Gear Ratios - Arm

Something I would like to do is, because of space, use a different gear ratio for my arm other then the usual 1:5 or 1:7, All season I have been using a 1:3-3:5 or a 1:8.3 final. This is really nice except for my redesign, the gearing is really big. I have thought about using a 1:3-1:3 or 1:9 which is ideal in size but I don’t exactly know about the whole friction, stress on motor etc etc. Can someone help answer these questions?

I am not going to be using a 6-bar linkage, rather an 8-bar linkage which means that I can get to the 30" position at a less of a degree which i know affects how the gearing will be.
I have also tried this gearing and have had a lot of problems with the high strength gears breaking but this was because I had the whole arm to the intake, in steel, with aluminum will I still have that problem? Some ways to prevent if I do?

Thank you ahead of time to all who try and help me :slight_smile:

If you were happy with the 1:3-3:5 then the 1:3-1:3 should be OK. The 1:3-1:3 will give you more torque and load your motors less but will trade off speed for the extra torque. The friction losses of both gear trains should be comparable since you have two stages of spur gears in each with about 3% to 5% torque loss per stage.

It would be nice to have a picture of what you are doing here.

Steel is OK provided you use it sparingly to still keep the weight a low as possible. If you use a lot of C channel instead of 1x25 beams (made of two 1x25 bars held together with .5 in or 1in aluminum beams) then your weight is probably too high and certainly would benefit from aluminum usage. I suspect that you are breaking gears when your lift is raised and the whole chassis rocks creating excessive dynamic loads . I.e. the robot tilts or is bumped hard. Perhaps you could comment on how and when the gears are breaking.

Generally even with steel you shouldn’t have trouble breaking gears if you have minimized the load, split the load between two towers and have balanced the load with elastic bands. This is really important especially with steel structure since it weighs about 3x that of aluminum arms.

Here is an example of a robot built almost entirely without C channel:

https://vexforum.com/gallery/files/2/4/0/7/img_0583.jpg

There are a few more images in the gallery.

minus the 5x25 c channel in the middle, right? :wink:

and what if another robot drove into your arm sideways?
wouldn’t the whole thing bend?

Yeah it is more likely to bend, and I don’t like the idea of using the 1x25 “beams” the aluminum is lighter and stronger than that IMO. Though it is a good weight loss on, what would be a very heavy arm…

When I need to do a lot of math on an arm joint and don’t feel like doing it all manually (which is… all the time), I use John V-Neun’s mechanical design calculator.

http://www.chiefdelphi.com/media/papers/2059

The versions just include FIRST motor specs from that year, which you don’t need for VEX. The sheet of interest is the “Rotary Mechanism” sheet, which you can use to design a rotary gearing system for a rotary joint (such as an arm).

The specs for VEX motors are:
Spec Voltage - 7.2 (you must also change “applied motor voltage” to 7.2v for vex batteries)
Free Speed - 100 - This is the same for both 269 and 393 motors
Stall Torque - 0.96 for 269, 1.53 for 393
Stall Current - 2.6 for 269, 3.6 for 393
Free Current - 0.18 for 269, 0.15 for 393

If you are using multiple motors, add their stall torques together in the stall torque cell, but leave the rest of the cells alone. This will show you the estimated current draw for one motor. If you mix 269 and 393 motors, this current will be off, and you will have to calculate it on your own, but you can use it as a guideline.

You can plug in your gear reductions to the table (Driving and Driven gears), using 1 and 1 for unused stages (if you have less than 4 stages of reduction).

Applied motor voltage should be vex battery voltage - 7.2
Lever length - It’s the length from the pivot axle to the point mass of the arm, which is not necessarily equal to the length of the arm. For simplicity, I would use the length of the arm from the pivot.
Gearbox efficiency - I leave it at 70%, maybe this is too conservative, but it seems fine.
Applied Load - This is the weight of the point mass. If you counterforce correctly (with rubber bands or latex tubing), this should only be the weight of game pieces plus a little extra for safety margins (e.g. for 2 game pieces, use 1.5lbs for two barrels + a little extra). If you don’t counterforce enough, you will need to increase the design load as the motors will be lifting more of the arm weight.

The spreadsheet will give you information about rotational speed loaded and unloaded, as well as stall load. A rule of thumb says the stall load should always be 4x or more the applied load you estimated earlier. You can also look at the current to see if you will trip breakers, although that is very stall dependent (how much you hold the load in place with the motors after reaching the target position).

There are also a few other sheets for other mechanism types, I’ll let you play around with them on your own.

We have run several robots with this type of construction for gateway and never had a problem mixing it up. The arms are a little more compliant in the side direction but this can be an advantage in that it gives a little in collisions and can help prevent tipping. They can always be stiffened up in any direction of loading if one is worried about it. Actually, most of the 1x25s in the picture are made of aluminum. So this robot is really light in addition it has a two speed pneumatic transmission. I don’t know for certain if anything ever bent permanently on this robot, but I seriously doubt it.

If you haven’t put together one of these beams, try it and you will be suprised how strong they are. Stiffness can be adjusted by adding more cross beams.

The picture is one of three robots built by team 599d for gateway and has since been replaced by a four position pneumatic lift robot that uses 3 pistons per side. We are hoping it will be an excellent skills challenge robot. The first robot had a Titan 1103 type slide lift with a claw. The primary designer is Cedric… and his mind never stops creating.