- ▸Sealing-grade flatness within 0.0009mm: face micro-roughness must be smaller than the fluid film, or no film can form
- ▸One interference fringe order is about 0.0003mm (half-wavelength order of monochrome light); 0.0009mm equals three fringe orders
- ▸Reading rule: straight, evenly spaced fringes pass; each fringe's deviation counts one order; a closed bull's-eye is about one order per ring, multiplied by ring count
- ▸The flat test needs a clean, dry face with no oil film, and a monochrome sodium lamp or dedicated LED flat-light source
- ▸Three consequences of lost flatness: initial leakage, start-up noise, and abnormal graphite ring wear
The fluid film between mechanical seal faces is only a few tenths of a micron thick, so face flatness must be pressed to the same order — that is where 0.0009mm comes from. This article explains how flatness is measured, how to read fringes, and why this figure is a core factory-inspection item for seal rings.
1. What 0.0009mm Means
0.0009mm is 0.9 micron. For reference: a human hair is about 70 microns across, so 0.9 micron is roughly one-eightieth of that. Seal face films typically run 0.5-3 microns; if face flatness exceeds the film thickness, high spots touch while low spots open, and sealing fails outright.
Standards frameworks (such as GB/T 566 and API 682) set overall mechanical seal performance requirements; face flatness is enforced as a manufacturer's factory-inspection item with a typical acceptance line of within 0.0009mm — no more than about three fringe orders.
2. How the Optical Flat Works
An optical flat is a glass disc whose own flatness is an order of magnitude better than the tolerance being checked. Placed against the seal face, the face's micro-irregularities create varying air-gap thickness; reflected monochrome light interferes:
- Every half-wavelength of air-gap difference produces one bright-dark fringe
- Sodium light is about 0.00059mm in wavelength, so one fringe order is about 0.0003mm
- A 0.0009mm flatness limit allows up to about 3 fringe orders
3. Reading the Fringes
3.1 Basic Patterns
- Straight, evenly spaced fringes: the face is very flat with only a slight wedge — pass
- Uniformly bowed fringes: flatness error; the bow as a fraction of fringe spacing times 0.0003mm gives the value
- Closed circular bull's-eye: spherical crown or hollow; about 0.0003mm per ring, multiplied by ring count
- Oval or asymmetric patterns: local edge collapse or chipping
3.2 Operating Points
1.The face must be clean and dry: oil films and dust create false fringes
2.Use a monochrome source (sodium lamp or dedicated LED flat-light); white-light fringes carry color mixtures that are hard to judge
3.Place and press the flat gently — never slide it; a scratched flat is scrap
4.Graphite is opaque, so you read the reflection between the graphite surface and the flat; a light polish on the face helps when needed
4. What Lost Flatness Costs
1.Initial leakage: a freshly installed seal leaks from first run because the high-spot contact zone cannot hold pressure
2.Start-up noise and heat: a broken contact band gives unstable friction
3.Abnormal wear: local pressure on high spots far exceeds design values, carving uneven wear grooves into the graphite ring within a short period
This is why flatness is a mandatory factory-inspection item. Huahao Sealing was founded in 2006 with its factory in Lu'an, Anhui; every batch of seal rings is flat-tested and dimension-checked piece by piece before shipment under full-process quality control, and out-of-tolerance parts do not leave the factory. For re-verification before fitting, press a flat on the cleaned face and read the fringes — a one-minute check.
5. Summary
Flatness of 0.0009mm is not dogma — it is a physical requirement set by film thickness. The optical flat is cheap, intuitive and gives an answer in a minute, making it the most practical incoming-inspection tool for buyers. When ordering made-to-drawing non-standard rings, write the flatness acceptance requirement into the drawing so both sides check against the same ruler.