Analysis and Optimization of a Mechanical Object

A shopping cart or trolley is one of the most common purely mechanical household objects used today. It is supplied by large stores and supermarkets to assist customers transport goods within the shop. It provides convenience as it eliminates the cumbersomeness associated with customers having to carry the goods. However, despite its exceptional ubiquity, the trolley’s mechanical design has remained considerably unchanged for tens of decades earning it a fitting nickname, the granny cart. This case study will explore the mechanical functioning of the shopping cart design. It will outline its strengths and weaknesses and propose changes to its structure to improve its functionality. Particularly, the study endeavors to replace optimize the traditional ball-bearing drive wheel complex to provide it with an infinite lifetime. The new design will feature an air bearing approach which is will minimize the effects of traditional challenges such as friction and wear. A stress analysis for the system shall be provided.

Analysis and Optimization of a Mechanical Object

Summary

One of the most obvious reasons for the lack of motivation to modify common design of a shopping cart is the disability to change consumer behavior. As research indicates, most consumers prefer something they are accustomed to (Symons, 2014). As has been noted in many retail markets, the primary driver for change in any common component is its ability to affect profits. As a new design is unlikely to increase margins, most establishments lack incentive to institute changes to the cart. Additionally, the lack of significant changes to the granny cart is due to its exceedingly handy design. The trolley appears to satisfy its functionality characteristics and as a consequence, is not often considered for improvement. By the look of things, there are seemingly no new significant changes that can make it more effective, or are there?

Design Optimizations

Several designs have been fronted to attempt to reconfigure the shopping trolley’s mechanical design. Engineers have offered comprehensive insights as to how it can be modified. Unfortunately, none of these prepositions have ever been implemented, at least not in a large scale sense. One such prominent case is the IDEO shopping cart.

Retrieved from https://www.ideo.com/post/reimagining-the-shopping-cart.

Figure 1. IDEO Shopping Cart

One of the most sensational innovation concepts of the late 20th century was the development of a new shopping cart concept. The design solved traditional engineering concern of maneuverability Its nestable steel frame lacked a bottom and instead, featured removable plastic baskets (Ng, Lim, Danapalasingam, & Tan, 2015). The baskets were to increase the shopper’s flexibility, as well as protect goods from damage and promote brand awareness. Additionally, the cart featured a dual child seat that had a swing-up tray as the baby’s play surface as well as a cup holder. Furthermore, it had back wheels which simplified the trolley’s movement and steering process. For a moment, it appeared to be a great idea but soon, it fizzled into oblivion.

Introduction

As Black, Clemmensen & Skov (2010) explains, the lack of a fiscal impetus has significantly limited the prospect of seeking a new design. No one cares that there can possibly be a better product as most believe the current one’s operability is adequate. They don’t even seem concerned of its rudimentary design or of the fact that it can be significantly more efficient than it already is. The article seeks to investigate the mechanical operation of the traditional shopping cart with a view to providing a comprehensive alternative as to how it can be optimized. It shall include schematics as well as drawings and other illustrations of the same. Perhaps the key reason that its design will remain unchanged is its seeming non-noticeability (Kamble, Meshram, Thokal & Gakre, 2014). Most people spend an average of half an hour doing their household shopping. They perceive the carts as a regular item that will always exist in the shopping scene and have no need of having it improved. This article will propose several adjustments to its mechanical functioning and appearance hence improving it functionality.

Historical Development of the Shopping Trolley

The average shopping cart is efficient due to its size and basic construction. It overly simplistic design as well as customizable size has made the item immensely usable, inspiring its adoption the world over. There are few adjustments that can be factored in to increase its function. Continuous streamlining has ensured that the few glaring anomalies have been fixed. Initial designs feature maneuverability problems which were subsequently solved by adjusting wheel movement. The other key challenge the initial trolley was noted to consume a lot of space, a challenge that was quickly corrected by making it collapsible. Invented by Sylvan Goldman in 1937, the shopping cart was an invention occasioned by need.

