Answer : when there are more than 2 and then they become a block and tackle.
A single pulley is seldom used on its own, generally several of them are used in the commonly known ‘block and tackle’ systems, an individual pulley being a block and, in an assembly of more than one block with rope or cable threaded through, they are termed tackle.
The more pulleys that are used in the block and tackle system, the easier it becomes to lighten the load.
There are, of course, several types of blocks available:
- Stainless steel blocks which have the great advantage of long life and virtually no maintenance
- Ball bearing blocks which give low friction and maximize the load bearing performance
- High capacity blocks used when strength is of paramount importance
- Special purpose blocks, e.g. those that can be permanently mounted within walls
- Snatch blocks, used to ‘snatch’ some of the load as they can be inserted anywhere along the load-bearing cable/rope without having to remove the load to re-thread the cable/rope
- Blocks manufactured from different materials for different uses
It would be very difficult to imagine how life would be without the invention of the age-old block and tackle!
News and Updated
Google Plus: https://plus.google.com/u/0/+Ronstanindustrial_hardware