Componentry manufactured by SelfLube is used principally to guide or control some form of linear motion, often under considerable load. The range of possible applications is very broad: dies, molds, fixtures, special machines and heavy equipment of all types. Our components have an extremely long life, often exceeding that of the tool or machine in which they are installed. Virtually all of our sales are for new construction.
Many of our components are self-lubricating. Self-lubricating components are initially more expensive than conventional components; however, they possess the advantage of having permanent lubrication built in. No additional lubrication will be required, ever. In terms of total life cycle cost (i.e., when the cost of performing periodic greasing is considered), self-lubricating components are always less expensive than conventional components - often considerably so since there is the risk of the lubrication not being applied.
Graphite plugs provide self-lubrication. Graphite has some unusual properties that make it an excellent lubricant. Chemically, it is one of three common allotropic forms of carbon (amorphous carbon and diamond being the other two). In contrast to diamond, which has a very dense and strong three-dimensional crystal structure, graphite has a two-dimensional crystal structure - strong in two dimensions but weak in the third. Its atoms are arranged in parallel sheets that are easily sheared off, giving graphite its characteristic slippery feeling. For example, if you were to rub a pencil lead between your fingertips, it would feel oily. That feeling is the sliding and peeling off of crystal sheets.
In the self-lubrication process, it is precisely these sheared off crystal sheets that provide the lubrication, somewhat like a piece of tissue paper between two pieces of glass. Initially there is no lubrication but as the two mating surfaces (e.g., bushing and shaft) move in relation to each other, a minute amount of graphite becomes distributed over the wear surface and acts as a solid lubricant. It will stay there a long time due to graphite’s superior stability. Although graphite sublimes at 10,000 F in a reducing atmosphere, it will oxidize in air at about 500 F, so it is important to maintain temperatures well below this threshold. In addition, graphite has a near-zero coefficient of thermal expansion. When a graphite plug is embedded into a metal part, the metal will expand but the graphite plug won't. The plug loosens, which restricts the temperature range to about 200 F if the plugs have any open surfaces (i.e., not constrained by a mating part).
In general, we don't recommend it. The only exception is that when new parts are fitted together, it takes a number of cycles to distribute the graphite over the wear surface. Some customers find that it is useful to wipe a little light oil (never grease) on the wear surface, which then acts as a temporary lubricant until the self-lubrication action gets started. Beyond that, any additional lubrication is actually harmful to graphited components. Liquid lubricants tend to attract dirt and grit, causing premature wear of the graphite. Grease is a particular problem; graphited components should never be greased.
Many of our components are made of aluminum bronze, one of the best bearing materials available. It has a combination of characteristics that is pretty hard to beat: toughness, high tensile strength and a low coefficient of friction when mated with a moving element made of hardened steel.
It also exhibits a property called formability. In the case of a slight misalignment - in a bushing, for example - aluminum bronze has the tendency to slightly reform or redistribute itself along the axis of movement thus correcting (or partially correcting) the misalignment. Friction, galling and seizing are all significantly reduced. A steel bushing, in contrast, does not have this property and any misalignment is permanent. When it comes time to replace a worn bushing, the steel bushing is far more likely to damage the pin as well. All and all, aluminum bronze is a far better choice than steel.
Aluminum bronze refers to a family of copper alloys (most commonly C954 and C959) that have a chemical composition of about 85% copper, 10% aluminum and 4% iron. Mechanical characteristics are:
| C954 | C959 | 1018 (ref) | |
|---|---|---|---|
| Tensile strength (typical): | 90,000 psi | 110,000 psi | 64,000 psi |
| Yield strength (typical): | 32,000 psi | 60,000 psi | 54,000 psi |
| BHN Hardness: | 179 | 286 | 126 |
Individual applications differ, but standard handbook PV values for aluminum bronze bushings (either graphited or conventional) are as follows:
Max PV = 40,000
Max P = 4,500 psi
Max V = 225 fpm
Note: PV measures the performance capabilities of bearings. P is the load on the bearing in pounds divided by the projected area of the bearing. For a sleeve bearing, the projected area is length x ID. For a short slide running on a long wear strip, one would use the area of the slide area. V is velocity in surface feet per minute. Values are for 72 F.
The coefficient of static friction for a graphite aluminum bronze component is similar to that of a conventionally lubricated aluminum bronze component.
CFs = 0.10 - 0.16
SelfLube engaged Detroit Testing Laboratory to perform independent testing of its products in a touch metal stamping application. Using precision laboratory measuring equipment, they concluded that no wear could be detected after 250,000 cycles of operation.
Clean Environments: For applications that require clean operation, our self-lubricating componentry represents an excellent choice. They completely eliminate liquid lubricants in the form of grease or oil, thus significantly reducing the chance of product contamination. In addition, there is no need for an operator or maintenance person to perform periodic lubrication activities, which eliminates another potential source of contamination.
Dirty Environments: Interestingly, our self-lubricating componentry also works well in very dirty environments. With no liquid lubricants, there is far less opportunity to attract contaminants to bearing surfaces. In addition, there is no need for an operator or maintenance person to perform lubrication activities in dirty or potentially hazardous areas.