Properly Lubricated Chains Last Longer
Chains are often given little attention as components in machines or logistics systems. For the overall system, however, they are of great importance: without a drive chain the machine will not run, and without conveyor chains the logistics will not work. Renold, the world’s leading chain manufacturer, offers tips on how to make chains last longer with the help of the correct lubrication.
Ever since it was invented by Swiss engineer Hans Renold in 1880, the roller chain has proved itself time and again as the most efficient method of power transmission. In essence it is comprised of a series of precision bearings made of inner and outer link plates, bearing pins, bushes and rollers. But in addition to these well-known parts, there is also another important component in the roller chain which is often overlooked: the lubricant. This article examines the different options available for the lubrication of standard roller chains for achieving an optimum wear life and takes a look at the solutions available to engineers when lubrication is either undesirable or not possible.
Lubrication as a time and cost factor
The correct lubrication is an operative and economic factor that can hardly be overestimated: in most applications a properly lubricated chain should last for at least 15,000 operating hours before it should need changing due to wear. In many cases, however, chains do not last nearly as long in various applications. In around 60 percent of cases, poor or insufficient lubrication is the cause of chain failure and is therefore the single biggest risk factor. The consequence: higher costs due to longer downtimes, increased material and maintenance costs and reduced overall system performance. Downtimes in particular can be a significant financial cost factor. Manufacturing industries calculate that the downtime costs of a production machine can be up to €50,000 per hour – costs which can be saved with carefully thought-out and responsibly safeguarded lubrication. Tests have shown that chains that are properly maintained can last up to 60 times longer than chains that are incorrectly lubricated or not lubricated at all.
Frequent lubrication errors
Correct lubrication starts before the installation stage is even reached. Well-lubricated chains are already provided with optimum basic lubrication at the factory and, depending on the application, this is normally through hot grease immersion. Immediate relubrication in the system will often lead to a deterioration in the initial lubrication, especially if a thin oil containing a solvent is used which immediately washes the initial lubrication out of the chain. During operation, the large number of failures which are due to lubrication errors show that correct chain lubrication continues to be a process in which many errors are made.
The wrong lubrication methods and the use of the wrong lubricants are some of the most common sources of trouble. Above all, the right choice of lubricant is critical for achieving optimum efficiency. Heavy oils and greases, for example, are too stiff to penetrate deep enough into the chain to reach the actual working surfaces of the chain. Applying grease to the outside of a chain will only act more as a sealant, impeding the work process as a consequence and resulting in higher wear. This ultimately leads to the premature failure of the chain. In more precise terms, what this means is that the desired film of lubricant is often only formed on first-time use.
The lubricant can pass between the spaces between the plates and form an even film as a result. Further relubrication thereafter, by contrast, will result in the development of increasingly thick layers of grease on the plates and rollers. With the accumulation of dust as well, the chain becomes increasingly thickened and encrusted, which in turn prevents the lubricant from reaching the small spaces in the chain joints. From the outside the greased chain might appear fine, but the important components on the inside of a chain will be running with a high level of friction or, in the worst case scenario, dry.
Care also needs to be taken with the use of lubricant sprays as an alternative. These often use thinners which are generally volatile, in other words they quickly evaporate after application. What they leave is a viscous film of lubricant which is not capable of flowing and which, in a similar way to encrustations, prevents a smooth process and therefore contributes to wear.
When it comes to correct lubrication, therefore, it is always important to examine the chain pins of the connecting link. Ideally the entire surface of the chain pins should be completely covered with a film of lubricant. In addition, the surface should be uniform and as smooth as glass on the load side when the lubrication product has been removed. Oil is used for the standard relubrication of chains, because removing the chain and hot-dipping it again is too complicated for all but a few applications. As a general rule, oil with a viscosity of 150 to 450 cSt is recommended. In many cases lower viscosities provide insufficient lubrication effect where there is high surface pressure in the joint, whereas higher viscosities seldom reach the chain joint between the pin and the bush.
Correct chain lubrication
Given the design of a chain, lubrication products are required to pass through a narrow gap between the plates. Most suitable for the purpose are liquid lubrication products without thinners, which are capable of being applied so that sufficient quantities get into the chain joints. However, this procedure alone involves plenty of potential for errors. One of the most common lubrication errors is in the treatment of the surface and the chain roller. These normally only need minimal lubrication because they are not exposed to much friction or movement. Nevertheless, when it comes to troubleshooting, time and again it turns out that lubricant has been applied particularly generously here. Which chain lubricant is the most suitable for each application depends on a variety of factors.
The conditions in the surrounding area, for example, play an important role in the choice of lubricant. If there is a large amount of dust in the ambient air, such as lime, talc or flour, for example, initial lubrication with a waxy grease should be the preferred option. This has the disadvantage, however, that the chain joint is often not reached on relubrication. Waxy lubricants should therefore be specifically used where relubrication is difficult or even impossible and there is a risk of the lubricant being drawn out of the joint by dust because of the nature of conditions in the surrounding area.
