 The
typical fundamental components of a shaft seal are: -
The sealing lip, consisting of a flexible membrane ending
in an edge, made of elastomeric material, designed to
wrap around the shaft and thus exert a sealing action
(Par. 2.1)
- The metal case, designed to provide the shaft seal
with the necessary rigidity for a stable coupling with
the relative housing bore (Par. 2.2)
- The Garter spring, acting as a complement to the
fundamental action of the sealing lip (Par. 2.3).
2.1 Materials
used for the sealing lip
The material used for the sealing lip is a mixture
of one or more basic elastomers and a variety of ingredients,
such as: reinforcing fillers, plasticizers, antioxidants,
accelerators, etc. This is for the purpose of providing
it with certain properties, such as:
- Compatibility with the fluid contacted
- High degree of elasticity
- Wear resistance
- Low friction coefficient.
A familiarity with the materials is essential to help
the designing specialist make the proper selection of
the most suitable materials for the application of interest.
The main qualities of the compounds ROLF uses for producing
its shaft seals are:
NBR
ACM
MVQ
FPM
HNBR
EPDM |
nitrile
rubber
polyacrilic rubber
silicon rubber
fluorinated rubber
hydrogenated nitrile rubber
ethylene-propylene rubber |
(acrylonitrile-butadiene)
(polyacrylate)
(polysiloxane)
(vynilidene-fluoridehexafluoropropene)
(acrylonitrile-hydrogenated butadiene)
(ethylene-propylene) |
(Identification according to the ISO R 1629 standard
of March, 1971).
NBR - Nitrile rubber
The most widely used elastomer in most current applications.
It is particularly recommended in case of contact with:
- Paraffin-based (aliphatic) oils
- Mineral oils and fats (oils for engines, gearboxes,
differentials, etc.)
- Hydraulic oils
- Water and aqueous solutions (lyes).
The temperature range varies from -30°C to + 120°C.
ACM - Polyacrylic rubber
This elastomer is recommend for use with:
- engine oils even if containing additives and sulfur
- transmission oils
- hydraulic oils.
The temperature range varies from -25°C to + 150°C.
MVQ - Siliconic rubber
Due to its chemical composition (high molecular weight
chains of appropriately modified polysiloxanes), this
series is particularly resistant toward atmospheric
agents, light and ozone. It also exhibits an excellent
high- and low-temperature resistance, so that its field
of application covers a broad range. Despite its less
than fully satisfactory tear and abrasion strength,
its low friction coefficient amply compensates for the
relative effect. It is recommended for:
- resistance to atmospheric agents, ozone, etc.
- mineral oils
- glycol-based fluids.
Never use with petrols.
The temperature range varies from -55°C to + 180°C.
FPM - Fluorinated rubber
This elastomer has exceptional heat and chemical resistance.
Its properties remain indefinitely stable up to about
200°C. It offers excellent performances in contact with:
- aliphatic hydrocarbons
- aromatic hydrocarbons (toluene, benzene, xylene)
- vegetable and mineral oils and fats, even if containing
additives
- chlorinated solvents
- ozone
- light and atmospheric agents.
The temperature range is from -30°C to + 200°C.
HNBR - Hydrogenated nitrile rubber
The chemical structure of this elastomer (obtained
by hydrogenating an appropriate type of NBR nitrile
rubber) allows achieving, especially if vulcanized with
a peroxide system, an average heat resistance 30°C above
that of nitrile rubber, and an excellent abrasion resistance.
Its resistance to oils and solvents is on average slightly
superior to that of nitrile rubber, except for special
cases. It is therefore recommended for:
- heat resistance
- ozone resistance
- abrasion resistance.
The temperature range is from -40°C to + 150°C.
EPDM - Ethylene-propylene rubber
This rubber is based on ethylene-propylene plus a
third (diene) monomer which allows its reticulation
with sulphur. Due to its chemical structure, it has
a peculiar resistance to fluids such as water and steam
and environments such as ozone, which recommends its
use for:
- water, up to boiling point
- steam
- particular hydraulic systems, such as braking systems
- ozone
- atmospheric agents
- bases
- polar solvents at ambient temperature.
The temperature range is from -50°C to + 150°C.
2.1.1 - Thermal expansion of elastomers
The thermal expansion coefficients of elastomers are
decidedly superior to those of metals (see Table belowe).
