Rhino track failier?
 

My team purchased the Rhino Tracks from Andy-Mark this year, but after driving with them, the pulleys (am-3298) have started to fall apart. The plastic is chipping around the bearing. Is anyone else having similar issues?

Re: Rhino track failier?
 

sorry pic in not working

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Try attaching the image to the post as an attachment, or linking it differently.

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[IMG]blob:https%3A//mail.google.com/82aaea0c-d18f-40c0-8001-74d3ed496baf[/IMG]

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copy this link into google without the [IMG]

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Try again. Link does not work at all.

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Upload your image to imgur.com and provide the link here. We're not logged into your Google account, so we can't see images in your email.

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there it is, thanks

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Did you use locate when you assembled them? Because that looks. Exactly. Like. When a student used locate on our wheels. One year some got. On the plastic and the locktite eats. A. Component of the plastic. And they fall. Apart

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We just had the same breakage today. We are checking for more signs of weakness. Did you have more than one break??

Re: Rhino track failier?
 

these wheels where assembled with 6 bolts, 3 per side. we followed the manufactures construction. I think that it may come from a combination of over-tightening of the the track, or the force of the robot slamming into the ground after crossing a defense. We have broken 5 of these wheels so far. Is this happening for anyone else?
-thank you for any info you can give

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We haven't done a lot of defense crossing yet. Today we were code testing when we found the broken piece. We weren't even sure where it came from until I read your question. Keep us posted :(

*update we found cracks on several wheels, some are in advanced stages but not broken yet. placement on the track was not a factor. we also do not believe it is a tension issue.

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I agree with cmwilson13, it looks like you used superglue or Loctite on these.

Some plastics react badly (called crazing) to superglues (CAs in particular) and Loctite.

The bearings should be a slight press-fit into those pullys and should not need any glue. If they are a little loose, use a fender washer and spacer to keep them there.

Re: Rhino track failier?
 

no adhesives, only bolts. this is a very dense plastic, when it breaks it cracks it is a lot like obsidian, it forms very shiny surfaces

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Just checked all that we have for our second bot and they have cracks in every one. Looks like it is from too tight of a fit with the bearing. Waiting for a reply from AndyMark.

This looks very very bad. Hopefully Andymark resolves this quick or there will be a lot of angry people with stationary robots.

Re: Rhino track failier?
 

My team hasn't had an issue like this yet, and we attached a cinderblock and a lot of lead weights to our chassis and went over all the obstacles full-speed. So it's definitely either over-tightening, or you guys just got a bad batch.

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What position were the pulleys in when they failed?

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The first one is the drive wheel, but we found them in seven of eight positions. We were doing basic driving when we found the broken piece. When we checked the pulleys we found cracks in all of them. We did some hard testing this afternoon and all of them are worse. All were installed per manufacturer's instructions.

This is not what I want to hear!!! Please please keep us updated on what you all figure out and also what andymark has to say about this!

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It's a new product this year and as with any new product there are going to be problems. Especially for a complicated system like a tank tread system, teams spend many seasons and off-seasons trying to perfect their design. It seems that the pulleys, are the same type of material as the Hi-grip wheels, and there is barely any material to cushion any impacts. With the impacts that tank treads will receive in this years game, it seems that maybe plastic just isn't fit for this purpose and maybe Aluminium Pulleys would be better( at a significant monetary cost).

I'm sure that the guys at Andymark, are doing their best to find a solution to this problem.

It seems that due to the dead axle design, it would be quite simple and quick to change out broken pulleys, especially driven ones and replacements aren't too expensive either $8

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Cyclical loading from tension in the belt? The HiGrip wheels obviously have constant contact with the carpet, but it would seem like the load on the pulleys would change slightly as the teeth engage and disengage. The counter argument is that AM sells 42-tooth HTD pulleys made out of the same stuff, but it seems like HTD profiles engage smoother than standard timing belt profiles.

