How are bearing balls made?

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The importance of  crushing load test

As an important part of ball bearings, steel balls have a great influence on the life, accuracy, vibration and running characteristics of ball bearings. When the bearing works, the contact surface of the steel ball and the raceway is actively small, and the compressive stress per unit area is large, generally up to 150-500 kg/mm2.

The points on the contact trajectory of the steel ball to the raceway are cyclically stressed. Under normal circumstances, the number of cycles per minute is more than tens of thousands of times. Under the repeated action of cyclic stress, the is prone to fatigue damage. This phenomenon is called contact fatigue, which is one of the main features of the normal damage of the steel ball in the bearing. When contact fatigue reaches a certain level, it will lose its ability to work.

The crushing load of the refers to the pressure value of the steel ball when it is broken. It is an important indicator of the comprehensive performance of the steel ball. There are many factors in the processing of the steel ball affecting the crushing load value of the steel ball, such as the original metallographic structure of the steel ball, the microstructure of the heat treatment and different processing methods, the geometric precision of the surface and the processing time of the steel ball. And test methods for measuring the crushing load of steel balls. The crushing load inspection of steel balls is a very important task in the quality inspection of steel balls, which also plays a significant role in the life and dynamic performance of the bearings.

How are bearing balls made?

Bearing balls are the component of a ball bearing that permit smooth, friction-free rotary motion. They are typically manufactured out of steel, but can also be made from other materials such as silicon nitride ceramic, plastic or even glass.

Steel bearing balls, such as , are easily the most common ball material, with tens of millions produced annually. They are near-perfectly round and are polished with a mirror-like finish to provide precision rotary motion in machines, power tools, motors, power transmission and hundreds of other applications.

Have you ever wondered how they are made? The answer is a multi-step process that converts a slug of unhardened steel to a hardened, ground and polished round ball.

The slug

Metal bearing balls are manufactured from steel wire or rod. During the first step in the process, the wire or rod is cut into small pieces called slugs. The volume of material for the slug is slightly larger than that of the finished ball. The excess material is removed in subsequent machining steps. At this stage, the material has not been hardened and is somewhat malleable. This is important so that the slug can be formed into a spherical shape in the next manufacturing step.

Heading

The slug is then fed to a header machine where the balls are put through a cold-forming process called heading. In cold heading, the slug is placed between two semi-spherically shaped dies and shaped under pressures between 10 and 20 tons. It is a high-speed process and can be very loud, especially for large-diameter balls, requiring ear protection for machine operators. Heading forms the slug into a spherical shape called a raw ball. There is a little excess material that still needs removal.

Flashing

The excess material, called flash or burrs, needs to be filed away. This is done in the flashing process, also referred to as filing. This removes the flash, but they are not yet perfectly round and the metal is still soft. There are two different machining variations that manufacturers use, but both involve rolling the balls between metal plates.

Soft grinding

Some manufacturers will then put the balls through a soft grinding process. It is referred to as soft because the material is not yet hardened. This operation is similar to the flashing process but an abrasive grinding stone is used instead of one of the metal plates.

Heat treating

Now that the balls have had the excess material removed and are relatively round, they are put through a heat-treating process to harden and strengthen. The balls are heated to approximately 1,500° F and then quenched in an oil bath. They are heated again to a much lower temperature. approximately 325° F, resulting in a ball that is through-hardened.

Descaling

After heat treating, the balls are discolored and covered with oxide deposits. To remove these deposits, the balls are put through a process called descaling. Descaling involves using a chemical agent, usually an acidic compound, to remove the build-up on the balls.

Grinding

Now that the are hardened and relatively round, they are nearing the end of the manufacturing process. The next step is to grind the balls to size them and improve roundness geometry. The process is similar to the soft grinding process used earlier — the balls are rolled between a steel plate and a fine-grit grinding wheel.

Lapping

Finished steel balls, including .The last step in the process is to lap the balls. Lapping is a superfinishing process that improves the surface finish or geometry and removes small amounts of material so that the balls attain the tight, required tolerances.

The balls are rolled between two hardened steel plates, with one fixed and one rotating at a low speed. The balls are now perfectly round and have a mirror-like finish.

Washing and sizing

Now that the manufacturing process is complete, they are washed to remove any leftover residue. Lastly, the clean balls are inspected for scratches and sized. Even though the balls all have the same nominal outside diameter, small variances still occur. The ball grade defines the geometric tolerances of the bearing. The lower the grade number, the tighter the tolerances. The number of the grade is related to the sphericity and diameter variation of the balls. The tolerances are extremely tight and the variation between balls is quite small.
 
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