A snatch block is a rope pulley with a side plate that swings open, so the wire rope can be laid into the sheave from the side. With a closed block you have to feed the whole rope end through the housing; with a snatch block you open the plate, drop the rope into the groove and close it again. On site that saves a lot of time, especially when the rope is already attached to a load or a winch. The Dutch term is klapschijf.
Snatch blocks are made in a wide range of capacities — from light models of a few tonnes to heavy blocks rated at several dozen tonnes.
Where it is used
- Redirecting a pull — turning the pull of a winch into another direction, for instance to drag a load around the corner of a building
- Building a tackle — combined with other sheaves to reduce the pulling force needed
- Temporary lifting points — in erection work, demolition and moving heavy components
- Frequent rope changes — where the rope route changes often, a snatch block is far more practical than a closed block
Points to watch
- Work out the force on the anchor point — with a 180° turn (the rope runs back the way it came) the force at the anchor point is roughly twice the line pull; at 90° roughly one and a half times. The anchor point is often loaded more heavily than the load itself
- Lock the side plate — the hinged plate must actually be secured with its pin, bolt or clip before the block is loaded
- Sheave diameter — a sheave that is too small for the rope diameter accelerates rope failure; follow the manufacturer’s ratios
- Inspection — lifting gear has to be inspected periodically; before use check for cracks, deformation and wear in the groove
- Never stand under the load — a rope failure or a block letting go whips out dangerously
Note: in metalworking the Dutch word klapschijf is sometimes used for a flap sanding disc — an entirely different product.
Related terms
- Winch
- Hoisting beam
- Treadwheel crane
- Jack
- Steel
- Scaffold
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