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FRC Bearing Load Ratings (Climber Winch)
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? |
Re: FRC Bearing Load Ratings (Climber Winch)
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. |
Re: FRC Bearing Load Ratings (Climber Winch)
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 |
Re: FRC Bearing Load Ratings (Climber Winch)
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?
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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. |
Re: FRC Bearing Load Ratings (Climber Winch)
I agree with the request/focus on bearing ratings, but I'd be less concerned about bearings on a simply-supported climber shaft failing than I would a wheel bearing, particularly in a WCD-type application. The climber shaft should see very low (if any?) dynamic loading, where-as the drivetrain bearings will often see a wide variety of dynamic loadings... even if the climber never falls and the robot lands on its wheels. ;-) Additionally, the drivetrain bearings experience a lot more cycles/use given that they're used almost continuously during the match and are often turning at higher speeds.
Regardless, it'd be good to use more than experience from other applications! Quote:
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Re: FRC Bearing Load Ratings (Climber Winch)
Just to clarify, our "bearing" will be either a hole in a piece of wood, or a chunk of plastic if the wood doesn't work out. At 1 or 2 revs/second, and a total of under 5 turns per use, I think we'll be OK :)
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-nevermind-
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For the sake of your climbing speed I really hope not... Though in general I think a plain bearing could be a robust solution to this problem. Especially if it was coupled with a dry-film lubricant. |
Re: FRC Bearing Load Ratings (Climber Winch)
The trick is to balance the friction right where it needs to be....there should be about 7 times as much torque available, as needed to climb (stall torque vs the torque at design climbing speed), so I think we'll be ok.
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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. |
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The idea of using 1/2" shaft to support a winch drum to lift over 100 lbs, sounds a bit crazy to me.
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I totally understand....many folks seem to be rather focused in their conceptions of how robots are built.
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Re: FRC Bearing Load Ratings (Climber Winch)
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.
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Re: FRC Bearing Load Ratings (Climber Winch)
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. |
Re: FRC Bearing Load Ratings (Climber Winch)
For a hole that size, I would just use a file to make a hex shaped hole in an aluminum bushing.
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Re: FRC Bearing Load Ratings (Climber Winch)
If you use 5mm hex stuff a lot it may be worth investing in a 5mm hex broach (McMaster has them). Some teams have ground their own broaches from old machine tooling for a lower-cost option. (Not that I have anything against filing, if you making just a few it's a great option)
To directly answer your question, I recall seeing a team last year with 3D-printed wheel hubs, so printed parts can be used in high-stress situations. Your success will depend on the printer, materials, and printing pattern, not simply that its 3D printed. You can also do fun things like soaking the part in epoxy to make it ~100% dense and thus stronger. |
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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. |
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This is coming from the person who was EXTREMELY skeptical and made sure we printed half a dozen extras. Like with a lot of things, YMMV. On the topic of climbers, this year 'Snow Problem designed one that used 4 1/2" hex bearings and a ~29" run of 1/2" 1018 steel hex shaft, with a bolt attached to a shaft collar serving as the "hook" for a loop. So far it has failed twice: once was the rope (turns out a random cord from a surplus store is NOT what you should be making your rope out of), and once from shearing the bolt, which was 1 1/2 or 2", 8-32 SHCS, alloy steel, and costs us about 10 cents and 10 minutes (if that) to replace. |
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