Forces in scissar lift
Forces in scissar lift
(OP)

Hi
I need help, how can i calculate force in screw and in rods ?
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RE: Forces in scissar lift
“Do not worry about your problems with mathematics, I assure you mine are far greater.” Albert Einstein
RE: Forces in scissar lift
I think direction of Rb force is correct but Ra is not. Only 2 unkown forces Ra , Rb so i didnt use sumM=0. I think im doing huge mistake.
RE: Forces in scissar lift
Where are the dimensions of the scissor lift ? There are several ways to do this but you need some dimensions and angles to work with, is this a student post?
“Do not worry about your problems with mathematics, I assure you mine are far greater.” Albert Einstein
RE: Forces in scissar lift
Cheers
Greg Locock
New here? Try reading these, they might help FAQ731-376: Eng-Tips.com Forum Policies http://eng-tips.com/market.cfm?
RE: Forces in scissar lift
============
"Is it the only lesson of history that mankind is unteachable?"
--Winston S. Churchill
RE: Forces in scissar lift
Cheers
Greg Locock
New here? Try reading these, they might help FAQ731-376: Eng-Tips.com Forum Policies http://eng-tips.com/market.cfm?
RE: Forces in scissar lift
Force in screw depend on angel alfa, Qmax is when alfa=25deg, for alfa>25deg Q=const=Qmax. How can i calculate forces in screw and arms correct ? This structure work as lattice i think.
RE: Forces in scissar lift
“Do not worry about your problems with mathematics, I assure you mine are far greater.” Albert Einstein
RE: Forces in scissar lift
This is still bad because i got 2 unkown forces Ra and Rb and 3 equation (x=0 y=0 M=0).
RE: Forces in scissar lift
“Do not worry about your problems with mathematics, I assure you mine are far greater.” Albert Einstein
RE: Forces in scissar lift
Note that 169/tan25 = 362.4, which I rounded to 362.
BA
RE: Forces in scissar lift
What is the force required to crack the nut? It is a vertical force caused by a beam hinged at the left with a 227mm cantilever and a 135mm back span. Say that force is F. The 'nut' in our case is a diamond shaped frame of length 270mm and height 270*tan25o. Two corners are tied together by a horizontal member to prevent them from spreading.
You should be able to calculate all of the forces using statics.
BA
RE: Forces in scissar lift
I didn't go through your calculation sheet, but at a glance, it appears far too complicated. Let E-T know if we can help.
BA
RE: Forces in scissar lift
Thanks for the explanation. However, is friction in the foot pin enough before engaging the load and is it enough if the load and foot are not precisely aligned? I have only used symmetrical scissor jacks. This type doesn't give me a lot of confidence.
Fred
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"Is it the only lesson of history that mankind is unteachable?"
--Winston S. Churchill
RE: Forces in scissar lift
I'd say you were correct the first time. The structure is unstable as shown. It needs more support to prevent it from rotating about the bottom pin. That is why I added a roller support at the left end in my model.
Friction should not be relied upon to prevent rotation. The "foot" on the ground can't help because it's below the pin. A socket joint at the top could help, provided the vehicle being jacked is capable of providing adequate lateral support. That is not specifically stated, and should not be relied upon.
I share your lack of confidence in the jack as drawn.
BA
RE: Forces in scissar lift
volvo
Cheers
Greg Locock
New here? Try reading these, they might help FAQ731-376: Eng-Tips.com Forum Policies http://eng-tips.com/market.cfm?
RE: Forces in scissar lift
The models shown below are much better, although I wouldn't trust any of them on a steep gradient.
The main difference between your model and these is the fact there are two pins instead of one, above the base plate providing a degree of stability. The assembly could collapse in an orthogonal direction too. For that, there is the width of the base to provide some stability.
BA
RE: Forces in scissar lift
Cheers
Greg Locock
New here? Try reading these, they might help FAQ731-376: Eng-Tips.com Forum Policies http://eng-tips.com/market.cfm?
RE: Forces in scissar lift
So it relies on the wheels on the other side of the car being locked against rotation. Otherwise, it is not stabilized. I agree it isn't ideal.
BA
RE: Forces in scissar lift
To stick as close to reality as possible, the rolling support plane should be an arc so the support moves to the right as the jack is raised. But that still shouldn't change the forces seen by the members in the jack.
RE: Forces in scissar lift
Even the full scissor jacks you posted rely almost completely on the car being (relatively) rigid and at least two of its wheels remaining in contact with the ground for stability under load. The only real benefit of the stability added by their symmetric design and rectangular base is so you don't have to hold it up with one hand while you try to get it engaged with the car and the ground. In an unconstrained load situation, I wouldn't trust a scissor jack with a cinderblock.
RE: Forces in scissar lift
One extra roller support is needed somewhere to make the structure stable. It carries no load provided the upper pin is directly above the lower pin. It could be in the position shown and, as noted by by LionelHutz, does not change the internal forces or moments in the members.
Almost completely is not completely, handleman. The full scissor jacks I posted do not rely on wheels remaining in contact with the ground or the rigidity of the car. The applied load could be a sack of mashed potatoes; as long as the centre of gravity of the load remains inside the rectangular base plate, the structure is stable.
However, a slight lateral force could topple the scissor jacks if the resultant of the applied forces lies outside the edge of the base plate. If I were planning to work under the car, I too would want the additional support of some wood blocking or possibly a concrete block (not sure I would trust a cinder block).
The discussion of stability of the structure, while important, was probably not the concern of the OP.
BA
RE: Forces in scissar lift
The vertical roller support I have shown in my latest diagram prevents horizontal movement but permits vertical movement. Nothing idiotic about it.
BA
RE: Forces in scissar lift
Sorry.... Your first roller support was idiotic. Your second is a graphical representation of how this device actually works. I.E. this jack mechanism relies on external guidance of its load.
RE: Forces in scissar lift
The provision of outside supports to laterally stabilize the load is a wise precaution with any jack, on that we agree. However, the bases in the photos below are not pins, and they do offer some stability, albeit small. They are decidedly better than a pinned base.
Once again, you are mistaken, handleman. (No, you are not!!) You seem to be batting 1000 today. The horizontal roller in the first diagram is just as effective as the vertical roller in the most recent diagram. In both cases, they prevent rotation of the assembly about the lower pin.
EDIT: The horizontal roller in the first diagram prevents Node A from moving up or down, so it was a mistake.
Again, while very important to make the structure stable, none of this responds to the OP's question, which addressed forces in the rods, an exercise in elementary statics.
EDIT: My apologies, handleman.
BA
RE: Forces in scissar lift
RE: Forces in scissar lift