Molded rubber parts in hydraulic systems

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30.09.2019

 

Hydraulics are crucial across many technical applications. Basically, they are utilized whenever a system is required to transmit a force or energy with the aid of a liquid. Examples in which hydraulics provide power transmission are motor vehicles, agricultural machinery, brake systems, machine tools, presses, injection molding machines or lifting platforms.

Rubber components are essential for hydraulic systems to function. Without rubber molded parts such as seals, scrapers and O-rings, there would be no hydraulic systems.

There are many different areas of application with different stresses and strains, and as such, the demands placed on rubber materials are high. Each hydraulic system works with its own specific media or fluids, temperatures and pressures, to which the rubber parts must be tuned. When selecting an elastomer, the focus must be its mechanical properties; chemical and media durability; and thermal resistance.

In this blog post, we discuss the basics of molded rubber parts required by hydraulic systems, describing the special demands placed on the performance of the elastomers, and the main materials suitable for use in hydraulics.

Key rubber components in hydraulic systems

Hydraulic systems would be unthinkable without molded rubber parts. They are central to the effectiveness of a hydraulic system and fulfill a range of functions determined by the type of system and its design.

Basic design of a hydraulic system

Figure 1: Typical rubber parts in a hydraulic system. Source: Flow Control

In a hydraulic system, a piston rod moves continuously in and out of a cylinder. The space between the piston and the hydraulic cylinder is a critical area where hydraulic fluid may escape from the cylinder enclosure into the environment. Rod seals, made of rubber, function to prevent leaks in precisely this area, and in addition act as a seal for the piston in the hydraulic cylinder (Figure 1). As a rule, rod seals are composed of grooved rings or sealing lips. Generally, a longer external sealing lip at the external rim of the grooved ring functions as the static cylinder seal. The inner seal lip is often shorter. This seals the movable piston and as a result is subject to dynamic stresses.

When in operation, the piston moves up and down the tube of the cylinder dividing the cylinder tube into two chambers. To seal these two cylinder chambers from each other, piston seals are used (Figure 3). The most important task of the piston seals therefore is to act as a seal between the piston and cylinder tube and prevent leakage. In addition, the piston seals have a stabilizing effect on the piston and keep the movement efficient and smooth. 

Example of a typical rod seal

Figure 2: Typical rod seal design. Source: Simrit Catalogue, Edition 2004, p. 4.2 (Freudenberg Simrit KG)

Example of a typical piston seal

Figure 3: Typical piston seal design. Source: Simrit Catalogue, Edition 2004, p. 4.3 (Freudenberg Simrit KG)

 

Depending on the type of hydraulic cylinder, a distinction is made between single-acting and double-acting piston seals.

Single-acting piston seals need only seal in one direction of movement. This is how simple hydraulic cylinders work. Only one side of the piston is in contact with the hydraulic fluid. In other words, the work of the piston is in one direction only. Pressure is created on one side of the piston and that is at the point where the single-acting piston seal has to form a seal. It prevents fluid leaking from the piston for one active direction of movement. Grooved rings are often used as single-acting piston seals. The inner sealing lip is static, whereas the outer, shortened sealing lip ensures dynamic sealing.

On the other hand, for double-acting hydraulic cylinders, there are two active directions of movement. These cylinders have an opening at each end, so hydraulic fluid is supplied to both cylinder chambers. Examples of these are differential cylinders, synchronous cylinders and tandem cylinders. In these cases, pressure acts on both sides of the piston surface. They are actively moved in two directions and, as such, must be sealed in two directions. This task is performed by double-acting piston seals that seal dynamic pressure from both sides. In some applications, double-acting piston seals also have two single-acting piston seals, with an additional groove in the piston for the second seal.  

