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Plastic flexible couplings such as Paguflex (also known as double loop or twisted-eight couplings) are an excellent solution for connecting two rotating shafts while managing inevitable alignment imperfections. That’s why it’s especially important to understand how to choose the right double-loop coupling for your task.
Using a high-quality elastic element—made of polyurethane (PUR) or thermoplastic elastomer Hytrel®—enables these couplings not only to transmit torque efficiently but also to act as a mechanical filter: they absorb torsional vibrations, compensate static and dynamic misalignments, and crucially reduce wear on expensive components such as bearings, seals, and motors. The result is smooth motion, zero backlash, and reduced maintenance.
Larosi supports industry with a wide range of Paguflex® couplings with metric or inch bores, including customized options. Below is the essential technical guide to help choose the ideal Paguflex model for your application.
1. Understand the Operation and Structural Advantages of Paguflex
Paguflex® is a medium torsional stiffness coupling that excels in control and light-to-medium load applications. Key advantages:
- Double Loop/Twisted-8 Design: This unique elastic geometry flexes in multiple directions to compensate misalignments while maintaining high positioning repeatability and low recovery stress on adjacent bearings.
- Zero Backlash Torque Transmission: Crucial for encoders, servomotors, and machine tools. Paguflex transmits torque by controlled elastomer compression, eliminating angular play typical of mechanical couplings.
- Vibration and Noise Absorption: The elastic element damps torsional vibrations and harmonics from the motor or load, reducing structural noise and prolonging component life.
- Maintenance-Free: The non-metallic construction eliminates the need for lubrication or periodic adjustments.
2. Define Load Requirements and Service Factor (Fs)
Proper selection cannot be based on nominal power alone. Correct sizing must include safety margins:
a) Torque Calculation and Importance of Service Factor (Fs)
Calculate nominal torque and apply a Service Factor (Fs). The coupling must be selected for a Selection Torque greater than the motor’s nominal torque:
Selection Torque = Nominal Torque × Fs
Fs varies by load type:
- Low Fs (1.0–1.2): Uniform loads (e.g., centrifugal pumps).
- Medium Fs (1.3–1.5): Variable loads (e.g., mixers, light machine tools).
- High Fs (1.6–2.0+): Heavy loads, frequent shocks, rapid starts/stops (e.g., presses, reciprocating compressors).
Tip: Always choose a coupling with a Selection Torque above the average service torque—oversizing by 20–30% provides better fatigue life for the elastomer.
b) Type of Motion and Dynamic Stiffness
Analyze the duty cycle: motion with frequent reversals or high inertia requires an elastic element with excellent reverse bending fatigue resistance—properties that Hytrel® maintains under repeated dynamic stress.
3. Dimensional Considerations and Installation Space
Dimensional verification is essential for installation and maintenance:
- Distance Between Shaft Ends (DBSE): The space between shaft ends determines overall coupling length. The double-loop design often allows compact axial installation.
- Outer Diameter: Ensure the coupling’s outside diameter doesn’t interfere with guards or nearby structures.
- Ease of Mounting: A major advantage of Paguflex is the ability to install or replace the elastic element without moving metal hubs or adjacent machinery—reducing machine downtime.
4. Check Shaft Sizes and Tolerances
Proper hub-to-shaft fit prevents fretting and false wear:
- Shaft Diameters and Bores: Verify actual shaft diameters (often with tolerance like 12 mm H7). Paguflex offers a wide range of metric and inch bores, including combinations (e.g., 1/4″ × 5/16″) to pair heterogeneous components.
- Manufacturing Tolerances: For precision (e.g., servomotors), hubs need tight concentric tolerance (H7) to maintain zero backlash.
- Fixing and Hub Type: In addition to set-screw fixation, consider clamping hubs for higher torque and rotational speed applications.
5. Analyze Misalignment Limits and Their Consequences
Paguflex provides excellent compensation, but operating limits must not be exceeded:
Typical Paguflex Misalignment Limits:
- Angular: up to ±15° — exceeding this leads to overheating and accelerated elastomer fatigue.
- Radial: around 3 mm — excessive radial load generates harmful forces on bearings and shortens life.
