Seals.pptx

What is and what is the
function of a rubber seal
Luis Antonio Tormento
12/07/2022
Luis.tormento@outlook.com

Introduction
• The Increasing of the speed of mechanical systems, driven by the desire for
greater productivity, leads to higher operating temperatures and reduced
fluid viscosities. This, coupled with higher pressures, causes an increasing
tendency for fluid to leak. This leak in fuel systems that handle highly
flammable solvents cannot be overlooked as there is a high probability of a
fire hazard.
• For this reason it has become common practice to include a safe leak path in
the system design, to an escape or collection point, in order to minimize
risk.
• Seals prevent fluid from escaping from a hollow cylinder when a shaft
penetrates the cylinder wall. Most commonly, the axis will have a rotary or
linear motion. If a seal is not made for functional requirements, or installed
and maintained properly, it can fail, causing fluid loss. The two main
functions of a seal are to keep the fluid in while keeping dirt and debris out.

Introduction
• Fluid leaks in hydraulic and lubrication systems are a common occurrence
in many mechanical equipment, and are undesirable and embarrassing
events that fuel the rising cost of any equipment manufacturing and
maintenance activity.
• Fire and accident hazards are often direct consequences of such leaks. In
many situations, despite the anticipation of such dangers, leaks are
neglected and the leak system is not attended to, as operational engineers do
not take the threat of accidents seriously.
• For years, the leak has received low-level attention from corporate
management at many industrial manufacturers. One reason is that leaks are
often seen as unavoidable and corrective actions such as repair and
replacement are only done when convenient.

Introduction
• Unsupervised leaks can result in downtime, affect product quality,
pollute the environment and cause injury. System vibration, pulsation,
and thermal cycling are common causes of system leakage. It can be
assumed that any type of fitting or fitting can leak, regardless of the
type of pipe that is used, especially when mechanical vibration is
present. This 'vibration fatigue' is an unavoidable factor that can be
aggravated by poor metallurgical consistency in the construction
material, undue stress imposed on the connection, other system design
features or simply improper installation. Usually to avoid such failures
we use a rubber ring with physical and chemical characteristics to
resist and prevent the leakage of the internal fluid.

Function of Seals
Figure 1 – Representation of a sealing seal.

Function of Seals
• The most common failure of these seals is blisters caused by
• Compound mixing process (occluded air)
• Product hardness (generally low)
• Permanent deformation (usually low)
• High elongation at break (greater than 200%)
• Low crosslink density (which causes high deformation)

RUBBER PROPERTIES FOR
FUNCTIONAL SEAL
• Rubber seals are used in a variety of applications where the forces acting on them are very different. To
optimize the performance of a seal, the forces acting in a given application, along with the functional
properties that are required to respond to them, need to be understood.
• When a rubber seal is used in an external application, such as an 'O' ring, it is usually stretched over the piston
in a groove where it will typically initially be at a maximum 8% stretch.
• The sectional diameter of the ring will be reduced due to this stretching and, when the piston is mounted in
the cylinder bore, this section will be compressed giving a sealing effect due to the recovering force of the
rubber. Stress relaxation will occur when the seal is under constant tension for an extended period and
permanent deformation will be observed. If this occurs in a reciprocal application on a piston head, the 'O'
ring will be pushed back into its housing and may extrude outward on both sides or be spirally twisted.
• Alternatively, compression-type seals operate by distorting under compression load, and the hardness
specified for such application must be sufficient to ensure adequate retention of the seal pressure.
Compressive strength can be increased by incorporating one or more layers of fabric into the rubber section,
rather than increasing the composite hardness as this can adversely affect other properties. Dimensional
variations due to contact with fluids can be adjusted to achieve a small positive expansion that can maintain
sealing efficiency, compensating for wear and defined compression. The choice of compound will depend on
the effect of the fluid the seal is in contact with, the operating temperature, and mechanical conditions such as
pressure, relative velocity and abrasion.

FUNCTIONAL SEAL
• In designing seals for specific applications, the engineer must first
understand the limitations of the physical properties of the rubber
material to be used in order to avoid applying stresses (applied loads)
and strains (strains) that exceed these limits. The strengths of rubbers
are considerably lower than those of metals, plastics or wood, but their
elasticity is much greater. The ability of rubber to return to its original
shape and dimensions after a deformation is often called resilience or
memory. Resilience implies fast payback, while memory implies slow
payback. In seals, resilience is important because it allows a dynamic
seal to adapt to variations in the seal surface. Memory, which implies a
slow return, is the same called creep.

