You can reduce the load on your gearbox by adding a counterbalance to your arm. Ideally, you want the counterbalance to carry all of the gravity load, with the motor only needing to do acceleration, deceleration, and precise position holding.
That’s the fun part about all of this…not being an engineer, you might not understand all the stuff you need to know to design a mechanism like this. Learning by failure is a great teacher, but it sure is frustrating! If you would like to understand what the basic problem is that’s causing the bearing failure, you probably ought to post the whole robot design, along with details of the area that is failing, and folks here can help you understand how it might be improved, or why the basic design is causing problems.
hint: using 1/2" hex shaft to move long arms, is kind of asking for trouble.
Agreed. 1/2" hex is overused/abused in FRC. We are using 1.188" bearings for our arm shoulder, with a pulley transmitting the torque through a bolt pattern, not the shaft. Other joints use 6806 (30mm id) bearings, common in bicycle headsets. It’s taken a lot of machining to do that, but on the other hand, the kids learned a lot of machining.
What are you doing that’s busting through bearings?
Post photos and then we can actually provide real help.
If it is an arm joint, I’ve learned from 973 and others to just use big ol aluminum tubing as the dead axle, bushings, and sprockets attached to the arm.
We’re not busting through bearings yet. We were busting through 3/8" hex shaft (I didn’t design this thing, one of our “engineers” and a couple of students did), so we’re substituting 3/8" steel hex shaft, and I’m just worrying ahead of time about the bearings being the “next thing to go”, because I can feel how they get wonky when pressed into a hole (in 1/8" aluminum plate) that is a little too small. So I’m worried about the same thing happening when the steel shaft is pulled up against them (shaft in bearings mounted in two pieces of aluminum plate, with sprocket on shaft between two plates, carrying #35 chain).
They were going to use belts but were worried that we don’t have any experience making custom-length belts, so went to chain which we’ve used in the past.
I did the calculations (Google is my friend and I am not afraid of math, so I think I found the right formula), and the 250:1 gearbox driven by two NEOs should have plenty of torque to lift the ~15 lb., 1 meter long arm. But the designers didn’t check the characteristics of all the materials they spec’d for the apparatus. Super-big oops!
I highly recommend my design caculator to help choose the proper number/type of motors and gear ratio for your application. Off the top of my head that sounds like a reasonable combination, but I’m not at my computer to check it right now.
We (well, the guys designing the gearbox) did use a calculator (I think Camnetics? something like that) to design the motors and gear ratios. They just didn’t put enough thought/work into selecting the shafts holding it all together.
Without running the numbers, you are almost certainly exceeding the torque that a 3/8 hex shaft can transfer, regardless of the shaft material or bearings used.
As others have noted, on really high reduction mechanisms like robot arms, using hollow round aluminum tube stock is the best axle material.
Lastly, bearings should never feel wonky after being pressed in. This can occur if your bearing holes are either way too small, or if you pressed them in incorrectly and damaged the races.
Whenever you press in bearings, you should only press on the part of the bearing that is not moving against the pressed object - if you are pressing a bearing onto a shaft, only press on the inner race. If you are pressing a bearing into a housing, only press on the other race… I usually use sockets as a jig to ensure I’m only pressing on the correct parts of the bearing.
OK, so is this because of the force on the corners of the shaft? How do I find out how much torque a 3/8" hex shaft can transfer? We were using 3/8" because we couldn’t get the gears we needed with a 1/2" hex bore. Guess we should have used round bore with keyed shaft?
The whole arm is about 20 lbs, with most at the far end, and is 1.12m long. There is a small (2" dia) sprocket on the bottom (on a 3/8" steel shaft), connected by chain to a 4" dia sprocket at the ‘shoulder’ (on 1/2" aluminum hex shaft, held to the arm with hex hubs at the ends of the shaft bolted to very solid brackets).
This has been incredibly helpful. My one concern is attaching the round shaft to a drive shaft. Would you use a set screw to mount the round shaft to a hub? Would you place a key in the shaft?
Pictures would be so helpful.
Here is where the bearing we have is failing.
When we drive off the platform we ended up hearing weird noises from the bearing . Another problem could be our installation method but I am not sure.
I mean a picture of the whole robot… It’s really hard to tell what that is supporting.
We were considering an arm with around 120 ft. lb. of torque at the pivot. The original design had a large sprocket at the centerline of the chassis and live axles going out to the arms. The axles on each side would be around 8" long.
Neither AndyMark nor VEX could give me a torque rating for the 1/2" hex shaft they sell so we decided to do some testing. We clamped an 8" section of 1/2" aluminum hex shaft in a vise. I used a torque wrench with a 1/2" socket to apply torque to the shaft. With the torque wrench set to 50 ft. lb., we got around 15 degrees of twist without the torque wrench clicking. After removing the torque, the shaft straighten itself. A local team was not so lucky in 2015. Their arm shaft ended up permanently twisted.
That’s the chassis of the robot. the back left wheel to be precise.
How many wheels on the robot? How heavy is it? How long is the shaft, and how far out is the wheel? What type of wheels? How many wheels? Does it seem like the issue is just with one wheel, or with all of them?
When you say “weird noises”, can you quantify that any more? What, exactly, are you hearing? Are there any noticeable changes when you rotate the wheels by hand?
This is a picture of our chassis.
The final weight of the robot is 108 lbs. This issue seems to be on both side of the back of the robot(the pneumatic wheels)
We hear unusual sounds like a grinding of metal. This happened right after we drove off the lvl 2 platform.
Perhaps get a high-quality bearing with a super thick inner ring and replace or sand that ring into a hex (or replace the inner ring with a hex bearing). This is not intended to be a professional solution.
Hex bearings are an odd ball thing that really only sees use in Robotics competitions. They have recently been appearing in light weight combat robotics, but they are generally not used in industrial applications.
If you want truly high load bearings you are going to need to get round bore bearings and make reducer bushings if you have to use 1/2" Hex. Mcmaster has a large selection of bearings with specifications. If you can wait a week or two for delivery, you can go to ebay. If you are really hurting on time and can be a little flexible, tractor supply sells bearings in store.
If you are doing something like a 4 bar lift this year, you will need around 600ft lbs of torque which is going to be a few hundred pounds of tension in a chain. This means you will probably need a bearing with a static load rating of 1000lbs or greater if you want a reasonable level of reliability. This means you are probably going to have to move away from hex shaft and move up to at least a 5/8in ID bearing to get that load capacity.
I’d love to see your calculations (I’m not criticizing)