A magnetic coupling transmits torque contactlessly via a magnetic field. To keep the two sides completely separate, a non-magnetic wall sits between the inner and outer rotors: the containment shell. In pump engineering it is also known by its German name Spalttopf, and in compact permanent magnet couplings as the barrier wall (Begrenzungswand).
The containment shell is the safety-critical component of the entire coupling. It hermetically seals the fluid and determines the system’s leak-tightness, pressure resistance, corrosion resistance, and efficiency. Choosing the right material is therefore not a minor detail, but rather determines whether the system operates safely, leak-free, and economically.
Key takeaway: Stainless steel is the pressure-resistant standard for non-critical media, but it produces eddy current losses. Hastelloy offers the highest corrosion resistance, ceramics ensure leak-free operation with aggressive or pure media, and PEEK is ideal for lightweight, corrosion-resistant applications with limited pressure. The selection table and design guidelines below will help you make the right choice.
Function of the containment shell
The drive motor drives the outer rotor (drive magnets). Its magnetic field penetrates the containment shell and couples with the inner rotor running in the medium—without any mechanical connection. The can thus separates the “wet” area (medium, inner rotor) from the “dry” area (drive, outer rotor).
This gives rise to two demanding requirements that are in conflict with one another:
- Magnetically permeable: The field must pass through the wall with as little obstruction as possible. The material must be non-magnetic, and the wall should be as thin as possible to keep the air gap small.
- Mechanically sealed and secure: The wall must be able to withstand operating pressure, temperature, and chemical attack over the long term—which calls for greater wall thickness and high-quality materials.
It is precisely this trade-off—thin and permeable versus thick and rigid—that makes the choice of material and wall thickness central to every containment shell design. The wall thickness directly affects the magnetic air gap and thus influences the transmissible pull-out torque.
Comparison of Materials
Four material groups dominate the design of containment shells. The table compares their properties—the key factor is always the interaction between the medium, pressure, temperature, and efficiency requirements.
| Material | Corrosion resistance | Temperature | Pressure | Eddy currents | Costs |
|---|---|---|---|---|---|
| Stainless steel (e.g. 1.4571, 1.4404) | Good (standard media) | High | High | Yes (losses) | € |
| Hastelloy (Ni-based) | Very high (acids, halides) | High | High | Yes (losses) | €€€ |
| Ceramics (Zirconia ZrO₂) | Very high (virtually inert) | Very high | High, but prone to brittle fracture | No (lossless) | €€€ |
| PEEK / Plastic | High (depending on the medium) | Limited | Limited | No (lossless) | €€ |
The specific limits (permissible pressure, maximum temperature, list of media) depend on the design, wall thickness, and manufacturer, and must be determined on a case-by-case basis.
Eddy currents & heat generation
When the magnetic field rotates relative to an electrically conductive containment shell wall (stainless steel, Hastelloy), eddy currents are induced in the wall. These currents generate heat and are lost as power dissipation. The losses increase with speed, magnetic field strength, and wall thickness.
Practical tip from TEA:
With a metal containment shell and higher speeds, heat dissipation is critical. If the heat is not dissipated, the temperature rises—in extreme cases, to the point of partial demagnetization of the magnets and thus a loss of performance. For high speeds, high power ratings, or tight efficiency specifications, a non-conductive ceramic or PEEK containment shell — the low-loss alternative.
Non-conductive materials completely eliminate eddy currents: no loss, no field-induced self-heating of the wall. The trade-off is higher material costs and—in the case of ceramics—brittleness, which requires a design that minimizes impact and thermal shock.
Material Selection by Medium & Application
| Requirement / Medium | Recommended material | Reasoning |
|---|---|---|
| Standard media (water, oils, neutral liquids) | Stainless steel | Cost-effective, pressure-resistant, readily available |
| Corrosive acids, halides | Hastelloy or ceramics | Stainless steel is susceptible to pitting corrosion; Hastelloy and ceramics are resistant |
| High RPM / Efficiency is critical | Ceramics | No eddy current losses, minimal self-heating |
| High temperature | Ceramics or stainless steel | Durable; plastic is not an option |
| High operating pressure | Stainless steel / Hastelloy | Metal can withstand the highest pressures |
| Low pressure, weight- and cost-sensitive, corrosive | PEEK / Plastic | Lightweight, chemically resistant, loss-free |
The hermetic seal makes the magnetic coupling particularly attractive in applications where leakage is unacceptable—such as with toxic or environmentally hazardous substances. This comparison is explored in depth in the guide Magnetic Coupling vs. Mechanical Seal.
Design: Wall Thickness, Clearance, and Safety
Wall thickness is the key design parameter. It affects three properties simultaneously:
- Torque: Any increase in wall thickness increases the magnetic air gap. Since the transmissible torque decreases disproportionately with the gap, a thicker wall results in a noticeable loss of torque.
- Compressive strength: A thicker wall can withstand higher internal pressures and provides greater protection against bursting.
- Losses (metal only): In conductive materials, eddy current losses increase with wall thickness.
In practical terms, this means: as thin as is permissible for pressure and safety, but as thick as necessary. For torque sizing, an additional safety factor should be applied (typically 1.5–2.0× the operating torque) to account for start-up peaks, load fluctuations, and the temperature dependence of the magnets. For detailed sizing, the product page of TEA PMKC magnetic couplings (with containment shell selection guide) provides a sizing assistant.
Common design errors
Error 1: Material not compatible with the medium
Standard stainless steel used in chloride-containing or highly acidic media is prone to pitting corrosion and can perforate the containment shell. Media compatibility must be verified before selecting the material—if in doubt, use Hastelloy or ceramic.
Mistake 2: Neglecting heat dissipation in metal containment shells
Eddy current losses cause the metal containment shell to heat up. Without adequate heat dissipation, the temperature rises until, in extreme cases, the magnets become partially demagnetized and torque drops. At high speeds, provide a heat dissipation path or use ceramic materials.
Error 3: Wall thickness designed for compression only
If you design the wall based solely on pressure, you’ll overlook its impact on the air gap and torque. A wall that’s too thick increases the gap and reduces the transmissible torque—causing the coupling to slip sooner than intended. Consider pressure, torque, and losses together.
Mistake 4: Using ceramic without considering brittle fracture
Ceramic is loss-free and chemically inert, but prone to brittle fracture. Impact loads, installation stresses, or thermal shock (rapid temperature changes) can cause cracks. Install ceramic containment shells with structural shock isolation and avoid sudden temperature changes.
Inspection & Maintenance
Because the containment shell is the only component between the medium and the environment, monitoring it is an essential part of any maintenance schedule. Key inspection points:
- Visual inspection: Cracks, corrosion marks, signs of erosion, and discoloration on the inner wall.
- Temperature: An unusually high operating temperature may indicate increasing eddy current losses, a reduced air gap, or incipient damage.
- Leak-tightness: Monitor for leaks during critical processes; check immediately if there is a drop in pressure.
- Replacement Planning: For safety-critical applications, establish a defined replacement interval rather than operating the equipment until it fails.
The type of coupling—synchronous or hysteresis—operating behind the containment shell affects load and thermal behavior. The differences are explained in the article Hysteresis Coupling vs. Permanent Magnet Coupling.
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