During the regular movement of the piston rod out of and into the hydraulic cylinder, foreign particles may be transported into the hydraulic system. Therefore, rubber wipers are used to prevent the ingress of dirt or swarf, which may contaminate the hydraulic fluid and result in damage to the hydraulic system. Either single or double wipers can be used. Scrapers with a scraping edge should ensure that dirt particles from outside do not penetrate the hydraulics. Accordingly, the wiper edge is directed outwards. In addition to protecting against dirt intake, double-acting scrapers also have an extra inner sealing edge to reinforce the sealing system. Whether single or double acting scrapers are used depends on the requirements of the hydraulic application.  

Without oil or lubricating film for the cylinder tube or piston, hydraulic components will fail due to excessive wear caused by friction. When designing the seals and scrapers, it is important that for each pass of the hydraulic piston, the film is completely pulled back into the hydraulic system.   

In addition, when a hydraulic system is in operation, lateral forces may occur that cause the moving piston to come in contact with other housing parts. Guides or guide elements guide the piston rod while absorbing lateral forces and preventing unintentional metal contact. The aim is to allow as little friction as possible and keep wear to a minimum in the hydraulic process.

Depending on the type of application and the requirements on the hydraulic system, a variety of materials and designs of guide elements can be used. These include both metal and non-metal guides. Elastomers are becoming increasingly popular for the non-metallic guides.

 

Hydraulics: a real challenge for rubber parts

Rubber parts are generally exposed to extreme conditions in hydraulic systems. A rubber material must be selected capable of withstanding a range of factors simultaneously, so as not to be the cause of hydraulic failure.

Heat: If the rubber parts integrated in the hydraulics system, such as the rod or piston seals, are exposed to a high temperature for too long, they can become brittle, wear out more quickly and lead to sealing failure. The oil temperature in a hydraulic cylinder is generally around 80° C. However, in some cases, this can easily rise to 110° C placing the rubber parts under significant stress. The temperature of seals like the grooved rings is often way above the oil temperature as they are exposed to constant friction in the hydraulic system.

In addition, there is a risk that the active rubber molded parts disintegrate and get into the hydraulic system. The resulting impurities can cause the hydraulic system to fail.

There are big differences in the temperature resistance of elastomers (see also: Materials Overview). For example, there are materials that can withstand temperatures above 200° C (which may not be suitable for hydraulic applications for other reasons). It is therefore important to be aware of the limits of the selected material at elevated temperatures. It should be remembered that in general the properties of an elastomer change significantly whenever temperature rises or falls (see also: The Value of Technical Data Sheets).

Media/chemicals: In order to choose a material that is going to be right for an application, you have to consider the media with which the rubber part comes into contact. If, for example, the material is not compatible with the hydraulic fluid being used, then swelling or decomposition can result leading to poor performance or even failure of the hydraulics. 

Common hydraulic oils are based on mineral oil, so it is important to select an elastomer that has good resistance to mineral oil. However, there are critical applications in which it is necessary to dispense with mineral oil since it is flammable and the risk of combustion high. For example, in civil aviation or mining, mineral oil-based fluids cannot be used as the risk of fire and associated effects is too great. Also, in some applications, such as in foundries, power plant turbines or rolling mills, there is a risk that in the event of failure of the seals, when hydraulic fluid could come into contact with hot metals or other components, there could be a fire.  In such cases, recourse is made to low-flammability hydraulic fluids. These are classified into different groups (HFA, HFB, HFC, HFD), all of which have different compatibilities with rubber materials. For example, NBR (acrylonitrile-butadiene rubber, or nitrile rubber for short) has good resistance to the HFC group low-flammability hydraulic fluids, but only moderate resistance to the HFA group. NBR is not resistant to HFD (see also: NBR: acrylonitrile butadiene rubber).

Pressure: In hydraulic applications, rubber parts such as seals, scrapers or guides may be subject to high pressure and constant pressure changes. In addition, there are lateral forces and deflections. The important factors in the calculation of the stress of the rubber parts are system pressure, size of the hydraulic cylinder and its lifting force, piston speed and the resulting friction. 

