three 269 motors do not turn right out of the package. 2 of them in both directions and a third in one direction.
Kevin,
Our 269 Motors do not have the ability to “Back-drive”
This means that they can only be turned electrically (under power), not mechanically (by hand)
Please test your motors with one of our microcontrollers and inform me on your results.
-Eli
OK they work fine when connected. But why do I have some 269 motors that can be had driven in both directions, some that are locked in one direction and some that are locked in both directions but all work fine when powered.
Hi Kevin,
I understand why this might be confusing or frustrating. Let me try to explain it to you.
The VEX 2-wire Motor 269 consists of a small DC motor, and a multistage spur gearbox.
To start with, think about the small DC motor by itself. If you put you fingers on the tip of the output shaft and try to spin the motor by hand (disconnected from power) there is some SMALL amount of torque required to spin it. This torque is not constant. As the motor rotates, the friction of the motor brushes varies depending on their position. In some spots this friction is higher than others.
This small amount of friction is multiplied by the gear ratio of each stage of the gearbox. As such, the small amount of friction in the input becomes a large amount of friction at the output.
Additionally, each gearbox stage has friction – stages closest to the input have the most affect on the overall output friction since they get increased by the most gear ratio.
The variance in brush friction is why “sometimes the motors backdrive, but sometimes they don’t.” If the motor brushes are in one position, the friction might be such that the motor backdrives, whereas in another position it won’t.
As to why motors will back-drive in one direction but not another – DC motor characteristics vary slightly between forward and reverse.
Another contributor:
When you lock a spur gear (1) and load its mating gear (2) (say, the friction from the motor holds one in place while you try to backdrive the other) the 2nd spur gear will try to act like a planetary gear on the 1st. It will literally try to roll around the 1st gear. The axle it’s on will see that force, and may flex a little bit.
When you backdrive the 2-wire Motor 269, you are trying to accelerate the DC motor. The rotational intertia of the motor makes it try to stay in one place, and in that instant – we have a situation like the one I described above. Now, since the shafts are not all “in-line” in the 2-wire motor 269’s gearbox. This means that when you backdrive the motor really hard in one direction and the spur gears act like planetary gears (as discussed in the paragraph above) they behave differently in one direction than the other. In one direction, the gearbox actually locks, as the gears try to shove together, while in the other direction the gearbox actually loosens up a little bit.
Now this doesn’t happen in big, major ways – just small subtle shifts. However if these subtle shifts happen close to the input stage (by the motor) they can be felt in big ways at the output.
All in all, whenever a gearbox+motor combo (referred to as a gear-motor) has a very high gear reduction, it won’t back-drive as well as one with less reduction.
For a few years now we’ve posted that “the 2-Wire Motor 269” probably won’t back-drive. (Yes, depending on manufacturing variances, sometimes they do.) That said, we know that user’s think this is bad, and it is one of the reasons we’ve discontinued the 2-wire Motor 269 and started transitioning people to the 2-wire Motor 393.
The 2-wire Motor 393 (in addition to being more powerful) has:
- Less internal gear-reduction
- In-line gear reductions
- Larger axles for each gear stage (since there is more room in that size motor for this)
This means (for the reasons we discussed above), it back drives very smoothly, and behaves like people want it to.
Hopefully this answers your question about why different motors behave differently when it comes to back driving.
Let me know if you have any other questions.
-John