Rhino track failier?

[Kevin Sevcik]

Originally Posted by Andy Baker

Due to the speed of things happening, if folks want to post input here on what else to do to this Rev4, it may or may not get implemented.

Andy B.

Rev 4 improvements sound good, I think. Possibilities for the hopefully unnecessary Rev 5:

• If the cracks are starting at that thin section cleared for the bolt head, make it thicker by switching to #8 screws or flat-head screws. There would obviously be potential sourcing issues for either.
• If Richard's idea pans out, modify the mold to fit standard sleeve bearings all the way through. McMaster has 3/4" OD, 3/8" ID, 1" long sleeve bearings. That should thicken the wall plenty, but might transfer the failure to shock-loading of the oilite bearing.

Which reminds me to tell Richard to thoroughly shock-test his solution. I've seen some oilite bearings shatter on some of our subsystems before. There's a gray colored iron enhanced version that's more shock resistant that you might want to try. Your McMaster part number would be 2938T39, which is actually 0.20 cheaper per than your original bearing.

[Kevin Sevcik]

Originally Posted by team222badbrad

If that's the case I think it would be better to change the screw so it has a smaller diameter and can be tightened without a socket such as a hex head, but I doubt you can source that in self threading style.

I just seems that's the area that needs to be modified to make the most strength in addition to the ribs.

Any thoughts on a material change such as Nylon?

Fastener Superstore apparently has them in flat head or pan head with (shudder) phillips drive. Assuming these are "Taptite" screws.

[Richard Wallace]

Originally Posted by Kevin Sevcik

Which reminds me to tell Richard to thoroughly shock-test his solution. I've seen some oilite bearings shatter on some of our subsystems before. There's a gray colored iron enhanced version that's more shock resistant that you might want to try. Your McMaster part number would be 2938T39, which is actually 0.20 cheaper per than your original bearing.

Thanks, Kevin!

One question: the bearing you cited is rated for higher loads, but at much lower RPM compared with the one I was looking at. Actually neither is rated for maximum wheel speed in the Rhino drive, but the 6338K465 is closer, rated for 700 lb at 250 RPM, while the high load bearing you cited is rated for 1500 lb at 80 RPM.

Are the RPM ratings critical here? I understand that the material composition of the high load bearing makes it less susceptible to shock.

[Kevin Sevcik]

Originally Posted by Richard Wallace

Thanks, Kevin!

One question: the bearing you cited is rated for higher loads, but at much lower RPM compared with the one I was looking at. Actually neither is rated for maximum wheel speed in the Rhino drive, but the 6338K465 is closer, rated for 700 lb at 250 RPM, while the high load bearing you cited is rated for 1500 lb at 80 RPM.

Are the RPM ratings critical here? I understand that the material composition of the high load bearing makes it less susceptible to shock.

I think McMaster's ratings are highly conservative. Based on PV and V max numbers I can pull off the web, the max SFM should be 225, which translates into about 2300 RPM. Not that you could handle a full robot load at that speed. What RPM max do those Rhino Drives run at? Give me that and I can give you a max load.

[Richard Wallace]

Originally Posted by Kevin Sevcik

What RPM max do those Rhino Drives run at? Give me that and I can give you a max load.

Max Rhino drive RPM is the team's choice. AM options for gearing with the TB Mini (easiest to mount on a Rhino module) range from 12.75:1 down to 5.95:1. Since the Rhino timing pulleys are 1:1 (42 teeth on drive and driven), track pulley speeds will be in the range 415 - 890 RPM. My team plans to use a ratio between these extremes, so about 670 RPM max is the target for us.

[Andy Baker]

Originally Posted by team222badbrad

Any thoughts on a material change such as Nylon?

We are thinking of this, but the shrinkage of nylon as it comes out of the mold is far different than polycarbonate. The whole mold would have to be remade if we were to go with nylon.

Andy B.

[Kevin Sevcik]

Originally Posted by Richard Wallace

Max Rhino drive RPM is the team's choice. AM options for gearing with the TB Mini (easiest to mount on a Rhino module) range from 12.75:1 down to 5.95:1. Since the Rhino timing pulleys are 1:1 (42 teeth on drive and driven), track pulley speeds will be in the range 415 - 890 RPM. My team plans to use a ratio between these extremes, so about 670 RPM max is the target for us.

PV of the SAE 863 is 35000. V @ 900 RPM is 88.5 SFM. So P is 395.5 psi. Projected area for one of those bearings is .375 sq in. So max load on one bearing is 148.3 lbs. You're using two, so max load per pulley is about 300 lbs. That's combined load from tension and weight, but I think you should be fine regardless.

[team222badbrad]

Just to inspire some thoughts:

In the radio control truck/car world all manufactures use "standard" plastics.

