Why Choose Self-Lubricating Bearings?

Introduction 

Bearings are a vital part of almost every piece of machinery, enabling rotating and sliding motion. A common type is the Plain Bearing, also referred to as Bushing or Sliding Bearing. For a Plain Bearing to work effectively it is important that adequate lubrication is applied. This can often be achieved by using a Self-Lubricated Bearing.

Lubrication is often the last thing considered in any design, but it should be treated like any other major component. Most lubricants after all can be considered liquid engineering. A lubricant is a medium introduced between two components that slide relative to each other. The task of the lubricant is to prevent the two components from coming into contact and therefore abrading each other.

 

In this blog we will be discussing the benefits of Self-Lubricating Bearings, four different types of Self-Lubricating Bearings, and aspects to consider selecting a s Self-Lubricating Bearing.

 

Benefits of Self-Lubricating Bearings

Self-lubricating bearings enjoy significantly longer maintenance and replacement intervals, greater machine uptime, higher throughput, and cleaner operation. They also save money by eliminating the need for expensive lubrication systems and lubricants.

Utilising self-lubricating bushings can provide several benefits for end-users to be aware of.

 

These include:

  • There is no need for oil holes and grooves in the design of the bearings. This eliminates the cost of machining the holes and grooves in the bearings.
  • Self-lubrication eliminates the need for an extra oiling system.
  • They reduce the machinery running cost since it has fewer maintenance problems.
  • Self-lubricating bearings are mostly maintenance free reducing the cost of preventive and routine maintenance.
  • Their mechanical design is simple to manufacture, with thin wall thickness; higher load capacity, and excellent wear resistance, making them more economical.
  • They are environmentally friendly as they do not need extra oil and waste management issues.

 

Types of Self-Lubricating Bearings

  • Bronze Embedded Solid Lubricant Bearings

 

Bronze embedded Solid Lubricant bearings are a common Self-Lubrication bearing suitable for high load, low velocity applications. Lubricant, normally in the form of Graphite or PTFE, is embedded into Bronze Plain bearings as polka-dots covering the surface of the bearing in a systematic pattern. As the bearings slide the solid graphite or PTFE lubricant is deposited to the bronze surface of the bearing creating a lubricant film to achieve a hydrodynamic condition.

 

Graphite has some unique properties that make it an excellent lubricant. Chemically, it is one of three common allotropic forms of carbon, with amorphous carbon and diamond being the other two. In contrast to diamond, which has a very dense and strong crystal structure, graphite has a two-dimensional crystal structure, strong in two dimensions but weak in the third. Its atoms are arranged in parallel sheets that are easily sheared off, giving graphite its characteristic slippery feeling. It also has a near-zero coefficient of thermal expansion. This enables it to be an excellent solid lubricant option especially in high temperature conditions.

Sintered bronze bearings are made of porous bronze or iron and are usually filled with oil. During operations, the lubricant which is already impregnated in the bearing material is released through pores from the sliding layer of the bearing thereby lubricating the contact surfaces. The lubricant is dispersed quite uniformly over the contact layer providing low friction between the moving surfaces.

 

When the motion stops, the micro-porous material acts as a sponge to reabsorb the oil, preventing excessive build-up of the lubricant on the shaft and eliminating any messy dripping that comes with manual oil applications.

 

A specially coated surface is added at the top of the sliding layer to provide a low friction at the start-up of the operations to allow time for the impregnated lubricant to reach the bearing surface.

 

Metal-Polymer Bearings typically consist of a metal layer that provides high mechanical strength, a sintered bronze layer then overlayed with a PTFE layer or high performing thermoplastic. These bushings typically have a thin impregnated layer of modified PTFE that is mechanically bonded to the backing. PTFE lowers friction, improves wear resistance, and increases dynamic load capability and performance life.

Composite is woven from Polyester cloth with embedded Moly or PTFE lubricants and high-performance resins. Polyester Composite contains unique properties to achieve self-lubrication and strength while being environmentally friendly. Composite is laminated polymer materials made by impregnating textiles with thermosetting resins. Solid lubricants are added to the resin to provide evenly dispersed lubrication throughout the material. As sliding begins under load, the solid embedded lubricant is spread from the bearing onto the mating surface to create an ideal hydrodynamic condition.

 

Polymer Composites offers high load capacity and better dimensional stability than most other non-metal bearing materials. Suited for high load, slow speed applications, Composite is the desirable choice for applications where there is water exposure, servicing is difficult and when vibration dampening is required.

