Splat

[HELP] Gearbox Design

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Hi all, I've recently come out of my Lego dark-ages, and have collected a few Lego sets (mostly Technic), but now I want to design and build my own creation (MOC).

I want to build a tank, driven by 2 XL motors (joined by a differential), and a 3rd XL motor used as a subtractor to control the steering. I also want to include a gearbox with the following specs:

  • 4 speeds:
    - 3 speeds send power to the same output which goes on to rotate the tank treads.
    - the 4th speed outputs to a second drive-train, to control a winch (while the tank treads can rotate freely).
  • Gear selection should be able to be made with one M Motor (via a linear actuator or similar).
  • Gear selection should be able to be made on the fly, without having to stop driving between gears. To do this I have to be careful about the spacing between the gears to avoid binding.
  • As the tank is driven by 2 XL Motors, the gearbox needs to be robust enough to handle the torque involved.
  • I want the gearbox to be as compact as possible, but I understand that these things do take room.

I have the 8865 Test Car, so I started by basing my gearbox off that. The problem with that design is that all the gears in that set have square edges (double bevel gears weren't available in 1988), so it can be a bit hard at times to mesh the gears properly when changing them. Therefore I wanted to use bevel gears, or a combination of bevel and square edge gears that mesh a lot easier when changing gears.

I have also taken a lot of inspiration from Sariel's tanks and gearbox designs. Thanks Sariel :thumbup:

I have come up with two different gearbox designs that I like so far (the M Motor and wheel in the pics below is just to represent the input and output axles):

Design #1:

This design uses only 20T, 16T, and 16T with clutch gears. All the gears are spaced out a bit to prevent the gears from binding, and these spacers would be replaced with liftarms and attached to the chassis to stop the axles from bending.

00020000.png

Third gear
- 20:16 > 20:16 = 0.64:1

00020001.png

Note that the 16T with clutch is only there to help hold the gears in place to prevent them from jumping under high resistance situations.

Second gear
- 20:16 > 16:20 = 1:1

00020002.png

First gear
- 16:20 > 16:20 = 1.5625:1

00020003.png

Again, the 16T with clutch is only there to help hold the gears in place to prevent thm from jumping under high resistance situations.

Winch
- 16:20 > 20:16 > 16:16 = 1:1

00020004.png

The gearing to the winch can then be altered later, probably with a worm gear.

Design #2:

This design uses a greater variety of gears, including a 24T clutch gear, but I'm not sure it this is the best position to use it though.

00010000.png

Third gear
- 12:24 > 24:12 = 1:1

00010001.png

Note that the 24T clutch is only acting as a idler gear here. It won't slip as both 12T gears are meshing with the outside of the 24T clutch gear.

Second gear
- 12:24 > 20:16 = 1.6:1

00010002.png

First gear
- 12:24 > 12:24 = 4:1

00010003.png

Winch
- 12:24 > 20:16 > 16:16 = 1.6:1

00010004.png

The gearing to the winch can then be altered later, probably with a worm gear.

Pros and cons:

  • Both of these designs can be controlled by a single M Motor and linear actuator.
  • The second design is quite smooth when changing gears, but the gears aren't as supported as in the first design, so they might jump under high resistance situations, although I'm hoping that the 24T with clutch will help here.
  • Both design are 10 studs long (excluding the chassis), but the central axle needs to be about 16 studs long (two axles joined together), and they need to slide +/- 4 or 5 studs either way, which takes up a lot of room.

  1. Which design do you think is better?
  2. Can you think of a better design that fulfils the specs above? It doesn't have to be a linear gearbox, but I want the gear changes to be controlled by a single M Motor. Is there some cool gearbox design that you think might work that I haven't come across yet?
  3. Is that the best placement of the 24T with clutch within the second design? I could never really work out where to put the 24T clutch gear for optimal performance. Should I remove it completely and replace it with a standard 24T gear?

As the gearbox will be buried deep within my MOC, any help in getting this right is appreciated. Thanks.

My Brickshelf folder with larger images.

Edited by Splat

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Why do you use clutch gears in the first design? What do they do (of course, not counting the one used for the winch output, whose use is clear)? What happens when you replace them by bushes?

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Why do you use clutch gears in the first design? What do they do (of course, not counting the one used for the winch output, whose use is clear)? What happens when you replace them by bushes?

Hi Erik. Thanks for your question.

The middle axle is so long between supports, and when there is lots of resistance in the drive-train (like when the tank is going up-hill), it is likely to bend a little bit with the amount of torque from 2 XL Motors. This will cause the gears to start jumping, and the teeth will grind down. The reason that I used the 16T clutch gears is so that the 20T gear on the middle axle is held in place by gears on both sides, which should help to hold the middle axle in place and stop the teeth from jumping and grinding quite so much. They have no function apart from that, so you could replace them by bushes, but then the middle axle might bend.

