Machine Elements: Design Project -...
Transcript of Machine Elements: Design Project -...
Machine Elements: Design Project
Drag Chain Conveyor
Matthew Gray, Michael McClain,
Pete White, Kyle Gilliam,
Alejandro Moncada and Matthew Gonzalez
Executive Summary
The logistics of many industries use drag chain conveyors to move heavy loads short distances.
The purpose of this project is to analytically define the characteristics and design parameters for
our own drag chain conveyor system.
The starting parameters used for our general design are listed below:
Maximum load of 3000 pounds (treated as a 4 feet x 4 feet wooden pallet)
Maximum speed of 35 feet per minute
Load travel distance of 10 linear feet
Conveyor frame is rigid
Neglect weight of wooden pallet
Chain idealized
Drive Sprocket parameters are taken from manufacturer data
The following components were the focus of our design:
Pinion driven by the motor
Main drive gear
Shaft connecting main drive gear
Bearings supporting the shaft
Drag chain conveyors operate by rotating a shaft that drives two or more sprockets, which in
turn, drive the chains. The chains are connected around a drive sprocket and a free rotating
sprocket (an idler sprocket) at the opposite end of the conveyor. A load to be moved rests on top
of the chains and as the chains rotate around the prescribed loop, the load is moved from one
point to another.
Our design approach was to work backwards from our starting parameters to determine the
forces necessary for our conveyor to operate as prescribed. These forces were then used to
analyze the key components of our conveyor.
From our force analysis, we were able to determine that it should take the conveyor no longer
than 17 seconds to move the 3000 pound load 10 feet. The total force required for our conveyor
was calculated to be 776 pounds.
Our sprocket design was based on our decision to use an ANSI No. 50 chain for the conveyor.
With the properties of this chain, we were able to calculate the pitch diameter of the sprocket by
using the American Chain Association Sprocket Design Guidelines. This diameter was then used
in selecting an actual sprocket to be used in our design. Finally, we determined that a torque of
81 pound-foot on each sprocket is required to satisfy our initial parameters.
The initial parameters used for our gear design are listed below:
20 degree pressure angle
Standard full depth gear
21 teeth for pinion
55 teeth for gear
2.62:1 gear ratio
3 inch pitch diameter for pinion
These parameters were used to calculate the geometries of both the gear and pinion. After
determining the geometries of the gear and pinion, a thorough stress analysis was conducted on
the gear set. Notable calculations from this analysis include the rotational speed of the pinion and
the number of cycles of both pinion and gear. The pinion was calculated to have a rotational
speed of 45 revolutions per minute, while the number of cycles of the pinion and gear in a 5 year
span were determined to be 27 million and 10 million respectively.
Other calculations that comprised this stress analysis include: bending stress, surface stress,
bending fatigue strength, and surface fatigue strength. After conducting the stress analysis of the
gear set, the safety factors of the pinion and gear were determined. We then re-evaluated the
design and re-iterated the process.
The final step in our gear design was selecting the material of the gear set. Our choice of material
is shown below:
Gear Material (Table 12-20 Norton)
o AGMA Grade 2 (Premium Grade)
o Class A5
o Carburized and case hardened
o Brinell Hardness = 710 (corresponds to 64HRC)
o Qv = 5
With this material, the final safety factors of the gear and pinion are as follows:
Gear
o Bending Safety Factor = 4.76
Pinion
o Bending Safety Factor = 3.98
o Surface Fatigue Safety Factor = 2.38
Shaft Design Parameters:
1.75 inch diameter
2 โ 3 inch overhang on the outside of each sprocket
SAE/AISI 1010 Cold rolled steel
As with the gear set, a stress analysis was conducted on the shaft. The values we calculated for
the maximum stresses on the shaft are as follows: 7188 psi (bending stress), 219 psi (shear
stress), and 1850 psi (torsional stress). After calculating the shaftโs maximum stresses, we
proceeded to determine its safety factor. With the shaftโs material properties and maximum
stresses, we used the maximum shear stress theory to get a safety factor of 5.4.
In designing the bearings, we first had to determine the total force being subjected on each of the
two bearings. The bearing closest to the drive gear was calculated to have a total force of 379
pounds, while the other bearing was calculated to have a total force of 85 pounds. Using the
higher total force, we determined the dynamic load rating. The value of this rating was calculated
to be 1296 pounds. Finally, using the dynamic load rating we were able to determine the safety
factor of the bearing, which was calculated to be 15.
Report Outline
I. Introduction & Overview
A. Identification of Need
B. Definition of Problem
C. Drag Chain Conveyor System
II. Analysis
A. Nomenclature
B. Preliminary Design
C. Force Analysis
D. Sprocket Design
E. Gear Design
1. Gear and Pinion Stress Analysis
a. Bending Stress Calculations
b. Surface Stress Calculations
c. Bending Fatigue Strengths
d. Surface Fatigue Strengths
2. Safety Factor Calculation
3. Material Selection
F. Shaft Design
1. Shaft Stress Analysis
a. Bending Stress Calculation
b. Shear Stress Calculation
c. Torsional Stress Calculation
2. Safety Factor Calculations
3. Shaft Key Stress Calculations
G. Bearing Design
1. Force Calculations
2. Dynamic Load Rating Calculation
3. Safety Factor Calculation
III. Summary of Safety Factors
IV. Conclusion
V. References
VI. Appendix
VII. Engineering Drawings
Introduction & Overview
Identification of Need
The logistics of many industries require the use of human operated forklifts to transport heavy
loads. Implementation of automated conveyor systems has proven to be an exceptional solution
to minimizing human interaction in moving heavy loads short distances. These conveyor systems
are commonly used to move heavy pallets of finished products or raw materials from one area of
a manufacturing facility to another.
Definition of Problem
The purpose of this project is to analytically define the characteristics and design parameters for
a drag chain conveyor system. The criteria we chose for our design process came from our
research on existing conveyor systems. The assumed maximum load of 3000 pounds was based
on an average of maximum loads for these conveyor systems. In addition, these loads are
typically placed on 4 feet ร 4 feet (l ร w) wooden pallets. For our design, we will also assume
that the 3000 pounds maximum load takes the form of this pallet. We defined 10 linear feet as
the distance that this load must be moved. This distance keeps the conveyor system small enough
to be shipped as a finished product via motor freight. It also allows simple installation in a
manufacturing facility with no need for additional setup or assembly. Again, based on average
values found for existing systems, we chose our translational speed for the system to be 35 feet
per minute.
