The Worm Gearbox

[xkill]

So, some time ago I was reading this:
Ds3 Supercar: Like Being Thrown Out Of A Plane
And this part got stuck in my head:
“If you see the start-line of a rallycross circuit and look down at the black tyre marks, you will see the cars go off at a slight twist. As they leave the line the back wheels try and come round. The reason for that is because the engines are normally inline and so powerful that the internal inertial motion of the engine pulls the car to one side. But with a transverse car, as the engine is facing the direction of travel it gives you more even lateral grip. It’s better off the start, but also through the corners. You’ll see that inline cars like the older C4 drift through corners, whereas these just grip and go. It’s not so dramatic, but it’s so much more effective.”

So my idea was:
Could I make this in a 2.2s rig?

Only with a worm gear... So I got to work and this is where I am at the moment:





If I would be using a normal 540 motor it would be way to wide, so I plan on using a outrunner motor since they come in so many sizes.
In the pictures I'm using this motor:
A28L brushless Outrunner 920kv
But I also have mounting for a 35-45mm motor.
As it is on the pictures it's 59mm wide.

The best thing is that worm gears don't need drag brake, is very simple and compact!

My plan for this is to make a specific chassis for it so the motor can stick on the side, pretty much as the cars still using the axial gearbox.

I still have a lot of work to to, gearing will be a challenge, as well as chassis design, but I'd like to know what you think.

[crawl-o-matic]

I think this is a cool idea. The biggest problem I see is CG. I think you'd be much better off flipping the whole gear box on its head.

[Jus]

Seal it up so that you could run gear oil and I'd buy one. Pretty cool.

[crawl-o-matic]

sticky marine grease is all you need for smooth, cool operation...but it should be sealed none the less...any dirt or sand in the worm gear and you're 100% screwed.

[crawl-o-matic]

BUT you have to get that motor to the bottom and scrap the dog bone cups...needs 5mm output shafts. Oilite bushings would also be golden at either end of the worm gear due to the nature of how worm gears exert torque (think of a screw, push/pull) as opposed to bearings which are designed to handle roational forces.

[Arctic Fire]

Research and Development;

Multi-million dollar Axial - 0
RCCrawler member with CAD - 1

I like it, flip it, output shafts, seal it.

[Calderwood]

Originally Posted by crawl-o-matic

BUT you have to get that motor to the bottom and scrap the dog bone cups...needs 5mm output shafts. Oilite bushings would also be golden at either end of the worm gear due to the nature of how worm gears exert torque (think of a screw, push/pull) as opposed to bearings which are designed to handle roational forces.

Or better yet, use angular contact bearings.

[gunder463]

can i get 2 yet?

[SilmarilS]

And you will have a heavy power loss at high speed... Since you would be using it direct drive with the motor.
Ask any Losi Worm axle users about friction/heat in the worm gear, and they are way slower than your design!

[crawl-o-matic]

There's a HUGE difference between diff worm gears and primary drive worm gears. The setup won't even get warm let alone hot as long as it's lubed properly. There won't be any loss of speed because of the wide range of gearing choices available as opposed to no choices for the Losi worm drive diffs. In fact, with the right gearing and motor, you could get some insane speed (coupled with insane torque) out of this setup.

[crawl-o-matic]

Originally Posted by SilmarilS

Since you would be using it direct drive with the motor.

The worm gear is acting as a pinion...

[xkill]

Thanks for the input!

Wile I was replying here, early in the night, my laptop just wet kaput....
So I cant work on this anymore for the time being.

But I had this picture already on photocucket!

Here we can see the motor comparison:
Outrunner 28mm
Outrunner 35mm
Standart 540



The distance from the bottom of the motor to the bottom of the box are:
13.8mm
10.3mm
9.9mm

I think that I'll have to use the 35mm outrunner that is about 1800Kv on 3s. I was even looking for a airplane ESC that are very compact and run it on 4s but they don't have reverse...

If I flip the box upsidedown the ouput shafts will rotate in the oposite direction for what I need for the SCX axles... Can I rotare the axles?
Another option would be to lower the motor by putting one pinion on the worm gear and another on the motor. This would also give me some gearing option but in the end It wouls also make it rotate the oposite way...

As far as the bearing, you are totally right! I didn't think of that... Would bushing be OK? Does anyone know if there are any 5x9 (flanged) bushings or of those fancy bearing that you guys mentioned?

I think that with grease it would be ok like any other gear... As far as selling... I'm still trying to see if it works and I don't have a way to produce them.
It has dog bones because I went ahead to make ans exact copy of the gear I have, but it will be changed (in some way...) to 5mm outputs.

Right now this is a 25:1 gear but I have the option to go to a 40:1.
I think I will have to make a big compromise on gearing.... I don't know if the motor will have torque with the 25:1 and I'm sure it wont have speed with 40:1... The outrunners don't have a lot of Kv...
Only testing will say.

[crawl-o-matic]

25:1 with an inrunner will be great...there's no need for an outrunner. Actually, a good brushed motor will work fine, too (maybe even better at the end of the day). 25:1 will leave gearing options wide open for the diffs, so no worries there. Flanged bushings are easy to come by and are very inexpensive to boot. I'm not familiar with the other bearings that were mentioned, but Barry really knows his stuff, I'm sure they would work great but I have a feeling they're pricey but I can't say for sure. In terms of direction, it's not a problem having the motor on the bottom...just reverse the polarity by switching the wires, switch it with transmitter, or turn literally just rotate it east/west.

