- ▸Graphite bushings are brittle materials with fracture toughness of only 1.0-1.5 MPa·m^(1/2); interference fit must be strictly controlled within 0.02-0.05 mm to avoid assembly cracking
- ▸A bore-based H8/f7 clearance fit is recommended, with minimum clearance 0.020 mm and maximum clearance 0.075 mm (for Φ50 mm bushing), balancing running flexibility with concentricity requirements
- ▸High-temperature conditions (above 200°C) require thermal expansion differential compensation: steel shaft coefficient 11.5×10⁻⁶/°C, graphite 4.5×10⁻⁶/°C; for every 100°C rise per 100 mm fit length, 0.07 mm compensation is required
- ▸Antimony-impregnated graphite bushings recommend surface roughness Ra 0.8-1.6 μm with mating shaft Ra 0.4-0.8 μm — excessively low roughness actually disrupts the formation of the graphite self-lubricating transfer film
As a key component of sliding bearings, graphite bushings are widely used in gear pumps, centrifugal pumps, compressors and vacuum pumps. Unlike metal bushings, graphite bushings are typically brittle materials that are extremely sensitive to fit tolerances. In our years of graphite bushing manufacturing at Huahao Sealing Co., Ltd. (霍邱县华豪密封件有限公司), we have encountered many early failure cases caused by improper fit tolerance design. This article systematically introduces the design principles and calculation methods for graphite bushing fit tolerances.
1. Material Characteristics of Graphite Bushings
1.1 Brittleness Characteristics
The fracture toughness of graphite materials is only 1.0-1.5 MPa·m^(1/2), approximately 1/50 that of metallic materials. This means graphite is extremely sensitive to local stress concentration, and excessive assembly interference or thermal stress can cause axial cracking or spalling. Therefore, the design of graphite bushing fits must follow the principle that "clearance is better than interference."
1.2 Elastic Modulus
The elastic modulus of ordinary graphite is 10-15 GPa, and antimony-impregnated graphite can reach 15-20 GPa. Compared to steel (210 GPa), the elastic modulus of graphite is only 1/15 of steel. This means graphite bushings deform very little under assembly stress and cannot absorb machining errors through elastic deformation, demanding higher machining accuracy.
1.3 Thermal Expansion Coefficient
The thermal expansion coefficient of graphite is 4.0-5.0×10⁻⁶/°C, far lower than steel (11.5×10⁻⁶/°C), copper (17×10⁻⁶/°C) and aluminum (23×10⁻⁶/°C). In high-temperature conditions, metal parts expand more than graphite parts, and the fit clearance decreases with rising temperature, requiring thermal expansion compensation to be reserved in design.
2. Fit Tolerance Selection
2.1 Fit Between Bushing Bore and Shaft
The fit between the graphite bushing bore and the shaft is the key to the sliding pair. A bore-based H8/f7 clearance fit is recommended. For a Φ50 mm bushing:
- Bushing bore: Φ50 H8 (+0.000/+0.039 mm)
- Shaft diameter: Φ50 f7 (-0.025/-0.050 mm)
- Minimum clearance: 0.025 mm
- Maximum clearance: 0.089 mm
For low-speed heavy-load conditions, an H7/f7 fit (clearance 0.025-0.075 mm) can be used to improve concentricity; for high-speed light-load conditions, an H8/e7 fit (clearance 0.050-0.114 mm) can be used to increase the lubricant film thickness.
2.2 Fit Between Bushing OD and Housing Bore
The fit between the graphite bushing outer diameter and housing bore must consider assembly stress. An H7/d8 clearance fit is recommended, with interference 0.02-0.05 mm, achieved by adhesive bonding or mechanical fixation (snap rings, pins) for axial positioning. Direct press-fitting with interference is strictly prohibited, as it causes radial cracking of graphite bushings.
2.3 Length Tolerance
For graphite bushing length, h11 tolerance is recommended (-0.000/-0.160 mm for Φ50 mm), and H12 for housing bore depth, ensuring 0.5-1.0 mm axial float to avoid bottoming out under thermal expansion.
3. Thermal Expansion Compensation Calculation
3.1 Calculation Formula
Thermal expansion compensation ΔL = L × (α_metal - α_graphite) × ΔT
Where:
- L: fit length (mm)
- α_metal: thermal expansion coefficient of metal part (1/°C)
- α_graphite: thermal expansion coefficient of graphite part (4.5×10⁻⁶/°C)
- ΔT: temperature difference (°C)
3.2 Example Calculation
Φ50 mm steel shaft with graphite bushing, working temperature rises from 20°C to 220°C, ΔT=200°C:
ΔL = 50 × (11.5×10⁻⁶ - 4.5×10⁻⁶) × 200 = 50 × 7.0×10⁻⁶ × 200 = 0.07 mm
That is, when assembled at 20°C, the clearance must be ≥0.07 mm to avoid seizure at 220°C.
3.3 Multi-Layer Sleeve Compensation
For high-temperature conditions (>300°C), a multi-layer structure of graphite bushing + metal sleeve + graphite bushing can be used, with the metal sleeve's elastic deformation absorbing the thermal expansion differential.
4. Surface Roughness and Assembly Process
4.1 Surface Roughness
For graphite bushing bores, Ra 0.8-1.6 μm is recommended, and for outer diameters Ra 1.6-3.2 μm. The mating shaft surface roughness should be Ra 0.4-0.8 μm. Excessively low surface roughness (Ra < 0.2 μm) actually disrupts the formation of the graphite self-lubricating transfer film, intensifying dry friction and wear.
4.2 Assembly Process
1.Cleaning: Clean the fit surfaces with anhydrous ethanol before assembly, removing burrs and foreign matter
2.Lubrication: Apply molybdenum disulfide or PTFE dry film lubricant to fit surfaces during assembly
3.Press-fitting: Use a dedicated fixture with evenly distributed force points, press-fitting speed not exceeding 5 mm/s
4.Inspection: Check clearance with feeler gauge after assembly, check concentricity with dial indicator (≤0.02 mm)
Conclusion
The fit tolerance design of graphite bushings is the foundation of reliable sliding bearing operation. Huahao Sealing Co., Ltd. provides high-precision graphite bushings with bore tolerances controlled to IT6 (tolerance 0.016 mm at Φ50 mm) and offers customized tolerance design solutions. Please contact our technical team — we will provide the optimal fit tolerance recommendation based on your operating conditions.