Need engineering help

bones1 · Sep 15, 2006

I need help calculating the correct lift cylinder length for this application.The distance from the two pivot points where the cyl will go is 18"This is where it will have to attach to the upright and the moveable part.
The bar at the end of the pivot, if you measure in a straight line to the end of travel position is 35".
Trying to figure the collapsed and extended length of the cylinder I need.I found one with 22'' collapsed length and 40''extended length(18 stroke)but somehow I can't quite figure this out.Also found a 20'' collapsed and 32'' extended.How would you measure for this?Somehow I think it will come up short.The cylinder pivots at both ends as well.

john_bud · Sep 15, 2006

bones1,

can you please add distances to your drawing? Also, do one for the retracted cylinder and a second drawing for the extended cylinder.

Thanks!
jb

bones1 · Sep 16, 2006

Hi John.Thanks for looking.Some numbers below.

AndyMA · Sep 16, 2006

Bones

There is a fundamental issue with your application. It will work until the upper arm reaches almost full extension (approaches 180 degrees). The applied force to do what you want will start decreasing at 135 degrees. At that point, the cylinder will be able to supply decreasingly smaller amounts of lift since it has an ever shrinking moment arm to work upon. Take a look at an engine hoist and see the limits of its lift ability. If you need what you are picturing, then I suggest you investigate adding an offset to either end of the cylinder from the main axis of the fully extended lift.

Andy

patrick_g · Sep 16, 2006

That plus the cylinder has finite diameter and can't shove the moving part to near vertical as shown. need to stand off the attachment points of the cylinder and probably use gussets to reinforce the cylinder attachment so as to not try to bend the member the cylinder is attached to. I bet if you make a scale model, just 2D, out of cardboard and nail it to a scrap of wood to create pivot points you will see what is required pretty easily. Pat

bones1 · Sep 16, 2006

Points well taken.I will add offsets to the mounting points and see how it goes.I will go with a 22'' closed and 40'' extended unit.Thanks for your input.

MossRoad · Sep 17, 2006

Here's something I found handy while building my backhoe. Make a wooden mock up out of 2x4s and use cardboard tubes that can slide one inside the other. It really helps to visualize things and take some measurements.