This article will describe the basic influences and parameters that need to be taken into account when designing seals for hydraulics. We will delve into the realm of physics and theoretical knowledge that form the basis for seal design, depending on the operating conditions.
Mechanics of Seals
The requirement for seals in hydraulic cylinders is to prevent fluid flow across two surfaces during relative motion and to maintain a high level of sealing performance throughout its lifespan and under the operating conditions for which it was designed.
During sliding movement, a drag flow develops, and due to the hydrodynamic pressure increase, the seal lifts off the guide surface, leaving a thin liquid film between the seal and the guide surface. The thickness of this liquid film is governed by the following formula:
- s – liquid film thickness
- K – coefficient (≈ 2.3)
- n – liquid viscosity
- v – speed
- L – length of the surface that is in motion
- p – pressure
The amount of liquid passing through the seal during movement can be considered as a form of leakage.
Pressure
The pressures acting on the seal are those induced by the hydraulic pump (Pp) and those caused by the movement of the cylinder, called “drag pressure” (Pt):
- Ptot – total pressure
- Pp – hydraulic pump pressure
- Pt – drag pressure
- K – constant factor
- – liquid viscosity
- v – speed
- L – surface length
- s – clearance
Drag pressure, especially in cases of tight fit, can sometimes be higher than the pressure generated by the pump itself, causing rapid seal damage. During operation, the sealing element may be exposed to various constant pressures, which, even for a short time, reach very high intensities. These additional loads subject the seal to high operating stress. These must be taken into account when choosing the right sealing system.
Pressure load – less than 50 Bar
Low pressure is the most critical situation for good sealing system performance and is a situation where major leakage problems can occur. In these cases, the seal lips are not sufficiently expanded by the fluid, and the oil film between the edge and the dynamic surface reaches excessive thickness. Choosing the right seal profile and material can significantly reduce the risk of leakage in such a situation.
Medium pressure – 50 to 150 Bar
The pressure range between 50 and 150 bar is one of the most favorable, and under such conditions, almost all types of seals guarantee good performance, albeit with varying lifespans depending on the seal material.
High pressure – above 150 Bar
At high pressure or high pressure surges (impact loading), the seal typically performs well. The fluid pressure itself activates the seal lips, ensuring very good sealing performance. High pressure, on the other hand, reduces the lifespan of the entire sealing system. Under these operating conditions, many cases of wear and extrusion occur, causing premature seal failure. Material selection is very important because it must be resistant to extrusion and wear.
Speed
The speed between the seal and the moving surface is a critical factor that must be considered when selecting a seal and has a significant impact on the performance of the sealing system. Leakage can be considered proportional to the square root of the speed, although this cannot be precisely predetermined as it depends on many other factors, starting with the appropriate seal selection, fluid type, temperature, and surface quality.
Horizontal axis: Speed
Low speed – less than 0.05 m/s
At low speed, leakage problems do not occur, but problems with rapid wear and irregular movements (stick-slip) are likely. In the low-speed range, the hydraulic pressure generated by the movement is not sufficient to create a permanent film of liquid, and the edge of the seal comes into direct contact with the sliding surface, increasing the risk of rapid wear and irregular movement. “Stick-slip” is a noisy vibrational movement caused by repeated sticking and slipping between the seal and the sliding surface. The correct choice of seal profile and material (e.g., PTFE, with a low coefficient of friction) can reduce the problem and also improve fluid and speed control.
Medium speed – 0.05 ÷ 0.3 m/s
This is the ideal situation, where there are neither irregular movements nor excessive losses typical of high speed. Under these conditions, the hydraulic pressure generated by the movement is able to ensure a permanent film of liquid between the edge of the seal and the guide surface, ensuring precise fluid control and proper lubrication of the seal. The thickness of the liquid film, proportional to the square root of the speed, generally does not reach a thickness capable of causing unwanted leakage.
High speed – above 0.3 m/s
As the hydraulic pressure generated by the movement increases, the seal lifts off the guide surface, allowing an excessive amount of liquid to pass through. The situation becomes particularly critical when low-pressure phases are combined with high speed. In such a case, the seal is subjected to deformation. Under such conditions, it is necessary to choose a seal that ensures sufficient tightness even in the absence of pressure.
Temperature
The system temperature is a critical factor that must be considered when selecting a seal and has a significant impact on the extent of losses. As a result of friction, the temperature at the edges of the seal is higher than the system temperature, although this cannot be precisely predetermined as it depends on many other factors, starting with the material, seal profile, fluid type, and surface quality. Because the viscosity of the liquid is inversely proportional to the temperature, leakage can be considered proportional to the square root of the reciprocal of the temperature (see chapter “Mechanics of Seals”):
Horizontal axis: Temperature
Low temperature
At low liquid temperatures, the hardness of the sealing material increases, and the seal loses elasticity, allowing an excessive layer of liquid to pass through.
Medium temperature
This is the ideal situation, where the liquid has the ideal viscosity for preventing losses through sufficient lubrication. At this temperature, materials do not change their mechanical properties enough to affect the performance of the sealing system.
High temperature
The sealing material becomes more elastic, the volume of the seal increases, and the viscosity of the liquid decreases, thereby limiting losses. At the same time, however, there is insufficient lubrication, and wear and the risk of irregular movement increase. Increased attention must be paid to the temperature limits of the materials. At the limit values, the seal loses its elasticity.
