Gear questions

[Johnny1360]

I want to turn some tires at the fastest possible speed. I will be using 3 medium motors, they do turn fastest right? There will be very little load on them, just the gear train and tires. So a couple of questions and I have been experimenting, since I am going to gear up, which gear type has the least friction and does anyone know how far I can up the ratio before the gears and wheels drag it down? I will be using 56x26 balloon tires and again there will be no load other than the gears and tires themselves. 

[Imanol BB]

Hi, if you are looking for the maximum performance then you shouldn't just consider gears and ratios, one important thing that we don't have with Lego is bearings, the axles rotate directly inside pinholes, its basically plastic to plastic movement which obviously creates lots of friction and hence power losses, so gearing up alone won't be necessarily enough, my advice for you would be to place the motors to have the most direct power transmission to the wheels, try to avoid using gearboxes or rooting the drive axles through too much holes, you should see Didumos Greyhounds buggy:

You will notice that the 4 motors drive the wheels almost directly, rears just go through 1 set of gears each, and front ones just use a U-joint, the are no gearboxes/transmissions, no differentials, not too much places where the power of the motors will be lost as friction, only the hubs are the major point of power loss, even though that this MOC uses 3rd party power supply it still has an outstanding performance for those motors, one other example is RM8 Trophy truck:

He used 2 buggy motors that literally drive the wheels directly, there are not even gear involved, the result is a very efficient power transmission from the motors to the wheels, so that's my advice, don't only consider types of gears and ratios, consider the other friction sources which will drastically affect the performance of your model; i hope this information results useful, i'm also interested in what kind of project you have in mind, i wish you luck with it.

[Johnny1360]

Thanks for the very insightful reply. I have been using Sariel's gear ratio tool to figure out exact ratios, very helpful tool. I would hook the motors to tires directly but I want the tires to turn much faster than the motors, I am using mediums as they have high rpms, I have no buggy motors. I have tried three gear trains together 20 tooth bevel to 12 tooth bevel, for a ratio of 1:4632 which I think may be a bit much for the motors. 

As far as what I am working on, it's a motorised paper airplane launcher. So not much load on the tires, I just realize there is a point of diminishing returns as far as gearing and was looking for some insight on what is the best I can expect. 

I do have many other things to work out as well, chiefly at this time, how to not mangle the plane. I think I should just try straight motor to tire drive for now and gear up later. So thanks for your input. 

My experiments so far seem to indicate that first I should get the plane to launch then add speed. 

Edited May 6, 2019 by Johnny1360

[sirslayer]

@Johnny1360 you can ues the same setup on the Lego set 76112 Batman Rc, instead of using the Powered Up equipment you can use the Lego PF equipment and install the 2 M motors like the 76112 set.. !!

[Johnny1360]

Thanks, though I am not sure how I could incorporate that set, I will check out the instructions. 

What I have in mind is something like a rail gun, with opposing sets of tires spaced equally down the length of the rail, with a sort of channel for the wings to slide through. I will be shooting a paper airplane through it, more like a missile really. 

I would like to get some serious speed on the wheels and basicly  they will be the only source of friction or load on the motors

I am using the 56x26 wheels because with six holes inbetween the axles, the tires barely touch each other. 

Could someone please tell me the gear ratio of a new style differential driven with a 20 tooth bevel gear? I just can't seem to figure it correctly and I assume quite of few builders have already worked it out. 

Edited May 6, 2019 by Johnny1360

[Zerobricks]

What about using train motors? 

[Johnny1360]

That would be great, unfortunately I have none and I am not buying any parts just for this project but yes those would be great. I do have a rather large parts selection though. 

As I have mentioned, I built a small POC medium motor 800/1200 rpm with a 1:4.632 ratio for a theoretical tire speed of 5558.4. That was only one tire, I want to drive two tires per motor. I have my doubts on that gearing working, will probably reduce it some. 

I was thinking of differentials, which is why I wanted to know their gear ratio.

Anyway this is just a stupid little POC thing for me and I don't really care what it looks like. I do want to work out the gear ratios though and for it to actually work. I do realize friction will be a huge factor for speed and there will be a point of diminishing returns. It does seem like bevel gears have less friction so far.

Just fooling around a bit until August. 

Edited May 6, 2019 by Johnny1360

[Johnny1360]

Okay can somebody please confirm my math? A new 26 tooth differential with 12 tooth gears inside of it driven by 20 tooth bevel gear gives me a ratio of 1:1.666. Or am I just confused? 

