Our idea was to build a motorized rail cart to explore the many abandoned rail lines across the country. We built the structure at Tech Shop (http://www.techshop.com) from the ground up, starting with the hull. The concept was a truss fame with eight tubes and transverse stiffeners. The chassis would split apart so a single person could carry each half as they would a rickshaw.
Step 1: Frame Prefab
After going through some initial concepts, it was decided to build a truss frame.
The tube strength was tested by placing tubing sections in a bender and measuring the deflection with a dial indicator. The test is representative of the point load case. Based on this test, we were able to select an appropriate tube size. We then cut out the transverse channels from a solid piece of sheet metal which was to be bent into a C-channel shape. The 2D template was created using Solidworks and then exporting to a dxf format where it was imported and used in the CNC plasma cutter. Features were placed in the part to mark bend center-line.
The 2D shapes were then placed in a punch press. The punch press has a set of dies that can be used depending on the length of bend, bend radius, and material thickness in use. The vertical range of travel must be adjusted so that there is something somewhat greater than the material thickness clearance between the dies at maximum travel in the cycle. After bending the channels, they were welded to make a seamless channel.
Step 2: Welding the Frame
The frame was setup on saw horses and welded together starting with the bottom part, then the sides. The side channels were welded onto the bottom channels in order to transmit bending moment, but primarily compression from the seats, which are supported by the top tubes. The height of the top tubes is at a comfortable sitting height. After the frame was welded together, it was painted to keep the bare metal from rusting.
Step 3: Brakes and Drivetrain
In this step, we recycled wheel bearings from a Honda Civic to make something strong enough to support at least five full sized people. The caliper bolts were identified as a strong point to tie into with a truss. The steering knuckle joint was used as a third support point and way to adjust wheel toe. The wheel toe is important to tinker with because it adjusts how the train rides on the track. The spline the half shaft normally goes into was adapted to a pulley and driven from the "pumpkin" which houses the final drive reduction to the gearbox.
The brake housing was welded together out of aluminum using a fixture to align everything. The driveshaft is keyed and supported at 3 points using flanged ball bearings. The Tees here were the original way we planned to tension both the chain and pulleys. The outer tees are still in use for pulley tension, and the all-thread rod below tensions the chain now, with ball end joints on each end to make alignment easy.
Step 4: Finalizing
As with any project, considerable time was involved in testing and improving things. We went through sets of seats, fabricating a driver console consisting of a gas tank, control switches and a throttle/brake actuator. The friction throttle is modeled after a boat and holds the cable at a specific setting. This has been tested and preformed well on the track.