FUNCTIONAL SEAL
• In designing seals for specific applications, the engineer must first understand the limitations of the physical
properties of the rubber material to be used in order to avoid applying stresses (applied loads) and strains
(strains) that exceed these limits. The strengths of rubbers are considerably lower than those of metals,
plastics or wood, but their elasticity is much greater. The ability of rubber to return to its original shape and
dimensions after a deformation is often called resilience or memory. Resilience implies fast payback, while
memory implies slow payback. In seals, resilience is important because it allows a dynamic seal to adapt to
variations in the seal surface. Memory, which implies a slow return, is the same called creep.
• The reversibility of the elastic deformation of rubber is known as hysteresis. If, by a given elongation before
breaking, the stress is relaxed and the specimen can retract, the retraction energy is less than that used for
elongation. This phenomenon is hysteresis. Thus, hysteresis indicates that the energy recovered by the rubber
during shrinkage is distinctly less than the energy used in stretching. If a rubber sample is subjected to several
stretches followed by shrinkages, it will be seen that the rubber sample finally reaches a limit, this is called
permanent deformation.
• In practice, permanent hardening also occurs during compressive stresses. For a sealing application, the
rubber should decay or relax less, and the permanent deformation should be as low as possible. In the case of
metals, the creep phenomenon manifests itself differently. For example, when a vertical metallic bar is
subjected to a load 'p' for a certain period of time 't0', it produces an elongation equal to 'd0'. After time 't0', if
the load is kept constant and the time increases, the bar increases gradually and slowly. This elongation effect,
that is, the increase in stress on the metal bar while the load (stress) does not change, is called the creep
behavior of the metal.

FUNCTIONAL SEAL
• The wear and abrasion resistance of a rubber seal when in contact with
a moving surface is related to other physical properties such as
hardness and tear resistance, and thermal deformation.The resilience
of rubber provides a firm, durable seal. Generally, in high pressure
application it facilitates sealing improvements. Rubber seals used
under high pressure must have high tear resistance, hardness and
modulus to prevent rubber gasket extrusion. It is usual practice in this
type of pressure system for the rubber seal to be supported by a high
hardness rubber in order to avoid extrusion.

FUNCTIONAL SEAL
• Seal manufacturers develop their own suitable rubber compounds, which
have the chemical, physical and swelling properties to meet the functional
requirements and working conditions of the application. The compounds
used in the manufacture of seals are derived from base rubbers such as
natural rubber, NBR, Neoprene, IIR, SBR, XNBR, viton, silicones and
polytetrafluoroethylene.
• Of the properties exhibited by the various types of rubber compounds, the
most critical relate to how they change when installed as seals and during
service. All physical properties change with age and exposure to variations
in temperature, fluid type, pressure and other factors which can include
corrosive chemicals and fumes and gases. Compounds with the least
tendency to change their properties, whether chemical or physical, are easier
to work with. More adaptable and versatile seals can be produced with these
compounds.

BEHAVIOR AT LOW TEMPERATURE -
GLASS TRANSITION (Tg)
• A fundamental property of an elastomer is the glass transition temperature
(Tg), which differs from one to the other. For example, for NBR the Tg is -
25°C. Generally speaking, this means that above -25°C the material behaves
like rubber, but below -25°C the material behaves more like glass. When
glassy, NBR is about a thousand times more rigid than it is when
rubberized. When glassy, a hammer blow to natural rubber will cause it to
shatter like glass, while when rubberized, the hammer will likely bounce.
• At normal temperatures, rubber chain molecules are in a constant state of
thermal motion; they are constantly changing their configuration and this
movement makes them reasonably easy to stretch. Note that as the
temperature decreases, currents become less flexible and the amount of
thermal movement decreases. Eventually, at the glass temperature
transition, all main chain movement ceases. The material no longer has the
properties that make it rubber and behaves like glass.

BEHAVIOR AT LOW TEMPERATURE -
GLASS TRANSITION (Tg)
• For all practical engineering uses of elastomers, we require good
flexibility and therefore it is essential that we only use them at
temperatures that are comfortably above the glass transition. This is
generally no problem for NBR with a Tg of -25°C, or cis-butadiene
rubber with a Tg of -108°C. But many elastomers, especially those
designed to be highly resistant to heat or oil, have much higher Tgs
and this must be taken into account when selecting them. For example,
some fluoroelastomers, which have excellent oil and heat resistance,
have a Tg not much below room temperature. This can result in
problems if a component that needs to work in high temperature also
needs to work in colder conditions.