I hope that explanation helps. Let me know if you have any other questions.

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Sorry to bump and old topic, and to double post, but I have some more developments on my gearbox design that I thought that I would share with you all. Rather than create a new topic, I thought that I would reply to this one.

Over the last 6 months or so, in between other projects, I have been trying to design a gearbox that has 3 speeds plus a 4th gear that sends output to a secondary drive-shaft to control another piece of equipment, such as a winch or crane.

I designed many different gearboxes that failed for one reason or another, some of which are shown here:

failedattempts.jpg

The main problems with these gearboxes are:

  • some used long axles, and long axles can flex if not supported properly,
  • some used square edge gears that don't like to mesh nicely with other square edge gears (8T, 16T, 24T), and
  • some used a sliding axle to change gear, which means you need twice as much space in your MOC to allow the axle to slide back and forward.

As a result, they were big, hard to control, and the gears would jump, grind, and/or bind too easily.

Another failed attempt, using a worm gear to change gears:

It works, but it can't handle much resistance before the gears jump. :sad:

I also tried to come up with some other methods of controlling a gearbox, which can be seen in these two YouTube videos:



Unfortunately these methods take up a lot of room, and while they are great proof-of-concepts, they aren't too practical in real-life.

After much trial and error, I have finally designed a gearbox that I am happy with: :classic:

00000002.jpg

(In the following photos, notice the position of the Dark Bluish Grey 16T Clutch Gear in the middle - click for larger images)

Secondary Drive-shaft = 1.66 : 1

00000004.jpg

Neutral

00000005.jpg

1st Gear = 2.77 : 1

00000006.jpg

2nd Gear = 1.66 : 1

00000007.jpg

3rd Gear = 1.00 : 1

00000008.jpg

It fulfills all of the criteria that I set out in my first post above, and also includes a neutral gear too.

For each gear position, it is always a bevel gear meshing with a square tooth gear (no square on square combinations) so the gears mesh really smoothly. There is also a little bit of space between each gear to prevent the gears from binding.

On my YouTube page I posted the following description:

A design for a Lego Technic gearbox that has 3 speeds plus neutral, and a secondary drive-shaft (red). The secondary drive-shaft can be used to control another piece of equipment such as a winch or crane.

The gear changes are smooth, and the teeth don't jump, grind, or bind at all. As the motion of the gear change is linear and sequential, it can easily be controlled using a Linear Actuator and a M-Motor (as shown in the video), or remote controlled using the Lego Power Functions system.

The gear ratios are:

Secondary Drive-shaft = 1.66 : 1

1st Gear = 2.77 : 1

2nd Gear = 1.66 : 1

3rd Gear = 1.00 : 1

Different gear ratios can easily be achieved by changing where the drive motor is attached.

The gearbox mechanism can be built into a chassis of a vehicle quite easily. The linear actuator takes up a lot of space, but if you remove the Secondary Drive-shaft and the Neutral positions, then the gearbox can be controlled using the smaller Mini Linear Actuator.

More images are available on Brickshelf :

Please let me know what you think, or if any further improvements can be made.

Edited by Splat

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On 3/3/2012 at 9:18 PM, timr said:

wow that is really complex and intricate.

Thanks timr :sweet:

The gearbox isn't that complex or intricate, and I think it is a fairly elegant solution. The bulk of it is mainly due to the frame around it, but in a proper MOC this frame would become part of the chassis of the vehicle.

Looking back at the video, I can see that it might be a bit difficult to understand what is happening in the gearbox, so I have created a series of CAD images that show what the gears are doing at each gear position. Basically, the Linear Actuator is just moving the central dark bluish grey 16T clutch gear back and forward along the central axle, and in doing so different gears are engaged.

(click to enlarge)

00000010.jpg

00000011.jpg

00000012.jpg

00000013.jpg

00000014.jpg

In the pictures above you can see that the gears that are engaged always go in a straight line through the central 16T clutch gear. It takes a lot of force to get these gears out of line, so there is little chance of the gears jumping at all.

The 1st, 2nd, and 3rd gears all have an even number of gear steps, which means that the output always rotates in the same direction when any of these gears are used. The secondary driveshaft currently rotates in the opposite direction.

On 3/3/2012 at 9:18 PM, timr said:

Can it handle any force on the output?

In my initial tests (and shown in the video at 2:17), if I stop the output axle from turning it stalls the motor rather than letting the gears jump, grind, or bind. The video shows me using M-motors, but I suspect that if I used an XL-motor the universal joints would come apart before the gears give way. I'm not sure exactly how much force the gearbox can handle before something 'bad' happens though.

