Drive Belt Help!!

Ok, so I have not been able to use the Gates belt calculator, is there something I need to download before I can access this?

Either way, I need suggestions for my friction drive. I am only doing one reduction from motor to roller. I have a 10mm axle on a 300kv motor good for about 2.5 kw. My tire is a 26 inch slick, and the roller is 1.3 inches in diameter. The motor will be powered by 48 volts. What gearing would be most efficient for the motor and still get about 25-28 mph? Which belt pitch, belt width, pulleys?

Any help will mean much progress, as this is the last part I need.

thanks

I do not have a calculator for your ratio with me, but I can tell you 5mm HTD pitch belt in 9mm width should be best for the torque and RPM you are looking for. They are also quite efficient at that RPM and torque.

Off the top of my ehad, I can tell you roughly 2 to 1 ratio will get you in the ballpark.

Matt

Pretty much as Matt said:

@ 48 volts you need about 32:1 reduction.

Your drive roller gives about 20:1, on a 26" wheel, so the reduction between the motor and this needs to be about 1.6:1

Something like a 20t to a 32t pulley would do, for 5mm pitch GT2 Powergrip, or equivalent.

Anyway, your roller will slip before the belt, for sure......

With a friction roller your wheel size doesn't matter, because the surface of the roller travels at the same speed as the surface of the wheel, ignoring a minute amount of deflection and stretching of the tire itself. You've got 1.3"x pi for a 4" or 1/3 ft circumference roller. 25mph x 5280ft /60min = 2200ft/min, so you need 6600rpm with your roller for 25mph. If the motor really turns 300rpm/volt under load, then 48V gives you 14400rpm, so the 2/1 reduction was right on target.

I question the motor rpm under load, but more importantly, I question the ability to use 48v. That's essentially what the RC guys call 13s, 13 series lipo batts at 3.7v each. Aren't those limits more typically 10s with RC motors?

John

That's an excellent question John. I just figured I could go to the voltage capacity of my controller, which is 50 volts. Maybe I should gear it to run at 36 volts instead, and just push more amps. I wonder what voltage CNCAddict will recommend for his motor? I know alot of people aren't gonna be happy with 10s voltage, that will require some very high C rate batteries to get the speed that most of us are going for.

etard said:

That's an excellent question John. I just figured I could go to the voltage capacity of my controller, which is 50 volts. Maybe I should gear it to run at 36 volts instead, and just push more amps. I wonder what voltage CNCAddict will recommend for his motor? I know alot of people aren't gonna be happy with 10s voltage, that will require some very high C rate batteries to get the speed that most of us are going for.

Click to expand...

If the max power is say 3kw, it doesn't really matter if the voltage is 24V or 48V, it's going to take the same batteries for the same range and efficiency. These motors seem to be spec'd on current limits, so it's easier to get to higher currents at lower voltage simply by paralleling batteries. If you're planning on running significant power with RC motors, it seems to me that it requires either very large format packs or more expensive per wh high power batts, since they require high current for best performance. It's the least of my concern, since I'm lucky and have 24 A123 cells, almost 2kwh of high power konions, 1kwh of nicads, 1kwh of 36V ping packs, plus another kwh of lifepo4 in route for testing.

John

John in CR said:

If the motor really turns 300rpm/volt under load, then 48V gives you 14400rpm, so the 2/1 reduction was right on target.

Click to expand...

John,

Where does it say 300rpm/volt under load? You're right, you can't simply gear for the no load speed... I use 85% of no load rpm.

Miles said:

John in CR said:

If the motor really turns 300rpm/volt under load, then 48V gives you 14400rpm, so the 2/1 reduction was right on target.

Click to expand...

John,

Where does it say 300rpm/volt under load? You can't simply gear for the no load speed... I used 85% of no load rpm.

Click to expand...

Miles,

I wasn't disputing your number, though you confused the issue by talking about wheel diameter and leaving out your 85% assumed reduction of motor rpm for load. I've seen wheel diameter discussed on several occasions when people are talking friction drives, so I did want to point out that speed will be the same with a 16" tire as a 26" one. BTW is the 15% a pretty good number for us to use for all brushless motors?

John

John

You're quite right about my unnecessary step - I just did it the way I normally do, without thinking

85-90% is peak efficiency speed. I use 85% for gearing and it usually works out about right.

Thanks for pointing that out. I went and looked at a couple of the motor performance reports I have for different motors, and you hit the nail on the head with 85% of no load speed being the speed at maximum efficiency. Now if I can just figure out how/if maximum efficiency changes with voltage.

John

John,

A good way to learn about this stuff is to play with one of the motor calculation programs.

Much of this discussion is exactly why I run Lipo cells. They (most of them anyway) are rated for at least 10C. It is not tough to find 30C lipo packs. So, amperage is a non-issue from my perspective. I think I need to keep in mind the fact that most bike guys run 3C packs.

