Oil Seals and Wire Ropes

In this tutorial, you will learn about two supportive mechanical elements, Oil seals and Wire Ropes, and understand their design process. You will understand the basis of the oil seals as a machine element and how they are graded and also learn about the stresses involved in the application of wire ropes.

Contents:

  1. What are Oil Seals?
  2. What are Oil Seals made of?
  3. Oil Seal Dimensions
  4. Applications of Oil Seals
  5. What are Wire Ropes?
  6. Rope Lays and their Effects
  7. Stresses in Wire Ropes
  8. Applications of Wire Ropes

What are Oil Seals?

An oil seal is any mechanical device made of an elastomeric material that is used to prevent leakage of fluid between two machine components. It serves to prevent leakage of expensive lubricating oils and prevent the entry of contaminants and other foreign particles into the mechanism.

  • Due to the radial pressure of the garter spring, the sealing lip rubs over the surface of the rotating shaft and prevents leakage.
  • The magnitude of contact pressure between the sealing lip and the rotating shaft is the most important parameter, affecting the performance of the seal.
  • When the magnitude of pressure is too large, there is excessive friction, resulting in high temperature and rapid wear of the sealing lip. On the other hand, when the contact pressure is low, there is excessive leakage.
  • Oil Seals are cheap, compact, and easy to install. They can be used for a wide range of lubricating oils and can tolerate to some extent misalignment of the shaft and vibrations.

What are Oil Seals made of?

The following diagram shows an Oil Seal.

Oil Seal

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An oil seal normally consists of three basic components: the rubber sealing element, the metal insert, and the garter spring. The metallic casing of the oil seal is made of carbon steel, aluminum, or brass. The garter spring material is usually carbon steel or stainless steel. The rubber compounds used for the lip are as follows:

  • Nitrile compounds for general purpose lubricants up to a limiting temperature of 120°C. These compounds tend to harden in high-temperature applications.
  • Silicon compounds up to a limiting temperature of 175°C. These compounds, however, have poor mechanical properties and are subject to damage easily during rough handling and installation.
  • Fluoro-elastomer compounds for a wide range of lubricants up to 200°C. They are costly compared with other rubber compounds.

Oil Seal Dimensions

Oil Seals have grades associated with them which define their characteristics and dimensions. The following figure demonstrates an example of a possible oil seal grade.

Oil Seal Grade

Applications of Oil Seals

Oil seals find widespread application due to their properties. Some applications of Oil Seals are listed below.

  • Oil seals can function in high-temperature conditions. As a result, they are ideal for use in fluid environments. Oil seals perform best at temperatures ranging from –70°C to 260°C.
  • Oil seals are intended for use in low-friction situations. The lubrication in the oil guarantees that the sealing lip does not come into contact with the shaft and prevents the sealing lip from burning.
  • Oil seals may be utilized for lengthy periods thanks to sealing materials such as PTFE and fluoro-elastomer and are safe for an operation involving prolonged application.

What are Wire Ropes?

Wire ropes are extensively used in hoisting, haulage, and material handling equipment. They are also used in stationary applications such as guy wires and stays. The wire rope consists of several strands, each strand comprising several steel wires. The number of wires in each strand is generally 7, 19, or 37, while the number of strands is usually six.

  • The individual wires are first twisted into the strand and then the strands are twisted around a fiber or steel core.
  • The specification of wire ropes includes two numbers, such as 6 ¥ 7 or 6 ¥ 19. The first number indicates the number of strands in the wire rope, while the second gives the number of steel wires in each strand.
  • The advantage of wire ropes is high strength to weight ratio, silent operation even at high velocities, and greater reliability.
  • The central portion of the wire rope is called the core. There are three types of cores—fiber, wire, and synthetic material.
    • The fiber core consists of natural fibers like sisal, hemp, jute, or cotton.
    • The steel core consists of another strand of soft wires with lower tensile strength.
    • The wire core is used where the wire rope is subjected to severe heat or crushing conditions.
    • Plastic cores are used in special-purpose wire ropes. It can be a plastic-impregnated fiber core, plastic-covered fiber core, or a solid plastic core.

