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Saito2

Moly grease use and diff durability question (TXT)

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Two part question about TXT axles and greases. I've read several time on here that Tamiya Moly grease is better suited on metal to metal gearing. If that's the case, when I rebuild my TXT-2 axles, would it be better to substitute the Tamiya Ceramic grease I typically use with Tamiya Moly?

Secondly, Tamiya Anti-wear grease is often suggested to slow down diff action (a'la limited slip effect). Would slowing down diff action on the TXT be detrimental to its diff durability? Specifically the the 3-pronged star shaft inside the diff that supports the spider gears has been know to break in the past. So which is kinder to that shaft, loading it down with sticky grease to slow spider gear movement or allowing the spiders to spin freely but rapidly when the axle "diffs-out" (un-loaded wheel spinning)?

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I typically use Moly grease for metal gears and Cera grease for plastic gears.  In my TXT axles I typically pack the diff completely full with automotive wheel bearing grease to get as much of a llimited slip effect as possible without totally locking them.  Have never broken a set of diff gears or axle shaft in a TXT that I can remember.  I would think that minimal grease in  there would cause more breakage from the hard stops and starts in the gears when unloading a tire and having it come back down.  Hard to say for sure, but I've had good luck packing the diffs full and prefer the way it drives like that.  I've had way more issues with the tiny screws that hold the diff together, they seem to back out no matter what...

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52 minutes ago, 87lc2 said:

I would think that minimal grease in  there would cause more breakage from the hard stops and starts in the gears when unloading a tire and having it come back down. 

Good point. I hadn't considered the shock load involved in a potential sudden hook up.

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It's funny you brought this topic up today.  Stopped at my parents for dinner on the way home from work and being that my father always builds his RCs box stock I took his Agrios out for a drive.  The diffs are wide open with just the recommended amount of grease, I could tell as soon as I drove it.  To be honest I didnt like it at all.  I much prefer the way a packed diff works on that truck, it will still unload a tires from time to time, but only in pretty agressive maneuvers.  I like how the packed diffs push through the turn a bit where unpacked diffs pretty much try to spin the inside tire right away.  

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Moly sticks to metal surfaces. It fills imperfections. It can withstand a lot of pressure because it binds molecularly. And it's very slick. Ceramic grease is microscopic glass beads that should work like bearings.  It does work like that when it's working between two teeth that are giving, like plastic gears.  But ceramic is harder than aluminum, which eats up aluminum pinions like sand-blasting.  In theory, moly should be superior to sand-powder (ceramic).  I haven't used it so, a theory is all I've got. ^_^

 

[[ 3 prong shaft ]]

I thought about that question before, but I hadn't thought through it until now.  It took me a while to organize my thoughts, but I think it should be alright. (I haven't found any engineering article or anything. I was just trying to follow the logic)  

[Strength]  I would say the 3 prongs of a star shaft encased in a diff case is stronger than 1 nub of a dog bone. You know how when the wheel is steering at a funny angle, only one nub of the dog bone could be doing most of the work?  That one end of the pin is strong enough to break the drive cup.  "2mm stainless pin" vs "three prongs of 3mm pot-metal encased in a gear together," I'd say the star shaft would win all the time.  (if you calculate the area of both, it's 3.14 vs 21 square millimeters)  Material strength wise, they should be fine.  

Using Anti-Wear grease could put tiny bit more burden on the 3-pronged star shaft in terms of sticky grease "twisting" the shafts because bevel gears are turning and shafts would want to twist along with them.  I don't think that's a strong force.  I use sticky grease on Juggernaut 2, it's fine.  (As a random thought, if star shafts are made of 4 pieces, like 3 nails coming out of 1 ring, each shaft could twist with respective bevel gears, and the 3 prongs would wear less. But that could raise the production cost for gears that's not all that active. I've color-coded in red at the bottom). 

nxsZfoy.jpg

[Unloading saves]  The star shafts take the most force if you do a full forward while going full reverse (no differential action).  Most cars would wheelie.  Actually, the worst abuse would be going full throttle, jump, and land backward on full throttle the other way.  The tip of each prong could break (color-coded green at the bottom diagram). That could even break a drive cup or a dog bone, maybe even the star shaft.  But imagine losing traction on one of the wheels at the moment of landing.  The explosive energy that could go to the 3 prong, would leak out to the wheel that's not in contact.  The differential action would actually save the 3 prong shaft, instead of breaking it.  

What if we make 3 pronged shaft out of toothpicks?  It's going to break in normal driving even without any differential action.  On the other hand, what if you lift the car up and turn one wheel, without turning the motor?  The diff can work without breaking the toothpick shafts.  Going forward or backward is a burden on the star shaft, not the differential action.  

