CHAPTER 3 BODY DESIGN & CONSTRUCTION
3.2 Leverage / Flipper Arms
plate bent to create a 8 x 8 mm C-channeling. This channeling was used for a rim around the edges to allow better sealing between the halves. A 45° angle was cut on the C-channeling at the corners of the chassis, and bent to follow the curve. This C-channeling was kept in position with phenolic resin.
Straps were made from 40 mm strips of 0.8 mm mild steel. These were spot-welded together to allow a tight fit over each half of the chassis. These straps assisted in pulling the two halves of the chassis towards each other with clips.
Elastrosil LR 3001/55 silicone which can withstand temperatures of 230 °C and is flexible at -55 °C, was used to seal the opening between the two halves of the composite body. This helped prevent moisture and heat entering into the body of the robot.
After the complete assembly of the robot, aluminized Kevlar was glued to the body of the robot. The aluminized Kevlar added to the strength of the robot. A test was performed on the aluminized Kevlar with a blow torch. The aluminized Kevlar was able to withstand the flame for periods of about one second, which is similar to a flash flame duration in a disaster scenario. A flash flame is caused in a scenario when a fire is burning with difficulty in an oxygen deficiency room. As a door is opened and oxygen is made available to feed the fire, the fire will cause a blast out of the door until the room is filled with enough oxygen for the fire to continue burning in it's previous environment.
The mechanical properties of the materials used give the design a significant advantage compared to previous USAR robots, in that it is able to withstand high temperatures of 200°C, and to protect the electronics components and modules within the chassis from the falling debris [19]. The strength of the body also allows for the transportation of medical equipment that might be needed for the rescue operation, which is not possible with other medium-sized USAR robots. CAESAR's construction is compact in size and lighter in weight by 14 % and 33 % respectively, in comparison to other medium-sized USAR robots such as the ATR-X-50 [18].
Two 08B and one 06B sprockets were solidly attached to each main drive shaft. This provided supports against the translation of the shaft along the turning axis.
As is seen in the exploded diagram, two flipper arms rotate around a supported axis provided by the main 12 mm shaft. Each flipper rotates on a needle roller bearing and is held against a needle thrust bearing by a 8 mm shaft which holds the two flippers together with four nuts. The needle thrust bearings allow freedom of rotation of the flipper with respect to the mounted sprocket and the main drive shaft simultaneously. Teflon could also have been used for this application, but was not used due to to costs and the quantity that had to be ordered. Diagrams of the needle roller bearing, needle thrust bearing and ball bearing are shown in figure 3-15. Their selection were determined by availability, and strength properties of the required shaft and flipper arms, derived from the analysed simulations.
A bracket is attached to the body at the outer ends of the shafts and supports the shaft with another thrust needle roller bearing. The bearing is held against the outer 08B sprocket to constrain translation along the axis of rotation.
The shaft attached to the flipper arms, which extends through the body, can rotate within a normal ball bearing. The ball bearing is fastened in a machined bracket which is specially made to slide into both the top and the bottom half of the body. To keep the flipper stationary, a 06B sprocket was attached to the flipper via a grub screw which would thrust against the lower portion of the ball bearing.
At the end of the flipper arms a 8 mm shaft provides support for the 08B sprocket.
The shaft is held by four nuts stationary with respect to the flipper in rotation and translation. The sprocket rotates on a press-fitted ball bearing constrained by two cir-clips.
The flipper arm was made by cutting a square mild steel tube in half to form two U- beams. These U-beams were then welded to a cylinder machined to house the bearings. Stress analysis of the arms was performed to determine if it is able to withstand the weight of the robot. As the front and back each is fitted with two arms, each arm could carry half of the weight on it. With an initial estimation of the body weight of 25 kg, and taking a safety factor of at least 5.5, the force applied to an arm is 674 N. This analysis is found in figure 3-16. A safety factor of 5.5 was considered, so that should a single flipper arm have to carry the full load, the flipper arm will not deform due to the strain. The safety factor must also consider the force of a payload that might be on the robot.
This analysis indicates that the highest stress concentration is at the welded section with a value of 37.8 MPa.