Grabs are fun. Here is a simple clamshell grab which uses the scoops from, e.g.,
42023 Construction Crew, or 8069 Backhoe Loader. It can be operated manually, or by a motor.
Here is a demonstration of the motorized version:
To build it, first make two scoops. They are identical, so lay out two times these parts:
I chose some coloured parts, just to make it easier to read the instructions. You should of course use parts in whatever colour you want.
Put the first tan 3-axle with stop into the cross-axle hole of the 2-length grey liftarm, and through the connectors of the scoop:
Having made both scoops in an identical way, bring out some more parts:
Connect the scoops offset by half a unit length:
Perhaps you dislike offsetting the scoops, but note that their teeth mesh better that way:
The 9-length yellow beams are for connecting the scoop to the crane, or whatever you are building:
Use the dark grey 4-axles to connect the yellow beams:
Get some more parts, note that you need three black 4-axles, not one:
Use the dark grey 4-axles to attach the black connectors to the yellow beams:
The grey 8t gear will be driven by the worm gear, eventually.
More connectors are needed to hold the worm gear:
Finally, the worm gear goes in with the 5-axle, and you're done!
If you want to motorize it, use this as a hint:
Another view of the motor driven mechanism:
Here is what the scoops look like when using less colourful pieces: