[HELP] old differential grinds on forward direction only

[scynox]

hi everyone,

I have a weird problem about old differential http://rebrickable.com/parts/6573/technic-gear-differential-24-16-teeth/

the yellow 8t gear grinds with old differential 24t side if the XL motors run at full speed in 4th gear in my gearbox only if forward direction. Weird thing is, on backward no problem on same speed. On other gears (1,2,3) they work fine as the whole setup runs slower. During the build I tested under light load (build %30, only chassis) and it worked fine but with heavy load (build %90, almost complete) now it grinds a lot. Learning about the gearbox problem when the model almost complete is really demotivating.

Do you have any idea why it behaves like that? How can I solve this issue?

Incomplete LXF but the gearbox is there: http://www.bricksafe.com/files/scynox/temp/dif_problem.lxf

Note XLs are geared down on 4/5 and 1/2 ratios to the driver rings (not shown in LXF yet)

thanks

 

 

 

[Saberwing40k]

Are you absolutely sure it only happens in forward? I found your issue, and it is the fact that your gears are in the center of two beams that are secured only at the ends. When the gear system is loaded, the beams flex, causing the gears to skip. Somehow, the system is loaded so that in reverse, the weight of the vehicle forces the gears to stay meshed, but in forward, they come apart. Unluckily, this is not an easy fix. I'll detail more on this later, right now it is really late at night where I am.

[Rudivdk]

Edit  -double post with the above-

Edited February 13, 2017 by Rudivdk

[letsbuild]

3 minutes ago, Rudivdk said:

Might be a bracing issue on the 8tooth gear... Is there a way to replace the dbg axle connector below the differential with something to brace the axle with the 8tooth gear some more? Axles tend to flex a little under stress

@Saberwing40k already said it is a bracing problem.

[Rudivdk]

Was typing at the same moment, I see it now yes... 

[Didumos69]

1 hour ago, Saberwing40k said:

Are you absolutely sure it only happens in forward? I found your issue, and it is the fact that your gears are in the center of two beams that are secured only at the ends. When the gear system is loaded, the beams flex, causing the gears to skip. Somehow, the system is loaded so that in reverse, the weight of the vehicle forces the gears to stay meshed, but in forward, they come apart. Unluckily, this is not an easy fix. I'll detail more on this later, right now it is really late at night where I am.

I can confirm this. I looked into the LXF and it appears to me that the vertical connectivity is quite far from the vertical gear meshes. This could cause the issue, but doesn't really explain the difference between forward and backward. What you could do, is try to sandwich the diff. See my modified LXF (you have to get the bracing right yourself ). EDIT: This way you can actually do without the axle right underneath the diff, I updated the image and lxf-file..

Edited February 13, 2017 by Didumos69

[scynox]

Thanks a lot guys for help. Although I cannot test your suggestions now immediately, I believe the "13L DBG beam bending" theory seems very plausible. Only happens one direction, I can confirm this. In my test runs I did not cause any force to stop the car so perhaps if encounters such a force that tries to stop the XLs, maybe grinding will happen in backwards also.

@Didumos69 unfortunately I use these holes in the 5x7 frame, your solution might fix the problem but the design will be changed a lot. I have 2 black (in LXF yellow http://rebrickable.com/parts/15100/technic-pin-connector-hub-with-1-pin-with-friction-ridges-lengthwise/) there and it is necessary in my current design. Still this is a good idea to sandwich the diff. thanks for idea.

I added a new test LXF: https://www.bricksafe.com/files/scynox/temp/dif_problem-2.lxf

For a quick dirty fix, I can add the orange 3L beams to make sure the parallel 13L DBG beams are not bending away from each other but on the other side of the diff, perhaps too far away. This is something I can add without breaking down whole model. Do you think it would work enough to prevent gear slipping?

thanks

 

 

[Jurss]

In first picture, next to the small gear (yellow), there is 2L axle connector. Maybe it is possible to put something in that place, what could reinforce the structure, keep more that gear in place.

You could build some support to that axle form L or T shaped parts.