10-09-2014, 10:29 AM
THE SHORT BUS
THE SHORT BUS.doc (Size: 554.5 KB / Downloads: 729)
ABSTRACT
This project addresses the potential hazards of using a hydraulic floor jack and improperly or the total lack of blocking the vehicle to keep it from falling in the case of hydraulic failure. The goal was to design, build, and test a prototype floor jack that incorporates jack stands as a mechanical safety feature. The main problem was to develop an ergonomic and low cost solution while ensuring the safety and reliability of the system throughout its full range of motion.
The prototype built consists of a standard 3.5 ton floor jack with a toothed jack stand support member attached to each side that rotate with the main jack arm. A ratcheting mechanism on the support members is capable of supporting the arm throughout its full range of motion, from 14.6 cm (5.75 in.) to 50.8 cm (20 in.) vertical, and allows the jack arm to be raised freely. The ratcheting mechanism consists of a latch that prevents the support members and the jack arm from lowering while it is engaged. A system of cable pulleys connects a release mechanism on the handle to the latches of both support members and allows the user to disengage the latches and lower the vehicle when desired by axially compressing the handle. This design was chosen because it was deemed to be simpler, cheaper, and safer than any alternative solutions considered.
INTRODUCTION
Safety is of primary concern when performing automobile repair or maintenance that requires access to the underside of the vehicle. The average amateur mechanic does not have easy access to the expensive lift tables and floor pits available to professional mechanics. The most common solution is to use an inexpensive hydraulic floor jack to raise the vehicle. However, the hydraulic jacks currently on the market do not have a mechanical backup system preventing the vehicle from falling in case of sudden hydraulic failure. Therefore, blocks or jack stands are placed underneath the vehicle to guard against such accidents. Many users, however, do not block their vehicles or block them improperly, which can lead to vehicle damage, injury, and most importantly, death.
DESIGN PROCESS
Patent and Product Search
As the initial design process step, a search for relevant patents and existing products was performed. No existing products were found in the marketplace. This can mean one of three things. Either the project is addressing a problem that has not been previously been examined, there is no market for such a safety device, or previous safety jack designs have been unsuccessful. The patent search resulted in a number of relevant patents. Some of the key ones are as follows: Patent 3759488, where a spring loaded block is used to prevent the hydraulic piston from retracting once the jack was raised. Patent 6929248 describes a process similar to our gear latch design, where a non-hydraulic lever-arm is used to lift the jack, and is held in place by a quarter-circle gear on the side of the jack. Patent 5183235 describes a hydraulic piston that is used to lift a load. The piston is notched on two sides and spring loaded latches are used to prevent the piston from retracting. It appears that previous inventors have not been able to produce a safe jack at a minimal cost, and/or previous inventors have not been able to make a safe jack work as intended.
Design Concepts
A number of concepts were produced to meet the ideology of the problem statement as stated in the introduction. The concepts are shown below with a description as well as the advantages and disadvantages of each.
Locking Arm with Pin
The locking arm with pin concept, shown in Figure 1, works by placing a track along the bottom side of the jack that allows an arm attached at the seat to be able to lock at various positions as the seat is raised. The advantage to this design was the large number of locking points in the track. The disadvantages were mechanical disadvantage when the jack is in a lowered position and also the long range of motion needed to release the mechanism
The Replacement
The replacement arm design, shown in Figure 2, came from an idea to replace the secondary arm of the jack with a new arm with teeth. This would allow a third arm to lock at several positions and would not require a lot of motion to release. The disadvantage was from the large bending moment placed on the replacement arm since this additional arm is used for stabilization purposes
Finger Latch System
This system, shown in Figure 4, evolved from the geometry of the jack’s primary and secondary arms. These two arms produce a parallelogram and as the jack is raised and lowered, they slide past each other while remaining parallel. This means they will move closer to each other as the jack is lowered. By placing fingers on the primary arm, the system is able to lock into a track located on the secondary arm and prevents the system from lowering due to hydraulic failure or release. The compactness of this design made it desirable as well as the added safety of having numerous fingers that could release in ratcheting steps. However, this is also a disadvantage because of the complexity involved with releasing the fingers when needed
Concept Selection
The final design was chosen from a list of concepts by comparison of eight criteria. The importance of each criterion was determined by pair-wise comparison. The criteria were, in order from most to least important: lock reliability, the size of the system, ergonomics, number of locking positions, cost, force range, number of parts and aesthetics. The number of parts was chosen as a representation of system complexity and number of moving parts. System cost is relative cost of manufacturing, taking into account factors such as the assembly, material type, and any precise machining necessary. The force range is the capacity of the safety mechanism to support the rated load throughout the range of motion of the jack arm. Aesthetics is how well the jack appeals to the eye. Lock reliability is the reliability of the safety mechanism that stops the jack arm from lowering in case of hydraulic pressure and also the reliability with which it can be released. Ergonomics is the operational ease of use of the device, particularly the safety of the release mechanism.
