Saturday, March25, 2017

Blocks and Mechanical Advantage


This is the second part of a two part tutorial on mechanical systems made from blocks and line. The first part is HERE I will explore the mechanical advantage of various systems, and shows the general principles on how to set these systems up, including how to thread the line through the blocks, or reeve them. It will go from 1:1 to 6:1 with simple systems and up to 24:1 with cascaded systems. The second part will explore some more unusual systems.

All block systems are comprised of a stationary part and a moving part. These terms are self explanatory. Most all tutorials on block systems show the fixed part at the top and the moving part, attached to some weight, at the bottom. In other words, they are systems that lift a load. It is left to the reader to realize these can be turned around and used to pull down. On a boat, the majority of block systems pull down. These include the mainsheet, vang, and Cunningham. For that reason, I am detailing systems that pull down as well as the more conventional diagrams that pull up. Some block systems pull sideways for example outhauls and reef systems. For those you can decide which drawing is most appropriate to your application.

Creating the diagram of a system that pulls up was a bit of a challenge. I needed something that represented an upward force and a weight of course always pulls down. What I decided to use was a super anti gravity filled balloon. This anti gravity gas pulls up with a force just as strong as the downward force supplied by the weight on the drawings. In use, you can replace my anti gravity gas balloon with your mast or sail.

Cascaded Systems

8 to 1 cascaded Vang System

A cascaded system has many advantages. It uses simpler blocks and is more likely to avoid twist and tilt issues. The disadvantage is that they take up a lot of room so are best for situations where there is a lot of space compared to the amount of motion you expect. A vang or backstay are examples of such applications. In the system pictured there are three 2:1 systems to the total advantage is 2 x 2 x 2 = 8. You can also cascade 2:1 with a 4:1 to get 8:1. There are a large number of useful possibilities.

24:1 mainsheet fine course system

Resource id #14

This mainsheet system is not only a cascade for the fine control, but feeds the other end of the line out through a turning block so that the major distance can be adjusted quickly with a 6:1 system. The fine is 4:1 so the net is 6 x 4 or 24 to 1.

Special or Unusual Reeving

Resource id #19
Sometimes special reeving makes since. In this case, when both ends needed to exit the fiddle blocks on opposite sides for a dual ended system, a simple reversing the direction of one of the sheaves did the trick. In this case, if you start with both ends and draw where you need them to go, and then continue with them to the next block, the reversal of the line between the two inner sheaves presents itself as the solution.

Capsizing Blocks

If you put a very unusual load on a block it can capsize and create a very high friction situation. I did this once by hanging a fiddle block on a single block. The line then fed out at 90 degrees from the single block. Consider that single block. It had 5 times the tension in the line pulling it down and one times the tension pulling it sideways. The result was that it capsized and tried to turn sideways. This happens because the force on the line going out at 90 degrees pulls on the sheave on the wrong side of the pivot point of the block. It is an extreme form of twist and not one that is particularly obvious. The solution was to provide some forward force to orient the single block in its natural position, at 45 degrees to both the incoming and outgoing lines.

Ad by Google

The information on this web site has not been checked for accuracy. It is for entertainment purposes only and should be independently verified before using for any other reason. There are five sources. 1) Documents and manuals from a variety of sources. These have not been checked for accuracy and in many cases have not even been read by anyone associated with I have no idea of they are useful or accurate, I leave that to the reader. 2) Articles others have written and submitted. If you have questions on these, please contact the author. 3) Articles that represent my personal opinions. These are intended to promote thought and for entertainment. These are not intended to be fact, they are my opinions. 4) Small programs that generate result presented on a web page. Like any computer program, these may and in some cases do have errors. Almost all of these also make simplifying assumptions so they are not totally accurate even if there are no errors. Please verify all results. 5) Weather information is from numerious of sources and is presented automatically. It is not checked for accuracy either by anyone at or by the source which is typically the US Government. See the NOAA web site for their disclaimer. Finally, tide and current data on this site is from 2007 and 2008 data bases, which may contain even older data. Changes in harbors due to building or dredging change tides and currents and for that reason many of the locations presented are no longer supported by newer data bases. For example, there is very little tidal current data in newer data bases so current data is likely wrong to some extent. This data is NOT FOR NAVIGATION. See the XTide disclaimer for details. In addition, tide and current are influenced by storms, river flow, and other factors beyond the ability of any predictive program.