Our Design Process:
When it came to designing Off-Road Rescue, we looked towards similar products on the market for inspiration. The prominent current market rescue trailers (like the one seen in figure 3.1) are relatively simple in design, offering basic suspension and little adaptability. These trailers often exceed 10 ft in length and require a heavy duty tow vehicle to fully utilize its capability. The extremely long profile makes it difficult to navigate through densely packed woods and often require specialized rescue litters to fit its mounting points. Using the general form of these rescue trailers, our team designed away many of the faults inherit to their construction and improved heavily on their usability and functionality.
Figure 3.1, Currently Available Off Road Trailer With Rescue Litter Compatibility.
The finalized design for our project is composed of three main sections: the main frame, suspension and tilting basket rails. The frame is comprised of 1″x1″ square stock steel tubing and is constructed in a one piece design. The pneumatic suspension controls the height of the trailer and allows it to sit with over 15 inches of ground clearance when fully extended and sit with 5 inches of ground clearance when fully retracted. This ability to lower down to the ground combined with the tilting rails is designed to allow for easier loading and unloading of a injured individual. The following sections cover each of these three areas in detail.
The initial design for the frame was a unibody style constructed out of 1×1 steel square stock. This design was chosen as it provide optimal strength for the frame while having a reasonable weight and being an inexpensive material. Originally the frame had several extra cross members, a lower box that sat below the main frame to hold all of the electronic hardware for the system,and a flat stub nose front. The extra cross members were removed when it became apparent that the extra rigidity they provided was not necessary due to the high strength the steel provided. The lower control box and stub nose were removed and replaced with the triangular front during the build phase so that the trailer would have increased ground clearance. The triangular front also allowed for greater strength for the tongue of the trailer as all of the turning forces would be placed directly into the frame, this also allowed for greater deflection of obstacles around the side of the trailer so that both the hardware and passenger were better protected.
There were three types of suspension considered for use on our rescue trailer: Independent Double wishbone, straight axle with leaf springs and a trailing arm suspension. Each type of suspension has its own strengths and weaknesses, but in the end we decided to implement a form of an independent trailing arm suspension design. Our finalized design integrates two main force control members; a fore and aft control member and moment force member (See Figure 3.4). The most dangerous force a suspension member can face is a parallel (lateral) force to the axle. The fore-aft member designed into each control arm transmit all of the forces into the suspension mounts on the frame and, when combined with 3/4″ schedule 40 mild steel pipe, makes the control arm extremely durable and rigid. Extensive finite element analysis was performed on the suspension to confirm its design would meet and exceed all force requirements (See Figure 3.5).
Figure 3.4, Trailing Arm Suspension Design
Figure 3.5, FEA Analysis of lateral force on wheel axle.
Incorporated with our suspension design in a pneumatic strut / shock absorber. The strut consisted of four main components; coil spring, pneumatic cylinder, lower spring mount and upper spring mount (See Figure 3.6). The total weight of our combined system determined the spring rate of the coil springs and pneumatic cylinder size required to achieve our ground clearance and ride quality goals. The orientation of the pneumatic cylinder allows to pushrod to be actuated without causing binding within the spring when lowering to the trailer to the ground.
Figure 3.6, Pneumatic Strut Design
Link to PPFS: Team17 PPFS (Updated 4/2017)
Eventual Link to Final Report