Flying Bobs Wes Drennan

Introduction

810.jpg

1

The Flying Bobs ride is commonly found at carnivals and amusement parks around the world. It consists of a set of arms, extending radially from a central shaft. A car hangs freely at the end of each pair of arms, and is free to rotate about an axis tangent to the path of travel. The arms contain roller followers that rest on a stationary curved track, which acts as a disk cam. As the central shaft rotates, the changing height of the cam profile causes to cars to swing out sideways as they travel around the profile of the cam.

Problem Statement and Assumptions

For the sake of simplicity, the free swinging motion of the car will be neglected to reduce the system down to a single degree of freedom. The ride is typically designed to swing the riders to a maximum angle of 90 degrees when the rotation of the central shaft has reached its peak angular velocity. When the car is at maximum swing, the vertical component of the acceleration at the tip of the arm is assumed to be equal and opposite to the acceleration due to gravity, leading to a moment of zero lateral force from the rider's perspective. The goal of this project is to design a disk cam profile based on this specified acceleration, along with the constant angular velocity of the central shaft and the radius of the disk cam. The profile is assumed to follow a sinusoidal curve with two periods, so the maximum and minimum accelerations will occur at the lowest and highest points of the track respectively.

Methods and Calculations

The following Matlab code is used to solve for the profile of the cam, which outputs a three-dimensional graph to represent both the shape of the track and the path followed by the end of the follower arm, as well as a two-dimensional graph of these two curves with respect to the angle of rotation.

From the official technical specifications2, the input angular velocity was found to be 14 rpm.
The ride's dimensions were also approximated from the images and data provided.
The cam profile is evaluated at 128 points.

%Wes Drennan
%DOM II
%10/6/2010
%Cam Design: Flying Bobs Carnival Ride

clc
clear all

%Assume simple harmonic motion of follower arm with respect to angle theta

%Independent Variables:---------------------
%Radii and angular velocity from source below:
%<http://www.bertazzon-america.com/matterhorn.php>

%radius to cam surface (m)
r1= 6.096 ;

%radius to cars (m)
r2= 8.2296 ;

%angular velocity (rpm)
omega= 14 ;

%height of follower at pivot point (m)
z0= 3 ;

%number of iterations of theta
iter= 128 ;

%acceleration due to gravity (m/s^2)
g= 9.80 ;

%-------------------------------------------

omega_rad=omega*2*pi/60; %(rad/s)

beta=2*pi/8;

L=(g*(1/omega_rad^2)*4*(beta^2)/(pi^2));

i=[1:iter+1];

theta=2*pi.*i/iter;

%z0 redefined to optimize pressure angle
z0= L ;

P_prime=L*(1-cos(2.*theta));

figure (1)
plot(theta,P_prime,'--')
axis([0 2*pi 0 2*L])
hold;

P=r1.*(P_prime-z0)/r2+z0;

figure (1)
plot(theta,P)
xlabel('theta(rad)')
ylabel('P(m)')
legend('end of follower arm','cam surface')

hold off;

x1=r1.*cos(theta);
y1=r1*sin(theta);
x2=r2.*cos(theta);
y2=r2*sin(theta);

z= L*(1-cos(pi.*theta/(2*beta)));

figure (2)
plot3(x1,y1,P,'.-')
hold;
plot3(x2,y2,z,'-')
axis ([-r2 r2 -r2 r2 0 2*r2])
pbaspect([1 1 1])
grid on

%show platform and vert. support lines
for i=[1:1:iter+1];
plot3([x1(i) x1(i)],[y1(i) y1(i)],[0 P(i)])
plot3([x1(i) x2(i)],[y1(i) y2(i)],[P(i) z(i)])
end

%show follower lines
for i=[1:8:iter+1];
plot3([0 x1(i)],[0 y1(i)],[z0 P(i)])
end
xlabel('x(m)')
ylabel('y(m)')
zlabel('z(m)')
legend('inner ring = cam surface', 'outer ring = end of follower arm')

clc

%accerleration(g) at end of follower arm
a_c=r2*(omega_rad^2);
fprintf('Measured at end of follower arm')
fprintf('Neglecting swinging movement of cars')
accel_g_min=sqrt(a_c^2+g^2)/g
accel_g_max=sqrt(a_c^2+(2*g)^2)/g

Results

Based on the input parameters included in the code above, the following graphs were produced to define the cam profile:

Bobs_Graph1_cut.jpg
Bobs_Graph1_side2_final.jpg
Bobs_Graph1_side1_final.jpg
Bobs_Graph2.jpg

The code also outputs the minimum and maximum accelerations at the tip of the arm (measured in multiples of g), which occur at the highest peak and lowest valley respectively on the cam's profile.

Measured at end of follower arm, Neglecting swinging movement of cars
accel_g_min =

    2.0635

accel_g_max =

    2.6940

Results

The resulting value of 2 meters for the maximum height of the cam coincides with the height of the actual ride based on the source images, so the original approximation of zero vertical acceleration at the peaks seems to be a valid assumption.

The values of acceleration, which fluctuate between approximately 2.1 and 2.7 g are well within the limits of human comfort. Based on personal experience, these values seem to be in accordance with the accelerations typically produced by the actual ride.