Delrin Blocks as elevator bearing replacement

#1

One of the debates amongst our leadership and design team of late has been about replacing traditional elevator bearings with CNC machined delrin blocks to be used as linear slides. On last year’s elevator we used the traditional bearing approach, which worked well and never failed us all season, but was a huge time suck of machining and assembly resources. The delrin blocks on the other hand require only one setup on the mill and two screws to mount. Additionally, this would save us about 2.5 pounds of weight (which would have saved us a fair number of cheese holes at SFR).

All of that seems very attractive, but I am concerned about the reliability and longevity of this approach. I am afraid that overtime, the elevator may bind up and become less efficient. I think that we may see galling of the aluminum elevator members by the delrin sliders. Does anyone have prior experience?

One of our mentors had a case in which delrin butter pump rotors badly galled the stainless steel pump housing in which they rotated. You would think the plastic would lose and and the metal would win, but the reality was the opposite. As stainless steel wear particles embedded in the plastic, the wear couple essentially became stainless-on-stainless, and that wear couple is notoriously bad. It was sort of a runaway process. Aluminum-on-aluminum is also notoriously bad as a wear couple, hence my concern.

I did some googling, which suggests hard-anodized aluminum would work. However, if that route could be avoided, that would be fantastic.

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#2

Have you considered using something like this: https://www.woodcraft.com/products/slick-strips-3-width

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#3

We used delrin blocks on our elevator in 2011. I would not recommend it. We had some real issues with it, and burned out several motors. Our elevators in 2008 and 2018 used all bearings, and we’re much more reliable, with fewer issues and no burned out motors between them.

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#4

The coefficient of friction for roller bearings is around 0.002 (reference).

The coefficient of friction between delrin and other materials varies between 0.14 and 0.40 (reference).

Granted, the bearing pressure (normal force) is fairly small, but even so, you’re looking at significantly more power required to move at the same velocity.

Plus downsides such as having to worry about thermal expansion under heavy usage, and risks of misalignment or jamming.

I would say the weight of a bearing approach is worth the speed and lower risk.

#5

W19-132 (ELEVATOR SLIDE BEARING) v2.step (35.4 KB)
here is the delrin slide in question. I had some thoughts about keeping the overall design and embedding bearings inside, but it seems as thought that would be very difficult to do.

#6

We used delrin (or similar plastic, I think we used scrap from a sponsor) to link our carriage to the 2nd stage last year and we really didnt have any issues. We notice that it seemed to handle impacts better than some brackets we had for bearings. We did replace them towards the end of the season due to being worn down and not sliding as smooth as they use to. We did spray frequently a dry teflon layer on the tubes to make them slide even better. We did a similar setup with our claw in 2017, delrin blocks to have our tapped square stock slide on.

They have seemed to work for us, but we are going to all rollers this year for our elevator. We do have a function that we are looking at using Delrin for.

#7

here is an embedded bearing version. I have to check for crashes in CAD because it is a bit wider now, but it should require only two milling setups and then some holes drilled in the side.

#8

Maybe not the most compact way to get the bearings on an elevator, but I have to say that is a nice job on the packaging. With the right material I could totally see these as a 3D printed part.

#9

I think that using delrin blocks, or any sliding-contact plain bearings, will be a real bummer. The performance difference between a coefficient of friction of 0.2-0.3, sliding plastics, and essentially 0, with a roller bearing, will be dramatic.

I’ve seen several examples of minimal-setup elevator bearing blocks. Even some bearing blocks (plates, really) with only 1 setup required.

Our blocks aren’t quite as simple, but are super strong and a comparable version served us well last year. There is one setup to mill, and a second setup to drill 1 hole. They are pretty quick to make. Bearings, pins, nuts, and bolts are all from McMaster.

You can find more details (including PNs) by going to our build thread and viewing the models directly in OnShape:

#10

Though I can’t speak much for using Delrin or the difference it will have, last year we used Nylatron E-Blocks as a replacement for bearings for a lot of the same reasons your team is looking at. We also wanted to avoid the bearings wearing into the metal and creating small grooves as the season went on. We barely ran into any issues throughout the season and if you’re looking to save weight/machining time I think it’s a decent replacement if it’s done right. The only warning I’d give is just to make sure your elevator is sturdy and doesn’t have any flex as it reaches its max height, earlier on in the season our elevator definitely wasn’t as rigid and the friction that was placed on the nylatron worked against the motors and just made the whole system very inefficient. Here’s a top view of our elevator to get a better idea of how we applied it -

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#11

I’ll admit that I’m no expert on this, but in 2018 we used Delrin blocks the entire season and encountered no issues. If anything DID happen that I wasn’t aware of, then it must not have been a major issue, because the bearings were never an issue in match play and I had never heard any discussion of repairs needed. I was skeptical at first, but it seemed to serve us well the whole season. I do know that we lubricated the elevator often to ensure quick sliding. Do your research though, what we did might not translate as successfully over to what you’re doing.

#12

Last year: Our elevator was half Delrin (sorta) We made our elevator stages out of 1/16" aluminum, but that wasn’t very sturdy (obviously). So we coupled the aluminum with Delrin. The Delrin acted as a slider and did that job very well. I would recommend it as a simple cheap solution. You don’t need fancy tools to cut it either so it works for any team.

#13

For what it is worth, Delrin/acetal blocks can work just fine in a vertical lift. One of the tricks to making them work is to have a hard-anodized surface for them to ride on. If you try to use a 6xxx or 7xxx series aluminum without some sort of hardening you have a larger chance of disappointment.

Over the years, I have built a few elevators using 8020 and their slides. The work fine if you run the cable/chain correctly but a bearing setup is certainly superior.

The one advantage of a high friction system is that it takes less liner force to hold it in position when it is in position. The efficiency makes it harder to move thus it is easier to keep it from moving.

#14

Last year we ran delrin blocks on 1/16 square tubing. We made them in the shape of a H where the top of the H was bolted fast to the one side and the bottom of the H slid along on the adjacent tube. The bottom of the H was shortened so that it just straddled the tube it was sliding over. Made for a compact and light weight assembly. The friction was fairly high though and contributed to chains skipping.

#15

IIRC (and please correct me if I didn’t) both Chaos and the Cyber Knights used 3-4x 775s to power their lifts. They had the power to deal with any added friction from using plain bearings instead of roller bearings. They also configured their lifting tensile members (chain for Chaos, rope for Cyber Knights) to pull close along the center of action of their lift elements, minimizing any wracking torque applied to the lift elements. Furthermore, they were both part of #teamtoothpick, using a single center lifting column, rather than a wide-stance frame sort lift like most other teams did. This also helps reduce/eliminate the potential for wracking in secondary sorts of ways.

Not surprising that these two excellent teams independently arrived at many of the same design decisions to make a solid-bearing lift work effectively. These are design considerations that can be often ignored if roller-element bearings are used in a lift. @pKlopp, take notes from these two teams if you pursue a solid bearing approach.

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#16

Might check out 148’s 2018 elevator. It used lots of 3D printed rollers and parts on round tubing. Can download the CAD from their website.

#17

While these cool, solid bearing blocks are really nice, you really don’t need intense machining to use ball bearings for elevator supports. You can easily take care of both side and axial bearing support using two pairs of flat plates mounted to the elevator tubing. See the 973 RAMP videos or the VexPro product for this purpose.

That said, you CAN use solid delrin for bearing support - pay close attention to your surface finishes and be sure to gear the elevator with a little more torque than you think you might need. They can certainly be a pain if there is any extra friction or misalignment, though, so bearings or even delrin rollers are really preferred.

#18

Essentially yes, we used 2 deCIMates geared 4:1, deCIMates have a natural gearing of 4:1 anyways, so we added another 4:1 stage to basically make it a 16:1 gearbox, 4 775’s in total. I don’t think you need that much power for this year but I guess it all depends on what mechanisms you’re riding along your carriage. Also while I think going a single mast helped us out I’d be wary of using it again, a lot of TLC went into making it work the way we wanted and there’s better elevator designs from last year to take inspiration from, as another user already said 148 is a great starting point, especially if you’re worried about weight like OP mentioned.

#19

IIRC (and please correct me if I didn’t) both Chaos and the Cyber Knights used 3-4x 775s to power their lifts. They had the power to deal with any added friction from using plain bearings instead of roller bearings. They also configured their lifting tensile members (chain for Chaos, rope for Cyber Knights) to pull close along the center of action of their lift elements, minimizing any wracking torque applied to the lift elements. Furthermore, they were both part of #teamtoothpick, using a single center lifting column, rather than a wide-stance frame sort lift like most other teams did. This also helps reduce/eliminate the potential for wracking in secondary sorts of ways.

There are certainly different design considerations with a friction lift vs a bearing lift. I agree with having the sliding elements exceptionally close to the lifting mechanism (chain, rope, string, leadscrew, etc). We chose to overcome binding concerns by having dedicated chain run for each vertical column to overcome binding related to have one side of the lift move up faster than the other. The two chain runs were driven by the same gearbox and shaft down in the base of the robot. This meant that the two sides didn’t need to be connected by anything other than the gripper and chassis to prevent them from separating.

The choice of 4x 775 pros was driven by the desire to have the lift take ~1s to go from floor to full height while still having the torque to perform a climb without toasting the motors. If we had chosen CIMS instead then we probably would have needed fewer as we would have been able to run them closer to stall. The system needed about 5% pwm to hold position. In 2007, we (frc 1100) ran the same lift design with just 2x of the lower power fisher-price motors (effectively an AM-9015) and had no issues. This was also one of the heaviest lifts I have ever built. The extra power helped me sleep better at night.

We did have mechanical issues with the system last year, but they were mostly from trying to put chain in a space that was too small for it and having improper tensioning techniques.

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#20

I will also say that I would try to use a bearing lift as a starting point. We started with one last year but couldn’t get parts and didn’t have the ability to custom machine them to make the design work. This led us to switch to a solution that had been built many times before but was not ideal for the task at hand.