It is impossible, therefore, to merely consider the
geometric shape of a shaft seal and its total radial
load at ambient temperature, because its operating conditions
and lifetime may substantially vary, depending on the
change of the modulus of elasticity induced by a temperature
change.
Material Thermal
expansion coeff in m/m°C-1
Steel
12 x 10-6
Aluminium
24 x 10-6
Brass
18 x 10-6
73 NBR 004
110 x 10-6
70 ACM 301
100 x 10-6
70 EPDM 601
170 x 10-6
75 HNBR 103
115 x 10-6
80 MVQ 501
180 x 10-6
73 FPM 401
150 x 10-6 |
2.2 Metal
case
Its function is to offer the shaft seal the necessary
rigidity to enable a stable coupling with its relative
housing seating. With reference to the elastomer, it
may be of an inner (see par. 2.2.1), an outer (see par.
2.2.2) or a part-coated type (see par. 2.2.3).
2.2.1 - Inner metal case
This solution includes the following advantages:
- It eliminates the risk of corrosion
- It avoids damaging the seating, even if made of a
light alloy, thus affording a better opportunity of
substitutions without damages.
2.2.2 - Outer metal case
This type of case was designed for applications requiring
high pulling forces and automated motions based on magnetic
systems. In time, it has also been shown that in order
to achieve a reliable seal, a ground outer finish and
a finely machined seating was needed in addition to
the use of sealing materials. Its cost was considerably
higher than that of a coated type. It was therefore
decided to use it only in combination with high-quality
compounds, where most of the cost increase is compensated
by the savings in elastomer materials.
At any rate, ROLF solved the problem by producing
its seals with their outer surface coated only up to
half of its height, as detailed below.
2.2.3 - Part-coated metal case
This solution involves coating the outer case up to
about half of its height. This coating is a result of
vulcanization and can be plain or corrugated to better
fit the assembly forces required by the customers.
The resulting advantages are:
- excellent locking-in in the housing
- savings of high-quality materials
- ease of assembly
- safety in operation
This type of locking is advisable for projects requiring
a particularly challenging application.
2.2.4 - Nature of the materials used for the case
In its standard version the metal case consists of
a medium/deep draw steel sheet according to the UNI
EN10130 or DIN 1624 standards, of a thickness commensurate
with the size of the shaft seal. Where a resistance
to corrosive fluids is required, it can be supplied
as made from
- Stainless steel, to DIN 17440/tab. 1.54401 or AFNOR
Z6 CND 17.11 standards (ex AISI 316)
- Brass, to UNI 4894 standards.
2.3 Spring
The spring has a function that is complementary to
the fundamental action provided by the sealing lips.
In fact, heat, mechanical deformation and chemical action
of the fluids affect the original properties of the
rubber. As a result, the original radial force exerted
by the sealing element tends to decrease. The function
of the spring is to counteract this tendency. The spring
is a closely wound helical spring in toric form and
possesses a calculated initial pre-loadinging force.
This is supplemented by a stabilizing heat treatment
performed at a higher temperature than the operating
one, which makes it possible to achieve:
- at the design stage: the safety of using the most
suitable radial force for the expected application,
- at the operating stage: a guaranteed stability of
the radial force itself. The temperature effect actually
determines, in the course of time, not merely an alteration
of the rubber's original characteristics, but also a
decrease of the mechanical properties of the steel constituting
the spring.
2.3.1 - Nature of the materials constituting the spring
The choice of the materials constituting the spring
depends on the type of fluid the Garter spring comes
in contact with. In the standard version it consist
of a phosphatized, high strength piano wire steel to
UNI 3823 or DIN 17223 standards. The standard springs
undergo a programmed bedding-in process which allows
a precise evaluation of the radial force at the design
stage. The use of springs of different material may
be considered for particular applications. For instance,
in cases requiring a seal against corrosive liquids
such as seawater, detergent or acid solutions, a stainless
steel spring can be employed, conforming to the following
standards:
- DIN 17007, Table 2: 1,4300 or AFNOR Z10 CN 18.09 (ex
AISI 302)
- DIN 17007, Table 2: 1,4401 or AFNOR Z6 CND17.11 (ex
AISI 316)
- DIN 17007 Table 2: 1,4571 or AFNOR Z8 CNDT 17.12
The use of phosphorous bronze springs, while having
the same chemical resistance as stainless steels, is
not recommended because of the instability of its dimensional
characteristics and the uneven decay of its load capacity.
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