Kudos goes out to Peak Rescue Institute for the help.
The purpose of this lesson is to give the student rescuer a more thorough understanding of mechanical advantage. It will review basic concepts learned in the technician course and add advanced concepts such as the T-method, system efficiency, haul system progressions, and counter-balance haul systems.
Basic Mechanical Advantage Review
A quick review of basic principles of MA
• need for haul systems
• advantages and disadvantages
• manual vs. mechanized haul systems
• theoretical vs. practical
• simple, compound, and complex systems
• required personnel
A quick review of systems learned earlier
T-Method ~ What is it?
A method for calculating the mechanical advantage of a haul system. It works by tracing the distribution of tension (where we get the T in the T-method) throughout the system.
Why learn it?
Unlike other methods of calculating mechanical advantage, it does not require the user to identify the system as simple, compound, or complex.
How to do it:
1. Start at the haul team and assign them the ability to pull with 1 unit of tension or 1T.
2. Travel along the rope away from the haul team and toward the load and label the amount of tension at key points along the path of travel.
3. The amount of tension at the haul team is 1T and does not change until the rope encounters a junction such as that created by a prusik or cam device attached to the rope.
4. When the rope goes around a pulley, the tension remains the same on either side, but the output tension transferred through the pulley and onto the prusik (or another rope grab) is double the tension in the rope (i.e. it is equal to the tension on both sides of the pulley added together). This is true if the lines are running parallel with each other.
5. Where a prusik from the output side of a pulley is hitched to another strand of rope, the tension at that junction equals the tension on the prusik plus the tension on the strand of rope to which it is hitched.
6. Learn this method by labeling the tension at key points (e.g. 1 T, 2 T, etc) including the haul team, all sides of a pulley, the junction of a cam device and the rope, the anchor, and the load. Get in the habit of doing this mentally after you have practiced on a system or whiteboard.
7. The mechanical advantage is the ratio of tension at the load to the tension at the haul team. For instance, if there are 5 units of tension at the load, it is a 5:1.
8. Check your work by comparing the tension on the load, anchor, and haul team. The load tension should equal the haul team tension plus the anchor tension or the load tension plus the haul team tension should equal the anchor tension depending on which way the haul team is pulling.
Accounting for tension at the anchor is important for two reasons:
1. To check our work.
2. Informs us of forces acting on our anchors and system components. Mechanical Advantage systems do not usually multiply forces on the anchor. Change of directions will. These are important concepts to understand.
Peace on your Days