Kingcab/madcap diff research

[Muso31 122]

3 hours ago, bavee said:

26.10 for my used one.

 

1 hour ago, Kol__ said:

Brand new one (for my Monster Racer build) is 26.15mm as well.

Cheers both, the thing I don't understand here is for the diff to be 0.8 module it has to be 26.40mm. Either I'm missing something, or Tamiya just don't work to fine tolerances with these components. I think I'll stick to an accurate metric tolerance rather than shrink the new diff design by 0.25mm to match the Tamiya one. This would allow a perfect mesh to be formed with the idle gear if a new one of those is to be designed.

I'm wondering if these minor differences in tolerance are contributors into the reliability of the gearbox, but I'm inclined to say they must know more than I do! Still a bit of a head scratch though - I'd be interested to see if the more expensive components have a finer tolerance, as an exact/precision mesh is less likely to fail under load. The more I work on this, the more I suspect Tamiya simply viewed these as toys

[Saito2 6686]

12 minutes ago, Muso31 said:

Cheers both, the thing I don't understand here is for the diff to be 0.8 module it has to be 26.40mm. Either I'm missing something, or Tamiya just don't work to fine tolerances with these components. I think I'll stick to an accurate metric tolerance rather than shrink the new diff design by 0.25mm to match the Tamiya one. This would allow a perfect mesh to be formed with the idle gear if a new one of those is to be designed.

I'm wondering if these minor differences in tolerance are contributors into the reliability of the gearbox, but I'm inclined to say they must know more than I do! Still a bit of a head scratch though - I'd be interested to see if the more expensive components have a finer tolerance, as an exact/precision mesh is less likely to fail under load. The more I work on this, the more I suspect Tamiya simply viewed these as toys

Just some ideas running through my head concerning all this. Like I said, I'm not an engineer, so I may be way off base. What are the thermal expansion characteristics of the fiber reinforced Polyamide 6 the original gears were likely produced from? How much do transmission gears even heat up in operation? Could this be the reason for the looser tolerance? Furthermore, what about the shrinkage rate of the gearbox housing when removed from its mold and how consistent is it across the many parts trees produced for these buggy lines? The gearbox housing is likely ABS or PC (both have similar shrinkage rates regardless). Could the looser tolerance be a safeguard against stack-up?

It has been my experience in nearly all things Tamiya does that their design philosophy is to run things on the looser side rather than risk ill-fitment or binding ensuring a trouble-free (if not somewhat sloppy) build experience for the end user.

[Kol__ 3377]

40 minutes ago, Muso31 said:

Cheers both, the thing I don't understand here is for the diff to be 0.8 module it has to be 26.40mm. Either I'm missing something, or Tamiya just don't work to fine tolerances with these components. I think I'll stick to an accurate metric tolerance rather than shrink the new diff design by 0.25mm to match the Tamiya one. This would allow a perfect mesh to be formed with the idle gear if a new one of those is to be designed.

You know when installing a pinion gear, you shouldn't have the mesh too tight, a papers thickness gap some say. You need that little bit of play. Does that thinking follow onto the other gears in the transmission? Do you need some free play to ensure smooth operation? What's the diameter of the idle gear that the diff gear meshes with? Perhaps that is also a little (0.25) below the perfect mesh measurement and the two gears sized like that combine to create a smooth running mesh? I think this is to reduce friction and stop the gears overheating from too much contact.

[Muso31 122]

1 hour ago, Saito2 said:

It has been my experience in nearly all things Tamiya does that their design philosophy is to run things on the looser side rather than risk ill-fitment or binding ensuring a trouble-free (if not somewhat sloppy) build experience for the end user.

I think you could be right... This link https://khkgears.net/new/gear_knowledge/gear_technical_reference/design-of-plastic-gears.html says:

On the other hand, these plastics are subject to greater dimensional instabilities, due to their larger coefficient of thermal expansion and moisture absorption. These should be taken into consideration when plastic parts are designed. Therefore the design engineer should be familiar with the limitations of plastic gears. It is usual that plastic gears are brought into use after a practical test.

