Original equipment manufacturers (OEMs) often overlook a simple solution that can have a positive, long-term impact on profitability for themselves and their customers: the elimination of bearing lubricant. By eliminating lubrication systems where possible, OEMs can reduce production costs, while at the same time make their equipment more marketable and less expensive to operate.
What are the issues with bearing lubricant? According to a major ball bearing company, 54 percent of bearing failures are lubrication-related. In a study by the Massachusetts Institute of Technology, it was estimated approximately $240 billion is lost annually in the US alone due to downtime and repairs to manufacturing equipment damaged by poor lubrication. Moreover, improper bearing lubrication or re-lubrication accounts for up to 40 to 50 percent of machine failures.
By eliminating lubrication from machinery, OEMs can minimise the costs and risks associated with maintenance. At the same time, costs related to the proper disposal of oil can be eliminated and the initial expenditure for ancillary components and processes (grease lines, nipples, manifolds, etc) can be decreased.
Hidden costs of lubrication
Proper lubrication delivery is critical for the operation of ball bearings, and most require continued maintenance for re-lubrication. The re-lubrication process typically requires scheduled machine downtime, which increases maintenance costs and causes a loss of production time. In addition, re-lubrication maintenance practices often fall short.
While some processes are automated, the majority of re-lubrication is performed manually using a grease gun. This seemingly simple task actually involves a number of critical steps to ensure proper lubrication delivery, including correct amount of grease, the right grease gun, proper cleaning, and careful storage and handling conditions, just to name a few. In addition, it is critical to use the same grease for the entire lifespan of a bearing.
The Technical Training Division of Life Cycle Engineering conducted a study that found 80 percent of maintenance workers surveyed scored less than 50 percent when it came to the basic technical skills needed to perform their job; ‘bearing lubrication’ was noted first on their list of tasks.
Additional parts may be required to protect bearings from contaminants. According to Monition Limited, while the leading cause (36 percent) of bearing failures is due to inadequate or incorrectly specified lubrication, an additional 14 percent of failures are as a result of lubricant contamination. Seals, wipers or scrapers may be needed to protect bearing lubricants in dirty and dusty environments. These have a limited lifespan, are not always effective and can also increase friction in the application
So, we’ve looked at hidden component costs but there are more costs to consider and these are often not taken into consideration when specifying bearings.
A major oil company in the US studied the time required to manually lubricate a single grease point. The results showed manual lubrication takes an average of three minutes per point. The average machine has 20 grease points to maintain. This correlates to a total labour cost of $7,300 annually for maintaining 20 grease points on one machine, every day, seven days per week.
When a bearing fails prematurely, a number of actions may need to be taken. Replacement of bearings, shafts, and even motors can be very costly. If the machine needs to be taken off-line, expenses can potentially skyrocket. Lubricant disposal is also costly and has been estimated by Valin to amount to 20 percent of the cost of annual lubricant expenditures.
Self-lubricating plastic bearings
There is a lower cost, easier-to-maintain machine component that eliminates the total cost of bearing lubricants – high performance, dry running plastic bearings.
Self-lubricating plastic bearings are made from high-performance polymers and, unlike rolling-element bearings, slide instead of roll. They consist of a base polymer, which is optimised with fibre reinforcement and solid lubricants. The fibre reinforcements increase load carrying capabilities and wear-resistance, and the solid lubricants are transferred from the bearing to the micro finish of the shaft in order to reduce friction. No external oil or grease is needed for their operation; self-lubricating bearings operate completely dry.
They are particularly suitable for applications in labs and food processing machinery that require clean, oil-free operation. Plastic bearings also perform well in dirty environments since there is no oil to attract dust and dirt. They can be used on shafts made of softer materials such as anodised aluminium, which has excellent corrosion resistance and is usually less expensive and easier to machine than case-hardened material or stainless steel. As well as costing a fraction of their rolling element counterparts, plastic bearings do not require the machining and other processes required to install ball bearings.
Of course, self-lubrication plastic bearings are not suitable for all applications. Applications where high loads with high speeds occur, for instance, can lead to excessive frictional heat build-up and wear. Also, since self-lubricating plastic bearings slide (unlike ball bearings that roll), linear applications with higher coefficients of friction may result in uneven movements for highly cantilevered loads or drive forces.
Plastic bearings have a higher running clearance than ball bearings - sometimes in the region of 25 to 50 microns - and therefore are not ideal for applications needing extreme precision. Furthermore, plastic bearings are not recommended for applications that are exposed for long periods to temperatures exceeding 250°C.
That said, let’s take a look at a few examples of where high-performance plastic bearings are currently being used, in preference to their metal equivalents.
Agricultural machinery - A manufacturer of agricultural equipment has been using plastic bearings on its ‘Pick Planter’ for a number of years. This machine creates individual planting row units using walking gauge wheels to deliver a consistent planting depth.
Oil-impregnated bronze bearings with graphite plugs were used to facilitate this movement until they began causing severe problems. They were even requiring replacement two to three times a season. The bronze bearings were experiencing high wear and premature failure due to the very abrasive conditions in which the equipment was operating. And operating close to the sea, high salt content in the air was causing bearing corrosion and seizure.
By replacing all 144 bronze bearings with iglidur self-lubricating plastic bearings supplied by igus, the pick arm’s lifespan was increased by 500 to 600 percent while the actual bearings cost was 70 to 80 percent less than the bronze bearings.
Packaging machines - Another manufacturer that has benefited from converting to using plastic bearings specialises in vertical, form, fill and seal packaging equipment for handling a wide range of products – from green beans to confectionary to detergent. The machines are capable of reaching up to 160 cycles per minute and withstanding loads up to 7kg, while operating at speeds of 4m/s.
The manufacturer had been using metal linear ball bearings. After the metal bearings scored the shafts and leaked grease on some of the machines, the company decided to replace them with self-lubricating drylin R linear plain bearings supplied by igus. To date, the linear bushings have surpassed the ten million cycle mark on some of the company’s packaging machines with little to no noticeable wear.
Medical equipment - In the quest to improve the way prostate cancer is detected and treated, a team of researchers from the Worcester Polytechnic Institute have developed a specialised magnetic resonance imaging (MRI) compatible piezoelectric actuated robot.
To facilitate different types of motion, the robot uses both the drylin linear guide system and iglidur plastic self-lubricating plain bearings. The linear guides facilitate translational motion of the positioning module, which provides gross positioning for the robot’s needle driver. The needle driver is a vital part of the system, as it enables the rotation and translational movement of the needle cannula: a flexible tube inserted into patient for MRI-guided diagnosis and therapy.
The needle driver has a needle guide sleeve, a collet locking mechanism and passive optical tracking fiducial frame. Two plastic plain bearings are used in the front and rear of the driver to constrain the needle guide. The bearings enable the robot’s motor to rotate the needle using the collet mechanism by way of a timing belt. This rotating needle reduces tissue damage while enhancing targeting accuracy. Another ten plain bearings were used to create a revolute joint, also known as a ‘pin joint’ or ‘hinge joint’, to provide single-axis rotation.
The linear guides chosen are comprised of hard anodised aluminium rails and carriages and high-performance plastic sliding elements – essential non-magnetic materials that are MRI compatible. The linear slides operate without messy lubrication, which is important in a sterile medical environment; they also feature a lower-profile for applications where installation space is an issue.
The specific plastic plain bearings used were an ideal choice for the robot, as they are made from FDA-compliant polymers specifically designed for applications involving contact with food or drugs.
Rob Dumayne is a director at igus