Conveyor Coupling: Which Type and How to Size a Gear Coupling
How to select gear couplings for conveyor drives: torque, start/stop duty, and alignment.
To select a gear coupling for a conveyor system, calculate design torque as 9,550 × kW ÷ rpm × service factor and choose a coupling rated above it. A 75 kW motor at 1,480 rpm with a service factor of 2.0 needs 968 Nm. Size a gearbox-to-pulley coupling separately: after a 20:1 reducer, torque is about 20 times higher.
Which coupling type suits a conveyor?
The coupling depends on where it sits in the drive and how much shock the conveyor sees. The low-speed shaft between gearbox and head pulley carries the most torque and is where gear and chain couplings are used. The high-speed shaft between motor and gearbox carries far less torque, and on light conveyors a flexible elastomer coupling is usually enough there.
| Coupling | Where it fits on a conveyor | SMI range |
|---|---|---|
| Full gear coupling | Gearbox to head pulley, and motor to gearbox on heavy, long or loaded-start conveyors | FG-100 to FG-119, 559 to 1,666,744 Nm, bores to 710 mm |
| Chain coupling | Low-speed, high-torque positions where a compact, easily split coupling is wanted | 3812 to 40028, 99 to 717,000 Nm, bores to 670 mm |
| RRS pin bush coupling | Motor to gearbox where the rubber bushes should absorb starting shock | See the product page for sizes |
| HRC coupling | Motor to gearbox on small conveyors with smooth loads | SHRC 70 to SHRC 280, 31 to 3,150 Nm, bores to 130 mm |
| Star coupling | Light conveyors and small drives | SJC 75 to SJC 295, 9.8 to 1,279 Nm, bores to 100 mm |
| Brake drum gear coupling | Inclined or reversing conveyors that need a brake on the drive | See the product page for sizes |
Whatever the type, size it on design torque, 9,550 × kW ÷ rpm × service factor, using 1.25 to 1.50 for a belt conveyor from the coupling service factor chart, and add 0.25 if it starts more than four times an hour.
Conveyor Drive Challenges
Operating Conditions
Conveyor couplings must handle:
- Frequent starts and stops causing torque spikes
- Loaded starts with belt carrying material
- Environmental exposure to dust, moisture, temperature
- Continuous operation for extended periods
- Misalignment from thermal growth and foundation issues
Coupling Location
Typically installed between:
- Motor and gearbox (high speed)
- Gearbox and head pulley (high torque)
- Both positions in large systems
Selection Process
Step 1: Calculate Design Torque
Formula: Design Torque = (Motor Power × 9550) / Speed × Service Factor
Example:
- Motor: 75 kW at 1480 RPM
- Service factor: 2.0
- Design torque: (75 × 9550) / 1480 × 2.0 = 968 Nm
Step 2: Determine Service Factor
| Conveyor Type | Service Factor |
|---|---|
| Light duty, uniform load | 1.5 |
| General purpose | 1.75-2.0 |
| Mining, heavy loading | 2.0-2.5 |
| Frequent starts, loaded | 2.5-3.0 |
Step 3: Consider Speed and Size
Match coupling to position in drivetrain:
| Position | Speed | Torque | Coupling Type |
|---|---|---|---|
| Motor-Gearbox | High | Lower | Standard full gear |
| Gearbox-Pulley | Low | Higher | Heavy duty gear |
Special Considerations
Starting Torque
Conveyors require higher starting torque:
- Belt and material inertia
- Static friction breakaway
- Motor starting characteristics
Rule: Starting torque may reach 200-250% of running torque
Alignment Challenges
Conveyors experience alignment changes (see the gear coupling alignment checklist) from:
- Thermal expansion of long structures
- Belt tension variations
- Foundation settling
- Pulley wear and bearing play
Solution: Select couplings with adequate misalignment capacity
Environmental Factors
| Environment | Coupling Features |
|---|---|
| Dusty | Enhanced sealing, labyrinth design |
| Wet | Corrosion-resistant finish |
| Hot | High-temperature grease |
| Cold | Low-temperature grease, ductile materials |
Recommended Coupling Types
Head Drive Applications
High-torque, low-speed position:
- Full gear couplings
- Heavy-duty designs
- Robust sealing for dust
Motor-Gearbox Connection
High-speed, moderate torque:
- Standard gear couplings
- Good for acceleration loads
- Precision balanced for high RPM
Take-Up or Tail End
If driven:
- Moderate duty couplings
- Spacer type for maintenance access
- Good dust exclusion
Installation Best Practices
Alignment Targets
| Parameter | Maximum Allowable |
|---|---|
| Angular | 0.5° per gear mesh |
| Offset | 50% of allowable |
| Axial gap | Per coupling spec |
Mounting Sequence
- Install motor and gearbox on machined pads
- Mount coupling halves to shafts
- Perform rough alignment
- Complete final cold alignment
- Record readings for reference
- Recheck alignment after heat stabilization
Maintenance Schedule
Routine Inspection
| Interval | Check |
|---|---|
| Daily | Unusual noise, vibration, leaks |
| Weekly | Temperature during operation |
| Monthly | Visual condition of seals |
| Quarterly | Relubrication, alignment verification |
| Annually | Comprehensive inspection |
Common Issues
| Symptom | Possible Cause | Action |
|---|---|---|
| High temperature | Misalignment, low grease | Realign, relubricate |
| Vibration | Coupling wear, imbalance | Inspect, replace worn parts |
| Noise | Dry operation, tooth wear | Lubricate, check wear |
| Grease leakage | Seal failure, overfilling | Replace seals, correct fill |
SMI Conveyor Coupling Solutions
We offer gear couplings specifically suited for conveyor applications:
- Full and half gear couplings in 20 sizes, FG-100 to FG-119
- Rated 559 to 1,666,744 Nm, with bores up to 710 mm (size chart)
- Enhanced sealing options
- Quick delivery for standard sizes
Contact SMI for conveyor coupling selection support and quotations.
Frequently asked questions
Where does the coupling sit on a conveyor drive?
Usually between motor and gearbox, and sometimes between gearbox output and the drive pulley shaft. The torque is very different in the two positions — a 15 kW motor at 1,440 RPM is about 100 Nm at the motor shaft but roughly 2,000 Nm after a 20:1 reducer. Always state which position you are specifying for.
What service factor should a conveyor drive use?
Around 1.25 for a uniformly fed belt running up to ten hours a day, 1.4 to 1.5 for continuous running with non-uniform feed, and 1.5 for bucket elevators. Add more if the belt regularly starts loaded, since breakaway torque is far higher than running torque.
Should I use a shaft mounted reducer instead of a foot-mounted gearbox and coupling?
On conveyors and elevators, often yes. A shaft mounted speed reducer sits directly on the driven shaft with a torque arm, removing the base plate, the coupling and the alignment job. We manufacture SMSR units in 5:1, 13:1 and 20:1 ratios covering 0.5 to 50 kW.
Super Mech Industries — Odhav, Ahmedabad, Gujarat 382415, India. Phone: +91 63510 70577. Email: smigvs@gmail.com. ISO 9001:2015 certified.