Is it safe to use two electric motors connected to one axle? No, that destroys the motor Even you feel it or not. My dad told it and elctronic controling of motors is his professional so he knows what he talk about. He told that the motors will burn up inside of you connecting in paid toghether on one axle. ( three gears, the gear in the middle on Axle and one motor on each side and that is not safe. One is allways working harderr than the other and both motors suffer for that and burns up inside. This is of course power functions I talk about now since it is so unusal to connect to motors in pair, but that is not safe. 

But it is possible to do it anyway without destroying the motors. A differensial gear between to two motors that working in pair is safe. That does not destroy the motors. But you need a differensial gear between if the motors shall last long and be good long time and do not be destroyd. (Anyway not of that reason) 

Edited December 20, 20168 yr by Helge Johan

Connecting two motors in a high precision or heavy duty industrial environment does seem to not be recommended, from what I could find on Google.  As well as electrical problems and motor life problems, it seems that there can be problems with vibration and uneven gear wear if the motors are not precisely matched.

But in a toy like Lego, I don't think attaching two motors to the same axle or gear train is a very likely problem for motor life   Except if you do it wrong and reverse one motor against the direction of the other   Or you run the motors continuously for hours or days at a time.  

A more likely problem with connecting two L or XL motors together is that they will easily twist axles or break gears.  

The differential tip is good, I have tried this before, and yes you do need a strong differential, it's very easy to smash the inner gears.  

 

Some people has got their motors totally buned up of this reason. My dad said that it has nothing to do with how powerful the motors are, and he is an engineer with 40 years experience in making motor control systems. I think he knows this better that you. As long they do not has a differensial between then they shall never be connected together, no matter what the size is.

if you shall choose between buying 3 gears in a differensial of buy two new motors...

Edited December 20, 20168 yr by Helge Johan

47 minutes ago, Helge Johan said:

Some people has got their motors totally buned up of this reason. My dad said that it has nothing to do with how powerful the motors are, and he is an engineer with 40 years experience in making motor control systems. I think he knows this better that you. As long they do not has a differensial between then they shall never be connected together, no matter what the size is.

if you shall choose between buying 3 gears in a differensial of buy two new motors...

Dear Helge Johan,

Please do not make assumptions such as "I think he knows better than you". Maybe Andythenorth is an electrical engineer also who works on motor control systems...

Again, there are differences between the metal industrial world and the toys plastic world, where phenomena might not be the same, and do not have the same effect on the parts.

I don't know that anyone had experience of broken motors that were connected directly. Any statistics available? 
I do couple motors very often for more than a year. Nothing went bad. Difference of RPM in different motors of the same type is very minor and it just forces one motor to turn slower just like it has some load. Lego motors can be stalled without any bad consequences for them.

And it is connected with how powerful motors are. Lego motors nominal voltage is higher than 9V, they do not work in full power. And if something went wrong there are several things that will occur before the motor itself will be burned: motor's thermal protection will cut the power, IR-receiver or SBrick will cut power, battery block will cut power, axles will be twisted, parts will be broken or anything else will occur but no burning of motor itself. 

All of my models that use at two or more motors for drive are hardcoupled. I have been doing this for the past three years, and I have noticed no problems with them nor any performance loss.

I'm almost certain it won't cause any problems. I've done far more dangerous things (like coupling a Large motor to a Medium motor with 5:3 gearing in between) without any effects on motor performance. If you're going to leave the thing running for hours, a diff might be good, but even in heavy-duty situations (in fact, I'd say ESPECIALLY in heavy-duty situations), the diff will cause more problems than it'll solve. I've stalled a motor for about 30 seconds with no problems whatsoever.

I should point out that the difference in speeds between identical motors is likely a few percent, which is negligible considering that loading a motor to max power output (torque times RPM) means loading it until it slows to half its RPM (varies between motors but this is a general rule of thumb that works for brushed DC motors).

There is nothing wrong with connecting two motors in parallel to the same axle.  MOCs do it all the time.  I have done it myself many times.  It will not burn up the motors.  You would not want to do it with different types of motors, but if they are the same type running at the same speed then there is no problem.  They will share the load.  It will not be perfectly evenly shared, but this is not a problem.  Keep in mind these are "dumb" DC motors, not brushless motors with a controller.  Philo already studied the effects of this extensively.

Nah

4 hours ago, wissamms said:

Maybe Andythenorth is an electrical engineer also who works on motor control systems...

Nah, I'm not   But I'm also not particularly concerned about burning out motors. As an adult with an income, M and XL motors are very cheap for me, and I have lots, and it's just a toy, so 'never do this' is not advice I would take.  

But perspectives vary: when I was a kid I had one 4.5v Lego motor and it was very very precious...and it eventually burnt out after years of abuse (questions like "what happens when you drive it with a 12v train controller?" - this is how you learn eh). 

I also learnt by experiment that 'my dad says x' is not a very successful argument, with either adults or kids, but eh we all have to learn by doing. 

I won't comment on whether this is a real problem for motors or not, other people have spoken on that. 

Edited December 20, 20168 yr by andythenorth

As I understand it, big alternating current induction motors with defined slip ratios won't generally play nicely together as one will end up doing all the work - the slip is slightly different between them in other words. Consequently, the reactive inductance changes, which alters the electrical frequency and then a voltage change. This tends up upset controllers trying to maintain constant frequency. 

The little dc motors here don't have that control mechanism and should happily react to the applied (or load if you like) torque and contribute input accordingly.

Each motor presents extra load to the other. The effect is nothing more. So the result is the total power available as output is less than the sum of each motor's power had it been put alone. I used 2 XL motors in all of my work on my TC10 entry for weeks and for extended times and they are still like bulls :-)

Ok. Good that you have so good experiences. But I make free MOC's for downloading and I use always a differensial between. Now I am working with a fork lift that already has to motors in pair with a differensial between. I make professional instructions also. 

 

 

Edited December 22, 20168 yr by Helge Johan

Ok so there is absolutely zero chance you damage lego motors as long as you use the same motor type and the gearing is 1:1 and in the correct direction, this principle holds true for even very powerful dc motors I have coupled 5+ horse power motors to spin a combat robot spinner with no ill affects, but ac on the other hand will cause problems especially with motor controlers