Newbie, seeking PFinfo

[Stardust56]

Hi all,

I'm new around here, having returned to a passion of 20 years ago after the 'dark ages'.

Technic trucks grab my fancy, with 1:17.5 scale being where I am to do a bit of modeling. I've started back with the heavy tow truck 8285, the crane truck 8421 and a technic formula one car. Most recently, the Excavator 8043 has arrived on my doorstep.

I'm quite taken by the detail of 2legoornot2lego's trucks and trailers, and having a look around, I see that most build using XL motors for the driveline function. My question is 'can I couple 2 M motors (from the excavator) to operate the driveline in the 8285 Tow Truck?'

You help is appreciatedin advance.

Cheers from DownUnder,

Brett

[mescalinum]

welcome to the forum!

My question is 'can I couple 2 M motors (from the excavator) to operate the driveline in the 8285 Tow Truck?'

hmm, maybe it is inefficient -but I'm not 100% sure- as there will be a motor which is slightly slower, and one which is slightly faster, which will be limited by the slower one.

making an adder -using a differential gear- is the proper way of adding the power of two motors (although it introduces the friction of differential gears)

you should try both, and don't forget to report your findings once you've done

[timslegos]

I would agree with the above post, the best way to connect two motors to drive one function is with a differential in between the two.

tim

[Splat]

You should also check out Philo's Lego Motor page. It will tell you the typical characteristics of the various 9V Lego motors.

Here are some key differences between the XL-Motor and the M-Motor:

• XL weighs 69g - M weighs 31g
  So two M-Motors are about the same weight as the XL-Motor
  
• XL spins at 220rpm - M spins at 405rpm (with no load)
  So if you use M-Motors in the place of an XL-Motor, you would have to gear it down about 2:1 to get the same speed as an XL-motor
  
• XL draws a current of 80mA, M draws a current of 65mA (with no load)
  So 2 M-Motors will draw more current than a single XL-Motor, so your battery will go flat quicker
  
• XL has 14.5 N.cm of torque, M has 3.63 N.cm of torque
  So the XL-Motor has about 4 times as much torque as an M-Motor
  

I'm not sure if this is correct or not (maybe someone can verify):

If you have two M-Motors, and slow them down 2:1, then you should get similar rpm and torque to a single XL Motor, but it would draw more current from the battery (so the battery won't last as long). Also, because you have to use Lego gears to connect the two M-Motors together (using a differential is the ideal way), and use Lego gears to slow down the rotation speed (rpm), the drive-train won't be as efficient, and will take up more room. So, in theory, 2 M-Motors should be similar to a single XL-Motor, but there are some disadvantages.

Lots more info on Philo's Lego Motor page and the Lego Power Functions site.

[Stardust56]

Hi guys,

Thanks for all the input. I'm currently grabbing some more parts to play with this, and also grabbing an XL motor. Clearly I will need at least one of these in my inventory.

Splat, that was a very convincing argument to buy the XL motor.

That said, Mescalinum and Timr, I'll throw a test chassis together to lu lethe 2 M motors and report back the findings.

Cheers and thanks,

Stardust56

[Dutch_EE]

Hello,

the only problem with motorising the 8285 tow truck, is the connection from the drive-axle to the differential. Those two at right angle placed 16 tooth gears only bearly mesh into each other. With my version of motorisation : a XL-motor with 8 -> 24 reduction, leads to gear stripping as soon as there is any resistance (rough carpet or even in a turn )

If you use an M-motor for drive, I would suggest at least 2 or three stages of reduction or, like me, use a second diff for the second rear axle. This how ever doesn't help the gear stripping :(

Here, a picture of my solution.

You can see the XL-motor, the 24-tooth gear, the 2 troubling 16 tooth gears and the diff. Also, the second diff is inserted.

Also, look here : My motorisation thread

Edited August 30, 2011 by Dutch_EE

[Sokratesz]

My default solution for gear slipping is to simply build it so they can't slip - meaning they are each supported by a rigid element that directly connects to the other.