The frame is assumed rigid as it is a non-moving part. In addition, it is typically overdesigned
compared to the moving elements of the conveyor system. The weight of the wooden pallet is
taken to be negligible with respect to the 3000 pounds load, and any flexure of the pallet is
ignored. The travel distance of 10 linear feet is measured from the center of the drive sprockets.
Since the design of the chains is outside the scope of this project, all chains used are assumed to
be idealized. This means that any flexure, stretching, or stress in the chain is ignored. The
sprockets that drive the chains are taken directly from manufacturerโs specifications to match the
drive chains.
All our initial parameters are listed below:
Maximum load of 3000 pounds (treated as a 4 feet x 4 feet wooden pallet)
Maximum speed of 35 feet per minute
Load travel distance of 10 linear feet
Conveyor frame is rigid
Neglect weight of wooden pallet
Chain idealized
Sprockets that drive chains are taken from manufactureโs information
The main elements to be designed are the pinion driven by the motor, the main drive gear, the
shaft connecting the main drive gear, and the bearings supporting the shaft. Many assumptions
and idealizations have been made in order to focus on the main elements required for this report:
a shaft, gear and bearing.
Drag Chain Conveyor System
Drag chain conveyors operate by rotating a shaft that drives two or more sprockets, which in
turn, drive the chains. The chains are connected around a drive sprocket and a free rotating
sprocket (an idler sprocket) at the opposite end of the conveyor. See the Engineering Drawings
section for a better illustration of the final design. These types of conveyors have many
variations and are used in many different industries around the globe. A common application of
these types of conveyors is in the area of material handling. A load to be moved rests on top of
the chains and as the chains rotate around the prescribed loop, the load is moved from one point
to another. Many different companies manufacture drag chain conveyors, however the design
principles are similar industry wide. The parameters that do differ among designs are usually the
component materials, load ratings, and speed of travel.
Analysis
Nomenclature
๐๐ = ๐๐๐ ๐ก๐๐๐๐ข๐ก๐๐ ๐ค๐๐๐โ๐ก ๐๐ ๐๐๐๐ ๐๐๐ ๐ ๐๐๐
๐๐ = ๐๐๐ ๐ก๐๐๐๐ข๐ก๐๐ ๐ค๐๐๐โ๐ก ๐๐ ๐โ๐๐๐ ๐๐๐ ๐ ๐๐๐
๐๐ฟ = ๐ก๐๐ก๐๐ ๐๐๐ ๐ก๐๐๐๐ข๐ก๐๐ ๐ค๐๐๐โ๐ก ๐๐๐ ๐ ๐๐๐
๐๐ = ๐ก๐๐ก๐๐ ๐๐๐๐ (๐๐๐ ๐ ๐๐๐) ๐๐๐๐๐๐๐ ๐ก๐ ๐โ๐๐๐
๐๐ = ๐๐๐๐๐๐ ๐น๐๐๐๐
๐น๐ = ๐๐๐๐๐ก๐๐๐๐๐ ๐๐๐๐๐ ๐๐๐ ๐ ๐๐๐
๐๐ = ๐๐๐๐๐ก๐๐๐ ๐๐๐๐๐. ๐๐ ๐๐ป๐๐ ๐๐๐ ๐๐ก๐๐๐
๐๐ถ = ๐๐๐๐๐๐ ๐ฃ๐๐๐๐๐๐ก๐ฆ ๐๐ ๐โ๐๐๐ ๐๐๐ ๐๐๐๐
๐น๐ถ = ๐๐๐๐๐ ๐๐๐๐ข๐๐๐๐ ๐๐๐ ๐ ๐๐๐ ๐ก๐ ๐๐๐ฃ๐ ๐๐๐๐
