Scissor Lift Mechanism
Scissor Lift Mechanism
(OP)
Hello,
I am designing a scissor lift mechanism with one arm 3 times the length of the other arms, with the remaining arms being symmetric. The mechanism is loaded at the end of the long arm. I am trying to determine what would be the force acting upwards at point A but it's proving to be a bit beyond my capabilities. Any help would be appreciated.
Many thanks
I am designing a scissor lift mechanism with one arm 3 times the length of the other arms, with the remaining arms being symmetric. The mechanism is loaded at the end of the long arm. I am trying to determine what would be the force acting upwards at point A but it's proving to be a bit beyond my capabilities. Any help would be appreciated.
Many thanks





RE: Scissor Lift Mechanism
RE: Scissor Lift Mechanism
RE: Scissor Lift Mechanism
RE: Scissor Lift Mechanism
RE: Scissor Lift Mechanism
RE: Scissor Lift Mechanism
For instance: if W moves 3 mm for every 1 mm of movement of A, your mechanical advantage is 1:3, and it will take 3N of force to hold 1N of weight.
The mechanical advantage will change as the mechanism extends and collapses. Take measurements at multiple heights and make a graph (in Excel?) to see the effect of height on mechanical advantage.
p.s. Yes, I can do the math. However, this method seems to work to help people understand the net results.
Looks like you would have to pull down at A to keep the mechanism from collapsin under its own weight.
RE: Scissor Lift Mechanism
To make it work, remove F and instead put a horizontal cylinder or jack screw between the first two end pivots.
Look at you car jack to see it.
RE: Scissor Lift Mechanism
RE: Scissor Lift Mechanism
it's "easy" to calc the external reactions with two applied loads, W and F ... sum moments at one support (which will tell you the reaction at the other support), then sum Fy.
but this isn't going to answer the question "what's F ?" the force required to balance W. for this i'd use energy/work ... the work done by F (F*dy) is equal to the work done by the weight (W*Dy). which now i read tick's post is what he's done for you ... but i think he's been beguiled by your sketch ... i think the mechanical advantage is 2:1, but a rough "finger" measurement of your sketch suggests that the 3L should be dimensioned to the joint ?? (which would make it 3:1)
to analyze the structure, the links are two force members (axial load only) and three force members (the three forces intersect at a point). so the top short link is a two force member, so you know the direction of the force (axial). the top long link is a three force member, you know the direction of two forces, so you know the direction of the 3rd force. and so on
Quando Omni Flunkus Moritati
RE: Scissor Lift Mechanism
am i right in thinking that you can delete the top diamond, or add another diamond above the existing one, without significantly changing the problem ??
Quando Omni Flunkus Moritati
RE: Scissor Lift Mechanism
RE: Scissor Lift Mechanism
Quando Omni Flunkus Moritati
RE: Scissor Lift Mechanism
You are right, never thought one would design a scissor mechanism with
negative mechanical advantage, since you need 5X the weight to lift it.
This is a first.
Rb,
Kinematics
You do it this way, nothing fancy
Geometry
Y1=5Lsin@ at the weight
y2=Lsin@ at the bottom pull point
dY1/dY2=5
F1dY1=F2dY2
F2/F!=5
Mechanical advantage 1/5
RE: Scissor Lift Mechanism
RE: Scissor Lift Mechanism
Cheers
RE: Scissor Lift Mechanism
The solution was simple: find the net mechanical advantage. The designer would sketch the framework in SolidWorks. Then, a macro in a spreadsheet would take the mechanism through various positions, "wiggle" it, and measure and graph the resulting mechanical advantage at each position.
Usually, the input was the length of a line representing a linear actuator, and the output was the net height of the system center of gravity.
The results of this method were great. Reality met calculations, the way they ought.
RE: Scissor Lift Mechanism
Thanks for sharing this analyzing method. Very nice. "a macro in a spreadsheet"... A question I have though: Did the macro take the mechanism sketch automatically through various positions? Or was it necessary for the designer to start up the macro again and again per each sketch step? Did you make a table in the spreadsheet first for the various positions that you want to know and then let the macro step through the table?
RE: Scissor Lift Mechanism