adding pulleys

[steffb503]

I have looked through lots of posts on the subject and I still can not come up with a formula for adding a pulley or two.
Can some one share a formula.
MA=length of arm/distance from fulcrum to plunger
Does the size of the pulley matter?
Does the distance from arm to pulley matter?
If so please give me any formula I might need.
There was a download that I am guessing had it all but I was not able to open it.

[DeejayDebi]

If I remember my physics correctly the size of the pulley does matter, as does the size, weight and length of the rope used to some degree but as small as they will be what is more significant is the number of sheeves or wheels used on the pully than the size of the wheels. If you were using  say 8 inch sheeves it would make a big difference in your calculations but as you will be using proabaly 1 or 2 inche sheeves at best it should be insignificant.

God I used to calculate this stuff at work everyday and now I just can't remember - sorry hon! Getting old!

[FarmerJd]

In general, the mechanical advantage of a pulley system is related to the number of weight bearing sections of rope not just the number of pulleys. Usually the number of pulleys is the same as rope sections but not always. The size of the pulley may have a very small practical effect on the friction involved but it is never used in calculating the mechanical advantage. The short answer is: count the pulleys and multiply that by the MA of the system with out pulleys (i.e. length of arm / distance from plunger to fulcrum) to find the new MA. This is assuming that the pulley is attached to the end of the arm or very close. Good luck.

[smolt1]

Also if the pully does not pull straight down XXXXXXXXXXXXXXXXXXXXXX
                                                                   x
                                                                   x
                                                                   x

then you have to take into account the angle  xxxxxxxxxxxxxxxxxxxxxxxx
                                                                      x
                                                                          x
                                                                             x

If the angle is less than 10 or 15 degrees from vertical then the error is under 10% ( cosine 15 degrees = .966 as compared to 1 for vertical)

Also as FJD said you need to hang the pully at the end of the arm exactly where the weight would go.

[steffb503]

So if I have a rope from end of arm down to a pulley and up again with a weight hanging from that  I multiply by one or two?
Formulas work better for me..
MA=5
angle from end of arm to bottom pulley is 10 degrees. I have a rope from end of arm down to pulley and up to end of arm.
Using a 10 weight.