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31 May 2012

The lubrication process: The right viscosity is vital for a proper, protective cushion

Lubrication is essential for keeping machines running. When a lubricant properly cushions moving parts and prevents metal-on-metal contact, it minimizes friction and wear. This reduces maintenance costs and downtime, and it can prolong equipment life. However, achieving proper lubrication can prove challenging. Many factors can affect the development of the protective cushion (also known as an oil wedge), including the application’s operating speed and load, and especially the lubricant’s viscosity. Understanding their interplay may help you ensure optimal lubrication and protection for your equipment.

Stages of lubrication

The lubrication process, going from no cushion at all to a sufficient cushion, generally takes place over three stages:

  1. Boundary lubrication — During start-up, operating speeds are relatively low and the load is high. The lubricant cushion is just beginning to form and is merely a thin film at this point. The probability of metal-on-metal contact is high, and so is the potential for significant friction and wear. This situation is known as “boundary” lubrication. Typically, machines will experience the most friction and wear during the start-up stage as well as during shutdown. (The speed and load conditions during shutdown typically are similar to those occurring at start-up.)
  2. Mixed lubrication — As operating speed increases, the cushion develops between moving surfaces as they begin to separate and more lubricant is drawn between them. Friction is reduced as the cushion of lubricant builds, but the cushion is not yet sufficient to prevent all metal-on-metal contact. This situation is known as “mixed” lubrication.
  3. Hydrodynamic lubrication — Continued increases in operating speed bring lubrication to the ultimate stage known as “hydrodynamic” lubrication. In this stage, surfaces are completely separated and properly cushioned with lubricant so that metal-on-metal contact is virtually eliminated. Friction reaches a theoretical minimum.

The goal for any lubrication strategy should be to reach the hydrodynamic stage as quickly as possible and to remain in that stage for as long as possible whenever the equipment is operating. After all, this results in the best equipment protection and the least amount of wear, minimizing maintenance costs and downtime.

The impact of viscosity, speed and load

For the proper cushion of hydrodynamic lubrication, you need a lubricant with the right viscosity for the application’s operating speed and load. Some important points to consider:

  • The viscosity needs to be high enough to support the load, but not so high that it impedes the movement of the system’s parts.
  • The slower the operating speed, the longer it takes for the lubricating cushion to develop. If your system’s operating speed is relatively low, you may need a higher viscosity to compensate for this and build the cushion in a timely manner. Otherwise, the system may experience excessive friction, wear and potential damage.
  • In high-speed operating conditions, the cushion will build faster, so a lower viscosity will usually suffice.
  • The higher an application’s load, the more cushioning support it requires. Therefore, high loads typically call for higher viscosities than low loads.

How high is “high”, and how low is “low”, with regard to speed, load and viscosity? How do opposing conditions in an application, such as a combination of high speed and high load, affect viscosity requirements and lubricant selection? What impact do other factors such as temperature and, in the case of refrigeration applications, refrigerant-lubricant solubility have on the choice of lubricant? The answers to these questions vary with each individual situation. Clearly, the interplay of an application’s specific characteristics will affect lubricant suitability. It is recommended that you seek expert guidance to evaluate your particular operating system and identify the right lubricant solution.

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