3e.
Design Decisions
After the analysis phase was completed there were many important aspects of the design that were ready to be finalized. A specific material and an appropriate weld were ready to be selected for the heavily loaded portions of the design. The exact form that the base frame and the rear restraint were to take was also ready to be decided upon. The function of the test rig proved it’s worth and was ready for further testing.
The extensive COSMOS analysis provided the necessary information to select a material for the heavily loaded portions of the prototype. As mentioned earlier some form of steel was to be used for these parts. A variety of different steels were researched as potential materials for the base frame, the ear, and the rear restraint connection. Since all of these pieces would be welded together it was decided that they should all be made out of the same type of steel to prevent the pieces from changing with respect to each other during welding. The most important consideration was strength and this is where the COSMOS results would dictate. The highest stresses seen in the analysis were where the Safety Angle meets the base frame and the mounting ear and were estimated to be as high as 64,000 psi. The ultimate strengths of the potential materials were compared and other factors were considered, such as cost, availability and weight. 1018 CD steel was selected. It has an ultimate strength of 63,800 psi, which is just below the estimated 64,000 psi COSMOS predicted. It is also readily available, rather inexpensive, and has the advantage of being easily workable. The base frame will be made out of 1018 CD square tube that is 1 x 1 x .065 in. The ear will be made out of 1018 CD 1/8 in. plate steel with a 1/8 in. collar between them. The rear restraint will be made out of 1018 CD ¼ in. plate steel.
The test rig proved that the general idea for the reclining mechanism would in fact work. After the test rig was completed it was tested with varying weight and it was able to recline a person from the starting position to a forty degree angle. So the use of the Acme threads and followers was decided upon as the method for reclining the seat. It was then tested further to ensure its durability and strength. These tests and their outcomes are discussed further in section 4f. Reclining Mechanism.
4. Design Execution
4a. Introduction
The only hurdle remaining in the design process was to integrate each subsystem together without exceeding the design constraints and determine their final dimensions. The final design of each subsystem will now be discussed in detail. Each subsystem will be discussed independently and the final dimensions will be given. The following section (Assembly) will discuss the final construction and integration of the subsystems.
4b. Base Frame
The base frame for the EZ Journey Car Seat is a key component to the final design. It houses the reclining mechanism and supports the entire seat assembly. It also absorbs the brunt of an impact load. The dimensions of the base frame were selected so that it would fit inside the specified constraints and still house the reclining mechanism.
The width of the base frame could not exceed 16 in. but this included everything that would be hanging off of the base frame. The mounting ear has a 1/8 in. plate that sits on the outside of the frame and a bolt head will be on the outside of the mounting ear. Therefore the tubing was spaced with an outside width of 15.375 in. apart. The length was selected as 18.419 in. so that the Safety Angle would clear the hinge and so the hinge would clear the gearboxes on the reclining mechanism. As mentioned previously the ears were mounted as far back on the frame as possible. The square tube on the front of the base frame was used to mount the pillow blocks that hold the ends of the acme threads. Figure 4b-1 shows the final design of the base frame. Dimensions and part details can be seen and appendix C-1 through C-8.
4c. Rear Restraint
The rear restraint assembly used on the EZ Journey car seat is one we have termed the Safety Angle. The Safety Angle is constructed of two pieces of .25” steel plate, one 4x5 in. and one 1.25 x 5 in. The two pieces were set at a 45-degree angle to each other and TIG welded together using a modified butt joint. Two 1.5” lightening holes are drilled in the large plate and five ½” holes were drilled in the small plate to conserve weight. For dimensions, see appendix C-9. Two Safety Angles were constructed, one for each side of the chair. The seat belt will be placed under the lip of each safety angle and secured with hook and loop fasteners. The lightening holes not only allow the piece to be lighter in weight but they do not affect its structural integrity appreciably. The final design of the Safety Angle can be seen in Fig. 4c-1 and the final dimensions can be referenced in appendix C-8.
