Going Gas Powered

Well, here goes. Taking the tank apart, pulling the electric motors, and swapping in a gas engine with a idler engaged pulley style transmission.  I’m tired of the 1hp from the electric motors, and not being able to have the tank really drive anywhere fun. Will probably go with a 3 to 6 hp engine in the back, and transmission in the front.

topOff

Here’s the basic plan for the transmission.  The motor spins 2 shafts in opposite directions.  Each has a pully on each side, which can be engaged with an idler, giving the 4 options:  Left Forward, Left Reverse, Right Forward, Right Reverse.
TransmissionPlan

To turn the 2 shafts in opposite directions, the current plan is to have the chain wrap around a sprocket on each shaft, and an idler to give the proper clearance.
CounterDriveSmall
The other option is to drive one shaft with the motor, and have it drive the other shaft in reverse via cogs/gears.  But I’m leaning toward the chain method.

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Controls and wiring

Well I got the switches in the control box wired, the levers installed, and the whole system all wired ready for testing.  I have a loose wire or the pass-through relays are acting funny, so it is working intermittently, so I am trying to track down the culprit.

 

I also installed the tracks and the drive belts.  While the electronics were working, I tightened them up, to see if the thing would roll!  And… drumroll…..  the belts slipped on the pulleys.  This is either because the pulleys are too slick being made of UHMW, or the belts just aren’t tight enough.  I will first try to just add some idler pulleys to tighten the belts, and hope that works.


Also, the total weight of everything seen, including suspension, tracks, and batteries, is 240 lbs, which I am pretty happy with.  The batteries are probably a good chunk of that weight, but that is how it works with electric vehicles.  Batteries are heavy, or expensive, one of the two.

Pulleys in place

I made the hubs that connect the pulleys to the drive sprockets.  I used a 3″ washer, a piece of steel tube, and a floor flange which matches the bolt pattern on the sprockets.

Then I bolted the hubs to the drive sprockets, and the pulleys to the hubs.  I fitted a test-belt (taped together) to figure out the length of the belts I need.  Everything lines up great, lookin good.  Now time to take everything apart to work on the lower hull, more to come on that soon…

Motors and wiring

Well after a few-month-long hiatus cause of moving houses, I am back to tank building.  I got an old mobility scooter that I can use most of, for the tanks powerplant(s).  It has two 24v motors, a speed controller, a governer (basically a 2nd speed controller), a 24v battery charger, and it even has a little transmission type of thing where the motors hook up to some output shafts.  All stuff that if bought separately would cost $Arm.leg.   So here it is:

 

Here is my attempt at a wiring diagram (electrical engineers please hold back your laughter).    The switches and relays in the diagram are DoublePole-DoubleThrow (DPDT), which basically means On-Off-On, and when wired a certain way, can be switched from Forward-Off-Reverse.    This is needed because the existing speed controller and wiring is setup to obviously power both motors the same direction.  I need the ability to have each motor run independent, directionally, of one another.  I will cover some steering and turning concepts later when I do the levers and stuff.  For now, here is the wiring.  Do enjoy.

 

 

Motors, gearing, and speed.

I have done some very preliminary research into what I should use for the electric motors.   Thanks to the wonderful popularity of those silly motorized scooters, there actually a decent number of affordable electric motors.  I will need 2 motors of course, one for each side.  This way each track can go forward or reverse, independently of the other.

The motors I’m considering at this point are 24v, around 250W and run up to like 2500 rpm.  They even come with small pulleys, around 1″, already on them  (I will have a future post about belts and pulleys versus chains and sprockets).

So, I wanted to get an idea of the top speed the tank would have, if the motors were running at 2500 rpm.   I am figuring a gear ratio of 6:1 (1″ pulley on the motors, to a 6″ pulley on the drive wheels).  I will also have approximately 6″ drive wheels, which, when driving a track, you can pretend they are actually just wheels on the ground, and the numbers will work out close enough (there is a slight difference due to the thickness of the track, but that’s not a huge deal for now).

Anyways, here’s the equation to figure out top speed:

(RPM/gear ratio)  x  60  x  WheelDiameterInches x Pi
———————————————————————-
5,280 x 12
= 7.4 mph with a 1:6 gear ratio
=5.4 mph with a 1:8 gear ratio

I am very happy with those speed estimates.  If they came out to like 30 mph, I would know I’d need to gear it way down or get lower revolution motors… likewise if it came out to 1 mph I would need to gear it up and so on.   Also, when finally selecting the motors, if it’s a high torque motor, but turns way less revolutions, I will know I can use more even gearing, etc etc.

Installing the motors is obviously far off at this point, but these things need to be considered.  For instance, I am leaving myself enough room for a 6″ pulley on the drive wheels, when I construct the frame, axles, and the other associated parts.    That’s about it for today, sorry, no cool pictures, but at least we did some handy math you can use when making your own tank.