Differentials: the “Achilles Heel”

[doug72]

Differentials - the “Achilles Heel” for my recent MOC of a mobile dockside crane.



The use of differentials in the caster steering units was not successful when under heavy load and steering due to the output axles pulling out of the 12T single bevel gears despite using 5.5 axle with stop and side beams to prevent any axial movement.

With the old style 16/24T differential you could use a half bush inside the diff casing to help prevent this - with the newer Z28 diff. this is not possible.


But the old 16/24 diff. cannot be used within a 5 x 7 frame or beam frame with ball joint because of its 4L width.

I have tried to use a LEGO Beam Frame with Large Ball Joint (92910) and side beams but was not suitable unless the ball section joint section is cut off  to give a “U” shape frame, to mount on a Z60 turntable.


Another possibility is remove one end of a 5 x 7 frame to give a 5 x 6 ”U”.

3D printing may be a solution rather than cut Lego parts.

Discussion, ideas & suggestions please.

[mocbuild101]

Would either of these work? (the DBG axles are 5.5 with stop)

The one on the left is 5x7, while the one on the right is only 5x5, but it's a lot weaker.

[Didumos69]

8 minutes ago, mocbuild101 said:

Would either of these work? (the DBG axles are 5.5 with stop)

The one on the left is 5x7, while the one on the right is only 5x5, but it's a lot weaker.

This will never work under high torque, because it is friction-locked. Parts will slide off the axles and cause the bevel gear to slip.

Edited February 7, 2018 by Milan
Removed quoted picture from previous post.

[Ivan_M]

There are countless ways how to enclose 5,5 axle into locked frame, here is one of the example:

Untitled #1

But for your build I would throw diffs away and i would power only 1 of the wheel

[Didumos69]

Here's an excerpt from my Tilting go-cart, showing everything that I think is important in bracing a differential and it's input and output axles. To avoid slipping gears and sliding axles...

• ...use a 5x7 frame to make sure there is no movement in the primary supporting pin holes whatsoever.
• ...make sure the input axle is supported not only by the frame but also further down the line. This wil keep the axle straight and reduce the transversal movement of the bevel gear
• ...make sure the output axles can't slide. This can be obtained by using axle connectors that can't go anywhere. In the excerpt below I combined that idea with the use of locked-in 5.5L axles to attach the wheels to.

Btw, supporting axles properly will not only reduce chances of slipping gears, but it will also result in a very smooth running gear system.

Edited February 7, 2018 by Didumos69

[Erik Leppen]

Using the 5x7 frame directly, like @Didumos69 did above, seems the best way to me. Locking the axles can be done further down the road. Really, t feels that the 5x7 frame and the differential are specifically designed to use together :)

The image above is similar to what I use in my Delta carrier right now, except my axles are 2 studs wider (one stud on each side). I also use dog bones for the wheel axles, but vertically and one stud further outward, and instead of 3L axles in the differential, I use 4L axles. Because the second "unit" of the 4L axle is in the frame, I used tan axles because they might fit just that slight bit tighter. Also, using 4L instead of 3L gives you a second stud directly outside the frame that you can use to support the 4L axle, before the axle joiner comes. That would also be the place to add a long beam as part of the chassis.Alternatively, you can use the 5x11 frame, of course, instead of 5x7.

Another tiny mention I would like to add is, that if you want even higher reliability, I would put the 20T gear on an axle with stop, with the stop on the inside. Which means you have to insert that axle with gear from the inside, which means it can't be longer than 5L unless you don't mind bending the axle when installing it, and you have to insert that axle before you add the diff. Then you're absolutely sure the axle can't slip out. To make sure the axle won't slip in as well, add two half-bushes somewhere on that axle (therefore, I think 5L is the best length).

Edit: also, there are two mould variants of this particular diff. Both variants have a recessed sleeve at the 28T gear side for the tan 12T gear to slide in, but in the older variant, this recess is deeper, to account for a "counterbore" for the hole in the 28T gear side to allow pins to be inserted. This deeper slit makes three of the 28 teeth a bit shorter (and also the support ring for the axle 0.1L shorter), which could maybe increase the chance of slipping there (although I never experienced that myself). So if you want to put large forces on the differential, maybe check whether you have the newer mould variant.

https://www.bricklink.com/v2/catalog/catalogitem.page?P=62821a
https://www.bricklink.com/v2/catalog/catalogitem.page?P=62821b

Edited February 7, 2018 by Erik Leppen

[schraubedrin]

Another point for @Didumos69's list should be:

• ... make sure the bevel gears are supported on the rear side (as is the case in his example. bad would be to replace the 20t gear with the flat version and a bushing)

To reduce unnecessary friction in @Didumos69's example, i would replace the Technic Pin Connector Hub with 2 Pins with a Technic Axle and Pin Connector Perpendicular 3L with Centre Pin Hole. This way the output-hole is guaranteed to be coaxial to your output-axle.

