What is a Rope Drive?
Rope drive is a form of belt drive, which is used for mechanical power transmission. Rope drives use multiple circular section ropes instead of single flats or V-belts. The rope drives are widely used where a large amount of power is to be transmitted, from one pulley to another, over a considerable distance.
It may be noted that the use of flat belts is limited for the transmission of medium power from one pulley to another when the two pulleys do not exceed 8 m. If a large amount of power is transmitted by the flat belt, it will result in an excessive belt cross-section.
The ropes for transmitting power are usually made from fibrous materials such as hemp, manila and cotton.
Types of Rope drive
The ropes drive uses the following two types of ropes:
- Fibre ropes
- Wire ropes.
The fibre ropes operate successfully when the pulleys are approximately 60 meters apart, while wire ropes are used when the pulleys are separated by 150 meters.
What is Wire Rope?
Wire rope is several strands of metal wire twisted into a helix forming a composite rope, in a pattern known as laid rope. Larger diameter wire rope consists of multiple strands of such laid rope in a pattern known as cable laid.
Wire rope is a complex mechanical device that has many moving parts all working in tandem to help support and move an object or load.
In the lifting and rigging industries, wire rope is attached to a crane or hoist and fitted with swivels, shackles or hooks to attach to a load and move it in a controlled matter. It can also be used to lift and lower elevators, or as a means of support for suspension bridges or towers.
Wire rope is a preferred lifting device for many reasons. Its unique design consists of multiple steel wires that form individual strands laid in a helical pattern around a core. This structure provides strength, flexibility, and the ability to handle bending stresses.
In stricter senses, the term wire rope refers to a diameter larger than 3/8 inch (9.52 mm), with smaller gauges designated cable or cords. Initially, wrought iron wires were used, but today steel is the main material used for wire ropes.
Wire ropes are made from cold-drawn wires to increase strength and durability. It may be noted that as its size decreases the strength of the wire rope increases. The various materials used for wire ropes in order of increasing strength are iron, cast steel, extra-strong cast steel, steel, and alloy steel.
Wire ropes were developed starting with mining hoist applications in the 1830s. Wire ropes are used dynamically for lifting and hoisting in cranes and elevators, and for transmission of mechanical power.
It is also used to transmit force in mechanisms, such as a Bowden cable or the control surfaces of an airplane connected to levers and pedals in the cockpit.
Clamp for wire rope
A wire rope clip, sometimes called a u-bolt clamp or u-bolt clip is used to clamp the loose end of a length of wire rope, once it has been looped back to form an eye. These fittings consist of a u-bolt and has a saddle secured by two nuts.
Generally, wire rope assemblies need at least two or three wire rope clips to secure the ends properly to the length of the rope.
Advantages of Wire Rope
The advantages of a wire rope are that they are more resistant to wear, have better crushing resistance, and high strength compared to a round-strand wire rope of equal diameter and classification. However, a swaged wire rope may have less bending fatigue resistance.
The wire ropes have the following advantages as compared to fibre ropes.
- Higher flexibility and more wear resistance.
- It has better-crushing resistance,
- They have high strength compared to a round strand wire rope of equal diameter and classification.
- It may have less bending fatigue resistance.
- Reduction of mechanical stress on both the rope and the sheave, thus extending the life of both the sheave and the winch drum
- Decrease of damages between the neighboring wraps in multilayer spooling – Better winding on drums
- Significant Cost Reduction due to reduced rope abrasion and increased service life with less maintenance expense
Construction of Wire Rope
Wire ropes are made from various grades of steel wire with tensile strength ranging from 1200 to 2400 MPa. The wires are first given special heat treatment and then cold drawn for high strength and durability of the rope. Specialized machines manufacture steel wire ropes.
First, strands such as 7, 19, or 37 of the wire are routed into a strand, and then a number of strands, usually 6 or 8, are rotated about the core or center to form the rope. The core may be made of hemp, jute, asbestos, or soft steel wire. The core must be continuously saturated with lubricant for the long life of the core as well as the entire rope.
Asbestos or soft wire cores are used when the ropes are subjected to radiant heat such as cranes working near furnaces. However, a wire core reduces the flexibility of the rope, and such ropes are used only where they are subjected to high compressions. Such as in the case of the wounding of multiple layers on a rope drum.
