[Gary Dillard]

Shayne:

Before giving you the short answer I'd like to discuss and explain "4-bars" a little bit here if you'll indulge me - someone asked what one was in another thread and I thought it more appropriate to answer here and refer him to this post. I searched and didn't find another thread that explains it - if anyone finds a better source please post it.

Generally speaking, a 4-bar mechanism is one where an input rotation or translation of one joint of a link or "bar" in an assembly results in a unique displacement path of some point fixed to another link or "bar". If you have two links pinned to your chassis or "ground" at separate points and pinned together at one point, you have a 3-bar; the ground between the 2 links counts as one bar and each link counts as one bar. The 3-bar has only one position and it is fixed. If you separate the 2 links and pin them to an additional link, you now have a 4-bar. For any rotational position of the first link, there is a unique position and orientation of the other 2 links (and therefore of any point on them). This is a traditional 4-bar, where the input angle is determined by the position of a driving motor on the input axis.

There are other mechanisms that are technically 4-bars but not usually referred to that way. If instead of adding an additional link to the 3-bar mechanism you made one of the links a linear actuator, you would have a slider crank (or crank slider since it's really the input) The slider itself is considered a bar (or was when I was in college ages ago).

If you attached an actuator to the traditional 4-bar as an input, you would actually have a six bar (there is only 1 bar for ground for all three fixed points), but this is really just an extension of a 4-bar; note that any additional 2 links (or link and slider) added to the stack will still only have one unique trajectory for a given input. Thus a scissors is really just a stack of 4-bars. Also note that a scissors doesn't have to move in a veritcal or linear direction; our lift last year actually started out facing away from the bar and then arced toward the bar.

Now to your questions....
I have a very smart friend who once told me "sliders are for people who couldn't figure out how to make a mechanism work". Usually it becomes a matter of packaging though; rotating joints are generally lower loss because you can use bearings rather than rolling or sliding surfaces, and you can concentrate the structure around the load which is always at the same point. However, there's no doubt that a good telescope or lift can be very compact for the same amount of displacement as a large mechanism. I've seen both used to great success, but probably equally as often have seen problems with both because of poor engineering (ourselves included). I like to think I get smarter every year at this so here's my biggest couple of pointers in working out the design:

1) Don't overlook out-of-plane loads. Mechanisms look great 2-dimensionally but don't behave that way. Dynamics of moving can put lots of torque and bending in directions that your mechanism isn't designed to accommodate.

2) Structures and mechanisms are frequently mutually exclusive. If you use small pins and bearings relative to the structure stiffness, every joint is a loss of stiff load path. This is the reason scissors are usually a poor choice - there's no good load path out of plane. Use big bearings and joints.

You asked the question about pneumatic loads, and you need to look at it from a different perspective with mechanisms - it's called virtual work. If you are lifting 10 pounds 8 feet you are doing 80 foot pounds of work. If your actuator extends 1 foot in order to do that, you have a mechanical advantage of 1/8, so it will take 10/(1/8) or 80 pounds of force for that foot. You can sum the component weights individually for this - how far does the tetra move, how far does the center of the first link move, etc. Here's where counter weighting and springs come into effect - the weight moving down is doing work, the tension spring retracting is doing work - those help based on their mechanical advantage.

It sounds from your discussion that it's just a simple slider crank, with the actuator attached to the lifting shaft. In this case the mechanical advantage is just the ratio of the lever arms as you said. However, consider using mechanical advantage to your advantage. When the arm is horizontal you have the highest load from the moment, and your vertical displacement per angle of input is the highest so your required work/force is very high. Try adding an additional linkage such that when the actuator extends, the output arm displacement speed is inverted - slowly at the side and quickly at the top when the moment is lower. It effectively averages the load requirement and lets you use a smaller actuator to get the same amount of work.

I hope this helps; good luck.