Feasibility Analysis of Cold Heading for Wind Turbine Bearing Steel Balls​

Abstract​

To address the issues of long processing time, low efficiency, high energy consumption, and insufficient microstructure density in traditional hot heading processes for large-sized steel balls used in wind turbine bearings, a cold heading forming process is proposed. Using steel balls with diameters of 50 mm and 65 mm as examples, a theoretical model for calculating the dimensions of the steel bar feedstock was established. The cold heading process was simulated, and the theoretical crushing load was calculated. The results show that the cold-headed ball billets exhibit distinct poles and ring bands, with uniform equivalent stress distribution. The theoretical crushing load for the 50 mm diameter steel ball meets the requirements. Practical processing verification confirmed that the crushing load of the 50 mm steel ball complies with standards, while the 65 mm steel ball demonstrates a dense internal microstructure and high strength. Both theoretical and experimental results validate the feasibility of cold heading for producing wind turbine bearing steel balls.

Keywords: rolling bearing; wind turbine bearing; ball bearing; wind turbine; steel ball; cold heading; crushing load; simulation

1. Introduction

With the rapid development of wind power technology, the capacity of wind turbines has increased from 2–3 MW to 6–10 MW, and the service life requirement has extended from 10 to 20 years. As critical components of wind turbine bearings, steel balls directly affect rotational accuracy, noise, and service life. Traditional hot heading processes for large-sized steel balls are inefficient and energy-intensive, resulting in inadequate microstructure density. Cold heading, however, produces steel balls with denser microstructures and higher strength, making it a promising alternative.

2. Theoretical Basis for Cold Heading

2.1 Technical Requirements

Wind turbine bearing steel balls require high reliability, long service life, and precision. Materials such as GCr15SiMn or GCr15 are typically used. After heat treatment, the crushing load and hardness must comply with GB/T 34891-2017. For this study, GCr15 steel was selected, with an elastic modulus of 219 GPa, density of 7,830 kg/m³, yield strength of 518.42 MPa, and Poisson’s ratio of 0.3.

2.2 Billet Volume Calculation

The cold-headed billet consists of three parts: spherical segments, ring bands, and poles. Its volume is given by:

Vbillet​=MDw3​+πBA(A+Dw​)

where Dw​is the billet diameter, Bis the ring band thickness, Ais the ring band width, and θis the cone angle of the billet.

2.3 Theoretical Calculation of Bar Dimensions

Based on volume consistency before and after cold heading, the bar dimensions must satisfy:

l=λd,d<Dw​<l

where dis the bar diameter, lis the bar length, and λis the compression ratio. For a 50 mm steel ball, a bar with d=34mm and l=79.5mm was selected; for a 65 mm steel ball, d=43mm and l=108mm were used.

3. Simulation of Cold Heading Process

A 3D model was imported into Deform software for simulation. The upper die moved downward at 12.7 mm/s, with a minimum step size of 0.03 mm. The required heading forces for 50 mm and 65 mm steel balls were 4.78×105kN and 8.09×105kN, respectively. The equivalent stress distribution was uniform, with higher stress at the poles and ring bands.

4. Theoretical Crushing Load Calculation

Using ABAQUS, a finite element model of the steel ball was constructed. The crushing load was calculated based on the work done during compression:

W=F⋅S

For the 50 mm steel ball, the crushing load was 1.16×106N, meeting the GB/T 34891-2017 standard.

5. Experimental Validation

Cold heading tests produced billets with clear poles and ring bands, smooth surfaces, and no cracks. After heat treatment, the hardness of the finished steel balls met standards (61–62 HRC after conventional tempering). Crushing tests showed that the 50 mm steel ball had a crushing load of 1,328.7 kN, while the 65 mm ball exhibited a dense martensitic microstructure with no cracks, confirming high strength.

6. Conclusion

Theoretical and experimental results demonstrate the feasibility of cold heading for wind turbine bearing steel balls. Future work will optimize bar dimensions to reduce machining allowances.