Specify the Right Lube-Delivery Line

Fig. 1. Cost should not be a factor in your lubrication-delivery-line choices. While the steel-tubing in this progressive-divider-lubrication system block took more time to install than plastic lines, the additional, but small, up-front cost will pay long-term dividends, especially if leaks or blockages occur.

The wrong lubrication-delivery line can compromise the reliability of your production equipment.

During lubrication-training workshops, I ask participants to name the components that make up a centralized lubrication system. Most will answer in the context of an automated-delivery system by citing the pump, reservoir, metering devices, and pump controller. Rarely do they actually include the lube-delivery lines in their answers.

Lubrication-delivery lines are important and integral components within centralized lubrication systems—be they state-of-the-art automated designs or simple, manual arrangements. Specifying the wrong type can put machinery reliability at risk.

The function of a lubrication-delivery line is straightforward: It must connect a bearing point to a lubricant source (indirectly from a meter or gang block, or directly from the pump) and allow the lubricant to be contained within the line to flow without constriction. As lube-delivery systems are hydraulic in nature, the line must also be capable of withstanding pressures ranging from hundreds to, in some cases, many thousand of pounds-per-square-inch (psi) of pressure.

Listen to the latest in a series of monthly lubrication-related podcasts with Ken Bannister. This edition of the podcast focuses on lubrication-delivery line matters.

Line size and material

Correct choice of size and material is essential if a lubricant-delivery line is to provide reliable service. For the most part, the line plays a passive role within a centralized system and is typically fixed to the side of a machine (the exception being where a lubricated part moves independently of a piece of fixed machinery, in which case, the line is used to provide the flexible connection.) Before a delivery line can be specified, however, a number of basic questions regarding the overall lube-system design must be answered, including:

Fig. 2.
The bundled plastic tubing in this progressive-divider system are difficult to individually trace from pump to the lube block. These types of lubrication-delivery lines are also difficult to physically attach to a machine’s frame and, consequently, more vulnerable to damage.

Is this system automated or manual? The answer is crucial in assessing line material, diameter, and wall thickness, which relate specifically to the line’s material-burst pressure rating.

• Manual systems designed to “gang” grease nipples in a central block can be lubricated by grease guns capable of developing as much as 15,000 psi.

• Manual hand pumps and automated systems operate at much lower pressures (between 100 and 2,000 psi).

What type of automated/engineered delivery system is specified? Some system designs require a single line size throughout, whereas others require a main and secondary line of different diameters and flow rates. For example:

• Single-line-resistance and pump-to-point systems are low-pressure systems designed to deliver the total amount of lubricant in one pump cycle. In such systems, i.e., total-loss, single-size-diameter delivery lines are sufficient.

• Single-line positive-displacement-injector, dual-line-injector, and progressive-divider systems require multiple cycles of the pump connected to a larger diameter main line used to rapidly fill the injectors/main distribution blocks, and smaller-diameter secondary lines that connect the metering outlets to the lubrication points,

• Re-circulating-oil systems usually require single-size-diameter delivery lines and a larger-diameter, return-line system.   

How many lubrication points are included in the system and where are they located on the machine? This question is required to map out a central pump location and injector or delivery block locations so the line distances can be measured for material take-off amounts, and in the case of long line lengths, to calculate pressure drop so the correct line diameter(s) can be calculated.

What lubricant type and grade/viscosity are you planning to use? The fact that grease requires higher pressure than oil to move through blocks and lines will affect the choice of line material type and diameter.

Fig. 3. If single-chamfered compression fittings designed for nylon lines are mistakenly used on steel lubrication-delivery lines that require double-chamfered fittings, seals can be compromised, causing leaks at the fittings. (Courtesy Bijur Delimon International, Morrisville, NC, bijurdelimon.com.)

In what type of working environment will the system be used? Ambient and working temperatures can affect line integrity. Furthermore, if unprotected, copper, brass, and plastic lines can be easily damaged in high traffic areas—especially where lift trucks are used regularly.

What is your budget? Cost should not be a factor in line choice. Figures 1 and 2 show progressive-divider blocks, one piped in correctly rated plastic tubing and the other in steel. While steel tubing (Fig. 1) takes considerably longer to install, the additional, but small, up-front cost can pay long-term dividends, especially when a problem, such as a leak or a blocked line, occurs. The plastic tubing (Fig. 2) is bundled together. making it difficult to individually trace a line from the pump to the lube block. In addition, these lines are difficult to physically attach to the machine frame and, consequently, more vulnerable to damage.

Although the steel lines used in Fig. 1 are dirty, they all have line-ID (identification) tags that make them easy to trace and troubleshoot. The steel-line system also looks more engineered and permanent in comparison with the bundled-plastic-line example.

Once you’ve gone through these six questions, present the answers to your lube-system designer or manufacturer/supplier. These resources can help you determine the best line material for a specific application.

Main problem causes

Problems in lubrication-delivery lines manifest as leaks or blockages. A leaking line will starve lubricant from one or many bearing points and seriously affect the associated production equipment’s reliability. Leaks are invariably found at connection points and line-bend areas. Keep the following in mind:

• Copper lines are very soft and can easily work-harden at bend points if significant machine vibration occurs.

• Nylon lines can be easily over-tightened or not cut square at the connection points. This can cause a leak at the compression fitting.

• If a single-chamfered compression fitting designed for nylon lines is mistakenly used on a steel line, which require a double-chamfered compression fittings (see Fig. 3), they can be compromised, causing a leak at the fitting.

• To reduce cost, nylon lines can be used as a substitute for flexible-hose lines in moving-bearing-point applications found on, among other things, machine slides and rams. Plastic lines, in most cases, are not rated for cyclic repetitive-movement duty.

Blockages in lubrication lines usually occur when they’re pinch-damaged from being hit by a foreign object that crimps or flattens the line shut. This situation causes a line backpressure that can blow the fitting or eventually stall an entire progressive-divider system, starving many bearings in the process. Steel lines offer the best defense against pinched lines.

Best practices

To ensure no bearing is starved after a lubrication-system implementation or line replacement, always pre-fill the lubricant line with the correct grease lubricant before final fastening to the bearing. Or, in the case of oil, operate the lube system and open all bearing points to ensure oil is flowing at each point before final tightening.

Finally, never forget that lubrication-delivery lines are a matter of choice. Reliable lube systems, in turn, depend on making the correct choice. 

Contributing editor Ken Bannister is co-author, with Heinz Bloch, of the book Practical Lubrication for Industrial Facilities, 3rd Edition (The Fairmont Press, Lilburn, GA). As managing partner and principal consultant for Engtech Industries Inc. (Innerkip, Ontario), he specializes in the implementation of lubrication-effectiveness reviews to ISO 55001 standards, asset-management systems, and training. Contact him at kbannister@engtechindustries.com, or telephone 519-469-9173.