A containment shield is a steel enclosure around the rotor, bolted to the floor, that must absorb the energy of the largest fragment a failed rotor can throw. A quarter of a 3 kg bowl at 8,000 RPM carries about 1.5 kJ and 47 N·s of momentum, and the carrier’s 3-6 mm steel cylinder is a starting point for rotors of that size, not a derived rule, so a shield must be oversized, anchored and tested, and the operator must stand outside the rotor’s plane regardless.
Part of Centrifuge Building
A containment shield is the barrier that catches the fragments of a burst rotor so they cannot reach the operator, the walls or other machines.
The Shield Has To Be Designed For Energy, Not For Splashes
The carrier article says to surround the rotor with a steel cylinder 3-6 mm thick, capable of containing fragments, extending above and below the rotor, and gives a minimum of twice the rotor wall thickness. These are practical starting figures for the small rotors in that article. They are not derived, and they disagree with each other for a thick-walled rotor: twice the 3.5 mm bowl wall is 7 mm, above the 3-6 mm range. So treat them as lower bounds and derive what the shield must absorb.
Worked Example: What A Fragment Carries
The reference bowl: 3 kg, rim speed 62.8 m/s (150 mm, 8,000 RPM). A burst may throw the whole bowl in pieces or break off a part. Take a quarter of the mass as one fragment, 0.75 kg (an assumption).
- Kinetic energy: ½ × 0.75 × 62.8² = ½ × 0.75 × 3,948 = 1,481 J, about 1.5 kJ.
- Momentum: 0.75 × 62.8 = 47 N·s. A shield bolted to a floor takes an impulse of this size, and a stop time of 10 ms means an average force of 47 ÷ 0.010 = 4,700 N, nearly half a tonne-force.
- The whole rotor energy is 5.9 kJ, as in Safety Considerations.
Bookkeeping, not design. A steel ring that stretches plastically absorbs energy. Taking a ring of radius 0.12 m, height 0.15 m and wall 5 mm (volume 2π × 0.12 × 0.15 × 0.005 = 5.65 × 10⁻⁴ m³) and assuming it can stretch evenly by 10% at a flow stress of 300 MPa (both assumptions), the energy it can absorb is 300 × 10⁶ × 0.10 × 5.65 × 10⁻⁴ = 17 kJ, about three times the energy of the whole bowl. It means the mass of steel is of the right order. It does not mean 5 mm is safe: a fragment with a sharp edge concentrates its energy on a small area and punches through, and a ring does not stretch evenly under a point load. The wiki has no formula for penetration, and a formula taken from memory would be a guess about a lethal failure.
What That Leaves You With
- Oversize the shield relative to the starting figures, and favor thicker steel over a larger diameter. Mass does the stopping.
- Enclose the whole plane of rotation, and top and bottom above and below the rotor. For a vertical-axis machine the fragments leave sideways.
- Anchor it to the floor with bolts sized for the impulse. A shield that slides or tips is only a heavy missile of its own. Bolt it to the floor and not to the vibration-isolated machine; see Vibration Isolation.
- Close the lid on every run, with the lid held by a catch the shield can load. An unclosed lid is the opening that fragments find.
- Use no glass or brittle windows. If you need to watch the run, use a mirror set outside the plane of the rotor rather than a hole.
- Keep people out of the plane of the rotor, even with a shield, and stand behind a solid barrier or an earth bank.
- Where the machine is big, put it in a pit with a heavy steel cover and an earth bank around it, so the soil does the stopping.
- Prove the rotor, not the shield, first. The first line of defense is the speed limit in Speed Limits and Material Stress.
What You Need
- Steel plate or rolled sheet, thicker than the starting figures, and welding or bolts to close it; see Stainless Steel Production for sheet and Steel Production for steel
- Anchor bolts and a heavy base
- A lid with a catch, an interlock if you can build one, so the drive cannot be engaged with the lid open
- A barrier or bank for the operator
Method: Fitting A Shield
Step 1: Compute the rotor energy (½ × m × v² upper bound) and a fragment energy.
Step 2: Choose thickness and mass well above the starting figures.
Step 3: Fit it with clearance between rotor and wall, enough that a rotor out of balance does not rub it.
Step 4: Anchor and close it.
Step 5: Run in stages, from low speed to working speed, with nobody in the plane, and inspect the inside of the shield for marks.
Common Mistakes
- A shield bolted to the machine frame on soft mounts. It moves with the failure.
- A light sheet lid. It is the weak side.
- A viewing window. It is the weak spot.
- No clearance. A shield that rubs the rotor causes the failure it is meant to hold.
Safety
A shield is a last resort, not permission to over-speed
No wiki figure establishes a safe shield thickness. A shield reduces the chance that a failure kills; it does not make a rotor safe to push beyond its limit. Stay out of the plane of the rotor, behind a barrier, for every run.
What’s Next
- Learn next: Safe Operating Procedures — checks before and after every run
- Combine with: Safety Factors — building in margin