[Olle P]

Originally Posted by crawl-o-matic

There's a HUGE difference between diff worm gears and primary drive worm gears.

Yes there is, with an advantage to the diff worms!
- Each diff worm only has to transfer the power to run a single axle, while a primary drive needs to transfer not only the power to two axles, but also the power lost in the diffs.
Since a worm drive is a worm drive the efficiency is the same in both cases, but the heat generated in a primary drive will be more than twice that of each diff drive. It thus takes more than twice the cooling power to keep it from overheating. If the worm is mounted directly onto the motor axle, the motor will get heated by the drive...

To avoid torque twist in a crawler, as is given as the primary objective here, using worm diffs is sufficient.

More reading about worm drives.

[xkill]

More than an effort to remove torque twist this is an effort to see if I the inertia of the motor will actually help the car to climb obstacles.
This is also the reason why I want the motor to rotate in a specific direction (clockwise when you look at the motor from the back).
I think I will still have some torque twist because I will still have ring & pinion on the axles.

I have no experience with worm gears so we will see if heat will be a problem.... It was something that didn't cross my mind, but makes sense.

As for the motors.
An inrunner or a (affordable) brushed are too long. On the other hand the outrunners are cheap, short, and are supposed to make good torque.

If my computer continues to work I will try to see if I can make something to lower the motor.

[Olle P]

Originally Posted by xkill

... this is an effort to see if I the inertia of the motor will actually help the car to climb obstacles. ...

Then why not just use a standard MOA configuration?

Or if "single motor" is a requirement: Belt drive!

As I, an engineer and MSc with some R/C crawler experience, see it, any attempt to use a worm drive and get positive effects from motor inertia is bound to fail. Either you don't get the effects or other technical requirements will add much larger negative effects. (Mostly due to added weight in the "wrong" place because of the cooling issue and/or severely limited motor effect for the same reason.)

I understand what you're after. Apart from driving the wheels the motor will have two effects:
1. Shifting pressure from the front to the rear axle or vice versa during acceleration and retardation. The effect will be small but could be a deciding factor while negotiating a slope. It will be beneficial either while going up or going down, and detrimental while going in the opposite direction...

2. Flywheel effects sloing down the roll in a side slope (good), and also add an understeering effect (bad).

[xkill]

Why not a MOA? Because I already have one of those and I've done pretty much every mod that I can to it...
Time for a new challenge and I don't have any 2.2s.

I have a belt driven drift car and I don't think a belt driven car would take the abuse of crawling....
In a bind the belt would jump tooth or snap... At least with me driving.
Chain driven, maybe!

So, worm gears warm up much more than other right? Didn't know that.
The weight distribution will be "the same" as any other 2.2s. At least front to rear. Left to right might be different, also depending on the motor that I'll use.

So far I haven't even build it and already I'm loving what I have learned.
Time to go shoping for some bushings! ;)

[crawl-o-matic]

Originally Posted by Olle P

Yes there is, with an advantage to the diff worms!
- Each diff worm only has to transfer the power to run a single axle, while a primary drive needs to transfer not only the power to two axles, but also the power lost in the diffs.
Since a worm drive is a worm drive the efficiency is the same in both cases, but the heat generated in a primary drive will be more than twice that of each diff drive. It thus takes more than twice the cooling power to keep it from overheating. If the worm is mounted directly onto the motor axle, the motor will get heated by the drive...

To avoid torque twist in a crawler, as is given as the primary objective here, using worm diffs is sufficient.

More reading about worm drives.

My 4,500 rpm worm drive saw stays cool because it's properly lubricated...same would be true here.

[crawl-o-matic]

Originally Posted by xkill

More than an effort to remove torque twist this is an effort to see if I the inertia of the motor will actually help the car to climb obstacles.
This is also the reason why I want the motor to rotate in a specific direction (clockwise when you look at the motor from the back).
I think I will still have some torque twist because I will still have ring & pinion on the axles.

I have no experience with worm gears so we will see if heat will be a problem.... It was something that didn't cross my mind, but makes sense.

As for the motors.
An inrunner or a (affordable) brushed are too long. On the other hand the outrunners are cheap, short, and are supposed to make good torque.

If my computer continues to work I will try to see if I can make something to lower the motor.

How about a reverse pitch worm drive...I think that'd work, but I'm not sure. And heat won't be a problem or I'll kiss my own ass
any attempt to use a worm drive and get positive effects from motor inertia is bound to fail I agree

Bottom line: this thing is going to be a smoooooth torque monster...and if the R&P's are geared correctly, it'll have nice pop, too

It needs a Holmes Puller...I'll get one to ya, don't worry about that!

[Olle P]

Originally Posted by crawl-o-matic

My 4,500 rpm worm drive saw stays cool because it's properly lubricated...same would be true here.

I bet the saw's drive is bigger and thus can spread the heat over more volume and cooling surface. I'd also be surprised if there's no form of cooling fan involved.
Proper lubrication = oil bath. => Adds weight and volume to a tranny.

Depending on the design of the drive (shape and materials used) it can, with optimum lubrication, have an efficiency in the range from 40% up to 80%.
Ideal design is unlikely for the crawler, so an expected efficiency is about 50%.
Roughly half of the power provided by the motor is thus turned into heat by the worm drive.