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The cyclical loading from the tension in the belt, would actually be very significant as during sharp acceleration the belt is in a way, trying to pull the pulley wheel towards it.

The belt is also steel-reinforced so there wouldn't be any stretch at all, and also the belt isn't going to slip. This isn't something that can be fixed by loosing off the belt tension either, because as soon as the belt is loosened, it'll be taken off by another robot extremely easily

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Small point, but a steel reinforced belt has more stretch in it than, say, a fiberglass reinforced belt like the HTD belts used for other drives.

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That is actually really interesting, thanks! I sort of assumed that because something is "steel" reinforced, it is going to stretch less than a fibreglass or kevlar reinforced belt.

So if it is a steel reinforced belt then how can it be a spliced endless loop belt, surely the weak point would be the splice? It seems difficult to join the steel strands, while they are embedded in the belt, because any heat welding would melt the rubber belt. So it is a crimp or something that is used to join the strands?

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I direct you to the Brecoflex page on their spliced belts. They say only about half the tension members carry the load through the spliced area.

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the steel cable will stretch over time if under constant pressure, my team did some testing last night and as long as you cant pull the track off the system by yourself they should be fine. track tension is not the main source of the problem but over tightening is definitely not helpful. (PS has anyone actually contacted AM yet?)
-thanks

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Do you have any fresh wheels to measure the bearing hole size prior to pressing a bearing in?

Since you have the problem I would reccomend that you contact Andymark so you can discuss the exact conditions that caused the failure. You are the team that knows the details and can answer questions as they are asked pertaining to the failure. That will be better and faster than another team searching for answers without having all the information in front of them.

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My team is not meeting on Sundays but tomorrow i will make sure that we contact AM, i do not want to start negotiating without having the parts in Question in front of me.

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We are talking about replacing them with aluminum. Glad we found it before any competitions! It makes me wonder about all the other teams using them who don't know or haven't had issues yet.

Wish we had the resources to machine some out of aluminum

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Why after two cases have come up with unknown cause of failure are people willing to tuck tail and run so fast? I would wait a day or two and see what comes of contact with Andymark. They understand build season and everything that goes into the process of building a robot, I'm sure they'll figure this out and help everyone know the cause and solution ASAP.

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Make that at least 3 cases, after seeing this we check our chassis and sure enough many of our hubs have cracks and some chips. What's interesting is even our second chassis has them and it hasn't even had the tracks run by anything except hand turning. Here're a couple of pics: http://1drv.ms/1Q467jC One has a piece of paper in the crack to highlight it.

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We've been really fortunate to have some wonderful parent helpers who can build stuff and know others who can build other stuff :)

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For those that do, the CAD model on AM website indicates a 1.124" bore with no draft.

Anyone want to place bets on the tool maker deciding to put a draft on that bore without telling anyone at Andymark?

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That was my thought when I posted that... it should be noticeable if it's there. What's a standard draft? In college we did 2.5 degrees, but we were a bunch of noobs when it came to making a mold.

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The layout print calls out that bore diameter as 1.1249 - 1.1254. Tight tolerance for a plastic molded part. Good fit range for a 1.125 bearing if the tolerance is held.

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I wonder about the fillet at the bottom of the bearing pocket in conjunction with clamping loads from a traditional dead shaft configuration.

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Thought I'd do some research, since I recall similar problems:
http://www.chiefdelphi.com/forums/showthread.php?t=84560&page=3&highlight=wheel+crac king
http://www.chiefdelphi.com/forums/sh...ht=wheel+crack
http://www.chiefdelphi.com/forums/sh...wheel+cracking

Most of the reports were from heavy shock loading from running into the old rock-wall like bump. Some were from applying loctite, which as noted in this thread, eats polycarb alive. Some reports were from over-tightening sprocket or assembly bolts. I think atleast one person blamed killing plaction wheels on hammering/forcing bearing into the wheel instead of hand pushing them in and stopping wherever they tightened up.

