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One of the major contributing factors to achieving high reliability in machines is proper lubrication.
Bearings operate on thin films of lubricant,which have to be maintained in order to ensure that the bearings design life is acheived. Lubrication reduces friction and wear by creating an elasto-hydrodynamic film of sufficient strength and thickness to support the load and separate the balls from the raceways preventing metal to metal contact.
The three primary principles of maximising bearing life are:
- selecting the correct lubricant.
- applying the lubricant properly.
- maintaining the lubricant in a clean condition.
Neglect or failure in any of these three areas will increase the risk of premature failure and will trouble free running.
The increased speeds and higher temperatures at which modern bearings often operate, combined with the demands placed upon them for improved accuracy and reliability mean that the process of selecting a suitable bearing lubricant is a significant decision.
Correct lubricant selection
- Reduces friction and wear by providing an elasto-hydrodynamic film that prevents metal to metal contact.
- Minimises cage wear by reducing sliding friction in cage pockets and land surfaces.
- Prevents oxidisation/corrosion of the bearing rolling elements.
- Acts as a barrier to contaminents.
- Serves as a heat transfer agent.
Bearing lubricants fall into three main categories – oils, greases and solid dry film.
The selection of a particular type of bearing lubricant is generally goverened by the operating conditions and limitations of a bearing system.
- The viscoisity of the lubricant at operating temperature.
- The maximum and minimum allowable operating temperatures.
- The speed at which the bearing will operate.
The primary advantage of grease over oil is that bearings can be pre-lubricated, eliminating the need and cost for an external lubrication system. Besides simplicity, grease lubrication also requires less maintenance and has less stringent sealing requirements than oil systems. The drawbacks of using grease are that it does not conduct heat away from a bearing as efficiently as oil as it tends to remain in proximity to bearing components. Grease can also increase the initial torque within a bearing and cause running torque to be slighlty higher. The speed limits for greases are generally lower than for oils due to the plastic nature of grease that tends to cause overheating at high speeds.
In certain applications the design of the bearing and selection of a suitable grease become very challenging. Here it is critical that the bearing supplier has the knowledge and experience to suggest a grease that ensures maximum relaibility of the bearings over long operating periods without re-lubrication. Current ‘greased-for-life’ bearing technology can consistently give 30,000+ hours of life.
- CASTROL NUCLEOL G 110 is a radiation resistant grease. Moderate to high speeds and temperatures up to 150centigrade, where the grease may be exposed to radiation doses of up to 109 rads
- Apezion Vacuum Grease
- Leadscrews can use a light consistency lithium-based grease, NLG1 aer0-grade
While grease lubrication is simpler than lubrication with oil, there are still applications where oil is the better choice. In high speed spindle and turbine applications for example, oil is supplied continuously and provides cooling as well as lubrication. A further example is instrument bearings with exremely low values of starting and running torque. These require only a minimal, on-time lubrication, each bearing receiving just a few milligrams of oil.
The limiting speeds for oil-lubricant bearings are governed by the size of the bearing and the design of the cage, rather than by the lubricant itself.
To ensure long life at high speeds, the lubrication system should provide for retention, circulation, filtration and possibly cooling of the oil.
In extremely harsh operating environments such as dry pump bearings, the challenge is to optimise the design of the bearings in order to make the best use of relatively poor lubrication.
Solid soft film lubricants
Solid soft films are primarily used to provide solid lubrication for bearings in extreme applications where traditioanl fluid lubricants would be rendered ineffective. They offer the advantages that their friction is independent of temperatures and they do not evaporate or creep in terrestial vacuum or space environments.
The solid soft film lubricant can either be applied directly to the surface or transferred by rubbing contact from a sacrifical source such as a self-lubricating bearing cage.
Self lubricating materials
The linear tables, rotary tables and elevator tables we design and manufacture at LG Motion Ltd all have their specific lubrication equirements to ensure maximum life in a given environment. With such a variety of applications from the Film Industry to Nuclear to Formula One, we have a wide scientific and industrial experience with motion technology and the importance of understanding lubrication requirements.
Dorset based inrekor Ltd has developed an innovative structural sandwich panel technology based upon an ARPRO® core material with external layers of steel or aluminium. With far reaching potential for lightweight and low-cost vehicle chassis and many other structural frame applications across all industries, the high strength to weight ratio inrekor® panel can be applied to small and large scale manufacture with minimal tooling investment. The finished panels are assembled and bonded to form final three-dimensional structures using a patented process that guarantees exceptional rigidity – offering geometric strength of up to ten times that of traditional honeycomb panels with considerable advantages in cost, design flexibility and ease of manufacture.
Reflecting the design possibilities and the production-volume flexibility of the inrekor system, the external metal surfaces for two-dimensional panels are typically manufactured using the latest water jet or laser cutting methods; for high volume production, punch-pressing these skins can be extremely cost-effective. ARPRO cores can be supplied in flat sheet form or specially moulded for large scale production. inrekor also supply fully assembled three-dimensional assemblies or ‘flat-pack’ panels for customers to assemble on-site. The technology is fully transferrable for Tier One automotive manufacturers to use directly for high volume production.
Joining these materials together to ensure optimum peel and tear strength is a critical process and inrekor has developed an automated plasma bonding technique that employs a large frame gantry positioning system, manufactured and supplied by motion control and machine building specialists LG Motion Ltd. The atmospheric pressure plasma treatment activates the surfaces to be bonded prior to the application of the adhesives and the lamination is completed using a separate high pressure cold press.
As part of the requirement specification for the gantry system, inrekor requested that flexibility was built into its design in terms of both motion control and mechanics to allow the machine to be adapted, if required, to suit the diverse production levels that the sandwich panel system offers.
As supplied, the LG Motion motorised gantry supports and positions the plasma head over the panel bonding area where flat sheets of core or skin material are supported on a wire-mesh bed. The vertical height of the plasma head is adjusted with a handwheel operated Z-axis stage and panels up to 3 m x 2 m may be processed using a simple PC programmed raster scan where the width of the plasma beam is tracked over the entire surface to be treated.
Using belt-driven linear actuators and Profile System machine framing components from LG Motion’s associate MiniTec UK Ltd, the lower X-axis is comprised of two parallel coupled actuators to prevent axis crabbing and supports a single Y-axis linear actuator for the 2 metre span. Motorised axes are open loop stepper motor driven, with in-line gearboxes for increased torque output. Motors are fitted with rear shaft knobs that allow operators to manually move the plasma head if required. Both X and Y axes include chain-type cable management and are fitted with over-travel limit switches for safety and datum switches for homing the gantry before the automated scanning process begins. A simple stepper motor control and drive system allows the operator to set-up individual axis sequential moves using a basic like programming language.
Future mechanical modifications, all made straightforward with the modular MiniTec linear actuators and Profile System, include adding rotary encoders to stepper motors to provide ‘position verification’ for improved position control, or to replace steppers with servomotors to allow more precision with higher scanning speeds. Control upgrades could be met with higher level motion controls that would permit compound X and Y moves for contoured positioning of the plasma head with the possibility to include GUI programming of complex shapes using customer supplied CAD data. Using controller I/O the plasma process could also be extended to control the plasma source as well as the Z-axis and include adhesive dispensing – further advancing and speeding the entire process.
LG Motion application engineers worked closely with inrekor Ltd from the outset of the project and provided assistance with motion programming as well as installation.