610kV Brushless Motor

[DKNguyen]

I was looking at some motors and found a brushless 610kV motor from Medusa Research. Isn't that ridiculously slow for a brushless motor? I think I read somwhere that a 75T was 750kV.

http://www.medusaproducts.com/motors/036.htm

There is also a 4:1 gearbox for it
http://www.medusaproducts.com/accessories/gearboxes.htm

I was wondering about people's thoughts on this motor as a suitable crawler motor. (Just assume that a strong enough ESC was available).

[JohnRobHolmes]

It is meant to run on 98v max. It is an inrunner as well, so it wont work for crawling being unsensored.

[DKNguyen]

An outrunner is needed for crawling? I just thought outrunners had advantages to inrunners, not that inrunners couldn't be used altogether. (I was thinking of using this on some Clod axles if that makes a difference).

Sensors are needed because crawlers operate at low speeds (or stall) right? Sensorless doesn't work so well for such low speeds?

[JohnRobHolmes]

Inrunners need sensors to start up in a bind, no matter what vehicle it is. If you used those motors in a clod you would have a fast beast or no power, depending on your voltage. Outrunners can start up in a bind since they only have to rotate 30 degrees or less per phase. A 2 pole motor has to rotate 120 degrees I believe. It just cant produce enough torque to start up without a sensor to tell the controller which phase is where.

If you want brushless and crawling to unite, you need a sensored system or an outrunner.

[DKNguyen]

INteresting. How does the torque/speed on an outrunner (which cannot be geared down right?) compare to the torque on a geared down brushed motor? It sounds like the torque of an outrunner is pretty high if it doesnt need gearing. It also sounds like outrunners don't need (or perhaps altogether incompatible) with sensors?

I'm trying to decide whether I want to go to the work of building my own brushless controller or just a regular H-bridge. I want something with the torque of a 75T, but can get the crawler going at 5ft/s or so on flat ground. Can you recommend any high torque, low speed (maybe even high voltage?) outrunners for a Clod? Money is no objective...it could even be a motor straight from the company that costs $400. Doesn't matter.

Do you happen to know the reduction on a Clod axle gearbox with a reference pinion (the number of teeth on the pinion gear is is directly proportional to radius of the gear correct? so then all the reduction is just inversely proportional to the twwth on the pinion gear)

[JohnRobHolmes]

If you are willing to build a controller, there would be nothing stopping you from installing a hall effect sensor on an outrunner motor. No company that I know of makes an outrunner with a sensor.

Reduction on a clod Clod Gear Ratio

WHEW!! that took a while to find!

Outrunners that I know work, anything that is 28mm stator diameter and at least 13mm long. You can go bigger

[DKNguyen]

I see. So if I used a 6 tooth pinion, the ratio would be 65:1, and I could pretty much use any "torquey" motor and I would probably be getting more speed than I need...I got a feeling it might snap some axles. If I used some motor that was like 700kV I would probably have an axle snapper...and it may still be faster than what I need. I guess what I really gotta start looking at is power draw.

What gear reduction + turns ratio combo starts to snap axles for you guys?

Actually, I should just probably build it to only run high speed on flat ground and stay beneath a certain speed during climbing so it doesnt snap.

[JohnRobHolmes]

yeah, a large outrunner will snap clod axles if you arent careful. If you want crazy power I would suggest the Twin Force axles and a beefy tranny.

[Kamikaze]

Originally Posted by DKNguyen

What gear reduction + turns ratio combo starts to snap axles for you guys?

I started snaping on demand when I put the 6 tooths in with the 35 turns. The 9 tooths would stall but were very fast, 9 tooths with 55 turns just plain blew.

[DKNguyen]

[quote=Kamikazeo9 tooths with 55 turns just plain blew.[/quote]

Why? Too slow?

Wow, 6T pin, 35T will snap axles? I got the impression you needed like 6T with 55T or 75T to snap axles.

This may seem like a silly question, but when you say axle snapping, do you mean actually twisting the axle until it snaps? Or the main drive shaft (the one between transmission and differential whatever it's called? he drive shaft between the wheel and differential? The wheel hub? The transmission gears?

I want axle snapping power, not for climbing torque but to be used to get "human scale" fast walking or jogging speeds on flat ground when the load is low. I guess I should just stop worrying since it seems it's almost impossible to get a motor that runs too fast with 65:1 ratio (or isn't strong enough for that matter)

[Reflection]

I'm pretty sure Kamikaze means actually snapping the shaft in two. Kamikaze runs a Clod so theres no driveshaft,he's refering to the actual axle shaft.

If you want power a Hyperion Z3025-8 is where it's at. It'll pull a lathe motor inside out without a strain and sling a 20 pound truck around like it's nothing. If I tightened up my slipper anymore,it'd shread everything plastic Since I removed the gear reduction all together,the only plastic in my entire drivetrain is an Emaxx spur and Revo driveshafts. I'm currently running 52:1 final drive on 9 cells. It's got almost to much power honestly. If you don't watch it,you'll break something fast. The gear ratio and cell count I'm running now seems to give a near prefect combo for me. AWESOME low end power and GREAT wheelspeed.

[Tanis]

I'm betting a pair of z3025's in a clod would give..."Watch this...SNAP" on demand.

[Reflection]

Originally Posted by Tanis

I'm betting a pair of z3025's in a clod would give..."Watch this...SNAP" on demand.

Probly wouldn't be able to even get the "watch this" part out

[Tanis]

[DKNguyen]

Would these prevent axle snap?
http://rc4wdstore.com/product_info.p...roducts_id=476

Or would they just make for a more expensive and spectacular axle snap?I can only assume it has the same reduction as a Clod axle...but I honestly don't know.

Raptorman mentioned a slipper. Does such a thing exist on for a Clod? or only transmission drives? Maybe use a nylon set screw or plastic pinion or something so that it will break before anthing else does. Maybe I should go for a little brushless rather than a brushless the size of a 550.

Im kind of wondering how the axles break in the first place (like the maneuver). Do you just climb up a slope and stall the motors until the shaft breaks. Or is there something you can actually do on flat ground to break the axles?

[Tanis]

I would have to say, more expensive snap. Those motors really pack a punch. Raptorman only has 1 on his twin crawler, which is shaft driven. His slipper is on his tranny. 1 motor per axle would be ridiculous power

[DKNguyen]

Since the only reason I want a brushless is speed over level ground...I wonder how fast it would go with two lol. It'd probably tear itself apart at the slightest turn because without a diff all the motor speed turns into torque during the skid.

[Tanis]

If you ran 2, you would need 2 esc's, one for each motor.

[DKNguyen]

Yeah I know. I'm building my own anyways (it's actually a robot, not a rock crawler). I'll be damned if I can come up with some sensors and path finding algorithm smart enough to rock climb on it's own.

I'll probably get a Johnson and Mabuchi 550 first while I mess around with a brushles and it's driver since they are so cheap (and I can have the torque/speed/current curves for them all). I wish I could get those for some of the brushless motors out there.

[Tanis]

Sweet