Line Angles Line angles affect how much tension is on the lines as well as the snatch blocks, chains, straps and anchors. The larger the angle between the incoming and outgoing lines to the snatch block, the more line tension on each side of the line will increase and tension on the snatch block will decrease. When the lines are parallel, the force multiplication is concentrated on the snatch block and line tension is reduced. The reason the line tension increases with wider line angles is that you are starting to produce lateral forces; when the line angle reaches 120 degrees, you are no longer reducing line tension.
Sheave Friction
Number 128 on Reader Card
Number 181 on Reader Card
All snatch block sheaves create friction when the rope moves around the sheave. Even the most efficient snatch blocks operate at about 98 percent. This means that if the fall line has 100 lbs. of tension when it enters the first snatch block, the outgoing line will have 102 lbs. of tension on it. The snatch blocks we use in the towing industry are generally about 90-percent efficient, so the line tension on the exiting line would be 110 lbs. This is not enough to be concerned with when only using a few snatch blocks. However, when dealing with heavy loads and many snatch blocks, friction may become a factor. Snatch blocks in theory are simple, but real-life applications require a thorough understanding of the basics to use them effectively and safely.
Editor’s note: In the May 2012 issue, Abejuela covered pulley systems (snatch blocks) and how they can be used to multiply or redirect force.
Terry Abejuela has 30 years of light-duty towing-and-recovery experience; he's also been a light-duty level 1 instructor for the California Tow Truck Association since 1998. Number 145 on Reader Card
16 • June 2012 - TOWMAN.COM
