Q: What are the advantages and disadvantages of cold-heading tapered rollers for rolling bearings?
A:Cold-headed tapered rollers are produced at room temperature by utilizing a die to force a metal billet segment to undergo plastic deformation, thereby filling the cavity formed by the die and the punch (male die). The advantages are as follows:
① Enhanced Mechanical Properties: Since the metal undergoes plastic deformation at room temperature during cold-heading, work hardening inevitably occurs. Furthermore, the original grain flow lines of the bar stock remain uncut; any minor internal defects within the metal can also be compacted, which contributes to improving the strength of the roller.
② Material Conservation: Cold stamping is inherently a plastic forming method that generates minimal or no chips. If the process parameters are optimized, the amount of flash (excess material) can be kept very small, thereby resulting in significant savings in metal materials.
③ High Productivity: Generally, cold-heading machines feature a high degree of automation and simple operation. Once properly set up, production efficiency is quite high, typically ranging from 70 to 100 rollers per minute.
④ Precise Shape and Dimensions, Low Surface Roughness: The shape and dimensional accuracy of cold-headed rollers are primarily ensured by the precision of the dies and the machine tool settings. During the cold-heading process, the metal surface is burnished by the smooth surfaces of the dies under high pressure; consequently, the surface roughness of the rollers is low, typically ranging from Ra 0.25 to 2.0 µm. If the processing parameters are well-adjusted and quality remains consistent, the rollers may even bypass the soft grinding stage and proceed directly to heat treatment.
Since the cold-heading process utilizes cylindrical bar segments, during the formation of tapered rollers—prior to the final shaping—most of the roller's body is subjected to a combination of uniaxial compressive stress and biaxial tensile stress. The only exceptions are the chamfered area at the small end of the roller and the central core region, which are subjected to triaxial compressive stress. The magnitude of the tensile stress increases as one approaches the large end of the roller. Experimental studies indicate that tensile stress promotes intergranular deformation within the metal, thereby reducing its plasticity; conversely, compressive stress facilitates intragranular deformation, which enhances the metal's plasticity.
Consequently, if the design of the billet or die geometry is flawed, the material quality is substandard, or the cold-heading process parameters are inappropriate, cold-headed rollers frequently exhibit cracking in the chamfered area at the large end. Furthermore, during the cold heading process, factors such as friction between the raw billet and the die surface, internal material inhomogeneity, and unreasonable die geometry can induce additional internal stresses within the cold-headed roller. These stresses reduce the metal's plasticity, increase its resistance to deformation, and generate residual stresses within the roller. Such residual stresses can lead to dimensional and shape deviations in the roller, as well as a degradation of its processing performance. In particular, the additional stresses induced by external friction exert a detrimental influence on both the quality of the cold-headed roller and the manufacturing process itself. External friction primarily occurs in the axial region of the chamfer at the roller's small end; this friction compromises the internal and surface quality of the roller at the chamfered section, while simultaneously accelerating die wear and reducing die service life. Consequently, during the cold heading process, efforts should be made to minimize friction between the workpiece and the die. The primary factors influencing friction during cold heading include material properties, die geometry, surface finish, and lubrication effectiveness. Since the cold heading of rollers is performed at room temperature, the material's resistance to deformation is substantial. This is particularly true when the billet material fills the die cavity to form the roller blank; at this stage, the entire solid body of the roller is subjected almost exclusively to triaxial compressive stresses, resulting in extremely high deformation resistance. As the size of the roller increases, the required heading force also increases; consequently, the maximum permissible dimensions of cold-headed rollers are often constrained by the power capacity of the cold heading equipment and the structural strength of the die. Moreover, if the degree of cold deformation applied to the roller exceeds the maximum allowable deformation limit of the metal material itself, cracks will form on the circumferential surface of the roller, resulting in the production of defective parts. Therefore, for rollers of varying shapes and dimensions, an appropriate degree of deformation must be carefully selected.
About LJHB:
LuoYang JinShuo Precision Bearing Co., Ltd. is mainly committed to the R&D and sales of high-precision bearings and other special products, and provides bearings and transmission parts with high performance for customers' equipment. With quick response, from pre-sales technical communication to on-time production delivery, as well as perfect after-sales service, we win the recognition and trust of customers.
Our main products include cylindrical roller bearings, tapered roller bearings, slew bearings, gear drives, spindle bearings, crossed roller bearings, turret bearings, thin-wall bearings, tandem roller bearings, spherical roller bearings, etc. The bearing dimensions rang from 100mm to 6000mm, the accuracy reaches P5, P4 and P2 levels. The bearing are widely used in mining and metallurgy, industrial equipment, industrial gearboxes, petroleum equipment, cranes, medical equipment, robotic equipment, etc. Among them, cylindrical roller bearings with better performance and high-precision deep groove ball bearings have been explored for high-speed tubular strander rotating cable equipment, which have completely replaced imports, saving customers costs and time, and improving work efficiency. It has been recognized and widely praised by customers.
Contact LJHB:
Address: Yiyang Bearing Industry Zone, Luoyang City, China 47100
Tel/Wechat: +86-18037970383
Whatsapp: +86-18037970383
Fax: 0086-379-65199160
E-mail: lyjh@lyjhbearing.com
Website: www.chinajhbearing.com