4d. Seat Hinge
The seat hinge is another key component to the design of the EZ Journey Car Seat. It serves many functions and also must survive a strong impact load. The hinge is machined from a piece of 3” round steel rod with a 5/8” center hole to accept a bolt. It has a single 5/16 hole drilled through the outer portion which lines up with an array of 3 5/16” holes on the inside hinge to allow the seat back angle to be varied. There is also a hole that locks the seat in the folded position for transportation of the EZ Journey. It has also been designed with a male and a female half so that the two halves will stay together in the event of an impact. The inside half of the hinge is the female while the outside is the male. This design increases the bearing area that makes the connection between the seat and the base frame stronger. A 5/8” hole has been bored in the 12 o’clock position of the hinge which acts as a receiver for the seat back assembly the final design of the hinge can be seen in Fig. 4c-1. Final dimensions and tolerances can be seen in appendices D-2, D-3, and D-4.
4e.
Seat Frame
The seat frame is not expected to be heavily loaded even in high-speed impact so it was designed mainly for functional purposes. Basically this means providing suitable areas for a variety of different things to be attached to the EZ Journey. The seat bottom had to have a flat top that different seat bottoms could be attached to so that the user could specify their own individual seat. The seat back had to be designed in a similar manner. The seat bottom and seat back also had to allot space for the attachment of any additional items that the user may need. These additional items can be attached to the seat back or seat bottom provided they do not weigh more than the equipment limit of 20 lbs. Otherwise added equipment should be mounted to the base frame. The final design can be seen in Fig. 4e-1 and the final dimensions for the seat frame can seen in appendices D-5 through D-10.
4f. Seat Bottom and
Seat Back and Harness System
The client was given the option of having DDI custom build these subsystems or simply specify want he would prefer. Ben Sutcliffe has used a variety of different options in his lifetime for these three components and opted to simply select what he wanted. He supplied DDI with information on where to find what he wanted. In the case of the seat bottom Ben currently owns two of the types of seat he specified and was able to donate one of them to DDI for this project. The harness will be bought from Agassiz Health Care Supply. Originally it was decided that a four-point harness would be used. Unfortunately the Federal regulations for these types of harness are too difficult to work around and a simpler choice of a two-point belt will be used.
4g. Reclining Mechanism
Although the concept used in the reclining mechanism was proven and decided upon, further design changes were made to improve its function. Tests were done to determine its true output abilities for this application and it was optimized so that maximum lifting force could be achieved in the final prototype. It consists of four main components: the motor, the twin gearboxes with acme thread, the acme thread followers, and the lift rods.
The lift rods are connected on one side to the acme thread followers and to the bottom of the seat frame on the other. It is by the action of the acme thread sliders pushing forward against the lift arms that rocks the seat to a reclined position.
First the test rig was utilized to perform test on the mechanism. It was important to know just how much force the acme thread and follower could push because this was directly related to how much weight it could life provided the geometry was known. A test was done where the test rig was stripped down to just the base frame and the motor/acme thread assembly. It was held in a vertical position between two tables and weight was hung from the sliders. The motor was then run and the speed that the sliders traveled at and the amperage that the motor pulled was recorded. This test showed that the motor could lift 100 lbs while drawing 10 amps. It appeared that under normal operation the motor would draw between seven and eight amps but had no problem drawing 10 amps for short periods. The motor would only be required to recline a heavy load at startup and therefore would only need to draw 10 amps for short periods.
Next, the actual force that the follower needed to have was determined. Again the test rig was used to determine what kind of force the lift arms needed to push the seat up with. A scale was placed under the seat portion of the test rig and a rod that was placed a specific length down the seat bottom rested on the scale. Different people of different body types sat in the test rig and the scale reading was recorded. Therefore the force a human body applies at a specific distance from the hinge axis can be translated into the moment a human body applies around the hinge access. This is where the optimization of the reclining mechanism comes in. There were many variables involved in the optimization of the seat so an Excel spreadsheet was used to make decisions about the final dimensions of the reclining mechanism. The first aspect of the reclining mechanism that was decided upon was the connection point of the lift rods on the seat bottom. It was clear that the further forward the connection of the lift rod to the seat bottom was, the less force would be required from the acme followers. The spreadsheet analysis also immediately showed that the steeper the starting angle of the lift rods the more force that would be transmitted from the follower to the seat bottom.