Not saying any of these are the are the actual solution, just putting it up to inform the speculation.

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The glossiness and types of fractures I can tell from these pictures indicate the material may be glass-filled. Definitely would make the pulleys super stiff, but definitely suseptible to impact failures, as the glass in the resin makes the plastic surprisingly brittle.

Additionally, while the spec'd total tolerance may be 0.0005" (0.0127mm for those of us versed in those units), this is a basically an unmoldable range. It would be tough for an injection mold to hold that steady over many thousands of parts, even things like the HVAC of the factory or running at night vs running at day could have that effect. I envision few suppliers would sign up for that tight of a tolerance.

That being said, molded press fits have been common in AM and VP parts for years without issue- so this may not be part of the failure at all. Just wanted to add some realism to the discussion regarding tolerance and injection molding.

-Brando

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Ah, I see what's up. The drawing (which I didn't bother looking at, of course) is up to rev 3, which creates the bearing pocket. The model does not have that feature, and shows a straight bore. I thought it was a little odd to have a inch long perfectly straight bore in a molded part.

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AM specs the material as black polycarb. That's consistent with their other black plastic parts, none of which seem glass filled.

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All,

Thank you to the Ginger for starting this thread. During this past weekend, we have received emails from three teams concerned about the same issue. I won't list the team names, as they may consider it confidential information.

This post is a review what we are doing. I will post updates in this thread regarding how things are progressing.

Customer Service:
If anyone has cracked hubs for these Rhino Track Drive modules, please send a note to sales@andymark.com. We will send out* replacement pulley halves. It will help us if you take pictures of the pulley fracture and tell us which pulley it is. Please refer to the pulleys numbered as such in this picture:



In house testing:
We have a test mule here at AndyMark that has been tested many times. Our initial tests (before kickoff) did not show cracking. We drove this mule over the wooden defenses after kickoff and it seemed to do fine. As it drove more last week, seemed to have more resistance to driving the tread. Today, when we drove it, the 120a breaker tripped. We took it apart and found that there were pulley fractures on both sides, on pulleys 1 and 4. Pulleys 2 and 3 did not have fractures. They had a few hairline cracks, but nothing was wrong with pulleys 2 and 3 to affect performance. Here is a picture of our pulleys we took off today:



This picture shows that the front (#1) and back (#4) pulleys were fractured. Since the middle two pulleys fared much better, we think this fracturing is due to the higher forces on the front and back pulleys with regard to belt tension. We are building up the test mule with new pulleys in order to make a test today. We will be testing the chassis with a slightly looser belt tension to see if we can find a belt tension sweet spot between the belt not falling and the pulleys breaking.

Pulley improvements:
The bearing fit is good, in our opinion. It's a light press fit with no taper. The radius at the bottom of the bearing hole is smaller than the radius on the bearing. Since we are able to recreate this failure here, I don't suspect that a customer was inadvertently using thread locking material (which is always a no-no with polycarbonate).

Material: This material is black polycarbonate with no filler. We have tried fiberglass fill years ago with omni wheels, and we learned that while strength is increased with fiberglass, impact resistance is significantly reduced. We could try to make this part out of some sort of Nylon or a co-polymer using Nylon, but that would require an entirely new mold and would take much time to try.

Mold tool improvements: We can take away metal from the mold to add plastic to the pulley. We will look at improving the mold in 3 ways:

1. Adding a rib in the pocket between the mounting holes
2. Raising the counter-bored level for the screw head so that the top of the head is slightly under the face of the pulley (we believe that we can do this without changing the screw length)
3. Increase the radius on the ribs supporting the inner circle bigger

Corrective Action:
There is enough evidence here to justify that we need to improve this mold. Once we get a new part designed, I'll post a pic of it on this thread with the additions highlighted. Since our molder is here in Kokomo and works with us very closely, I am confident that we can make a mold improvement and have parts available within 8-10 days. Thank heavens this part wasn't made overseas.

