Can any master give me some idea

[Danger]

I'm a bit busy to get into it too much right now, but it certainly is possible to use a rack and pinion with a scissors mechanism.
As for calculations, I have no clue about any of that stuff. If you want to skip down to Mechanical Engineering, though, Timma300 has a thread entitled "Scissor Lift Force" that deals with it.

 

[Danger]

Okay, I'm fueled with pizza and ready to roll.
I can see why you would want an easier way to lower the thing when it's not under load. While the rack and pinion would work, it would be horrendously complicated and possibly even heavier than a motor/battery combination. With the possible exception of an epicyclic gear train, a lead-screw gives the best mechanical advantage for it's size and weight.
A possible compromise comes to mind, though. Perhaps you could use a dual-acting lead-screw as I suggested earlier, but build small locking ratchets into the followers to allow them to "skip" down the threads for retraction. They could be locked in for powered lowering under load.
I like your diagram. Just make sure that the two sets of arms don't lie in the same vertical plane, or they'll be scraping on each other.

 

[sporty]

Finally you are back Danger,thanks a lot.. i need a lot of your help.. so u mean it really complicated if using rack and pinion. I also worried that the gear tooth will wear easily if the load support about 70 to 80 kg. I really have no idea in linear actuator, i wish to know the mechanism and operation inside. and i also don't know how to design the structure if using linear actuator, where to put it.
epicyclic gear train, how is the combination with the lead screw in order to lift up? can you show me some picture or draft? I really wish to use the things where it can reduce our applied force needed to lift a things up.
build a small locking ratchets into the followers to allow them to "skip" down the threads for retraction, it sound nice, but how is this mechanism work? as what i know the follower also have the thread, how it can skip it? Please explain to me more. Really need your help. Thanks a lot.
Forgive me for bring a lot of trouble to you.
For timma300 the site he show i went in before, but due to the problem i face is my scissor linkage is not equally length so it is not symmetric, and 2 pinned linkage have different degree of tilt, so ,i feel a bit crazy because i can't get any calculation on any site. What should i do. I am too stress.

 

[Danger]

I'm pretty inebriated right now, having just returned from a pool match, but I'll try to sort things out in a limited fashion before I pass out.
First off, I suggested an epicyclic gear train in lieu of a lead-screw, rather than in conjunction with one. That would give some sort of advantage if using a rack and pinion approach.
The complications involved in using a rack and pinion are incredibly formidable.
As for the "skipping" followers... traditionally, most followers do indeed have threads that match the screw drive rod. I'm going to say this in the simplest way that I can, to try to compensate for our language differences. Think, if you will, of a solid piece of pipe that just barely fits over the screw rod and can slide up and down it. Now, drill and tap a hole in the side of that pipe. Screw in a bolt with a rounded end, until it stops just short of the interior diameter of the pipe. The whole thing can still slide up and down on that screw rod. Now, when you want to engage, you just twist the bolt a couple of times until the rounded end catches in the threads of the screw rod. That whole pipe is then a follower.
A linear actuator is any device that provides a straight-line force in any particular direction. As it is normally considered, as has been done in this thread, it consists usually of a simple lead-screw contained within a cylindrical metal housing.
I really have to bag some ZZZ's now, but I'll try to get back to you within a couple of days.

And by the bye... chill a bit. Stress does no good for anyone, and is counter-productive when you are actually trying to achieve something. Think like a Vulcan.

 

[sporty]

thanks a lot t oyour reply danger, you make me more understand on skipping follower, but is it too troublsome for a person to screw the bolt to follow and unsrew it frequently? or is it have other mechanism to substitude the screw bolt to lock the thread?
Now i am more understand to the linear actuator, actually i need the mechanism for analysis for my project, my project is to design a mechanism for analysis so if i using actuator, it is only one things and no mechanism to analysis in my report.I propose wheelchair seat lift to my lecture because i saw a lot of leg broken patient, aging people like my grandma in my home , i wish to help them too beside can finish my project.
Before this, i bought a ratchet with rack and pinion mechanism, it is a device use to hold tightly the line to tighten the box. so i thinking to modified it by put the rack to a rectagular hollow bar and fixed the bar on side of wheelchair fixed also the pinion ratchet too. and that rectangular bar i open a slot at one of its side to pin the linkage with rack. so they can move together to lift a human. but i am not sure is it can work for my mechanism . with help in reduce the applied force by patient.

and my linkage also got problem, because it is unsymetric scissor linkage. different length for the leg. and one of the linkage is both end moveable. So i don't know wheather it will work ?
[PLAIN]http://www.fileupyours.com/view/295883/example1.bmp. Further Question is in my picture.
you are very kind danger. thanks for your advice. and your helpfulness.

 

[Danger]

you are very kind danger. thanks for your advice. and your helpfulness.

It's not so much that I'm kind, as it is that I love an opportunity to both assist someone and exercise my mind at the same time. And believe me, most of my ideas are way off base. I have no education, so everything that I come up with is something that I would do with available tools and supplies. Anything that I've ever built was based upon car parts, typewriters (my favourite), VCR's... I believe that there was part of a goat involved in the last one...
Anyhow, back to the chase...
Rather than using the set-screw situation that I mentioned originally, the main thought was to utilize an over-centre cam with the "follower" built into it. I mentioned the set-screw only to illustrate the point about not needing full thread engagement.
I really am too burned out right now to post any more, but I'll get back to you tomorrow.

 

[sporty]

thanks a lot Danger, have a nice dreams..

 

[Danger]

I haven't given up on you, pal... but have also not yet slept. I promise that I will get back to you as soon as I am able. I don't even feel tired, but I know that I am impaired by endorphins and lactic acid. My cozy little bed is very attractive to me right now. I will get back to you.

 

[Danger]

Jeez, but it's been a while. Sorry... I got sidetracked a lot. Still, I've been thinking on this quite a bit. It suddenly struck me a couple of weeks ago that there might be some way to accomplish the design using roller chains, but I haven't figured it out yet.

 

[Know_seeker]

i think that u should consider using a mechatronics system for ur purpose. i think this is what is meant by linear actuator in the previous post. To implement this u will need some type of a controller , a electrical motor(actuator) there (are DC Motors , Stepper Motors , Servomotors , u can choose among them depending on their price ,torque, and wether u accuracy is important in ur system , i think for ur application a stepper motor is suitable ), and u will need a switch to activate the system.
now a electrical motor usually outputs a rotational movement so to convert it to linear movement u could use rack and pinion (this is only an idea ).
for controller u could check arduino microprocessors because it is easily to deal with and they provide Pulse Width Modulation signal for the motor.
i hope that i helped