Let's say that at full throttle, the star prongs get 1kg of force, split to both wheels.  If the force is transferred to only one wheel, does it get more force?  That one wheel (and tire) itself take on the full force.  Instead of 0.5kg, it now gets full 1kg. And you do a funny steering and only one wheel gets all the force from 1 nub of a dog bone?  That drive cup can break.  

But what about the drivetrain?  The motor does not work twice as hard.  It does the same work, gears do the same work, I think the star shafts do the same work.  For sure, the bevel gears in the diff would suddenly work very hard.  But because the diff tries to "unload," even if you have clay, the 3 prong shaft would be less stressed than going forward with full traction. 

Below is an animation.  Note the speed of the 4 prong shafts (essentially, the speed of the spur gear).  It's same whether or not the diff works.  If one wheel is completely unloaded on ice, the star shaft should get less burden, because it's not pulling the car. 

[One wheel driving]

What if it pulls with just 1 wheel because it's packed with differential clay?  I think the star shaft itself would not be burdened any more than when the diff is not working (by going straight).  Clay will change shape and heat up little bit due to physical forces on it.  However, that much energy was taken away from the star shaft. It should get less force than going straight at the same RPM. 

 

Ultimately, what you are worried about is the bevel gears being pushed around because of clay or grease.  It would be a problem if only one side of the bevel gets the force.  But it's got clay or grease on both sides.  If you assume bevels are turning counter-clock wise below, where I circled would be squishing clay.  But there is another side that's squishing too.  So it's balanced.  The pivoting axle would get more force, but ultimately one side pushing should be canceled by the other side. (Color coded blue below)

1iYUl6S.jpg

bYqUk0n.jpg

Now my head hurts. I'll go watch "Kipo and Wonderbeasts." (yes, it's for children, and I like it.)  

4zj6iZk.jpg

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So it sounds like I'm the only person on here so far who's actually managed to snap one of the 3 pronged pieces, just confirming that is possible, and that it is a weak point in the transmission. @Juggular has come up with a lot of useful info here, namely I was not aware of the differences between the moly and ceramic grease, and have always greased everything with ceramic grease. I think I'll be amending that in the future in light of this info to use moly grease on metal gears. 

Back to the 3 pronged shaft. I run my TXT-1 with open diffs. On the front I have a TT-01 1 way roller fitted to take the strain off the transmission when cornering on high grip surfaces. I have the rear steering locked out so the dogbones have both parts of the pin in contact. I managed to break it by using a 1/8 brushless motor in it on 3S. The rear spider snapped of 1 pin at the base. As others have said, it takes all the shock load when accelerating hard or landing off a jump with the throttle on (Hard braking usually causes the rear end to come off the ground on a normal TXT, mine with the one way roller just causes the rear tires to lock up, but most of the weight is being transferred to the front axle anyway so no problem there). It's on my list of things to do to make a steel replacement.

Just to talk about the dogbones for a minute. Just a minor correction, the pins in the TXT-1 / Jugg are 3mm not the usual tamiya 2mm. It is possible to snap the end off one of the dogbone driveshafts by having the diffs locked or very tight. When landing off a jump, if you land on one wheel, all the momentum in the truck is trying to turn that locked axle through one dogbone, and I managed to twist the end off a TXT axle doing just this (The dogbone pin continued to rotate whilst the axle remained stationary. I don't think it was even a jump. I just cartwheeled it backwards down a steep hill and was unlucky enough to land on one wheel (I had very small motor pinions fitted at the time, so it took a lot of force to turn over the transmission). This is why I don't run locked diffs any more.  

 

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Thank you all for the very valuable input. My Agrios has sat for some time and I intend to go through it, de-powering it in the process. Its gone from a Sidewinder 4600 brushless (briefly) with the punch control turned way up, to currently twin brushed 15T HPI Firebolts and will be going to twin Tamiya Sport Tuned motors. I may possible give 4wS a try at those reduced power levels. I'm kinda looking for Clod on 8.4v type speeds. More realistic. In light of this, I'm probably way over-thinking this axle thing as usual, but its great fun to learn.

7 hours ago, Juggular said:

Using Anti-Wear grease could put tiny bit more burden on the 3-pronged star shaft in terms of sticky grease "twisting" the shafts because bevel gears are turning and shafts would want to twist along with them.

Hadn't considered this, but would probably use Moly on the shafts themselves and Anti-wear on the spiders/bevels.