* - While you may need cracked pulleys sent to you very soon, you may also wish to wait until we get this mold changed and have new, stronger pulleys. Please stay tuned for timing regarding when we can do this.

I am warning our ops and customer service folks that we may be sending out many replacement pulleys soon.

It will be an interesting week. Please stay patient with us as we solve this issue.

Sincerely,
Andy B.

Re: Rhino track failier?
 

Andy, thank you for the prompt attention to this developing issue. I'm sure many teams are going to be watching this situation as it develops.

One additional data point request for Andy and others that have seen the failures. What was the total weight of your robot? Are these weighted 120lb machines or are they failing with lighter basic rolling chassis as well?

Re: Rhino track failier?
 

Just want to highlight this level of Customer Service in any industry is unparalleled. The insight and full honesty into what is going on makes AndyMark the company that it is.

Andy- sounds like the addition of ribs is a solid solution path. For whatever my two cents is worth, I agree with the solution.

-Brando

Re: Rhino track failier?
 

We have yet to test our VexRhino (custom drive using AM pulleys and belts).

We will let you know if we have any failures.

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To echo Brandon's feelings. This could well serve as a prime example of a customer service goal for a business in any industry to emulate.

I just sent a link to Andy's post to our other mentors on our team telling them that this is one of the main reasons I like volunteering with, working with and associating with the people that make FIRST what it is.

Thank you Mr. Baker

Re: Rhino track failier?
 

Andy, I think Rhino customers really appreciate the open approach AM is taking to address this. I agree that a mold improvement is likely to solve the problem.
---------

Another approach, more in the quick-fix direction, might be to distribute the tension and shock loads on the pulley over a larger area. This might be accomplished by inserting a tight fitting sleeve (maybe aluminum tube) through the pulley bore (0.96 inch), and then sleeve type bearings (maybe oil-lite bronze) into the ends of the sleeve to accept the 3/8 inch bolt spindle.

I have ordered some pulleys to try this, along with aluminum tube and bronze bearings. Tube and bearing part numbers from McMaster-Carr are 9056K28 and 6338K465, respectively. I plan to turn the tube O.D. for a tight fit to the pulley bore, then bore (drill) it to a tight fit for the 5/8" bearing O.D., and face to 2.175" long. After a light press to fit the tube into the pulley, the bearings can be inserted and then the assembly should replace a damaged one. I don't know how much extra friction the sleeve bearings will add, relative to the 1416ZZ ball bearings -- current draw will tell that tale. If this approach works it might be used to repair damaged pulleys.

I am just an electrical engineer fiddling with mechanical things based on FRC experience. I would appreciate critique of this idea, especially from those with stronger Mechanical Engineering background and/or more FRC design experience.

Re: Rhino track failier?
 

Agreed. Always impressed by AM customer service. And great job sourcing locally, 1 week is a pretty impressive lead time for a mold change.

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Thank you all for your understanding, support, and patience.

Richard, this is brilliant! Thank you for posting the idea. We may try something similar here in our tests. You're an ME in disguise!

Regarding the mold changes, we are implementing 4 changes to the pulley.



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.

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The only issue that I see, looking at the drawing for the pulleys is that the center bore is relatively loosely held at .95-.97. You may want to open the pulley bore slightly to prevent the need for matched sets of tubes and pulleys to get the desired light press fit.

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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?

Re: Rhino track failier?
 

Good question, Cliff.

We also discussed this issue. The distance of thread engagement was 0.485 with the current version. With the new version (Rev4), the engagement will be lessened by 0.125, resulting in 0.36. Since there is a taper on the end of the screw, the last 0.15" (approximately) can't be relied on with regard to screw thread engagement. So, we now have about 0.21" of engagement. Since it's a 10-24 screw (with 24 threads per inch), that means we now have 5 threads of engagement at each screw. With 6 screws all having this same engagement, we believe that this will be enough. However, we will make a test and report back if this is good or not.

