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.
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.
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.
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.