<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0 Transitional//EN"> <HTML><HEAD> <META http-equiv=Content-Type content="text/html; charset=US-ASCII"> <META content="MSHTML 6.00.6000.16414" name=GENERATOR></HEAD> <BODY id=role_body style="FONT-SIZE: 10pt; COLOR: #000000; FONT-FAMILY: Arial" bottomMargin=7 leftMargin=7 topMargin=7 rightMargin=7><FONT id=role_document face=Arial color=#000000 size=2> <DIV> In a message dated 4/27/2007 12:41:05 P.M. Eastern Daylight Time, gnewell@ameritech.net writes: <DIV> <DIV> <BLOCKQUOTE style="PADDING-LEFT: 5px; MARGIN-LEFT: 5px; BORDER-LEFT: blue 2px solid"><FONT style="BACKGROUND-COLOR: transparent" face=Arial color=#000000 size=2>Tom,<BR><X-TAB>        </X-TAB>This is an important post to me. Thank you for taking the time! I'd like to ask if the jerky feel from motor driven hoists is present in air motors too. I understand that there is difficulty with the electric units but I wasn't aware of the same problem with air driven units. Also, what is a "differential" chain hoist? Thanks again!<BR><BR>Greg Newell<BR></FONT></BLOCKQUOTE></DIV> <DIV></DIV>Hi, Greg -</DIV> <DIV> </DIV> <DIV>I have no experience with air-powered hoists.  I could imagine that might be OK if there's a way to bleed air pressure into the system for start-up and if the rest of the mechanical arrangement is friction-free enough that is doesn't require too much build-up of pressure to start the lift.  Remember that pneumatics depend on a very elastic medium - air.  If you go this route, I'm sure we'd all be glad of your feedback.</DIV> <DIV> </DIV> <DIV>Ah, the differential chain hoist.  Let me give you a description and a couple photos.  There is a single continuous length of endless chain (not the separate operating chain and lift chain of the common geared chain hoist).  This chain runs over two wheels on the upper fixed pulley, and one on the lower pulley, thus forming two loops.  One of the loops is the 'load loop' (containing the lower pulley) and the other is the 'operating loop' (no lower pulley).  The upper (fixed) pulley consists of two 'wildcat' wheels cast as a single unit.  (A 'wildcat' is a pulley which has teeth cast in it to mesh with a certain gauge of chain link.)  The two wildcats turn as a unit.  One of them has one more tooth and accommodates two more links of chain than the other.  (Mine is 11/10 I think.)  Thus, pulling the chain on the 'operating loop'  subtracts or adds (depending on whether you're lifting or lowering) two links to the 'load loop'  for each revolution of the wildcat.  Now remember that in lifting a load you must shorten both sides of the 'load loop' in this typical pulley arrangement.  </DIV> <DIV> </DIV> <DIV>So (using the ratio of my hoist) your total mechanical advantage is 20:1.  That's the theoretical advantage; there's some minor loss due to friction.  I have many times used this rig to lift a whole S&S 'D', just under 1,000 lbs.  And I supposed that the pull required was perhaps 70 lbs., about all it's comfortable for me to pull.  Lifting a typical plate, say 350 lbs., probably requires about 20 lbs. of pull.</DIV> <DIV> </DIV> <DIV>Attached is a photo.  It shows the whole setup including the differential hoist.  You might notice that there are actually two parallel I-beams.  The larger one is structural to the building, and I used to hoist from that.  The smaller I-beam, carrying the trolley and hoist in the photo, was added alongside by me.  Its sole purpose is to gain some eight inches of lifting 'head room'.</DIV> <DIV> </DIV> <DIV>I'm sending a second photo (attached to a seperate posting) that shows the teeth on the 'wildcat' pulley.  Notice that the wheel on the left is a little smaller than the one on the right.  That is the 'differential' from which this rig gets its name.</DIV> <DIV> </DIV> <DIV><FONT lang=0 face=Arial size=2 FAMILY="SANSSERIF" PTSIZE="10">~ Tom McNeil ~<BR>Vermont Piano Restorations<BR>VermontPiano.com<BR><BR>346 Camp Street<BR>Barre, VT 05641<BR>(802) 476-7072</FONT></DIV></DIV></FONT><BR><BR><BR><DIV><FONT style="color: black; font: normal 10pt ARIAL, SAN-SERIF;"><HR style="MARGIN-TOP: 10px">See what's free at <A title="http://www.aol.com?ncid=AOLAOF00020000000503" href="http://www.aol.com?ncid=AOLAOF00020000000503" target="_blank">AOL.com</A>. </FONT></DIV></BODY></HTML>