The key aim of lubrication is to decrease heat and wear occurring between contacting metal surfaces when in relative motion. Although heat and wear cannot be entirely eliminated, lubrication can help reduce them to acceptable or negligible levels. As excessive wear and heat are typically associated with the force of friction, both impacts can be minimised when the friction coefficient is reduced between surfaces in contact.
Additionally, lubrication is used to prevent rust and other corrosives and reduce oxidation. It is also used to insulate in transformer applications and transfer mechanical power in hydraulics and seal parts against contaminants like dirt, dust, varnish, and excess moisture. There are two key principles which govern lubrication, boundary lubrication and hydrodynamic lubrication.
What is hydrodynamic lubrication?
While boundary lubrication involves an oil film that is not adequate to totally stop any metal-on-metal contact, hydrodynamic lubrication features a continuous and full-fluid layer of protection that effectively separates sliding metal surfaces. Hydrodynamic lubrication is considered the most common of the two lubrication principles and applies to almost all kinds of constant sliding action in cases where extreme pressures aren’t involved.
Regardless of whether the sliding action occurs on flat metal surfaces, (like with most thrust bearing applications), or whether the metal surfaces happen to be cylindrical in nature, (like journal sleeve and plain bearing applications) the principle remains much the same.
While you might expect that as one part slides over, or on another, the lubricating oil film existing between them would swiftly be scraped off, under certain conditions regarding reciprocating motion, this doesn’t occur at all. Engineers understand that when the correct design is deployed, this continuous sliding motion supplies the means of construction and maintaining a robust oil film.
Hydrodynamic lubrication involves a thicker lubricant film than that produced in boundary lubrications. As a result, it is sufficiently viscose enough to ensure the metal surfaces remain separated avoiding direct metal-on-metal contact preventing wear and heat issues occurring.
Hydrodynamic lubrication is considered more effective in applications with heavy loads or high relative speeds, as such conditions generate the required pressure to effectively maintain full film lubrication.
What are the benefits of hydrodynamic lubrication?
Hydrodynamic lubrication is referred to often as “stable lubrication”. The key benefit of hydrodynamic lubrication is that it can provide continuous protection to parts that would otherwise endure friction when working. The constant barrier is maintained preventing heat and wear build up while sealing components against unwanted debris or water contamination. The continuous film also defends metal surfaces from corrosive forces like rust.
It is deemed an exceptional method of lubrication as it’s possible to achieve friction coefficients as low as 0.001 where no wear occurs between moving components. Special care must be taken regarding lubricants being heated by the friction as viscosity is always temperature dependent. Options used to mitigate heat issues include selecting a lubricant with viscosity index improve additives or adding a cooling reservoir to lubrication cycles.
Hydrodynamic lubrication can achieve extremely low friction coefficients. As a result, it reduces energy loss caused by friction. The lubricant creates a film in between surfaces, keeping them apart effectively and preventing unwanted wear.
It is also especially effective working at high relative speeds, under heavy loads and other conditions that produce the pressure required to maintain the protective fluid film it provides. Extremely suited to high speeds, lubrication is enhanced as operating speeds are increased rather than lessened.
Hydrodynamic lubrication also offers valuable damping properties that can reduce vibration and noise while offering additional benefits for bearings. The damping effect results in smoother operations, with the film absorbing shock loads and evenly distributing forces. This protects bearing surfaces and enhances overall robustness, particularly in applications involving dynamic load changes. The vibration reducing effect also allows machinery to operate more quietly, improving work environments.
The effects of decreased friction and wear also mean that hydrodynamic lubrication, supplied by high quality lubricant brands like Kluber and Shell, also promotes long service life for equipment and components.
What are the disadvantages of hydrodynamic lubrication?
To remain effective, hydrodynamic lubrication has specific requirements. Fluid viscosity is key and must be high enough to maintain sufficient film thickness to keep contacting surfaces separated at desired operating speeds. The fluid in use must be able to adhere to the metal contact surfaces for transfer into the pressure area to effectively support the load.
Viscosity is the property of lubricants that defines their thinness or thickness and quantifies their resistance to flow. In hydrodynamic lubrication, lubricant viscosity is critical as it directly dictates the lubricant’s capacity to form and then maintain full film lubrication.
However, fluid viscosity is not constant and can change when subjected to various factors. Hydrodynamic lubrication product’s viscosity can be impacted by temperature changes. If the temperature decreases, the viscosity of the lubricant will increase, making it thicker however, temperature increases will reduce the viscosity and make it thinner.
Changes in pressure also affect viscosity. Pressure increases also increase lubricant viscosity, so it is more resistant to flow. Shear is the force that is applied parallel to surfaces that are in motion. In terms of hydrodynamic lubrication, shear present decreases viscosity, and makes the lubricant increasingly more fluid-like. Finally, time can take its toll on lubricants and as they become less stable their viscosity is impacted.
As a result, alterations in viscosity directly impact how effective thermodynamic lubrication is.
To work properly, the lubricating fluid must distribute itself comprehensively and the operating speed must be high enough to allow both formation and maintenance of the full-film lubrication. As a result, thermodynamic lubrication is not suitable for slower speed operations and lighter loads.
Thermodynamic lubrication cannot work effectively when contacting surfaces are not smooth as sharp asperities can disrupt stability of the fluid film.
A drawback of hydrodynamic lubrication is its dependence on speed and temperature. To work effectively, it needs the relative motion between surfaces to generate sufficient pressure. At extremely low temperatures, lubricant viscosity can increase, which may inhibit the creation of a full film.
Hydrodynamic lubrication offers poor performance at low speeds because a minimum sliding velocity is required to generate the pressure that forms the load-carrying film. With no movement, the oil film does not form, and there is an increased risk of metal-on-metal contact.
This lubrication method doesn’t hold up well against start/stop challenges. Like at low speeds, it is ineffective during frequent stops and starts, leading to friction and wear. It is also prone to instability because of its self-reinforcing cycle regarding viscous heating. This increases fluid temperature and effectively lowers viscosity, thinning the fluid film and causing further heating.
Unlike some other lubricating methods, ancillary equipment is required for hydrodynamic lubrication to maintain optimal lubricity, manage heat, and ensure adequate lubricant supply. This includes equipment like heat exchangers for cooling lubricants, reservoirs and pumps for delivery, filters for purification, and monitoring sensors, adding to the complexity and cost of operations.
Hydrodynamic lubrication also has a sensitivity to contamination. Contaminants can disrupt the lubricating film’s formation and cause abrasive wear, while aiding detrimental chemical reactions like corrosion and oxidation.
