Why is this a bearing? (Terra Scorcher)

[Saito2 6686]

Soooo...I'm not the brightest bulb in the box, but considering the BA18 shaft has a flat at the end that plugs into a corresponding "D" shaped hole in the counter gear and thusly doesn't/can't rotate on said shaft, why is a BA11 ball bearing inserted here? Seems like a waste of a perfectly good bearing but maybe I'm missing something.

[El Gecko 1344]

I'm not familiar with the chassis, but judging from the step above that we can partially see in your pic, it looks like it's for the other side of the diff gear/diff yoke?

[Saito2 6686]

The counter gear meshes with that diff when you close up the gearbox halves. The BA18 shaft exits the side of the box to run the prop shaft. The BA11 bearing fits into the back of the counter gear, but the counter gear itself is locked/keyed to the shaft by the "D" shaped hole. The counter gear doesn't rotate on the shaft, rather the whole shaft spins with the counter gear locked to it.

[El Gecko 1344]

Sorry, was looking at the wrong BA11. You're right, since the counter gear is keyed, it definitely doesn't spin on that shaft. So why put a bearing inside the gear? The only reason I can think of is that it might be a tighter fit on the shaft vs. a bushing which is most likely a looser tolerance. And maybe Tamiya thought that the larger circle of the bearing would be a better mating surface for the gear, rather than molding the plastic all the way down to the shaft size. Possibly for longevity and strength, since that single spur gear drives both gearboxes. Just not for the function of the bearing actually spinning.

[Frog Jumper 3661]

Personally, I think its a left-over from a previous design iteration...

Terry

 

[WillyChang 1814]

Me suspects T has been repurposing BBs (+shims) for duty as shaft spacers in some reres rather than remaking the OG’s custom tube part. 

BiTD we’d use bronze 850s for that job if we lost the proper spacer. 

[El Gecko 1344]

3 hours ago, Frog Jumper said:

Personally, I think its a left-over from a previous design iteration...

Terry

 

 

2 hours ago, WillyChang said:

Me suspects T has been repurposing BBs (+shims) for duty as shaft spacers in some reres rather than remaking the OG’s custom tube part. 

BiTD we’d use bronze 850s for that job if we lost the proper spacer. 

Indeed, we have been missing the difference between re-re and original. The vintage manual for the Terra Scorcher shows two pieces there instead of a single ball bearing: E1 which looks like a D shaped bushing, and SW4 which is called a Thrust Washer in the sidebar. Also listed is the "Ball thrust bearing bag" which isn't even mentioned on the re-re manual.

[Saito2 6686]

That's a bingo. I never thought to look back at the original manuals. Thanks.

Now I'm looking at the original manual and wondering why they choose an E1 spacer and thrust washer combo to start with originally. There really isn't any thrust load going on there. The counter gear itself is straight cut, so no load there. The counter gear itself along with the E3 bevel gear on the other side of the shaft, while keyed to the shaft, still "float" on the shaft. Even if the other E3 bevel imposed thrust loading, the friction point would be between the E-ring and the BA12 850 bearing in the gearcase, not the backside of the counter gear and the E-ring. 

Adding more confusion, Tamiya correctly placed an E1 spacer and thrust washer behind the bevel in the re-re, where it would actually see thrust loading. However, in the original Terra Scorcher manual, this is a 1150 ball bearing.

I know this is probably too detailed analysis for "just" and RC car but I like to know why things are done the way they are. Unfortunately, with Tamiya, I never know if things are done for cost (i.e. its cheaper to do it this way now vs how we did it back then) or actual improvement (i.e. we didn't do it right the first time, so we're making a subtle correction with the re-re).

I'm also debating (if the tolerance is equal), should I not put a plastic bushing in the backside of the counter gear to reduce rotating mass?

[Frog Jumper 3661]

1 hour ago, Saito2 said:

That's a bingo. I never thought to look back at the original manuals. Thanks.