๐น๐ = ๐๐๐๐๐ ๐๐๐๐ข๐๐๐๐ (๐ก๐๐ก๐๐)๐ก๐ ๐๐๐ฃ๐ ๐๐๐๐
๐๐๐ 1 = ๐๐๐ก๐โ ๐๐๐๐๐๐ก๐๐ ๐๐ ๐ ๐๐๐๐๐๐๐ก #1
๐๐๐ 2 = ๐๐๐ก๐โ ๐๐๐๐๐๐ก๐๐ ๐๐ ๐ ๐๐๐๐๐๐๐ก #2
๐๐ ๐1 = ๐ก๐๐๐๐ข๐ ๐๐ก ๐ ๐๐๐๐๐๐๐ก #1
๐๐ ๐2 = ๐ก๐๐๐๐ข๐ ๐๐ก ๐ ๐๐๐๐๐๐๐ก #2
๐๐ ๐ = ๐๐๐ก๐๐ก๐๐๐๐๐ ๐ ๐๐๐๐ ๐๐ ๐ ๐๐๐๐๐๐๐ก๐
๐๐ โ๐๐๐ก = ๐๐๐ก๐๐ก๐๐๐๐๐ ๐ ๐๐๐๐ ๐๐ ๐ โ๐๐๐ก
๐๐๐ = ๐๐๐๐๐๐ก๐๐๐ ๐๐๐ก๐โ ๐๐ ๐๐๐๐๐๐
๐๐ = ๐๐๐ก๐โ ๐๐๐๐๐ข๐ ๐๐ ๐๐๐๐๐๐
๐๐๐ = ๐๐๐๐๐๐ก๐๐๐ ๐๐๐ก๐โ ๐๐ ๐๐๐๐
๐๐ = ๐๐๐ก๐โ ๐๐๐๐๐ข๐ ๐๐ ๐๐๐๐
๐๐ก๐ = ๐๐ข๐๐๐๐ ๐๐ ๐ก๐๐๐กโ ๐๐ ๐๐๐๐๐๐
๐๐ก๐ = ๐๐ข๐๐๐๐ ๐๐ ๐ก๐๐๐กโ ๐๐ ๐๐๐๐
๐๐ = ๐๐๐ก๐โ ๐๐๐๐๐ก๐๐ ๐๐ ๐๐๐๐๐๐
๐๐ = ๐๐๐ก๐โ ๐๐๐๐๐ก๐๐ ๐๐ ๐๐๐๐
๐๐ = ๐๐๐๐๐ข๐๐๐ ๐๐๐ก๐โ
๐๐ = ๐๐๐ ๐ ๐๐๐ก๐โ
๐ = ๐๐๐๐๐๐๐ข๐
๐ = ๐๐๐๐๐๐๐ข๐
๐ท๐ = ๐ค๐๐๐๐๐๐ ๐๐๐๐กโ
๐ท๐โ = ๐คโ๐๐๐ ๐๐๐๐กโ
๐ก๐๐ก = ๐๐๐๐๐ข๐๐๐ ๐ก๐๐๐กโ ๐กโ๐๐๐๐๐๐ ๐
๐๐ = ๐๐๐๐๐๐ก ๐๐๐๐๐ข๐
๐ถ๐๐๐๐ = ๐๐๐๐๐๐ข๐ ๐๐๐ ๐๐ ๐๐๐๐๐๐๐๐๐
๐๐ก๐๐๐๐ = ๐๐๐๐๐๐ข๐ ๐ค๐๐๐กโ ๐๐ ๐ก๐๐ ๐๐๐๐
๐ถ๐ ๐ = ๐๐๐๐๐๐๐๐๐
๐๐ = ๐๐๐๐ ๐ค๐๐๐กโ ๐๐ ๐ก๐๐๐กโ
๐ = ๐๐๐๐๐กโ ๐๐ ๐๐๐ก๐๐๐
๐ถ = ๐๐๐๐ก๐๐ ๐๐๐ ๐ก๐๐๐๐
๐๐ = ๐๐๐๐ก๐๐๐ก ๐๐๐ก๐๐ ๐๐ ๐๐๐๐ ๐๐๐ ๐๐๐๐๐๐
๐๐ = ๐๐๐ก๐๐ก๐๐๐๐๐ ๐ ๐๐๐๐ ๐๐ ๐๐๐๐๐๐
๐๐ = ๐๐๐ก๐๐ก๐๐๐๐๐ ๐ ๐๐๐๐ ๐๐ ๐๐๐๐
๐๐๐ฆ๐โ๐ = ๐๐ข๐๐๐๐ ๐๐ ๐๐ฆ๐๐๐๐ ๐๐ ๐๐๐๐๐๐
๐๐๐ฆ๐โ๐ = ๐๐ข๐๐๐๐ ๐๐ ๐๐ฆ๐๐๐๐ ๐๐ ๐๐๐๐
๐๐ก = ๐ก๐๐๐๐๐๐ก๐๐๐ ๐๐๐๐๐ ๐๐ ๐ก๐๐๐กโ
๐๐ = ๐ก๐๐๐๐ข๐ ๐๐ ๐๐๐๐
๐พ๐ฃ = ๐ท๐ฆ๐๐๐๐๐ ๐น๐๐๐ก๐๐
๐๐ฃ = ๐๐ข๐๐๐๐ก๐ฆ ๐น๐๐๐ก๐๐
๐พ๐ = ๐ฟ๐๐๐ ๐ท๐๐ ๐ก๐๐๐๐ข๐ก๐๐๐ ๐น๐๐๐ก๐๐
๐พ๐ต = ๐ ๐๐ ๐โ๐๐๐๐๐๐ ๐ ๐น๐๐๐ก๐๐
๐พ๐ = ๐ด๐๐๐๐๐๐๐ก๐๐๐ ๐น๐๐๐ก๐๐
๐พ๐ผ = ๐ผ๐๐๐๐ ๐น๐๐๐ก๐๐
๐พ๐ = ๐๐๐ง๐ ๐น๐๐๐ก๐๐
๐ฝ = ๐ต๐๐๐๐๐๐ ๐๐ก๐๐๐๐๐กโ ๐บ๐๐๐๐๐ก๐๐ฆ ๐น๐๐๐ก๐๐
๐ผ = ๐๐ข๐๐๐๐๐ ๐บ๐๐๐๐๐ก๐๐ฆ ๐น๐๐๐ก๐๐
๐๐๐ = ๐๐๐๐๐๐๐ ๐ ๐ก๐๐๐ ๐ ๐๐ ๐๐๐๐๐๐
๐๐๐ = ๐๐๐๐๐๐๐ ๐ ๐ก๐๐๐ ๐ ๐๐ ๐๐๐๐
๐๐๐ = ๐ ๐ข๐๐๐๐๐ ๐ ๐ก๐๐๐ ๐ ๐๐ ๐๐๐๐๐๐
๐ถ๐ฃ = ๐ท๐ฆ๐๐๐๐๐ ๐น๐๐๐ก๐๐
๐ถ๐ = ๐ฟ๐๐๐ ๐ท๐๐ ๐ก๐๐๐๐ข๐ก๐๐๐ ๐น๐๐๐ก๐๐
๐ถ๐ = ๐ด๐๐๐๐๐๐๐ก๐๐๐ ๐น๐๐๐ก๐๐
๐ถ๐ = ๐๐๐ง๐ ๐น๐๐๐ก๐๐
๐ถ๐ = ๐ธ๐๐๐ ๐ก๐๐ ๐ถ๐๐๐๐๐๐๐๐๐๐ก
๐ถ๐ = ๐๐ข๐๐๐๐๐ ๐น๐๐๐๐ โ ๐น๐๐๐ก๐๐
๐ธ๐ = ๐๐๐ข๐๐โฒ๐ ๐๐๐๐ข๐๐ข๐ ๐๐ ๐๐๐๐๐๐
๐ธ๐ = ๐๐๐ข๐๐โฒ๐ ๐๐๐๐ข๐๐ข๐ ๐๐ ๐บ๐๐๐
๐ฃ๐ = ๐๐๐๐ ๐ ๐๐โฒ๐ ๐๐๐ก๐๐ ๐๐ ๐๐๐๐๐๐
๐ฃ๐ = ๐๐๐๐ ๐ ๐๐โฒ๐ ๐๐๐ก๐๐ ๐๐ ๐๐๐๐
๐๐๐๐= corrected AGMA bending strength for gear
๐๐๐๐= corrected AGMA bending strength for pinion
๐โฒ๐๐= uncorrected AGMA bending strength
๐พ๐ฟ๐ = ๐ฟ๐๐๐ ๐น๐๐๐ก๐๐ ๐๐๐ ๐๐๐๐๐๐
๐พ๐ฟ๐ = ๐ฟ๐๐๐ ๐น๐๐๐ก๐๐ ๐๐๐ ๐๐๐๐
๐พ๐ = ๐๐๐๐๐๐๐๐ก๐ข๐๐ ๐น๐๐๐ก๐๐
๐พ๐ = ๐ ๐๐๐๐๐๐๐๐๐ก๐ฆ ๐น๐๐๐ก๐๐
๐๐๐๐= ๐๐๐๐๐๐๐ก๐๐ ๐ ๐ข๐๐๐๐๐ ๐๐๐ก๐๐๐ข๐ ๐ ๐ก๐๐๐๐๐กโ โ ๐๐๐๐๐๐
๐โฒ๐๐๐= ๐ข๐๐๐๐๐๐๐๐ก๐๐ ๐ ๐ข๐๐๐๐๐ ๐๐๐ก๐๐๐ข๐ ๐ ๐ก๐๐๐๐๐กโ โ ๐๐๐๐๐๐
๐ถ๐ = ๐๐๐๐๐๐๐๐ก๐ข๐๐ ๐น๐๐๐ก๐๐
๐ถ๐ = ๐ ๐๐๐๐๐๐๐๐๐ก๐ฆ ๐น๐๐๐ก๐๐
๐ถ๐ฟ = ๐๐ข๐๐๐๐๐ ๐ฟ๐๐๐ ๐น๐๐๐ก๐๐
๐ถ๐ป = ๐ป๐๐๐๐๐๐ ๐ ๐ ๐๐ก๐๐ ๐น๐๐๐ก๐๐
๐๐๐ = ๐๐๐๐๐ก๐ฆ ๐๐๐๐ก๐๐ ๐๐๐ ๐ต๐๐๐๐๐๐ ๐๐ ๐๐๐๐๐๐
๐๐๐ = ๐๐๐๐๐ก๐ฆ ๐๐๐๐ก๐๐ ๐๐๐ ๐ต๐๐๐๐๐๐ ๐๐ ๐๐๐๐
๐๐ถ๐ = ๐๐๐๐๐ก๐ฆ ๐๐๐๐ก๐๐ ๐๐๐ ๐๐ข๐๐๐๐๐ ๐น๐๐ก๐๐๐ข๐ ๐๐ ๐๐๐๐๐๐
๐๐๐๐๐ฅ= ๐๐๐ฅ๐๐๐ข๐ ๐๐๐๐๐๐๐ ๐ ๐ก๐๐๐ ๐ ๐๐ ๐ โ๐๐๐ก
๐ = ๐๐๐ฅ๐๐๐ข๐ ๐๐๐๐๐๐๐ ๐๐๐๐๐๐ก ๐๐ ๐ โ๐๐๐ก
๐ = ๐๐๐ ๐ก. ๐๐๐๐ ๐๐ข๐๐ก๐๐๐ ๐๐ฅ๐๐ ๐ก๐ ๐๐ข๐ก๐๐ ๐๐๐๐ ๐๐ ๐ โ๐๐๐ก
๐ผ๐ = ๐๐๐ ๐ ๐๐๐๐๐๐ก ๐๐ ๐๐๐๐๐ก๐๐ ๐๐ ๐ โ๐๐๐ก
๐๐ = diameter of shaft
๐๐๐๐๐ฅ= max shear bending stress
๐๐๐๐ฅ= max shear force
๐ด๐ โ๐๐๐= area under shear force
๐๐ก๐๐๐ ๐๐๐ = max ๐ก๐๐๐ ๐๐๐๐๐ ๐ ๐ก๐๐๐ ๐
๐๐ โ๐๐๐ก = ๐ก๐๐๐๐ข๐ ๐๐ ๐ โ๐๐๐ก
๐ฝ๐ = ๐๐๐๐๐ ๐๐๐๐๐๐ก ๐๐ ๐๐๐๐๐ก๐๐ ๐๐ ๐ โ๐๐๐ก
๐๐๐๐ฅ = ๐๐๐๐๐๐ ๐ก ๐๐๐๐๐๐๐๐๐ ๐ โ๐๐๐ ๐ ๐ก๐๐๐ ๐
๐๐๐๐๐ = ๐ ๐๐๐๐ก๐ฆ ๐๐๐๐ก๐๐, ๐๐๐ฅ๐๐๐ข๐ ๐โ๐๐๐ ๐โ๐๐๐๐ฆ
๐๐ท๐ธ = ๐ ๐๐๐๐ก๐ฆ ๐๐๐๐ก๐๐, ๐ท๐๐ ๐ก๐๐๐ก๐๐๐ ๐ธ๐๐๐๐๐ฆ ๐โ๐๐๐๐ฆ
๐1 = ๐๐๐๐๐๐๐๐๐ ๐ ๐ก๐๐๐ ๐ 1
๐3 = ๐๐๐๐๐๐๐๐๐ ๐ ๐ก๐๐๐ ๐ 3
๐โฒ = ๐ฃ๐๐ โ ๐๐๐ ๐๐ ๐๐๐๐๐๐ก๐๐ฃ๐ ๐ ๐ก๐๐๐ ๐
Preliminary Design
Our initial design was based on existing conveyor technology. After reviewing existing conveyor
systems, we were able to make some justifiable preliminary design decisions. Many drag chain
conveyors contain tensioning devices. These devices put an extremely high tension on the chains
in order to minimize the flexure and keep the chain from riding on the metal frame. This aim at
reducing metal on metal wear has its flaws. As a load is applied, the tension in the chain
increases, and more force is transmitted to the sprockets that drive and guide the chain. This
causes increased fatigue on the sprockets and connecting shafts. Another issue with tensioned
conveyors is that when a load is applied on the chain, it may flex so much that there is contact
between the chain and the conveyorโs metal frame. This can cause extreme friction and
premature failure of the frame and/or chain. We opted for a design that utilized Ultra High
Molecular Weight (UHMW) Polyethylene wear blocks below the chains. These blocks have low
coefficients of friction and wear extremely well. This decision reduces the need for extremely
high tension in the chain, which in turn, reduces the overall fatigue on the system. The UHMW
wear blocks can be easily replaced at regular intervals to prevent excessive wear on the chain and
conveyor frame.
In order to simplify the amount of analysis required, we chose a chain used by existing drag
chain conveyors with sprockets that match the chainโs properties. We will analyze the force
acting on the chain and UHMW blocks to determine the amount of torque required at the shaft.
The shaft diameter is determined by the sprockets that match the drive chains. However, all other
design aspects of the shaft will be analyzed and a suitable material selection will be made.
Finally, the drive gear and bearings will also be designed from scratch based on the forces
required to transmit the load at the prescribed 35 feet per minute.
Our approach to analyzing the assembly was to work backwards. From defining our load and
desired travel speed, we calculated the force required to move the load. This force calculation
was the foundation in designing the components of the conveyor.
Force Analysis
The first step in our design was to use the given information to find the forces transmitted
throughout the system. By analyzing the 3000 pound load acting downward on the chains and
using the coefficient of kinetic friction of the UHMW block, we were able to determine the force
needed in the chains to move the load at 35 feet per minute. The 3000 pound load is applied
evenly between the 2 chains at 1500 pounds per chain. Each 1500 pound load is distributed
over 4 feet of chain. The detailed calculations of our force analysis are shown below:
๐๐ฉ = (1500 ๐๐
4 ๐๐ก) โ (
1 ๐๐ก
12 ๐๐) = ๐๐. ๐๐
๐๐
๐๐ (Eq. 1)
๐๐ = (4๐๐
๐๐ก) โ (
1 ๐๐ก
12 ๐๐) = . ๐๐๐
๐๐
๐๐ - Data from ANSI Chain # 50 (Eq. 2)
๐๐ = ๐ฟ๐๐๐ + ๐ถโ๐๐๐ ๐ค๐๐๐โ๐ก = 31.25๐๐
๐๐+ .33
๐๐
๐๐= ๐๐. ๐๐
๐๐
๐๐ (Eq. 3)
๐๐ = (31.58๐๐
๐๐) โ (4๐๐ก โ (
12 ๐๐
1 ๐๐ก)) + (
.33 ๐๐
๐๐) โ (6 ๐๐ก โ (
12 ๐๐
1 ๐๐ก)) = ๐๐๐๐. ๐ ๐๐ (Eq. 4)
๐๐ = ๐. ๐๐ ๐๐๐ ๐๐ป๐๐
๐๐ = WT = ๐๐๐๐. ๐ ๐๐
๐ญ๐ = ๐๐๐ = (0.25) โ (1539.6 ๐๐) = ๐๐๐. ๐ ๐๐ (Eq. 5)
๐๐ = (35๐๐ก
๐๐๐) โ (
1 ๐๐๐
60 ๐ ) โ (
12 ๐๐
1 ๐๐ก) = 7
๐๐
๐ (Eq.6)
โ 10 ๐๐ก โ (12 ๐๐
1 ๐๐ก) = 120 ๐๐ โ (
1
(7๐๐
๐ )) = ๐๐. ๐๐ ๐๐๐๐๐๐ ๐ ๐๐ ๐๐๐๐๐๐ ๐๐ ๐๐๐๐
โ ๐น = ๐๐ โ ๐น๐ถ โ 384.9 ๐๐๐ = [3000 ๐๐+(
.33 ๐๐
๐๐)โ(10๐๐ก)โ(
12 ๐๐
1 ๐๐ก)]
32.2 ๐๐ก
๐ 2
โ (๐) (Eq. 7.a)
๐น๐ถ โ 384.9 ๐๐ = 94.41 ๐ ๐๐ข๐๐ โ (๐) โ ๐ = ๐๐
๐๐ก (Eq. 7.b)
๐น๐ถ โ 384.9 ๐๐ = 94.41 ๐ ๐๐ข๐๐ โ (๐๐
๐๐ก) โ โซ (๐น๐ถ โ 384.9 ๐๐
๐ก
0)๐๐ก = โซ (94.41 ๐ ๐๐ข๐๐ )๐๐
๐
0 (Eq. 7.c)
โซ (๐น๐ถ โ 384.9 ๐๐17.14 ๐
0)๐๐ก = โซ (94.41 ๐ ๐๐ข๐๐ )๐๐
7๐๐
๐ 0
(Eq. 7.d)
โ ๐น๐ถ(17.14 ๐ ) โ 384.9 ๐๐(17.14 ๐ ) = 94.41 ๐ ๐๐ข๐๐ โ 7๐๐
๐ (Eq. 7.e)
โ ๐น๐ถ =1
17.14 ๐ โ [(660.87
๐ ๐๐ข๐โ๐๐
๐ ) โ (
๐๐ก
12 ๐๐) + 6597.186 ๐๐]=388.11 lbs (Eq. 7.f)
โ ๐น๐ = 388.113 ๐๐ ๐๐๐ ๐ ๐๐๐ โ 2 ๐ ๐๐๐๐ = 776.226 ๐๐ (Eq. 8)
Sprocket Design
Once the force to move the maximum load of 3000 pounds was calculated, we were able to use
this information along with idealization of the chains to calculate the torque and angular velocity
required at the sprockets and the shaft. The chains were idealized to have no stretch and it was
assumed that any force on the chains was transmitted by the sprockets. We began with an ANSI
No. 50 chain, which has a pitch of 5/8 inch.
By using the properties of the ANSI No. 50 chain, the AGMA equations and the assumption that
each sprocket has 40 teeth we were able to calculate a pitch diameter of the sprockets. The bore
size was assumed to be 1.75 inches. This starting value would determine the diameter of the shaft
and its feasibility is found later in the calculations. The calculation for finding the pitch diameter
of a sprocket was found by using the following equation:
๐๐๐ 1 = ๐๐๐ 2 = ๐โ๐๐๐ ๐๐๐ก๐โ
sin (180
๐)
=(
5
8๐๐)
sin(180
40)
= 7.966 ๐๐ (Eq. 8)
Based on this pitch diameter, the assumed number of teeth and bore size, we chose part
#6236K744 from McMaster-Carr, a large material and component supplier. From this pitch
diameter and our calculated total force, we were able to find the torque required at each sprocket
to move the load at 35 feet per minute.
๐๐ ๐1 = ๐๐ ๐2 = ๐น๐ (๐๐
2) = (244.268 ๐๐) (
7.966 ๐๐
2) = 972.9 ๐๐ โ ๐๐ (
1 ๐๐ก
12 ๐๐) = 81.076 ๐๐๐๐ก (Eq.9)
Thus,
Tsp1 = Tsp2 = 81.076 lb * ft
๐ป๐๐๐๐๐ = ๐ป๐๐๐ + ๐ป๐๐๐ = ๐๐๐. ๐๐ ๐ฅ๐๐๐ญ (Eq.10)
The rotational speed (angular velocity) of the sprockets is easily found through basic dynamic
relationships. The chain must move at 35 feet per minute, and this speed corresponds with the
tangential velocity of the sprockets at the pitch point or pitch diameter distance. We calculated
the angular velocity of the sprocket and used the same value for the angular velocity of the shaft.
Since both components are connected to each other, they share the same rotational speed.
๐๐ = (๐๐๐ 1
2) โ ๐๐ ๐ โ ๐๐ ๐ = (
2โ35 ๐๐ก
7.966 ๐๐โ๐๐๐) โ (
12 ๐๐
๐๐ก) (
1 ๐๐๐ฃ
2๐ ๐๐๐) = 16.78 ๐ ๐๐ =ฬ 17 ๐ ๐๐ (Eq.11)
Thus; ๐๐ฌ๐ฉ = ๐๐ฌ๐ก๐๐๐ญ = ๐๐ ๐ซ๐ฉ๐ฆ
Gear Design
In designing our gear and pinion mesh, we referred heavily to Machine Design โ An Integrated
Approach by Dr. Robert L. Norton. Our starting point was to define the pressure angle for the
gear teeth. We chose a 20 degree pressure angle based on the fact that is the most widely used
pressure angle in industry. Table 12-5 (Norton) shows the minimum number of pinion teeth to
avoid interference between a full depth pinion and gear with a 20 degree pressure angle. We
wanted to avoid interference so that we could eliminate the need to undercut the teeth. This saves
time in drafting and manufacturing. We chose the range of 16 teeth minimum for the pinion to
101 teeth maximum for the gear; this range would give us more gear ratio options. We chose a
standard full depth gear teeth because the equations for the geometry of this gear with a 20
degree pressure angle are readily available in Table 12-1 (Norton). Other standard tables have
also been established for several factors, including the AGMA bending geometry factor J and I.
We referred to these tables to narrow down our gear ratios. In the end, we chose the number of
teeth to correspond with a pre-defined J and I values from the AGMA tables to further ease
calculations. Because we also know that the motor will be at a lower torque and higher speed
than the shaft requires, we estimated a required gear ratio of at least 2 to 1.
Referring to the AGMA tables for bending and surface geometry factors and our teeth range to
avoid interference, we were able to choose a desirable number of teeth for both the gear and
pinion. We decided on 21 teeth for the pinion and 55 teeth for the gear, giving us a 2.62:1 gear
ratio. We assumed a pinion pitch diameter of 3 inches. This would roughly make the gear pitch
diameter very close to the pitch diameter of the sprockets. Once these factors were established
we could now find the geometry for the gear and pinion.
Gear and Pinion Dimensional Calculations
One of the most useful properties for calculating the geometry of a gear is the diametral pitch.
Once this diametral pitch is defined, all remaining gear and pinion geometry can be found based
on this value. To find the diametral pitch, we used our assumed starting pinion pitch diameter
and number of teeth on the pinion.
๐๐๐ =๐๐ก๐
๐๐=
21 ๐ก๐๐๐กโ
3 ๐๐= 7
๐ก๐๐๐กโ
๐๐ (Eq.12)
In order for gears to mesh properly, they must have the same pitch angle and diametral pitch.
Using this information with our assumed 55 teeth on the gear, the pitch diameter of the gear can
be calculated.
๐๐๐ = 7๐ก๐๐๐กโ
๐๐=
๐๐ก๐
๐๐=
55 ๐ก๐๐๐กโ
๐๐โ ๐๐ =
55 ๐ก๐๐๐กโ
(7๐ก๐๐๐กโ
๐๐)
= 7.857 ๐๐ =ฬ 7.86 ๐๐ (Eq.13)
With the known pitch diameters and diametral pitches of both gear and pinion, we can now
calculate the remaining dimensions of both the gear and pinion.
For quick and simple calculations for the gear geometries, we created a Microsoft Excel
spreadsheet (See appendix) that included all of the formulas for calculating gear geometry from
Table 12-1 (Norton). These values apply to both the gear and pinion in order to ensure the gears
mesh and transmit power properly. The calculated gear geometries are listed below:
Pc = 0.446 in
Pb = 0.4215 in
Pd = 7 teeth/in
a = 0.1429 in
b = 0.1786 in
๐ซ๐พ=0.2857 in
๐ซ๐พ๐ = 0.3214 in
๐๐๐ = 0.2244 in
๐๐ = 0.0429 in
๐ช๐๐๐๐ = 0.0357 in
๐พ๐๐๐๐๐ = 0.0357 in
๐ช๐๐= 0.0500 in
Another critical property needed for gear design is the face width. Face width (๐๐ ) is roughly
expressed as a range function of diametral pitch (8/Pd < ๐๐ < 16/Pd) (Norton). For our design, we
used the following to find a starting face width:
8
๐๐< ๐๐ <
16
๐๐ โ
87 ๐ก๐๐๐กโ
๐๐
< ๐๐ < 16
7 ๐ก๐๐๐กโ
๐๐
โ ๐. ๐๐ ๐๐
๐๐๐๐๐< ๐๐ < 2.29
๐๐
๐๐๐๐๐ (Eq. 14)
We took the average of this range and defined our starting face width (๐๐) of both gear and
pinion to be 1.714 inches.
๐พ๐ = 1.714 in
At this point, we have calculated all of the dimensions of the gear and pinion. We can now find
the length of action of the gear mesh. Note that โ๐โ represents the addendum length and is the same
for both the gear and the pinion.
๐ = โ[(๐๐ + ๐)2
โ (๐๐๐๐๐ โ )2
] + โ[(๐๐ + ๐)2
โ (๐๐๐๐๐ โ )2
] โ ๐ถ๐ ๐๐(โ ) (Eq.15)
๐ = โ[(1.5 + .1429)2 โ (1.5๐๐๐ (20))2] + โ[(3.93 + .1429)2 โ (3.93cos (20))2] โ 5.43 sin(20)
Thus, Z = .7037
With the length of action, we can calculate the contact ratio (mp) and ensure that it agrees with
typical values for spur gears. According to Norton, the contact ratio is calculated using the length
of action (Z), the diametral pitch (Pd), and the pressure angle (ร).
๐๐ =๐๐๐
๐cos (โ )โ ๐๐ =
7โ.7037
๐cos (20)= 1.67 Thus; mp = 1.67 (Eq.16)
This contact ratio falls within the acceptable range for spur gear sets of 1.4 to 2 (Norton). The
gear contact ratio ensures that more than one tooth of the gear carries the load at any given time.
Although true in theory, in reality, there will be times when one tooth carries the entire load. This
occurs at the center of the mesh where the load is applied at a lower position on the tooth. This
position is referred to as the highest point of single tooth contact (HPSTC) (Norton). We
assumed this information when we chose our number of teeth based on the geometrical bending
factors (J and I). This assumption will provide a more accurate stress analysis.
Gear and Pinion Stress Analysis
The first step in beginning a stress analysis is to gather the required values for the calculations.