Figure 3 Goldman's Improved Design

Retrieved from http://invention.si.edu/reinventing-shopping-cart

As Rajeshkumar, Mohanraj, Varatharaj & Scholar, 2016) explain, the subsequent prototype was a metal frame that held two wire baskets. Goldman found a mechanic who could mass produce the product. Coming to terms with the groundbreaking innovation he had just conceptualized, Goldman sought to market his edge. He conceptualized the phrase, “No Basket Carrying Plan," a marketing concept that would soon go viral decades later. He had introduced a new dimension in self-service.

Throughout the years, Goldman continued to make adjustments to his initial design. However, it was advice from his business partners George O'Donnell and Fred Taylor that would ensure the trolley became a permanent fixture. They conceptualized the idea of the nesting cart

Throughout the years, new developments have been fronted. Perhaps the most common are the electric and driverless cart. Developed in 2012, the driverless cart sought to eliminate the traditional role of customer’s effort (Ng et al., 2015). Through near field communication, the customer could be able to steer the trolley without necessarily having to steer it. However, the new carts are mainly designed for indoor use (Symons, 2014). As such, they may be insufficient for use in the rugged outdoor terrain.

Mechanical System

The mechanical system of the granny cart is especially simplistic. It features a massive wire-grid cage mounted on a platform with four rubber wheels. Currently, it has a capacity of not more than 150 pounds. In some trolleys, the wheel movement is often aided by ball bearings while others are not.

Th proposed system features the inclusion of a driver less function. It will also integrate an electrical component to power the mechanical function. The driverless cart is fitted with a Windows Kinect kit which enables it to detect and evade obstacles. It also features an electric drive train connected to a Windows 8 tablet. The tablet acts as the control console. Users have a virtual track pad on their tablets with which they see an animated layout of the space before them. They then steer the trolley with the control buttons on the tablet.

Figure 4. Tablet-controlled Shopping Cart

Charging is availed by the trolley’s motor function. As it moves around, it generates charge through a diverse dynamo system that then recharges it allowing it to operate for a lot longer without require charging. The systems are principally ideal for moderate loading. Loads exceeding 700 pounds may have a negative loading characteristic, where the force exerted downwards by the shopping may not be able to match its reaction, hence, affecting the electrical motor system. This is as per Newton’s third law of motion. The cart will feature larger wheels as opposed to the traditional trolley with tiny wheels. This will afford them the ability to move over great distances with fewer revolutions hence, limit the amount of energy and the associated wear and tear. They will be driven by a gear system.

Figure 5. The electro-mechanical Drive System

The driver cog approach is mainly inspired by the idea that the supermarket is not an active transport environment hence does not need a more elaborate drive mechanism. Furthermore, the gear drive system requires a minimal amount of energy to start, and sustain optimal functioning.

Functionality of the Part

The proposed trolley will dramatically improve load capacity. It will allow for a larger cage or holder design. Additionally, it will feature a segmented basket apportioned to feature major shopping categories. The largest bottom sections will be labeled to be used for dry foods and heavy household conveniences. The wheels of the trolley are responsible for nearly all of the mechanical effort of the construction. They support the weight of the structure while maintaining the mechanical movement function. They are under constant stress and are consistently required to hold the massive, overwhelming weight that is commonly associated with shopping. As inflatable tires would present a total new challenge, using a ball bearing construction is almost an obvious choice. However, as a half-decade of using this approach has shown, there no way of avoiding the problem of wear and tear, commonly associated with friction. As long as the structure will continue to utilize ball-bearings, this problem is bound to occur. As such, the adoption of the air bearing will significantly revolutionize how the trolley’s traction system operates. Instead of bulky bearings, the new structure will employ the use of an air cushion. The space within the wheels that would traditionally be covered by balls will be filled with air. The construction of the system will entail a pressurized system where the axle and the rest of the wheel have no other connection other than the fastening.

