Miniature flexible jaw couplings with bores from 3 mm to 16 mm are now standard catalog items across MJC, L, XO, and SDP/SI families, with rated torque spanning roughly 0.7 Nm to 130 Nm and outer diameters from 15 mm to 51.5 mm [S5][S7].
The format is consistent: two hubs (aluminum, steel, or stainless) clamp a thermoplastic or rubber spider that absorbs angular, parallel, and axial misalignment while transmitting torque. Hub material and spider durometer, not bore size alone, set the torque ceiling.
Three-Piece Construction and Spider Options
Every miniature unit referenced uses a three-piece body: two identical or stepped-bore hubs plus a single elastomeric spider. Ondrives' MJC-30-BL-16-16 combines anodized aluminum hubs with an 80 Duro blue polyurethane spider at 16 mm x 16 mm bores, 30 mm OD, weighing 44 g and rated 70.80 lb-in (8.0 Nm) at up to 21,000 rpm [S1].
Spider material is the single biggest spec swing. Lovejoy's 2025 jaw catalog (December 2025) covers Neoprene, NBR, Hytrel, and polyurethane inserts, and explicitly warns that pushing past catalog ratings, ignoring alignment, or running with damaged elastomers can throw parts at speed [S4]. For miniature bores, the practical limit is usually the spider, not the hub: Martin's jaw-coupling catalog notes horsepower ratings must be multiplied when Hytrel or bronze spiders are substituted for the default NBR/neoprene element [S6].
Bore Range Coverage: Where 3 to 16 mm Sits in Each Family
SDP/SI's anti-vibration and disc-type miniature couplings cover 3 to 16 mm bores in aluminum or steel, with outer diameters 15 to 39 mm and lengths 18 to 55 mm, including Fairloc hub options for repeatable shaft mounting [S5]. Amironic's MJT-BL miniature curved-jaw line spans 3 to 55 mm bores, but the low-torque disc-style subset starts at 0.7 Nm and is the natural fit for the 3 to 16 mm band [S7].
Hayes' XO Series is the dedicated miniature tier: max bore 5/8 in (16 mm), 1-3/8 in (35 mm) OD, 2.00 in (51 mm) assembled length, in aluminum or steel, with Neoprene or Hytrel spiders and torque up to roughly 3,996 in-lb at the top of the 60 Series [S3]. The 10 Series jumps to 1 in (25 mm) max bore and 2.03 in OD, so the XO line is where the 16 mm ceiling actually lives [S3].
Selection Criteria Beyond Bore Diameter
Bore diameter alone is not a selection criterion. Engineers must also check rated torque, max rpm, angular offset, and bore tolerance. The MJC-30-BL-16-16 lists bore tolerance +0.027 mm, max angular offset 1 degree, temperature -4 to +140 degF, and no electrical isolation [S1]. Lovejoy's rule of thumb is equally blunt: if the bore is not large enough for the shaft, step to the next size that fits both driver and driven shafts [S4].
Material stack-up matters because misalignment, chemical exposure, and temperature all interact with spider durometer. Comparing the three common spider choices on the same L090 body illustrates the trade-off. Sizing and selection workflow, including service factors, is laid out in the Lovejoy JW series selection process [S4].
Comparison: Common Miniature Jaw Coupling Specs
The four entries below are the most-cited miniature three-piece jaw couplings in the 3 to 16 mm bore band. All values are taken directly from the cited catalogs. [S1]
Comparing materials against the four decision criteria: aluminum MJC, SDP/SI disc-type, Hayes XO aluminum, and Hayes XO steel deliver different trade-offs, while spiders range from 80 Duro polyurethane through NBR to Hytrel. Hytrel raises torque ceiling and temperature, but at the cost of lower damping and higher price per spider [S6].
Use Cases: Stepper Motors, Encoders, Small Servos
Stepper and small BLDC drivetrains in the 3 to 8 mm shaft range are the dominant application. SDP/SI markets the disc-type line directly at robotics, automation, and motion control where zero backlash and high torsional stiffness matter more than raw torque [S5]. The Hayes XO Series and the MJC miniature curved-jaw are both aimed at precision robotics, biomedical, and industrial machining duty cycles, with aluminum as the default and steel as the uprate for higher inertia or shock loads [S3].
Pump, gearbox, and conveyor retrofits are the second cluster. Lovejoy's L090 with NBR spider is cross-compatible with Martin, Browning, and TB Woods L-Jaw hardware, so it drops in as a replacement hub on existing 7/16 in to 1 in shafts [S2]. Lovejoy also covers L, LC, Al, SS, RRS, RRSC, C, and RRC variants, with stainless (SS) specified where washdown or corrosion rules out aluminum [S4].
Limitations, Misalignment, and Failure Modes
Three-piece jaw couplings are not zero-backlash, and they are not for high-cycle start-stop shock beyond their catalog service factor. Lovejoy's catalog warning is explicit: exceeding catalog ratings, ignoring alignment, running with damaged elastomers, or skipping inspection after start-up can throw parts at speed and cause serious injury [S4].
Temperature and chemical exposure bound the spider choice. The MJC polyurethane spider caps at +140 degF and is not electrically isolating; for higher temperatures or for any need to break ground loops between motor and driven shaft, specify a different material or add an isolating sleeve [S1]. Bore tolerance is also tight: +0.027 mm on a 16 mm bore is an H7-class fit, so the mating shaft should be machined to a matching tolerance, not a press-fit ground stock [S1].
Standards, Sourcing, and Spare-Spider Practice
Jaw couplings sit in a grey zone between AGMA gear standards and ISO 21940 balance grades, so most OEMs publish in-house ratings rather than a unified ISO torque class. The Lovejoy JW-11 selection process is the closest thing to a de facto reference and is updated December 2025 [S4]. Hayes publishes XO, 10, 20, 30, 40, 50, and 60 Series dimensional drawings, all keyed on bore and OD in metric and imperial [S3].
Spare-spider stocking is the field practice that keeps a line running: NBR and 80 Duro polyurethane spiders wear out before the hubs do, and a 3-piece L090 or MJC body can be re-used by replacing the insert only [S1][S2]. Ondrives publishes tier pricing for the MJC-30-BL-16-16 at 1-4 ($80.14), 5-9 ($77.73), 10-24 ($76.93), 25-49 ($75.33), and 50+ ($73.72) pieces, which is the kind of break point a maintenance buyer will want to know before issuing a single-unit PO [S1].
For reference designs that already pair a miniature jaw coupling with a precision linear guide or ball spline, the Miniature Ball Spline 4-8 mm Load Ratings: 2026 Spec Reference sits naturally alongside this bore range, since the same 3 to 8 mm shaft band drives both component choices. Where corrosion rules out standard aluminum hubs, the cross-reference in ISO 12944 corrosivity categories C1 to CX and coating durability ranges is a more useful input than picking stainless by habit. The spider-versus-hub trade-off also overlaps with the Low-Friction POM Grades: PTFE vs Silicone Internal Lubricants Compared article when the application pushes toward self-lubricating, low-wear interfaces around the same 3 to 16 mm envelope.
For component-level specifications, see jaw coupling, measurement test 3, and pressure transmitter.