Today, respected lubrication manufacturers like Kluber, Mobil and Millers are pushing the boundaries of equipment efficiency and lifespan. Fluid engineers formulate solutions that allow components to move more freely and stay protected for longer extending their active service life to the benefit of the businesses that use them. Many products are designed for hydrodynamic lubrication, a lubricating method that forms a film between moving metal surfaces.
The robust film is designed to reduce friction and prevent the moving parts becoming worn. Without appropriate lubrication, damage occurs over time leading to many unwanted outcomes. These include but aren’t limited to mechanical downtime, repair costs and the expense of loss of productivity, replacement parts or irreparable damage to an expensive asset resulting in lost investment.
Hydrodynamic lubrication is a lubrication technique that uses a thick film of lubricant to separate two moving surfaces, reducing friction and wear. It’s also known as full film or thick film lubrication
What is the basic theory of Hydrodynamic Lubrication?
Sometimes called thick film, full film or stable lubrication, hydrodynamic lubrication creates a fluid film that separates two surfaces while they are in motion, preventing friction and stopping wear. The basic idea behind hydrodynamic lubrication is that fluid pressure forms a wedge in between the metal surfaces to lift them apart so they don’t contact one another. How durable the lubrication film is referred to as the film strength.
It’s understood that many people might think that after lubrication is applied and parts start to move in a sliding motion that the film formed would quickly be removed in the process. However, this is not the case in a reciprocating motion when certain conditions like viscosity and operating speed are met. Informed engineers know that as long as the mechanical design is correct, the continuous sliding motion of moving parts facilitates the formation and maintenance of a strong thick fluid film that protects them from friction and wear.
Why is Hydrodynamic Lubrication important?
There are two main lubrication methods, hydrodynamic and boundary lubrication. Unlike boundary lubrication that forms a film that cannot completely stop surface-to-surface contact, hydrodynamic lubrication creates continuous and full-film of protection that keeps moving metal surfaces apart.
Hydrodynamic lubrication is classed as the most common lubrication technique and is used in countless equipment applications where sliding action is involved. As it supplies a thicker and more viscous film than boundary lubrication, it is effective in preventing metal-on-metal friction and wear.
It is considered the optimal choice for processes with high relative speeds and heavier loads, as these conditions support effective full film lubrication.
What are the two conditions for the occurrence of hydrodynamic lubrication?
There are two key conditions that enable hydrodynamic lubrication to take place – low-contact pressure and thick lubricant film. In relative motion, the metal surface needs low contact pressure and the film formed between parts must have a thickness that keeps them separate.
Other factors can also play a role however, like viscosity, geometric conformity and speed. The lubricant’s viscosity must let the hydrodynamic state be maintained working at a wide selection of operating conditions. The components lubricated also need to conform geometrically, and sliding speeds must be higher to generate a film that is thicker.
What are the advantages of Hydrodynamic Lubrication?
Perhaps the most prominent advantage of hydrodynamic lubrication is its ability to supply continuous protection to moving metal components that would experience considerable friction otherwise. The thick lubrication film forms a constant and durable barrier in between parts, preventing abrasive forces that lead to wear. As well as prevention friction, the lubrication method also reduces the build-up of heat, which in turn reduces damage and seals components protecting them from unwanted contaminants like water and unwanted particles like varnish, dirt and other debris that impairs lubrication effectiveness and clogs moving parts.
The stable lubrication film also defends machinery against rust and other types of corrosion. In operating environments that includer moisture and oxygen, equipment containing parts made from iron or iron alloys like steel are at risk of rust (iron oxide). Rusted components lose their structural integrity, making them more susceptible to damage but also cause a build-up of debris that can interfere with the smooth operation of equipment. Full-film lubrication stops rust taking root on parts reducing mechanical downtime and extending the service life of machinery and its working parts.