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Rope Lays and their Effects

The lay of the rope refers to the manner in which the wires are helically laid into strands and the strands into the rope. If the wires in the strand are twisted in the same direction as the strands, then the rope is called a Lang’s lay rope. When the wires in the strand are twisted in a direction opposite to that of the strands, the rope is said to be regular-lay or ordinary-lay. The following figure shows the two types of the lay of wire ropes.

Lays of Wire Rope
  • Regular-lay ropes are more popular than the Lang’s-lay ropes. The balance resulting from the opposite direction of twisting the strands to that of the wires is advantageous.
  • Regular-lay ropes offer the following advantage of more structural stability, resistance to crushing, and distortion. They have less tendency to rotate under load.
  • In Lang’s-lay ropes, the same direction of twisting results in outer wires being bent on a larger arc of a circle. Lang’s-lay ropes are difficult to handle and install. They are less resistant to crushing and distortion. Lang’s-lay ropes are likely to untwist unless both ends are permanently fastened.

Stresses in Wire Ropes

Analyzing the stresses in a wire rope is a complicated process due to multiple factors. Each wire is subjected to direct tensile stress due to the load being raised as well as due to bending stresses.

  • When the wire rope passes around the periphery of the sheave or the drum, the length of the wires in the outer portion of the rope increases, while that in the inner region decreases. This results in additional tensile stresses in outer wires.
  • The bending stresses in the individual wire are given by the following equations
    σb = \(\frac{M_b d_w}{2I}\)
    Where dw is the diameter of the individual wire.
  • The failure of the wire rope is mainly due to fatigue or wear while passing around the sheave. The bending and straightening of the rope as it passes over the sheave results in fluctuating stresses leading to fatigue failure. The individual wires slide on each other and over the sheave resulting in the gradual wearing of the load-carrying material.
  • The amount of wear that occurs depends upon the pressure between the rope and the sheave. The following figure shows the equilibrium of forces in the vertical direction.
    Stresses in Wire Ropes
  • From the above figure, we get the following relation
    2P=pdrD
    Where pdr represents the force per unit length of the wire rope and P is the tension in the rope.

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Applications of Wire Ropes

Wire ropes find application in the hoist and haul mechanisms where large forces are in play due to very heavyweights. Depending on where they are used, wire ropes have to fulfill different requirements.

  • Running ropes like stranded ropes are bent over sheaves or drums are mainly stressed by bending and tension.
  • Track ropes (full locked ropes) must act as rails for the rollers of cabins or other loads in aerial ropeways and cable cranes. In contrast to running ropes, track ropes do not take on the curvature of the rollers.
  • Wire rope slings (stranded ropes) are used to harness various kinds of goods. These slings are stressed by the tensile forces but first by bending stresses when bent over the sharp edges of the goods.
  • Wire ropes are used dynamically for lifting in cranes and elevators and transmission of mechanical power.
  • They are often used to transmit forces such as in Bowden cables or the control surfaces of airplanes.
  • Static wire ropes are often used to support structures such as suspension bridges or as guy wires to support towers.
  • Aerial tramways also rely on wire rope to support and move cargo overhead.

Key Points to Remember

Here is the list of key points we need to remember about “Oil Seals and Wire Ropes”.

  • Oil Seals are used to prevent leakage of oils and other fluids from the mechanical systems and prevent any contaminants from entering the system.
  • An oil seal normally consists of three basic components: the rubber sealing element, the metal insert (case), and the garter spring.
  • Oil Seals are graded by the application they are used in. They are identified by an alphanumeric series that characterizes the seal.
  • The wire rope consists of several strands, each strand comprising several steel wires.
  • There are two types of lays for a wire rope, the regular-lay, and Langs-lay for wire rope.
  • Wire ropes find application in the hoist and haul mechanisms where large forces are in play due to very heavyweights.

If you find any mistake above, kindly email to [email protected]

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Manish Bhojasia - Founder & CTO at Sanfoundry
Manish Bhojasia, a technology veteran with 20+ years @ Cisco & Wipro, is Founder and CTO at Sanfoundry. He lives in Bangalore, and focuses on development of Linux Kernel, SAN Technologies, Advanced C, Data Structures & Alogrithms. Stay connected with him at LinkedIn.

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