Shear legs

Shear legs (also known as shears) is a simple lifting device composed of two or more poles or legs, which are joined at the top and spread apart at the bottom to form a stable A-frame. This structure is used to support a pulley or block through which a lifting rope or cable can be run. Shear legs are a form of Gin pole and have been used historically in various applications, including the lifting of heavy objects during construction, in shipyards for mast stepping and unstepping, and in rescue operations.

Design and Operation

The basic design of shear legs involves two or more long, straight members, typically made of wood or metal. These members are tied, hinged, or bolted together at the top to form a point, from which a lifting device such as a Pulley or hoist can be suspended. The legs are spread apart at the base to provide stability. The angle at which the legs are set, and the distance between them at the base, determines the load capacity and stability of the shear legs.

To operate, the shear legs are positioned over the load to be lifted. A lifting tackle, consisting of ropes, cables, or chains, and pulleys, is attached to the load. The other end of the lifting tackle is connected to a hoist or winch. When the hoist is operated, the load is lifted off the ground and can be moved to a new location.

Applications

Shear legs have been used in a variety of applications where temporary lifting solutions are needed. Some common uses include:

• Construction Sites: For lifting heavy materials such as steel beams, trusses, and concrete panels.
• Shipyards: For mast stepping (installing) and unstepping (removing) on sailing ships.
• Rescue Operations: To lift heavy debris during search and rescue operations following disasters.
• Theatrical Productions: For rigging heavy set pieces or equipment.

Advantages and Limitations

The primary advantage of shear legs is their simplicity and ease of construction, which allows them to be quickly assembled on-site using readily available materials. They are also relatively inexpensive compared to more complex lifting devices such as cranes.

However, shear legs have limitations in terms of lifting capacity, which is determined by the strength and length of the legs, as well as the stability of the base. They also require a significant amount of space for setup and operation, which can be a limitation in confined areas.

Safety Considerations

When using shear legs, it is important to ensure that they are properly designed and constructed for the load to be lifted. The base must be stable, and the legs must be securely fastened together at the top. The lifting tackle must be in good condition, and all operations should be carried out under the supervision of a competent person.

See Also

• Crane
• Derrick
• Hoist
• Pulley