Now I'm looking at the original manual and wondering why they choose an E1 spacer and thrust washer combo to start with originally. There really isn't any thrust load going on there. The counter gear itself is straight cut, so no load there. The counter gear itself along with the E3 bevel gear on the other side of the shaft, while keyed to the shaft, still "float" on the shaft. Even if the other E3 bevel imposed thrust loading, the friction point would be between the E-ring and the BA12 850 bearing in the gearcase, not the backside of the counter gear and the E-ring. 

Adding more confusion, Tamiya correctly placed an E1 spacer and thrust washer behind the bevel in the re-re, where it would actually see thrust loading. However, in the original Terra Scorcher manual, this is a 1150 ball bearing.

I know this is probably too detailed analysis for "just" and RC car but I like to know why things are done the way they are. Unfortunately, with Tamiya, I never know if things are done for cost (i.e. its cheaper to do it this way now vs how we did it back then) or actual improvement (i.e. we didn't do it right the first time, so we're making a subtle correction with the re-re).

I'm also debating (if the tolerance is equal), should I not put a plastic bushing in the backside of the counter gear to reduce rotating mass?

Use a brass bushing and lots of cera-grease…

[Saito2 6686]

I realized the scope of this thought process is leading down a path I've been before:

So here's the other page involving the bevel gears involved from the re-re Terra Scorcher manual:

 

So, a quick run through most of the Thundershot/Terra Scorcher manuals reveals the Thundershot used plastic bushings everywhere, including in the backside of the counter gear which was my original query. It also uses E1/thrust washer combos in both places for the prop shaft outdrive, concerning the E3 bevels, just like the re-re Terra Scorcher manual does pictured above.

The re-re Terra Scorcher obviously uses bearings everywhere, including the useless one in the backside of the counter gear and the E1/thrust washer combos pictured above in the prop shaft outdrive assembly.

The original Terra Scorcher used bearings everywhere except the backside of the counter gear where an E1/thrust washer combo is present like the re-re. Actual thrust bearings are used on the BA17 prop joint shaft and an 1150 ball bearing is found inserted into the A4 case rather than the E1/thrust washer combo used in every other model and every other instance.

Whew. 

Of course the link I posted also listed this (above), which basically tells you to use bearing in place of everything. So, that's basically what I'm going to do except the backside of the counter gear. A fresh plastic bushing actually has less slop than a ball bearing in this location, more precisely indexing the counter gear to the shaft. Since it never rotates, it should never wear. It also saves a good deal of rotational weight. 

 

..or I'll totally change my mind tomorrow.

[AJB123 107]

55 minutes ago, Saito2 said:

So, that's basically what I'm going to do except the backside of the counter gear. A fresh plastic bushing actually has less slop than a bear in this location, more precisely indexing the counter gear to the shaft. Since it never rotates, it should never wear. It also saves a good deal of rotational weight. 

 

..or I'll totally change my mind tomorrow.

Don’t change your mind. There has to be at least one valid use for all of those unused white plastic bushings we must all have, stashed away!!

But also, yes, I’d have thought that a bushing would work better than a bearing when what’s wanted is just a spacer with no rotating motion.

[Saito2 6686]

Doing some research, it seems radial ball bearings are capable of taking some thrust load (though I am curious what % of ball circumference is supported axially by the inner and outer races internally in a typical Tamiya ball bearing) 

Since the center raised boss on the back of the bevels is small enough to contact only the inner race of a typical 1150 ball bearing, using them in in the A4 case/housing as Tamiya did with the original Terra Scorcher should be feasible (not to mention the prop output shaft as many here have already done). My only concern is the ball bearing is about .008"-.009" thicker than the thrust washer combo. This will slightly bring both bevels into closer contact with each other. Normally a not much of and issue but with the quite poor molding found on these gears, the tighter tolerance my exacerbate the problems already there. I will have to be extra vigilant in filing/shaping the gear teeth here in this case.

Edit: I just realized much of my meandering topic has been cover already in Frog Jumper's TRF Fire Dragon post. Sorry for the redundancy if anyone read this far. Hopefully you saved some time and gave up before reading this far, lol.