7 hours ago, Juggular said:

Ultimately, what you are worried about is the bevel gears being pushed around because of clay or grease.  It would be a problem if only one side of the bevel gets the force.  But it's got clay or grease on both sides.  If you assume bevels are turning counter-clock wise below, where I circled would be squishing clay.  But there is another side that's squishing too.  So it's balanced.  The pivoting axle would get more force, but ultimately one side pushing should be canceled by the other side. (Color coded blue below)

That's what was in my head and that explanation helps me sleep at night :).

57 minutes ago, MadInventor said:

So it sounds like I'm the only person on here so far who's actually managed to snap one of the 3 pronged pieces, just confirming that is possible, and that it is a weak point in the transmission.

Thanks for confirming I'm not totally crazy. I knew I had read that somewhere, lol.

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8 hours ago, MadInventor said:

I managed to break it by using a 1/8 brushless motor in it on 3S

Thank you. It is very helpful to know the mechanical limit.  I would have thought it could break with 1/10th motor at 3S in tricky circumstances. (I suppose it still could)  But suspecting that there would be a limit, and getting a firsthand report are two different things.  Hats off to an explorer who has gone beyond the envelope. (If I ever go with a 1/8th motor, I should keep it at 2S).  

 

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I ran one of my TXTs on a 1/8 Castle Brushless system for a long time, 2200kv motor I believe.  Mostly ran it on 2S, with the occasional 3S pack through it and never hurt a differential gear or axle shaft.  I did have the diff screws back out a few times as I mentioned before.  Even when they backed out and I heard the diff crunch nothing broke, but I knew it was time to tear it down again and re-secure.  Even with red loctite they kept backing out.

Saito - I recently de-powered that TXT as you are doing now and went with stock 27t Tamiya motors.  On 2S it leaves a bit to be desired, but on 3S it's perfect.  Just enough power, motors stay cool, and nothing has broken.  I also really like the Tamiya torque tuned motors, I put them in a stock Bullhead last year and it feels like it has a lot moe punch than my stock Clodbuster on 27t motors.  

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1 hour ago, 87lc2 said:

I ran one of my TXTs on a 1/8 Castle Brushless system for a long time, 2200kv motor I believe.  Mostly ran it on 2S, with the occasional 3S pack through it and never hurt a differential gear or axle shaft.  I did have the diff screws back out a few times as I mentioned before.  Even when they backed out and I heard the diff crunch nothing broke, but I knew it was time to tear it down again and re-secure.  Even with red loctite they kept backing out.

Saito - I recently de-powered that TXT as you are doing now and went with stock 27t Tamiya motors.  On 2S it leaves a bit to be desired, but on 3S it's perfect.  Just enough power, motors stay cool, and nothing has broken.  I also really like the Tamiya torque tuned motors, I put them in a stock Bullhead last year and it feels like it has a lot moe punch than my stock Clodbuster on 27t motors.  

My motor is a 2000KV Hobbywing motor, and just like you say, 2S is very mild, but on 3S you get a good turn of speed and the motor does not heat up at all, and suits the truck well.

I did briefly try mine on 4S to do an entry for the Tamiyaclub speed run, and well, if you can you've got to try it at least once :D . It was way too much power. Snapping the throttle open would cause huge wheelies with the rear bumper dragging the floor.

 

 

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2 hours ago, 87lc2 said:

I also really like the Tamiya torque tuned motors, I put them in a stock Bullhead last year and it feels like it has a lot moe punch than my stock Clodbuster on 27t motors.  

Considering these too as I've had good luck with them and they are a bit cheaper. 

Its interesting how many of us have experienced different issues with the axles. I've read about the star shaft breakage and the screws backing out in the past but personally have had issues with the MR16 drive gear (as its called in the TXT-1 manual). The gear has a D-shaped hole that slips over a flatted shaft (its the first non-bevel gear in the axle). The D-hole gradually eggs out and gets sloppy. I've installed shim stock with a special Locktite to take up the clearance and prevent the slop. I've unfortunately seen pics were the hole/center of the gear actually burst out in failure mode. Someone on Facebook supposedly makes a steel replacement but I don't do the Facebook myself. The truck could use some form of slipper clutch with all those diecast gears in it.

 

 

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1 hour ago, Saito2 said:

Considering these too as I've had good luck with them and they are a bit cheaper. 

Its interesting how many of us have experienced different issues with the axles. I've read about the star shaft breakage and the screws backing out in the past but personally have had issues with the MR16 drive gear (as its called in the TXT-1 manual). The gear has a D-shaped hole that slips over a flatted shaft (its the first non-bevel gear in the axle). The D-hole gradually eggs out and gets sloppy. I've installed shim stock with a special Locktite to take up the clearance and prevent the slop. I've unfortunately seen pics were the hole/center of the gear actually burst out in failure mode. Someone on Facebook supposedly makes a steel replacement but I don't do the Facebook myself. The truck could use some form of slipper clutch with all those diecast gears in it.

