While sheaves may be a relatively small piece of hardware, they play a crucial part in mechanical engineering and lifting equipment. They are used in tandem with a rope, belt, or cable to lift items with a crane.
Essentially, a sheave is a wheel with an open groove that a rope or cable fits around so it can rotate around the exterior. One end of the cable is attached to an item that needs to be moved, while the other is attached to a fixed object, like the base of a crane. The cable smoothly moves over the wheel of the sheave as it is wound up on the fixed object, lifting the other object into the air.
Sheaves are also commonly known as pulleys, and when two or more pulleys are used together for extra strength, this is known as a sheave block. Rope pulleys were actually used as far back as the 1500s in order to hoist water out of wells, and they were likely used to lift and move heavy rocks that created Stonehenge in the UK.
Over the centuries, the overall design of sheave blocks has really not changed too much. These days, they are typically made with an alloy mix of iron and steel. Some models also add carbon and silicon for extra durability.
The most common design for a sheave block that is used in lifting equipment is comprised of seven parts:
First, there is a hook at the top which connects the entire block to the lifting equipment piece, such as the top of a crane.
Next, there is a strap that surrounds the sheave block so the rope or cable stays in place.
There is also a protective shell surrounding the sheave.
Inside is the sheave or pulley, which looks like a wheel. There is a pin that runs through the sheave so that it can rotate.
The opening between the interior sheave and exterior shelf is called the swallow, and the bottom opening of the shell is the breech.
You certainly want to purchase a sheave block that is durable and strong enough for whatever task you will be using it on. You also want to look out for a sheave with a smooth radius to reduce friction between the pulley metal and the cable or rope. It is also best to buy a sheave with rounded outside diameters as any sharp corners could damage or wear down the rope as it slides over the pulley.
Finally, look for a sheave that has a thicker base and thinner side grooves. This provides better support while allowing the cable to move smoothly over the top.
Now, if you are in the market for lifting gear supplies, you may be a bit overwhelmed with the options available. There are lots of types and styles – and some are better for specific jobs than others.
So, here’s everything you need to know about picking out the right shift block for whatever job that you have at hand.
Types of Bearings
A bearing is an axle inside of the pulley which helps to minimize friction as the cable or rope moves back and forth within the sheave block. There are several different types of bearings that are used for different applications.
- Bronze Bearing: Best for moderate weight loads with slow line speed.
- Roller Bearing: Suitable for heavier loads and high speeds. For radial loads only.
- Full Complement Cylindrical Bearing: High speed, heavy loads, and continuous operations.
- Tapered Roller Bearing: Heavier loads and high speeds. It can be used for continuous operations on axial and radial loads.
There are also different shapes and styles of the grooves within the sheaves. These different styles are made to accommodate different types of ropes or cables, and it is best to choose a groove size that is 5% to 10% wider than the diameter of the rope used.
It is also very important to keep tabs on the groove’s wear and tear. Over time, it can wear down, especially if they are used with wire rope products – which cause additional abrasions. Excessive wear can break down ropes and cables much quicker.
The API style groove profile is 30 degrees wide, the Aise style is 35 degrees wide, and the European style is the widest at 45 degrees.
There are also different shape styles of grooves. U-shared profiles are best for use with ropes since it prevents any pinching and allows the rounded shape to glide smoothly. V-shaped profiles are best for wire slings and cables.
Normal Duty Sheaves
As the name implies, normal duty sheaves are designed for moderate weight loads and “normal” duty applications. These sheaves are available in 76mm through 406mm (3” to 16”) wide sheaves and are often protected with sheave guards so the rope or wire being used stays in place.
A normal duty sheave block is best for lighter loads and slow line speeds, such as hand-operated applications.
There are five distinct types of bearing options on normal duty sheaves. Bearings are smaller circular pieces of metal that fit inside of the pulley to help it rotate.
Different bearing styles are designed for various applications.
- Self-lubricating bronze bushings are best for pulleys that are difficult to reach for re-lubrication and are often best at low speed, intermittent operations.
- Plain bore sheaves are best for infrequent usage and light loads and must be regularly re-oiled.
- Plain bronze bushings require frequent lubrication but are designed for heavier loads and continuous operations at slow speeds.
- Straight roller bearings are optimal for medium loads and high-speed operations. Regular lubrication is required, especially if it is in continuous operation.
- Tapered roller bearings are designed for heavy loads and high-speed, uninterrupted operation. These do require regular lubrication, particularly if they are in continuous use.
Normal duty sheaves are made from machined ductile iron, which offers distinct advantages over other types of metals –, particularly steel. Ductile iron has a higher silicon content, which offers it more machinability and will often last longer than tools made of steel.
Normal duty sheaves are commonly used in:
- Sailing equipment
- Farming machinery
- Some construction, but not for heavy load-bearing equipment
Heavy Duty Sheaves
Heavy-duty sheave blocks are best for heavy-duty applications with thicker ropes and cables and heavier weight bearings. They are available in 102 mm through 305 mm (4” through 12”), and are often used in work environments where the sheave block will be operating continuously.
Heavy-duty sheaves use closed die forging, which means that the hot metal is poured on dies (or molds) and forged into particular shapes. This helps to strengthen the shape of the tool since it does not need to be welded together in multiple pieces.
These are often used for:
- Material handling equipment
- Construction equipment
- Crane slings
Domed sheaves are reinforced for extreme duty applications. These sheave blocks are welded in a circular pattern, which reduces additional stresses made by welding ribs. Domed sheaves are also the largest and are available in 610 mm (24”) and larger sizes.
Domed sheave blocks have the greatest strength and rigidity of all sheave types and are made for applications with high working stress, heavy loads, and continuous operation.
Domed reinforced sheaves are often used in:
- Specialty equipment applications
- Large lifting equipment
- Large cranes and construction equipment
Choosing the right equipment for the job at hand will always ensure that the task will be easier and most likely more successful. Sheave blocks play a crucial role in numerous types of applications, and they can make it possible to do anything from hoisting up a sail to moving large pieces of machinery from one point to the other.
If you have any further questions regarding sheave blocks, their applications, and which style or size you need to, please reach out to us at Hollow Houston Inc. Our team of experts can help you find the correct pieces of equipment for your specific applications.