All Stressed Out

Making a scale tank is not a new idea. However, making one without a frame is. I have yet to see anyone's scale project that didn't have a frame. Real tanks don't have a frame. The armor is thick enough to provide ample support for the vehicles weight and any stress that may be encountered. That's not to say that there are not "frames" inside a tank, i.e. support frame for the transmission, main gun, engine & etc. Most of these sub-frames merely provide attachment points for the various equipment in a tank. The big reason none of the other scale projects are frameless is mostly due to weight. A frame with a skin of plywood or sheet metal is significantly lighter than steel plate. But being hard headed, I decided to buck the trend and go the heavy metal route.

The big question was what was the minimum thickness the steel plate needed to be, but still provide adequate support? From I planning standpoint, I started out with 1/4" plate. I was pretty sure that if the whole skin was made from 1/4" plate, everything would be strong enough. That would put the weight of the body (outer skin only, not tracks, suspension, engine & etc) right at about 2000 lbs. Which means by the time I added everything else in, I was looking at about 5500 lbs. 5500 lbs is starting to push the limit of my suspension, hydraulics and tracks design. So in an effort to reduce weight, I asked for some help from an engineer who frequents the scaledtanks.com forum.

He took my CAD design and fed it into a program to evaluate stress on the body of the tank. We went through several different combinations of steel plate thickness before we came up with a winner. The combination I finally settled on was the floor of the tank in 1/4" plate and the rest of the tank in 1/8" plate. In addition, there was some angle iron added to specific joints to help reinforce high stress areas.

In the first picture, you can see the effect of a 2.5 G (61625 Newtons) strike on the left front road wheel axle hub.

The color scale shows the "buckling" of the plate. Blue, green and yellow are ok; red is bad. As can be seen, there is just one small area of red in the front corner of the right side sponson. The next two pics are of different angles.

So what does all of this mean? By using the thinner plate with some reinforcement in key welded joints, approximately 800 lbs and $600 in material costs will be shaved off of the project. I'll call that a victory any day of the week. :)