Four-Bar-Lifter Dynamics

[nick]

Ewan, the rear arm length c is important even for initial lifting force calculations whenever b is not equal to b1, because even though the top bar starts horizontal its angle it is not generally fixed.

The formula for the coefficient of lifting force with respect to torque for a frictionless 4-bar linkage infinitesimally above the fully flat position is

(a+(a-c*((a*e/(c*d)-a*e*(d2+c2-b12)/(2*c*d3))1/2-(-a/d+a*(e2+d2-a2)/(2*d3))1/2))*(b-b1)/b1)-1

where d = b1 - c and e = a + b1 - c = base length

This equation does not give valid output when d is exactly equal to zero or if b1 = 0.

One obvious point of conflict with your formula is that when the front and rear bars are of equal length lifting force is *independent* of both b and b1. With your example if the rear bar is one third the length of the front bar the lifting force before friction is now 29kgf...

Anyone ever thought of a wedge shaped 4-bar-lifter?

something like this:

http://members.lycos.nl/rwnl3/technical/td/wapen-schets.jpghttp://members.lycos.nl/rwnl3/techni...pen-schets.jpg

[armadrillo]

This may be a dumb question but:

Does a 4 bar lifter have to powered by pneumatics?

No it can be powered by anything that can rotate either the front or rear arm, pneumatics are really one of the least popular ways of powering a 4-bar lifter at the moment the more common ones are by a geared down motor or linear actuator.

Chris - ^_^

storm2 is a perfect eample of an electrically powered 4bar lifter. A motor with a worm gear on the end, directly onto the front bar (i think thats how it is anyway)

[nick]

If only it were that simple!
Theres a spur gear stage before *and* after the worm!

Nicks right - Storm IIs lifter is a little more complex than that.

Storm II uses an S28 Magmotor which drives a large worm through a pair EN36 of spur gears to reduce the speed. That worm in turn drives an aluminium-bronze wormwheel which is coupled to a MOD3 small gear which drives the final output quadrant gear to which the front lever arms are attached.

The front axle which carries the lifter arms also drives a 10k pot through a pair of crossed helical gears. The potentiometer provides positional feedback to the speed controller (like a giant servo) and can either be configured to provide end-stop positions or a fully positional servo mode).

Were currently upgrading the arm from an S28-150 to an S28-400 motor and the strength of the gears to prevent them from breaking as weve recently suffered a couple of failures.

The arm moves from its fully down to fully up position in around 0.3 seconds, a speed which will be improved on with the S28-400 which can provide more power, and thus for the same weight we can get more speed and thus thrust.

You can see more details of the arm here http://www.teamstorm.com/heavies/storm2/series7/weaparmour/index.htmhttp://www.teamstorm.com/heavies/sto...mour/index.htm on our website.

Ed
http://www.teamstorm.comhttp://www.teamstorm.com

[ewan]

Re Nick:

I can see that you have compiled a much more accurate equation - which I would no doubt advise those fabricating more costly and complicated systems to use - but my guide so far was much created to provide an easily understandable rough calculation means for the practicle application of four-bar lifters in most given instances. Of course, as you have found, it may not be suitable for some cases.

My second much more complicated equation should give a solution to the problem that you have encountered with my equation. I hope that I can construct an online calculator which will allow people to enter the figures required without any knowledge of the equations behind them to solve their 4-bar requirements. I dont think I shall get round to this until Summer or thereabouts though as Im piled under far too much work at the moment and for much of the forseeable future.

Ewan

[ewan]

Look like someones beaten me to the calculator :wink:

http://www.hyperdynamic.co.uk/4bar.xlshttp://www.hyperdynamic.co.uk/4bar.xls

Way to go mark mellors, brilliant, I shall save a copy

(Message edited by ewan on April 17, 2006)

[aliboy]

Here is IO ( now Wazio owned by Chad New ) It makes good use of the 4 bar lifter in a 12 pound robot
http://www.teamcosmos.com/io/build.shtml
http://www.teamcosmos.com/io/1.5/gallery.shtml

(Message edited by aliboy on April 17, 2006)