Knowlage of Snatch blocks & wire rope pulleys

 

Snatch blocks & wire rope pulleys

Snatch blocks and wire rope pulleys are common lifting mechanisms used for rigging. Using a snatch block in a block and tackle application reduces the amount of force required to lift an abnormally heavy object.

A pulley system is used in conjunction with wire rope or other types of rigging ropes, by looping the rope through the pulley (around the sheave) several times before hooking to the object. The force required diminishes proportionately with the amount of pulleys or “loops” used during the hoist. 

More about Snatch Blocks & Wire Rope Pulleys

A wire rope pulley is a block that can be opened on one side to quickly and easily insert a wire rope without having to remove the load or change direction of the cable.  Called a rigging block because it contributes to or “snatches” the mechanical force of pull, it offers a unique combination of power and flexibility. When used with a winch, a block lets you pull or lift heavy objects by “breaking” the pull span between the object and the winch. This cuts the direct pull load in half, which doubles the amount of weight your winch can pull. 

          

With so many different variations, sizes and weight limitations, it’s important to know what to look for when choosing a snatch block for the job. Factors to keep in mind when choosing a snatch block for your rigging needs:

  • Check the Working Load Limit (WLL) of both the snatch block and the wire rope. If the WLL of the pulley is not compatible with the WLL of the rope, it can create a dangerous situation if either one should fail.
  • Coordinate the size of the sheave in the snatch block to the diameter of the wire rope. If the wire rope is too large for the sheave, the block can crack. Generally, the wire rope-to-sheave size ratio should be 12:1 in order to hold the wire properly while under load.
  • Know your numbers. Because a snatchblock can cut the direct pull load in half, choose a rigging pulley that has a rating of double the pull of the winch you’ll be using with it.

China LG carrys a variety of styles of snatch blocks & pulleys; check the specifications for each product for break strength and load limits. We also carry a wide selection of wire rope in various sizes.  Along with heavy duty snatch blocks, we also offer smaller rigging snatch blocks and pulleys in various designs and sizes, including stainless steel snatch blocks, square blocks, single and double pulleys, swivel block and more.  

BLOCKS——Warning

CAUTION: NEVER EXCEED THE WORKING LOAD LIMIT. Read important information below and here. READ THE FOLLOWING WARNINGS AND SAFETY INFORMATION CAREFULLY. EVEN EXPERIENCED PERSONNEL NEED TO UNDERSTAND THIS INFORMATION. Failure to follow warnings and instructions can result in serious injury or death.

Blocks are devices used for lifting or lowering loads, or for changing direction of the pull on the line. They consist of a sheave, two side plates, and an end fitting which is either a hook with a latch or a shackle.

 

IMPORTANT SAFETY INFORMATION
ASSEMBLY:

A number of potential hazards exist when working with blocks. These hazards can cause serious harm to both humans and property. However, these hazards can be avoided if the block systems are assembled, used, and maintained properly. The following directions should Always be followed in order to prevent serious injury or death.

  1. Always make sure that the hook supports the load. The latch must NEVER support the load.
  2. Always check for wear or damage before rigging block systems. Check for cracks, rust, corrosion, deformation, intentional alterations, chips, warps, or any other physical signs of damage or wear. When in doubt about the extent of wear or damage to a product, take the product out of service immediately. Only laboratory tests can determine the fitness of a damaged or worn part. Thus, you may not be able to tell whether a part with any amount of damage is safe to use. Discard any part that appears worn or damaged.

USE:

  1. Know and follow OSHA rules.
  2. Know the weight of the load and the line pull required.
  3. Make sure that the correct wire rope grade and diameter is used. ConsultWire Rope product page for information.
  4. Never exceed the Working Load Limit (WLL). The Working Load Limit indicated is the greatest force or load a product can carry under usual environmental conditions. Shock loading and/or extraordinary conditions must be taken into account when selecting products for block systems and especially in determining the Working Load Limit for a specific application.
  5. The Working Load Limit of a product is affected by many factors. Among these are: Overloading, misuse, intentional abuse or alteration, corrosion, deformation, operation in extreme environments, and simply wear. In order to determine whether the product can continue to be used at the published WLL, regular inspections must be performed. These inspections also will determine whether a product must be withdrawn from service.
  6. Avoid side loading blocks. These products generally are intended for tension or pull. Side loading must be avoided, as it exerts additional force or loads which the product may not be able to withstand.

MAINTENANCE:

In order to ensure peak efficiency and extended service life it is necessary that blocks be inspected and maintained at frequent intervals. Consider the following points:

  1. Inspect products for any signs of wear or damage. These include:
    • Worn sheaves, bushings, side plates, pins, hooks, or shackles.
    • Hook latch for proper fit and operation. Deformed latches must be replaced.
    • Deformed side plates, pins, hooks, and shackles. Worn, deformed or damaged parts of the block and any other part used in the assembly must be replaced. If necessary, the entire assembly needs to be removed from service.
  2. Blocks supplied by Edwards Wire Rope are furnished with bronze bushings. This type of bushing is not self-lubricating. These blocks are intended for infrequent and intermittent use with low line speeds. Follow this lubrication schedule as a minimum.
    • Moderate use: Every 8 hours.
    • Intermittent operation: Once a week.
    • Rotate sheave during lubrication process.

LOADS:

Throughout this catalog the term Working Load Limit is used. It refers to the maximum load or force which a product is designed to support in general service when the pull is applied in-line. There are, however, other terms used in the industry which are interchangeable with the term Working Load Limit. These are:

Other terms applicable, such as Shock Load, Proof Load, and Design Factor are discussed in detail in the General Warnings section.

IMPORTANT:

The total load on the snatch block, and therefore also on any fitting which is attached to the block, is usually considerably greater than the actual load lifted. The deciding factor in determining the total load on the block is the angle between the lead line and the load line.

Multiplication Factors for Snatch Blocks*
Lead/Load Factor Lead/Load Factor Lead/Load Factor
Line Angle Line Angle Line Angle
2 60° 1.73 130° 0.84
10° 1.99 70° 1.64 135° 0.76
20° 1.97 80° 1.53 140° 0.68
30° 1.93 90° 1.41 150° 0.52
40° 1.87 100° 1.29 160° 0.35
45° 1.84 110° 1.15 170° 0.17
50° 1.81 120° 1 180° 0

 

Questions:
Choosing the right block and all the equipment to go along with it can be confusing. If you have any questions about using snatch blocks, choosing wire rope blocks, working load limits, etc., contact our sales specialists. They can help you find just what you need to get the job done safely.