Figure 6. The Trolley Load Sectioning

The middle section will be marked for carrying lighter effects and foods such as cereals and other food substances. The top section will mostly be utilized for grocery, hot foods, and other lighter substances. It will be sectioned with light PVC boards. The partition of the trolley into section is mostly in place to influence consumer behavior.

The proposed model sought to meet on primary function, mechanical design optimization.

Loading Conditions

Figure 7. Design Loading Conditions

The proposed trolley will sustain a maximum load of 600 pounds. This will exert a maximum force of 600x4.44822= 2668.932N. Therefore, the structure would need an excessively sturdy base and axle system. To conserve electrical energy as well as alleviate stress on the structure, the design will incorporate large wheels. They will span 300 millimeters in diameter and will be made of tough ribbed rubber.

Material Selection

The materials used for the proposed trolley design would include a stainless steel wire-grid cage, rubber wheels, an electromechanical drive system as well as, a Windows8 powered tablet. Preliminary designs had incorporated tough poly vinyl chloride frames but this proposition was shelved in favor of a more reliable stainless steel structure.

The front wheels will be made light duty. They will feature moulded polypropylene centers with ribbed tires. The rear wheels will feature triple ball-bearing twist castors. These castors will have wheels with nylon or rubber content. The axle will be made of aluminum bars to sustain the super heavy loads that will be place on it every time and again. The wheels will be centered and puncture proof.

The trolley will have a 150mm ground clearance. This will allow it to sufficiently maneuver around the busy supermarket shelves at relatively faster speeds. It will also offer a better handling and more energy efficiency. The height of the trolley will be 1050mm with a width of 640mm. PVC partitioning will be used to section the different load categories. It will have an ASTM D2240-00 scale OOO hardness.

Stress Analysis

To ensure certainty in the designs infallibility, the trolley’s powering system was subjected to a plethora of tests. The frame and wheels were subjected to a compression stress analysis aimed at determining their breakdown point. As the wheel system is perfectly circular and uniaxial, principle and applied stresses were not calculated separately since they would be equal. The fatigue strength amplitude calculated in the presence of massive static stress was noted to be tending towards two as opposed to the ideal unity. As such, the Goodman’s safety factor can be correctly stated to be 2.

FEA Study: Design Optimization/ Improvement

Figure 8. Drive System Stress Graph: A graph of normalized stress amplitudes against normalized mean wheel drive axle stress

Rolling element and sealing tests were conducted on a sample ball bearing to determine dynamism and impact and structural analysis. The graph below shows the stiffness and theoretical modeling

Infinite Lifetime of the Air Bearing

Conventional bearings are restricted to traditional problems such as bearing fault, friction and wear. As a result of the constant movement they are subject to, their lubrication handling tends to minimize. The traditional trolley wheeling system is perennially plagued with this challenge. However, this article proposes the application of a different approach to the problem. The new construction will feature bearings with a thin film of pressurized air. The film will avail an extremely low friction load-bearing medium between the axle and the wheel. In essence, these two surfaces will not touch. As such, problems such as consistent lubrication, static friction and lacking backlash, particulates, and wear will be effectively prevented.

Additionally, this will present higher precision in wheel-axle positioning making the system exceedingly efficient. Air shall be the only fluid and will flow through the bearing to its surface. This design of the arrangement is in that, as air continually escapes, from the gap, the pressure between the bearing’s surface and that of the axle is sufficient to support the load placed on the trolley. Aerodynamical bearings will be best suited for this job mainly because they create their own air cushion as they move along. Additionally, the design will exhibit low air consumption while retaining maximum stiffness.

