Gear Ratio Question

[ACEGUY]

I am currently thinking of using 2 Ampflow a40-300 for my drive as well as possibly running an Ampflow A28-400 motor to power a spinning disc.

The problem I am having is figuring out gear ratios. I have looked at the downloadable attachment at the Ampflow site but it doesn't really make sense to me.

Any help?

[maddox10]

A40-300 4000rpm@24V.

What is the end speed you want to reach with what size of wheels?

For example. A standard 6" solid polyurethane wheel and 25kph.
4000rpm*.152m*3.14=1914meters per minute
1914*60= 114840 meters per hour
114840mh/1000= 114,8Kph
114.8Kph/25Kph=4.5
Result = gearratio 1 to 4.5 15 tooth gear on the motor and a 68 tooth gear on the wheel.

[ACEGUY]

A40-300 4000rpm@24V.

What is the end speed you want to reach with what size of wheels?

For example. A standard 6" solid polyurethane wheel and 25kph.
4000rpm*.152m*3.14=1914meters per minute
1914*60= 114840 meters per hour
114840mh/1000= 114,8Kph
114.8Kph/25Kph=4.5
Result = gearratio 1 to 4.5 15 tooth gear on the motor and a 68 tooth gear on the wheel.

what I'm looking at is a 10" pneumatic wheel with a top speed of 15mph, that's the hope anyway.

[maddox10]

Short answer. A 1 to 8 gearratio.

[ACEGUY]

Short answer. A 1 to 8 gearratio.

Thanks,

I was wondering does the same calculation to find the drive gear ratio work with finding a spinning weapon's gear ratio?

[maddox10]

Yes.

[ACEGUY]

I have been wondering, I know that E Tek seems to be the go to motor for HW spinners, but could the Ampflow A28-400 work for a spinning disc or bar?

[maddox10]

In the end, any motor can work for a spinner motor. As long it doesn't fry itself due wrong gearratios compared to the mass and diameter it has to swing around.

If you load an electric motor too much, it will draw more ampères, so, convert more electric power into mechanical power and heat.

Problem is in efficiency. An electric motor is most efficient in converting electric power into mechanical power at a certain RPM.

For example.
A Bosch GPA750 is about 78% efficient when drawing 24V 40A = total 960W.

78% is converted to mechanical power = 748W. The 212W not converted in mechanical power is converted into heat. That is why a Bosch 750 has a fan, as it cannot keep running without that fan, as it has to get rid of 212W of heat (that's 8 normal soldering irons). For us, it's not that big a deal. We run for 5 minutes and have at least 15 minutes before the next fight.
But overvolting increases the strain, and most of us run the Bosch 750 24V or equivalent on 36V. Keep the 78% efficiency in mind.
Increase the voltage and the ampères drawn will increase as much. Overvolt by 50% (24V+12V) will increase amp draw by 50%(40A+20A)
36V*60A= 2160W. 78% of that is 1680W.
Left over in heat is 480W.... That's a small bathroom heater going full blast.

A Bosch 750 is an industrial motor and is build for a useable RPM range "nominal +/- 15%", as the nominal rpm@24V is 3300rpm on 24V, or 4900rpm@36V you can see where it will go wrong. Drop below 2700/4000RPM, and you're increasing the inefficency.
At stall (motor doesn't have to power to get it turning) all the electrical power drawnwill be converted into heat. Efficiency 0% as a motor, close to 100% as a copper melting device.

[overkill]

The Ampflow motor only has a quarter of the power output of the Etek and way less low speed torque. It will obly reliably power a spinning weapon 1/4 the weight that an Etek can. Another thing to remember when comparing motors is their mass and size; For the same amount of power output the Etek will heat up slower than the Ampflow due to its larger thermal mass.

[ACEGUY]

in an unrelated question what gauge of wire is used in a HW? Is there a separate wiring for the drive and the weapon has its own wiring?

[maddox10]

I can't speak for others. But I aim for 20A per mm². I also rather double up thinner wires than go up a size. Thinner wire solders easier, is easier put un corners and so on.

[blazerbotics]

Gearing calculator; http://howto.killerhurtz.co.uk/calculator.htm

[ACEGUY]

Gearing calculator; http://howto.killerhurtz.co.uk/calculator.htm

Thanks for the link but, I have the drive gear ratio figured out, now I'm trying to figure out what gear ratio for the weapon.

[emisnug]

Thanks for the link but, I have the drive gear ratio figured out, now I'm trying to figure out what gear ratio for the weapon.

For a very rough estimate, use the Run Amok Spinner Weapon Calculator. Bear in mind that it's only as good as the information you put in.

Let's take, for example, the following:

- 457.2x101.6x25.4 spinning bar (18'' x 4'' x 1'' because most of the online calculators are American save for this one, plus bars are a bit more simple than discs to work out)
- Ampflow A28-400, as you suggested.
- 8s Battery (29.6V)
- Mild Steel Material

Thanks to the Ampflow website, we know that the Ampflow A28-400 has the following characteristics:

Diameter (inches)	3.0
Length (inches)	6.7
Peak HP	4.3
Stall Torque (oz-in)	3560
Efficiency	83%
Nominal Voltage*	24V*
RPM @ 24V	4900
No Load Amps	4.4	4.4	2.1	2.5	3.2	2.1
Resistance (Ohms)	0.044	0.064	0.076	0.081	0.089	0.190
Kv (RPM/Volt)	205	257	189	287	240	237

We know that our battery puts out 29.6V, and the Kv (RPM/Volt) is 205 - which gives us a no-load RPM of 6068, assuming 100% efficiency. In real terms, again helpfully provided by Ampflow, the efficiency of that motor is 83%, which gives the no load RPM as

5036RPM (rounded down)

Pop that into the calculator with a 3:1 gear ratio and you'll see that the bar above will give you a spinning bar at 1679RPM, with a tip speed (which must be under 250MPH for Robot Wars) of a hair under 90MPH and with a kinetic energy of 2.5ish KiloJoules of energy at the tip of the bar. You'll want a fast spin up time (larger gear ratio, such as 5:1) so that your weapon is up to speed before your bot hits someone else in the arena.

2.5KJ is not enough for a heavyweight. It's barely enough for a featherweight.

(as a guide, the feather weight that I'm designing at the moment has over 15KJ of energy)

Hope this helps