Date: 2025 05-May 01

Clayton 0:

How to properly size a flex coupling between a rigid motor shaft and a pump shaft with a single floating bearing

ChatGPT 0:

To properly size a flexible coupling between a rigid motor shaft and a pump shaft with a single floating bearing, you need to consider both mechanical performance requirements and alignment tolerances. Here’s a structured approach:

⚙️ 1. Understand the Application Parameters

Gather the following:

• Power transmitted (kW or HP)

• Shaft speeds (RPM)

• Shaft diameters (motor and pump)

• Type and magnitude of misalignment:

  • Angular

  • Parallel (radial)

  • Axial movement (important with a floating bearing)

• Distance between shaft ends (DBSE)

• Duty cycle and start/stop frequency

• Environmental factors (temperature, dust, fluids, etc.)

📐 2. Account for the Floating Bearing

A single floating bearing on the pump shaft means:

• The pump shaft will move axially during thermal expansion or load changes.

• The coupling must accommodate axial movement without transmitting axial loads to the motor bearing (which is likely fixed).

• This rules out rigid or limited-flex couplings.

✅ Use a coupling with high axial flexibility, such as:

• Double-flex disc couplings

• Bellows couplings

• Flexible grid couplings

• Elastomeric (jaw or tire) couplings (depending on precision needed)

🔧 3. Coupling Sizing Procedure

Step A: Calculate Required Torque

T=9550×PNT = \frac{9550 \times P}

Where:

• TT = Torque (Nm)

• PP = Power (kW)

• NN = Speed (RPM)

Apply a service factor (SF) based on:

• Load type (uniform, moderate shock, heavy shock)

• Start/stop frequency

Tdesign=T×SFT_{design} = T \times SF

Step B: Match Shaft Diameters

Ensure the coupling can:

• Fit both shaft diameters

• Be bored or bushed accordingly

Step C: Check Misalignment Tolerance

• Angular: usually < 1°

• Parallel: up to a few mm

• Axial float: ensure it handles the full axial movement range of the pump shaft

📏 4. Final Selection

Choose a coupling that:

• Transmits at least the design torque

• Fits both shafts (or is available with reducers)

• Accommodates the expected misalignment

• Allows enough axial float (floating bearing movement)

🧰 Tip: Safety and Installation

• Proper alignment during installation reduces coupling and bearing wear.

• Use dial indicators or laser alignment tools if high precision is needed.

• Check mounting length and clearance for axial movement.

Would you like a quick example using specific values?

Clayton 1:

https://www.rexnord.com/contentitems/techlibrary/documents/cp5-004_productsheet

Here is a link describes Flak Steelflex Grid couplings, one of the products my water treatment plant is considering to retrofit a shaft.

We need to choose a proper type and size of flexible coupling.

However, this is not an isolated issue. Our motor, a E-M Large Vertical Induction Motor suffers from vibration. This is likely due to the motor being oversized for the original concrete pile support. The Centrifugal raw sewage pump is the same, but the motor that drives it has been changed to this larger E-M, with a VFD, compared to the original version, which may have had a mag drive or direct drive.

Also, we are worried that a highly flexible coupling, included with vibration and misalignment, could cause the pump shaft to "flop".

ChatGPT 1:

Thanks for the detailed background—that’s critical for making a smart coupling choice. Based on what you've shared, here's a focused engineering breakdown:

🧩 Key Constraints from Your Setup

1. Motor Upgrade Implications

• The oversized vertical E-M on a possibly under-rated concrete pile is introducing structural resonance or harmonic vibration.

• VFDs often contribute additional torsional pulsation and shaft current issues.

2. Pump-Side Weak Point

• The single spherical roller bearing on the pump is floating—great for thermal expansion, but poor at axial constraint or resisting bending moments from misalignment or unbalanced loads.

• A flexible coupling that transmits too much misalignment or shock will cause shaft deflection, possibly leading to rotor instability or "flopping".

🔧 Rexnord Falk Steelflex Grid Coupling Consideration

The Steelflex Grid is a medium-flex coupling designed to:

• Absorb shock and dampen vibration (good)

• Handle moderate misalignment (good)

• Transmit torque efficiently (good)

• Have limited axial float (can be a concern if axial movement is large)

🟡 Risk: It’s more forgiving than a rigid coupling, but still transfers some lateral and angular loads. If your shaft alignment is imperfect and the pump bearing is sensitive, it may cause the rotor to deflect dynamically.

