FRC Bearing Load Ratings (Climber Winch)

[Lil' Lavery]

There was a thread last year that talked about the RPM ratings for hex bearings, but it doesn't appear that either primary supplier of FRC hex bearings followed through with posting load ratings on their product pages. While those numbers may once again come into play for flywheel shooter designs, I'm interested in the load ratings for bearings for purposes of climber design. Particularly the hex bearings, which seem to be of a less standardized nature compared to the round bearings (which are closer matches to what are found in industry). With many of the winch-based climber designs, many teams are going to be supporting the weight of their robot with two bearings. While hex bearings have been used for supporting drivetrain loads, that robot weight has been split among four or more axles and eight or more bearings. I'm curious if teams are going to be getting themselves into trouble with bearing loads on their climber winches.

In terms of round bearings, this Amazon listing has FR8ZZ bearings listed as having a static radial load rating of 2.4kN (~540lb), which pushes it beyond all but the most extreme shock loading conditions on a climber. Anyone have any data for the hex version of this bearing?

[asid61]

Going off the load ratings for the 6800 bearings (10x19x5), which have smaller balls (like the larger inner race hex bearings) I would estimate them to have at the very minimum 200lbs static load or 400lbs dynamic load, which should be more than suitable for FRC considering that climbing is at its highest force during the climb/dynamic rate and that you're using two bearings. Plus, those ratings are for 1 million cycles, which I doubt any climber will see.
That doesn't take into account the stress concentrations on the hex, but it also doesn't account for the increased width or anything.

[BJC]

I also do not know the load ratings. However, this would be a situation where Thunderhex is ideal. The thunderhex bearings are round so you are far less likely to crack the inner bearing race upon high/impact loading while maintaining hex shaft functionality.

Cheers, Bryan

[MrForbes]

We're planning to support weight with a 2" to 3" diameter tube, riding in some sort of plain bearing (bushing), are we doing something wrong?

[Ari423]

Originally Posted by MrForbes

We're planning to support weight with a 2" to 3" diameter tube, riding in some sort of plain bearing (bushing), are we doing something wrong?

As long as your bearings are rated for 154lbs plus a factor of safety at the RPM you're using them at and the tube has a sufficient wall thickness that it will not bend under load, then you should be fine.

[Kevin Sevcik]

Originally Posted by MrForbes

We're planning to support weight with a 2" to 3" diameter tube, riding in some sort of plain bearing (bushing), are we doing something wrong?

Is the bushing itself going to be 2-3" ID? McMaster's rating for a 1" ID, 1" long Delrin bearing is 400lbs @ 25 rpm. Bronze bearings will have much higher ratings. So you definitely don't have to worry about too much load. You may have to be careful about the speed rating, though. Too fast and too much load can cause galling and extremely rapid failure. You should be able to find max P(ressure), V(elocity) and PV values for any bearing you get, and calculate out what your actual values for that are.

[Chris is me]

Originally Posted by MrForbes

We're planning to support weight with a 2" to 3" diameter tube, riding in some sort of plain bearing (bushing), are we doing something wrong?

This seems like a far more mechanically durable / safe solution than using a hex shaft for climbing, if your team has the design and manufacturing "chops" to do it. Bushings are in general far stronger than bearings in terms of peak loading, and the rotational losses just aren't a big deal on a high power mechanism like this. A tight, crappy bushing might even help with backdriving a little.

I would be quite nervous about supporting the weight of your robot on an aluminum hex shaft this year, depending on the implementation. Have spares ready, at the very least.

[Lil' Lavery]

Originally Posted by BJC

I also do not know the load ratings. However, this would be a situation where Thunderhex is ideal. The thunderhex bearings are round so you are far less likely to crack the inner bearing race upon high/impact loading while maintaining hex shaft functionality.

Cheers, Bryan

Originally Posted by Chris is me

I would be quite nervous about supporting the weight of your robot on an aluminum hex shaft this year, depending on the implementation. Have spares ready, at the very least.

To be clear, with the proposed climber design that inspired this comment for my team, we were planning on use 1215 steel hex, not aluminum (including Thunderhex). Given that compression of balls in our intake caused deformation in a ~24" piece of Thunderhex last year, I am definitely shying away from Thunderhex and any aluminum shafts as a primary weight bearing piece of our climbing mechanism.

Turning down the corners of the steel to fit into a Thunderhex bearing is a possibility, as is hex broaching a larger round bushing to fit the hex.

[MrForbes]

The idea of using 1/2" shaft to support a winch drum to lift over 100 lbs, sounds a bit crazy to me.

[Chris is me]

Originally Posted by MrForbes

The idea of using 1/2" shaft to support a winch drum to lift over 100 lbs, sounds a bit crazy to me.

It's not that it's the optimal design, but it's a widely accessible design; some teams and some winch designs may not really work through a stronger method.

[MrForbes]

I totally understand....many folks seem to be rather focused in their conceptions of how robots are built.

[Lil' Lavery]

Depending on which concept we opt for, we may well go with a larger diameter tube based design. One of our concepts calls for elements protruding multiple inches from the surface of the winch drum, so a smaller diameter helps reduce packaging concerns. Thus the desire to use a 1/2" hex shaft.

[Karibou]

Originally Posted by MrForbes

The idea of using 1/2" shaft to support a winch drum to lift over 100 lbs, sounds a bit crazy to me.

We did it last year. 1/2" aluminum hex shaft (not sure what alloy, just whatever was laying around), connected to the frame by 4 bearings in VEX brackets (two on each side of the robot). Granted, the winch spools (one on each side of the robot) holding the load were sandwiched pretty closely between the bearings so they were well-supported.

We did bend a shaft during prototyping when we tried lifting using one spool in the center of the base we were using (which was probably about 28" wide) with the shaft only being supported on the ends by bearings. Still no damage to the bearings, but the shaft was definitely shot.

[Ari423]

Slightly off-topic but does anyone have experience using 3D printed bushings? We want to 3D print a 5mm hex bushing to interface with REV Robotics' 5mm hex shaft[1], but we want to make sure the 3D printed bushing won't crack under load[3]. We are planning on printing a few and testing them by hand, but if anyone has any info on this, I would appreciate it.[4]



[1]http://www.revrobotics.com/rev-41-1349/[2]
[2]I know they sell bushings for that purpose, but we don't want to have to wait for them to ship overseas.
[3]To be clear, this is not the load of the climber. This is a small flap we are actuating with a REV Smart Robot Servo. The flap is only taking a few pounds of force total.
[4]Both empirical and anecdotal information is welcome. If anyone has any ideas on how to lathe and mill that small of a hex bushing that would be welcome too.

[MrForbes]

For a hole that size, I would just use a file to make a hex shaped hole in an aluminum bushing.