Knowledge

Production Process of Gear Shafts

The manufacturing process involves multiple steps, with the goal of obtaining parts that meet requirements for dimensional accuracy, shape accuracy, positional accuracy, surface roughness, surface hardness, and microstructure. The process flow depends on the material, batch size, precision requirements, and structural characteristics (integral vs. assembled).

1. Raw Material Preparation:

- Round bar stock: Used for small and medium-sized gear shafts, cut to the required length by a sawing machine to form the blank.

- Forgings: Particularly important! The vast majority of medium carbon steel and alloy steel gear shaft blanks are forged. Forging eliminates casting defects, refines grains, increases density, and makes the metal flow lines more reasonable (improving strength and toughness). Main methods:

Open die forging: Suitable for single-piece, small-batch, and large gear shafts.

Die forging: Preferred for mass-produced small and medium-sized gear shafts. Offers better flow line distribution, higher dimensional accuracy, and less machining allowance.

Precision forging: Produces gear shafts with teeth closer to the final shape (net shape/near net shape).

- Castings: Mainly used for cast iron or ductile iron gear shaft blanks. Methods include sand casting (for single-piece and small-batch production), die casting (for batch production), and precision casting (investment casting, pressure casting).

 

2. Pre-heat Treatment:

- Objective: Eliminate forging stress, improve the coarse and uneven microstructure after forging, reduce hardness, and enhance machinability.

- Methods:

Annealing/normalizing: Most commonly used. Normalizing is more frequently used, with a slightly faster cooling rate than annealing, resulting in a finer microstructure and better overall performance.

Quenching and tempering: An important subsequent process for medium carbon steel and alloy steel gear shaft blanks after forging. Improves machinability and initially enhances performance.

 

3. Rough Machining:

- Objective: Remove most of the allowance and form the basic shape.

- Operations: Typically performed on a lathe to turn the outer cylindrical surfaces, end faces, and chamfers, etc.

 

4. Semi-finish Machining:

- Objective: Prepare the clamping reference surfaces for subsequent finish machining and heat treatment.

- Operations:

Turn the reference journal (with higher precision requirements), end faces (to ensure end runout), and center holes (for subsequent grinding and other operations requiring two-center clamping).

Mill keyways or splines (if present on the gear shaft). Sometimes done before heat treatment.

Leave uniform allowances for subsequent tooth surface machining.

 

5. Final Heat Treatment: (Core Process)

- Objective: Ultimately determines the load-bearing capacity, wear resistance, and service life of the gear shaft!

- Key methods (depending on material type):

Quenching and tempering: For machined blanks/rough-machined parts, quenching followed by high-temperature tempering to obtain a uniform tempered sorbite microstructure. Provides excellent comprehensive mechanical properties (balance of strength, toughness, and ductility). Suitable for medium carbon steel and medium carbon alloy steel gear shafts (such as 45, 40Cr).

Surface hardening: Induction heating at medium or high frequencies followed by rapid quenching (usually only for the toothed portion). Produces a high-hardness surface martensite layer and a tough core. Suitable for integral gear shafts (such as medium-frequency quenching of the teeth of 45 steel after quenching and tempering). Characterized by a shallow hardened layer and relatively controllable deformation.

Carburizing and quenching: For finish-machined or semi-finished low-carbon alloy carburizing steel parts. Long-term carburizing in a protective atmosphere furnace (to obtain a 0.5-2mm high-carbon surface layer), followed by quenching and low-temperature tempering. Result: Extremely high surface hardness and wear resistance (HRC 58-62), with the core maintaining high toughness (HRC 30-45). This is the standard process for high-speed and heavy-load gear shafts (such as automotive gearboxes). The process is complex, with relatively large deformation, and requires precision grinding afterward. Materials such as 20CrMnTi.

