This can be a bit of a brain teaser if you haven't thought it through (or if you have ;-) ). > I may be wrong (my name is "wrong way ray" after all) but all pulleys would > move same rate if the rope is same width its entire length. But I agree > still they should not have common shaft. If the rope got rained on and part > was in the sun and part was not, then the rope would not be the same > thickness its entire length because of swelling (wet) or shrinking (dry). > Wrong Way Ray (Raymond Choat) Yes - you are wrong - but I was too at first (no great surprise ;-) ) and it is not fully intuitive. The rope velocity relative to pulleys which support the load - which is what matters here, increases by a quantum amount as it passes through each load bearing pulley !. I was initially disturbed by this apparent breaking of the laws in such a simple machine - clearly I was seeing something from the wrong point of view. And that is exactly what the problem was - the frame of reference changes for different parts of the rope. For our purposes the rope velocity is measured relative to the pulleys as they are what are turning and we wish to establish if they are turning at the same rate. However, the pulleys are moving vertically as well and taking the vector sum of the various support velocities plus the rope velocities makes sense of the system. For a graphic demonstration of the velocity differences - Take a pice of string about 500mm / 18 inches long. - Tie a loop in one end.to make a circle about 40mm / 1.5" in diameter. - Place loop over left index finger. - Hold left hand about 150 mm above a door know or drawer handle etc. - bring string down and around the knob and up again and over left index finger again. - slowly pull down on end of string - Note rate that your left hand or a mark anywhere on the first piece of the string is descending compared to the pulling hand. The right (pulling )hand has exactly twice the velocity of the left hand. While this result is an obvious one, actually seeing the string haveing different velocities in two adjacent parts may be 'interesting". Is that anything like "wrong way Mollison" (I think his name was)(who definitely wasn't going the wrong way). Russell > > ----- Original Message ----- > From: "M. Adam Davis" > To: > Sent: Friday, September 21, 2001 4:45 PM > Subject: Re: [EE]: Pulleys > > > > You should NOT use a pulley block in this manner with a common shaft. > > If you intend to use a block to amplify the force, then the pulley > > closest to the object to be lifted will move very little relative to the > > pulley closest to the pulling force. > > > > -Adam > > > > Russell McMahon wrote: > > > > >>> If the pulleys are fixed to a common shaft, they have to be the > > >>> same > > >>> size. Otherwise, any will work. > > >>> > > >> The pulleys *must* be able to rotate independently anyway. If they > > >> > > >are all > > > > > >> the same diameter than the rotation rates have to be different. > > >> > > > > > > > > >In PRACTICE I'm sure it is very wise to allow each pulley to rotate > > >independently as any slight differences in pulleys or rope will lead to > rope > > >slip or tension changes etc which could cause losses that markedly reduce > > >the efficiency. > > > > > >My mental picture (which may be wrong) suggests that in THEORY, with > equal > > >size pulleys, each group of pulleys can be fixed to a common axle. > > >This is because the rope must maintain a constant tension and velocity > > >throughout and this would occur in these (theoretical) circumstances. > > >With care one could perhaps use a single rotating axle with spacer disks > > >between ropes (Teflon?) although even resisting the side forces which > occur > > >along the axle due to the slight horizontal offset in each rope could be > > >enough to introduce unacceptable losses. The fact that pulley blocks that > I > > >have seen all seem to use multiple independent pulleys suggests that this > is > > >the result of practical experience. > > > > > > > > > > > > Russell > > > > > >-- > > >http://www.piclist.com hint: The PICList is archived three different > > >ways. See http://www.piclist.com/#archives for details. > > > > > > > > > > > > > > > > > > > -- > > http://www.piclist.com hint: The PICList is archived three different > > ways. See http://www.piclist.com/#archives for details. > > > > > > > > -- > http://www.piclist.com#nomail Going offline? Don't AutoReply us! > email listserv@mitvma.mit.edu with SET PICList DIGEST in the body > > > -- http://www.piclist.com#nomail Going offline? Don't AutoReply us! email listserv@mitvma.mit.edu with SET PICList DIGEST in the body