Often these plastic parts such as suspension arms, bearing blocks, etc. will break easily and often.

It is known by many to upgrade these parts made by an aftermarket company called RPM. http://www.rpmrcproducts.com/

This company guarantees their products from breakage and are made of a Nylon material.

I've never broken one of their parts, but many OEM parts made from "standard" plastics.

I'm not saying you should change the plastic, but I'm just offering some experience I have with RC's.

They survive the impacts because of the flexible nature of the Nylon.

With that said I do not know how positively or negatively that may have an impact.

I also like the idea of inserting a bushing into the pulley until a more permanent fix is determined.

We will probably source something tonight as a bushing would at least allow the pulley to remain fully supported on the axle if a failure were to occur at the bearing.

[KrazyCarl92]

Originally Posted by Andy Baker

We are thinking of this, but the shrinkage of nylon as it comes out of the mold is far different than polycarbonate. The whole mold would have to be remade if we were to go with nylon.

Andy B.

This may not necessarily be the case. Nylon 6-6 (AKA Polyamide) can shrink at a wide range of rates depending on what filler is used with the resin. The desire to avoid fillers for fear of reducing impact strength was mentioned, but some fillers can actually improve impact strength.

So it may be possible to mold Nylon 6-6 parts with your existing mold and get close to the same shrink rate. However, the problem may be over-constrained due to the compounded effect of fillers on both shrink rate and impact strength (i.e. there may not exist a filler+resin for which the impact strength is improved, while maintaining your current geometry).

[jfox100]

Chuck, Andy,

We discovered right at 'going home time' over the weekend that we also have a pulley failure. We don't yet know how many have failed, but at least one has. We'll learn more tonight. My initial guess of how to fix/prevent it is to fill the cavities in all the pulleys with bondo/liquidnails/epoxy/similar in order to reinforce the bearing mount holes.

Jim

[Jon K.]

Originally Posted by camtunkpa

Andy the revision looks pretty good. Are there going to be longer bolts included in the upgrade or do you feel the current bolts will be long enough even with the bolt recess change?

Today we tried to duplicate what the new pulley will behave like with the shorter thread engagement. We produced these results using the same screws, but using 3 #10 washers that were approximately .04 thick.

Once this was in place, we screwed a standard AndyMark hub, with a hole drilled through the bore, to the pulley. We then used wire ties to form a loop which we could use a force gage with.

Clamping one half of the pulley in a table top vise, we pulled on the other with the force gage and maxed out at 104.2 lbf. We did not want to push our force gage further as it was only rated to 110 lbf. At 104.2 lbf, we did not see any separation between the halves, and believe that the reduced thread engagement should suffice.

To verify this, we duplicated the washer set up with 8 more of the Rev 3 pulleys and have placed them on our test bot and have been running that around today, as well as doing other testing with it.

I have not been involved with that testing, however we will have more information on that coming soon.

[The Ginger]

I just want to say thank you Andy for the help and customer service. This just goes to further show how FIRST is more than robots.

[aJASONt_angles]

Hey All,

My name is Jason and I'm an AndyMark Intern this year and mentor on Team 4272. I've been doing testing on the Rhino Track drive pulleys and their durability.

We put on all new pulleys and bearings on our test mule after discovering the previous ones were broken. We tightened the track belt on one side as much as we were comfortable with and left the other side a bit looser. The pulleys were assembled with 3 washers under each screw to mimic the thread engagement of the new mold.

Here are the tests we performed in the order we performed them. In between tests we drove the track drive around on carpet and concrete. The test mule was weighted down to 120 lbs.

30 Rockwall crossings, full speed on concrete.
30 Rockwall crossings with fully tightened front bolts, full speed on concrete.
Dropped from 2.5 feet onto concrete.
10 Rockwall crossings, full speed on concrete.
Hammer to the side of front bolt 3 times each side.
Dropped from 2.5 feet onto concrete twice.
Ran into metal bar full speed five times.
5 axial hits with weighted cart.
5 front end drops from 2.5 feet.
Crashing into various things in the warehouse.

Throughout the testing we stopped between tests to look at the pulleys. The pulleys showed no signs of fracture externally on either side of the track drive.

Here is a video of our axial hit with the weighted cart.

More testing is being done today and I will update those of you on Chief Delphi as soon as I have more results.

- Jason

[Coach Seb]

Did anybody look at data regarding what type of transmissions is used on the failing systems? We are using the toughbox mini, i know some team are using different trans and was wondering if this could be a common failure pattern... Knock on wood, ours still look ok...

[dradel]

With all the various responses my brain has turned to jelly !! So are we saying new pulleys are on tap, or are we saying put washers between the screw heads and the pulleys? Sorry I am so lost here but has been a stressful build season for us this far with snow days and other issues