Nitriding/ nitrocarburizing: Performed at lower temperatures (500-570°C), with minimal deformation (sometimes used as the final process), resulting in a hardened layer with high hardness, high wear resistance, and excellent fatigue resistance (shallower than carburizing, 0.1-0.6mm). The hardness can reach up to HV 1000-1200 (approximately HRC 65-70). It is suitable for applications that require minimal deformation and high fatigue strength (such as precision machine tools and plastic extruder screws). Commonly used materials include 38CrMoAl and other nitriding-specific steels containing Al, Cr, and Mo, or quenched and tempered structural steels. Before nitriding, the parts need to be precisely machined to the final dimensions or semi-finished with a small amount of grinding allowance (such as 0.1mm).

Tempering: Parts that have undergone quenching or carburizing and quenching must be tempered (at low or medium temperatures) to stabilize the microstructure, relieve stress, and adjust hardness.

 

6. Finishing:

- Objective: To achieve the final dimensional accuracy, geometric shape accuracy, positional accuracy, and surface roughness as per design requirements. Mainly to correct the deformation caused by heat treatment.

- Key processes:

Turning and grinding: Recondition the center hole to prepare for subsequent fine grinding.

Outer diameter grinding: Core process. Grind the journal (where the bearing is installed), positioning surfaces, and mating surfaces for gears to the final dimensions and accuracy (up to IT6 for high-precision requirements). Ensure roundness and cylindricity.

Inner diameter grinding: (If it is a hollow gear shaft)

Gear profile finishing (gear grinding):

Gear grinding: Precision grinding (sometimes heavy scraping) of the gear tooth surface after heat treatment to correct the deformation and achieve the highest accuracy (5-6 grades according to GB/T 10095) and surface finish. Mainly used for precision gear shafts made of medium carbon steel/alloy steel after surface quenching or carburizing and quenching/nitriding. It is costly. An essential process for high-precision, high-speed, and heavy-load gear shafts.

Sometimes honing or grinding is used as a supplement or alternative for lower precision requirements.

Splining grinding: If splined connections are present.

Superfinishing/polishing: For extremely demanding bearing mating surfaces to further improve surface roughness and shape accuracy (such as Ra 0.1um).

Dynamic balancing: For high-speed rotating gear shafts (such as engine crankshafts, high-speed reducer shafts), dynamic balancing correction must be performed on a dynamic balancing machine.

 

7. Inspection:

- Throughout the entire process: incoming material inspection, in-process inspection (hardness and case depth before and after heat treatment), and final product inspection.

- Key items: critical dimensional tolerances, roundness/cylindricity of the journal, radial runout/axial runout, gear profile accuracy (tooth shape, tooth direction, tooth pitch error), surface roughness (Ra 0.8-0.2um for bearing mating surfaces, Ra 0.8-0.4um for gear surfaces), heat treatment hardness and case depth (effective hardened layer depth, surface/core hardness), metallographic structure (Martensite grade, retained austenite, presence of abnormal structures such as coarse grains, decarburization), etc.

- Equipment: calipers, micrometers, dial indicators, roughness meters, optical projectors, coordinate measuring machines (CMM), gear measuring centers, hardness testers (Rockwell, Brinell, Vickers, surface Rockwell), metallographic microscopes, etc.

 

8. Cleaning, rust prevention, and packaging:

- Remove machining residues (chips, coolant, grinding particles).

- Apply rust prevention treatment (oil coating or vapor phase rust prevention).

- Package as required.

 

Differences in production processes between integral and assembled gear shafts (taking commonly used quenched and tempered steel/carburized steel as an example)

Integral gear shaft:

- Raw material -> Forging -> Pre-heat treatment (normalizing/annealing) -> Rough turning -> Quenching and tempering -> Center hole correction/semi-finish turning (to the final heat treatment reference) -> (If there are keyways/splines -> milling) -> Rough gear profile machining (rolling/cutting) -> Final heat treatment (quenching/carburizing and quenching/nitriding) -> (Fine grinding of reference surfaces) -> Fine grinding of all outer diameters/journals -> Fine grinding of gear surfaces/finish turning

 

If you have any questions, demands, need new parts to be developed or to improve your supply chain, please feel free to contact us at info@castings-forging.com