Without proper design and construction, the rubber parts may be overstressed, resulting in damage such as cracks and ultimately complete failure. Failure of a rubber seal or other elastomer part may result in damage to the entire hydraulic system. When selecting the material to be used, therefore, different mechanical variables need to be taken into account. At low pressures of up to 10 MPa, the use of elastomer parts is usually unproblematic. At higher pressures, it should be checked whether reinforcements are necessary, for example, fabric reinforcement.

Contamination: The sealing functionality of the hydraulic system can be severely affected by dirt and deposits in the hydraulic oil or other hydraulic fluids. Metallic abrasion may cause metal shavings to penetrate the system. Other foreign particles, such as sand, gum residues, or oxidizing agents due to the aging of the oil also pose a risk to the sealing function if they are trapped under the sealing edge or have damaged the rubber part. The result is leaks that negatively affect the efficiency of the hydraulic pump, motor and cylinder. Control valves to control flow and pressure can also be damaged by contamination. In the worst case, it can lead to complete failure of the hydraulics. 

Elastomers for hydraulic applications

The challenges described above elucidate the high demands placed on elastomers in hydraulic systems. Users and designers should be aware of these demands when selecting the materials for rubber parts in order to decide which materials are right for their application. Consideration must be given to the temperature range of the application, the type of hydraulic fluid being used, the fluid pressure range, the sliding speeds and the dimensions of the hydraulic machine.

Since hydraulic systems are often complex systems and downtime and maintenance are costly, the rubber parts and the rubber compounds that they require must combine several properties. On the one hand, a long service life of the seals or scrapers is expected for optimum time between maintenance. At the same time the rubber parts must be designed so that they have a high function safety. The material used must be resistant to hydraulic fluid and resistant to mechanical stress regardless of whether the temperature is high or low as the scrapers and piston and rod seals are exposed to constant friction and enormous pressure. Also, during operation of the hydraulics, an umbalance or eccentricity may occur between the piston rod and the hydraulic cylinder, which acts accordingly on the molded rubber parts.

NBR (acrylonitrile-butadiene rubber) and FKM (fluorocarbon rubber) have proven to be suitable materials for the rubber parts used in numerous hydraulic systems.

Piston and rod seals, scrapers or guide elements are often made of NBR or FKM. However, EPDM (ethylene-propylene-diene rubber), HNBR (hydrogenated acrylonitrile-butadiene rubber) and FFKM (perfluoro-rubber) are also sometimes used. 

Nitrile rubber (NBR) has good resistance to mineral oil, various fats, water and hydraulic fluids. NBR combines this with mechanical properties such as high tensile strength and good abrasion resistance, which are invaluable in hydraulic applications. In addition, NBR is very well suited for use in the low temperature range down to -40° C. The material can also be used for an oil temperature of approx. 80° C, which is generally the case in a hydraulic cylinder. Incidentally, in a previous blog entry we looked in detail at the strengths and weaknesses of nitrile rubber (see here).

If a hydraulic system requires improved high-temperature performance from the rubber material or even better wear resistance, then, for example, the more expensive HNBR (hydrogenated nitrile rubber) can be used for the rod or piston seals. However, at temperatures above 110° C, FKM (fluorocarbon rubber) is likely to be the material of choice, also known under its trade name Viton™. FKM is ideal for use in a variety of hydraulic applications, such as automobiles, aircraft or chemical processing.     

Sealing elements for hydraulic systems can also be designed as an assembly of parts in order to increase service time and functionality, and reduce wear. For rod seals in hydraulic cylinders, for example, the sliding ring may be made of PTFE, while the O-ring is made of an elastomer such as NBR or FKM. Experience in the production of parts with rubber-metal connections or rubber-plastic connections is advantageous for manufacturers of molded parts for hydraulic systems.    

Please do not hesitate to contact us for more details.

If you have a question about this blog post or would like us to discuss a particular aspect of elastomers in an upcoming blog, please email us on info@hepako.de   

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