Lubrication

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Lubrication is important aspect of every CNC machine and the RepRap is no exception. Proper lubrication will make your machine run smoothly, will suppress or even remove ugly noises and reduce wear and tear of your components. This article is a practical overview of lubricants compatible with the RepRap project, but it may be useful to understand the physics. Lubricants work by being slippery, and forming a barrier between two friction surfaces. Thick, viscous lubricants can maintain a barrier without squeezing out even under high pressures. Due to their viscosity, however, they may be more sticky than slippery. There is a trade off, and RepRap requires much thinner lubricants than axle grease or engine oil. Wikipedia: Lubrication is a good overview of the theoretical mechanisms of lubrication.

For each lubricant, there are several factors to take into account:

  • Viscosity/squeez out pressure- a thick grease might clog up your bearings, but would be fine on a threaded rod. (see NLGI Grade)
  • Reactivity - will the lube soften or dissolve your plastic? Most lubricants are formulated for steel, but not all. It might also react if combined with other lubricants.
  • Friction - all lubricants are designed to reduce friction, but some get closer to zero than others. Some are formulated for ultra-high speeds & heat reduction, for example.
  • Maintenance - RepRap doesn't wear out lubricant anywhere near as quickly as your car, but dust and grime will accumulate in a sticky oil.
There was a recent discussion in the forums regarding lubrication. 
The information on this page might need to be updated. 
[Reprap-Compatible Grease]
--Buback 22:21, 25 October 2010 (UTC)
"Linear bearing compatible lubrication?".

Contents

Lubricants by application

The main parts of the RepRap machine that need to be lubricated are:

  • Threaded Rod
  • Bearings and bearing rails
  • Bushings (if used instead of bearings)

Threaded Rod

Both Darwin and Mendel RepRap models have multiple Z-stage drives constructed using threaded rod and a captive nut, though some experimental or RepStrap machines use only one. There are RepRap clones that have X and Y also made with threaded rod and a nut. Friction between nut and threaded rod, especially under the load of the Z platform in Darwin based models is huge and if not lubricated the Z-Stage will produce ugly squeaking sounds and rub off metal powder, and soon your threaded rods will show signs of wear and tear (especially if you use a lot of Z-Motion). Properly lubricating the rods will reduce the wear and tear on the rods (not to mention reducing the ugly sounds). So far, high viscosity PTFE filled oil (super-lube for example) shown best results here.

Bearings

There are two major types of bearings found in RepRap machines.

  • Roller bearings
  • Linear bearings

Roll bearings

These bearings are usually closed and have their own lubrication, so no additional lubrication is needed.

Linear bearings

Linear Bearings come in open and close package. The closed ones have their own lubrication and no additional lubrication is needed while open ones need additional lubrication. high-viscosity PTFE filled oil (super-lube) for example shown best results here. Synthetic Gear Oil also shown very good results. On the other hand, TheGremlin recommends low viscosity lithium soap based lube when using bushings. High viscosity greases, such as axle grease, can clog up roller bearings and cause them to slide instead of rolling. This will wreak havoc on your expensive precision ground linear rods. He recommends a grease NLGI Grade 1 or 0.

Bushings and ball joints

Selecting a lubricant for a ball joint is not as complicated as selecting a lubricant for a bushing, because ball joints tend to be metal on metal. Depending on the type of a bushing (PLA on Chrome, PTFE on Chrome, ABS on Chrome ..) you will need to find a compatible lubricant. This is because some oils do not interact well with some plastics. In most cases when using PTFE on Chrome no lubrication is needed, but some PTFE filled oil will help a bit, especially if you use PTFE inserts for X or Y movement. Multipurpose grease also shown nice results. Because the smooth rods are in the open air, most oils and greases will accumulate dust. Silicone based dry lube is a potential solution to this problem, according to TheGremlin's post.

