In Focus

Good chemistry makes for sound operation

Good chemistry makes for sound operation

Cooperation between lubricant and component manufacturers plays a vital role: an example from the field of pneumatics

It is a frequent occurrence in many sectors of industry that component developments are stalled because material specialists were not involved at an earlier stage. A lubricant, for example, that was selected without checking if it matches the particular tribo-system encountered in the component, may cause unpredictable problems and additional costs when the component is put into operation. Such problems can easily be avoided if the design engineer and the lubricant specialist cooperate at an early stage. Customers can then benefit from compo¬nents that function reliably with reduced operating costs.

Not only are the development phases for new components becoming shorter and shorter - the products have to meet more stringent require¬ments as well. Components are expected to fulfill several functions at the same time, they must not pollute the environment, and at the end of the day production costs should be lower than before. The lubricants involved are expected to function without problem over a long time, in many cases over the whole lifespan of the component. This is only possible if they have been tuned carefully to the tribological system as a whole and the application environment in which they are to be used.

Pneumatic drives and control systems pose taxing requirements in this respect. They incorporate a variety of different elements such as cylinders, motors or valves, which are in turn made of different materials, so the tribological system that results is often highly complex. Their operational reliability and service life are strongly influenced by the contact between the sealing element and the moving components, mostly made of metal, for example the piston rod or the cylinder wall. A decisive factor for maximum service life and proper function is the right pre-start lubrication of all components that move relative to one another, e.g. piston rod, cylinder wall, valve slide and sealing elements. Furthermore, the lubricants used for reducing friction and wear must also be tuned to the ambient temperature, sliding speed and normal forces, just to name a few.

Tribo system of a sealing system

When pneumatic components fail during operation, the ensuing costs can be substantial - the repair costs for the pneumatic unit can by far exceed the costs of a lubricant that is precisely tuned to the component/tribo-system used, or the replacement of a failed seal. As operating pressures, temperatures, sliding speeds and stroke frequencies rise continuously, the points where shafts or rods are taken through housing walls have proven the most critical areas for design engineers and tribologists.

Complex pneumatic components

Taxing conditions

Selecting a speciality lubricant that suits a particular design and application of pneumatic components takes a lot of expertise and knowledge of tribology. There is a variety of factors that influence the processes in the various gaps and spaces that make up the tribological system:

  • intermolecular forces
  • thermal transmission and conduction
  • friction and wear
  • chemical and electro-chemical corrosion.

These factors must be considered while bearing in mind:

  • cylinder and rod materials
  • different seal materials and sealing lip geometries
  • contact surface pressure and surface micro-geometry
  • acting pressures
  • mounting position and situation.

Additionally, there are various environmental factors acting on the component and the friction point, e.g. low and/or high temperatures or aggressive media in liquid, gaseous or abrasive form, such as sand, etc. Only an early cooperation between design and tribology engineers leads to the development of sound products that truly benefit the customer.

Oil-tight means less energy consumption

A major requirement in the development of pneumatic components is the prevention of leakage. While a component is in operation, some leakage may go unnoticed, for example when water leaking from a pump evaporates on the spot. There are less trivial types of leakage, though, which have to be remedied with considerable outlay in terms of time and costs. Compressed air, for instance, is expensive to generate and leaks therefore extremely costly. There is a rule-of-thumb saying that you have to reckon one to two Euro-cents for each cubic metre of compressed air generated. Each additional bar of compressed air can be expected to increase energy costs by approx. 10%. This means costs can rise enormously, especially when leakages go unnoticed, e.g. in connecting lines, check- and control valves, quick-fit joints, maintenance units or the terminal equipment. Besides professional maintenance, it is high-quality design elements, namely in terms of the materials used, sealing elements and lubricants, that are indispensable for economical operation, with replacements becoming necessary only at long intervals or not at all. Consequently, when looking at the costs what matters is not so much the cost of air compression as such, but rather how much energy is needed to make the compressed air available at the point of use. On average, approx. 33% of compressed air is generated only to vanish through a large number of small leaks which combine to a significant loss.

An example calculation visualises the losses to be expected - and hence the potential for improvement: A medium-sized company operates a 100 kW compressor station with a total runtime of all compressors of approx. 6.000 hours a year. Based on a power tariff of 6.5 Euro-cent per kWh the annual energy costs to be spent on compressed air will amount to approx. Euro 39.000. If the typical leakages can be prevented, the operator will need 30% less compressed air, resulting in savings of roughly Euro 12.000 on energy alone.

Oil-tight over 6 000 kilometres

Klübersynth AR 34-402, a synthetic special grease for pneumatic drives and their seals, underwent a lifetime test with highly convincing results. The following parameters had been used for the test:

  • Mounting position: horizontal
  • Cylinder type: VDMA cylinder
  • Compressed air: dry and de-oiled
  • Temperature: + 20 °C
  • Pressure: 8 bar
  • Speed: 1 m/s
  • Load: 0 kg mass at the rod
  • Power reduction: none
  • Targeted distance: at least 6.000 km

The test was performed without a transverse load to prevent wear on the guide influencing the friction results.

A well-known measure for the durability of pneumatic seals is the amount of leakage occurring after a certain operating time. The Pneuko M test cylinders from Merkel-Freudenberg Fluidtechnic showed no significant wear after 6.500 km. The pneumatic cylinders were checked for leakage throughout the test duration. No leakage was detected even after 6.000 km. Only after approx. 6.500 km, some minor leaks of approx. 0.6 l/h on the piston side and approx. 0.9 l/h at the rod side were detected.

The synthetic special grease Klübersynth AR 34-402 is highly adhesive and free of heavy metals or solid lubricants. It contains a package of active substances that is carefully tuned to the range of possible applications, making for excellent, balanced product performance combined with high operational reliability and a long service life.

Klübersynth AR 34-402 is frequently used for pneumatic control and sealing elements such as in ISO single- and double-acting cylinders with and without piston rods, in pneumatic rotary drives, short-stroke and compact cylinders, valves as well as the associated sealing elements. Besides a relatively wide service temperature range of at least - 30 °C to +130 °C, piston speeds of approx. 0.01 m/s to > 1 m/s are possible - sufficient service life was attained even exceeding 5 m/s. Further characteristics are minimum breakaway torques that help to prevent the so-called "Monday morning effect" after prolonged standstill as well as stick-slip, which is a plus when piston speeds are low, and during positioning.

Klübersynth AR 34-402 has proven its worth in combination with piston-, rod- and dampening-seal profiles of various shapes and materials, such as NBR, HNBR and PU materials. Klübersynth AR 34-402 can be easily applied using the known methods and metering equipment. For pneumatic control or sealing elements with special geometries, the grease is also available in the softer NLGI consistency class 1 (Klübersynth AR 34-401) to ensure that all friction points are sufficiently wetted.

Combined know-how

To prevent leakages from being caused by brittle seals, it is essential that the seal and the lubricant be compatible. The companies Merkel Freudenberg Fluidtechnic and Klüber Lubrication, both members of the Freudenberg Group, have been engaged in a close cooperation in the field of pneumatic components for many years. This cooperation also involves well-known manufacturers of pneumatic elements and research institutes of universities. Klüber Lubrication and Merkel Freudenberg Fluidtechnic offer their mechano-dynamical test rigs for tests that replicate application and component realities as near as possible. The combined knowledge of all the specialists involved enables the development of seal-and-lubricant combinations for the various tasks encountered in pneumatic systems. Their operators will eventually benefit from the sophisticated design of the pneumatic drives, the valves and their seals. They can trust their machinery to operate reliably over a long time and cut their energy costs considerably.