BEHAVIOR AT HIGH TEMPERATURE
• The upper temperature limit at which a rubber can be used is generally
determined by its chemical stability and will therefore vary for
different rubbers. Rubbers can be attacked by oxygen or other
chemical agents and, because the attack results in a chemical reaction,
degradation will increase with temperature.

DEGRADATIVE CHEMICAL REACTIONS
• Degradative chemical reactions are generally of two types. The first
are those that cause molecular chain breakage or cross-linking,
softening the rubber because they weaken the network. The second are
those that result in further crosslinking, hardening the rubber and often
characterized by a hard and degraded film formation on the rubber
component. Selection of a suitable rubber and the use of chemical
antidegradants can reduce the attack rate.

STRETCHING - A REVERSIBLE
PHENOMENON
• It is a known fact that elastomers can stretch, but that does not explain
why, when the stretching force is removed, the material returns to its
original shape. This can be explained by thermodynamics.

FLUID RESISTANCE
• The structure of an elastomer comprises a network of chains, which means that there are
gaps between adjacent chains. In fact, the elasticity of rubber depends on a substantial
thermal movement of the currents, which would not be possible if the currents were very
close together. The free volume available in the rubber means that some liquids can enter
the rubber and cause swelling - sometimes in large amounts. For example, the ability of
oil to swell natural rubber is well known.
• A given liquid will have a specific solubility in any rubber, and knowledge of this will
allow designers to avoid excessive swelling when a rubber comes into contact with fluids
while in service.Solubility parameters were developed by Charles Hansen as a way to
predict whether one material will dissolve into another and form a solution. They are
based on the idea that "similar dissolves as ', where one molecule is defined as being' with
'another if its chemical bond is similar.
• The Hildebrand solubility parameter provides a numerical estimate of the degree of
interaction between materials and can be a good indication of solubility, particularly for
non-polar materials such as many polymers. Materials with similar solubility parameter
values are likely to be miscible.

INCOMPRESSIBILITY
• Another property of rubbers that distinguishes them from other solid
materials is their incompressibility. For most practical purposes, except for
use under very high pressures, elastomers do not change their volume
significantly when deformed. A rubber band can stretch by 600%, but if its
volume were measured in the stretched state, it would be found almost
identical to its unmagnified volume.
• This has important implications for design with rubbers, as the stiffness of
components can be controlled, not just by changing the stiffness of the
rubber itself, but across multiple designs. This phenomenon, known as the
form factor effect, is described in more detail in several textbooks and leads
to great design versatility. In particular, it allows the engineering of rubber
components as well as rubber bonded metal seals to be designed with
different and controlled rigidity.

'O' RINGS AND OIL SEALS
• 'O' rings
• In all industrial fluid and hydraulic systems, pumps, pistons and pipe connections,
there is a requirement to seal against leakage of operating fluids or fuels. An O-
ring (thick solid) with circular cross section) known as a rubber "O" ring,
primarily made from nitrile rubber, polychloroprene or fluoroelastomers, has
several advantages in these applications. These universally accepted toroidal seals
are lightweight and flexible and, under compression, will deform to follow the
contours of the component parts to be sealed. 'O' rings are commonly used in
rotating shafts, reciprocating and oscillating applications.The main operation of
the 'O' ring can be described as controlled deformation. This deformation occurs
because all 'O' rings are manufactured to allow an initial tightening or deformation
of approximately 8–10% of their transverse diameter. This basically means that
the 'O' ring is made to be larger than necessary, causing it to roll and distort when
pressure is applied. This deformation grip flattens the 'O' ring into intimate contact
with the sealing surfaces and provides the sealing action.

• No special tools are required to fit the "O" rings. They are typically
fitted into a rectangular shaped groove machined into one of the two
components to be sealed and the sealing effect is achieved by
squeezing the 'O' ring between the bottom of the groove and the
opposite face. Groove size is designed to provide the correct amount
of interference (ie press fit or tight fit). 'O' rings provide effective
sealing in both directions with constant or variable pressure, high
vacuum and extreme temperatures.