The operating temperature also plays an important role. Table 1 shows the optimum lubrication viscosity for each temperature range. Mineral oils are suitable for most applications. However, if grease lubrication is required, it is particularly important to heat the grease until it reaches a liquid state and then to immerse the chain into it until no more air bubbles are seen. Chains lubricated using this method need regular cleaning and relubrication. Chain temperatures in excess of 100 degrees Celsius should be avoided as a general rule because of lubricant limitations.
Table 1: Lubricant viscosity for different temperature ranges
In special cases chains can be used in temperatures of up to 250 degrees for some applications and deliver good performance. This requires the use of special oils for high temperature applications. One good means of improving the effectiveness of the lubrication and its cooling effect is to use forced-feed lubrication with a higher oil volume (up to 4.5 litres per minute per chain strand) and to provide a method for the external cooling of the oil. For very high temperatures (e.g. oven chains), graphite or molybdenum disulphide can be used as an admixture in support of the oil or in spray form. Dry lubricants are often used in these cases because relubrication is difficult and has an adverse effect on the product in the oven.
The four types of lubrication
There are four general methods of lubricating chains: manual, drip, oil bath and oil flow. The choice of method depends on the operating speed of the chain and the power transmitted.
In manual lubrication, the lubricant is applied to the chain by brush, oil can or spray can. This tends to be little used in the industry because the manual work involves considerable time and effort. Although it varies from application to application, it can generally be assumed that the procedure needs to be repeated at least every eight operating hours. The quantity of oil and frequency must be sufficient to keep the chain wet and it is essential to make sure that the new, clean oil is able to penetrate into the joints in order to lubricate the bearings. This means that the oil needs to be directed into the spaces between the inner and outer plates, ideally at the point at which the chain connects with the sprocket on the lower strand. However, this method of lubrication is considered to be error-prone because of the human factor involved and is therefore only used on the odd occasion as opposed to use for continuous operation.
IMG 1 - Manual Lubrication
In drip lubrication, the oil is dripped into the space between the inner and outer plates of the chain by a drip oiler. It is important to check the oil in the reservoir on a regular basis and to make sure that the lubricant flows at the required frequency. This is generally in the region of four to twenty drops per minute, depending on the speed of the application. Also particularly important here is the application point of the oil: the oil will only be able to penetrate between the inner and outer plates and reach the bearing surfaces if it is delivered to the edges of the chain. This type of lubrication is especially recommended for chain speeds between 1.5 to 8 m/s.
IMG 2 - Drip Lubrication
In bath lubrication, the lower strand of the chain is run through an oil bath in the drive enclosure. The oil level needs to cover the lowest point of the chain whilst operation is in progress. Another form of bath lubrication is the so-called centrifugal disc, in which an oil bath is used but the chain runs above the oil. A disc picks up the oil from the sump and delivers it to the chain via baffle plates. The plates need to have circumferential speeds between 180 and 2240 metres per minute. Bath lubrication is particularly suitable for speeds ranging from 4 to 8 m/s.
IMG 3 - Bath Lubrication
The fourth type of lubrication is forced-feed lubrication in combination with an oil bath, in which a constant supply of oil is delivered to the chain via a tube. In this method it is important to check that the nozzle holes from which the oil comes are aligned with the edges of the chain. The spray tube needs to be positioned so that the oil is delivered to a point on the chain just before the drive sprocket comes into action. This ensures that the oil is centrifuged by the chain and the impact of the rollers on the teeth of the sprocket is damped. Forced-feed lubrication is particularly suitable for providing effective cooling and shock absorption for high speeds.
IMG 4 - Forced Feed Lubrication
Special chains without lubrication
Although there are different types of lubricant and lubrication, there are some applications for which lubrication is generally difficult and undesirable. In many industries, e.g. the food, paper and electronics industries, the possible contamination of the end product by lubricant is not acceptable. This problem can be solved with maintenance-free chains which manage without lubrication for the duration of their entire working lives and which are certified for use for specific applications.
The Syno chain, for example, is a solution which has been developed by Renold for use with zero external lubrication. A sintered bush was developed for this purpose, featuring a lubricant which is only delivered to the friction surface when the chain is running. The moment the application stops, the lubricant returns to the bush and there is no possibility of product contamination. This so-called "dry-to-touch" surface of chains such as the Renold Syno is suitable for most applications in which additional lubrication is not allowed to be used. These chains have their limitations, however, when it comes to speeds over 3 m/s and high surface pressure in the joint. Chains with sinter bearing bushes should not be used in dusty environments either, because the dust draws the lubricant out of the joints.
In applications involving mainly frozen food, the use of standard chains is not possible without restrictions because of the increased risk of corrosion due to melt water. The effective solution here is to use chains with polymer inner links. In this case the inner link is made of special plastics, while the pins and the outer link plates are made of stainless steel. In most cases these chains do not need any bushes or rollers at all, with the entire inner link being made as a single piece requiring no lubricants.
When it comes to choosing a plastic chain, it is essential to bear in mind that although it offers distinct advantages in terms of hygiene and weight, it is not as strong as a standard chain. As a new alternative, rust-free chains with brass bushes are now also offered. These permit higher levels of surface pressure than chains with plastic inner links, but they require a film of water in the joint for lubrication. In the food segment with dry ambient conditions, nickel-plated chains with sintered metal bushes containing approved lubricants are also an option, similar to the Syno previously mentioned above.