Wheelbarrows are critical tools in the fields of construction, landscaping, home improvement, and gardening. Most wheelbarrows are designed around the idea of toppling the wheelbarrow over or strenuously lifting the handles to unload it and are typically equipped with a single wheel in front. While this is a simple concept, it can lead to out-of-control loads and accidental spills. Aspect Engineering designed a new wheelbarrow with the goals of providing the stability, loading capacity, and ease of use that typical wheelbarrows fail to provide. The "Rolling Barrel" has a half of a 55-gallon drum mounted on a frame to hold the transportable contents, which can then be rotated about its center of mass to unload it. The prototype verifies that the design goals have been met.

1. Introduction and Problem Statement
2. Design Partition
3. Conceptual Ideas
4. Patent and Product Search
5. Selection Analysis
6. Layout and Part Drawings
7. Engineering Analysis
8. Prototype Review
9. Conclusion

The ability to transport a large amount of cargo safely and efficiently is important in the design of any transport tool. The typical wheelbarrow design has accomplished this with some success, but with enough creative engineering and consideration, this concept can be improved. The capabilities of the basic wheelbarrow can be largely surpassed when the bar is raised to new capacities and functionality.

In a typical wheelbarrow, a container is mounted in an "A" frame, which rests on a single wheel in front and two legs in the back, and has two handles protruding out past the legs for maneuvering the wheelbarrow. This design is simple, and many people have little or no difficulty using it.

But problems arise when the wheelbarrow is loaded heavily. The user must lift the handles to rotate the load about an axis far from the load’s center of mass. At times, this means the wheelbarrow cannot be unloaded accurately, safely, or at all. Another problem is the instability associated with a single wheel for support while transporting contents. The wheelbarrow can easily tip over, especially when the wheelbarrow is heavily loaded to one side, or being rolled across a slope. This can usually be attributed to a high center of gravity, inadequate lateral support, and the inability of the wheelbarrow to adjust for such conditions.

The new design will have a lower center of mass, increased lateral support, and a mechanism to ease the unloading of the wheelbarrow. It will also be comparable in cost, durability, and carrying capacity.

A function tree was designed to break up all possible concepts into the required subsystems. This was done to ensure that all aspects of the wheelbarrow are taken into consideration and to ease the design process. Figure 1 shows the wheelbarrow subsystems: a tub to contain the materials to be moved, a frame and handles to support and maneuver the tub, an unloading mechanism for removing the materials from the tub, and the wheels and axle to move the materials.

Figure 1: Function Tree for Wheelbarrow Design

The first two ideas are simple additions to the basic wheelbarrow design. The "Pump and Dump" design shown in Figure 2 is based on the basic wheelbarrow, but instead of lifting the load by the handles, the operator steps on a lever that flips the load over the front wheel.

Figure 2: "Pump and Dump" Conceptual Idea

The "Adjustable Handle" design shown in Figure 3 is also based on the basic wheelbarrow, but the handle used to move and lift the load can slide up or down for increased leverage while unloading and allows the operator to lift using leg muscles rather than back muscles.

Figure 3: "Adjustable Handle" Conceptual Idea

The next two designs are based around using a half of a 55-gallon drum with increased lateral support from two wheels in front instead of just one. The "Splitting Barrel" design shown in Figure 4 has a drum that separates along the bottom to allow the load to fall through to the ground, thereby using the stored potential energy to unload the contents. The operator simply unlatches the barrel quarters from each other at the bottom.

Figure 4: "Splitting Barrel" Conceptual Idea

The "Rolling Barrel" design shown in Figure 5 has a drum that is rotated about its center of mass by way of a handle or crank. The barrel is unloaded by applying a small force to rotate it upside-down, thereby allowing the stored potential energy to unload the contents.

Figure 5: "Rolling Barrel" Conceptual Idea

The final concept, shown in Figure 6, is much like a box on wheels. "Old No. 5" has a door on the front that opens, and a mechanism to push/pull the contents through the door.