✅ Engineering Recommendation

Given your situation, prioritize a coupling that:

• Dampens torsional vibration (from VFD + motor)

• Minimizes radial and angular force transmission

• Allows axial float without pushing into pump

🔍 Options Comparison:

🧠 Strategy for Your Retrofit

1. Measure Actual Misalignment & Axial Float Range

   • If you’re >0.5° angular or >0.5 mm radial misalignment, grid couplings may overburden the pump bearing.

2. Conduct a Vibration Survey

   • Confirm vibration source—VFD drive train harmonics? Structural resonance? Motor imbalance?

   • Consider modal analysis of the motor/pile foundation.

3. Consider Intermediate Shaft or Spacer

   • A spacer with flexible coupling ends may decouple vibration and protect the pump from misaligned forces.

   • Helps isolate pump from axial loads due to thermal drift or misalignment.

4. Try a Torsionally Soft, Axially Compliant Coupling First

   • You might trial an elastomeric coupling with good misalignment tolerance and torsional damping.

   • Evaluate motor and pump side bearing loads post-install.

Would you like help comparing specific Rexnord or Lovejoy part numbers based on your motor torque and shaft sizes?

Clayton 2:

We do not want to oversize our grid coupling, because it should serve as a mechanical fuse, so that neither shaft is damaged.

Please provide a roadmap for identifying a properly-rated coupling, based on torque and shaft input values. Provide modular equations, the high-level theory overview, and typical parlance and jargon necessary for shopping for properly-rated couplings.

ChatGPT 2:

Excellent—treating the coupling as a mechanical fuse is a smart, preventive design approach, especially when you have sensitive equipment like a pump with a floating bearing. Below is a complete, structured roadmap to help you select and specify a properly-rated flexible coupling—specifically a grid-type like the Rexnord Falk Steelflex—without oversizing it.

🧠 High-Level Theory Overview

A coupling must:

• Transmit torque between shafts

• Accommodate misalignment (angular, radial, axial)

• Dampen vibration or shock

• Act as a fuse: it should fail before damaging shafts or equipment

Oversizing a coupling reduces its compliance and may prevent it from failing under fault conditions—bad for fuse logic.

🧰 Roadmap to Proper Coupling Sizing (Grid-Type)

🔹 1. Gather Input Parameters

🔹 2. Calculate Design Torque

First, calculate nominal torque:

🧮 Nominal Torque Equation

T=9550⋅PNT = \frac{9550 \cdot P}

Where:

• TT = Torque in Nm

• PP = Power in kW

• NN = Speed in RPM

• Use 63025 instead of 9550 if power is in HP

🔹 3. Apply Service Factor

Account for dynamic loads using a Service Factor (SF):

Tdesign=T⋅SfT_{\text{design}} = T \cdot S_f

Typical Service Factors (per AGMA / Rexnord):

🟨 For your VFD + raw sewage pump setup, consider S.F. = 1.5 to 1.75

🔹 4. Match Coupling Based on Torque Rating

Use the manufacturer's coupling catalog (e.g., Rexnord CP5-004) to:

• Identify a model that has a rated torque ≥Tdesign\geq T_{\text{design}}

• Make sure the bore sizes can accommodate both shaft diameters

• Ensure the max misalignment tolerances are compatible

Avoid going too far above TdesignT_{\text{design}}; this ensures it can still act as a mechanical fuse.

🔹 5. Confirm Misalignment and Axial Float

Check:

• Max angular misalignment (°)

• Max parallel (radial) misalignment (mm or in)

• Max axial float (mm or in)

🧭 These values must cover real shaft movements plus installation tolerances.

🔹 6. Shaft Fit and Keying

Verify:

• Bore range: Will the coupling fit your shaft diameters?

• Keyway sizes (match to shaft standard—e.g., ANSI or DIN)

• Interference or clearance fit needed for your operating conditions

You may need bushed hubs (e.g., Taper-Lock or QD style).

🔹 7. Check Environment and Safety Requirements

Environmental conditions may require:

• Stainless steel or epoxy coatings (corrosion)

• Dust covers or guards (safety)

• Non-lubricated variants if grease leakage is a concern

🛠️ Terminology & Jargon When Sizing or Purchasing

📊 Sample Example

Let’s say:

• Power = 100 kW

• Speed = 1475 RPM

• SF = 1.5 (moderate shock, VFD, sewage)

• Shaft diameters = 90 mm (motor), 80 mm (pump)

Step 1:

T=9550⋅1001475=647.5 NmT = \frac{9550 \cdot 100}{1475} = 647.5 , \text

Step 2:

Tdesign=647.5⋅1.5=971.3 NmT_{\text{design}} = 647.5 \cdot 1.5 = 971.3 , \text

Now:

• Choose a grid coupling rated for ~1000 Nm

• Make sure it fits 80 mm and 90 mm shafts (or order with specific bores)

• Confirm it handles expected misalignment and axial movement

Would you like me to help you match this to Rexnord Steelflex part numbers or sizes (e.g., T10, T20, etc.) based on your real pump/motor specs?