Each concept was evaluated as follows. Each of the six group members rated each concept using a relative scale from one to five for each criterion. These values were then averaged, weighted and summed to give the total concept scores. Table A.1 in the Appendix shows an evaluation matrix of all the design concepts. Two concepts came away from this process as viable solutions, the Jack Stand Jack and the Gear Latch Jack. Since both concepts had scores that were very close, the group reconvened and discussed the potential advantages and problems with each concept. The Jack Stand Jack was chosen because it was a simpler design with less mechanical disadvantage and it was projected that it would be more reliable and have a better weight capacity.
Design Description
As mentioned before, the chosen design operates on the principle of incorporating static jack stands with the standard floor jack. Jack stand arms, or toothed support members, are attached to the end of the support arm at one end and the jack frame at the other. The ratcheting mechanism housed in the support sleeve allows the support members to freely slide one way but not the other. The ratcheting mechanism consists of a latch that is held against the support member teeth using a tension spring. Therefore, the support arm can be freely raised but cannot be lowered while the ratcheting mechanism is engaged. The mechanism to disengage the latch from the teeth is connected to the handle using bicycle cables to allow easy and safe lowering of the vehicle. The release is operated by pressing down on a sleeve that fits loosely around the jack handle. The sleeve presses down on the compression ring mounted against the backing plate. The compression ring pulls a cable that is routed to the support flange. A cable lever, mounted on the cable flange, is used to achieve mechanical advantage and allow for easier release of the locking mechanism. Figure 8 shows a model drawing of the final design. The sleeve is attached on to the handle using a bolt and groove, allowing the handle and sleeve to rotate together while the sleeve can move axially with respect to the handle. When the handle is rotated, the hydraulic valve is engaged and disengaged
Engineering Analysis
As a basis for the analysis, 1045 cold rolled steel was used as the material of choice due to its high yield strength and low cost. It was determined to design the jack to support a total rated load of 31140 N (7000 lbs). A safety factor of 3 was used in the analysis because it is an industry standard for load carrying parts. The engineering analysis separated the components of the jack into two subsystems. The first is the support system consisting of the support members, support member sleeves, latch, support flanges, support axles, and top plate. The second system is the release mechanism consisting of the latch, latch lever, cable lever, cable lever flange, backing plate, compression ring, handle, and cables.
For the support system, the key concern was the load that it could safely support. Detailed engineering analysis was performed on the critical load carrying components consisting of the support arm and teeth, bottom sleeve pin, support flange with pin housing, and top plate. Even though the total system was designed to withstand a load of 31140 N (7000 lbs), there are two support members so this divides the load for each member to 15600 N (3500 lbs). However
Release Mechanism
The release mechanism is shown in Figure Cable.1 in the appendix. For the release mechanism, an exponential friction estimation was used that is a function of the coefficient of friction, the tension on the cable and the angle of contact for the pivot point. The list of pivot points and their associated angles of contact are shown in Table Cable.1. Because the coefficient of friction was estimated to be 0.3 for contact with pivot points, friction between the cable and the cable sleeve elsewhere were ignored. Also included in the analysis was the torsion of a spring used to keep the release latch in contact with the support member. Adding up the friction from the 5 pivot points, the tension from the torsion spring and including the 4:1 mechanical advantage designed into the cable lever, the total force required on the handle to release the locking mechanism is only 91.6 N (21.6 lbs) as shown in Table Cable.3. These forces were calculated with input variables shown in Table Cable.2.
