Acetyl with Bronze Bushing - Lubrication

Technical Technical Discussion
#1

Hello All,
I have been prototyping a deferential swerve module and have run into a bit of a snag. I am using press-fit acetyl spur gears for convince. I trimmed part a spur gear’s hub length for space constraints, this of course limits the amount of friction on the shaft and the gear could slip. I could try to use loctite or various adhesives for working with acetyl, Delrin, HDPE, etc but I would prefer not to. A cross-hole & pin in the shaft is also not an option.

I was not able to source a bearing with the required ID and OD for the spur gear hub and gearbox plate respectivly, however I was able to source a bronze bushing that will work with my space constraints (if this was competition hardware I would redesign the plates to accommodate a proper bearing). Here lies my question - What lubrication (if any) would be required for acetyl-bronze contact?

Additional info:
Shaft diameter: 3/8"
Shaft max RPM: 950 (free)
Axial load: Minimal
Radial load: Minimal

#2

Wait, why do you have Acetyl-Bronze contact? Is the bushing sitting static on the shaft and the acetyl gear is spinning on the bushing?

#3

I originally trimmed the hub of the spur gear, the 1/4 in shaft this gear is press fit on went through a 1/4" x 1/2" fanged ball bearing. The spur gear had less griping power on the shaft than it was designed for. I want to remedy this. The spur gear has a 3/8" extended hub. I can’t source a 3/8" x 1/2" fanged ball bearing (I did not do an extensive search, but checked the usual suspects and no dice, defiantly a specialty bearing). I did find a bushing that meets these sizing requirements. The extended hub of the spur gear would be in contact (max 950 rpm) with the bushing while the ID of the hub is acting as-designed-press-fit on the 1/4" shaft.

This is by no means a production thing, I am just trying to limit the chance of slippage on the shaft during testing and that throwing everything out of wack.

#4

I’m not sure a caster wheel would even need a bushing… (Hey, you said “deferential swerve module”, which means that it defers to others, and a caster is the best fit for that :p;).) Correct spelling, “differential”. Comedic misspellings means engineers are around, no big deal.

Anyways, on to the technical question. Let’s see if I can understand this:
You have acetal spur gear(s) on a shaft, possibly sliding around, and you’ll be running the shaft through bronze bushings in the gearbox plate. You want to minimize friction between the spur gear(s) and the bushing(s), so you want to know what lubricant to use.

Well… Delrin is (practically) its own lubricant, in some forms. And if you plan ahead and get something like these, the bushings are already lubricated (SAE30 oil). Or if you really plan ahead you can get some “dry-running” sleeve bearings (bushings) that are already plastic. I wouldn’t worry too much about it.

If I might make a suggestion, how much face width on the gear itself do you have left (and what’s the diameter of the gear)? It might be possible to put a setscrew down through the middle of the face to the axle.

#5

Eric asked some good questions. Also, he beat me to the deferential joke. Dang it. I’ll just add, have you looked at needle bearings? There may be one that fits your needs on McMaster.

#6

There likely isn’t one. I just checked. McMaster’s needle bearing ODs conveniently skip .5". You’re looking at 9/16" OD for a 3/8" shaft and 7/16" OD for 1/4" shaft.

Questions were about, basically, “how is the shaft locked from sliding around, you can exploit that”, “can’t have the spur gear sliding around, see below”, and another one that I judged unnecessary to ask. (Edit deleted most of 'em.)

#7

Here, these cross sections will probably help.


The shaft is locked from any axial translation (end-shake) - beyond what is needed for clearance.

This bearing/bushing does not experience any real loads. It is primarily for alignment.

My indicated “.001 interference fit” is just an approximation - not an actual measurement.

Scale is way off in these cross sections.

[Sorry for previous typos - it was late :rolleyes: ]

#8

What’s the part number for the bushing? Especially if it’s one of the Oilite impregnated bushings I don’t know if you need any lubrication for this. Both acetal and the bushing are somewhat self-lubricating, particularly for a short run time.

#9

Servo city special AGB-A2-16. Not listed as oilite, if it was I wouldn’t be asking.

I am well aware most of the material properties at play, save plastic-bronze interactions. I am lost here.

I could turn my own bushing from brass and go after it with a polishing compound as well…

#10

Without seeing the entire design concept it’s hard to understand all the space constraints and shaft retentions.

A metal spur gear can be machined down thinner, and still have better shaft retention capability than plastic.

Cothron already mentioned needle bearings.

Metric bearing have a better availability of thinner ID to ODs. If you went to a 6mm shaft, there are flanged bearings with either a 10 or 12mm OD (under 1/2 inch).

One thought is to use a 3/8 Hex shaft and a standard 3/8 Hex Vex Aluminum gear. Except for the gear area of the shaft, you could machine down to round as needed on a lathe.

#11

Silly question: Would it be possible to increase the diameter of the hole in the plate to accommodate a bearing that would fit the hub of the gear?

#12

Something I considered, but I figured I would try the $5 solution first. And I got curious about the acetyl-bronze interaction.

Short answer yes, I was sure to design the plates with clearance. Long answer I do not have a large enough 4-jaw for my lathe (I could do a super-glue arbor though).

#13

I would not worry about adding lubrication to that interface.

I would just run it, and see what else breaks :slight_smile:

(did you see how the AndyMark wormbox works? how the plastic gears are retained to the shafts? they have a notch in the gear, and a roll pin through the shaft. Exactly like the Chamberlain garage door opener, which they were originally used in)

#14

I reworked some spacing in the gearbox and was able to get everything to fit w/o using the bushing technique. The 1/4"x1/2" flanged ball bearing is supporting the shaft. Thanks for the responses. (Doesn’t mean I lost intrest in the bronze-acetyl relationship though, perhaps it’s viable for real low cost/ light duty gearboxes)