Ive bin Googling for the past couple days trying to find some info on
construction details of parallel grippers but i havent found much
other than sites that sell them. In paricular im looking for info on
parallel grippers with independant "finger" drive/movement. Im under
the assumption that they use some sort of oppositely threaded
screwgear in which half of the gear is threaded opposite to the other,
or am i completely out of the ballpark with this assumption. Any ideas
Yes that is it.
You use two smooth rods for shafts to stabilize the two fingers.
The third rod is threaded 1/2 left hand and 1/2 right hand. Each finger is
threaded appropriately to match of course.
A gear motor spins the threaded rod and it then causes the fingers to move
in or out.
Next some hall effect sensors and or limit switches are used to avoid going
too far one way or the other.
You can mount some micro switches or a flex strip on the fingers to
determine when to stop the fingers as they move inwards too.
Depending on how strong you need it to be determines the size and weight on
You could probably use two same threaded rods with a reversing gear or two
motors running in opposite directions
to the same effect, but it doesn't seem to look as neat though.
( note: pardon my bad english, not my native language)
For low-tech solution, you dont need ball-screw type mechanism. If you have
a chance, rip apart a broken CDROM drive. You can find "linear gear" ( ? )
from inside the CDROM opening mehanism, use two of them on the opposite
sides of gearhead, and youre good to go. My attempt at ASCII art:
O * O
where the * is motor shaft with gear, "\/\/\/\/" is "linear gear rod", and
"O"-s are gripper fingers.
This way you get parallel "synchronous gripper". It has the advantage ( or,
in some cases disadvantage ) of automatically centering the parts that you
If you want asynchronous, with independent fingers, you could use two
different motors somewhat like this
The suitable motors could be small steppers, like found in 3,5" floppy
drives. BtTW, those come with ball-screw type mechanisms, easiest thing to
try if you wanna try hand at independently moving fingers.
H is the stepper motor scavenged from floppy drive, --- is the screw, | is
the "bearing" and O-s are the fingers
Industrial automation often uses pneumatic grippers, with small cylinders to
actuate the fingers, but thats good only if you have one fixed type of part
to pick up. Sometimes, grippers with "linear servos" are used, those have
the serious disadvantage of having low torque, plus they were quite bulky
( last time i had anything to do with such )
Actually I had this same thought too.
I took apart a couple of old CD rom drives to get at the screw drive
But I needed to get a couple of small gear motors to drive the screw shaft,
such as those from www.solarbotics.com
I then wound up going the simpler singe screw drive rod method, but I might
still build it up using the dual rod method.
I got the left hand taps and dies from
there was someplace that actually already had prethreaded brass and steel
rods in 4-40 6-32 8-32 and 10-32 sizes
but now I can't seem to locate them anymore. But http://www.mscdirect.com
does show some to check out.
I took a plain brass rod and threaded both ends towards the center with LH
and RH dies. But the alternate was to use a short threaded tube in the
center to connect the two rods together.
I used brass/bronze inserts threaded for the grippers and a couple of
threaded short tubes as bushings on the ends of the rods.
One end has a small gear motor coupled to the threaded rod one one end.
I made my gripper arms one peice full length, I don't know why I never
thought about having removeable finger ends.
Mmm... i was questioning that myself, but would we still require a
left-hand threaded nut for half of the rod ? Isnt half of the rod
threaded lefthanded while the other is righthanded ? Im treading new
You've gotten some ideas for using jackscrew-type mechanisms, and those
are fine (but can be slow).
Anyway, to throw out a different variation: parallelism of a gripper can
be achieved using two pivoting rails for each finger. An example of this
is on page 411-412 of my book Robot Builder's Bonanza, Second Ed. The
illustration shows rather small pads for the grippers, but the "grip"
can be made longer.
A single servo motor, with a double-arm horn, is used to open and close
the gripper. One end of the servo arm goes to the left rails, and the
other end of the servoarm goes to the right rails. Use ordinary aircraft
servo linkage to connect the servo arm to the rails. For maximum
leverage and least binding-up, connect the linkages only to the outer
rails on each side. Let the inner rails be used just to keep the gripper
With this approach there's no need to buy anything special, and there's
nothing to machine. It's all at the hobby and hardware store. Select a
servo with the torque you need. GWS makes servos with a 110 oz-in
torque, for example. A standard Futaba S-148 or S3004 will have over 45
oz-in at six volts. Given the right size of gripper your robot can
readily lift a 12 oz can of soda/beer.
Author: Constructing Robot Bases (Forthcoming)
Robot Builder's Sourcebook, Robot Builder's Bonanza
I just remembered where I saw prethreaded left and right hand threaded rods.
www.smallparts.com has them.
I was looking for something else when I saw them there.
of course you still need to make some threaded nuts or something. But they
might have those too.
You can make a simple joiner for the center section, heck maybe a tube
threaded and a brace for the center
would make it even more stable too.
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