Hey guys, I’m new to the Vex system and been experimenting a bit with the tank tread drive. I don’t have the Cortex micro-controller to test my build, so I’ll probably hook up an Arduino to it next week.
I’m more interested in speed, but want to use only 1 motor 393 on each side (possibly using the high speed configuration). I didn’t really think much about the gear ratio to be honest and was just working with the space I had. The length of the robot is not going to change because it’s a desirable length for me. I did think about compound gearing with a possible combination of a chain system to drive the front sprocket, but couldn’t think of a way to fit it in without increasing the width of the drive system (which is something that I don’t want to do). Some photos below. I accidentally installed the track upside down, so ignore that Also, ignore the other side.
What do you think of the system? What do you feel I’m doing wrong? Am I putting too much load on the motor? Is my gearing good? Is the contact area between the track and the ground too little (it’s tilted a bit in the pic)? I welcome criticism.
Also, if someone has the time and resource to build this and test it for me, that would be great. I used:
1 x 60 tooth gear
2 x 36 tooth gears
2 x 12 tooth gears
1 x 2-wire motor 393
Tread (47 links)
5 x 3" square shafts
10 x shaft collar
2 boogie wheels
2 sprockets
2 x [R16] Chassis rails
The rest should be straight-forward. Two of my bearing flats were orientated in the vertical direction and I couldn’t use the rivets for them. I will design and cut my own chassis plates if this design is a good one.
Selecting gear ratios is actually a very open process - there really isn’t a right or wrong answer, it depends on what you want the drive train to do. Higher gear ratios mean greater speed, a lower ratio means more torque (pulling or pushing power) which give better acceleration.
You’ve mentioned using an arduino ( ), so another factor in your design might be additional electronics. As a rule of thumb, slower robots with greater acceleration function better autonomously, as the robot will react to it’s control inputs faster.
As for your current implementation, it looks very solid. I did notice that there is no support to stop the gears sliding along their shafts - if they became mis-aligned, they would slip, which could stop the drive from working :(. Using collars or spacers on the axle to stop the gears moving along it would help stop this.
The robot will not be used autonomously. I’m planning on controlling it with a PS3 controller as I also don’t have the Vex controller. For me, the goal is for speed and control. So, I’ll have to find some sort of common ground.
As for the gears sliding on the shaft, I’m aware of that I didn’t have enough spacers to use, so I thought that I just won’t put them on for now. Next week I’ll be getting more spacers, so I’ll install them
With a robot that small and light, with 2 393s at a 5:1 speed ratio you probably won’t have any issues and it should be pretty quick. Just be wary of those vertical bearings and keep in mind that you can always change the gear ratio if needed. One thing we’ve done in the past about bearing fitment if needed was to cut off one of the holes of the bearing, making it 2 holes long. As long as you get that 1 screw in really well and it’s not a high strain axle then you should be in good shape. The last thing I have to say is that if you find you want to change the gear ratio, keep in mind that you don’t necessarily have to make it fit between the two drive sprockets on the treads. Since the treads are basically a chain, gearing the drive motor to both sprockets is really only a redundant measure that might add some extra strength.
Thanks a lot for your reply! Some very helpful tips
Regarding the vertical bearing, I’m thinking of getting a piece of metal strong enough and the same thickness as the vex ones, and designing it to accommodate my design.
But wouldn’t gearing the front sprocket as well make it seem as if there was another motor there (but of course, with much lower power) ?
I can’t wait to test it and see if there’s a noticeable difference.
No, you can really only get as much power as your motor can push, period. On a wheeled system it sometimes makes sense to gear the motors to all wheels because it can increase traction, but in the case of a tank drive system your traction will be the same no matter how many sprockets you drive. It basically is good to gear it to them if it’s simple and easy to accomplish, but it doesn’t really add too much in the way of real benefits.
Another thing I noticed is that on those bearings where you have two axles and one pop rivet, I would replace the pop rivet with a screw. Pop rivets are OK when there’s two of them on one bearing but we’ve found they’re not strong enough to hold the bearing well with just one.
No it’s fine as it is! But if you end up wanting to change the gear ratio it will give you more options if you don’t have to make it fit between those two set points.
My opinion.
More weight (axles, shafts, shaft collars, screws, nuts) , more friction (from these gears), more complexity can give more potential for failure. Unless they give some real advantage, I would remove them if it was me.
Thanks for your opinion, Paul. I was thinking of the same thing if it’s not really gonna give me any real advantage. And I sometimes don’t trust the shaft collars too
So I completed the building of the robot today with an Arduino hooked up to it. I tested the robot autonomously on a tiles surface, and it seems that the tread was slipping quite a bit.
Have any of you encountered this problem on this kind of surface?
I know Vex has an upgrade kit which provides rubber links for more traction. How good are those?
I mean, what are generally some good methods of preventing this slipping?
Try to see if you can zip tie anything grippy to some of the links of tread. it may not be possible due to the tread needing to be able to spin with full mobility. If that doesn’t work, just get the vex tank tread addition kit or whatever its called with the grippy links, they’ll work just fine.
Is it important that I put the grippy or rubber links on all the tread links? Cause that will be an expensive solution I have around 47 tread links per track.
No, that is not necessary. A grippy link every 5 links should be fine.
You may also try using anti-slip mat on the treads, that is flexible and will increase your traction.
Honestly, I haven’t checked that yet. But it’s pretty much the same sized base as in the earlier pictures but with a different profile of the track. Also less gears and the Vex battery on top.
I’m using an Arduino micro-controller and a small breadboard for now, so that should make it a bit light compared to other Vex robot I’ve seen on the net.
I’ve installed some traction links on the tread (4 treads apart). Is it supposed to move like this? I mean, the movement isn’t smooth, unlike when I don’t have the traction links on.
you do now a 1 to 7 gear ratio skips allot .Try a 1 to 5 or do a 1 to 7 gear ratio using high streaghth gears so it wont skip .And our grade 10 team was experimenting with the speed and power of a normal 363 and a speed 363.the speed 363 was faster and stronger too, which makes no sence at all but it happened this way when we experimenting .If you go to youtube and watch rabots with tank tread drive go very very slow .If you use a high speed 363 with a 1 to 7 gear ratio then this will work.
You can trust me .This is my 2nd I have been in the vex robotics .My team has won the rookie of the year award last year .And this year we have won the design award and the juge award.
Also, ignore the other side.
I didn’t have enough spacers to use, so I thought that I just won’t put them on for now. Next week I’ll be getting more spacers, so I’ll install them