binghamton university solar charging station

Binghamton University

Solar Charging Station Binghamton University

Thomas J Watson School of Engineering

ME 580A: Design of Mechanical Elements

Andrew Rafalowitz

12/19/2013

1

Using the PTC Creo parametric software suite, the task for this project was to design,

simulate, and analyze a solar charging station that utilizes a dual axis slewing drive to position a

solar panel toward the sun. Creo Parametric was used to design all of the components. Creo

Mechanism was used to simulate the motion of the building. Creo Simulate was used to perform

a Finite Element Analysis that would determine if the parts would fail.

The design of the slewing drive required an understanding of how the system was going

to be loaded. The solar panel and frame that it is mounted in is creating an axial force on the

lower slewing drive and the pole that the entire system is mounted on. In addition, the weight of

the top slewing drive also adds an axial force onto the bottom slewing drive and the pole that

they are both mounted to.

The pole could potentially buckle from the weight placed on it by the slewing drives and

solar panel. A buckling analysis was performed to verify that the critical load was not reached by

the weight of the solar panel frame and slewing drives. The buckling analysis showed that the

critical load for column buckling is 9.15*10^6 N. The maximum axial load on the pole was

shown to be only 3700 N, which means that the column will not buckle. A wind analysis on the

pole was also performed to show that the pole will not topple over in heavy wind. Upon

completing the wind load analysis, the pole was shown to only move 1.9*10^-2 mm in 60 mph

winds.

The slewing drives were studied and were shown to be able to sustain the axial and radial

placed on them, respectively. The axial configuration was shown to be better at minimizing

stress in the slewing ring.

The bolt that connects the flange on the solar panel frame proved to be a crucial part to

analyze. The flange bolt must sustain a torque from the slewing gear and must also sustain the

weight of the solar panel frame at the flange connection.

Overall, the maximum von Mises stress on the components studied are as follows: axial

slewing ring 3.5 MPa, radial slewing ring 5.5 MPa, pole - 0.3 , flange bolt - 60. These results suggest that the heaviest loading is coming directly from the slewing drive.

The safety factors for the components analyzed are as follows: axial slewing ring 49, radial

slewing ring 31, pole - 924, flange bolt - 4. These findings suggest a stronger bolt should be

used to connect the frame to the slewing drive.

Executive Summary

2

Table of Contents

Introduction 3

System Design ....4

Analysis ..6

Results ...13

Discussion of Results ....14

Conclusion ....15

Works Cited ......16

Appendix I ....17

Appendix II .......23

Appendix III ......48

3

Introduction

My senior capstone project for the academic year is to design and build a solar tracker on

the Binghamton University campus. To increase the solar radiation incident on the solar panel, a

tracking system will be implemented that tracks the trajectory of the sun throughout the day.

Tracking the sun requires rotating and tilting the solar panel to a specified position. To provide

rotation and tilt, a dual axis slewing drive will be utilized in our design. This report will analyze

critical components of the slewing drive to ensure the safe operation of our charging station.

The slewing drive is essentially a gearbox that contains gears, bearings, a motor, and a

housing to transmit torque while also holding axial and radial loads. The slewing drive is driven

by a motor that rotates a worm that is meshing with the teeth of a gear. A ring is connected to the

top surface of the gear, so that as the gear rotates so will the ring on top of it. The gear has

another function; it also serves as the outer ring of an internal bearing system. The inner ring of

the bearing is grounded into the housing so that it will remain stationary.

To ensure the solar charging station is safe for public use, it is imperative to determine

the failure modes of operation. Since the solar panel will be located on top of a pole, it is crucial

to know that the structure will be structurally secure and will not buckle due to the weight of the

panel and frame. The weight of the solar panel and frame will be axial and radial loads on the

slewing drive. The slewing drive must be designed so that it will not yield from these loadings. If

wind speeds become too large, the station could potentially tip over. The design of the pole must

be strong enough against intense wind loading. Lastly, the bolts that connect the frame of the

solar panel to the slewing drive must be strong enough to withstand the torque from the slewing

drive and the radial load from the weight of the solar panel.

4

System Design

Before designing the entire system, it was necessary to plan out the design of the dual

axis slewing drive. As it is the core mechanism of our project, a thorough slewing drive design is

needed that factors in many external loading scenarios. As shown in Figure 1, the slewing drive

consists of a housing which contains a gear, ball bearing, seal, worm shaft, grease nipples, bolts,

motor, outer ring, and housing enclosures. All of these components come together to provide the

torque needed to rotate the assembly above it.

Figure 1. Exploded view of bottom slewing drive

5

The balls inside the bearing are made from silicon nitride because of the high fracture

toughness and low density of this ceramic. The bearing ring, gear ring, worm, and outer ring

were chosen to be made from SAE No. 65 bronze. This type of bronze contains a high tin

composition and is especially useful in low speed applications. The rest of the slewing drive is

made from steel to ensure durability. Grease nipples were placed near the connection point

between the worm and gear to maximize lubrication and reduce wear.

After completing the design of the slewing drive, it was then necessary to design a way to

connect another slewing drive that would provide the tilt axis for the solar panel frame. A base

with an upright arm was then bolted into the original slewing drive. The second slewing drive

will then be fitted and welded into the space provided in the arm. Figure 2 shows how the dual

axis slewing drive is assembled.

Figure 2. Dual axis slewing drive

6

Analysis

A buckling analysis was performed to determine the critical load to induce column

buckling in the steel pole. It was first necessary to determine the slenderness ratio to classify the

column. As shown in Appendix III, the calculations show that the Euler formula is not applicable

to our pole. The parabolic J. B. Johnson formula was used to determine the critical load for

buckling. The critical axial force to induce unstable bending is 9.15 * 10^6 N. Since the weight

of the components above the pole is less than 4 kN, buckling does not pose a threat to the pole.

After showing the pole would not buckle from the weight placed on top of it, it was then

necessary to show that the pole would not yield from intense wind loading. It is also desirable to

have the pole deflect as little as possible during such conditions. A finite element analysis was

performed to show the pole could survive the harsh wind conditions. A wind speed of 60 mph

was selected as the magnitude for the wind. The force on the pole from the 60 mph wind is

obtained from the drag equation, as shown in Equation 1.

Where,

is the density of air is the wind speed is the drag coefficient is the surface area

The drag coefficient for the pole is 0.42. The area of the pole in contact with the wind is

approximately 1.24 m^2. The drag force on the pole is 228.6 N. The wind loading, axial dead

loading, and constraints on the pole are shown in Figure 3.

7

Figure 3. Axial and radial loads and constraint on the pole

The compressive axial force acting on the top of the pole was obtained from summing all

the weights of the components placed on top of the pole. The von Mises stress distribution and

displacement of the pole from the static analysis is shown in Appendix 3. The axial load on top

of the pole was found to be approximately 3.9 kN. The pole was constrained at the base only and

is completely free to move at the top.

8

An analysis of the loading on the slewing bearing was performed. The loading can either

be a tilting moment, axial load, or a radial load. A combination of the aforementioned loads is

also possible. Since there is a negligible change in the location of the center of gravity as the

solar panel tilts, the effects from tilting moments may be ignored. The tilting moment is also

ignored because of the inherent symmetry found in the slewing drive assembly. The weight of

the solar panel and supporting frame was then resolved into radial loading on the top slewing

drive and axial loading in the bottom slewing drive. Figure 4 shows how axial loads and radial

loads are applied to the respective slewing drives.

Figure 4. Axial and radial loads acting on the slewing drive

The figure shows that the slewing drive has the force applied to the interior bearing ring

of the slewing drive. This is due to the fact that the interior ring is completely motionless and is

bolted into the housing of the slewing drive.

9

A finite element analysis was performed on the top slewing drive interior ring with a

radial bearing load. The magnitude of the load comes from the dead weight of the solar panel and

frame. The magnitude of the radial load was 3109 N. Since the chosen slewing drive rotates at

only 0.48 rpm, the effects of dynamic loading have been ignored. The radial loading has the

largest magnitude in the middle and follows a parabolic path as it tapers off in magnitude when it

reaches the sides. Figure 5 shows the radial load and constraints on the top slewing ring. The

slewing ring is crucial in taking the applied loading off of the components that are constantly

moving.

Figure 5. Radial load and constraints on the top slewing ring

10

A finite element analysis was performed on the bottom slewing drive interior ring with an

axial bearing load. The magnitude of the axial load comes from the dead weight of the solar

panel, frame, and top slewing drive. The axial load magnitude was 3735 N. The axial loading has

the same magnitude applied to the outer surface of the slewing ring. Figure 6 shows the axial

load and constraints on the bottom slewing ring. The slewing drive is better at taking axial

loadings as opposed to radial loadings, which is why the slewing drive on the bottom takes an

axial load instead of radial load.

Figure 6. Axial load and constraint on the bottom slewing ring

The von Mises stress distribution and displacement fringe plots for both load types are

shown in Appendix III.

11

The last component that was analyzed was the flange bolt that connects the solar panel

frame to flange on the top slewing drive. The flange bolt is made from steel and is experiencing

shear loading from the slewing torque and loading from the solar panel and frame. The region of

the bolt that is in contact with the flange is where the dead loading is applied. The bolt is critical

for the overall integrity of the connection between the top slewing drive and the solar panel

frame. The bottom surface of the bolt is constrained by its connection into the gear and flange.

Equation 2 shows how the supplied torque of the slewing drive places a shear stress on the bolt.

Where,

T is the torque from the slewing drive

P is the shear force on the bolt

r is the radial distance from the center of the flange to the bolt

n is the number of bolts on the flange

Since the radial distance was 101.6 mm, there are 8 flange bolts, and the torque is 727 N-

m, the shear force on each bolt is approximately 894 N. This shear force was only applied to the

surface that was in contact with the gear. Figure 7 shows the two loads on the flange bolt and

how it is constrained in the slewing drive. The finite element analysis fringe plot results for von

Mises stress and displacement are in Appendix III.

12

Figure 7. Shear load and dead weight load and constraints on flange bolt

13

Results

Maximum von Mises Stress (MPa)

Slew Ring (Axial load) Slew Ring (Radial load) Pole Flange Bolt

3.5 5.5 0.27 60.57

Table 1. Maximum von Mises stress for all critical components

Maximum Displacement (mm)


5.7*10^-4 4.0*10^-4 1.9*10^-2 3.4*10^-3

Table 2. Maximum displacement for all critical components

Factors of Safety


49 31 924 4

Table 3. Safety factors for all critical components

14

Discussion of Results

The tabulated results prove that all of the components will not yield from the loading they

are subjected to. From Table 1, it can be seen that the slewing drives are better are handling axial

loads since the von Mises stress is much lower for the axial loading. Table 1 also shows that the

pole barely received any loading from the wind and axial loading. The flange bolt had the most

stress of all the components studied. This is to be expected since it is receiving a large shear load

from the output torque of the slewing drive.

Table 2 suggests that all of the parts have experienced negligible amounts of

displacement. This is in agreement with the small stress values found in Table 1.

Table 3 yields the values of the safety factors for the studied components. The flange bolt

had a safety factor of only 4 which suggests a stronger bolt choice may have been wise.

Additionally, the safety factor for the pole suggests that the wind loading will not be a problem.

The safety factor for the axial loading is also greater than the safety factor for radial, which is to

be expected.

15

Conclusion

The design of the dual axis slewing drive was shown to meet the safety requirements of

the charging station. The results for the pole suggest that the pole could support more weight.

This fact could eventually lead to more solar panels being added to the frame to increase energy

output. It is possible to determine the maximum number of solar panels that could be placed on

the pole before it would begin to yield or buckle. It is also possible to add stronger bolts or to

simply make room for more of them to reduce the shear force they experience. Another option is

to pick a slewing drive that has an output torque that is less than the output torque used in this

study.

16

Works Cited

Olave M, Damian J, Serna A, Sagartzazu X. Design of Four Contact-Point Slewing Bearing With

a New Load Distribution Procedure to Account for Structural Stiffness. J. Mech.

Des.. 2010;132(2):021006-021006-10. doi:10.1115/1.4000834.

"Slewing Ring Bearing & Loads Selection." Slewing Ring Bearing, Loads Selection, Special

Bearing - Rollix. Rollix, n.d. Web. 19 Dec. 2013.

17

Appendix I Assembly Drawings

23

Appendix II Detail Drawings

48

Appendix III Graphs and Calculations

54

Verification

binghamton university solar charging station

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