 

 

It actually says in the manual not to glue the tires on. I think it was the intention that tires slipping inside the wheels were a cheap form of slipper clutch.

I've also seen the diff screws backing out and the wear in the MR16 drive gear you describe.

I've just finished up making a new input shaft for the rear axle on My Jugg 2. I was going to just fit a TXT item in it, but I can't find any for sale at the moment so I ending up doing my own mod. I drilled the splines out of the first bevel gear and loctited it to a new 6mm shaft. I was a bit dubious about sticking brass to steel with the loctite but it grabbed in a couple of minutes and I think it's going to hold well. That being the case, it would be easy to do the same with the second bevel gear (to attach to a 5mm shaft, and then get a steel replacement for MR18. Trouble is, I have no idea what the module is for that gear to order another. They're fairly beefy so I'm thinking mod 1.5 or mod 2.0, but the sample gears are expensive to buy to just find out.

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Out of curiosity, what are the gears in the TXT axle made from? Most of the gears appear to be the typical Tamiya pot-metal but the first two large bevels feel dense/heavy. Chromed brass perhaps? Was this the case in the smaller, failure prone Juggernaut 1 bevel gears or were they pot metal as well (in addition to not having properly supported shafts)?

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On 11/22/2020 at 3:46 PM, Saito2 said:

Out of curiosity, what are the gears in the TXT axle made from? Most of the gears appear to be the typical Tamiya pot-metal but the first two large bevels feel dense/heavy. Chromed brass perhaps? Was this the case in the smaller, failure prone Juggernaut 1 bevel gears or were they pot metal as well (in addition to not having properly supported shafts)?

The first 2 gears are chromed brass. I know because my first TXT's chrome flaked off the bevel gears, and I drilled the splines out of another. Definitely brass. The diff gear and the gear it meshes (MR16) are the usual tamiya cheese / pot metal.

I also have some information to pass on with respect to the diff gear and the MR16 drive gear. I took one of my axles apart and got busy with the calipers.

Distance between the diff axle and MR16 shaft = 26.5mm centre to centre (32mm edge to edge when measured with a 6mm and 5mm shaft in situ)

MR16 gear  = 16 teeth

Diff gear = 28 teeth

Module of gears = 26.5 / (28+16)/2 =1.2045

It's actually module 1.2, which gives a theoretical distance between the gears of 26.4mm. The extra 0.1 I measured is to provide a small degree of clearance between the gears (I use 0.1mm gap in all the gearboxes I've made when using module 1.0 and module 0.8 gears)

That's not very helpful as module 1.2 is not an industry standard size, and there is no D.P. equivalent either. Can't buy module 1.2 gears anywhere in the world on EBay. Thanks Tamiya........ 

It's not all bad news however, as, if you take mod 0.8 (Tamiya standard size gear teeth for most buggies) 33 * 0.8 = 26.4. Hey presto, a match. I wonder if they originally designed the axle for 0.8 gears and then found they shredded themselves with pot metal or plastic.

So as long as the number of gear teeth adds up to 66 (33*2), it's possible to use mod 0.8 gears as replacements (I'm thinking custom fits for one way rollers, or for the rear end, one long shaft going all the way through the axle from end to end (Rear steering lockout and diff lock).

To get the gear ratio the same as the existing axle:

existing axle = 16/28 = 0.571

New axle = 24 / 42 = 0.571

Funny how you can get an exact match in module 0.8. Why they didn't use module 1.25, which was an industry standard and would have only made the gears 1mm further apart?  I guess they'd already tooled up the axle dies before thinking 'Ahh, this might  a problem using mod 0.8'

 

 

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Fascinating stuff, though frustrating why Tamiya always seems to find a way to make it difficult, lol.

4 hours ago, MadInventor said:

he first 2 gears are chromed brass. I know because my first TXT's chrome flaked off the bevel gears, and I drilled the splines out of another. Definitely brass. The diff gear and the gear it meshes (MR16) are the usual tamiya cheese / pot metal.

Were the bevels in the Juggernaut 1 chromed brass or standard pot metal? I know they appear smaller but I was wondering if the chromed brass gears were another upgrade against wear in the Jugg2/TXT series. Slightly off topic, is chromed brass also what Tamiya makes their standard shinny diff outdrives out of? They appear to get eaten away by the steel cross pin on the dogbone end as power levels increase. My guess is brass is tougher than pot metal yet easier/cheaper to machine than steel but uses the hardness of chrome (plating) to create a harder wear surface.

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