Full vs Half Gear Coupling: Selection Guide
When to choose full gear or half gear couplings for your drive application.
A full gear coupling is flexible at both ends; a half gear coupling is flexible at one end only. A full gear coupling has two gear hubs, so it takes angular and parallel misalignment plus axial float; a half gear coupling has one gear hub and a rigid flange hub, so it takes angular misalignment at the gear end only, not parallel offset.
SMI makes both in the same 20 sizes, FG-100 to FG-119, with the same torque ratings and speed limits.
Full vs half gear coupling: comparison table
| Full gear coupling | Half gear coupling | |
|---|---|---|
| Construction | Two hubs with crowned external teeth, meshing with internal teeth in the sleeve: two gear meshes | One hub with crowned external teeth and one rigid flange hub (no teeth): one gear mesh |
| Flexible ends | Both | One; the other is a rigid flange |
| Angular misalignment | Up to 1.5° per gear mesh (3° total) | Up to 1.5° at the single gear mesh |
| Parallel offset | Yes | No, not on its own |
| SMI sizes | FG-100 to FG-119 (20 sizes) | FG-100 to FG-119 (same 20 sizes) |
| Rated torque | 559 to 1,666,744 Nm | 559 to 1,666,744 Nm |
| Maximum speed | 7,600 rpm at FG-100, down to 500 rpm at FG-119 | 7,600 rpm at FG-100, down to 500 rpm at FG-119 |
| Maximum bore | D1: 35 mm (FG-100) to 710 mm (FG-119) | Gear hub D1: 35 mm to 710 mm; rigid flange hub D2: 45 mm (FG-100) to 260 mm (FG-110) |
| Order it for | Normal motor-to-machine drives with real misalignment | Floating-shaft and flanged arrangements where the second flexible element is elsewhere in the drive |
| Size chart and product page | Full gear size chart · Full gear coupling | Half gear size chart · Half gear coupling |
Both types are sized the same way: design torque = 9,550 × kW ÷ rpm × service factor, then the first FG size whose rated torque covers it. The selection guide gives the service factor for each type of driven machine.
Full Gear Coupling
Design Description
A full gear coupling features:
- Two hubs with crowned external teeth
- One or two sleeves with internal straight teeth
- Two gear meshes providing maximum flexibility
- Complete misalignment accommodation in all planes
Key Advantages
| Advantage | Benefit |
|---|---|
| Axial movement | Accommodates thermal expansion |
| Shock absorption | Better load distribution |
| Flexibility | Handles varying operating conditions |
Best Applications
- Rolling mills with reversing loads
- Equipment with significant thermal growth
- Applications with foundation settlement
- Systems requiring maximum reliability
Half Gear Coupling
Design Description
A half gear coupling features:
- One hub with crowned external teeth
- One rigid flange hub (no teeth)
- Single sleeve connecting to gear hub
- One gear mesh providing flexibility
Key Advantages
| Advantage | Benefit |
|---|---|
| Compact design | Shorter overall length |
| Reduced inertia | Lighter construction |
Best Applications
- Floating shaft arrangements
- Flanged drives where one end is already rigidly located
- Drive trains where the second flexible element is provided elsewhere
Selection Criteria
Choose Full Gear When:
High misalignment expected
- Foundation settling likely
- Thermal growth significant
- Dynamic deflection present
Maximum reliability required
- Critical production equipment
- Limited maintenance access
- High consequence of failure
Shock and reversing loads
- Rolling mill drives
- Crusher applications
- Heavy industrial machinery
Choose Half Gear When:
One end is already rigidly located
- A flanged drive
- A floating shaft arrangement
The second flexible element is provided elsewhere
- Parallel offset is taken by another flexible element in the drive train
Do not choose a half gear coupling to save cost: it is not a cheaper substitute for a full one on a normal motor-to-gearbox drive, because it cannot accommodate parallel offset by itself.
Installation Considerations
Full Gear Coupling
- More forgiving of alignment errors
- Requires proper axial gap setting
- Both gear meshes must be lubricated
- Longer installation time
Half Gear Coupling
- Demands better initial alignment
- Simpler axial positioning
- Single lubrication point
- Faster installation
Maintenance Differences
Both types follow the same routine: daily and monthly checks, re-greasing at the interval the duty calls for, an annual alignment check and a strip-down every 3–5 years. The half gear coupling has a single lubrication point; on a full gear coupling both gear meshes must be lubricated. The gear coupling maintenance schedule sets out every check and interval.
Common Mistakes
Selecting Full When Half Would Work
- Increased maintenance effort
- Excess weight and length
Selecting Half When Full Required
- Premature wear from edge loading
- Reduced coupling life
- Higher vibration levels
Contact SMI for assistance selecting between full and half gear configurations for your specific application.
Which one to order from us
The comparison table at the top of this page sets the two side by side, with links to each size chart and product page.
The mistake to avoid: a half gear coupling is not a cheaper substitute for a full one on an ordinary drive. It cannot accommodate parallel offset by itself, so fitting one where a full coupling belongs puts that offset into the bearings instead. If you are unsure which arrangement you have, send a photograph of the drive and we will tell you.
If you want the geometric reason a second mesh is what enables parallel offset in the first place, our technical library works through why two meshes accommodate offset and one cannot.
Frequently asked questions
What is the difference between a full and a half gear coupling?
A full gear coupling has a flexible gear mesh at both ends, so it accommodates angular and parallel misalignment plus axial float. A half gear coupling has one flexible gear end and one rigid flange, so it handles angular misalignment at one end only and cannot accommodate parallel offset on its own.
When is a half gear coupling the right choice?
Where one end connects to something already rigidly located — a floating shaft arrangement, a flanged drive, or where the second flexible element is provided elsewhere in the drive train. It is not a cheaper substitute for a full coupling on a normal motor-to-gearbox drive.
What sizes do you make?
SMI full gear couplings run FG-100 to FG-119, rated 559 to 1,666,744 Nm, with finished bores up to 710 mm. Half gear couplings share the same FG-100 to FG-119 sizes and ratings, with one flexible hub and one rigid flange.
Do full and half gear couplings use the same size chart?
For ratings, yes: both use the FG-100 to FG-119 series with the same torque ratings and speed limits. The half gear coupling chart adds the rigid flange hub — its maximum bore D2, from 45 mm at FG-100 up to 260 mm at FG-110, and the hub dimensions C1 and ØB1, which the spec sheet lists up to FG-110.
Super Mech Industries — Odhav, Ahmedabad, Gujarat 382415, India. Phone: +91 63510 70577. Email: smigvs@gmail.com. ISO 9001:2015 certified.