A safety factor for the overall design was developed. The results were extrapolated to show the relative amount of stress the system would exhibit after extended use. Safety factors of the different components of the system were compared after their parameters were put into the system. It was established that the most optimum wheel size would be 300mm (Ng et al., 2015). This would not only counteract the massive forces of inertia but would increase the overall system efficiency. The decision to use an aluminum frame as opposed to steel was based on its high carbon characteristics. A frame of 640mm by 700mm and a 30mm thickness would comfortably react with equal intensity to a downward compressive force of 3000 Newtons. The axle shall be mounted on a wheel axle that shall drive the entire system. As a consequence, the axle shall bear the entire 600lb load.

Bearing and Shift Optimization

To optimize mechanical drive system of the system, one must ensure that the wheel and axle systems are improved as well. The trolley is a fairly simple mechanical device and one that does not require elaborate adjustments to its structure. However, as the proposed system proposes an electromechanical drive system, it is critical that the bearing and shift mechanisms be optimized as well. This section shall an impact analysis of the wheel to determine the safety factors critical in optimizing the bearing and shift mechanism. The main change that will occur in this system is the optimization of the bearing system. The conventional ball bearing arrangement is often troubled by problems of continued lubrication, wear, backlash, and particulates. As such, it is fundamentally limited in terms of having an extended lifetime. However, by optimizing it to work akin to an aerodynamical bearing, these challenges will be averted.

Impact Analysis of Each Wheel

The impact analysis shall be based on the deflection equation, . It shall assume that all wheels on the axle are constrained. A solution for the value of F shall be found using the equation , as the value of F has been expressed inform of s. The force of impact on each wheel will be 25175N. A wheel stress of 203.667MPa was determined using the shaft stress equation. This was immensely high compared to the 85MPa stress that was noted on the conventional shopping trolley. This value when compared to the shaft’s yield strength, a value of 1.33 for the safety factor is realized.

Discussion

Figure 9. Trolley Wheel Stress Analysis

It was noted that both Matlab and finite element analyses produce the similar results. The 300mm wheel was tested against the downward stress of 2668.8N designed to be the optimum applied load. As each wheel is being analyzed separately, an expected load of about 667 newtons will be exerted. The FEA compression breaking stress was determined for the frame to determine aluminum bending or wheel collapse. These two parameters would determine the yield strength of the two components under duress and without the electrical powering that the system would need. A 10 percent difference was noted between the ideal values and the experimental safety factors.

Figure 10: Matlab bearing fault analysis

Conclusion

The optimization process determined that the amount of effort utilized customers who utilize the shopping cart was set to dramatically reduce. The fact is mainly inspired by the fact that they would eradicate the forces of inertia as well as the downward compressive force that they have come to associate with pushing the shopping cart. While the new design is not set to drastically influence shopping behavior, it will certainly reduce the effort a customer would use when out shopping. The aerodynamical bearing approach is set to increase the lifetimes of th trolley infinitely. As such, manufacturers will no longer grapple with the challenges of having to recycle or remanufacture the drive system. The design is not set to be taken by the market with energy as it solvers a miniscule problem but it is certainly a start to a new dispensation of household technical revolution.

References

Black, D., Clemmensen, N. J., & Skov, M. B. (2010). Pervasive Computing in the Supermarket: Designing a Context-Aware Shopping Trolley. International Journal of Mobile Human Computer Interaction (IJMHCI), 2(3), 31-43.

Kamble, S., Meshram, S., Thokal, R., & Gakre, R. (2014). Developing a Multitasking Shopping Trolley Based On RFID Technology. International Journal of Soft Computing and Engineering (IJSCE), 3(6).

Ng, Y. L., Lim, C. S., Danapalasingam, K. A., Tan, M. L. P., & Tan, C. W. (2015). Automatic human guided shopping trolley with smart shopping system. Jurnal Teknologi, 73(3).

Rajeshkumar, R., Mohanraj, R., Varatharaj, M., & Scholar, U. G. (2016). Automatic Barcode Based Bill Calculation by Using Smart Trolley. International Journal of Engineering Science, 2539.

Symons, T. J. S. (2014). U.S. Patent Application No. 15/024,239.