Let's Go Design is an interactive web video series presented by SolidWorks, and hosted by Jeremy Luchini, a SolidWorks employee. According to SolidWorks, this web series “brings CAD fanatics from around the world together to collaborate on innovative design projects.”
The most recent project on Lets Go Design was a “hot rod baby buggy.” The goals of the project were that it had to be cool and fun, couldn't endanger the baby, and would run on multiple terrains, including sidewalks. What they came up with was a tracked all-terrain electric vehicle. Here is a picture I took of it at SolidWorks World:
After SolidWorks World, the Let's Go Design crew took the Hot Rod Baby Buggy out for a little off-road testing. And they busted it good. Here are some still pictures, captured from YouTube, showing what happened:
The first two pictures show the track on the right side picking up a chunk of mud and grass. It gets run between the track and the idler sprocket at the rear of the buggy, and binds up the track, bending the idler sprocket spindle.
Most all of the mechanical parts of the hot rod baby buggy were designed in SolidWorks, except, notably, the drive axle assemblies and the spindle and hub assemblies used for the ider sprockets—which were sourced from golf carts. Here you can see the spindle for the idler sprocket. Seems a bit... spindly... doesn't it?
It's not so interesting that the spindle failed. It's more interesting to consider why it failed.
One of the basic requirements for a tracked vehicle is that the tracks be able to turn in different directions, allowing in-place pivoting. In this project, the Lets Go Design crew used motor/differential/axle assemblies from golf carts. They modified the assemblies, by removing one axle shaft from each, and welding the differential spider gears. They used one assembly up front, to drive the right track, and one assembly in the back, to drive the left track. Spindle and hub assemblies were bolted to the side of the axle assembly where the axle shaft was removed, to hold the idler sprocket.
At first blush, this seems like a clever way to drive the tracks. But let's pause for a moment, and draw (crudely) a couple of quick diagrams, showing major forces caused by the track on the drive and idler sprockets when moving forward. (These are force vectors, not direction vectors.) Notice anything significant?
When you drive a track from the front, the idler sprocket is put under major forces when the vehicle is moving forward. Double the forces that the drive sprocket is under. With this drive arrangement, it was a foregone conclusion that the right idler sprocket spindle (which came from the front of a golf cart) was going to get tweaked the very first time the track got bound up with foreign matter (dirt, rocks, sticks, mud.)
In episode 3 of this project, Stephen Endersby, a SolidWorks simulation guru, stopped by the show, to be a “voice of reason.” He looked at the drive axles, and warned Jeremy that if there was too much flex on the drive sprocket inner spacer, there could be a binding problem. He didn't, however, mention the much more serious problem with the idler sprocket spindle. At least, not on video.
The Let's Go Design crew had at their disposal some of the finest CAD and CAD engineering software available. They were all experts in its use. They all had engineering backgrounds. And they had access to advisors and community members with deep experience in nearly any engineering discipline you could imagine. Yet, they still made a neophyte mistake in this design.
Why? I think it comes down to what their goal was: to showcase SolidWorks software. If their goal had been to showcase not just tools, but also the engineering process, the results might have been different.
I give SolidWorks a lot of credit for doing this web series. It was entertaining, and often thought provoking. I'd like to see more. But I'd like to suggest a slight change to the series' name. Something that reflects a part of the product development process that shouldn't be forgotten: