.0 1650
27 400,000,000 1,150 270–380 830 50 380 1200 440 ±9.0 30' 2.0 1950
28 500,000,000 1,100 290–400 875 50 400 1250 450 ±9.6 20' 2.5 2200
29 630,000,000 1,000 320–425 935 60 400 1280 480 ±10.0 20' 2.5 2300
30 800,000,000 930 340–440 1030 60 440 1380 500 ±10.4 20' 2.5 2600
31 1,000,000,000 880 380–460 1080 66 460 1440 520 ±11.0 20' 2.5 3500
32 1,250,000,000 820 400–500 1160 70 520 1620 580 ±11.6 20' 2.5 4800
33 1,600,000,000 740 420–560 1290 82 570 1740 600 ±12.4 20' 3.0 6100
34 2,000,000,000 680 460–600 1410 92 570 1740 600 ±13.0 20' 3.0 7600
35 2,500,000,000 620 480–650 1530 105 610 1900 700 ±13.6 20' 3.0 8600
36 3,020,000,000 570 500–710 1670 115 730 2210 750 ±14.4 20' 3.0 11000
37 4,050,000,000 520 600–780 1830 125 800 2450 850 ±15.0 20' 3.0 14700
38 5,300,000,000 480 650–860 2000 130 800 2500 900 ±15.6 20' 3.0 21000
39 6,600,000,000 430 700–945 2200 140 960 2920 1000 ±16.0 20' 3.0 26700
40 8,100,000,000 400 800–1030 2400 140 960 2920 1000 ±16.4 20' 3.0 32000

How to Select an SJM Diaphragm Coupling

Design Torque Calculation

TC = K x 9,550 x P / N    ≤    Tn

K = service factor  |  P = rated power (kW)  |  N = speed (rpm)  |  Tn = nominal torque of selected size

When to Choose SJM over DJM

  • Shaft radial (parallel) offset exceeds the DJM single-pack's indirect compensation capacity
  • Long shaft spans where thermal growth or foundation settlement routinely produces radial drift
  • Both angular and radial misalignment are present simultaneously
  • Applications where re-alignment after each maintenance shutdown is impractical
  • Turbomachinery trains where bearing-to-bearing distances exceed 600 mm

Spacer Length C

The spacer length C determines the total coupling length L and the effective radial compensation. Standard C values are listed in the parameter table. Custom spacer lengths are available to suit specific shaft-end gaps — specify the required C dimension when ordering. Increasing C beyond the standard value increases radial compensation proportionally.

Metal Diaphragm Coupling Series Overview

All diaphragm coupling types in the GBC range share the same stainless steel diaphragm pack technology and steel hub materials. Browse the full diaphragm coupling range for selection guidance.

DJM — Single Diaphragm, Key Connection

9.8–8,100,000 N·m · 41 sizes · Single pack · Shorter length · Axial and angular compensation

SJM — Double Diaphragm, Key Connection (this page)

9.8–8,100,000 N·m · 41 sizes · Dual pack · Radial offset to 3.0 mm · Long shaft spans

DJM-Z1 — Single Diaphragm, Locking Device

33–1,270 N·m · 7 sizes · Interference-fit Z1 locking device · Zero backlash · CNC drives

DJM / SJM Taper Sleeve Type

Cone-hub connection · CNC machine tool feed shafts · Replaces Z1 locking device

DJM / SJM-YP — Locking Disc Type

33–8,100,000 N·m · Z7B locking disc · Single and double pack

SJM-P — Double Diaphragm with Brake Disc

420–25,410 N·m · Integrated disc brake D0 315–1,000 mm

JM — Single Diaphragm with Intermediate Shaft

Rod-type · 40–180,000 N·m · Large compensation

JMJ — Double Diaphragm with Intermediate Shaft

Rod-type · Maximum misalignment · Long shaft spans

SJM Coupling Industry Applications

Turbomachinery — Gas and Steam Turbines

Large turbine-driven pump and compressor trains involve shaft spans of 1–4 metres where thermal growth routinely produces radial offset of 1–3 mm. The SJM's dual-pack design absorbs this growth without transmitting bending moments to the turbine or driven equipment bearings — a critical requirement for API 671 coupling applications.

Large Centrifugal Pumps — API 610 Service

Multi-stage boiler feed pumps and pipeline booster pumps operate at 3,000–6,000 rpm with shafts that drift radially as operating temperature stabilises. The SJM accommodates this cold-to-hot offset transition while maintaining torsional stiffness, protecting pump impeller clearances from misalignment-induced vibration.

Power Generation — Large Generator Sets

Generator coupling applications involve high torque, high speed, and shaft arrangements where alignment tolerance is difficult to achieve and maintain. The SJM provides the zero-lubrication, zero-maintenance operation essential for long continuous run plant, with radial compensation that accommodates foundation creep over months of continuous operation.

Marine Propulsion — Shaft Line Couplings

Shipboard propulsion shafts are subject to hull flexing, temperature change, and installation tolerances that produce both angular and radial misalignment. The SJM's all-metal construction withstands humidity and temperature cycling, and its lubrication-free design eliminates the pollution risk of grease-lubricated alternatives.

Pulp and Paper — Large Refiner and Mixer Drives

Disc refiners and high-consistency mixers transmit cyclic shock loads while running on foundations that are subject to vibration-induced settlement. The SJM's dual-pack configuration tolerates the resulting radial misalignment growth while the stainless steel packs resist the corrosive washdown environments common in pulp mills.

Chemical Process — Agitator and Mixer Drives

Large agitator drives in reactors and blending vessels often have poor shaft alignment due to the difficulty of setting baffled vessel nozzle flanges. The SJM allows more generous installation tolerances than a single-pack type, reducing installation time and the risk of premature bearing failure from forced alignment.

Installation and Maintenance

Installation Procedure

  1. Hub fitting: Press or heat-shrink hubs onto both shaft ends (induction heater ≤120°C). Torque setscrews to specification.
  2. Initial alignment: Align shafts to within the SJM's compensation limits using laser alignment equipment. The SJM tolerates more initial offset than the DJM, but good alignment maximises diaphragm service life.
  3. Spacer installation: Place the centre spacer between the two hub flanges; confirm spacer-to-flange gap corresponds to dimension C in the parameter table.
  4. Diaphragm pack mounting: Fit one pack at each end. Torque all bolts in a cross-pattern in three passes to the specified value. Correct bolt torque is critical — both under- and over-torquing damage the diaphragm at the bolt holes.
  5. Final check: Rotate two full turns by hand; verify smooth rotation, correct C dimension, and no metallic contact.

Fault Diagnosis

Symptom Likely Cause Action
High 1× vibration Radial misalignment exceeding SJM limit; unbalance Re-align; check coupling balance
High 2× vibration Angular misalignment; damaged diaphragm Check angular alignment; inspect both packs
Diaphragm crack at bolt hole Bolt under-torqued (micro-slip) or over-torqued (fatigue) Replace pack; verify bolt torque with calibrated wrench
Spacer contact noise Misalignment causing spacer eccentricity Re-align; check spacer running clearance

SJM vs Alternative Coupling Solutions

vs Gear Coupling: A gear coupling can tolerate similar radial offsets but requires grease re-lubrication every 6–12 months. In steam turbine or compressor applications, lubricant degradation at high temperature is a chronic cause of premature failure. The SJM eliminates this risk entirely, with no wear-dependent maintenance life.

vs Elastomeric Coupling: Elastomeric couplings are limited to +80°C in standard polyurethane grades and cannot match the torsional stiffness of metal diaphragm types. In turbomachinery applications requiring precise rotordynamic calculations, the SJM's predictable stiffness allows accurate critical speed analysis — elastomeric elements introduce variability that complicates this analysis.

vs DJM Single Pack: For applications where shaft radial offset is the dominant alignment challenge, the SJM is the correct choice. The DJM single-pack type remains preferred where space is constrained and shafts are well-aligned — it delivers the same zero-backlash, zero-lubrication characteristics in a shorter overall length.

Customer Case Studies

Australia — LNG Plant, Gas Turbine Driven Compressor

Installed SJM-18 on three gas turbine driven refrigerant compressor trains. Thermal growth on these machines produces 1.8–2.2 mm radial offset between cold and operating conditions. The SJM handles this growth without transmitting bearing loads, and we have eliminated the twice-yearly grease changes our previous gear couplings required. No unplanned stops in 22 months.

Rotating Equipment Superintendent, Western Australia AU

★★★★★

Netherlands — Large Boiler Feed Pump

SJM-14 and SJM-15 on three high-pressure boiler feed pump sets at a combined cycle power station. The station runs at high load factor; we cannot afford alignment-related failures. The SJM's dual-pack design tolerates the shaft movement we see during load changes without the vibration penalties that drove us away from gear couplings in 2022.

Turbine Engineer, Rotterdam NL

★★★★★

Japan — Steel Mill, Large ID Fan

SJM-20 to SJM-22 on induced draft fan drives at a blast furnace. Fan bearing temperatures can shift shaft position by 1.5–2.5 mm radially. The SJM absorbs this without the periodic realignment events that the previous gear coupling arrangement required after each fan maintenance stop. Maintenance downtime for these couplings is now zero.

Mechanical Maintenance Manager, Chiba JP

★★★★☆

Brazil — Pulp Mill, Disc Refiner Drives

SJM-10 through SJM-13 on chip disc refiners. The combination of high cyclic torque from wood knots, wet corrosive environment, and poor foundation stability made the previous elastomeric coupling unreliable. The SJM stainless steel packs handle the shock, resist the chemicals, and tolerate the foundation settlement we get during the wet season.

Asset Reliability Engineer, Mato Grosso do Sul BR

★★★★★

Frequently Asked Questions

▶ Why does the SJM provide more radial compensation than the DJM?

The SJM uses two diaphragm packs separated by a rigid spacer. Radial shaft offset causes both packs to deflect angularly in complementary directions — the resulting moments cancel at the spacer, so no bending force is transmitted to the shaft bearings. A single-pack DJM accommodates radial offset only indirectly through its angular flex, which generates a residual bending moment. The dual-pack arrangement is the only way to achieve true zero-moment radial compensation.

▶ What is the C dimension in the SJM parameter table?

C is the length of the centre spacer that separates the two diaphragm packs. The standard C value for each size is listed in the parameter table. The total coupling length L equals the sum of the two hub dimensions and the spacer C. Custom spacer lengths are available for non-standard shaft-end gaps — increasing C proportionally increases the coupling's radial compensation capacity.

▶ Can an SJM coupling be balanced for high-speed applications?

Yes. GBC offers shop-balanced and site-balanced SJM assemblies for applications above 5,000 rpm or where residual unbalance must meet API 671 specifications. Dynamic balancing is performed on the complete coupling assembly (both hubs and spacer). Specify the required balance grade (G1.0, G2.5, or API 671 Grade) when ordering.

▶ How often should SJM diaphragm packs be inspected?

Visually inspect both diaphragm packs at each machine shutdown, at minimum annually. The main failure modes — fatigue cracking at bolt holes from chronic misalignment, and fretting from under-torqued bolts — are visible to the naked eye. The diaphragm pack can be replaced without removing the hubs from the shafts; only the pack mounting bolts need to be undone.

▶ Can the SJM be used as a drop-in replacement for a gear coupling?

In most cases, yes, subject to dimensional compatibility. The SJM offers a longer overall length than an equivalent gear coupling due to the spacer, but the bore range, hub OD, and bolt circle are often compatible with existing shaft arrangements. Supply GBC with the existing coupling's dimensional drawing and shaft data; our team will confirm dimensional fit and calculate the correct SJM size for the application.

Specify SJM Double Diaphragm Couplings for Your Drive Train

GBC supplies all 41 SJM sizes with full dimensional documentation, custom bore and spacer configurations, balance certification, and material test certificates. Send your motor power, speed, shaft diameters, and alignment data and our engineering team will confirm the correct size, spacer length, and bore mark within 24 hours.

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The SJM Type Double Flexible Diaphragm Coupling uses two stainless steel diaphragm packs separated by a centre spacer to deliver twice the misalignment compensation of the single-pack DJM — including direct radial offset up to 3.0 mm. Spanning 41 sizes from 9.8 N·m to 8,100,000 N·m at up to 20,000 rpm, it is the preferred zero-lubrication solution for long shaft spans, turbomachinery, and drives where multi-directional misalignment cannot be avoided.

Contact Us

Email

sales@australia-drive.com

stuart_hamill@generalbearing.com.au

Tel: +61 3 9070 7991

SKU: 85060fc6d511 Categories: , Tags: , , , ,

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