Lubrication Methods for Universal Joints
Universal joints, primarily the cross-type universal couplings and needle bearings, require frequent lubrication during use. Improper lubrication can lead to wear, indentations, and peeling. Unfortunately, many maintenance workers have misconceptions about the lubrication of cross-type universal couplings, often incorrectly using grease, which not only fails to lubricate effectively but may also cause damage.
1. Why Should Universal Joints Not Use Grease for Lubrication?
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Calcium-based Grease Structure: Over extended machine operation, the base oil in calcium-based grease can separate, causing the oil film to fail and not perform its intended function. Furthermore, grease has low fluidity, and once the oil film deteriorates, it’s difficult for a new one to form. This can result in semi-dry or dry friction, leading to wear.
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Grease Evaporation Forms Soap: Grease can evaporate and form soap-like substances that hinder the movement of the needle bearings, preventing proper lubrication. Additionally, there is no oil drainage system, so the grease cannot be removed or replenished easily.
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Weak Oil Film Strength: Grease has poor oil film toughness, which makes it difficult for the universal joint to form a good oil film during operation. Therefore, gear oil should be used instead.
2. Advantages of Gear Oil
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High Oil Film Strength: Gear oil forms a strong oil film with excellent elasticity, providing better lubrication under operational conditions.
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Good Oiliness and Extreme Pressure Properties: Gear oil has superior oiliness and extreme pressure (EP) properties, enabling it to establish a boundary lubricating film, which is essential for reducing friction in universal joints.
3. How to Lubricate Cross-type Universal Couplings
Due to the poor sealing of steel cups and the high fluidity of gear oil, gear oil can easily be thrown out, leading to semi-dry or dry friction. To address this, lithium-based grease can be used in combination with regular maintenance practices to break down and clean the universal joints.