Andy B.

Re: Rhino track failier?
 

Thank you for the kind words, Andy! I will post results of our attempt at this quick-fix.

Agreed. We will run a 61/64 31/32 inch ream through the plastic parts before pressing.

Re: Rhino track failier?
 

Would filling the hollow spaces in the pulleys with something like epoxy work to mitigate this?

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Do the failures seem to be coming from the weakest area as shown by the arrow? It seems that way, but we haven't busted any because we haven't driven any... Busy work on more important things. :D

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Yes, that would help also. Good idea!

Yes, you are correct, Brad. That is where the failure is happening.

Andy B.

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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?

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It seems like the fact that there's an edge rather than a chamfer/fillet where the well for the screwhead begins (radially out from the center) might be a point of failure. All of the fractures pictured are at the depth of the well, so I think having an edge there to bend around is contributing somewhat. Just a guess though.

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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.

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

Re: Rhino track failier?
 

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.

Re: Rhino track failier?
 

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.

Re: Rhino track failier?
 

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.

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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.

Re: Rhino track failier?
 

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.

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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.

Re: Rhino track failier?
 

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).

Re: Rhino track failier?
 

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

Re: Rhino track failier?
 

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.

Re: Rhino track failier?
 

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. :)

Re: Rhino track failier?
 

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

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...

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

Re: Rhino track failier?
 

Jason,

Thanks for the updated information. From your description it sounds as if you folks are driving your test mule much like our team drivers.

Of course we encourage them to "Drive it like you stole it" and that a robot can always be repaired BUT a match cannot be replayed.

From the looks of some of the frame parts in the video you guys have taken the "crashing into various things" portion to heart!

Re: Rhino track failier?
 

For Andy and co. -

PC/ABS may be a good alternative option to try as well. Shrink should be very similar to PC that is already being utilized so you can more than likely drop it right into your current tool. It will decrease the notch sensitivity of the material relative to impact failures (ie: making the part less sensitive to the microscopic flaws in the molded material).

Just an idea for the pile!

-Brando

Re: Rhino track failier?
 

Washers are a test of a potential modification of the mold that would result in less thread engagement of the screws. It's purely to determine if pulleys from the new mold would require longer screws.

Ok now I'm caught up. Like I said been a bit stressful with the snow

Re: Rhino track failier?
 

Just a thought.

If the tight fit between the bearing and the plastic hub is causing issues, could teams take a reamer to their existing plastic hubs and change a tight fit to a slip fit? Doesn't solve all the impact issues, but could reduce some of the stress in the hub.

Kudos to the AM crew for being on top of this. FRC teams are lucky to have so many dedicated suppliers!

-Mike

Re: Rhino track failier?
 

You probably could ream the hole out, but I wouldn't risk causing damage to any presently undamaged parts. The reamer could put unnecessary stress onto the hole and possibly fracture it. Depending on 3D printer access I would print a new part with a stronger material and a slightly adjusted hole diameter. Just my advice.

Re: Rhino track failier?
 

You're not 3d printing a stronger part out of anything short of sintered metal. So that's not really an option.

Re: Rhino track failier?
 

It's possible... :-)

https://markforged.com/

Re: Rhino track failier?
 

I'm clearly not keeping up with the times if we're up to printing FRP. Still, their claim is the material is stronger than aluminum PER kg. A better strength-to-weight ratio is often a good thing, but if your limiting factor is volume, not weight, then it's not the important metric. In this case, the pulley geometry is already defined, so the important metric is strength per unit volume, which I suspect aluminum or sintered metal would better at.

Re: Rhino track failier?
 

No offense, but do you know what reaming is?

Reaming is an operation using a special tool used to produce a hole diameter of a very accurate size. Reaming is done by removing a few thousandths of an inch of material from a hole using a spinning cutter of a precise size. Very little stress is imparted onto the part to do this; certainly less stress than drilling a hole, and it's a reasonable suggestion here.

