Elevator Sheave | جنرال کابین

2024 June 24

Read time : 14 m

Types of Sheaves

Elevator sheaves are categorized into two types: deflecting and reversing. Depending on the type of elevator, different sheaves are used. In some elevators, a reversing sheave optimizes the alpha angle. In other elevators, sheaves are used to change the direction of the hoisting ropes and guide them. The grooves in these types of sheaves do not create traction and only serve to guide the hoisting ropes.

General Specifications of Elevator Sheaves

The diameter ratio of the sheave should be at least 40 times the nominal diameter of the hoisting rope.

Cast iron is used in the manufacturing of elevator sheaves, offering the following advantages:

Good castability
High compressive strength
Use of dampers to reduce vibrations
Excellent sliding properties
Ability to stop or rotate in an emergency
High resistance to corrosion

The hardness of the cast sheave depends on its chemical composition, and the acceptable hardness for the traction sheave is between 160 and 200 Brinell.

Polyamide Sheave

Due to the heavy weight of cast iron materials and the challenges encountered during the casting, machining, finishing, and painting processes in the production of elevator sheaves, the entire process of manufacturing cast iron sheaves is fraught with complexities and numerous challenges. Those knowledgeable in this field understand the characteristics and issues associated with producing cast iron sheaves from start to finish.

The issues and problems with cast iron sheaves are as follows:

Need for a furnace with an appropriate temperature
Transporting raw materials to the casting site
Mixing and melting various materials
Preparing the mold
Mold setting with casting sand
Injecting molten materials
Cooling the cast materials and removing the sheave
Turning and machining
Wear and tear on various turning tools
Preparing the shaft and bearing seats
Separating defective sheaves with voids and cracks
Painting

Given the extensive workload and high costs involved in the production stages of cast iron sheaves, industrialists and manufacturers of sheaves or elevator pulleys have considered alternative materials. Some major companies have replaced casting materials with PVC nylon, polyurethane, and high-density polyethylene (HDPE) to produce elevator sheaves. Many manufacturers have recently started producing and replacing plastic sheaves instead of cast iron sheaves.

MC Nylon

MC Nylon is one of the plastic materials that has widely replaced steel and cast iron in the industrial world, especially in the elevator and escalator industry, and it offers the following advantages:

Lower cost and greater savings compared to cast iron sheaves
High hardness and resistance to friction
Long lifespan against corrosion
Lightweight and easy to transport, making installation easier for technicians
Reduced noise, low vibration, and oscillation
Smooth movement of hoisting ropes over it, providing a quiet and smooth ride for elevator passengers
Prevents hoisting rope wear and increases its durability and lifespan
High resistance to moisture and direct sunlight
No need for oiling or greasing
Available in white and yellow colors
No wear and production of fine dust, environmentally friendly

One of the crucial elevator components is the idler or traction sheave. This sheave refers to the pulley used for transmitting motor power and moving the hoisting ropes in the elevator.

The timing of idler sheave replacement depends on the wear level of the elevator sheave. If the bearing of the sheave encounters a problem, it is possible to replace the bearing; however, in such cases, the best course of action is to replace the elevator idler sheave. Signs that indicate the need for replacement include:

Presence of iron filings around the motor sheave and elevator idler sheave
Formation of spiral grooves in the sheave grooves, indicating the passage of the hoisting rope over the grooves
Indentations of the hoisting rope inside the grooves

Factors Influencing the Installation of the Deflector Sheave

Design and Use: The deflector sheave must be designed and utilized according to the nominal speed and load of the elevator.
High Resistance: The deflector sheave must be resistant to bending and torsional stresses.
Precise Angle: The angle between the deflector sheave and the gearbox sheave must be precise and calculated.
Bearing Balance: The bearing of the deflector sheave must be balanced without any play.
Sheave Health: The deflector sheave must be healthy and free from any cracks or roughness.
Appropriate Grooves: The grooves of the deflector sheave must be larger than the diameter of the wire rope.

Corrosion and Damage of the Sheave

The reasons for the corrosion and damage of the deflector sheave are as follows:

Imbalance of the Counterweight and Cabin: An imbalance between the counterweight and the elevator cabin is one of the main reasons for sheave corrosion. To prevent this issue, installers must carefully balance the counterweight and cabin.
Dry Wire Rope: A dry elevator wire rope can lead to sheave corrosion. The service technician must check the wire rope monthly and lubricate it if necessary.
Misalignment of the Motor Sheave and Deflector Sheave: The motor sheave and deflector sheave must be aligned and in line with the elevator cabin. Otherwise, the deflector sheave will suffer from corrosion over time. To prevent this issue, installers must ensure the alignment of these two components during the elevator installation.

Alpha Angle

The alpha angle of the wire rope is one of the critical angles in elevator installation calculations that must be calculated precisely. Wire ropes play an important role in the traction of the elevator; the motor pulley must perform the dual functions of moving and shifting the elevator and changing the direction of the force.

The alpha angle, or Angel of warp, is formed between the main motor sheave and the deflector sheave and must be selected in such a way that, during the rotation of the wire rope on the sheaves, it is in the best condition in terms of safety, speed, balance, and power. The cabin and counterweights are suspended on either side of the motor sheave, and this system is designed and executed through wire ropes by the sheaves.

If an appropriate alpha angle is not considered during the installation of the elevator deflector sheave and gearbox sheaves, it will cause the elevator to slip.

Formula for Calculating the Alpha Angle in Elevators

The calculation of the reverse bend in the suspension system plays an important role and must be considered during the elevator design. There are two methods, approximate and precise, to determine this angle, which are discussed below.

Approximate Method for Calculating the Alpha Angle:

An approximate method used to determine the alpha angle in an elevator has an error range of 10 to 15 degrees. The minimum alpha angle for an elevator should be 150 degrees, and the maximum should be 180 degrees.

To calculate the alpha angle, follow these steps:

Measure the length of the wire rope on the gearbox motor sheave.
Multiply the obtained value by 360.
Divide the calculated value by the circumference of the gearbox sheave.

(3.14 * sheave diameter) / (360 * length of the wire rope lying on the motor sheave) = alpha angle

α = (L * 360 / πd)

Accurate Method for Determining the Alpha Angle

To determine the alpha angle using this method, the sine of the complementary angle is used; therefore, the alpha angle can be measured with high accuracy. The parameters Rs, Rp, I, and h respectively represent the radius of the traction sheave, the radius of the deflector sheave, the horizontal distance between the centers of the two sheaves, and the vertical distance between the centers of the traction and deflector sheaves.