[Imanol BB]

30 minutes ago, Johnny1360 said:

Okay can somebody please confirm my math? A new 26 tooth differential with 12 tooth gears inside of it driven by 20 tooth bevel gear gives me a ratio of 1:1.666. Or am I just confused? 

The differential has 28 teeth, when driven by a 20T gear there will be a 1.4:1 reduction

[Johnny1360]

Ah yes 28 teeth my mistake, so then the gears inside have no further effect on gearing, since they are all the same? Thank you very much for clearing that up for my age addled brain. 

[Johnny1360]

Yet another stupid question for anyone reading. Would I be better off adding speed (gearing up) before the differential or after? I assume after just not sure. 

[sirslayer]

@Johnny1360, depending on your hardware, the Lego part,

Technic, Steering Portal Axle, Housing
Item No: 92908

from bricklink website is a solution to provide extra height for the wheel hubs is a gearing add on that is mounted after the differential.. the reinforced frame of that lego part also provides strength when you gear down for torque, torrque is a very strong force and most "homemade" wheel/steering portals  would bust apart and using the Lego technic part 92908 would give the strength to gear down for torque and its all geared past the diff., ideally inside the wheel rim.. If you need to do "high speed" which speed is inversely proportional to torque, the default output of the Lego M motor is 275 rpm at 9 volts ( 405 rpm at 12 volts!!!) and gearing for more speed out of this under powered motor would be only recommended for very very light weight.. like a paper plane hauler ... but without the battery pack..  the weight of the batery pack might be to heavy for a single 1 M motor to pull.. check out this video..  this guy is a master of Lego techinc gears!!!  

 

Edited May 7, 2019 by sirslayer

[Johnny1360]

Certainly a lot of interesting stuff there @sirslayer appreciate the info, I sure thought the medium motor turned faster than that, I may have to recheck my math and add some more gears.

I do have a few portal axles but I don't really think I can put them to use, as I will be gearing up not down and I will not be using hubs either. 

As far as load is concerned the tires will be freewheeling barely touching another tire. There will be three pairs 13 studs apart along a line, with each pair being powered by 1 m motor. The only load will be the tires themselves with the gear train. The plan is for them to move a paper airplane/missile down a channel at high speed, in order to poke little Johnny's eye out. 

Right now I have to work out how to offset the power units 1/2 stud from the final drive due to tire placement and channel width. 

Thanks again and I will recheck that motor rpm thing. 

[Jurss]

You could try to use some 2 speed gear box, there are even automatic possible. 

Whit some more direct gearing You could start those wheels, and then switch to higher gearing for speed, so that engine and all mechanism has more softer start.

[Johnny1360]

That does sound like a very good idea, since the Speed I am looking for has the most load just starting  up and my POC module does seem to be right at the limit. I have plenty of room so I will see what I can do. Maybe three 2 speed transmissions with one shifter link so I don't have to do them all separately. 

Damn now I have yet another redesign, oh well that is were all the fun is and I think it will be a great idea. 

[AndyCW]

Another option, would be to make a heavy flywheel.  Use the motors to spool up the flywheel(s) and the use a trigger mechanism to nudge the aircraft into the launcher.  This way you are not using the torque of the motors to accelerate the plane, but rather the inertia of the flywheel.

[Johnny1360]

That is also a great idea, although I think I am too far into my design at this stage to make such a major change. I will keep it in mind though and might end up doing something along those lines. I kind of consider the tires to now be some sort of flywheel. Maybe I should put some weight inside the tires, half bushs or 2 L axles, hmm we'll see. 

As I will just be using one sheet of standard paper I don't think it will drag it down much if any at all. Hopefully it doesn't just shred the paper, my first attempt mangled it pretty good.

Love hearing all these ideas, so thanks everyone, now I feel like I really need to make this thing work. When I started I was just messing around with some gears and motors and didn't really care. 

Edited May 7, 2019 by Johnny1360

[Jurss]

If You will use some larger wheels, then they will be already pretty good flywheels. 

Also, larger wheels outer part will move faster than for smaller wheels. Take this in to account, maybe You don't need to turn them so fast, when choosing larger wheels. 

[Johnny1360]

Good point and you are absolutely right, originally I had planned to use larger wheels with flatter tread. I ended up choosing these though because with six holes between the axles the tires just barely touch, the only tires that do this without going to some weird 1/2 hole off-set or something. Which is necessary for moving the paper through the mechanism. I actually tried with them further apart and closer together and this seems to be the sweet spot. 

Edit: I am kind of bummed as I could not make time to work on this today, even though I thought about it all day. 

Edited May 8, 2019 by Johnny1360