The ‘T-system’ for calculating Mechanical Advantage
At their simplest, pulleys can be viewed as levers. This becomes more obvious if you consider the static system (where everything is at rest).
To understand the ‘T-system’ for calculating Mechanical Advantage always start with the input (the effort end of the system) and assume that force of 1 unit of tension (or 1 T) is applied.
If a rope is continuous and has no attachments then the tension is of the same magnitude throughout. At any change in direction in the rope (ie a pulley), an opposing force is required to pull against the ropes going through that change. At any attachment to a rope (ie a rope grab) tensions are added.
Ideal Mechanical Advantage (IMA) is the predicted MA assuming that all losses in the system can be ignored. We assume that all pulleys are 100% efficient and that ropes running over
objects such as edges and carabiners experience no friction.
Theoretical Mechanical Advantage (TMA) is what we can calculate when we attempt to model and include losses in the system. We may make broad assumptions and attribute frictional losses to components and changes in direction of ropes.
Actual Mechanical Advantage (AMA) can only be measured during the operation of the system. It cannot be predicted however, with careful consideration, it should be fairly consistent with the TMA.
Need more information on the gear used in this video?
The larger sized Prusik-minding pulley (PMP) Swivel 2.6 is ideal for industrial rescue use where maximum pulley efficiency and strength is important. The rotating swivel aligns the rope with the direction of the pull. By combining the swivel with the pulley, overall system length is minimized, increasing travel. The patented pivoting side plate allows the rope to be quickly inserted.
- Prusik-minding side plate opens without having to disconnect the pulley from the anchor
- A double-detent button prevents accidental opening of side plate
- Sealed ball bearings provide maximum efficiency
- Aluminum sheave
- Works with rope sizes up to 13 mm
- UL Classified to NFPA 1983 (2012 ED) – General Use