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Hey everyone.

I built a compact subtractor as I thought I would give it a go.

Unfortunately I doesn't work very well, doing 1-3 turns before the current protection kicks in and stops.

Any ideas why? Is it cos I used the L motor and not another XL? I wanted it to turn on the spot so I thought a faster motor would work, and it does, it just cuts out. :hmpf_bad:

This is taken with the BB and IR not in place.

Cheers.

16192182265_43053b38c2_b.jpg

16006399277_c2b032159e_b.jpg

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I don't see nothing wrong with the gear setup, I think your problem is in IR receiver. Did you put two XL on one output? I hope you used V2 receiver at least. Try using two receivers, one xl per each, I gues there is data on how much current each motor eats up and how much a single output and the whole receiver can take. There is also BB current limit.

I really like your setup otherwise, and will definetly try it out ;-)

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At first glance I would say that the gears are not braced enough. Maybe I'm wrong, but the structure seems to be only 1 layer thick (so it can easily twist) and there's a floating 3L beam. Floating beams never worked for me, especially when there are 8t gears.

BTW did you try to spin the input axles by hand without the motors?

Edited by Lipko

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Are you sure the motors are not fighting on another,eg they turning and forcing the connecting gears to work against one another ?

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If you look closely there is no gear forcing in the setup, which is really great. If you use 3 L motors und ypu csn put them all on ine side, do you gain a bit more space for bb. You'd have to turn the diffs around and the whole middle gear section with it...really cool!

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It looks like the gearing from the L-motor to the diffs is too high. The diff cage has about 3:1 higher ratio than the XL motors turning the bevel gear. This would lead to overtorque and tripping the current trip of either the L-motor or the IR Receiver.

A better torque balance would be an L-motor on each diff and an XL motor for the steering.

If I were building a subtractor then I would take the outputs from the cages of the diffs.

The principle is: always gear down for LEGO motors driving land vehicles. One of the few times I gear things up is for propellers.

The PF motors were designed with full-voltage speeds in the range 110 to 400 rpm. The most efficient use of a motor is for it to do half its no-load speed under load.

Mark

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How about this, u do need an extra switch to flip the rotations of the xl's though...

16193073991_5b3d641338_z.jpg

Edited by dikkie klijn

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How about this, u do need an extra switch to flip the rotations of the xl's though...

16193073991_5b3d641338_z.jpg

Thats awesome! but yes you do need a switch on one of the XL's. I do have a switch.

Does this work for you? Do you get stalling? I'm very curious to know! :grin:

To answer some questions. I'm using V2 and don't seem to have mechanical issues, I'm thinking its more a power problem but don't under stand under almost no load 2XL and a L cut out.

Yes Lipko that is a floating 3L but using new 8t I don't think its a problem, the frame is only 1 stud think but bracing seems ok. :sweet:

It looks like the gearing from the L-motor to the diffs is too high. The diff cage has about 3:1 higher ratio than the XL motors turning the bevel gear. This would lead to overtorque and tripping the current trip of either the L-motor or the IR Receiver.

A better torque balance would be an L-motor on each diff and an XL motor for the steering.

If I were building a subtractor then I would take the outputs from the cages of the diffs.

The principle is: always gear down for LEGO motors driving land vehicles. One of the few times I gear things up is for propellers.

The PF motors were designed with full-voltage speeds in the range 110 to 400 rpm. The most efficient use of a motor is for it to do half its no-load speed under load.

Mark

Ahh thanks very much! I sorta understand, I thought something like this was happening. So gearing down the L would work better? But if it is slower it wont turn on the spot, correct me if I'm wrong.

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No stalling :D, but i do use the small lithium battery box. But its running smooooooth :tongue:. But i think you could make this thing really interesting by using 2 receivers controlling all of the motors independently.

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Even though I appreciate the compact solution, I have to say that I am a bit sceptical about using all those 12T bevel gears in the drivetrain.

An XL motor driving through more or less only 12T bevel, and perpendicual through a diff, you are bound to get some gear destruction sooner or later!

But that's only my 2 cents...maybe your vehicle is not going to meet any obstacles so it's all good :classic:

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I built a subtractor some time ago based on ideals in Sariel's excellent book "Unofficial Lego technic guide"

It uses only two motors, the XL to run both tracks with a M motor when running slowing down one track for large radius turns.

With XL motor stopped & M motor running the tracks run in opposite direction allowing turns on the spot.

To avoid using small bevel gears I used knob gears for final transmission to the track drive wheels.

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How about this, u do need an extra switch to flip the rotations of the xl's though...

16193073991_5b3d641338_z.jpg

If i'm not mistaken: There's no need for a switch, he could also flip the tan gear on one of the sides and it would also work?

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No because without a switch the L motor lets the output shafts of the subtractor turn in the same direction as the xl's would

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No because without a switch the L motor lets the output shafts of the subtractor turn in the same direction as the xl's would

Aha, see ya :wink:

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Even though I appreciate the compact solution, I have to say that I am a bit sceptical about using all those 12T bevel gears in the drivetrain.

An XL motor driving through more or less only 12T bevel, and perpendicual through a diff, you are bound to get some gear destruction sooner or later!

But that's only my 2 cents...maybe your vehicle is not going to meet any obstacles so it's all good :classic:

HAHA, valid point! After I get it running we shall see how long they last.

No stalling :D, but i do use the small lithium battery box. But its running smooooooth :tongue:. But i think you could make this thing really interesting by using 2 receivers controlling all of the motors independently.

I shall give that a shot! Cheers.

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instead of use The switch why don't You change The yellow gear positIon? i've never tried To make a subtractor But this looks quite simple

ohw sorry I didn't See ore post, But in You use 2 receivers You Could invert The channel direction

Edited by PKW

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Now built a more compact subtractor drive which eventually have all motors driven through a turntable.

So far have chassis with subtractor unit built - see Flika album:

https://www.flickr.c...57650656511358/

Photos show motors in the chassis for testing the subtractor unit.

It took some time to get the gear ratios correct.

Both motors drive at a 1:1 ratio through the differential units.

An L motor drives both tracks as shown by Red line.

The M motor provides the steering either by slowing one track down if L motor running

or with L motor stopped by turning tracks in opposite directions for turns on the spot as shown by green line.

Final drive to the tracks is by two 24:1 worm drive gear boxes.

The L motor provides ample power for both tracks but could be substituted by an XL motor.

Now working on moving both motors into the upper rotating body by using a dual drive through the turntable.

Using Sariels double-axle turntable transmission as a starting point (as on shown on pages 96 to 100 in his excellent book)

I modified it to elimiate the fragile single bevel 12T gear - replacing them with double bevel 12T gears which more robust.

This also avoids the need to use the 4 right angle drive housing p/n 4558692 and has a very rigid structure.

Edited by Doug72

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