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.

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…

Making bogey mounts

Before I can have a rolling chassis, I  need to make the frame mounts for the 3 bolts that attach the bogies to the frame.

Here are the mount  pieces:

They will be welded to the frame like this:

Bolting the bogies on, to hold the mount pieces in place while I weld them on:

Here they are, all of one side bolted on, ready to be welded… they really make this chassis start to look like something.

Once the mounts are all welded in place, I will remove the bogies, add the spacers, bogey arms, springs, upper rollers (which support the track), and finally the wheels.  I am currently using 1/4″ bolts.. I’m a little worried they will be weak, so I might drill them out and replace them with 3/8″ or even 1/2″ bolts.  I could also just replace the bottom center bolt making it the load bearing bolt.  We shall see.

One other note… the wood hull will also need to have these same holes drilled, so the bogies can mount on the outside of the hull.

Frame again

Frame coming along nicely, and drivewheels are done…

Clamping second side to completed first side:

Both sides done set out to decide width:

Notice the (soon to be) supports across the bottom of the frame.  The drive sprocket teeth are shaved to a point, and drive wheels are fully assembled (except bearings):

Welding the frame

I am making the frame, and doing a little reshaping of the drive sprockets right now.  The frame is going pretty well.  My plan is to make 1 side, then make the other side on top of the first side, so they are as identical as possible.   Then, I will add the cross-members from one side to the other.

As for the drive sprockets, as mentioned in my last post, I am taking the corners off of them, so the treads clear as they come off.  I am just running the corners of the teeth, freehand, along the straight-cut router bit.  Doesn’t have to be perfect, just a little more clearance.

Drive Wheels done. Track time.

Finished the drivewheels.  Still need to get a few more bolts, but 2 works for now. They look pretty much awesome.

So, now that I have those, I can begin working on the tracks, since I can figure out the exact spacing, size, and shape of the overhanging treads, which the drive sprockets will turn.

I cut up a rubber doormat to simulate the rubber track, and made some test plywood treads and inner alignment blocks, which apparently are called “bells”.

I then countersunk some holes for wood screws which go through the tread, through the track, and into the bells.

Then I put it over a drive wheel to test how it rolls, if it catches etc.

It does slightly catch as the track is coming off the sprockets, so when I make the actual treads out of UHMW, I am going to router the corners off the ends of the treads, and round off the corners on the drive sprocket teeth.  Like so:

Drive Sprockets almost done

Ok, I finally made some really good progress on the drive sprockets.   I cut out all 4 (2 for each drive wheel).  I had to spend a little time making a homemade router table for my router, but once I finished that, I was in business.

So, after being cut out, due to the bending of jigsaw blade, they needed some cleanup done on them.   So I put a straight bit on the router, and just used it to shave the jagged edges, like this:

Then I put a chamfering bit on the router, which cuts a diagonal edge.  Before on the right, after on the left:

Here they are, they turned out great.

All I have left to do now, is drill the holes to bolt it to the drive wheels, and drill a 1-3/8″ hole in the center for the bearing to fit it.    Here’s the bearing I’m using:

Also, finished both hubs for the drive wheels, so they are almost done.   Once the drive wheels are finished, that is huge, cause I will be able to start working on the tracks, since I’ll know the exact spacing for the treads on the tracks.

Bogies and Roller Wheels

Bogies are the sets of roller wheels, which provide the track with its means of suspension.  I have considered a few different designs, including the two below which are loosely based on the M3/M4 Sherman tank.  I think for simplicity I will go with the One Spring design, since often times simple is better.

The main difference between the track and bogey system I’m using, versus from an M3/M4 Sherman, is I am using an inside, and an outside row of roller wheels.  These roller wheels will ride around 1 row of blocks on the inside of the track, which will keep the track from coming off.  An M3/M4 uses 2 rows of these alignment blocks, an inside and an outside, with just 1 row of roller wheels between those.     The reason to do it like I am, is having a few extra rolling wheels seems easier than making 2 sets of inner alignment blocks, where you need to do hundreds of them.  There are a few other reasons, but no one really cares.

Still not sure exactly what material to make the bogey pieces out of.  Leaning toward UHMW plastic, or possibly aluminum, trying to keep it fairly light.

UHM…W, I call it. UHMW the wonder plastic.

I have decided to use UHMW, which is a hard plastic, to make the drive sprockets and track treads out of.  And other stuff later on.   You can get remnant/scrap pieces, ($1 per pound in my case), which is a very fair price… after all, it’s plastic, not lead.

Why UHMW?   It is a thermoplastic polyethylene.   It has the “highest impact strength of any thermoplastic presently made.”   It has very low friction (less than nylon and acetal, and is comparable to Teflon), is self-lubricating, and is highly resistant to abrasion. It is odorless, tasteless, and nontoxic.  It is also very resistant to water, moisture, most chemicals, UV radiation, and micro-organisms. *

Basically it’s super tough.   BUT, the other great feature, is you can cut it with most woodworking tools… about the only thing you don’t wanna do is sand it with sandpaper.    I was also able to get it in black, which is nice, will give the tracks a nice realistic look.

I got 1/2″ thick for the treads.  I decided I will cut them 1″ wide, and I think the 1/2″ will be perfect… I had originally said 1″ thick, but I think that is overkill… it would weigh twice as much, and would dig or catch more into soft ground, which should not be necessary.

For now, I cut out one drive sprocket, just need too clean up the edges a little:

* – Copied verbatim or paraphrased from http://en.wikipedia.org/wiki/Ultra_high_molecular_weight_polyethylene

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