Forklift Load Capacity The forklift weighs 3500 lb and carries an 800-lb shipping container. There are two front wheels and two rear wheels on the forklift.(a) Determine the contact forces between the wheels and ground.(b) How heavy a load can be carried before the forklift will start to tip about

Question

Forklift Load Capacity

The forklift weighs 3500 lb and carries an 800-lb shipping container. There are two front wheels and two rear wheels on the forklift.(a) Determine the contact forces between the wheels and ground.(b) How heavy a load can be carried before the forklift will start to tip about its front wheels? (See Figure 4.21)

Approach
We are tasked with finding the wheel contact forces and the maximum load the forklift can carry before it tips forward. We assume that the forklift is not moving and then draw the free body diagram along with the positive sign conventions for forces and moments. We label the (known) 3500-lb and 800-lb weights that act through the mass centers of the forklift and container. The (unknown) force between a front wheel and the ground is denoted by F, and the (unknown) force between a rear wheel and the ground is R. On the side view of the free body diagram, the net effects of those forces on the wheel pairs are 2F and 2R. (See Figure 4.22)

Accepted Answer

(a) There are two unknowns (F and R), and therefore two independent
equilibrium equations are needed to solve the problem. We first sum
forces in the vertical direction

[latex] -(800 lb) - (3500 lb) + 2F + 2R = 0 \longleftarrow [\sum\limits_{i=1}^{N}{F_{y,i}=0} ] [/latex]

or F + R = 2150 lb, but a second equation is required to determine
the two unknowns. Summing forces in the horizontal direction will not
provide any useful information; so we use a moment balance. Any location can be chosen as the pivot point. By choosing the point to coincide with the front wheel, force F will be conveniently eliminated from the calculation. Taking moments about point A, we have

[latex] (800 lb) (24in.)- (3500 lb)(42 in.) + (2R) (72 in.) = 0 \longleftarrow [\sum\limits_{i=1}^{N}{M_{A,i}=0} ] [/latex]

from which R = 888 lb. Here, the 800-lb weight and the rear wheel
forces exert counterclockwise (positive) moments about A, and the forklift’s 3500-lb weight generates a negative moment. Substituting the solution for R into the vertical- force balance

F + (888 lb) = 2150 lb

returns F = 1262 lb.

(b) When the forklift is on the verge of tipping about the front wheels, the rear wheel has just lost contact with the ground, and R = 0. We denote the new weight of the shipping container that causes tipping as [latex]w[/latex]. The moment balance about the front wheels is

[latex](w)(24 in.) - (3500 lb)(42 in.)= 0 \longleftarrow [\sum\limits_{i=1}^{N}{M_{A,i}=0} ] [/latex]

The forklift will be on the verge of tipping when the operator attempts
to lift a [latex]w[/latex] = 6125 - lb container.

Discussion
It makes sense that the front wheel forces are larger than the rear wheel, because the load is ahead of the front wheels. The sum of the wheel forces also equals the combined weight of the forklift and load. The large weight necessary to cause tipping is also expected as part of the forklift’s design. As a double-check to our solution of part (a), we can determine the front wheel force without having to fi nd the force carried by the rear wheels. The key is to sum moments about the rear wheels at point B. The unknown force R passes through that point, and, by having a perpendicular lever arm with zero length, it will be eliminated in the moment calculation. The moment balance now reads

[latex](800 lb)(96 in.) + (3500 lb)(30 in.) - (2F)(72 in.)= 0 \longleftarrow [\sum\limits_{i=1}^{N}{M_{B,i}=0} ] [/latex]

We obtain F = 1262 lb through only one equation and without any intermediate steps involving R. In short, by carefully choosing the moment’s pivot point, we can reduce the amount of algebra by directly eliminating unknown forces.

Wheel forces: F = 1262 lb and R = 888 lb
Maximum tipping load: [latex]w [/latex]= 6125 lb

Question 4.7: Forklift Load Capacity The forklift weighs 3500 lb and carri...

This Question has been Answered.

Already have an account?

Login

Related Answered Questions