Applications of Self-Lubricating Bearings

Self-lubricating bushings have been widely used in textile machinery, steam and gas turbines, injection moulding machines, Primary Metal, medical equipment, the food industry, wind and hydro power generation, timber processing equipment, heavy-duty equipment, and a host of other industries. Below is a summary of industries and what self-lubricating bearings are typically selected:

  • Hydropower
    Hydropower stations run bearings in Wicket gates, Spillway gates, Intake gates and the Main Guides all require bearings that operate under high load while being exposed to waterways. To prevent waterway contamination from oil or grease these applications suit Polymer Composite bearings or Self-Lubricating Bronze bearings with PTFE embedded lubricant.
  • Primary Metal Industry
    In Steel Mills there is heavy loading on rollers with high amounts of heat. Getting lubrication distributed to these applications is difficult and dangerous so Bronze with embedded graphite plugs for lubricant are commonly used. Bronze and graphite can operate in temperatures up to 300°C. Without the requirement for regular servicing Steel Mills can operate with less downtime and with increased reliability.
  • Marine and Offshore Oil & Gas Rigs
    Rubber Bearings, Pintle Bearings, Fairleads and Stern Tube are common applications where self-lubricated bearings are used to avoid waterway contamination. Polymer Composite is a common choice in these applications as it self-lubricates, doesn’t swell and can withstand medium loading. Stern Tube can be made with water grooves to create a hydrodynamic condition allowing the Propeller drive shaft to spin at speed.
  • Medical Equipment
    With medical operating in conditions where sanitation is critical, it is advantageous using self-lubricating bearings, so no grease is spread and there is less requirement for service engineers to handle equipment. Metal-Polymer bearings are often used in Medical Equipment. These can handle the load requirements and their thin-wall design means equipment can be made compact and lightweight.
  • Kiln Trolleys
    Trolleys operating in ovens such as Ceramic Kilns or Timber Kilns utilise self-lubricating bearings as it is not possible for people to access the equipment to lubricate. Bronze with graphite plug lubricant can operate in these oven temperatures and can handle the weight and impact applied to Trolleys during loading movement.

Industrial steam turbine

Selection Aspects

Common reasons why Self-Lubricated bearings are selected are as follows:

  • Specifying Self-Lubricating Bearings

While designing a maintenance-friendly system or replacing a bearing that requires manual lubrication, the design and application factors that you should consider are similar. Maintainability adds a critical dimension impacting almost every variable. Self-lubrication options should be considered for just about every application or system.

  • Availability of Maintenance Support

Where end-user maintenance resources are limited, lubrication-free bearings are an easy choice. Independent testing shows that self-lubricating bearings exhibit no need for maintenance or addition of lubrication to the lube block during their operational life.

  • Life cycle

The possibility of consistent performance and maintenance-free operation can extend the life of the bearing by as much as three times when compared to a bearing that is lubricated just once, and not subsequently, during installation.

  • Accessibility

One of the key factors in determining whether to specify a self-lubricating bearing is the location of the bearing on the equipment. For lubrication of conventional bearings, more disassembly and reassembly is required to access it. Using a self-lubricating bearing will save time and money for maintenance in most applications.

  • Temperature

Vendors rate their bearings for operation in certain ranges. Operation outside those ranges could result in bearing failure. Some lubricants, for example, will dry up if the operating temperature is too high. Self-lubricating bearings normally run faster and therefore run hotter but since lubricant spill is not an issue they cause fewer problems.

Limeash Bricks Cart Kiln
  • Environmental Constraints

The ambient conditions in which the bearing has to operate also affects its maintenance-free capability. Operations in areas with high environmental dust or oil and grease may require minimal maintenance of the self-lubricating bearings.

  • Total Cost of Ownership

While the upfront cost of a self-lubricating bearing may be higher than that of conventional bearings, but the lower maintenance costs, elimination of lubrication assemblies, better performance, and reduced downtime makes up for it in the long run.

Conclusion

With inadequate lubrication being the most common and preventable cause of shortened bearing life and frequent failure, it is quite cost-effective if you consider self-lubricating bearings in your design.  In addition to maintenance savings and longer life, the lubricant-free bearings dispense the need for expensive lubrication systems and the cost of the lubricant itself, ensuring more environmentally clean operation. By considering these factors early on in the design process, maintenance costs can become one less thing engineers will have to worry about as they design equipment.

 

The Kormax Self-Lubricating Bearings Range

Kormax offers standard and custom self-lubricated bearings with world-class performance profiles. Should you opt for custom design, you can determine everything from what oil is used, to the design characteristics, tolerance level, and assembly method.

The Kormax Range of Self-Lubricating Bearings includes:

Download The Bushing Book

Get Started with Kormax

Let’s talk. We’ll answer your questions, provide direction and show you a solution that works for you.

Materials available in the New Zealand Catalogue are also available upon request.