Concepts
Mechanical Concepts
Go-Kart Heim Joint Steering Mechanism
Heim joint steering mechanisms are commonly used in go-karts. The design is much simpler than a rack and pinion but not as durable and does not allow for suspension.
Linear Bearings
This concept includes a flange mounted linear bearing to support the rack gear or rod as it exits each side of the case. It provides support for all side loading forces and prevents bending of the rack/rod.
Gearbox for Motor & Gearbox for Steering Wheel
This motor gearbox concept is proposed instead of just having a direct drive motor. A gearbox for the motor would decrease the torque required from the motor at the cost of a higher rotational speed or vice versa. A gearbox for the steering wheel can provide the steering wheel a stopping point by using a gear reduction where there is a gear in the gearbox that stops at one rotation while the steering wheel gear can rotate 2 or 3 times.
Square Rack Gear
This concept for the rack gear inside the case involves a square rack gear with the teeth milled into the top. This rack shape naturally prevents rack gear rotation but a rack like this would most likely need to remain within the case making the case much bigger than if the round rack gear was used.
Revolved Teeth Gear
This concept for the rack gear inside the case involves a round rack gear with teeth cut 360 degrees around so that the teeth are engaged even if the gear rotates. This is beneficial because the small rack gear rotation during steering won’t affect performance. However, the downside is this piece is extremely hard to manufacture.
Round Rack Gear
This concept for the rack gear inside the case involves a round rack gear with the teeth milled into the top. This option provides good performance as its cylindrical shape makes it easier for the rack gear to move in and out of the case. Rotation of the rack would affect the performance of this concept so design additions would be needed to counteract the rack gear rotation while steering.
Grease Lubrication
This concept has a greased rack gear which exits the case while turning in order to save space. This means that the grease will also exit the case but it is much more viscous than oil and won’t leak.
Oil Lubrication
This concept includes having the case filled with oil to lubricate the gears. The design does not allow the rack to exit the case since the teeth leaving the case would cause oil to leak out. Instead, the rack gear is attached to a rod which exits the case and a wiper shaft seal is used to seal the oil inside and clean the rod so no contaminants enter the gearbox.
Rack Key
This concept utilizes a key attached to the rack and a keyway milled into the bottom of the casing for the key to ride along. This is to prevent rotational movement of the rack gear.
Rack Pin
This concept utilizes a pin attached to the rack gear and a guide cut into the bottom of the casing for the pin to ride along. This is also to prevent rotational movement of the rack gear but this concept also has the added benefit of stopping the pinion from potentially running out of rack gear teeth and damaging the teeth of the pinion.
Rack Flat Base
This concept utilizes a flat milled face on the bottom of the rack so the rack gear can ride along the flat bottom of the case. This is to prevent rotational movement of the rack gear again. This concept is hard to manufacture and would result in a lot of friction.
Mechanical Concept Selection Matrix
Mechanical Concept Selection.pngWith all of these concept ideas and considerations, they had to be narrowed down. To do this the group decided to create a concept screening matrix. The group conducted concept screening by defining the most important selection criteria for the project. This selection criteria includes manufacturability, cost, size, durability, stability, weight, maintenance, performance, and ease of use. Each concept got a positive, negative, or neutral score for each of the criteria. In the end, the concepts with a positive net score would be approved to move onto concept selection.
With the remaining concepts, combinations had to be created to result in full system concepts. The group created three concept combinations from the remaining concepts. Then the best combination was chosen by creating a concept selection matrix as shown above. In this table, each selection criteria was given a weight to account for some criteria being more important to the design than others. Then each combination got a score of 1 to 5 for each of the criteria where 1 is the worst, 3 is neutral, and 5 is the best. In the end, the B, E, G, I, J, & M combination concept had the highest score so it will be the combination concept that the group will plan on following. This concept combination includes the round rack gear, grease lubrication, rack pin, linear bearings, aluminum case, and gearbox for steering wheel.
Electrical Concepts
Motor Connection
Three different motors are being considered: a brushed DC motor, a stepper motor, and a continuous rotation servomotor. The selected motor will connect to the pinion, which will move the rack gear linearly.
Encoding Input
A potentiometer could be used to sense the position of the steering wheel. This would be done by applying a voltage to the trace, and measuring the voltage of the wipe to determine the rotational position of the shaft.
A rotary magnetic scale could also be used to sense the position of the steering wheel. The rotary magnetic scale consists of a magnetic disk and a Hall effect sensor. When the disk rotates the sensor detects the change in the magnetic field and translates the changes into an angular position reading.
Encoding Output
Limit switches could be placed at the end of the rack gear to determine the end of travel of the rack gear. When the rack gear bumps into the limit switch, the signal can be sensed and the motor would be turned off. This allows for maximum steering angle, without potentially damaging the gears or case if the pinion runs off the rack.
Electrical Concept Selection Matrix
Electrical Concept Selection.pngA concept screening matrix was created to narrow down the list of electrical concepts. The following criteria were defined for the project: ease of assembly, cost, size, durability, accuracy, lifespan, performance, and versatility.
Then, a concept selection matrix was created for the concepts that were advanced from the screening matrix. The concepts and different combinations of the concepts were scored. The following criteria were used for the scoring: ease of assembly, cost, size, durability, accuracy, lifespan, performance, and versatility. A weight was assigned for each of the criteria and then each concept was given a 0-5 rating for each of the criteria. The combination with the highest weighted total score was selected to be developed, which in this case, was the combination of concepts A & F: stepper motor and potentiometer voltage divider.