Assembly methods of interference fit parts

InterferenCE fit parts achieve a tight connection by relying on the interference amount after the components are assembled. After assembly, due to the elastic deformation of the materials, pressure is generated between the mating surfaces. Therefore, during operation, there is a considerable frictional force between the mating surfaces to transmit torsional or axial forces. Interference fit assemblies are generally non-removable fixed connections. The assembly methods for interference fit parts are: (1) manual hammering, (2) press-fitting, (3) cold fitting, and (4) hot fitting.

1) Inspection Before Assembly of Interference Fit Parts

Interference fit parts must be re-inspected before assembly, and the results recorded.

(1) The interference amount should conform to the specifications in the drawings or process documents.

(2) The perpendicularity deviation between the end face of the relevant wheel or ring that abuts the shaft shoulder, and the end face of the wheel used as the assembly datum, and the hole should be within the range specified in the drawings.

(3) Relevant rounded edges, chamfers, etc., should not affect assembly.

(4) The mating surfaces should be free of burrs, rust spots, or scratches.

(5) When the hole of the enclosure part is a blind hole, the enclosed part must have a vent hole or groove; otherwise, assembly is not permitted.

(6) For mating parts with keyed connections, the positions of the shaft groove and hole groove, and the mated key must be re-inspected before assembly. Assembly can only proceed after confirming that everything is correct.

2) Assembly of interference fit parts

A.Manual Hammering Method: Suitable for small parts assembly with transition fits.

1. No hammer marks are allowed on the surface of the assembled parts.

2. Apply machine oil for lubrication during hammering.

3. Use soft metal or hard non-metallic materials as protective pads during hammering.

4. During hammering, the assembled part and the housing part must be coaxial, with no skewing allowed.

5. The assembled part must be tightly against the relevant limiting shoulders, etc., with a gap not exceeding 0.05mm.

B.Pressure Fitting Method: Suitable for medium and small parts assembly with small interference fits at room temperature.

1. The lead end of the pressure-fitted part must be tapered. If not specified in the drawing, the tapered end should be made with a taper of 1:150. The length is 10% to 15% of the total mating length. The empirical formula for calculating the pressing force F is F = KiL × 10⁴. Where:

i – measured actual interference fit (mm)

L – mating length (mm)

K – coefficient considering the material and size of the parts being installed

K coefficient ranges from 1.5 to 3.

2. When pressing a solid shaft with a blind hole, a venting plane with a depth greater than 0.5mm is allowed to be machined on the mating journal surface.

3. The mating surfaces of the parts being pressed must be lubricated with oil (white lead oil mixed with machine oil) before pressing.

4. During pressing, the center line of force should be coaxial with the center lines of the containing and contained parts. For slender shafts, the coaxiality of the center line of force with the parts should be strictly controlled.

5. When pressing a wheel and a shaft, the wheel rim must never be subjected to force alone.

6. After press fitting, the shoulder must be tightly closed. The gap should be less than 0.05mm.

7. When using a heavy object for press fitting, it should be pressed in smoothly and without resistance. Any abnormalities should be analyzed to prevent damage to parts.

8. When using a hydraulic press, the pressing force F must be checked to ensure that the pressure generated by the press is 1.5-2 times the pressing force F.

9. When using a hydraulic press, pressure changes should be recorded.

1) Pressure changes should be smooth. Any abnormalities should be analyzed to prevent damage to parts.

2) If the drawing specifies a maximum pressure, the specified value should be achieved; it should not be too high or too low.

3) When using a machine press, the speed should not be too fast. The pressing speed should be 2-4mm/s, and should not exceed 10mm/s.

C.Hot Fitting Method: Suitable for assembling parts with large interference fits.

1. Prepare for hot fitting. To ensure the smooth completion of the heat fitting process:

(1) Calculation formula for heating temperature T

T＝(σ＋δ)/ad＋T (℃)

Where d－Nominal diameter of fit (mm)

a－Linear expansion coefficient of heating part material (1/℃) For commonly used material linear expansion coefficients, see relevant manuals

σ－Maximum interference of fit size mm

δ－Required heat fitting clearance (mm) When d<200mm,

δ is (1"2)

When d≥200mm,

δ is (0.001"0.0015)d2

(2) Heating time is estimated based on the part thickness of 10mm requiring 10min of heating. The thickness value is calculated based on the smaller of the axial and radial dimensions of the part.