Lubricants by type

Oils, greases, and dry lubricants

Most lubricants can be broadly categorized into three types:

  • Oils - Thin liquids that flow, made up of long polymer chains. They don't squeeze out from under surfaces as easily as other liquids due to their long polymer structure. This means that rubbing surfaces don't get a chance to touch, unless they can apply enough pressure to squeeze out all the oil that separates them. Thicker oils have higher pressure thresholds, but their high viscosity resists motion slightly.
  • Greases - Lubricious solid particles suspended in a paste. Although generally thick, greases can range from "apple sauce" to "cheddar cheese" consistency. Thick grease takes much more pressure to make squeeze out, so it can handle much higher loads than RepRap will ever put it through. However, thick grease will bind up your printer. For reprap, we generally want thin grease (NLGI 1 or below)
  • Dry lubricants - Exists as free powder, dispersed in water/alcohol, or as a coating. Because RepRap rods tend not to be covered, dust collects on them. Sticky oil and grease only exacerbate this problem. Dry lubricants solve this, but may require slightly more work, depending on what is added to the powder.
    • Free powders don't adhere very well to the surface, and require frequent re-application.
    • Powders applied in a suspended medium will adhere slightly better than free powders once the dispersant evaporates.
    • AF (anti-friction) coatings are made by adding a binder to form a coating which can be painted on.
    • Self-lubrication works by embedding powder in the part, rather than applying it as a coating. As it wears it releases lubrication.

Oils and greases have been widely compared, however dry lubricants are in a class of their own. When too much pressure is applied and an oil is squeezed entirely out from in between two surfaces, the oil will flow back in to fill the dry spot once the part has slid past. This makes oils good for things like cars where varying loads are placed on parts. Greases and solids do not flow like this, but can require higher pressure thresholds to be squeezed out in the first place. Oils can also flow and carry away debris, whereas greases and solids aim to prevent the grinding that generated the debris in the first place. Oils have a high margin of error when selecting a viscosity, so require relatively little knowledge to apply and replace. Greases must be selected based on NLGI consistency (0 or 1 work well for the low loads RepRap requires). Some oils and greases can be corrosive to some plastics or even metals, whereas dry lubricants are less likely to react. Dry lubricants are not sticky and so don't collect dust, but offer less protection from rust when not applied as part of an anti-friction coating.

Dry lubricants are an enticing option for RepRap. It solves the dust buildup problem elegantly, and we really don't need motor oil or grease intended for power tools/industrial machines. The downside is that dry lubricants don't adhere well, so they may require more frequent re-application. If applied in an Anti-Friction coating, however, they will last much longer. (see the Wikipedia page for details.) If anyone has any experience using graphite or other powdered solid lubricants, please add your knowledge!

List

  • Graphite lube can be as simple as running a soft artist's pencil through your threaded rod. Most pencil cores are made of graphite mixed with a clay binder. While graphite is quite soft, clay is hard and abrasive; the OPPOSITE of the qualities we want for RepRap. (Although, at least in theory, the flat clay particles could reduce friction through the same planar slip mechanism as graphite.) Besides, graphite under 80% pure makes poor lubricant. (Wikipedia: Dry_lubricant#Graphite_source) Only types 5B and above meet this criteria (list of pencil compositions), so standard pencils (world HB, or US #2) are not suitable at only 68% graphite. In fact, the US system doesn't even include any high-graphite pencils because there is no value below #1. (Wikipedia: Pencil#Grading_and_classification). Artists suppliers offer solid graphite sticks at times with varying composition. EDM Electrode graphite is usually of a high quality. The hardness varies and there is usually no indication of the composition thought a high purity is to be expected. Grinding these in a ball mill would supply powder. Dry lock lubricants are often fine flake graphite in a small squeeze bottle intended for blowing into a keyhole. Electrode graphite could possibly be used directly to replace graphite stock that is intended for machining bearings. Pistons in air dash pots are sometimes made of graphite if they are used in glass cylinders. The vanes of dry vane pumps are often made from graphite as there is no option of a lubricant. High temperature anti-seize pastes sometimes contain graphite in a grease or other binder.
  • Silicone is generally used in grease form, but it is also commonly available dispersed in aerosol sprays, which are sold in hardware stores. Silicone grease that is often used with RepRap is actually meant to be used with rubber gaskets, O-rings and similar equipment. It is not "bad" and some users report good results with it. There is also silicone grease with PTFE that is reported to work well. In the case of dry lube sprays, the dispersant evaporates, leaving behind a layer of solid silicone. Silicone based liquid lubricants thicken under shear, keeping the surfaces from touching. This has its limits, however; silicone lubricants compress more and spread out more under pressure than other lubricants. As a result, they have a lower threshold for the pressure they can support. They are sufficient for plastic-to-plastic and metal-to-plastic applications, however metal-to-metal exceeds silicone's lubricating capacity. (How Silicone Lubricants Work) Solid silicone does not spread as much as silicone based liquid lubricants. This gives it a moderately higher pressure threshold, allowing it to be used under more severe conditions. (details on the exact physics on Wikipedia: Dry_lubricant#Silicone)
  • PTFE lubricants can withstand higher pressures than silicone, allowing it to be used for a metal-to-metal. It is used as a dry lubricant, in grease, or as an oil additive. PTFE is also highly hydrophobic, and so may help prevent rust even in dry lubricant form. It is sold in oil and grease form under the brand "SuperLube", and DuPont sells a "Teflon Non-Stick Dry-Film Lubricant". All of these should not react with ABS or PLA, but only the PTFE filled oil has been tested.
  • molybdenum disulfide (moly) is similar in feel to graphite, and can be used as a dry lubricant. Powder is frequently used in grease or as an oil additive, for applications like aircraft engines where failure would be catastrophic. It has been added to plastics like nylon, PTFE, and Vespel to form self-lubricating composites. More info at Wikipedia: Molybdenum_disulfide#Lubricant
  • low viscosity lithium soap grease may be hard to source locally, but works well for linear bearings
  • Multipurpose grease is good for use with bushings
  • Synthetic Gear Oil
  • Light oil reduced the squeaking sound but did not really help with friction and on the bushings it even increased friction (by attracting dust and creating sticky surface on the slider rod). CupCakeStrap has used light machine oil for the linear rails, which are aluminum rod and brass tube bushings. The oil worked very very well, but over a one-month period the oil formed some black gunk, and much of it leaked down from the rails onto the acrylic X stage, lightly discoloring it. The gunk is easily removed, but doing so dries out the rail.
  • WD40 is possibly the worst lubricant, tested on a Darwin based RepRap rapman. It "ate up" the Z threaded rods and produced huge amounts of ugly black gunk that was impossible to remove while not improving Z movement at all.

