Whenever fluid lubrication systems (oil / grease) come to their limits then solid lubricants can be the best solution.
Typical fields of application are:
- High temperatures up to 550°C
- Low temperatures down to minus 180°C
- Vacuum, high and ultra-high vacuum
- Aggressive environmental conditions (acids / leaches / bioactive substances)
- High hygienic / cleanliness demands
- Radioactive surrounding
- Low speeds
- Very high loads / shock loads
- Intermittent periodic duty / long standstills
Solid lubrication versus fluid lubrication
The goal of each lubrication system is to operate at heavy lubrication, the friction partners are completely separated by a lubrication film. There is fluid friction with lowest friction and wear. The friction coefficient µ depends on the bearing type and is between 0,0015 (DGBB) and 0,005 (ThNRB) at 10% load (Co/P = 10)
Boundary / mixed friction is caused by high loads – low speeds – high temperatures. The friction and the wear increases dramatically. If additives are not sufficient to improve the lubrication performance, dry lubricants are added to the lubricants.
Latest, if the fluid component of the lubricant has no function anymore (e.g. at operating temperatures above 260°C), a solid lubrication is the best option.
Solid lubrication always has a higher friction coefficient than fluid lubrication. Typical values are 0,04 to 1 at standard atmosphere. The friction coefficient highly depends on the Dry lubricant and the system of applying it in the bearing.
Die Feststoffschmierung hat immer höhere Reibwerte als Fluidschmierung, diese betragen zwischen 0,04 und 1 bei Normalatmosphäre. Die Reibwerte sind stark abhängig vom eingesetzten Festschmierstoff und deren Aufbringung (siehe Eigenschaften)
Relevante Festschmierstoffe und deren wichtigsten Eigenschaften
Mesh layer lubricants, graphite and MoS2
Graphite and MoS2 have a lamellar mesh layer structure with low adhesion between the layers in slip direction. Vertical to it the show an excellent compression strength.
Both lubricants have an excellent adhesion to metallic surfaces. In combination with a moderate price mesh layer dry lubricants are often the first choice.
Because of the outstanding characteristics MoS2 is the most frequently used dry lubricant.
Polymer lubricants and materials
The most common polymer lubricant is PTFE. In combination with with PFPE (Alkoxyflouro oil) high performance H1 greases are produced. These greases can be used up to 260°C (recommended max. constant temperature < 200°C) as fluid lubrication, with higher temperatures the PTFE produces a thin dry layer on the friction partner.
With Durolub G405 we offer a high performance grease which has achieved outstanding performance on a FAG AN2 test rig. A 6209C4, loaded with 3kN and operated with 55min-1 at 270°C showed a 60% higher operating life than any other product tested.
The temperature limits of PTFE must be observed carefully – above 300°C decomposes, generating poisonous vapour and corrosive decomposition products.
As material, especially for cages, there are mainly used polyetheretherketone (PEEK) und polyimide (PI). PEEK has been used for many years and is well approved up to 260°C. The material is easy to get with a good price – performance ratio.
PI is an extreme high performance material which can be used up to 300°C (peak up to 400°C) while maintaining extremely high chemical / physical properties.
Both materials are often combined with other dry lubricants in order to enhance the tribological properties.
For rolling bearings mainly silver and lead are used either as galvanic coating or within multi – layer CVD/PVD coatings.
Typical applications are in bearings used in turbochargers or in vacuum. They are used only in very demanding applications with serial demand.
Application of dry lubricants
Applying dry lubricants it is essential to observe:
- the dry lubricant layer should be evenly distributed to all friction partner
- the thickness of the dry lubricant layer should not be too high, any layer thickness above 1,5µm does not improve the lubrication performance but tends to cause problems (chipping)
- that a continuous relubrication is ensured, e.g. as transfer system
Application with powder / pastes or suspensions
With the rubbing system the dry lubricant (powder / suspension / paste) is filled into the bearing and distributed through low speed rotation of the bearing.
The dry lubricant layers are difficult to reproduce at the same quality, normally the layer thickness is bigger than the recommended max. 1,5µm.
Therefore, the rubbing system is used for plain and simple applications. The bearings should have a very high clearance to avoid any blocking.
Sliding lacquer is hardly used today. The thick bonding agent is rubbed off, the bearing does not rotate smoothly – operating life time is short
Application with polyglycoloil
The dry lubricant is applied in combination with polyglycoloil, Durolub G401 is based on MoS2 and Durolub G402 is based on graphite.
The polyglycoloil works as fluid lubricant up to 200°C and helps the dry lubricant to evenly distribute in the bearing. At higher temperatures the polyglycoloil evaporates and a dry lubricant layer is built on the friction partner.
With defined running-in processes the accuracy of the layer thickness can be guaranteed, the results are reproducable.
This procedure is one of the standard procedures for standardized kiln truck bearings, the Durolub S201 is built on G401 (MoS2) and S202 on G402 (graphite)
Application with alkoxiflourooil (mainly PFPE)
in combination with PTFE a widely used standard system. Besides H1 certification, this solution offers the highest possible operating temperature as fluid lubrication (260°C). For continuous operation such greases can be used up to 200°C, above 260°C the oil evaporates and the PTFE dry film continue to lubricate.
Transfer – lubrication (Durolub SLC and Durolub SLC-V for applications in vacuum)
Durolub SLC consists of a mix of dry lubricants and a high temperature resistant bonding agent. The compound is filled into the free space of the bearing and treated thermally. After hardening the compound rotates with the cage and constantly transfers lubricant to the friction partners (transfer lubrication)
At the begin of operation the friction is higher and reduces constantly until a approx. 1µm thick dry lubricant layer has been built up. The layer is built up perfectly in the right direction.
Based on the high volume of dry lubricant in the bearing long operating times can be achieved.
Durolub SLC bearings can be used from minus 180°C to 350°C. Higher peak temperatures just reduce the lubrication performance but do not make the bearing fail. The compound oxidizes faster.
Durolub SLC bearings can be used for easy applications (e.g. kiln truck bearings – Suffix S228) as well as for normal applications up to ndm 45.000.
Other examples for transfer lubrications are cages or segment cages made of dry lubricants.
Appplication with CVD/PVD technique (sputtering)
These processes produce extremely homogeneous and clean layers, the layer direction can be easily controlled, especially with MoS2.
The layer thickness can be accurately adjusted between 0,2µm to 1µm and can be reproduced easily.
There are multi-layer coatings, a combination of hard and tribological materials, with enhanced performance exactly adjusted to the demand.
The processes are expensive and are usually used on high performance bearing materials and ceramic rolling elements