PROTOTYPE REVIEW
Design and Prototype Difference
As a result of time constraints, budgetary constraints, and machining ability, the prototype differs in some ways from the design. The most significant difference is the load capacity, where the design has a load capacity of 31140 N (3.5 tons) with a factor of safety of 3, the prototype has a load capacity of 31140 N (3.5 tons) with a factor of safety of 2. One major reason for this change is because the support members and sleeves used are from a Torin production model 6 ton capacity jack stand. The dimensions of the teeth of this support member are slightly smaller in depth and width when compared to the designed support member. The diameter of the latch hinge pin attaching it to the sleeve was the limiting factor of the prototype forcing the factor of safety to be only two. Calculations predict that the latch hinge pin would be the first part to experience failure under a very heavy load and would fail in shear. Another factor that limits the weight capacity of the prototype is the thickness of the top-plate. The original top-plate design is a 2.86 cm (1.125 in.) thick isotropic steel plate, the prototype top-plate is made of 2.22 cm (0.875 in.) thick steel. The reason for this difference is the lack of suitable raw material at the time of manufacture. The prototype also lacks wheels underneath the support member flange, which are in the design. Because of budgetary constraints, it was impractical to purchase wheels that had a 15600 N (3500 lbs) capacity and also fit underneath the support member flanges. As a result, two 4.44 cm (1.75 in.) diameter carbon steel pipes with a wall thickness of 1.27 cm (0.5 in.) were used to support the flanges when under load. Also, throughout the prototype calculations, the effects of welding were neglected. There are many parts of the prototype that are welded together and the strength of the welds may not always be as great as the strength of the welded parts. No method was available in order to quantify the strength of the welds since testing the prototype until it broke was not an option due to safety concerns. Considering the strength of the welds, the safety factor for the prototype is likely little more than unity when supporting a 31140 N (3.5 ton) load.
Testing the prototype
The prototype was tested using a 2000 GMC Sonoma weighing approximately 14200 N (1.6 tons). The truck was lifted from the rear differential causing both rear tires to leave the ground. Assuming the weight distribution to be approximately 40% on the rear tires when not loaded, this translates to a weight of 5680 N that was supported by the jack. The truck was lifted off the ground using the jack’s hydraulic system. At maximum height, the hydraulic pressure was released and the truck descended 1 to 2 cm before the support arm teeth caught the latches of the safety mechanism. The mechanism exhibited no signs of high deformation and appeared to handle the load with ease. The hydraulics were re-engaged and the truck lifted slightly using hydraulic pressure to remove the load from the support arms. The jack handle was pressed down to release the latches while also opening the valve. The truck slowly dropped to the ground and no binding was observed in the system. This process was repeated four times with similar results. Due to safety concerns, the safe jack was not tested at higher loads.
FUTURE WORK
Future work on the jack would be focused on reducing weight in the system. This would include redesign of the floor jack to integrate the mechanical locking system into the floor jack. The cable assembly, backing plate, and support plate could be integrated into the body of the jack in order to lighten weight and improve aesthetics. Also, the support arms could be moved in closer to the midline of the floor jack through further redesign. This would reduce bending forces in the top plate, and thereby reducing the required size of the plate. The top plate weight could also be further reduced by using an aluminum-steel sandwich to increase rigidity while keeping weight down. It has been determined that the 1inch thick solid steel plate can be replaced with an aluminum-steel sandwich that is only 3.2 cm (1.25 in.) thick and weighs 45% less than the solid steel plate. That is approximately a 20 N (4.5 lb) weight savings. Honeycomb structures were also investigated for potential weight savings, however, because the projected manufacturing costs are so high, and the weight savings are not much more than those of the aluminum-steel sandwich, the honeycomb structure is not a viable solution.