In my EV3 robot, I have two rear axles connected to each other using this part mounted like this:

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The problem with this construction is that after a while of driving, the axles disconnect like this:

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Is there a sensible way to prevent this from happening?

  • Are you looking for a solution that's made with parts from the ev3 kit only? – Uli May 11 '20 at 19:23
  • Apologies if this is a dumb question but is "Get a longer axle piece" too obvious? I think they make them that long. – Darrel Hoffman May 11 '20 at 19:28
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    Unfortunately I don't have a longer one :< – d33tah May 11 '20 at 21:54
  • Hmm... Is the disconnect caused by turning? (i.e. if you make it drive only forwards and backwards, does the axel remain intact for longer?) It may be down to the difference in size of turning circle for each wheel, in which case the forces could be reduced or removed with the introduction of a differential – Chronocidal May 13 '20 at 14:34
  • @Chronocidal it's likely caused by turning. Could you link to some documents on the said differential? – d33tah May 13 '20 at 15:27

You can attach couple of half bushes or full bushes on inner side of the axle near Technic beam like on the other side you have done with a gear. If axle is still trying to slide - fill in the entire inner axle part with bushes.

enter image description here enter image description here

Here is picture of suggested fix. Here you can see extra half bushes added to fix the axle the same way gear fix the other axle from moving outwards. enter image description here

  • I filled all of it and it still slides. Thanks for the answer anyway :) – d33tah May 11 '20 at 12:05
  • @d33tah perharps I wasn't clear in describing the fix. I've added a picture. I cannot see how axle would slide again in such setup. – Alex May 11 '20 at 12:21
  • @Alex if the yellow half-bushes have subpar clutch power, the left axle could still move, especially given the forces that arise with such a long axle, no differential and rubber tires. However, applying more (half)bushes could increase the gripping, and if the whole length of the axle is filled with them, and the axles still slide out, then other solutions are needed. – zovits May 11 '20 at 14:12
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    @zovits Agree on more is better, but two half bushes are strong enough to keep the axle in place, unless half bushes are cracked. I assumed Technic beams holding the wheels don't move away from each other. If they do - your answer is the right added solution here. – Alex May 12 '20 at 8:06
  • @Alex Agree, it should be enough, and it is quite curious why does OP say that it does not work... – zovits May 12 '20 at 8:41

If you want to stick to your current design (despite its obvious flaws), then here's another idea. Let's go!

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As you can see, I tried to rebuild the crucial part of your framework.

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Alright. Now two things I've noticed. You're using axles with stop. And there's some play between the bush and the wheel.

Okay, firstly on axles with stop.

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Here's two pairs of axles. The two axles with stop are on top, the bottom ones are without stop bit. See that the top ones don't touch each other? See the gap? That's because of the stop and the fact that the wheel has no counterbore. Can you spot the inner rim on the liftarm (blue arrow)? That's what the wheel lacks.

So what about it? Well, here's the problem. Let's take a yellow connector and make them one long axle. But, remember the gap between them?

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Once you push them into the connector -all the way in till they 'hit the ground'- you will flex the red and yellow liftarms inwards. Maybe just a tad, but still. They will sure flex back, but because of the stop on the axle the wheel can't pop off. So the axle gets pulled out of the connector instead.

Next about the connector. You chose a connector with a pinhole. This piece is three studs long. So your wheel is now off by one stud. And I think you've therefore adjusted the wheel. That's why we can see this play.


If the application of (half)bushes as mentioned in Alex's answer don't cut it, consider building a frame that holds the ends of the grey axles together, like so:

enter image description here

  • It is better to replace red bushes near wheels with 15100 and liftarm than fixing from outside in such case. – Alex May 11 '20 at 14:27

I think the real problem is the design and the way you used the motors.

If you motorize only one of the wheels with the medium motor to get the other one to move, then the one that's closer to the motor may go faster and you'll probably end up with a slight deviation to the left, which also includes the other wheel snapping off.

Also, using the large servo motor just to steer a few degrees is not worth it, as the servo motor goes faster than the medium motor and you have two servo motors, so I recommend that you redesign your car and individually motorize the two back wheels. and since you said that you only have one servo motor, you should put one between the two axles to keep them together and to motorize both. Use the medium motor for steering instead.

See this car for ideas. Building instructions are included.

  • The problem is that I don't have two small servo motors anymore - one broke. This design is an attempt at working around that. Thanks for point out the problem though. Do you think I could hack around it? – d33tah May 13 '20 at 12:29
  • @d33tah Huh... perhaps you can put one large servo motor in between the two axles to motorize both AND to keep them together, or you can get the motor here and build the other car I've suggested: amazon.com/Lego-Mindstorms-Large-Servo-Motor/dp/B00E1QDP4W/… The lego site does not individually sell this motor unfortunately. – mindstormsboi May 13 '20 at 13:37
  • Why would my idea not work? Why downvote? – mindstormsboi May 13 '20 at 17:17
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    No idea, I think that your point is valid. – d33tah May 13 '20 at 18:29

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