Industrial gaskets and o-rings are essential elements of many mechanical devices. Their full efficiency is essential for maintaining the performance of the kinematic mechanism in which they are housed, but are we sure of their compatibility with the fluids they intercept? What are the risks of a change in their mechanical properties due to chemical incompatibility with synthetic oils? How do the structural properties change during contact in operation?
In this article we will talk about:
- the role of seals in mechanical devices
- how incompatibility between lubricants and o-rings manifests itself
- how to choose the most suitable lubricant according to the chemical composition of the rubbers
1) O-rings and sealing rings
The gaskets are used whenever it is necessary to separate two phases of different nature in all possible combinations: liquid, solid and gaseous. There is a multitude of dimensional variations and shapes, for simplicity and practicality we will analyze two essential types:
- O-ring: elastomer ring with circular section, designed to be inserted into a suitable housing and to be compressed during the assembly of two or more parts, thus creating a sealing gasket. This compression, added to that of the fluid, increases with increasing pressure, helping to improve the sealing effect
- Seal ring: separation element used for mostly dynamic applications, with the purpose of preventing the leakage of fluids, limiting the coefficient of friction and wear of the sliding parts.
2) Types of materials: choice of gaskets
The choice of a sealing element is primarily based on the expected operating conditions during the operation of the device. The wide range of materials and formulations available on the market now makes it possible to cover all possible combinations.
The table below compares some of the most common compounds used, and a score (from 1 to 5) referring to each of the reference characteristics:
- Tensile strength
- Elasticity
- Wear resistance
- Resistance to atmospheric agents and ozone
- Heat resistance
- Cold flexibility
- Permeability to gases
- Maximum continuous use temperature
From an initial analysis, the designer can make a preselection, evaluating which of the above aspects decisively affects the chosen direction.
3) Contact with lubricating fluids, why lubricate a sealing element
Seals can perform their function in two ways:
- Static: does not entail sliding of the seal on the counter-body in friction. The function is limited to protecting the device from any external contamination such as dust, fluids of different nature and gases which could accelerate the decrease of the functionality of the component in which they operate.
- Dynamic: the separation requirements are the same, but they are carried out in a context of relative sliding between the gasket and the counter-body.
In both cases the seals can come into contact with a lubricating fluid:
- Directly: if the application of the fluid serves to increase the performance of the device, such as in a cylinder or pneumatic valve, where it is necessary to reduce the coefficient of friction and mechanical wear.
- Indirectly, if the function is to separate a fluid, such as for the sealing rings inserted on the shaft of an oil-lubricated gearmotor, or in a hydraulic shock absorber.
The static and dynamic coefficient of friction values of elastomers towards many materials tend to be high. If this represents an aspect of strength due to the separation and sealing action they perform, it is undoubtedly a weak point with regard to the frictional wear that follows. This often leads to the need to apply a lubricating film if you want to improve the conditions during the dynamic action.
Through a correct choice and application of the lubricating fluid it is possible to obtain the following benefits
- Improvement of the sealing effect of the gasket
- Reduction of mechanical wear
- Reduction of the friction coefficient between gasket and counter-body
- Limitation of damage to the sealing lip of the ring during assembly
- Prevention of wear from running in the gasket generated by the lack of lubricating oil, during the first rotations of the shaft
4) Which lubricant to choose? What chemical composition?
Each contact between elastomer and lubricating fluid is governed by a more or less important chemical interaction, the evident effects on rubber can be distinguished in
- Swelling: incorporation of the base oil and its components in the rubber matrix.
- Contraction: migration of the components from the rubber to the lubricating fluid.
The two phenomena thus detected are the evident result of other important changes to the structure of the gasket, compared to the original values. After contact with the lubricating fluid, the seals can undergo important changes in:
- Hardness
- Volume
- Weight
- Tensile strength
- Elongation at break
However, the regulatory framework is very broad, customizable and simplified on the basis of actual design needs.
It is important to underline that in no case can a compatibility test alone provide a certain answer on the suitability or otherwise of a lubricating fluid in contact with a gasket. It is essential to analyze the results in the design context, where the gasket performs its function.
The result obtained after immersion of EPDM gaskets in mineral oil (Ø10 mm 70 Shore A for 24 h at 25 ° C) is reported below. The comparison has no legislative reference, but highlights aspects of compatibility and non-compatibility in a striking way.
The widespread presence of sealing elements in mechanical devices calls for attention to the aspects of chemical compatibility with respect to any lubricating fluids in contact.
In this article we have seen:
- What are the risks of an incorrect choice of a lubricating fluid in contact with the seals
- What are the structural characteristics of the gaskets to keep under control
- What are the empirical (immersion) and specific (ASTM D - 4289) tests that can be performed to provide immediate suitability answers