Friction
The friction between the dynamic seal and the sealing surface depends on many factors such as seal design and material, fluid, pressure, temperature, speed, and surface treatment.
The resulting frictional load may not be important for many applications (except for pneumatic cylinders, where minimal friction is required for optimal performance), but friction itself can be dangerous in terms of heat generation, which can cause degradation of the sealing material and lubricant film, or increase leakage by reducing viscosity. Seal performance in this sense is difficult to analyze in general terms, as it depends on a greater number of empirical factors that are typical of seal design.
In general, however, friction is proportional to pressure, but the coefficient of friction may vary with speed, temperature, material, and surface finish.
- K – empirical factor specific to the shape of the installed seal
- μ – coefficient of friction
- Pe – Equivalent pressure corresponding to the pressures acting in the system
- v – speed
- A – Contact surface of the seal (≈ π • Diameter • Radial cross-section)
Specific values of the K factor are difficult to obtain unless they are evaluated on empirical lines or based on comparative data. This formula can only be used directly to determine possible differences in performance and friction on compression seals of the same type and material, but with different sizes.
The coefficient of dry friction of typical sealing materials that rub against a smooth and dry sealing surface can be from μ=0.4 ÷ 1. For lubricated surfaces, the range is much lower, for example μ=0.02 ÷ 0.10. This is especially true for elastomers. Fabric materials and impregnated fabrics exhibit a similar value of “μ”, but with smaller differences, for example μ=0.04 ÷ 0.08 when lubricated. In general, the harder the material, the higher the friction, and the softer the material, the lower the friction, but this is well maintained at low pressure. The coefficient of friction “μ” is also influenced by pressure, but the direct relationship is not clearly determined. In general, friction will be highest at low pressure, with the smallest values being achieved at higher pressure.
The change in friction at different pressures also depends on the surface treatment, especially the manufacturing method in the case of cylinder and piston seals. The most significant increase in friction with increasing working pressure occurs with rougher surfaces and cold-forged structures, compared to ground and polished surfaces.
The common design of cylinders is ground, which leads to a precise surface with an average roughness between 0.25 μm and 0.625 μm. The biggest problem for the seal designer is the current tendency to use hydraulic cylinders made directly from drawn tubing without any subsequent treatment.
Friction and speed
Changes in friction with sliding speed are clearly defined and occur in three phases (see figure below):
- Static friction (direct contact between the seal and the sealed surface)
- Mixed friction (mixed dry and liquid friction)
- Liquid friction (liquid lubricating film between the seal and the sealed surface)
Horizontal axis: speed
Upon start-up, friction is higher because it must overcome the coefficient of static friction (area 1). After increasing the speed, a liquid film is inserted between the seal and the dynamic surface, thereby reducing the contact surface and thus friction (area 2). With a further increase in speed, the contact surface disappears, and friction increases (area 3) due to shear stress.
Wear and seal life
Due to different designs and manufacturing from different materials, sealing systems have different behavioral patterns with increasing pressure.
When using a hard material, the risk of damage from compression is reduced. On the other hand, a hard material does not have as good sealing properties as a soft material, especially at low operating pressure.
For the best sealing system that will be effective at high and low operating pressures, a seal made of several types of materials with different properties should be used. Ideally, a solid seal made of several materials that have increasing hardness and can reach maximum hardness at the back of the seal, where the joint is, would be ideal. However, this cannot be fully achieved, even though our seal designs are designed on the principle of many phases (multi-stage) with the aim of achieving the ideal.
Horizontal axis: surface roughness
The seal loses its ability to operate due to normal wear of the sealing material. This is greatest at start-up, at low speeds, and further due to erosion of the sealing material caused by fluid flow across the seal surface and impacts at the point of degradation.
The first signs can be traced at low pressure, when, due to wear, the seal is unable to maintain the required contact with the sealing surface. At high pressure, the deformation is more noticeable, and the seal meets the requirements as long as the pressure is maintained at the same level. Seal life cannot be accurately determined because it depends on many other factors, starting with the appropriate choice of seal for a particular job and proper installation.
Wear can increase with lack of lubrication, poor installation, excessive frictional temperature, too soft parts of the seal, etc. The normal life of a seal will vary considerably depending on the method of use, as the recommended acceptable conditions and type of seal also vary.
If the life of the seal is significantly lower than the average for a particular device, the wrong seal was probably chosen, and the operating conditions proved to be significantly more demanding than expected when choosing the seal.
Seal wear depends on the surface finish against which the seal rubs; it is therefore greatly influenced by the method of manufacturing the installation. The table shows the wear of seals on cylinder surfaces achieved by three machining methods.
Seal wear is graded from 0 for no visible wear to 10 for worn seals. These individual data were obtained after 100,000 cycles of cylinder operation at a working pressure of 250 bar.
It is clear from the table that with a polished cylinder surface in the range Ra from 0.08 μm to 0.7 μm, the wear is almost zero. In contrast, for cold-forged cylinders with a surface Ra from 0.4 μm to 1.25 μm, the wear is maximum.
We have completed the theoretical introduction to seal design. In the next article, we will open the topic of materials; their choice is another essential parameter that determines the functionality of the sealing element.