3D printed parts are more or less always substantially weaker than injection molded parts of the same geometry and material. There are very few material choices for 3D printers that are potentially stronger than injection molded polycarbonate, and it is reasonable to assume none of these materials / printers are within the reach of FRC teams.

And here's the kicker - in order to get accurate bore diameters using 3D printed parts, you would probably want to team the hole afterwards anyway.

Re: Rhino track failier?
 

This could be reamed out, of course, but I really don't think that a too tight bore is the problem. We can press bearings into the current pulleys we have (existing design, Rev3) with a strong push of a thumb. It's not a tight press fit.

Here is an update with our current status:

1. So far, 5 teams have contacted us regarding cracks or fractures on their pulleys. We are sending emails to those 5 teams today. Oddly enough, all of these orders were shipped out on Jan. 10th or 11th.

2. We are continuing with an improved mold for our Rev4 pulley. It has all of the improvements listed above, AND we were able to change the interior (non-bearing) bore to 0.754. This bore would be able to accept a bushing if the user would like to install one. We don't think it will be needed with the other improvements.

3. We will send out improved pulleys to the 5 teams who have sent us input regarding cracks and fractures.

4. As you can see from Jason's post above, we are quite confused about how we can't break these pulleys like we broke our original test mule. We are still testing. The current testing shows that our existing design is very robust for climbing over things and for surviving drops on concrete floor from a considerable height (2 feet).

5. If more teams are seeing cracks and fractures, please send a note to sales@andymark.com, telling us which pulleys are breaking (1, 2, 3, or 4), your Rhino Track Drive order number, and with pictures of the breaks. We will replace these pulleys with pulleys from the new mold.

6. We are hesitant to send out new pulleys to all customers due to the positive results we are seeing in #4 above. If customers want to contact us to get replacement pulleys, please do so by sending an email to sales@andymark.com. You will need to tell us your AndyMark order number (this saves us time) and tell us the number of pulleys requested.

Andy B.

Re: Rhino track failier?
 

Andy,

Do you have an estimate of when the new improved pulleys will be available and ready for shipment?

-Chuck

Re: Rhino track failier?
 

Andy,

Our pulleys would have gone out in that range of dates. We have yet to observe a failure. We've been checking them at fairly regular intervals.

I believe the reason we have yet to see a failure is related to our dead shaft design. We use a .375" OD tube between drive plates with a 1/4" bolt though the center, so the axle acts as a standoff. If spacers are fabricated properly it makes it impossible to put a clamping load on the inner race of the bearings in the pulley by tightening the bolt.

That's my two cents. I'll report back if we do see a failure.

This situation is an example of the world class customer service we've come to expect from Andymark.

Thank you.

Re: Rhino track failier?
 

I haven't looked but the belts could be extruded.

Re: Rhino track failier?
 

I know what reaming is thank you but I appreciate your concern, but injection molds can be structurally compromised from even small modifications. My team has modified the CADD file of the part, combining both halves into a solid part and filling in the empty space around the bearing socket. I will gladly share this part if it works after our testing.

-Jacob

Re: Rhino track failier?
 

It seems like you're misinterpreting the suggestion of reaming. The suggestion was to use a reamer on the plastic molded part after it ejects from the mold, not on the injection mold itself. This would allow the manufacturer to hit tighter tolerances on the hole diameter than can be achieved through molding alone, and without any risk to the mold. This is called "rework" and is often avoided in the real world because it is time-consuming and potentially labor intensive at high volumes (higher cost). It's not desirable, but if a manufacturer is in a position where it is the only way good parts can be sent out the door it is sometimes a reasonable course of action. However, Andy stated it is not clear whether this tighter tolerance would even make much of a difference.