[Didumos69]

21 minutes ago, Erik Leppen said:

Another tiny mention I would like to add is, that if you want even higher reliability, I would put the 20T gear on an axle with stop, with the stop on the inside. Which means you have to insert that axle with gear from the inside, which means it can't be longer than 5L unless you don't mind bending the axle when installing it, and you have to insert that axle before you add the diff. Then you're absolutely sure the axle can't slip out. To make sure the axle won't slip in as well, add two half-bushes somewhere on that axle (therefore, I think 5L is the best length).

Great addition ! I did this for the rear diff in my rugged supercar. Btw, the 5.5L axle can be inserted from the inside of the frame without any bending if you come in from the right side of the frame. With the right side, I mean the side that has no ridges to the sides of the pin-holes.

14 minutes ago, schraubedrin said:

To reduce unnecessary friction in @Didumos69's example, i would replace the Technic Pin Connector Hub with 2 Pins with a Technic Axle and Pin Connector Perpendicular 3L with Centre Pin Hole. This way the output-hole is guaranteed to be coaxial to your output-axle.

Edited February 7, 2018 by Didumos69

[zweihander]

Most official recent sets with solid or unsuspended axles with diff use restricted axle joiners to stop the axles sliding out. I think it's the best way for making the official sets as reliable as they can be. Examples: 42041, 42043, 42009

 

[doug72]

Thanks everyone for all the above suggestions, which in the own right all work.
Now attach them vertically under a Z60 turnatble, not so easy or robust and available space restricted once tires on both axles

1 hour ago, Ivan_M said:

But for your build I would throw diffs away and i would power only 1 of the wheel

Agree - I was thinking about that last night, power one wheel on each caster unit and leave the other one to spin independently to allow for differing rotational speeds when turning.

I will give it a try.

A usefull axle would be one with a 5L axle but with stop at 2L from one end allowing it to pass through  frame to 12T bevel gear and sandwhich the axle stop with another beam.

[Aventador2004]

1 hour ago, Didumos69 said:

Great addition ! I did this for the rear diff in my rugged supercar. Btw, the 5.5L axle can be inserted from the inside of the frame without any bending if you come in from the right side of the frame. With the right side, I mean the side that has no ridges to the sides of the pin-holes.

I thought I was the only one who might do that, no bending at all.

[doug72]

3 hours ago, Ivan_M said:

But for your build I would throw diffs away and i would power only 1 of the wheel

I have now modified one caster unit and seems to be the best solution - see image.
Also able to reduce the wheel speed by using 12/20 reduction.
The red axle connector is there to keep the 20T gear in place and is not connected to it.

Was able to remove caster unit from turntable OK without any major alterations.

Now modified all four caster units and a huge improvement with steering without any gears skipping - just need to remember to have all powered wheels rotating the same way.

Tried all steering modes including radial which is OK as long as its not too tight a turn.

I will make a new video & post on my Mobile Dockside Crane topic later.

Edited February 7, 2018 by Doug72

[Ivan_M]

Good to hear that

[doug72]

3 hours ago, Didumos69 said:

Here's an excerpt from my Tilting go-cart, showing everything that I think is important in bracing a differential and it's input and output axles. To avoid slipping gears and sliding axles...

• ...use a 5x7 frame to make sure there is no movement in the primary supporting pin holes whatsoever.
• ...make sure the input axle is supported not only by the frame but also further down the line. This wil keep the axle straight and reduce the transversal movement of the bevel gear
• ...make sure the output axles can't slide. This can be obtained by using axle connectors that can't go anywhere. In the excerpt below I combined that idea with the use of locked-in 5.5L axles to attach the wheels to.

Btw, supporting axles properly will not only reduce chances of slipping gears, but it will also result in a very smooth running gear system.

 

Thanks for this suggestion - will come in handy for my next MOC - a steered version would be usefull as well.

[Marxpek]

Did you consider using a wormgear and 8t gear?

It will need higher motor speed input, but i think that this will be sturdier.

Added: needs older diffs aswell... will be complex sorry...posted to early 

Edited February 7, 2018 by Marxpek

[sirslayer]

i have good success with this design from my Semi-Tractor design..  the only issue is that it needs to be secured very tight so any torque doesn't tear it off.. Haven't happen since id use a T shape beam on the input 

 

 

[ben20]

Try the older bush parts on the axles, they give more than double the force of 'normal' ones

Edited February 9, 2018 by ben20