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Powder Metallurgy (Sintering)

2023/01/31
What are oil-impregnated bearings? Explanation of their characteristics and manufacturing process
METAL COLUMN

What are oil-impregnated bearings?

Oil-impregnated bearings are bearings that have lubricating oil inside. When the shaft rotates, a pumping action occurs, causing oil to seep out from the bond molecules inside the bearing. This oil prevents metal contact (galling), and also acts as a cooling agent as it seeps out to the sliding surface of the bearing.
This is called the capillary effect. When the bearing is stationary, the oil that seeped out is reabsorbed into the bearing pores by capillary force.

Characteristics of oil-impregnated bearings

The characteristics of oil-impregnated bearings are that they are self-lubricating, so they can be used for long periods of time without oiling (lubrication), the oil content can be adjusted over a wide range, and solid lubricants can be used in combination. In addition, because oil is supplied from the entire surface of the bearing, seizure will not occur even if the oil runs out. Even if the oil runs out, oil can be supplied by penetration through the bearing wall.
In addition, its porosity provides excellent vibration absorption and quiet operation. For this reason, it is often used in motors and other parts that are prone to generating noise. In terms of cost, it is suitable for mass production as it mainly involves pressing and sintering, and it can keep costs low by eliminating the need for cutting processes, which are expensive.

Characteristics and applications of oil-impregnated bearings

Oil-impregnated bearing materials are mainly divided into copper-based, iron-based, and iron-copper-based. Copper-based materials are mainly used in audio equipment, office equipment, and motors.
Iron-based materials are slow, but can withstand high loads and are highly wear-resistant. They are mainly used in toys and household appliances. Copper-iron materials are intermediate between copper-based and iron-based materials, and are often used to reduce material costs. There are materials that can handle high speeds, have good conformability, and can withstand high loads, but what they all have in common is that they are wear-resistant.
It can be used in office equipment, audio equipment, and home appliances. It can be used in a wide range of environments by combining a variety of lubricants and bearing materials.

PV value

PV values are originally divided into P and V values.
The P value is the surface pressure and is calculated by dividing the maximum load (W) that can be applied to the bearing by the projected area (d x L) of the bearing.

The V value refers to the speed, which is the relative speed between the mating member and the bearing.
The PV value is the product of the P value and the V value, and is a reference value when checking for abnormal wear, such as melting and burning of resin due to frictional heat on the sliding surface.

The PV value at which these abnormalities occur is called the limit PV value. Using the product above the limit PV value means that it will eventually become unusable.

Manufacturing process

Oil-impregnated bearings are manufactured in the following manner.
Raw materials → Mixing → Molding → Sintering → Sizing → (machining) → Vacuum impregnation → Inspection → Packing → Shipping
First, the raw powder is selected and mixed in a mixer. Next, it is compressed from above and below in a press molding machine to create a green compact. The green compact is then fired in a sintering furnace to become a sintered body. After the sizing process, it is impregnated with lubricating oil in a vacuum oil immersion furnace (vacuum oil immersion) and is then completed.

Raw Powder

Generally, we use premixed powders sold by various material manufacturers (powder manufacturers). Sometimes bronze powder or carbon powder is mixed into the premixed powder in the next process.

mixture

Mixing refers to mixing two or more different substances. During the mixing process, the materials must be mixed properly. This is called segregation, and refers to the powders not mixing together and becoming separated. To prevent this, we recommend using a small amount of spindle oil. In the mixing process, a V-shaped mixer is mainly used to mix the materials for a specified period of time.

Molding

The molding process begins with a skilled technician setting the mold (upper punch, lower punch, die, core) into the molding machine. Because this is a very delicate task, it takes 1.5 to 2.0 hours to set up the mold (also called setup). Once the mold is set, a semi-automatic compression molding machine is used to pour the raw material (powder) into the mold and compress it with the upper and lower punches. The molded body (green compact) looks finished, but since it is simply powder that has been solidified, it will crumble into pieces if you squeeze the green compact too hard with your hand.

Sintering

Sintering involves baking and solidifying a green compact (green compact) in a sintering furnace (continuous furnace). The sintering furnace is divided into three stages: the preheating stage (to remove the lubricant added to the green compact), the main sintering stage (to bake at 800°C to 1300°C and bond the powder together), and the cooling stage (to cool the sintered compact). Since copper, iron, and stainless steel materials have different melting temperatures, some sintering manufacturers have multiple sintering furnace lines. In addition, stainless steel powder green compacts are sintered in a vacuum furnace.

Correction and sizing

Sizing, also known as shaping, is a process for further improving dimensional precision and strength.
The sizing process is almost the same as the molding process, except that the sizing die is placed in a compression press.
The sintered compact is then placed into a sizing die and recompressed.

The sizing process is an important step in oil-impregnated bearings.
First, to produce inner diameter dimensions with high precision requirements.
The other is to adjust the pores.

If the pores are not uniform, it can lead to too much or too little lubricant seeping out. For this reason, a table called the porosity rank is sometimes used to adjust the porosity distribution level in advance.

■ Sizing mechanism
When the sintered body is placed in a die and pressure is applied with the upper and lower punches, the material is pressed against the die and the upper and lower punches, correcting any deformation or dimensions of the material. There are two types of sizing methods: positive sizing and negative sizing.

・Positive sizing method
A method in which the material is made larger than the final product dimensions, and then during sizing it is pressed into a mold and rubbed against a die or core to achieve precision.

Negative Sizing Method
A method in which the material is made smaller than the final product dimensions and compressed inside the mold to press it against the mold surface, thereby achieving precision.
The sintered product is placed in the metal mold again and pressure is applied to improve the dimensional accuracy of the sintered product. For products that require even greater dimensional accuracy or strength, the metal mold is used again to recompress the product.

Post-processing and post-treatment

Post-processing of oil-impregnated bearings often refers to machining. To reduce costs, they are designed so that machining is not required, but machining is necessary when drilling or tapping a side hole is required. Also, when high accuracy is required for runout in oil-impregnated bearings, the runout may be removed using a lathe.

For oil-impregnated bearings, contact Kiyota

We will propose the optimal shape, production base, and logistics according to your specifications and requests. Since we have sales bases overseas, we can provide you with the best price in one stop. In addition, we accept orders for oil-impregnated bearings starting from a minimum of 100 pieces, so if you are having trouble dealing with small lots, please feel free to contact us!  

FAQ


Q: What are oil-impregnated bearings?

Answer: Oil-impregnated bearings are bearings that have lubricating oil inside them. When the shaft rotates, a pumping action occurs, causing the oil to seep out from the bond molecules inside the bearing.

Question: What is the manufacturing process for oil-impregnated bearings?

Answer: The manufacturing process of oil-impregnated bearings is as follows: raw materials → mixing → molding → sintering → sizing → (machining) → vacuum impregnation → inspection → packaging → shipping. We will introduce each process in detail.
For more details, click herePlease refer to.