Let me know if anyone is interested in instructions for this gearbox.

Edit: Nobody here on EB seems to want instructions, but a comment on my YouTube page asked for them so I have created a few quick steps which can be seen in my Brickshelf folder, as well as an LDraw file.

Edited by Splat
Fixed YouTube link after forum upgrade

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I know this is an old thread, but wanted to say thanks for making those instructions! Great job, and the design too. I will give it a go when I finish work. :)

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Hello splat!

I have modified one of your gearbox designs. (You'll know which one when you see it) It is now MUCH stronger, it doesn't skip gears, an it works under heavy loads.

http://imgur.com/a/KiAFj. As i could only find 3 wormgears out of my mess of technic, it couldn't reach all of ratios :sceptic: You'll just have to add in the extra wormgear.

The video http://www.youtube.com/watch?v=3C8M3HssoI0&feature=youtu.be

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I know this is an old thread, but wanted to say thanks for making those instructions! Great job, and the design too. I will give it a go when I finish work. :)

Thanks Doc_Brown :sweet:

I have modified one of your gearbox designs. (You'll know which one when you see it) It is now MUCH stronger, it doesn't skip gears, an it works under heavy loads.

http://imgur.com/a/KiAFj. As i could only find 3 wormgears out of my mess of technic, it couldn't reach all of ratios :sceptic: You'll just have to add in the extra wormgear.

Hi Someonenamedjon, I'm glad you could take this gearbox idea and make it work for you :thumbup:

I'm not sure about the worm gears fitting between those two beams though :sceptic:

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Why don't you just use LA for the selector?

Yes they may be a small bit of play but it should make your lives a bit easier.

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Why don't you just use LA for the selector?

Yes they may be a small bit of play but it should make your lives a bit easier.

To get the full range of gears for the gearbox that Someonenamedjon reworked, the selector needs to travel 6 studs.

Mini LA's only have an extension of 3 studs:

TBs_20101209_1e.jpg

The larger LA's have an extension of 5 studs:

dscn7774.jpg

So neither the Mini LA or standard LA have enough travel, unless you start to play around with levers or some other mechanism, which starts to make things bigger and more complex, and introduce more play in the system.

As mentioned above, this gearbox was considered a failure for my needs, but Someonenamedjon has been able to rework it to make it stronger and perform better :thumbup:

The final solution that I came up with and am happy with is still this one, which does use a linear actuator:

Edited by Splat

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What about a long 13 stud gear rack connected to the leaver?

That could work, but it means that you need to allow room for the gear rack to move back and forward.

One of my goals was to keep the gearbox as strong as possible, but also as compact as possible. Using a set of worm gears allows the gearbox to be more compact.

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I think you should just try both gearboxes to see how much torque they can each handle and then use the better one for the final model. You will need a freewheeling mechanism for the smooth linear gearbox, right?

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Hi! I just made your Gearbox with a few additions. I also plan in using it in my 4x4 I'm going to build and model after my 4 runner!

I'm going to use a xl motor for the drive and get rid of the u joints. Thank you for the instructions. They were extremely hard to follow tho. Especially with the same picture multiple times.

I will post a picture soon.

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Here it is!

th_2013-12-25152726_zps9e86501b.jpg

i think i mixed something up. I feel like the gears are out of order and i cant figure out whY!? EDIT: i fixed my issue, had the wrong set of gears split up.

Edited by mike4slund

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I'm going to use a xl motor for the drive and get rid of the u joints. Thank you for the instructions. They were extremely hard to follow tho. Especially with the same picture multiple times.

Here it is!

i think i mixed something up. I feel like the gears are out of order and i cant figure out whY!? EDIT: i fixed my issue, had the wrong set of gears split up.

It is probably a good idea to replace the universal joints - they were only added afterwards to attach the motors.

Yeah, these aren't the best 'instructions' that I have made - they are more like 'plans' rather than 'instructions'. They were mainly created so that I can recreate this gearbox at a later time, and can reuse the parts for other things until then.

I'm glad you got it working. Let us know how it turns out once you incorporate it into your 4x4.

Why not just use a rack and pinion for the shifter?

There are many, many ways to create the shifter. I have tried to make a lot of different types of shifters, but most of them suffer from the same problem - they take up a lot of space, including the linear actuator method.

If a rack and pinion is used in this design, then space has to be provided to allow it to slide back and forward, and a supporting structure also has to be created to make sure that it slides smoothly. It is definitely an option though.

I do have another method for the shifter that will be a bit more compact, but I need to perfect it before making it public.

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