- Axial: approximately 10–20 mm — exceeding this causes axial stress and potential decoupling.
Prevention Strategy: Choose a version (standard, PLUS, or specific series) that keeps operational misalignment under ~50% of the manufacturer’s maximum to maximize life.
6. Consider Environmental Conditions and Chemical Compatibility
Operating environment greatly influences elastic element life:
- Extreme Temperatures: Beyond +85 °C or below -30 °C demands Hytrel® (tolerates approx. -40 °C to +100 °C).
- Chemical Exposure: Oils, solvents, hydrocarbon environments — Hytrel® or high-resistance polyurethane is recommended.
- Ozone/UV Exposure: Outdoor or inverter environments produce ozone — Hytrel® offers better ozone resistance.
- Humidity/Corrosion: In marine or food processing (CIP/SIP) environments, stainless hubs prevent oxidation.
- Electrical Insulation: In systems with inverters or stray currents, isolated hub versions exploit elastomer dielectric properties.
7. Decide on Mounting: Keyway vs. Clamping
Mounting choice depends on torque and precision:
- Set Screw (without keyway): Suitable for very low-torque applications (encoders, instruments).
- Keyway: Essential for higher torque, frequent reversals, or shock loads — ensures torque transmission by shear, not friction.
- Clamping Hub: Preferred for high speed and precision; provides uniform shaft grip without keyway, eliminating backlash due to key play.
8. Evaluate Temperature, Shore Hardness and Aging
Temperature affects elastomer mechanical properties:
- Thermal Aging: Extended heat exposure accelerates polyurethane polymerization and stiffening, reducing damping capacity.
- Shore Hardness: Higher Shore hardness (e.g., 92A–98A) increases torsional stiffness (better precision) but reduces damping capacity.
- Operational Range: For operations near high temperature limits (>80 °C), Hytrel® ensures better stability of elastic properties.
9. Time Test: Durability and Total Cost of Ownership (TCO)
Choosing Paguflex® is a long-term economic decision:
- Reduced Maintenance: No lubrication and maintenance-free nature reduce interventions.
- System Life: Couplings isolate and compensate flaws, reducing the need to replace bearings and other parts. Well-sized Paguflex acts as a “sacrificial” but easy-to-replace component, protecting main assets.
- Standardization: Wide range of bores eases spare part standardization across varied machinery.
10. Summary of the Paguflex Selection Guide
Step by step guide on how to choose the right double-loop coupling:
| Step | What to Do | Key Decision Criterion | Purpose |
|---|---|---|---|
| 1️⃣ Load Analysis | Calculate Nominal Torque × Fs | Type of load (uniform/shock) and Service Factor. | Size the coupling to withstand torque peaks. |
| 2️⃣ Elastic Material | Check Temperature and Chemical Compatibility. | Use Hytrel® if T > 80 °C, or in presence of ozone, high fatigue, or frequent reversals. | Ensure long service life in harsh environments. |
| 3️⃣ Mounting / Fixing | Determine whether a keyway is required. | Keyway for high torque/reversals; Clamping for zero backlash and precision. | Guarantee reliable torque transmission. |
| 4️⃣ Bores | Check shaft diameters and tolerances. | H7 bores and concentric hubs for maximum precision. | Prevent play and premature wear. |
| 5️⃣ Misalignment Compensation | Estimate operational misalignments. | Paguflex PLUS or softer element for larger misalignments. | Protect bearings from excess radial load. |
Conclusion: How to Choose the Flexible Coupling
Selecting the right Paguflex® double-loop coupling means opting for a solution that optimally balances torsional stiffness, damping ability, and misalignment tolerance. To precisely select and optimize machine life and total cost of ownership, technical specialist support is essential. Larosi is available to assist with technical configuration, offering metric, inch, stainless hubs, or Hytrel® variants.
Contact us for tailored technical advice or a personalized quote.
Learn more about Paguflex
Functions and Applications of Elastic Shaft Couplings
Material and Specifications of Elastic Shaft Couplings
Origin, Evolution, and Warranty of the Registered Trademark PAGUAG PAGUFLEX
New Paguflex Counters Displays