Posted by Joe Johnson at 1/29/2001 9:43 PM EST
Engineer on team #47, Chief Delphi, from Pontiac Central High School and Delphi Automotive Systems.
In Reply to: also...
Posted by bill whitley on 1/29/2001 5:59 PM EST:
If you are trying to make a push-pull link make an arm
go more than 150 degrees or so you are really asking
for trouble.
The problem is that the links line up toward "top dead
center." This is not a problem if you have 3, 5 or 7
other pistons keeping the crankshaft going or if you
have inertia keeping the crankshaft going AND (this is
a very big "and"
if the piston is backdriveable
(meaning that the crank can make the piston move not
just the piston can make the crank move).
In your case, you have only one piston, I doubt you can
count on inertia to get you past TDC and worse yet, the
link (probably) cannot be backdriven.
I would not try this.
If you are only going under 150 degrees, you can use
the mechanism you want.
As to effeciency, it depends on the "lead angle," the
goodness of the thrust bearings holding the lead screw
and the slipperiness of the materials. The little lead
screw in the kit (approx 1/4" OD) is pretty good. If
you use ball bearings as your thrust bearings, I
suppose that you can get the efficiency up near 70-80%
For less steep lead angles and less effective thrust
bearings (more typical) I expect that you will have
effeciencies in well under 30% maybe as low as 10%.
To estimate effeciency, use the higher end if you have
a lead angle of 45 degrees or higher, use the middle
range for a lead angle in the 20-30 degree range, use
the 10% if your angle is near or below 10 degrees.
That said, very successful robots use this method.
Juggy from Team #1 has used this mechanism for 2 years
now to great success (They tell me it was their highest
repair item on their machine however).
As to figuring ratios: Simply measure the distance the
nut travels per turns of the screw. You can then
graphically estimate the angle that the driven link
will rotate when the screw rotates 1 turn. This number
will vary during the stroke so you will have to
estimate it at 2 or 3 places along the travel of the
mechanism. The "ratio" of your mechanism (from lead
screw to drive link) will be 360/(angle graphically
determined above). You will have to multiply this by
the efficiency of the lead screw to get the amount of
torque you are imparting on the driven link.
I think you will find that for a good efficiency the
ratios are not going to be too high and vice versa.
BOTTOM LINE:
In my opinion, it is almost always easier to used chain
and sprockets (perhaps 2 or 3 stages of chain and
sprockets) to get better range of motion and more output.
Joe J.