Mechanical advantage systems (MAS) are a HAVE TO KNOW SKILL SET FOR MOUNTAIN RESCUE. Knowing how to build MAS will allow rescuers to:
- Perform over the bank motor vehicle rescues
- Up-haul litter teams on steep angle or high angle rescues
- Build portable MAS (jiggers) for stranded climber pick-offs
- Perform up-hauls during crevasse rescue
- Rig guidelines and highlines
- Construct personal ascension systems for climbing ropes
- Construct telfer swift water rescue systems
- Pull wrapped boats off strainers
- Anything else that may require a few rescuers to lift a heavy load…
Simple MAS are some of the most common MAS used in mountain rescue. Simple MAS follow several rules:
- The traveling pulleys will move towards the anchor pulleys at the same rate.
- If the the knot at the end of the rope is attached to the anchor the MAS will be even (2:1, 4:1). If the knot at the end of the rope is attached to the load the MAS will be odd (3:1, 5:1).
- The amount of tensioned lines (when pulling on the MAS) will reflect the MAS. For example a 3:1 MAS will have 3 taut lines when the rescuers are pulling on the MAS.
- The number of pulleys need to build a simple MAS will be 1 less then the MAS (a 5:1 simple MAS requires 4 pulleys).
Click on any of the diagrams below to enlarge the image.
Simple 2:1 MAS
Simple 3:1 MAS
Typically teams may start of with a simple 3:1 MAS and if they need more pulling power they can add more pulleys to gain more mechanical advantage. This is typically done by adding 2 more pulleys to (or using 2 double pulleys) the system to create a simple 5:1 MAS.
Simple 3:1 MAS with Petzl ID
If you are transitioning from a lowering operation to a raising operation devices like the Petzl ID and the CMC MPD can be left attached to the anchor and serve as the anchor pulley and progress capture device. The BIG difference between the 2 devices is in their respective efficiencies as a pulley. The MPD works as a highly efficient pulley at about 90% efficient while the Petzl ID’s cam is only about 33% efficient as it doesn’t spin.
Simple 5:1 MAS
Many teams are now utilizing MPD’s into their raising and lowering rope systems. Make sure you are aware of the particular devices efficiency as a pulley if you will be using them as up-haul devices.
Always incorporate a progress capture device (safety ratchet) into your MAS so that if rescuers let go of the rope the load will not fall! This is known as the “whistle test”.
Most SAR teams use an 8mm sewn prusik loop on an 11mm rope as their progress capture device. Avoid or do not use rope grabs with teeth for progress capture (they have proven to cut the rope with moderate amounts of shock-loading)!
Calculating the MAS
A great method to confirm your MAS is what you think it is, is to use the tension or T-method. This method contrasts the units of input to the units of output to determine the MAS. Check out the diagram and video below to learn how to do the T-Method.
Friction and MAS
You can’t always get want you want…
Watch the video below to see how friction and pulley efficiency will reduce the ideal mechanical advantage.
Stay tuned this month for more on mechanical advantage systems including compound and complex systems.