A Snatch Block Pulley is simply a basic pulley that can be used to redirect a running rope or wire rope. Snatch blocks have to be one of the most commonly overloaded pieces of rigging equipment that we sell, and it often comes down to a lack of understanding the fundamentals of how they work. The reason for it is simple: most people tend not to know that the angle at which a snatch block redirects a line directly affects how much force the anchor point of the block will see.
The best way to envision this phenomenon is to start by imagining a line passing straight through the snatch block. In this unusual (and useless) arrangement, the snatch block will see no force whatsoever because again, the line is just passing straight through it. Now imagine pulling on the snatch block so that it redirects the line at a 90° angle. The amount of force on the snatch block (and its anchor point) will be equal to 1.41 times the line pull required to move the load attached to the line. This phenomenon is exacerbated as the angle of redirection steepens, to the point where when the line gets redirected at 180°, meaning that the lines coming to/from the snatch block are parallel with one another, the anchor point will see a force equal to 2x the line pull.
For example: If the snatch block is going to be used in a 180° configuration and is going to be used to lift a 2 ton load both the snatch block and the attachment point must be capable of at least 4 tons, so it is important that the proper snatch block is selected. Not knowing these facts can lead to selecting a mounting point that isn’t strong enough, but more commonly the snatch block itself won’t be strong enough.
A common misconception about snatch blocks is that they somehow reduce the line pull, which is true in some specific scenarios, but for the most part the only thing they do do (haha) for sure, as mentioned above, is endure increased loading and exert that extra force on whatever they are attached to. Just remember: a snatch block does not reduce the line pull required to move a load unless the snatch block itself can travel with the load.
So for another example: let’s say someone has erroneously concluded that with enough velocity, their quad will in fact make it through the slough. With the quad planted up to the handlebars in muck they now need to figure out how to get it back out with nothing more than a length of winch line, some slings and shackles, and a snatch block. A person might determine that it would be best to mount the snatch block to a tree and run the winch line through it back to a shackle on the stuck quad, but in actual fact the best strategy would be to attach the snatch block to the quad and run the winch line from the recovery vehicle through it and back to the tree. This arrangement will double the amount of force that the winch exerts on the stuck quad.
If you wanted to go one deeper, the use of a double sheave snatch block can offer even more benefit. Simply attaching the double sheave snatch block to the stuck quad and a single pulley snatch block to the tree can further amplify the pulling force of the winch. The recovery line must be reeved from the recovery vehicle, through one pulley on the stuck quad, back to the snatch block on the tree, back to the other pulley on the stuck quad and then attached back at the tree again. Stacking blocks like this can often lead to such a drastic increase in pulling power that the quad can nearly be extracted by pulling the recovery line by hand! At a very basic level the amount of force amplification is equal to the amount of lines going through the travelling block.
There's actually an excellent youtube video on how to use a snatch block https://www.youtube.com/watch?v=M2w3NZzPwOM
Northern Strands carries a variety of high quality Snatch Blocks and other Pulley's at our Saskatoon, Esterhazy and Regina locations. https://www.northernstrands.com/pdf/rigging.pdf
Northern Strands also provide in person Rigging Training that is offered at our Training Center or on site. We provide Crosby Rigging Training and a more comprehensive rigging course aptly named Below the Hook Rigging. You can find the full list of safety training courses and our Training Calendar here https://www.northernstrands.com/training.aspx
Did you know that Northern Strands is one of the largest suppliers of industrial equipment and rigging in Saskatchewan? We are locally owned and operated and have been been proudly serving Saskatchewan people and businesses for over 50 years!