- ▸Fit design goals: free assembly, no thermal seizure, no off-center wear under pressure, effective secondary sealing
- ▸Stationary rings use clearance fits with the housing; typical radial clearance is 0.1-0.3 mm, balancing float compensation against sway control
- ▸Rotating rings transmit torque via small clearance plus drive pins or drive collars, avoiding direct interference squeezing the graphite
- ▸Secondary seal compression is typically 15%-25%; more can wedge the graphite ring apart, less loses the static seal
- ▸Cumulative dimensional-chain error directly shifts face loading; poor tolerance allocation opens the faces or overloads them
- ▸Carbon graphite rings are typically machined to IT7-IT8 on bore and OD, with face flatness of 0.0009 mm
However precise the ring itself, wrong fit design still leaks. Tolerance and fit are the invisible half of seal reliability: they decide whether the ring can float, whether heat will seize it, and whether face loading lands in the design window.
I. Stationary Ring and Housing: Room to Float
The stationary ring (often carbon graphite) sits in the gland or housing with a small radial clearance for two reasons: to allow slight float following shaft sway without local face overload, and to allow thermal expansion without seizure.
- Typical single-side radial clearance: 0.1-0.3 mm, larger for big rings
- Axial position set by shims or a collar to place the face at design depth
- Fits of the H8/f8 clearance class are common (principle here; follow the drawing and standards)
The ring OD is not a sealing surface — sealing happens at the face and the secondary seal behind it. Excessive clearance lets the ring sway too much, concentrating face loading and accelerating wear.
II. Rotating Ring and Sleeve: Transmit, Don't Squeeze
For cartridge-type rotating rings (graphite mounted in a metal holder), the fit decides success or failure.
1.Shrink-fit mounting: heat the holder and press the graphite in; interference must be exact — too small loosens at temperature, too large cracks the ring (low tensile strength; a common root cause of cracked inserts)
2.Small clearance plus drive: keep a small clearance and pass torque through pins, forks or a drive collar so the graphite sees no assembly stress
3.Holder material should have a thermal expansion coefficient close to or slightly above graphite so the fit holds at working temperature
III. Secondary Seals: Compression Is the Key Number
O-ring compression is typically 15%-25%.
- Above 25%-30%: over-compression, reaction force wedging the graphite ring or distorting the holder
- Below 10%: insufficient contact pressure, static leakage
- Groove depth, width and cord diameter must be checked as one chain
IV. Dimensional Chains and Leakage
From shaft shoulder to seal face, the chain serializes shoulder position, collar length, spring free height, push-ring thickness, stationary ring length and gasket thickness. Their cumulative tolerance sets the face compression (spring compression).
1.Too little compression: face loading below design, leakage rises, faces may open
2.Too much: loading beyond design, PV up, wear and thermal-crack risk up
3.Practice: run tolerance analysis on the critical chain and keep cumulative error within ±10% of the design compression
Precision requirements for the ring itself: face flatness typically 0.0009 mm (within 3 light bands), runout and squareness of bore/OD in the 0.01-0.03 mm class. Huahao Sealing machines to customer drawings; when the fit chain is in doubt we recommend sending the assembly chain for review — a significant share of seal failures lies not in the ring but in fit and compression allocation.