Bellows Design & Selection Guide
Specifying the right edge welded bellows requires balancing five interdependent parameters: spring rate, stroke length, pressure rating, cycle life, and envelope dimensions. This guide walks you through each parameter so you can define requirements that lead to an optimal design — or simply send us your constraints and we’ll do it for you. For material-specific performance data, refer to our Material Selection Guide; for standard dimensions, see the AT-Series Catalog.
The Five Key Design Parameters
1. Spring Rate
The spring rate (k) defines how much force is required to compress or extend the bellows by a given distance. It is the most fundamental bellows parameter because it directly affects actuation force, natural frequency, and system stiffness.
Formula:
k = E × n × t³ / (D² × N)
Where:
- E = Modulus of elasticity (material-dependent)
- n = Number of convolutions
- t = Wall thickness (0.05mm–0.5mm)
- D = Mean diameter
- N = Geometric constant based on convolution profile
Practical ranges:
- Soft bellows: 0.5–5 N/mm — for low-force actuators and sensitive
instruments - Medium bellows: 5–50 N/mm — general vacuum and industrial use
- Stiff bellows: 50–500+ N/mm — high-pressure and structural
applications
Tip: If you’re replacing an existing bellows, always measure the spring rate empirically rather than calculating from dimensions. Manufacturing tolerances can shift the actual rate by ±10% from theoretical values.
2. Stroke Length
Stroke defines the total axial movement the bellows must accommodate — either compression, extension, or both.
| Stroke Type | Max % of Free Length | Typical Use Case |
|---|---|---|
| Compression Only | ~60% | Gate valve seals, linear actuators |
| Extension Only | ~25% | Tension-loaded vibration isolators |
| Bi-Directional (±) | ±30% | Motion feedthroughs, wobble compensators |
Critical rule: Always specify stroke as a percentage of the bellows free length (the active convolution length, excluding end fittings). Operating beyond 60% compression dramatically reduces cycle life.
3. Pressure Rating
Edge welded bellows can be designed for vacuum service (external pressure) or positive pressure (internal). Most Alpha Technology bellows are rated for:
- Vacuum service: UHV to 10⁻¹⁰ Torr (helium
leak-tested) - Positive pressure: Up to 10 bar depending on diameter and
wall thickness - Burst pressure: Typically 3-4x rated working
pressure
Pressure rating is inversely related to flexibility — thicker walls = higher pressure tolerance but stiffer spring rate. If you need both flexibility and pressure resistance, we can explore nested bellows configurations or reinforced end convolutions. See our Process & Design page for manufacturing details.
4. Convolution Geometry
| Profile | Shape | Best For | Trade-Off |
|---|---|---|---|
| Flat Diaphragm | Parallel discs, straight weld | Space-constrained, low stroke | Limited stroke capability |
| Nested Ripple | Pre-formed wave profile | General purpose, best balance | Slightly higher cost |
| Deep-Drawn | Cup-shaped convolutions | Maximum stroke per convolution | Wider OD for same ID |
For most applications, the nested ripple profile provides the best combination of stroke, cycle life, and dimensional efficiency. We recommend it as the default unless your constraints dictate otherwise. For a complete comparison of edge welded vs. hydroformed bellows, visit our Technical Guide.
5. End Fittings
End fittings determine how the bellows integrates into your assembly. Common configurations:
- Weld stubs (butt weld): For direct welding into vacuum
chambers or flanges. Most compact. - KF / ISO flanges: Standard vacuum flange interfaces for
modular assembly. - Custom flanges: Square, rectangular, or non-standard bolt
patterns machined to your drawing. - Threaded ends: For mechanical (non-welded)
integration. - Open ends: Plain tube ends for you to modify or weld
in-house.
Specification Checklist
When you reach out to us, having these parameters ready will accelerate your quote:
| Mandatory | Nice to Have |
|
✅ OD / ID (or one + wall thickness)
✅ Free length ✅ Compression or extension stroke ✅ Operating pressure (vacuum or positive) ✅ End fitting type |
☐ Target spring rate
☐ Required cycle life ☐ Operating temperature ☐ Process gas exposure (if any) ☐ Magnetic constraints (Y/N) |
Have your
parameters ready? Send us your requirements.