I would say "a reamer? oh that's easy enough to test on a few parts", but it sounds like AndyMark is unable to reproduce the problem on more recently molded parts (actually a good thing, but frustrating if you're troubleshooting the issue...). So it would be difficult to evaluate the effect of reaming without a failing baseline for comparison.

Re: Rhino track failier?
 

Going one step further, I'd like to point out that you can't ream a hole in the injection tool, because the tool has a positive feature to create a hole :)

-Brando

Re: Rhino track failier?
 

Just as a quick solution for any tram with a 3d printer, I would suggest printed pulleys out of abs plastic, we are running printed pulleys because we used different treads and have had no shattering problems. Our design is based on Andy's however it is longer to accommodate our longer tracks.

Re: Rhino track failier?
 

I know you meant on the Part itself, not the mold, but I'm concerned about shattering since although reaming is a light process, the injected plastic is very brittle and could snap or fracture under very light stress. Again, my team is modifying the 3D part to increase structural integrity before printing. If someone could try reaming and see how goes that would be great.

However, I do not think the problem lies in the tightness of the bearing recess, but in the thickness of it's walls. The recess has very little support along the edge of the bearing, and when it is under a shock load such as after crossing the rock wall, it shatters. This is what our 3D model adresses, by filling in the large gaps around the recess, we add extra, and constant support throughout the arc of the pulley's rotation. If anyone has thoughts on this I'd love to hear.

-Jacob

Re: Rhino track failier?
 

Also, beside the Pulley cracking issue, has anyone noticed any other problems?
Our tracks, which admittedly have not been run very much, have displayed no problems whatsoever as of yet, but we did get one of the first sets, so problems are bound to arise.

-Jacob

Re: Rhino track failier?
 

Another update:

Here is a picture showing our pulley improvements for Rev4. The guys working on mold said that they were working on adding the ribs yesterday, so this change is progressing well. We should see new parts in a couple of days.



Also, we are keeping a log and summary of what all is happening on this web page.

At this time, we have contacted (emailed) all of the customers who have contacted us regarding this issue. So, if your team has not heard from us about this and you think that you should have, please send an email to sales@andymark.com.

Sincerely,
Andy

Re: Rhino track failier?
 

Thanks Andy. I think that addresses the problem our modified 3D model fixed. It's nice to see you guys paying such close attention to this issue.

Re: Rhino track failier?
 

Another update for today:

We've got good news and bad news.

The GOOD NEWS is that the pulley mold modification is done and we have some first run pulleys. They look good so far, and we are testing them now.

The BAD NEWS is that there was much sinking around the inner bore when we reduced that bore to 0.754". So, we won't have Improvement #5 as I mentioned earlier today. We will have Improvements #1-4.

Our mold tool has is a two cavity mold. The tool makers changed the inner bore to 0.754" on one of them, but they left the second one at the original size. They are changing the one back today and tomorrow so that they are both at the original bore size. This eliminates the change which helped users add in a bushing if they wished to do so. With these new Improvements (1-4), I don't think that this bushing addition is needed. It would have been nice to have it, but since it caused sinking trouble with the material, we quickly gave up on that feature.

Sincerely,
Andy B.

Re: Rhino track failier?
 

Has anyone added a bogie wheel to help maintain a little more constant tension:eek: in the front of the rhino drive? We've got ours completed and I got a little nervous in the service when I only saw a turnbuckle tensioner. I'm just an old tanker and frankly don't care for having track casualties in the middle of competition. I'm sure I'm going over the top but...if it can crap out it will for our team. haha

Re: Rhino track failier?
 

This won't add anything to the solution of the problem with the plastic pulleys but I figured I would ask to see if it is an option. What would the cost look like if these were machined from aluminum instead of plastic injection molded and is it cost prohibitive? Some teams including mine would probably pay extra for metal rhino pulleys. The performance wheels run more than the plaction wheel products so is that a similar parallel?

Obviously it is a little late in the season for a new run of products but is that option on the table or is it just too expensive in this case?