Attached is my basic design for this years base. Gear driven by 4-393’s.
That looks incredibly heavy. What’s the gear ratio on that? 1.6:1?
Its not really as heavy as you would think, I still plan on hanging with it. It gets over the bump really easily and doesn’t catch. The gear ratio is that. I built it to be a powerhouse so that the motors won’t burn out at all and I can push other robots.
Your gear ratio(60/36) and wheels ratio( two ends / middle wheels=4/2.75) are not same. It will cause some degree of dragging.
Yeah I know the ratios are different, but the middle wheels are only touching the bump so they don’t drag at all because they are fast when they touch the bump and help it get over faster. The back and front wheel still go the same speed as each other so it works out
Definitely would get over the bump okay provided it was perpendicular during approach. I’ve seen many robots turn into trains or just totally beach themselves if turned inline with the bump.
Yeah this is an important thing to avoid. On my base we moved the front motors upward using a chain and sprockets so they don’t hit the base when straddling. I don’t think that would work on this base though…
So far I am putting on mouth and finalizing lift. The base has changed some, but I see how you might see the problem with being hit sideways on the bump. Actually I have accounted for this and the base does not get high centered on the bump. The wheels can actually pull themselves off if one side goes forward and one goes backwards which allows it to get off. So far I can hang with two motors just working on a small device which is in secret planning haha. And have two on mouth. If f all goes planned I should have two motors left over which i’m thinking of using for side to side movement which could be placed to increase autonomous max scores. But I have no problems with getting caught on bump.
Things you should caution yourself on.
Burning out:
-A 1.6:1 speed ratio is a very common ratio that doesn’t burn out often. Especially when motors are directly attached
-Power curves helps reduce the chances of overheating
-axle friction does the same
-Many teams have 6 motors. With the same engineering quality, they can easily outlast your robot
Push Fight:
-Many teams have 6 motor drives. You’ll be at a disadvantage
-Your drivetrain is wedged upwards, meaning that any robot which pushes you can cause you to lose contact with the ground
-Power transferred through gearing is quickly lost. If I remember correctly, each gear you use has about a 90% theoretical power transfer due to pressure angles. Not including pushing force lost to friction
-Axles are quite long. Even if you had the torque to Push robots, bending can cause you to lose power quickly or permanently handicap your drivetrain
-Omni Wheels have good traction in a single direction, however, if you are pushed from the side, or if an opposing robot turns while pushing, you can lose all of that traction.
-Even if your drivetrain can handle the stress, you need to make sure your intake and lift system can handle the same.
Some things off the top of my head to help out
-Rely less on gears and try chain and sprockets
-Drill your pillowblocks to eliminate unnecessary friction
-Move your pillowblocks inside your drivetrain
-Power curves through programming
-Gear for torque
-Apply screws through your wheels, connected to the axle
-Apply deadweight
-Apply a ramp
Those are some good points, I will look into those more as the year goes on. Thanks
Draco, you mentioned the efficiencies of VEX gears, is that just a generalization for spur gears or has someone actually done tests to calculate the efficiency of these gears? I have been looking for data like that without any luck for a while now. I was considering running some tests myself, but I just can’t find any time.
very nice, gone for a similar (ish) design aswell
Yes and no. To be honest, there’s a lot of uncertainty in my answers. According to the vex website, spur gears have a pressure angle of 20 degrees. So i just reasoned that it would be about 90% efficient. Torque is perpendicular force, so because gears have a slight angle, I believe you have to create a cosine trig function.
I then reasoned that
Cosine(theta)=Adjacent/ Hypotenuse
Cosine(theta)= Usable Torque/ Theoretical torque
Cosine(20 degrees) x (theoretical torque) = Usable torque
usable torque/theoretical torque= Efficiency per gear
Word of caution. Pretty much all of this was self taught, intuitive reasoning. With this however, I believe my result was around 90% (i think 90.1 to be more exact).
Now as for empirical tests, I haven’t done an exact test to determine this. The minimal empirical tests i’ve had were weak, if not irrelevant
However, I have seen a few things. For example, I helped develop a 10 pound robot in FTC geared with a slight speed ratio (I think that’s what came out to, 3 inch wheels on a 2:1 speed ratio) push an opposing with similar weights robot which had numerous gears(both vex and ftc have pressure angles of 20 degrees) with a torque ratio of 1.5:1 torque(40 tooth to 60 tooth) with the same amount of motors. Now, other variables may have been involved, such as a lower center of gravity and what not, however i would still think they were about the same. Photo below of another match, but both robots were involved(2425 robot with the ramp).
http://www.hydrarobotics.com/uploads/9/0/7/5/9075173/1357059955.jpg
Another test that i’ve done was making my AC generator. Though my axles were in perfect quality, I still couldn’t backdrive a 1:125 speed ratio. From this, I assumed that the power decay from pressure angles were a big factor.
Thanks for the info. 