A strand jack is a type of linear winch system used for pulling heavy loads from above. They are widely used in the heavy construction industry, especially for bridges, ports, and large-scale plant maintenance projects.
Lifting capacities vary depending on the models used. For example, The capacity range of Enerpac Strand Jacks is from 17 up to 1405 imperial tons (150 to 12500kN) per jack.
The main benefits of a strand jack over a crane are a much smaller footprint, and the ability to lift and lower with more precision. In addition, a single operator can control several strand jacks synchronously via computerized control from a central location.
How a Strand Jack Works
If you’ve ever pulled on a rope to lift or drag a load this will help you understand the working principle. Like your hands would do on a rope, they use a similar sequence of locking, pulling, and unlocking at the top and bottom of the strand jack, but instead of using a single rope, there can be anything betwHow Does a Strand Jack Work?een 1 to 84 steel strands included (the strand bundle). The strand jack is commonly positioned vertically on a support beam above the location where the load needs to be lifted, however, they can also be used in the horizontal orientation.
The lifting or lowering action is performed by a large hydraulic jack (cylinder) that extends and retracts while the top and bottom anchor systems in turn lock onto each strand, guiding them through the hollow cylinder.
The diagram below summarizes the operation sequence.
The video below shows strand jacks used during the construction of the Bay Bridge in San Francisco.
Components
Now you’ve seen the process, let’s look at the main components and other related equipment.
Integrated Strand Guide
Helps to keep the strands in position when exiting the jack
Main Hydraulic Jack
The hydraulic jack (cylinder) is the heart of the strand jack. Depending upon the model, this can extend and retract as much as 600mm (23.6 inches) with each stroke.
Top and Bottom Mini-Jacks
These include the anchor systems used to lock and unlock the strands. Each is automated and includes wedges, springs, and hydraulics.
Strand Jack Wedges
These are wedges used to grip the strands. The wedges lock and unlock on the strand through hydraulic pressure applied by the mini jacks. They are replaceable, and although long-lasting can be considered as consumables.
Strand Guide Tubes
Telescopic strand guide pipes keep the strands apart, and prevent ‘bird caging’.
Counter-Balance Valve
This serves two functions. The primary purpose is to provide load holding in the event of hose failure. It also provides smooth controlled lowering of the load.