We must first find the number of cycles for the gear and pinion. We already found that the gear
and shaft will spin at 17 revolutions per minute and we have the gear ratio, thus we can find the
rotational speed of the pinion and in turn the number of cycles. After finding the rotational speed
of the pinion, we will find the number of cycles for 5 years at 8 hour days and 250 working days
in each year. We decided this was a reasonable period of working time for such a system.
๐๐ = ๐๐ โ ๐๐๐๐ ๐๐๐ก๐๐ โ ๐๐ = 17 ๐๐๐ โ (2.62) = 44.54 =ฬ 45 ๐๐๐ (Eq.17)
๐๐๐ฆ๐โ๐ = 45๐๐๐ (60๐๐๐
โ๐) (
8โ๐
๐๐๐ฆ) (
250 ๐๐๐ฆ
1 ๐ฆ๐) (
5 ๐ฆ๐
1) = 2.7๐ฅ107 ๐๐ฆ๐๐๐๐ ๐๐ 5 ๐ฆ๐๐๐๐ (Eq.18)
๐๐๐ฆ๐โ๐ = 17๐๐๐ (60๐๐๐
โ๐) (
8โ๐
๐๐๐ฆ) (
250 ๐๐๐ฆ
1 ๐ฆ๐) (
5 ๐ฆ๐
1) = 1.02๐ฅ107 ๐๐ฆ๐๐๐๐ ๐๐ 5 ๐ฆ๐๐๐๐ (Eq.19)
Thus;
ฯp = 45 rpm
๐ต๐๐๐โ๐ =2.7x107 cycles in 5 years
๐ต๐๐๐โ๐ =1.02x107 cycles in 5 years
We assumed a gear quality value of Qv = 5, based on typical values found in Table 12-6 for
mining conveyor systems (Norton). With this assumption and the data calculated so far we can
now begin the calculation of various correction factors for the stress analysis of the gear and
pinion.
Gear and Pinion Bending Stress Calculations
The bending stress for a spur gear is defined by the AGMA Bending Stress Equation (Norton).
The equation is as follows:
๐๐ = (๐๐ก๐๐
๐๐๐ฝ) (
๐พ๐๐พ๐
๐พ๐ฃ) ๐พ๐ ๐พ๐ต๐พ๐ผ (Eq.20)
The Wt variable represents the tangential force on the tooth of the pinion and gear; other known
values include the face width and diametral pitch. The remaining variables used in this equation
are defined and justified below for both the gear and the pinion:
Tangential Forces (Wt) โ Same for both pinion and gear
This force is equal and opposite for both gear and pinion because they are in mesh. Because we
already found the required torque on the shaft we can use this value and the pitch diameter of the
gear to find the tangential force that acts on the gear and pinion teeth.
๐๐ก =๐๐
๐๐=
162.15 ๐๐๐๐ก
3.93 ๐๐(
12 ๐๐
๐๐ก) = 495.11 ๐๐๐ (Eq.21)
๐พ๐ = ๐๐๐. ๐๐ ๐๐๐
Bending Strength Geometry Factor (J) โ Table 12-9 (Norton)
Jg = 0.40 Jp = 0.34
Dynamic Factor (Kv) - Same for both pinion and gear
For gears with a Qv factor less than or equal to 5 this factor is defined as:
๐พ๐ฃ =50
50+โ๐๐ก in our case, the Vt term (pitch line velocity) is equal to 35 ft/min for both gears.
๐พ๐ฃ =50
50+โ35๐๐ก/๐๐๐= 0.89419 (Eq.22)
๐ฒ๐ = ๐. ๐๐๐๐๐
Load Distribution Factor (Km) - Table 12-6 (Norton) - Same for both pinion and gear
For face widths less than 2 inches, a Km factor is equal to 1.6.
Km = 1.6
Rim Thickness Factor (KB = 1) - Same for both pinion and gear
For solid-disk gears this value is always set to 1(Norton).
Idler Factor (KI = 1) - Same for both pinion and gear
This value is always set to 1 for a non-idler gear (Norton).
Application Factor (Ka) โ Table 12-17 (Norton) - Same for both pinion and gear
In our situation, we will be using an electric motor as the driving machine, which transmits a
uniform load. Torque at start up may cause slight shock; therefore the decision was made to
overdesign the component with a Ka value of 1.25.
Ka = 1.25
Size Factor (Ks) - Same for both pinion and gear
In order to be conservative we chose a value of 1.25, typically this would be set to 1 unless we
wanted to account for special situations (Norton)
Ks = 1.25
Now that all of the necessary factors have been found, the bending stress for the gear and pinion
teeth can be calculated. We employed Microsoft excel to speed these calculations as well as
those calculations for the surface stresses. The obtained values for bending stress on the gear and
pinion are found as:
๐๐๐ = ๐๐. ๐ ๐๐๐ ๐๐๐ = ๐๐. ๐๐ ๐๐๐
Gear and Pinion Surface Stress Calculations
Because the gear teeth experience not only bending stresses but also surface stresses or pitting
stresses, we must also take this into account for our design. The AGMA defines the pitting
resistance formula as follows (Norton):
๐๐ = ๐ถ๐โ(๐๐ก
๐๐๐ผ๐๐) (
๐ถ๐๐ถ๐
๐ถ๐ฃ) ๐ถ๐ ๐ถ๐ (Eq.23)
This equation is only applied to the pinion because it experience more revolutions and thus will
fail before the gear. The โCโ variables used in this equation correspond to the โKโ variables in
the bending stress equation with exception of (Cp), and (Cf). The value of (๐๐) is the pitch
diameter of the pinion, for reasons mentioned above.
Surface Geometry Factor (I)
The variable (I) represents the surface geometry factor, which takes into account the radii of
curvature of the gear teeth and pressure angle. This value is readily available from AGMA 908-
B89.
I = .102
Elastic Coefficient (Cp)
This value accounts for differences in tooth material and is found by (Norton):
๐ถ๐ = โ(1
๐(1โ๐ฃ๐
2
๐ธ๐)+(
1โ๐ฃ๐2
๐ธ๐)
) = โ(1
๐(1โ0.282
30๐ฅ106 )+(1โ0.282
30๐ฅ106 )) = 2276.72 (Eq.24)
Cp= 2276.72
Surface Finish Factor (Cf)
This is usually set to 1 for gears made with standard manufacturing methods (Norton).
Now that all of the factors have been defined, we may calculate the pitting resistance or surface
stress of the pinion.
๐๐๐ = 2276.72โ(495.11
1.714โ.102โ3) (
1.25โ1.6
.89419) 1.25 โ 1 (Eq.25)
๐๐ = ๐๐๐. ๐ ๐๐๐
Now that the bending and surface stresses have been calculated, we may choose a gear and
pinion material and determine the actual bending and surface strengths of that material. Once
determined, a safety factor will be calculated and re-evaluation made if required.