This optimization process showed that truly optimizing the chair would violate the compact design necessitated in section 2c. Design Constraints. The push rods were mounted to the seat bottom as far forward as possible without making the seat bottom so long that it hangs to far over the vehicles seat it is being used in. To increase the angle that the lift rods begin reclining the seat at the seat bottom needed to be raised as far up from the acme threads as possible. The seat bottom can’t be to far off of the vehicles seat before the occupant’s head would begin to hit the ceiling of a small car. The seat bottom was raised as far as possible to optimize the reclining mechanism and still allow ample head room for the occupant. Using these constrained dimensions it was determined that the follower would need to push with 300 lbs of force instead of the 100 lbs it could push in it’s current setup. Different options were discussed to overcome this problem. DDI decided the best way to solve this problem would be to make use of some further gearing to give more power to the acme threads. A three to one gear ratio was added so that the followers would have three times the pushing force. The added advantage of this setup was to slow the motion of the seat down. DDI felt that the chair traveled quite quickly without the further gearing and that the occupant may want a more controlled recline. The recline mechanism can be seen in Fig. 4g-1 and more detailed drawings can be seen in appendices E-1 through E-8.
4h. Limited Tilt Mechanism
It was judged necessary to include a mechanism to limit rollover in the event of a rear-end collision. The natural tendency of the unsupported seat is to roll towards the rear, due to a combination of inertia and impact forces. This is of course an unacceptable state of affairs, due to the risk of injury faced by the passenger as well as the risk of damage faced by the seat.
A simple, easy-to-build solution was created to solve this problem. It consists of only two parts welded to the backside of the main frame of the seat. The first part of the solution is a hinge, much like the standard seat hinge used by the seat reclining mechanism that is welded to the rear of the base frame. This supports and allows adjustment of a bar, 18” long and made of ¾” diam. round steel tubing, as seen in Fig. 4h-1. This bar pushes on the seat back, after being set in place as described in section 6a. To Put the Seat in A Vehicle. The final dimensions for the limited tilt mechanism can be seen in appendices F-1 and F-2.
4i. Head Rest
The headrest used was a custom unit salvaged from a motorized wheelchair. It is padded, and houses a plunger switch on each side of the occupant’s head. It is attached to the chair back by a piece of strap that fits into a slider bracket. The slider bracket has semi-circular cutouts placed horizontally that allow it to grip a piece of ¾ inch tubing. It has the two-fold purpose of supporting the occupant’s head and allowing control of the chair. This piece was useful in that it was not only free but the intended user is accustomed to its operation.
4j. Electrical Circuit
The electrical circuit for the EZ-Journey car seat needed to perform 3 main functions. These are:
- Switch control between user and attendant
- Allow user to control the seat through the head switch
- Allow attendant to control the seat through a toggle switch
Additionally, the following functions were deemed necessary to the circuit:
- Large, lighted, easily visible “kill” switch to cut all power
- Easily changeable fuses
- Motion limiting switch
- Cigarette lighter power plug
The head switch we had to work with for the project was a custom item that could handle only a fairly small current due to its fragile construction. The motor used for the seat requires a peak amperage of 15 Amps. Therefore, a switching circuit with relays needed to be designed to be able to turn the motor on and off with the minimal current that the head switch could handle. Siemens relays were selected since they have a rating of 15 Amps at 30 Volts. They also only require about 100 mA to actuate their relay coils. With this problem solved attention could be turned to the selector switch and the attendant control switch. 10 Amp 12 Volt DC rated toggle switches were used for this circuit acquired from Napa Auto Parts. When the circuit was shown to Dr. Marc Herniter, an NAU EE Faculty, he stated that it was of the utmost importance that DC rated switches be used since AC switches can malfunction with DC currents. It was decided that a master cutoff switch should be incorporated for safety reasons and a large red-lighted one was selected with a suitable rating of 30 Amps. A motion cutoff switch was salvaged from a Nissan power seat. This switch ensures that should the mechanism be completely forward or completely backward and power is still being applied (by the user or attendant) the motor will shut off rather than burn out. Surface mount fuse holders for the high amperage motor circuit and the low amperage control circuit were chosen since they make it far easier to locate and change fuses than their inline counterparts. A cigarette lighter plug (male) allows the chair to be connected to the vehicles electrical system or a suitable handheld battery pack. The schematic for the electrical circuit can be seen in appendix G-1.