I had problems with gear slipping in my wheel v2.0 (see sig) and solved it like this:

To the left there's two M motors each with a 12t double bevel attached driving the same 20t double bevel. I had problems with gear slipping until I rebuilt the frame the way it looks in this pic with a 1x5 studless beam covering all three axles so they physically cannot move apart.

- Sok.

Edited August 30, 2011 by Sokratesz

[timslegos]

My default solution for gear slipping is to simply build it so they can't slip - meaning they are each supported by a rigid element that directly connects to the other.

Unfortunately, sometimes i do this and the gears still slip . The key to limiting gear slipping is gear reduction.

tim

[Dutch_EE]

Unfortunately, sometimes i do this and the gears still slip . The key to limiting gear slipping is gear reduction.

tim

You mean, the opposite.

The reason the gears slip, is that there is too much torque going trough them. To prevent that, you need to increase speed, and with that decrease torque at those points.

The problem in the 8285 tow truck is, that I couldn't find an replacement gear set for the connection of the driveshaft to the differential. Also, if I change the

8 -> 24 reduction from the XL motor to 16 -> 16, the motor doesn't have enough torque to drive the truck. Sorry, I meant the 12 -> 12 gears. Or to make it completely clear where the problem lies, the transfer is

Motor -> 8 to 24 -> 12 to 12 (at 90 degrees, this slips) -> 12 to diff

[Sokratesz]

Unfortunately, sometimes i do this and the gears still slip . The key to limiting gear slipping is gear reduction.

tim

I'm quite sure it is physically impossible for interlocking double bevel gears to slip in that configuration without breaking anything, but I'd like to see it if it happened to you.

- Sok.

[timslegos]

You mean, the opposite.

The reason the gears slip, is that there is too much torque going trough them. To prevent that, you need to increase speed, and with that decrease torque at those points.

But doesn't increased speed increase resistance thus making the gears slip even more?

I'm quite sure it is physically impossible for interlocking double bevel gears to slip in that configuration without breaking anything, but I'd like to see it if it happened to you.

- Sok.

Well i haven't encountered a situation where the double bevel gears have slipped, but the 8t and the 24t gears have slipped before and i have actually broken many 8t gears.

tim

[Sokratesz]

Yeah okay but the small 8t gears are very weak compared to the newer style 12, 20, and 36t bevels :)

[timslegos]

Yeah okay but the small 8t gears are very weak compared to the newer style 12, 20, and 36t bevels :)

I definitely agree with you. i would like to use the new gears more but they require more gears because the gear reduction of 12/20 inst nearly as slow as the 8/24 combinations.

tim

[Sokratesz]

I definitely agree with you. i would like to use the new gears more but they require more gears because the gear reduction of 12/20 inst nearly as slow as the 8/24 combinations.

tim

If you need to gear down significantly, try using a worm gear or one of the old style prefab gearboxes.

- Sok.

[timslegos]

If you need to gear down significantly, try using a worm gear or one of the old style prefab gearboxes.

- Sok.

But that solution does not have as much torque as 2 or 3 8/24 gear combinations together.

tim

[mescalinum]

also the worm it is not efficient as spur gears, due to greater friction

[Sokratesz]

also the worm it is not efficient as spur gears, due to greater friction

Do we have any numbers on that? I've got the idea that with some grease, LEGO worm gears run fine.

[Nazgarot]

Do we have any numbers on that? I've got the idea that with some grease, LEGO worm gears run fine.

They do, and they have the nice ability that they distribute power over several teeth on the gear they are connected to, giving the gear a lot better torque handling. Where a 3x 8/24 gearing starts ripping teeth off the 8 tooth gears, the worm on a 24 tooth still is distributing the power over 2 or 3 more powerful teeth...

For low gearing 12 to 1 or lower I would clearly prefer the worm gear. But a good alternative to 8/24 gearing is 12/36 if there is room. Both gives 3 to 1 reduction, and the 12 tooth gear can handle a lot more torque than the 8 tooth gear.

-ED-