Clayton 3:

Sure.

ChatGPT 3:

Based on your application—a centrifugal raw sewage pump driven by a large vertical induction motor with a VFD—and your calculated design torque of approximately 971 Nm, selecting a Rexnord Falk Steelflex Grid Coupling that acts as a mechanical fuse requires careful consideration.

🔧 Recommended Coupling Size

Given your design torque, the Falk Steelflex 1050T10 or 1050T20 coupling is suitable: (Rexnord Falk - Steelflex Grid Couplings - HVH Industrial Solutions)

• Torque Rating: 3,850 in-lbs (~435 Nm)

• Bore Range: 0.5 to 1.875 inches

• Design: Double-flexing, close-coupled (1020T X 7/8 G-Flex Grid Coupling Hub, Rexnord Falk - Steelflex Grid Couplings - HVH Industrial Solutions, [PDF] Falk Steelflex Grid Couplings)

However, this torque rating is below your calculated design torque. To ensure the coupling can handle the required torque while still serving as a mechanical fuse, consider the Falk Steelflex 1060T10 or 1060T20 coupling:

• Torque Rating: 6,050 in-lbs (~684 Nm)

• Bore Range: 0.75 to 2.125 inches (1020T X 7/8 G-Flex Grid Coupling Hub)

This selection provides a balance between handling the operational torque and acting as a mechanical fuse in overload conditions. ([PDF] Falk Steelflex® Type T10 & T20 (Page 1 of 9) 428-111 1 ... - APRIL)

⚙️ Coupling Type Selection

• T10: Horizontal split cover, facilitating easier maintenance in horizontal installations.

• T20: Vertical split cover, suitable for vertical installations.

Choose the type that aligns with your equipment's orientation and maintenance preferences.

📌 Additional Considerations

• Service Factor: Ensure the selected coupling's torque rating, when multiplied by the service factor (typically 1.5 to 1.75 for your application), exceeds your design torque.

• Misalignment Tolerance: Verify that the coupling can accommodate the expected angular, parallel, and axial misalignments.

• Environmental Conditions: Consider any environmental factors (e.g., exposure to sewage, temperature variations) that may affect coupling performance and material selection.

📘 Resources

For detailed specifications and installation guidelines, refer to the Falk Steelflex Grid Couplings Product Sheet.

If you provide specific shaft diameters and operational details, I can assist in confirming the optimal coupling size and type for your application.

Clayton 4:

What is the smallest piece of equipment that the T10 would be appropriate for? The largest? What about the comparable range of service concerning the T20?

ChatGPT 4:

The Rexnord Falk Steelflex Grid Couplings—T10 and T20 series—are designed to accommodate a wide range of torque capacities and shaft sizes, making them suitable for various applications. Here's an overview of their capabilities:

🔹 T10 Series (Horizontal Split Cover)

• Smallest Size – 1020T10:

  • Torque Rating: 460 lb-in (52 Nm)

  • Maximum Bore: 1.125 inches (28.6 mm)

  • Typical Applications: Small pumps, fans, or conveyors with low torque requirements. (Rexnord-Falk Acoplamiento de Engranajes | PDF | Horsepower | Mechanical Engineering, Falk Steelflex Grid Couplings Redefining Total Coupling Value - Flipbook by 55344 | FlipHTML5, Rexnord Falk Steelflex | PDF | Pump | Electrical Grid)

• Largest Size – 1260T10:

  • Torque Rating: 8,250,000 lb-in (932,000 Nm)

  • Maximum Bore: 20 inches (508 mm)

  • Typical Applications: Heavy-duty industrial machinery such as large compressors, crushers, or rolling mills. (Grid Couplings – PB Traders)

🔹 T20 Series (Vertical Split Cover)

• Smallest Size – 1020T20:

  • Torque Rating: 460 lb-in (52 Nm)

  • Maximum Bore: 1.125 inches (28.6 mm)

  • Typical Applications: Similar to T10, suitable for small-scale equipment with low torque demands. (Gear Couplings - Couplings | Rexnord, Falk Steelflex Grid Couplings Redefining Total Coupling Value - Flipbook by 55344 | FlipHTML5)

• Largest Size – 1170T20:

  • Torque Rating: 660,000 lb-in (74,570 Nm)

  • Maximum Bore: 10 inches (254 mm)

  • Typical Applications: Medium to large industrial equipment where vertical installation and ease of maintenance are priorities. (Grid Couplings – PB Traders)

Note: The T10 series offers a broader range of sizes and higher torque capacities compared to the T20 series. The choice between T10 and T20 often depends on installation orientation and maintenance preferences.