'O' RING APPLICATIONS
• static application
• For static applications, the groove must be designed to allow the "O"
ring to contact all four sides of it when in position (Figure 2).
• alternative applications
• In a reciprocal application, the 'O' ring must make contact with the
bottom of the groove and the opposite surface. Clearance must be
allowed on each side to allow the gasket to move to either side of the
groove in accordance with the direction of pressure (Figure 3).

Figure 2

Figure 3

• When providing space for an 'O' ring, the annular groove must be
large enough to contain its full volume when subjected to diametric
pressures. If the groove cavity is too small, the 'O' ring will be
distorted, will tend to extrude along the gaps and be mutilated. At
pressures greater than 2000 psi, the 'O' ring may extrude along the
gaps unless an anti-extrusion ring made of rigid rubber is placed next
to it.

'O' ring cross section
figure 5 - Compression tolerance of an 'O' ring

'O' ring cross section
figure 6 – O-Rings Static and dynamic

'O' RINGS CROSS SECTION
• To give an example, if an 'O' ring is required to seal between a piston and cylinder diameters of 3.5 inches (9
cm) and 4.00 (10 cm) inches respectively, the nominal cross section of the ring is 0.25 inch (0.635 cm). But
the actual cross section of the ring should be 0.275 inch (0.698 cm), the difference of 0.025 inch (0.063 cm)
being allowed for compression as depicted in Figure 5.
1. Ventilation must be provided in the space between the 'O' rings to prevent fluid or lubricant from getting
trapped between them.
2. Provision should be made to lubricate O-rings that are not in contact with the fluid being sealed, using
oil-soaked felt rings on each side of the unlubricated ring.
3. Each 'O' ring must be in a separate groove.
• Anti-extrusion rings are recommended to be used when:
1. The clearance is greater than normal.
2. An 'O' ring of a particularly soft rubber compound is used.
3. Extra lubrication is used.
4. In all dynamic applications where pressure exceeds 1500 psi.
• Where pressure on the 'O' ring is being applied to one side only, then only one extrusion ring is needed. If
pressure is applied from both sides, then two anti-extrusion rings are fitted, one on each side of the ring.

'O' RINGS FOR ROTARY SEALING
APPLICATION
• When a rubber band is stretched to about 200-300% elongation (Figure 7), it contracts, and if it is warm, it
lifts the weight by about 0.25 inches (0.063 cm). The modulus of elasticity or the ability to carry a load
increases with temperature. Rubber under constant tension exerts greater stress. When using an 'O' ring to
seal a rotating shaft, an 'O' ring of smaller diameter than the shaft is chosen so that it fits and makes a
perfect seal. The seal is therefore in a stretched condition, resulting in the rubber rubbing against the
rotating shaft, generating heat. This heat, in turn, causes a stretched "O" ring to contract as the Joule effect
works to do so.The contraction or retraction of a rubber "O" ring fitted to a shaft causes a top loading unit
against the shaft due to the Joule effect noted above.
• Thus, the cycle of friction, heating and contraction (reduced tension or relaxation) of the rubber ring is
repeated until the loop occurs. This can occur within a few minutes at shaft speeds above 200 ft/min. The
presence of pressure within the sealing system will cause failure even at slower speeds as this increases the
'O' ring pressure on the shaft. Since rubber is a poor conductor of heat, the surface in contact with the
rotating shaft will char before the rest of the ring is affected to any extent. If assembly can be performed for
more than a few minutes in this condition, the shaft will be singed and blue in color with excessive heat
developed.
• Thereafter, cracks will appear in the ring as it deteriorates and the fluid being sealed leaks out. The remedy
followed by many seal designers is to specify the inner diameter of the seal to be slightly larger than the
shaft diameter by at least 3–5%. The oversized seal does not have the same tendency to heat up. During
operation, temperature levels decrease after a moderate increase.

'O' RINGS FOR ROTARY SEALING
APPLICATION
Figure 7 - Rubber under tension

OIL SEALS
• A standard oil seal consists of an outer circular metal disc with a
flexible rubber inner that is affixed to the metal during vulcanization.
The glued seal has no loose parts to allow oil leakage or entry of any
contaminants. This type of seal is more accurate and can easily be
installed in a smaller space. An example is shown in Figure 8.

