Drive-train shaft size

Technical Technical Discussion
1

I did a search but didn’t see what I am looking for at first glance.
I just recieved some samples from IGUS that are plastic bearings and such. Their website lists two FIRST teams who used them or other products in the last two years.

My question is two fold:

  1. What size of shaft do you use on your drive-train if it is a roller chain sprocket type? We have a lot of stock in 5/8 with a keyway since the SkyWay Beadlock wheels have that bore type. I am not an ME so I don’t feel solid on doing calculations but that seems to be extra beefy for FIRST class robots. Is 3/8 or 7/16 too weak?

  2. If a robot only competes in one regional and then the championships, would the IGUS plastic bearings hold up to the abuse? Pillow block sets of steel are a big weight user. We are not at a fabrication level to be able to use flanged bearing set into the frame.

2

In the past, our team has used either 5/8" or 1/2" keyed shaft with ball bearing pillow blocks. Pillow blocks are available with a lighter alloy frame instead of cast iron or steel. This year we used the kit of parts method which eliminates the pillow blocks. The 3/8" bolts worked fine. We did use grade 8 bolts though. Allot depends on the hub and bore options that are available for the wheels you chose. This year’s game wasn’t as abusive as some years games. Don’t know if the bolts would have survived some of the massive impacts we have taken in past years .

3

It all depends on the material. In 2004 we used a 5/8" aluminum keyed axle (3/16" key) , turned down to 1/2" on the ends to go in bearings. Over time, the keyway got a little sloppy and it got some play. But keep in mind there were large 12" wheels on this axle.

In 2005, we used a 1045 carbon steel 1/2" keyed axle (1/8" key). We had the wheels external (cantilevered out) and we bent the actual wheels but not the axles.

The problem when you go smaller than 1/2" is that your key size goes down (less than 1/8") unless you do an oversize key.

If you had a dead axle, or a hex axle, or a woodruff key maybe, then you could probably go down to 7/16 or 3/8.

As for calculations, I have no idea. In the short six weeks of FIRST Robotics build, I’m not even sure that something like that needs to be or should be calculated. Go with what you know will work, and try a little less if you are feeling brave. That is my rule of thumb. Of course it would probably never apply in the real world of engineering, but it FIRST it works beautifully.

Also, can you describe a little bit more exactly what your fabrication abilities/resources/capabilites are? With that info we can give you some more ideas for bearings and axles and stuff.

4

100 has used 3/8" and 1/2" shaft. Never have had problems with either. Had plenty of problems with poorly secured sprockets, ie: set screws only.

5

We have used nothing bigger then 3/8 for the last 4 years and haven’t had any problems. Now for arms we have use 1/2 inch.

6

Thanks for the input so far. We went back to the KOP wheels and shaft this season after a three years. The bolt on sprocket to the wheel eliminates the keyway issue of which was not in my thought process. Downsizing the shaft diameter would then require such a drive mechanism. We bent an axle this year but I am certian it was due to an over-zealous mechanic with the chain tensioner.

So 5/8 or 1/2 if using a keyway works, otherwise 7/16 or 3/8 can work if not using a keyway.

How about the IGUS plastic bearings? I’m looking through their catalog now and see uses in conveyor systems and the like.

Our team has no on-site milling or lathe capability. Our partners at Kelly USA, Lockheed Martin, have made some custom bearing blocks for mechanisms they help fabricate but that is it.

7

If you’re using 3/8" shaft, you definitely want to key it. NEVER rely on only set screws unless it’s absolutely the only way you can do it.

8

We used 5/8 aluminum for our stationary axles with pillow blocks/clamps on both ends. We directly attached the sprocket to the custom made wheels/hubs. We just used regular ball bearings inside the hubs and custom made wheels.

9

I think he meant having a dead axle with the wheel driven directly byt he sprocket.

10

It all depends on how you’re driving your wheels. If youre driving them with the bolt on sprocket style, the wheel just rests on the shaft and has its own bearings. A 3/8" steel shaft works fine for this, even 5/16" has been used with no problems.

However if you are using a shaft driven wheel, I would definitely use a 1/2" steel shaft with a 1/8" keyway. Just for security. If you want to use aluminum, then you should use a 5/8" shaft with a 1/8" keyway. Of course there are always variations. If you have a small 3" wheel, you can easily run it on a 3/8" shaft because it isnt taking so much torque, especially if the shaft is supported on both sides of the wheel.

Just use your common sense and ask yourself, what forces is this taking, and how big? If The wheel is run by bolt on sprocket, then the shaft only takes weight. If the wheel is shaft driven, then it takes torque and weight, so its gotta be stronger. If the wheel is smaller (high speed, low torque) then the shaft can be smaller than if the wheel were bigger (low speed, high torque). Just take all these things into account, and figure out the best shaft size.

A few notes: keyways are superior to set screws and roll pins in transmitting torque. I had to learn this the hard way freshman year. Also, the plastic skyway wheels can be stronger than the shaft they run on. You can break the shaft without breaking the wheel.

As for transmissions, it all depends on speed again. Your motors can only give a maximum power rating, so a high RPM shaft will take less torque than an low RPM shaft. Thus, my own transmissions have always had high RPM outputs, just to keep the shafts small (lowest RPM I’ve geared to was 1000 rpm at no load, on two CIMs). If you keep relatively high gearbox shaft speeds, you can easily have your entire transmission running 3/8" or 5/16" shafts. For shifters, I have always used 1/2" shafts on the shifter itself, for safety, since shifting can be rough.

As for your bearing question, I’ve always used steel bearings on everything. I dont know what type of bearings you have, whether they are thermoplastic or ceramic based plastic will change their load and rpm ratings significantly. It’s best if you can get the bearings specs, just to be sure.

Hope that helps.

11

I have always used 3/8 inch shaft supported on both sides, both with keyways driving a wheel and having a wheel ride on it with a sprocket bolted to the wheel. Never had one failure.

12

What size key and what diameter wheel?

YAY! This is my 3500th post!

13

The 3/8" bolts did work fine. We, however, were told during inspection that after the wear of two regionals, it would be best that we replace the bolts with bolts of stronger steel before championships because the ones the kit provided did have a tendency to warp after a while.

As for 3/8" shafts in general, we’ve never had a problem with them.

14

My team 1251 has always used 5/8 in steel for our drive axles and usually miller bearings and or a plastic material with a high oil content called UHMW for bearings. Generally for transmissions miller bearings and not the plastic. We have never had a problem with the 5/8in shaft. We are currently working on a gearbox for our arm from the past years bot which uses 3/8 steel internal shafts and a 1/2 in output.I know the gears will have a set screw and a key way however I am not sure of the size as of yet.

15

For our arm joints we used shafts that were 1" or more diameter. I can’t remember exactly. I know they took a 1/4" key. You can see here Massive torque when you take a van door motor and gear it down another 20:1 We were able to bore it out a bit inside to save some weight.

16

The IGUS bearings can stand quite a bit of abuse. We use them in many applications in my facility. The majority are used for linear motion in vertical and horizontal slides.

We use the G300 series with little wear on equipment that runs 24/7. We do not lube them as this picks up particles that then contribute to drag.

We have also experimented with the L1 in a vertical slide mechanism. Currently we have three years with no significant wear.

It is important that the bores for these bearings match the catalog. This provides the proper clearance for the shaft. This one of the reasons we have not installed any in our robots. You cannot produce those tolerances on a drill press : )

Read the recommendations carefully. Our experience concurs that a shaft without a high polish is best. We have shifted from Thomson ground shafting to stainless shafting with standard grind. The bearing has an initial wear that embeds its material into the shaft and that this increases longevity and reduces friction.

17

I’ve had great success in the past attaching sprockets to shafts using a keyless locking device known as a TranTorque Keyless Bushing. I’ve found them in MSC and they are produced by a company called Fenner Drives (www.FennerIndustrial.com)

The cool thing about them is that you don’t need a keyway or setscrew and they come in a whole range of sizes in both english and metric. I think the smallest I saw they make fits an 1/8" shaft, but they go all the way up to something like a 3" shaft or so.

Just a thought…???

18

Anyone have a good source for pillow block bearings with aluminum housings? Especially an economical one. Seem that most common ones have cast iron housings. Hopefully now that I’m setting up a lathe I can make my own bearing blocks, but sometimes it’s less hassle to buy.

19

One thing to take into consideration when looking at shaft selection is the support of the shaft. If your shaft-wheel is out there with no protection you’d better beef it right up. When you use a “protected” shaft(has two bearings/bushings/etc. on the two opposite sides of the wheel) you have to consider two things, the lenght of the shaft and the weight of the robot. The shorter the shaft the less flexx theyre will be and the smaller diameter you can get away. The longer that shaft gets the larger diameter you need to use to prevent the shaft from flexing/bending.

Another thing to consider is the material that the shaft is made out of. Aluminum would need to be a considerably thicker shaft than a steel shaft.

-Hope thats helpful and good luck
Pat

20

Here’s the only ones I’ve found.
http://www.cncbotparts.com/bearing_blocks.htm
http://www.teamdelta.com/products/prod5.htm

Have you considered stamped steel ones like this http://robotcombat.com/images/drivesystems/store_pillowblock2_lg.jpg instead of cast iron ones?

Also, I don’t believe pillow blocks are made on a lathe, a mill would be more like it, and a CNC one if you would like to get fancy.

EDIT, omg CD censors your username when I quote you. lol