If you need assistance selecting the appropriate coupling size for your specific application, please provide details such as torque requirements, shaft sizes, and operational conditions.

Clayton 5:

Which T10 or T20 sizes /models (1020T, 1030T, .... ,1250T, 1260T) might be appropriate for an induction motor of size 250 horsepower or larger?

ChatGPT 5:

For an induction motor of 250 horsepower (HP) or larger, selecting an appropriate Falk Steelflex Grid Coupling (T10 or T20) involves matching torque capacity and bore size to the motor’s output and shaft dimensions. Here's how to narrow down the appropriate models:

🔧 Step 1: Estimate Motor Torque

Assume:

• Motor HP: 250 HP

• Speed (N): 1,800 RPM (typical for 4-pole induction motors on 60 Hz)

• Formula:

Torque (lb-in)=63025×HPRPM=63025×2501800≈8753 lb-in\text{Torque (lb-in)} = \frac{63025 \times \text{HP}}{\text{RPM}} = \frac{63025 \times 250}{1800} \approx 8753 , \text

Apply a Service Factor (SF) (e.g., 1.5–2.0) for shock loads, pump misalignment, or overloads:

Design Torque≈8753×1.5=13,130 lb-in (minimum)\text{Design Torque} \approx 8753 \times 1.5 = 13,130 , \text{lb-in} \text

📘 Step 2: Filter Suitable T10/T20 Sizes

Based on torque and bore capability:

📐 Bore Compatibility

Check your motor and pump shaft diameters. Most 250 HP motors use shaft diameters between 2.375" and 3.375", which aligns well with 1070T to 1090T sizes.

✅ Recommended Sizes for 250 HP+

Depending on service factor and shaft diameter:

• Minimum suitable: 1070T10 / 1070T20

• Typical: 1080T10 / 1080T20

• Heavy-duty or high SF: 1090T10 / 1090T20

Would you like a sizing chart or torque curve visualized for these models?

Clayton 5:

Our Raw Sewage Shaft is made of 1045 steel and have a small bore of 5 inches on the motor side, a small bore of 5.5 inches on the pump side, and is stepped up to a large bore of 6.5 inches in the middle. We have the option of replacing this shaft or instead designing a custom solution that involves a flexible coupling.

Max bore was provided in the Step 2 table above - what about minimum bore? What determines max bore? Is it the physical size of the coupling? How is the coupling tightened and secured to the coupled shafts on each side?

Also, i misspoke before on the rating - it is 1200 hp or 1500 hp!

Test speeds are 350 rpm to 440 rpm.

Clayton 6:

Perfect — that low operating speed (350–440 RPM) is a critical and very helpful detail. It shifts the coupling selection entirely. Here's why:

⚙️ Effect of Low Speed on Coupling Sizing

Coupling selection is based on torque, not just power, and lower RPM → higher torque for the same HP.

Let’s recalculate:

📌 For 1500 HP @ 440 RPM:

Torque=63025×1500440≈215,789 lb-in\text{Torque} = \frac{63025 \times 1500}{440} \approx 215,789 , \text

📌 For 1500 HP @ 350 RPM:

Torque=63025×1500350≈270,107 lb-in\text{Torque} = \frac{63025 \times 1500}{350} \approx 270,107 , \text

➡️ Design torque (with SF = 2.0):

Design Torque≈430,000–540,000 lb-in\text{Design Torque} \approx 430,000 – 540,000 , \text

🧱 Now Match That to Coupling Sizes

You are now well into the upper range of the Falk Steelflex line.

➡️ Minimum T10 size for your needs: 1130T10 or 1140T10, depending on how conservative your design torque and service factor are.

🔩 About Your Shaft Bores (5", 5.5", 6.5"):

• These are not unusual in the upper Falk range.

• 1130T or 1140T will comfortably accept bores in this range.

• You can also order dissimilar bore hubs — e.g., 5.5" bore on pump side and 5" on motor side — with the same coupling sleeve in the middle.