Figure 6: "Old No. 5" Conceptual Idea

The wheelbarrow presented in this patent is specifically designed for carrying rocks and stones. The designer wanted to correct the problem of stability associated with transporting such heavy loads. As can been seen in Figure 7, a larger wheel, minimum 30 inches in diameter, and low center of gravity are proposed to correct the stability problem. Also a semi cylindrical container attached on pivot post is part of the design. When the box is tilted forward to dump the contents the center of gravity of the wheelbarrow is unaltered.

Figure 7: Sketch of US 5190351, Issued 2 March 1993

The dumping wheelbarrow, shown in Figure 8, has a design that allows the framework to remain stationary on the ground during the dumping process. The barrow tips forward along a pivot to allow the load to be unloaded. A hydraulic piston that telescopes out in three stages raises the barrow.

Figure 8: Illustration of US 4270786, Issued 2 June 1981

This device features a container that sits on top of a frame. The container is held in place by a releasable stop. When the stop is disengaged, the container is allowed to rotate forward to empty the contents. The movement of the container is controlled by an actuator. The device is shown in Figure 9.

Figure 9: Drawing of US 5897283, Issued 27 April 1999

This patent alters the handles of the conventional tub wheelbarrow. The handles can be extended longitudinally. They can also be rotated along the longitudinal axis. Since the handles can be positioned in so many ways, the ergonomics of the device is improved. The two wheels placed below the center of gravity, as shown in Figure 10, allow the wheelbarrow to be trailered more easily.

Figure 10: Illustration of US 5915706, Issued 29 June 1999

Sterling Handling Equipment, Inc. sells a variety of wheelbarrows for different applications. The industrial wheelbarrow, shown in Figure 11, is similar to the conventional wheelbarrow. The carrying capacity varies from 4.5 to 6.875 cubic feet. The barrow part is made from metal to increase durability.

Figure 11. Picture of Industrial Wheelbarrow

Also available is a specialized wheelbarrow for transporting concrete. The cart, pictured in Figure 12, has large 26" wheels to carry the heavy load. Also, reinforced rockers aid in dumping the concrete.

Figure 12. Picture of Concrete Cart

A third specialized wheelbarrow available is the brick barrow. The barrow is designed to carry bricks at construction sites. The brick barrow is shown in Figure 13. A dual wheel cart is also made.

Figure 13. Picture of Brick Barrow

The patent and product search was conducted to ensure that the group was not working on a design that already existed. None of the patents or products prevented the group from pursuing any of the conceptual ideas as viable options.

An evaluation matrix, shown in Table 1, was used to determine the best conceptual idea to pursue in satisfying the problem statement. The primary goals in the problem statement are given the greatest weight, while maintaining a comparably standard wheelbarrow is given less weight. The evaluation matrix shows that the "Rolling Barrel" design is superior to all others.

Table 1: Evaluation Matrix for Conceptual Ideas

"Stability" is a measure of the ability of the device to avoid tipping during use or while stationary. "Ease of use" is a measure of the ease with which the user can perform the intended functions of the device. Functions include loading and unloading. "Carrying capacity" is a measure of the volume and weight of material that the device can carry. "Maneuverability" is a measure of how easy the device can be moved while loaded. "Reliability" is a measure of how closely the device performs to its intended operation.

As stated previously, the goal of this project is to design a new type of wheelbarrow that is more stable and easier to maneuver and use. The overall layout of the new design is shown below in Figure 14. The overall layout drawing shows the entire wheelbarrow with all of the different components in place. The new design features a cranking system using the mechanical advantage to assist in the unloading process.