Self-Lubricating Composite

It would be interesting to add graphite or some other powder to ABS or PLA and extrude it with a filastruder. If quality filament could be produced, then it could be used to print gears, ball bearings, bushings, and other high-friction RepRap parts. Since the particles will be partially aligned in the extrusion process, it could be printed in such a way that the particles were already aligned in the direction that the shear force will be generated while the part is in service. Note that Conductive ABS is made by blending ABS with carbon fibre and carbon black, but NOT graphite. Perhaps graphite-infused (or carbon nanotube?!) filament could be adapted to also be conductive.

Industry uses composite lubricated plastics in certain applications. PTFE and Molybdenym Disulphide OR Graphite are used in some, one trade name is Vesconite.

Summary Table

Here's all the lubricants mentioned in this article, summarized in a table:

Lubricant type Form Viscosity Known Reactivity Friction Maintenance Sources Other notes
PTFE filled oil oil/solid  ? compatible with ABS and PLA  ?  ? Super Lube Good for threaded rods, bushings, and bearings
PTFE filled grease grease  ? compatible with ABS and PLA  ?  ? Super Lube  ?
low viscosity lithium soap grease? NLGI Grade 1 or 0 is recommended  ?  ?  ? May be hard to source locally Good for linear bearings
Silicone based dry lube Aerosolized solid N/A  ?  ? Dry, so no dust problems  ? Good for plastic bushings
Graphite solid suspension in isopropanol N/A  ?  ? breaks down and becomes a friction agent "after a bunch of years" Brand name "Neolube" Can also run a soft artists pencil through threaded rod
PTFE based dry lube solid N/A  ?  ? Needs somewhat frequent replacement (a mountain bike chain needs re-lubrication after 3-5 rides) Sold in hardware stores. Label may say Teflon instead of PTFE. Good for motorcycles and mountain bikes
Synthetic Gear Oil oil  ?  ?  ?  ?  ? Good for linear bearings
Multipurpose grease grease  ?  ?  ?  ?  ? Good for bushings
Silicone grease grease  ?  ?  ?  ?  ? Meant as a sealant for rubber gaskets, O-rings, etc.
Light oil oil  ? Bad for aluminium and/or brass; discolors acrylic Doesn't reduce bushing friction much?  ?  ? CupCakeStrap had poor results
WD-40 oil  ? Reacts with metal Doesn't reduce friction  ?  ? Designed for eating rust, NOT lubrication