OIL SEALS
• Figure 8
Figura 8 – Selo ligado a metal

OIL SEALS
• The spring shown in the figure is known as a circular spring and
maintains tension in the sealing lip of the seal. Alloy springs are
closed coil springs used in the form of a ring.The bonded seal depth
can be smaller and the gap between the hole and the outside - the
diameter can be changed to facilitate adjustment. The bonding of
rubber to metal is an important factor to consider in the manufacture
of such seals and must be carefully considered as failure to bond will
cause the seal to fail. The metal oil seal case is generally made of
deep drawing quality mild steel that allows for coining, punching,
stamping the steel to the required dimensions.

OIL SEALS
• The metal edge is finely ground after seal fabrication in a centerless grinder
to allow an interference fit in the oil stuffing box. A slight chamfer on the
outside diameter (OD) of the seal is desirable for easy assembly. The
sealing lip is prepared by polishing, grinding or cutting to ward off rubber
burr that occurs on the sealing edge. A thin sealing lip creates enough
pressure on the shaft to minimize spring load, leading to less friction while
maintaining effective seal performance. Coil spring plays an important role
in oil seal efficiency. If your tension is too high, heat will be generated
between the sealing lip and the shaft, resulting in rapid lip wear. If too low,
the spring will be ineffective and the sealing lip can be worn away leading
to fluid leakage.
• Another type of seal design has the metal wrapped in rubber (Figure 10)

OIL SEALS
Figure 10 - Closed metal rubber seal

OIL SEALS
• In this type of seal, wider tolerances are possible between the outer diameter of the seal
and the seal. Irregularities of the casing surface can be eliminated by the resilient rubber
layer on the outside of the seal. However, the rubber covered seal can be blown under
high pressure in a reciprocal application whereas, with a metal housing, there is no such
danger.
• No isolated physical property of rubbers is responsible for the successful performance of
an oil seal or 'O' ring. Ultimate tensile strength, elongation at break, modulus, shore
hardness, creep and stress relaxation under tensile and compressive loads are all
important physical properties that characterize a seal or O-ring. Compressive strength
and in conjunction with relaxation or deterioration of stress are important for an
effective seal. The difference in these properties in a swollen seal is highly critical. An
ideal swell value in a fluid medium is a desirable characteristic. Swelling decreases the
sealing pressure against the housing wall where the seal is attached, leading to leakage.
Swelling minimizes the physical properties of rubber. Seals made from polysulfide
rubbers have extreme fuel resistance but undesirable deformation at high compression.
The effect of temperature on the seal is an important factor. Swelling under stress can
increase at higher temperatures and a proper compounding technique must be adopted
to reduce this effect.

LIP SEALING PROJECT
• This type of seal has a sealing lip that is in contact with the shaft.
Therefore, it is important that the shaft surface is smooth and free of
debris prior to installation. Most lip seals have a press fit. Figure 11
gives examples of seal lip designs.

LIP SEALING PROJECT
• Figure 11

MECHANICAL SEALS
• These seals operate differently from standard oil seals. They are used
axially. Bronze, carbon or Teflon seal nose is coupled to a much harder
material (ie a hardness of 500 brinell) and held in position by a spring.
The harder material can be a tight fitting collar inserted into the shaft
or it can be a part of it. Contact surfaces must be true or at high
speeds, or surfaces may break contact and leakage may occur. A
mechanical seal and its application are shown in Figures 12 and 13.

MECHANICAL SEALS
• Figure 12 – Mechanical Seal

MECHANICAL SEALS
Figure 12 – Operation of a Mechanical Seal

MECHANICAL SEALS
• These seals are suitable for use at high speeds, temperature and
pressures eg 5000 ft per minute and 4000 psi pressure at 250 C. Wear
on the seal nose is negligible and carried by rubber moving axially
under coil spring pressure .
• A mechanical seal eliminates excess leakage, but a small amount is
acceptable as it lubricates the seal. This is accomplished by machining
the faces that are used in the sealing action to a very smooth
• These seals are primarily used for rotary seal applications.

REFERENCIES
1. https://en.wikipedia.org/wiki/Hansen_solubility_parameter
2. Ericks Techinical Rubber – Sealing Elements
3. Elastomers for Steam Service Presented at the High Performance Elastomers & Polymers for Oil & Gas
2010 - 5th International Conference, Aberdeen, Scotland, 277th – 28th April, 2010
4. Rubber Seals for fluid and Hydraulic Systems, Chellappa Chandrasekaran, 2010, Elsivier Inc

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