Figure 14. Overall Layout of the New Design

The cranking system also includes a pin that can be placed directly through the frame into certain holes in the main gear. This allows the operator to lock the barrel and keep it from rotating during transport or loading situations. When the operator arrives at the site where the contents are to be placed, the pin is removed to allow the barrel to spin freely and the contents to be unloaded. This cranking system is shown below in Figure 15. This system also incorporates a handle that slides in and out of the frame to engage and disengage the smaller gear with the larger gear attached to the barrel. This allows the handle to spin freely if the operator chooses to use the handle mounted to the barrel to unload the contents.

These features along with the innovative two-wheel design make the new wheelbarrow easier to load, unload, and transport many different types of materials.

Figure 15. Cranking System

The new design incorporates twenty-one parts that need to be cut from stock materials. The frame pieces are all to be cut from two-inch hollow square steel tubing. The drawings and dimensions for these parts are shown below in Figure 16.

Figure 16. Part Drawings for Frame Pieces

All of the frame pieces will be welded together with ½" holes drilled where holes are indicated in Figure 16.

The axle for the wheels, the rotating pins for the barrel, and crank pieces are all to be made of 0.5" steel round stock. The drawings and dimensions for these parts are shown in Figure 17 below.

Figure 17. Part Drawings for Handle Pieces, Axles, and Pins

The numbers beside the parts in the previous part drawings indicate where they are located on the plan views show in Figure 18 below. The numbers with the "X" beside them indicate the quantity of that particular piece needed to create the final product. The three #7 pieces will be welded together to make up the crank and handle assembly. The pins #8 and #10 will spin freely in the frame and be welded to the barrel.

Other parts needed to complete the wheelbarrow include the wheels and the gears. The wheels are standard wheelbarrow wheels, which can be found at any local hardware store. The gears can be purchased through most industrial supply catalogs. The gear that is mounted to the barrel is a six-inch gear and the gear mounted to the crank is a two-inch gear.

The axles shown, as #9 will be welded to the frame pieces shown as #5 and the wheelbarrow wheels will spin freely and independently of each other on these axles. The six-inch gear will be welded to pin #8 and the corresponding two-inch gear will be welded to the end of one of the #7 pieces.

Figure 18. Plan Views

There are two main objectives in this engineering analysis. First, the device must not fail under heavy loading. Therefore, the maximum stress and deflection produced in the crossbeam of the wheelbarrow from the load of the drum was investigated. Second, the forces required to unload the wheelbarrow were investigated. The analysis of the crossbeam is presented first.

First a maximum load was defined to be sand that would fill up the barrow and also be heaped above. The volume of sand would approximately be 55 gallons. The density of sand is 2300 kg/m³. So the total weight of the sand is 1060 pounds. With two crossbeams this weight is divided between the two making the force on one crossbeam 530 pounds.

Next, the maximum value of the moment in the crossbeam was found. The forces and moments acting on the crossbeam were defined as a downward force at the center of the beam, and an upward force as well as moment acting on both ends of the beam. Figure 19 is a diagram of the model.

Figure 19. Free Body Diagram of Model Crossbeam.

The model is a conservative because it does not take into consideration the braces under the crossbeam. From beam analysis the maximum moment was determined to be:

(1)

where P is the downward force acting on the center, and L is the length of the crossbeam.

The maximum stress occurring in the beam was found from the moment and is:

(2)

In this equation h is the height of the beam and I is the moment of inertia of the beam.

The maximum deflection of the beam occurs at the center. It is related to the downward force by Equation 3.

(3)

Young’s Modulus is represented by the variable E. For a solid beam the moment of inertia is given by

(4)

For a hollow beam the moment of inertia is

(5)

In both moment of inertia equations, b is the width of the crossbeam and t is the inner thickness of the hollow beam.

For a 2" x 2" x 28" steel beam the maximum stress 4 ksi. This stress is well below the yield strength of steel of 36 ksi with a safety factor of 9. Also the downward deflection of the beam at the center is 0.0078". In a hollow steel beam of the same dimensions with an inner thickness of 0.125" the maximum stress is 10 ksi. The safety factor for the hollow beam is 3.6. The deflection at the center is 0.019". Therefore, the solid steel beam was chosen to provide a higher factor of safety.

