Motion System Design

Primer: Chain lubrication reduces downtime

When it comes to roller chain drives, lubrication is the key to long life. But, you need to know the right type of lubricant and how to apply it too. Let us review.

Roller chain drives transmit power or convey products through diverse environments in applications such as material handling, agriculture, construction, mining, food processing, and automotive manufacturing. With simple periodic inspections, proper lubrication, and regular maintenance, these chains will give long service and may even outlast the machinery on which they’re installed.

Why lubricate?

Lubrication is the most effective method of extending the service life of chain drives. Its primary mission is to provide a film of lubricant between the pins and bushings in the chain links, thus preventing metal-to-metal contact and wear. But, it offers other benefits as well:
• Dissipates heat generated during operation.
• Helps flush wear debris and contaminants from the chain joints.
• Protects the chain against corrosion.
• Cushions the impact when a chain engages a sprocket.
• Enables smooth movement of the chain over sprockets.

Without lubrication, metal-to-metal contact causes rapid wear and elongation, reducing a chain’s service life. In addition, excessive heat generated by metal-to-metal contact can destroy the benefits of component heat-treatment.

With sufficient lubrication, chain parts function without generating excessive heat. Elongation due to wear is reduced. And, it is both uniform and predictable. As a result, chain life can be many times that of unlubricated or inadequately lubricated chain.

Selecting a lubricant

Roller chain lubricant is applied to the outer surfaces and allowed to flow into the pin-and-bushing joint. For this reason, coating the chain with a heavy, thick grease does little toward supplying the joint with the lubricant it needs. Oil does the job better.

The oil grade, viscosity, and purity are the most important factors for proper chain lubrication. Because some grades of oil contain additives or detergents that may foam or leave a residue in the chain’s joints, it is best to use nondetergent, single- viscosity petroleum oils selected primarily for the drive operating temperatures, Table 1. If available, antifoam, anti-rust, and extreme-pressure additives in the oil are beneficial. Impure or used oil may have degraded in service and may contain contaminants which can increase wear or damage the chain components. Therefore, this oil should be replaced.

Synthetic or food-grade lubricants are available for extreme temperature or food-handling applications. Generally, the cost of synthetic oil is significantly more than petroleum-based oil, so that petroleum oils have historically provided the best cost/performance ratio in most applications.

Choosing a lubrication method There are four primary methods of chain lubrication — manual, drip, oil bath and slinger disc, and oil stream applied under pressure.

Manual. Appropriate for drives operating at low speed (defined by Table 2), manual methods generally involve applying oil with a brush or spout-can approximately every 8 hr depending on the drive conditions and duty cycle. Oil should be applied to the inside of the chain loop so it won’t be thrown off by centrifugal force. The chain drive should always be stopped and power locked out before manually lubricating the drive.

Drip feed. A more continuous form of lubrication, the drip method is also adequate for low-speed drives. Oil is supplied to the chain from a reservoir or manifold system, typically at at rate of 4 to 20 drops per minute, Figure 1.

Oil bath and slinger disc systems. For moderate-speeds, the entire drive is contained in a housing with an oil sump. A short section of the chain travels through the sump to pick up oil, Figure 2. This method is often used in conjunction with a slinger disc which picks up small amounts of oil from the sump and slings it onto the chain.

Oil stream. For high-speed applications, a pump directs lubricant under pressure onto the chain at a high flow rate, Figure 3.

Horsepower rating tables published by the American National Standards Institute (ANSI) provide guidelines for selecting the correct type of lubrication system based on the size (number of teeth) and speed (rpm) of the smallest sprocket. Table 2 is a modification of this ANSI table. As a general rule, higher speeds and loads require more sophisticated lubrication systems.

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Periodic Inspections

Conducting regular inspections is the best way to monitor the performance of a chain drive and to determine if lubrication is needed. Only a few minutes per day (or per shift) is required to identify and correct problems before they cause unscheduled downtime. Here are some trouble signs to look for.

• Inadequate Lubrication. The two most common signs of inadequate chain lubrication are excessive slack — indicating the chain has elongated due to wear — or a red-brown discoloration (rust) located on the surface or seeping out of the chain joints. If this discoloration appears, remove the chain, then clean, lubricate, and reinstall it. If the chain is worn excessively or damaged, replace it. Before returning the machinery to service, increase the volume or frequency of lubrication or improve the method. For example, one could upgrade from a drip to a bath method.

• Chain wear (elongation). Pin and bushing wear causes chain to elongate. The chain industry standard for maximum allowable elongation is 3% of the chain’s nominal length. Elongation in excess of 3% can cause improper engagement between chain and sprockets, which, in turn, causes rough performance, sprocket damage, or chain failure. To check for excessive elongation, measure the length of a section of chain, Figure 4, and compare the measured value to elongation limits provided in ANSI tables.

To compensate for wear and elongation, one or more links can be removed as long as the basic chain length does not exceed the 3% limit.

• Sprocket Wear. This type of wear is more difficult to define than chain wear. However, sprockets should be replaced whenever tooth thickness is reduced or the tooth tips become hook-shaped, Figure 5.

Installing a new chain on excessively worn sprockets causes rapid chain wear and possible damage. As a general rule, replace the sprockets once for every third chain replacement.

• Chain Tension. Except in certain applications, chains should never be adjusted so that both top and bottom (tight and slack) spans are taut. The slack span relieves joints of loading during each revolution, permitting lubricant to flow into and around the pin and bushing area. For the recommended amount of slack at the midpoint of the slack span, consult the manufacturers’ catalog.

While you’re inspecting the chain, don’t forget to check the oil. Here’s what to look for:
• Check oil temperature every 8 hr. It should never exceed 180 F.
• Check oil level every 8 hr and add if necessary.
• Change oil every 500 hours (oil bath and oil stream systems).

If an inadequate lubrication system was selected initially or if the machine performance requirements increase (from the manual to the oil bath range of speeds, for example), it may be necessary to modify either the machinery or the lubrication system to achieve acceptable chain service life.

Beyond these daily checkups, a chain drive should be carefully inspected after its first 100 hours of operation, and then after every 500 hours, or more often if operating under harsh conditions. To avoid costly, unscheduled down-time, simply watch, listen, and lubricate!

Tables and illustrations for this article courtesy of the American Chain Association.

C.J. Roberts is the product manager-engineering applications for the Diamond Chain Co., Indianapolis.

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