First Build. Some questions whilst in design phase.

[Daniel Vernall]

Hi all just joined and am going to be starting my first build soon. Am currently in the design phase of my fw bot.
I'd like to be quite the behemoth in the fw category.

Regarding drive motors, I'm looking at using two 12" wheels with a couple of small non powered stabilizers.
To allow for quite a bit of pushing power would a simple cordless drill motor be sufficient for turning one of these wheels and provide a good bit of thrust. Each wheel will have its own motor.

Thanks for your help in advance!

[shakesc]

12 inch is a big wheel on a feather - 100mm / 125mm is much more common

You need to gear for around 10 mph top speed. Going much higher then you will likely have control issues

With the motors and wheels you also need to consider the torque you will get out of the motor when combined with the wheel.

In simple terms a cordless drill motor/gearbox and 12 inch wheels will have low torque (pushing power) and too high a top speed

[rob]

take a look at seraph, team saints FW. http://www.fightingrobots.co.uk/thre...l=1#post444455 give you an idea to wheel size and motors.

[Daniel Vernall]

Ah cool, thanks for the info. I have a specific design in mind with large wheels. I've been looking into motors. According to the info I've seen at 10mph a 12" wheel will be 280rpm.

I've seen multiple motors with around that operating rpm with different power requirements, starting from 6v up. Would I be looking at 12V or is it likely to burn out? I heard a good upgrade was to up to 18v?

[Daniel Vernall]

This is what I'm thinking so far:

Motors:
http://www.andymark.com/CIM-motor-FIRST-p/am-0255.htm

I need a 19:1 ratio gearbox for this setup to reduce speed and beef up the torque. The standard gearboxes to fit available seem to be around 14:1 (too fast/weak) or jump to 24:1(too slow).

I've worked out for my 19:1 sweet spot, I need a 12 tooth drive gear, 50 tooth large cluster gear, a 12 tooth small cluster gear and a 56 tooth output gear. That will give me a gear ratio of around 19.45:1.

What do you guys think to this for drive?

Does anyone around here make gearboxes btw (I've listed it on the wanted forum )?

[shakesc]

The Andymark gearboxes look nice but watch out for postage and tax to ship from the US
The other way, if you dont have access to machining equipment as geared boxes require high tolerances is to use chains and sprockets

Maddox does some nice machining

[shakesc]

PS: CIM motors are not that powerful for their weight

[Daniel Vernall]

I see, what would you recommend to achieve similar power with less weight?

[Daniel Vernall]

Just did a quick compare to the RS775 and here's my numbers:
I'll compare based on stall torque (I know I'd operate at no more than around 1/4-1/3 of this usually)

CIM:
5310 rpm
Stall Torque: 344 oz-in
w/
Gearbox: 19:1
279.5 rpm
Stall Torque: 6536 oz-in
Horse Power: 1.81
Weight: 45.12 oz

Banebot RS775
7300 rpm
Stall torque: 61.1 oz-in
w/
Gearbox 26:1
280.75 rpm
Stall torque: 1588 oz-in
Horse power: 0.45
Weight: 12.4 oz

[shakesc]

So 45 oz is 1275 grams
2 of those you are at 2.5 kg
Andymark Nano gearbox is 1.7 pounds = 770 gram
2 of those is 1.5 kg

So the gearbox and motors would be 4 Kg out of the 13.6 Kg you to play with.

You then need to start adding in electronics etc e.g. example battery pack
http://www.hobbyking.com/hobbyking/s...Lipo_Pack.html
0.565Kg

Once you subtract all this thats your budget for the chassis. It adds up quick

Have a look at for different types of motors and gearboxes
http://www.robotmarketplace.com/prod...rs_geared.html

A good alternative is Dewalt motor and gearboxes if you want some push.
These are readily available but arent that easy to mount without a kit like http://e0designs.com/products/dewut-...gearmotor-kit/

I recommend you read the postings on this forum and the Australian forum ( http://robowars.org/forum/ ) and consider what different designs and components people have successfully used.

[Daniel Vernall]

Ah cool I'm going to look into what you've mentioned and I'll get back to you .

Btw thanks for your time and help

[maddox10]

12" wheels, reasonable speed. In a feather? That's a large order.

Lowest RPM motors on 12V available atm are large brushless outrunners. But the problem with those light, affordable powerhouses are that they are absolutely unproven in combat for drives.

Next best thing are Graupner Speed 900's (actualy 51mm diameter Johnson motors), but those are getting very rare. With 6600 rpm @12V the slowest running usable motor for a featherweight.

[Daniel Vernall]

Ye, I'm beginning to see issues with my design. I've got a new design I'm working on. I'm still thinking of powerful motors for the drive. In your experience what sort of designs have good manoeuvrability? Increasing power to one side of the bot would make it turn but what are other mechanisms?

[maddox10]

There are 2 common ways to "steer".

Car style, using a turnable wheel to controle the direction. Robots of note. Onslaught and Mighty Mouse.

Tank style. What you proposed.

Uncommon or untried means I know off.


Tip Top used the disk as main propulsion, and by elevating a side of the robot tried to steer. Not really a succes.

Omniwheel track. I have seen a prototype at a early social/live event in the Oundle mill, way back a decade or more. 1 huge back wheel for propulsion, and a powered track in the front, 90° on the direction of drive; This track carried small wheels aligned with the back wheel.

Meltybrain spinner. 1 driven wheel, and 5 robots worth of brainpower and electronics.