Next, the analysis of the forces required to turn the barrel is presented. This analysis is performed for two different designs. The first is a simple handle design. The second is a gear design.

The Handle design consists of a barrel with a simple handle to rotate the barrel as shown in the following figure. The distance from the axis of rotation to the handle is the same as the radius of the barrel, or half of the width, W.

Figure 20. Handle Design

The force required to pivot the barrel is called F_h. The gear design consists of the barrel with one large gear mounted on the axis of rotation and a smaller gear with a handle as shown in Figure 21.

Figure 21. Gear Design.

The force required to rotate the barrel is called F_g. The ratio of the larger gear diameter to the smaller gear is called r. The handle length is L_h.

By summing the moments about the axis of rotation, the following relationship between the overall torque, T, required to rotate the barrel and the force, F_h, for the Handle Design is derived.

(6)

For the Gear Design, the overall torque, T, is a function of the gear ratio, handle length and force, F_g.

(7)

By combining equations 6 and 7, a ratio of the forces for the two designs indicates the mechanical advantage of the Gear Design over the Handle Design.

(8)

W=2’

r=3

L_h=0.5’

So, to turn the barrel with the Gear Design, only 67% of the force of the Handle Design would be needed. This may not seem like a big difference, but it can be significant at heavy loads. Therefore, a gear design was chosen to provide the user with an easier dumping mechanism.

Next, the maximum force required to turn the barrel in a "worst-case scenario" was examined. The worst-case scenario is depicted in the following figure. The barrel rotated by 90° and the load inside the barrel is concrete, which has not fallen out of the barrel yet.

Figure 22. Worst-Case Scenario of Static Loading.

The gravitational force, G, is acting on the center of mass of the barrel (including the load). The distance from the center of mass to the axis of rotation is called d.

(9)

Again, W is the width of the barrel. So, by summing the moments about the axis of rotation, the static torque, T_s, is a function of G and d.

(10)

Substituting for G with product of the density of the medium inside the barrel, r , the volume of the barrel, V, and the acceleration due to gravity, g, equation 10 becomes:

Substituting for the volume with the length, L, and width of the barrel, W, equation 5 becomes:

Using this equation along with W=0.6m, L=1.2m, r =2300kg/m³ and g=9.8m/s², the static torque required to turn the barrel can be calculated as follows:

Next, the dynamic torque required to rotate the barrel was investigated. Neglecting all friction, the dynamic torque to rotate the barrel, T_d, is a function of the angular acceleration, a , and the mass moment of inertia of the load in the barrel, J.

(11)

The mass moment of inertia, J, is a function of the density of the medium in the barrel, r , and the dimensions of the barrel, L and W.

(12)

a =30° /s

L=1.2m

W=0.6m

r =2300kg/m³

Since the dynamic torque is relatively small in comparison to the static torque it can be neglected. Thus, the maximum torque required to turn the barrel in a "worst-case scenario" was 490 Nm.

The prototype of our wheelbarrow was built to be dimensionally accurate and to demonstrate the functionality of the design. The frame is constructed of 2" x 2" wood pieces and is reinforced with L-brackets. The wheels are 8" diameter lawnmower wheels. The wheels are attached by a ½" steel rod held in place by two cauter pins. The barrel is half of a 55-gallon drum. Since an inexpensive gearing system could not be worked out, the handle design was built to demonstrate the basic idea. The prototype is shown in Figure 23.

Figure 23. Prototype

This wheelbarrow was designed with the goal of increasing the stability and the ease of unloading of the current design. Through our conceptual designs and the use of evaluation techniques, a design was chosen the meets both of these goals. The double wheels and low center of gravity provide for a much more stable wheelbarrow, and the innovative dumping mechanism allows for much easier unloading. Aspect Engineering is confident that this unique design will soon be the standard for personal and home use.