🛠️ Practical Tips:

1. ✅ Ask Rexnord or supplier for dual-bore hub compatibility for the 1130–1140T range.

2. ⚖️ Ensure grid size matches design torque, not just bore size.

3. 📈 At these torque levels, consider dynamic balancing and careful shaft alignment procedures, especially since your pump shaft has a single spherical roller bearing and could "flop" with too much misalignment or flex.

4. 🧩 You may still want the coupling to serve as a mechanical fuse — so avoid over-sizing beyond your design torque unless alignment and load dynamics are very unpredictable.

Would you like me to prepare a selection sheet template with all needed inputs (torque, speed, bore, service factor) so your team can compare vendor options side by side?

Clayton 7

Yes please provide a selection template / roadmap

Also provide links to relevant textbooks, sets of lecture notes, or other trustworthy and accessible technical sources which identify the relevant theory in terms of torque, power, shaft design, and machinery dynamics.

ChatGPT 7:

Here is a flexible coupling selection template and roadmap tailored to your 1500 HP, 350–440 RPM, large vertical motor & raw sewage pump system using couplings like the Rexnord Falk Steelflex Grid (T10/T20). It’s designed for practical use by your engineering team or a vendor rep.

🛠️ Flexible Coupling Selection Template (T10/T20 Steelflex Style)

📋 SECTION 1: Basic Application Data

⚙️ SECTION 2: Torque & Speed Calculations

Use these formulas to calculate required torque:

Nominal Torque:

Tnominal=63025×HPRPMT_{\text{nominal}} = \frac{63025 \times \text{HP}}{\text{RPM}}

Design Torque:

Tdesign=Tnominal×Service FactorT_{\text{design}} = T_{\text{nominal}} \times \text

🔩 SECTION 3: Shaft & Bore Requirements

📐 SECTION 4: Coupling Selection Table

🔧 SECTION 5: Mounting & Installation Preferences

📦 SECTION 6: Vendor Quote Checklist

✅ Include with vendor RFQ:

• Completed torque and bore worksheet

• Coupling model size preference (e.g. 1140T)

• Material and finish needs (e.g. stainless grid)

• Preferred mounting (tapered bushing vs. keyed bore)

• Any custom sleeve or spacer requirements

• Alignment expectations or flange standards

📌 Final Notes

• If you are designing this coupling to serve as a mechanical fuse, ensure it fails at a known, non-catastrophic torque below shaft yield strength. Consider requesting torque-limiting analysis from Rexnord or a third-party consultant.

• If you're worried about shaft flopping due to bearing freedom, you may want a T20 style with spacer to minimize transmission of angular misalignment.

Would you like a printable PDF version of this template or an Excel calculator based on your values?

Clayton 8

Yes please provide the Excel Calculator and the PDF.

Also provide links to relevant textbooks, sets of lecture notes, or other trustworthy and accessible technical sources which identify the relevant theory in terms of torque, power, shaft design, and machinery dynamics.

Chat GPT 8:

The Excel calculator and PDF template for your flexible coupling selection are ready for download:

• Download the Excel calculator template

• Download the PDF version of the template

📚 Relevant Textbooks and Resources

To further explore the theory related to torque, power, shaft design, and machinery dynamics, here are some trustworthy and accessible sources:

1. "Mechanical Engineering Design" by J.E. Shigley

   • Widely used for understanding basic mechanical design principles, including shaft and coupling design, torque, and power transmission. It includes examples of machinery dynamics and design practices for rotating equipment.

2. "Design of Machine Elements" by V.B. Bhandari

   • Covers topics related to the design of mechanical components such as couplings, shafts, bearings, and gears, with a detailed focus on power and torque calculations and design optimization.

3. "Introduction to Fluid Mechanics" by Robert W. Fox and Alan T. McDonald

   • While more focused on fluid dynamics, this textbook provides valuable insight into the flow dynamics in pump systems, which are crucial for selecting the right couplings and understanding pump-shaft interactions.

4. "Vibration of Mechanical and Structural Systems" by Maher N. Bassiouny

   • Offers in-depth discussion of vibration theory, which is key to understanding the interaction between couplings, motors, and shafts, especially in systems with misalignment or potential vibration issues.

5. Lecture Notes on Machine Design from MIT OpenCourseWare

   • Free online access to detailed lecture notes and resources on machine design principles, including shaft and coupling selection and power transmission systems.

   • MIT OpenCourseWare - Machine Design

These resources will help you deepen your understanding of mechanical design principles and coupling selection. Let me know if you'd like more specific information on any of these topics!