Classification of Wire Rope
According to the direction of twist of the individual wires and that of strands, relative to each other, the wire ropes may be classified as follows:
- Cross or Regular Laying Ropes: In these types of ropes, the direction of twisting of wires in the strides is opposite to the direction of the twist of the stand. These types of ropes are the most popular.
- Parallel or lang lay ropes: In these types of ropes, the direction of rotation of the strands in the strands is similar to the strands in the rope. These ropes have a better bearing surface but are easily split and twisted when loaded. These ropes are more flexible and the wearer is more effective. Since such ropes have a tendency to rotate, they are used in lifts and waved by guide methods and also as rope ropes.
- Composite or reverse-laid ropes: In these types of ropes, the wires in two adjacent strands are rotated in the opposite direction.
The direction of the laying of ropes can be right-handed or left-handed, depending on whether the strands form a right-handed or left-handed helix. But right-handed ropes are most commonly used.
Procedure for designing a wire rope:
- The following procedure can be followed while designing wire rope.
- First, select the appropriate type of rope for the given application.
- Find the design load by assuming a safety factor of 2 to 2.5 times the safety factor.
- Find the diameter of the wire rope (D) by equalizing the tensile strength of the selected rope for the design load.
- Find the diameter of the wire and the area of the rope.
- Locate the various stresses (or loads) in the rope.
- Find the effective stress (or load) during normal functioning, starting, and load acceleration.
- Now find the real safety factor and compare it to the safety factor. If the actual safety factor is within the permitted range, the design is protected.
Fibre ropes are made from fibers of varying length depending on their source. these are twisted up into yarns, and the twist given binds the fibres firmly together so that they hold by friction when the yarn is subjected to strain. The yarns are then laid up to form rope.
Ropes for transmission power are usually made of fibrous materials such as cannabis, manila, and cotton. Since hemp and manila fibres are rough, the ropes made of these fibres are not very flexible and have poor mechanical properties. The hemp rope has less strength than the Manila ropes.
When the hemp and manila ropes are bent over the sheave. The fibres cause some sliding, causing the rope to rub internally. To reduce this defect, rope fibres are lubricated with a tar, elongated, or graphite. Lubrication also makes the rope moisture-proof. Hemp ropes are suitable only for hand-operated hoisting machinery and for tackling rope tack, hooks, etc.
The cotton cord is very soft and smooth. Lubrication of cotton ropes is not necessary. But if it is done, it reduces the external wear between the rope and its edge grooves. It may be noted that Manila ropes are more durable and stronger than cotton ropes. Cotton ropes are more expensive than Manila ropes.
- Manila and cotton ropes typically have a diameter of 38 mm to 50 mm. The size of the rope is usually specified by its circumference or ‘circumference’.
- The ultimate tensile braking load of fibre ropes varies greatly. For Manila ropes, the average value of the ultimate tensile braking load can be taken as 500 D 2 kN, and for cotton ropes, it can be taken as 350 D 2 kN, where D is the diameter of the rope in mm.
Advantages of Fibre Rope Drives
The fibre rope drives have the following advantages:
- They serve smoothly, steadily, and quietly.
- They are slightly affected by external conditions.
- Shafts may be out of strict alignment.
- Power can be turned off in any direction and in degrees of the entire amount.
- They give high mechanical efficiency.
Application of rope drive
The use of ropes for hunting, pulling, fastening, attaching, carrying, lifting, and climbing dates back to prehistoric times.
Power transmission over long distances is the main application of rope drive. Rope drives are used to drive systems that are more than 8 meters in distance using a distance power transmission system. Metal ropes are used for distances beyond sixty meters. It is commonly seen in elevators and cranes.
Advantages and Disadvantages of Rope Drive:
Advantages of rope drive
- Significant power transmission.
- It can be used for long-distance power transitions.
- Ropes are strong and flexible.
- Provides smooth and quiet operation.
- It can run in any direction.
- Low-cost and economic.
- Precise alignment of the shaft not required.
Disadvantages of the rope drive
- Internal failure of the rope has no sign on external, so it often gets unnoticed.
- Corrosion of wire rope.