Remember, with high pole count RC outunners, you cannot assume they can be overvolted. In fact, most cannot. Normally RC motors are rated at 10S (ten 3.7 volt cells, or 4.1 volts at full charge). Not very many are rated for 12S (50 volts fully charged). CNCAddict's motors will all run very high voltage because they are 8 pole motors rather than 14 to 20 pole. That makes them not quite as smooth, but far more willing to run higher RPM.

Also, yes, no-load RPM is not a good number to go by. Also, some RC motors are frustrating in that you may do all the calculating and still have the gearing wrong. That happened to me with my recumbent. I am not sure why that is. But, I have found RC motors will run fine in a bike, but the you need to take a lower number than the no-load RPM. Miles mentioned 85% of no-load RPM. That is a good place to start. I have even found 70% of no-load to be more accurate in some situations. I do not think this is a motor issue as much as a motor/ESC communication issue. Even my efficient AXI ran at roughly 70% no-load RPM, but ran nice and cool.

That is my experience.

Basically, make sure you have a few different motor pulleys on hand for experimentation to get everything just right.

Oh, and I would not over-volt that motor. If it is a 14 to 20 pole motor, 10,000rpm is about the max.

Matt

Basicly I have been playing with SWBLUTO's ebike calc. and from that I calculated that with my motor a 32:1 reduction will be the best balance of range and speed. With my roller tire combination I get 20:1. Am I correct in saying that when combining 2 reductions, the effect on final drive is multiplied, not added? From what you are saying Miles, then I need only a 1.6:1 reduction between my motor and roller. John is saying that the tire diameter does not count as a reduction, I don't think this is correct. Could somebody hold my hand through this one?

Also, the motor manufacturer is saying only .03 ohm resistance in motor, is this a realistic number?

etard said:

Am I correct in saying that when combining 2 reductions, the effect on final drive is multiplied, not added? From what you are saying Miles, then I need only a 1.6:1 reduction between my motor and roller. John is saying that the tire diameter does not count as a reduction, I don't think this is correct. Could somebody hold my hand through this one?

Click to expand...

Yes, around 1.6:1 would be a good place to start.

Yes, you multiply the reduction ratios, so 1.6:1 X 20:1 gives 32:1

You can count the tyre as part of the drive, or not..... I did because I'm set up to calculate that way and it gives an easy comparison with other drives. You could also calculate directly from the rpm and diameter of the drive roller - as John said, the diameter of the tyre doesn't affect the result.

Here is online belt drive simulator that I use.

http://smarthost.maedler.de/maedlertools/maedler.html

Also, you can download all parts in 3D.

I wasn't able to get to that HAL, thanks though.

I decided not to overvolt to 48, but instead 36 volt. Playing with the calculations, I came up with two options:

As per Matt's recommendations I am going 5mm pitch (1/5th inch) with a 9mm belt probably GT2 (is this better?).

1. 13 tooth motor pulley to 17 tooth roller pulley

2. 15 tooth motor pulley to 20 tooth motor pulley

As I discussed, it is roller drive so I don't think belt slippage is much of an issue. Having said that, will the smaller diameter pulley set up work? I would like to keep it a small profile, and I am working with a 10mm motor shaft, and 1/2 inch roller shaft (double support), so I think I will be able to tension it pretty tight.

Any suggestions before I take the plunge?

A 13t 5M pulley is less than 20mm in diameter..... not easy to fix to a 10mm shaft. You could bond it on, I suppose..

I would go for the 15t motor to 20t roller, at least... 16t would be better.

GT2 has much greater torque handling capacity than HTD. In your case, as you say, the transfer from roller to wheel is the weakest link, though.

not that i can find my ass with both hands as far as this stuff is concerned but if you runlower volts and juice it with amps to compensate does that mean that the motor will get hotter?


Cheers,

D

Yes, more current can cause more heat; however it only matters if your wiring/windings are not a sufficient gauge. As your power goes up the resistance in your wiring becomes more apparent as more power is lost as heat. This is why most people up the voltage because a higher voltage will have less voltage drop over a given line due to increased push of the electrons.

D,
As I understand it these outrunners are designed to take alot of amps at a lower voltage. It seems like if you are about to pay $1000 for one of those Pletti's, you should get a full support team that can tell you exactly what the motor can take. And then there is me.... I bought a chinese 295 kv motor that I wont be able to overvolt, so I have to gear it for 36 volts instead of 48. I hope that putting more amps into it will allow me to attain the speed I want while still getting decent range, but who knows how it will act once I get it all setup. Likely I will have to get a few gears to get the proper balance of power and efficiency though.

Yeah it's a game of guessing, my chinese made motor hit it's limit at around 75 amps. It appeared to have thrown a winding at this point, but regardless of why the motor locked up it was certainly sizzling because it was only designed to handle 65 amps max.