The true cost of bearing lubrication

Treotham Automation Pty Ltd
By Matt Mowry, Product Manager, igus Inc.
Friday, 11 May, 2012

Today, machine and equipment manufacturers are feeling more pressure than ever to reduce costs without sacrificing machine performance and this balancing act can be difficult to achieve. 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 making their equipment more marketable and less expensive to operate for the end users.

What are the issues with bearing lubricant? According to a major ball bearing company, 54% of bearing failures are lubrication related (see Figure 1). In a study by the Massachusetts Institute of Technology (MIT), it was estimated approximately US$240 billion is lost annually (across US industries) due to downtime and repairs to manufacturing equipment damaged by poor lubrication.¹ Improper bearing lubrication or re-lubrication accounts for up to 40 to 50% of machine failures. By eliminating lubrication from machinery, OEMs can minimise the costs and risks associated with maintenance for the end user. 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, grease nipples, manifolds, etc) can also be decreased.

There is a lower cost, easier-to-maintain machine component that eliminates the total cost of bearing lubricants: high-performance, dry-running plastic bearings.

Figure 1: Types of lubrication-related bearing failures. Source: SKF, USA Inc.

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 lube, 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% when it came to the basic technical skills needed to perform their job;² and bearing lubrication was noted first on their list of tasks.

The main reasons re-lubrication maintenance practices fall short are:

Lubrication not being properly or consistently administered
Lubrication points not being easily accessible
Maintenance personnel not being properly trained
Incorrect or improper quantity of lubricant being used

Ancillary components for OEMs

Using lubricated bearings can increase manufacturing complexity and expenses. They often need to be fitted with grease nipples and manifolds, oil lines, and sometimes oil reservoirs and pumps. Not only are there extra costs associated with purchasing these components, there are also manufacturing costs associated with the machining and assembly of the mating parts.

Ancillary components for lubricated bearings include:

Grease fittings, grease nipples, oil lines and pumps
Wipers, scrapers, felt wicks and seals
Grease guns
Grease, oil and other lubricants

There are also additional parts required to protect them from contaminants. According to McNally Institute, the leading cause of bearing failure is due to contamination of the lubrication by moisture and solid particles. If as little as 0.002% water gets mixed into the lubrication system, it increases the probability of failure by 48%. Just 6% water can reduce the lifetime by 83%.³

Ball bearings require seals to keep oil in and unwanted water and liquids out, as well as wipers or scrapers to keep dust and debris out. Seals only last so long and do not perform well in dirty and dusty environments and can also increase friction in the application. In applications where dust and debris are prevalent during operation, seals and wipers may require frequent replacement.

The extra costs associated with lubricated bearings can include:

Gravity-fed oil reservoirs and lines: $50 to $300
Grease nipples: $2 to $40 (for four bearings and machining)
Seals, wipers and felt wicks: $4 to $12 (per bearing)
Automated oil reservoirs and lines: $1500 or more
Lubricant disposal

These are all things that are generally not required with self-lubricating plastic bearings.

Other costs not required for self-lubricating bearings

Labour

A major oil company 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 $7300 annually for maintaining 20 grease points on one machine, every day, seven days per week.⁴ Another source claims that the average plant employs 2196 bearings and spends $60,000 in re-lubrication costs per year; of that $60,000, $57,000 is used for labour alone.

Downtime

Improper bearing lubrication or re-lubrication accounts for up to 40 to 50% of machine failures. When a bearing fails prematurely, a number of actions may need to be taken. Replacement of the bearings, shafts, and even motors and other parts can be very costly. If the machine needs to be taken offline, expenses can potentially skyrocket. In a six-sigma lean manufacturing guide,⁵ it is estimated that the average cost for downtime is $500 per hour, and in some automotive and other high-volume production factories, downtime costs are considerably higher. In addition, unplanned downtime can cause a ripple effect that impacts a plant’s production schedule.

Disposal costs

According to Valin,⁶ proper disposal of lubricants by a process management company can amount to approximately 20% of the cost of annual lubricant expenditures. This means if a plant spends $50,000 per year on lubricants, they will spend approximately $10,000 in disposal costs. In addition, the cost of the lubrication itself can impact overall expenditures, as it is normally petroleum based and directly linked to the price of oil.

Figure 2: Comparison chart comparing ball bearings to self-lubricating plastic plain bearings.

Self-lubricating plastic bearings

Self-lubricating plastic bearings are made of 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 microfinish 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 an ideal solution for applications in labs and food processing machinery that require clean, oil-free operation. Plastic bearings also perform well in dirty and dusty environments since there is no oil to attract dust and dirt. They can be used on softer shafting, even anodised aluminium, which has excellent corrosion resistance and is usually less expensive and easier to machine than case-hardened material or stainless steel.

Figure 3: Self-lubricating plastic bearings are made from high-performance polymers, which are corrosion-resistant and maintenance-free.

Eliminate maintenance costs

Using high-performance, self-lubricating plastic bearings can significantly reduce maintenance costs, as well as reduce unplanned downtime due to bearing failure. OEMs that use self-lubricating plastic bearings are able to deliver a maintenance-free system that increases their end customer’s production throughput and the overall marketability of their product. In the event that a self-lubricated bearing does need replacement, the replacement part (a small, inexpensive plastic sleeve) can be purchased for a fraction of the cost of an entire recirculating ball bearing.

Lower production costs

Plastic bearings do not require the machining and other processes required to install ball bearings. They are less expensive and do not require grease fittings, lines or pumps. Plastic bearings also can be used on less expensive shafting, such as aluminium or cold-rolled steel. Some companies offer online calculators to predict bearing lifetime to ensure it is ideal for the application; this eliminates the need for testing and saves time and errors in material choice.

Applications not appropriate for self-lubricating plastic bearings

Of course not all applications are suitable for the use of plastic bearings. Examples of situations in which they are not appropriate are:

Applications with high loads and high speeds: These lead to excessive frictional heat build-up and wear.
Highly cantilevered loads: 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.
Extremely precise applications: Plastic bearings have a higher running clearance than ball bearings, sometimes 0.025 mm to 0.055 mm, and therefore are not ideal for applications needing extreme precision.
Extreme temperatures: Plastic bearings are not recommended for applications with long-term temperatures exceeding 250°C.

Other benefits of plastic bearings

Apart from lower maintenance and production costs, plastic bearings also offer:

Corrosion resistance
Lower cost than ball bearings
Good handling of contamination, often not requiring seals or scrapers
Suitability for very short-stroke applications, unlike linear ball bearings

Self-lubricating bearings are ideal for:

Harsh, extreme environments - dirt, dust, agriculture, outdoor equipment
Sensitive, clean environments - biotech, lab machines, medical equipment
Washdown applications - packaging, food processing
Weight-sensitive applications (aimed at reducing fuel consumption or lowering the inertia of moving parts)

Figure 4: Total cost of replacement parts over five years - plastic liners vs recirculating ball bearings. Based on ¾″ closed linear bearing and assuming replacement is required once per year.

Example of an application of plastic bearings

An example of the use of plastic bearings to replace traditional metal bearings is that of a packaging machine manufacturer that specialises in vertical, form, fill and seal (v/f/f/s) packaging equipment for handling a wide range of products: from green beans to candy to detergent. The machines are capable of reaching up to 160 cycles per minute and withstanding loads up to 65 N, while operating at speeds of 230 metres per minute.

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 igus DryLin R linear plain bearings. To date, the linear bushings have surpassed the 10 million cycle mark on some of the company’s packaging machines with little to no noticeable wear.

By Matt Mowry, Product Manager, igus Inc.