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Snatch Block Basics, Part 2

by Terry Abejuela

Example

In theory, snatch blocks work to reduce load by trading force for distance. Unfortunately, real life is not quite that simple. We also must understand the difference between simple and compound pulley systems and the effects of line angles and sheave friction.

A simple pulley system uses one rope and one or more snatch blocks. All traveling blocks move towards the anchor at the same rate of speed. In simple pulley systems (the most common system used in towing and recovery), theoretical mechanical advantage can be determined by counting the number of line parts attached to the load.

Theoretical mechanical advantage is the ratio of the output force a machine exerts to the input force that is furnished to that machine to do the work. A machine that has a 2:1 mechanical advantage means if the input force is 100 lbs. the machine yields 200 lbs. of output force.

If the fall line (the line from the winch) is attached to the load, it should be counted in determining theoretical mechanical advantage. Three line parts to the load would have a 3:1 theoretical mechanical advantage. This method only works with simple pulley systems.

Example No. 1 is a simple pulley system. Counting the lines to the load will determine the theoretical mechanical advantage (3:1).

Another method, the “T method, ” of determining theoretical mechanical advantage is to add the amount of tension on each line part. You need to add the amount of tension on each line part and divide the total by the line tension on the fall line. If you have 300 lbs. of load with a three-part line attached to the load, the fall line would have 100 lbs. of tension. There is 100 lbs. of tension on each of the line parts (300 lbs.), divided by the 100 lbs. of tension on the fall line (3). This pulley system has a 3:1 theoretical mechanical advantage.

The winch exerts 100 lbs. of input force that becomes 300 lbs. of output force on the load, causing the 300-lbs. load to move.

The T method works on simple and compound pulley systems.

Compound Pulley Systems

Acompound pulley system uses more than one rope with two or more blocks. They are made up of two or more simple pulley systems acting on each other. Counting lines to the load to determine theoretical mechanical advantage does not work in a compound pulley system; the T method must be used to correctly determine theoretical mechanical advantage.

Counting lines to the load in Example No. 2 you might come up with a 5:1 theoretical mechanical advantage, but because this is a compound pulley system it is actually 6:1. The load is 900 lbs. The fall

line from the winch goes through traveling snatch block No. 3 and is terminated at the anchor.

A second piece of rope is attached to the load and reeved through fixed snatch block No. 1 at the anchor, runs through traveling snatch block No. 2 at the load, and then terminates at snatch block No. 3. The three lines of the second rope are all attached to the load, so each line part supports 1/3 of the load, or 300 lbs.

Snatch block No. 3 is attached to one line part and only has 300 lbs. of tension. Because there is a two-part line attached to snatch block No. 3, the load is divided in half resulting in only 150 lbs. of tension on the fall line. The 900-lb. load is divided by 150 lbs. (load on the fall line), for a theoretical mechanical advantage of 6:1.