(3) Holding time is estimated based on 1/4 of the heating time.

2. Heating of the enclosure part. After the expansion reaches the requirement, the mating surfaces of the enclosure part and the package part should be cleaned quickly, and then heat fitting should be carried out immediately. 1. Operation must be swift and accurate, with hot fitting completed in one go without interruption. If any abnormality occurs, forced installation is not permitted; the fault must be rectified, and reheating must be performed before hot fitting.

3. After hot fitting, use reliable methods such as pulling, pressing, and pushing to bring the hot-fitted part close to the axial positioning surface of the enclosed part. After the part cools, the gap must not exceed 1000 mm of the mating length.

4. When installing a copper sleeve in a steel part, the enclosing part can only be hot-fitted once. It is not allowed to be reheated again as an enclosing part for secondary hot fitting after installation.

5. For hot fitting of gears with inlaid ring structures, if the gear ring has already been heated once, a second heating is required when hot fitting it to the shaft. Generally, oil bath heating should be used. If conditions are limited, electric furnace heating can also be used, but the temperature rise rate must be strictly controlled to ensure uniform temperature. Furthermore, the distance between the working outer surface and the heating element must be greater than 300mm; otherwise, it is not permitted.

6. When using oil bath heating, the oil temperature must be controlled 10-20℃ below the flash point of the oil. Using oil above or exceeding its flash point is strictly prohibited. Common oil flash points are shown in Table 7-86.

7. When using inductive heaters, appropriate equipment specifications must be selected, and the equipment operating procedures must be strictly followed.

D.Cold Assembly Method:

Suitable for components that cannot be heated or whose heating would cause changes in part precision, material structure, or affect the assembly of mechanical components.

1. During cold assembly:

Formula for calculating freezing temperature TI:

T1=2σ/a1d (℃)

In the formula:

σ—Maximum interference (mm)

d—Outer diameter of the enclosed part (mm)

a1—Coefficient of linear expansion of the enclosed part during cooling. For commonly used materials, see relevant manuals for the coefficient of linear expansion during cooling.

Formula for calculating freezing time t:

t= a'δ' (6～8)(mm)

In the formula, coefficients related to the material are found in relevant manuals.

Characteristic dimensions of the part to be frozen. That is, the maximum cross-sectional radius or wall thickness of the part (mm)

1) Calculate the freezing temperature T according to the formula.

2) Select the refrigerant. The temperature of the refrigerant must be lower than the required freezing temperature T1 of the enclosed part. When the diameter of the enclosed part is greater than φ50mm, liquid oxygen or liquid nitrogen refrigerant should be preferred. See relevant manuals for temperature values.

3) Calculate the freezing time.

2. When using liquid oxygen as the refrigerant for cold assembly, flammable materials and sources of ignition are strictly prohibited nearby.

3. Operators must wear appropriate personal protective equipment, including long-sleeved clothing, long trousers, protective glasses, leather gloves, and secure canvas foot covers.

4. Vent holes must be left in the refrigerant canister and cooling tank; do not block them during use to prevent pressure buildup and potential explosion. The inside of the tank must be clean, and the cooling tank must be placed stably and reliably.

5. Refrigerant must be used only as needed. Pour carefully to prevent spillage and splashing. The liquid level in the cooling tank must be sufficient to submerge the mating surfaces of the parts, but not too full; it should be 80cm below the top of the tank lid. Replenish any evaporated refrigerant promptly.

6. Use tools when placing or removing parts from the cooling tank. Use pliers or secure them with wire beforehand. Do not handle parts directly with your hands to avoid burns.

7. Freezing time is calculated from the moment the parts are immersed in the refrigerant. Initially, there will be a strong "boiling" phenomenon, which gradually weakens and disappears. The initial cessation of this phenomenon only indicates a small temperature difference between the part surface and the refrigerant, but it is not yet fully cooled. Complete cooling must be achieved according to the calculated time.

8. After the parts have reached the calculated temperature... Remove the parts and immediately insert them into the housing holes. The action must be swift and accurate. Ensure the parts are held concentrically; do not tilt. Correct any tilting during insertion by tapping with a copper rod or wooden mallet; use a wooden mallet for copper parts.

9. If installing many parts at once, remove one part from the cooling box and immediately insert it, replenishing the refrigerant as needed. Close the lid securely.