Packbot Flipper Design

I am in the middle of building a working replica of an irobot Packbot and am trying to work out how to build the flippers. My problem is that I am unsure how to tie in the treads to the ones on the main body without running a motor to the flippers. Also, I am trying to get them to flip just like the original Packbot. Any help or ideas would be great! Thanks.


I would also not like to cut or drill holes through anything. Keeps all the parts in good working shape and adds to the challenge :slight_smile:

You will need a coaxial mechanism for the front hubs. The central axle will handle the angle of the flippers, and the outer axle will be for the tracks.

Coaxial means the two axis are aligned with each other, but have independent motion. A good example of this is the hands of a clock. The hour-hand is mounted to the outermost hollow axle, and the minute-hand is coupled to an axle inside of that.

I know of two ways to do a co-axial mechanism with Vex:


*]Use the new turntables for the outer axle, and run a standard Vex axle through the center of that for the flippers. You would have to bolt HS sprockets to the turntable to engage the tank tracks, and you would have to drill-out the square axle hole on them so that the flipper axle can run through the center of them.

*]Use the same basic design as the swerve drive kit instead of the turntables. This is the same basic design as above, just using a different part for the hollow axle.

It is a shame the HS sprockets don’t use the same 1/4" hub inserts that the HS gears use. Then you could just pop in an idler insert and not have to drill anything out.

One other option that would not resemble a PackBot as much: Relocate the flippers so they are between the robot chassis and the main drive tracks. If you do that, the central shaft becomes the track drive shaft and so you wouldn’t need to drill out the sprockets. The turntable then becomes the axle which you bolt the flipper structure to. You might try experimenting with this design first since I think you can do it without modding parts.

Hope this helps,

  • Dean

Thanks so much! I was considering using the new ball bearing kits as well. for the bearings, is it the larger one or the smaller one that you recommend? I had an idea that would give it most of its range of motion (over 300 degrees) but would require a lot of gearing. Your ideas sound much simpler. Also if i am looking right, the swerve drive kit’s gears do not have the square shaft in them thus doing what I want. Thanks.

I had the smaller one in mind - it should nicely join to several sprocket sizes, and it will provide support for the central axle. But I suppose it depends on the scale of the model you are making - the large one could work too.

Right, though you still have to drill out the sprockets since the flipper axle needs to route through the center of the sprockets (unless you are relocating the flippers inward as I mentioned).


  • Dean

Thanks, I went ahead and ordered a set of high strength gears because they allow for circular inserts thus allowing the 2 drive shaft to run thorough the center bypassing the gear. I am planning on bolting the largest gear to the flipper arm and then placing the gear with the circular fittings on the drive axle and securing it with a collar. Then i’m going to use a set of gears on the side of the bot to control the flipping motion. Hopefully this will work. The logic sound correct to me. I’ll be sure to post some pictures of the build when they come in. Thanks again for your help.

What you are describing should fit together, but how do you plan to attach the tank tracks? They need HS sprockets to mesh with - they won’t fit onto the HS gears.

  • Dean

I am planning on attaching one of the sprockets that come with the tank tread kit tothe end of the rod. Like I said my idea sounds good but I do not know if it will actually work

Oh, I think I see what you are doing, with the “side gears” driving the flipper angle and preventing full 360° rotation of the flippers.

Seems like a solid plan - make sure to post pics!


  • Dean

actually with the high torque gears i just got the movement should be endless. The less than 360 was a previous theory that would have worked if i had more space to do so.

Makes sense - you can put the HS gear between the two track sprockets and drive it from a side gear that doesn’t interfere with the flipper rotation.

I think the biggest risk will be the amount of flex in the axles since the flippers will experience significant forces. If you have room, you should run a strap between the side gear axle and the main drive axle to ensure the gears stay meshed.


  • Dean