1 hour ago, Kol__ said:

You know when installing a pinion gear, you shouldn't have the mesh too tight, a papers thickness gap some say. You need that little bit of play. Does that thinking follow onto the other gears in the transmission? Do you need some free play to ensure smooth operation? What's the diameter of the idle gear that the diff gear meshes with? Perhaps that is also a little (0.25) below the perfect mesh measurement and the two gears sized like that combine to create a smooth running mesh? I think this is to reduce friction and stop the gears overheating from too much contact.

It is similar for the other gears, but it should be handled by both something called backlash, and the pitch diameter. The backlash is effectively the play between teeth, I've set that on the diff gear to 0.10mm so there is a tiny bit of play. For example, when moving from reverse to forward, the diff will need to rotate the teeth 0.10mm before making contact with the idle gear and moving forwards. But also, it's the pitch diameter that forms the mesh and not the outer diameter, so joining a pinion gear without the relevant gap would mean upsetting where the teeth should meet (at the pitch diameter). 

I'll set the outer diameter to 26.15mm, may as well keep it the same  if it's good enough for Tamiya..... 

 

...edit actually I'll edit the design to be the correct size once I know exactly which material will be used

[Muso31 122]

Thought an update is due on this....

First off Delrin (acetal) doesn't absorb moisture like nylon does, so it can be designed to be more precise as it is a lot more stable in its properties. So the diff will be machined in Acetal, I've had to redesign it to make it suitable for machining as injection moulding costs thousands for the templates (for mass production), and it doesn't offer any benefits over a milled component. So here's the new diff ready for testing before it goes into production:

I'll be testing the diff with 3D prints first. As for the diff housing... I've been working on the best tolerances for the shaft to fit inside the bearing. These are looking good, 3D printed for testing before being milled from a strong alloy:

 

 

Little bit of work to do but things aren't too far off release. Bear with!

[bavee 1032]

Again, thanks for your work! I ordered two of your A1 support brackets recently 😉.

[Muso31 122]

21 minutes ago, bavee said:

Again, thanks for your work! I ordered two of your A1 support brackets recently 😉.

Ah thanks @bavee much appreciated! You should have said,  I'd have thrown in some treats

[bavee 1032]

1 hour ago, Muso31 said:

Ah thanks @bavee much appreciated! You should have said,  I'd have thrown in some treats

Haha, save them for when the diff housings are ready and I can order them 😉. No rush!

[Muso31 122]

Update time, getting closer to some prod parts. First off the A2 bracket has proven a bit more of a headache than parts usually are, in that there's some small bevelled rises that have to fit exactly in the right place in the chassis of the cover won't sit as it aught to. Here's the latest part but I've made some tweaks to this revision which will need to be milled:

 

 

The planetary diff rev 1 is printed and all is looking great, quite pleased to see everything fitting together without any tweaks needed:

 

Here it is next to the SA version:

 

After I have a 3D print of the final rev, it'll be milled from Delrin and released publicly. Very happy with the diff as it's already working perfectly.

As for the diff housings, they're near release and should be available in Feb. 

Not too long now...

 

 

 

[Kol__ 3377]

Awesome mate! Great to see another update, you have been busy👍

On 1/30/2023 at 11:53 AM, Muso31 said:

A2 bracket has proven a bit more of a headache than parts usually are, in that there's some small bevelled rises that have to fit exactly in the right place in the chassis of the cover won't sit as it aught to.

Is that the bits that butt up against the big ol' 1680 bearings and holds them in place?

On 1/30/2023 at 11:53 AM, Muso31 said:

 

It looks so cool mate👍How come very thin around the edges of the countersunk screw holes? Guess the stronger material can take it.

[Muso31 122]

8 hours ago, Kol__ said:

Is that the bits that butt up against the big ol' 1680 bearings and holds them in place?

It's actually just the little raised parts near the 4x sunken oval shaped holes. These little raises have to be exact or it doesn't fit in the profile in the chassis. I've got a decent 3D printer now so I can check parts much faster and get them to production. 

8 hours ago, Kol__ said:

It looks so cool mate👍How come very thin around the edges of the countersunk screw holes

Maybe the photos over exaggerate it as it's similar to the original, the counter sunk profiles are the correct metrics for the type of screw and yes the alloy is pretty strong in this area on the part. 

In other news, someone anodised one of the gearbox plates, looks very nice... 

[Kol__ 3377]

Cool, nice one that you have a 3D printer..I've thought about one, what sort of cost am I looking at to get set up with one mate?

24 minutes ago, Muso31 said:

In other news, someone anodised one of the gearbox plates, looks very nice... 

'nuff said😂

[Muso31 122]

2 minutes ago, Kol__ said:

Cool, nice one that you have a 3D printer..I've thought about one, what sort of cost am I looking at to get set up with one mate?

Sadly they're not the cheapest things on earth. Mine was £800. There's some Ankermake M5's knocking about on eBay though that were kickstarter projects for around £500.

I'll use it for a lot of other things so it's worthwhile having one, depends what you need it for! 

I'll get more parts anodised in future, should be able to reduce the costs of these too once the first batch of stock is shifted. 

[Misterp180 257]

Amazing work!

I am always blown away by how smart people are. I wouldn’t know where to start with this stuff and greatful that people are out there doing this for the hobby 🙌🏻

[BuggyDad 4474]

Me too. Just caught up with this thread. Your engineering talent and perseverance both are quite something! 

I can see myself restoring a Madcap with your parts some day. 

Does your diff design take commonly available drive cups? 

[Muso31 122]

Thanks for the kind comments but I'm just using trade skills for RC as it's a hobby

1 hour ago, BuggyDad said:

Does your diff design take commonly available drive cups? 

Yes. The diff housings and the diff both have a 5.10mm diameter to house the cups which then mesh with either the large planetery gear when using the custom diff, or stock fixing when using the new alloy diff housings. 

Here's a video of an early version of the diff with the gears in action on the 3D print. This is driven from turning the drive cup, excuse the yellow fingers I just ate an orange  ...

It could be fine but I'd rather use the diff with a spur gear that can slip - that will be the next thing to design

[Muso31 122]

We have housings, for some reason the forum is playing up and I cant upload photos

Before releasing this to the public, @Saito2 @bavee @Kol__ @kontemax if you'd like a free set they're all yours as a thanks for the input. DM me a postal address.

...It's not completely free as you can test them to offer any thoughts on shims that may or may not be required   
 

[bavee 1032]

Sounds great! I can compare an original ball diff (which should be fine according to the seller), an Avante gear diff and an original ball diff with your housings.

[Kol__ 3377]

11 hours ago, Muso31 said:

We have housings

Awesome mate!😎

[Muso31 122]

Forum has fixed the issue with photo uploads. I'll try to get these tested properly tonight. Providing they run well (hopefully no reason not to) I'll release them to anybody who wants a set. They actually look a lot nicer than the pot metal colour...

 

[Misterp180 257]

Amazing work dude! Having stripped down my Madcap restore and seen the mess underneath this is all well timed 😎

[Muso31 122]

I'm done with my testing, as a round 2, if anybody would like to buy a pair of these please DM me. I've still not fully released them until the other gents have done their testing but happy enough to ship some out at this stage. 

[Collin 1956]

HI, I drop this question into this topic because it fits somehow. While I do my Astute restauration to original specs I have to renew thrust bearings. Cant remember where the original went...

Now am I right, there was one version with plastic ring from tree plus seperate ball bearings (Madcap)?
Another version, as showen in the manual, are complete thrust bearings with ring an ball, such as showen in the manual from Astute:


 

I would like to simply print this ring but I dont have any measures, also no idear about the so called "small balls". Can anyone please give me this numbers?

Thanks in advance.

 

[Muso31 122]

From the manual it looks the same as a madcap diff? If so the thrust washer is 6mm inner diameter (think its 10.2 outer from memory) and 0.8 thick.

By 'print' do you mean 3D print? I'm not sure I'd trust a 3D printed part in a diff. As for the ball size theres a post here... 

 

[Collin 1956]

17 minutes ago, Muso31 said:

From the manual it looks the same as a madcap diff? If so the thrust washer is 6mm inner diameter (think its 10.2 outer from memory) and 0.8 thick.

By 'print' do you mean 3D print? I'm not sure I'd trust a 3D printed part in a diff. As for the ball size theres a post here... 

 

Exactly. Madcap/Kingcap is same/simular to Astute. On Madcap manual it shows ball cages made of the tree material so it is ABS. Well, dont ask me about my thoughts but here I am okay for a display car. At least for the moment...