Hydrodynamic lubrication has been known to offer extremely low friction coefficients (0.001). Consequently, it is respected as an outstanding lubrication technique that can promote zero wear occurring in between components in motion. However, particular attention must be paid to operating temperatures. To remain effective and produce a thick film, hydrodynamic lubrication requires correct viscosity, and viscosity is impacted by temperature. However, there are options available that are employed to manage issues involving heat. For example, choosing a lubrication solution that includes a viscosity index (VI) improver additive, or including a cooling reservoir as part of the lubrication cycle.
The fluid film created in hydrodynamic lubrication can have a damping effect, helping reduce mechanical vibration. It also offers long service life in applications like hydrodynamic bearing and requires less maintenance, reducing running costs.
What are the disadvantages of hydrodynamic lubrication?
While hydrodynamic lubrication offers considerable benefits, it does have certain drawbacks that users must account for and overcome to achieve desired results. For example, to get effective results the hydrodynamic lubrication method has specific requirements.
Lubricant viscosity is of prime importance. Viscosity must be high enough that the film can maintain the right level of thickness to stop surfaces from making metal-on-metal contact at the operating speeds required. The lubricant being used must be able to stick to the contact surfaces for effective transfer to the pressure area so it can support the load.
Put simply, viscosity is a key lubricant attribute that determines how thick or thin it is and measures how resistant it is to flow. During hydrodynamic lubrication, viscosity becomes vital as it dictates the capacity of the lubricating fluid to generate and maintain a full film to protect parts.
It is important to remember that fluid viscosity is not fixed. It can alter when a lubricant is subjected to certain factors. For example, in hydrodynamic lubrication fluid viscosity may be affected by a change in temperature, whether it is higher or lower. A temperature decrease can result in a lubricant becoming thicker impacting how well it flows while a rise in temperature may result in the fluid thinning making it unable to achieve full film lubrication.
A change in pressure can also have a significant impact on fluid viscosity. A rise in pressure can increase the viscosity of a lubricant and make it more resistant to flow. On the other hand, shear decreases fluid viscosity. Shear is defined as force applied parallel to contact surfaces in motion. Regarding hydrodynamic lubrication, shear makes the lubricant in use increasingly thinner and fluid-like, preventing it from adhering to moving surfaces.
Time can also impact the effectiveness of fluids used for thermodynamic lubrication. In use over an extended period lubricants start to break down and lose their properties. As they become less stable, the fluid’s viscosity is negatively affected.
Consequently, any change to fluid viscosity impacts the effectiveness of thermodynamic lubrication directly.
Operating speeds play an important role in whether thermodynamic lubrication works correctly. To create and then maintain a continuous fluid film in between moving parts, the lubricant must be distributed comprehensively but the operating speed must also be high enough. Therefore, a disadvantage of thermodynamic lubrication is that it isn’t a suitable lubrication method for operations involving slower speeds but also in applications with lower loads.
For thermodynamic lubrication to succeed it needs contact surfaces to be smooth. While metal surfaces may appear smooth to the naked eye, at a microscopic level many contain peaks and troughs. While valleys in the metal cause little harm, sharp peaks known as “asperities” jut out and can disrupt the lubrication film’ stability, thereby reducing its effectiveness.
Common applications for thermodynamic lubrication
Hydrodynamic lubrication is especially suited to applications involving high relative speeds, heavy loads as the pressure generated assists with maintaining full film lubrication but is also suitable for large bearing diameters and forces. It remains the most often-used lubrication technique for sleeve bearings. Today hydrodynamic lubrication can be found serving a wide range of critical applications including motors, high-speed gear boxes, pumps and fans.
Sourcing appropriate fluids for thermodynamic lubrication
In sum, thermodynamic lubrication is a dependable form of lubricating equipment where specific loads and speeds exist. While it offers impressive benefits, it has some disadvantages that must be managed. If thermodynamic lubrication is the optimum method for your application, finding the correct lubricant to use is critical. Most operators will find that their original equipment manufacturer (OEM) can supply a recommendation on the best products to match their needs in terms of fluid viscosity and tolerance under specific operating conditions. However, professional lubricant distributors have extensive product knowledge and can also be a valuable source of information when advice and assistance is required.
