Calculations for Scissor Lift

In a case like this, a drawing would be most helpful. Each member of the scissors assembly is basically a simple beam supported at each end, and with a concentrated load at some point in between (commonly but not necessarily the midpoint). In addition to the loads imposed by the geometry, there will be friction and acceleration to consider, as well as safety factor. Also it will be suitable to build a prototype and actually test it to confirm its lifting capability.

Other issues include the closeness of fit at the joints, which affects how much the structure might sway when extended. Be aware also of operation on sloped surfaces or while moving; in either case the possibility of tip-over must be addressed. There are various strategies for such questions: speed or height or slope restrictions, counterweights, outriggers, etc.

The many variables make it impossible to give a simple answer, so be careful to consider as many such scenarios as possible. And good luck with your project.

The Website mentioned on Charlotte's original post shows a large (and rather elegant) scissors lift. Some comments about the calculations given:

1. Be aware that the angle Phi must be measured when the lift is at its lowest position.

2. The sketch shows a horizontal actuator, but the actual picture shows what appear to be two vertical hydraulic cylinders. The horizontal actuator is very suboptimal at the beginning of the lift. The actual device is better.

3. Only the Wa/2 part needs to be multiplied by the number of stages; the W part does not change.

4. The given formulas do not take into account friction and acceleration of the load, though they I think are correct as to the basic geometry.

There are other factors to consider in a design of this type. If this is a 1 of a kind it would be ok to design with a standard stress calculation and then over design the members to over compensate for the lack of precision in the calculation. In this case the cost would not be that high for the one piece. Keep in mind serious injury and even death are dependent on being correct. If this is a production product cost will be a factor. In that case someone who is versed in structural calculation should be consulted. This way the design would be strong enough and economical enough to market. I have not looked at the formula you were using, however I can tell you some of these are oversimplified. If that is the case they should only be used as an approximation.

Thanks a lot for replying. I'll try to be as carefull as possible. Luckily, this isn't real-life project, it's my final project before graduation so there'll be no harm to any real person :), but as a future engineer I tend to do the best possible 'job' I can :) And of course, your advices will be helpful.

Luckily, this isn't real-life project, it's my final project before graduation so there'll be no harm to any real person :), but as a future engineer I tend to do the best possible 'job' I can

It's not unheard this type project is found useful in real life too, please do consider integrity an ideal of merit in any endeavor.

I suggest you change your mind about engineering and study law or some other profession that will provide you with some job security. The best thing to come out of my engineering experience was that I did not let my daughter make the same mistake. She is a hospital aminstrator, while not perfect, is certaily more secure than engineering. Good Luck

Oh isn't your daughter one lucky girl to have such a father who believes in her! You know, I find your suggestion a bit insulting, especially cause I wrote before that I'm ending my faculty education, so - it would be a bit late for that even if I wanted to - and I don't by no means. If you know what my final project consists of, you would be aware (as am I) that it's impossible to do it by one person in a 2-month-period, especially perfectly cause I don't have enough time or knowledge or proper help for that. But that's another story. I think that being student of engineering/engineer means to be prepared for lifetime learning - which I'm trying to do right here right now by collecting as much as information and advices I can so I can do the best possible project. It's just a shame that some parts of my 'research' depend on some not-so-kind people.

Not wanting to bring kerosene to put out a fire but I do not think the words were unkind. Opinions and advice are just that. The advice given was good enough for their own family and they thought it was then good enough for you. So, I guess it could be said that you have been treated like family. Personally, I regret not going into formal engineering training. In the end the decision is for each of us to make for ourselves. We have all made choices that if given a chance we would do differently. And let's face it, this is a hard time for manufacturing in the USA. It is effecting everyone, not just engineers.

If you are in engineering be prepared for life long learning, no if's or but's about it. I'm on forums, reading text books, talking to engineers on a daily basics trying to optimize designs.

No body knows everything, even though they think they do. Oh and by the way making mistakes is part of the learning process.