Bending Fatigue Strengths for Gear Materials
In order to accurately design a gear set, one must ensure an acceptable safety factor. In order to
check the safety of our design we must find the associated AGMA bending fatigue strengths
associated with the gear materials. The AGMA publishes values; however they must be corrected
to account for the situational use of a particular gear set. This corrected Bending fatigue
strength(๐๐๐) is found by:
๐๐๐ = ๐พ๐ฟ
๐พ๐๐พ๐ ๐๐๐
โฒ (Eq.26)
where ๐๐๐โฒ is the uncorrected bending fatigue strength given by AGMA. This is found for Grade
2 gear materials by:
๐๐๐โฒ =6235 + 174*HB -0.126 HB2 (Eq.27)
Grade 2 implies a premium quality by the AGMA, ensuring a good homogenous microstructure
and purity of the metal used to make the gear. HB designates the Brinell hardness of the metal to
be used. We first decided on a steel that had HB = 300, this made our ๐บ๐๐โฒ = 47,095 psi. Other
factors in this equation are defined and justified below:
Life Factor (KL)
This factor is based on the life required by the part. This will be different for the pinion and gear
because they have a different number of cycles (N). An acceptable calculation of this factor for
commercial uses is given by Figure 12-24 as (See Norton):
๐พ๐ฟ = 1.3558(๐)โ0.0178 (Eq.28)
๐ฒ๐ณ๐ = ๐. ๐๐๐๐(๐. ๐๐๐๐๐)โ๐.๐๐๐๐ = . ๐๐๐๐
๐ฒ๐ณ๐ = ๐. ๐๐๐๐(๐. ๐๐๐๐๐๐)โ๐.๐๐๐๐ = ๐. ๐๐๐
Temperature Factor (KT =1) โ Same for both gear and pinion
For temperatures less than 250ยฐF, this value is set to 1(Norton). We are designing this for use at
no more than this temperature.
Reliability Factor (KR = 1.50) โ Same for both gear and pinion (Table 12-19)(Norton)
We initially chose a reliability of 99.99%, which corresponds to a value of KR = 1.50.
Now that all factors have been identified we can calculate the actual bending strength of the gear
and pinion.
๐๐๐๐=
๐พ๐ฟ๐
๐พ๐๐พ๐ ๐๐๐
โฒ =1.017
1โ1.5โ 47,095 = 31.93 ๐๐ ๐ โ ๐บ๐๐๐
= ๐๐. ๐๐ ๐๐๐ (Eq.26a)
๐๐๐๐=
๐พ๐ฟ๐
๐พ๐๐พ๐ ๐๐๐
โฒ =.9998
1โ1.5โ 47,095 = 31.39 ๐๐ ๐ โ ๐บ๐๐๐
= ๐๐. ๐๐ ๐๐๐ (Eq.26b)
Surface -Fatigue Strengths for Gear Materials
We previously calculated the surface-fatigue stress for the pinion and now must calculate the
actual strength of the pinion material in order to have a comparative analysis and formulate a
safety factor. This calculation will not be done for the gear because it relies on the number of
cycles of the gear or pinion being analyzed; in our case the pinion has significantly more cycles
than the gear and thus will fail before the gear does. The AGMA publishes surface-fatigue
strength values; however, like the bending strength values they must be corrected to account for
operational situations. The AGMA surface- fatigue strength calculation is given by:
๐๐๐๐=
๐ถ๐ฟ๐ถ๐ป
๐ถ๐๐ถ๐ ๐๐๐๐
โฒ (Eq.29)
where ๐๐๐โฒ is the uncorrected surface-fatigue strength given by AGMA. This is found for Grade 2
gear materials by: ๐๐๐โฒ =27000 +364HB. (Eq.30)
HB designates the Brinell hardness of the metal to be used. We first decided on a steel that had
HB = 300, this made our ๐บ๐๐โฒ = 136200 psi. The factors CT and CR are identical in value to the K
values with the same subscript. Other factors in this equation are defined and justified below:
Surface-Life Factor (CL)
The publish AGMA values for surface โ fatigue strength are based on 1 x107 cycles. So this
factor allows for correction based on the actual number of cycles of the pinion (2.7 x107). From
Table 12-26 (Norton) and classifying our design for commercial use, we can calculate this value
using the equation: ๐ถ๐ฟ = 1.448๐โ0.023 (Eq.31)
This makes our CL = .977
Hardness Ratio Factor (CH)
This factor takes into account the difference in hardness of the gear and pinion teeth, if one
exists. Because we are making both gear and pinion out of the same material with the same heat
treatment, this value is set to CH = 1 (Norton โ page 728).
Now that the factors have been identified and calculated, we may calculate the actual surface-
fatigue strength of the pinion.
๐๐๐๐=
๐ถ๐ฟ๐ถ๐ป
๐ถ๐๐ถ๐ ๐๐๐๐
โฒ = .977โ1
1โ1.5โ 136200 = = ๐๐. ๐ ๐๐๐ (Eq.29a)
Gear and Pinion Safety Factor Calculation
Now that the bending and surface-fatigue stresses and strengths have been computed, we can
calculate the safety factor for both the gear and pinion. These are basic calculations found by:
๐๐ = ๐ ๐๐
๐๐ and ๐๐ =
๐ ๐๐
๐๐ (Eq.32 and 33)
The safety factors for the pinion are:
๐๐๐ = 31.39 ๐๐ ๐
16.6 ๐๐ ๐= ๐. ๐๐ ๐๐๐ =
88.7 ๐๐ ๐
116.9 ๐๐ ๐=. ๐๐๐
The safety factor for the gear is:
๐๐๐ = 31.93 ๐๐ ๐
14.14 ๐๐ ๐= ๐. ๐๐
It is clear from the safety factors that our pinion will fail due to surface-fatigue stresses. There
are a number of values that we could change to increase the resistance to these stresses. These
values include: the reliability, the material hardness, the number of cycles and face width. In
order to speed the calculation process for changes we might make, we created a Microsoft Excel
spreadsheet (See appendix) that did all calculations based on the input parameters.
Gear and Pinion Material Selection and Final Values
After much iteration, we decided to reduce the reliability of our gear and pinion from 99.99 % to
99% and make the components from the following material, keeping the initial dimensions the
same, including the face width.
Gear Material (Table 12-20 Norton)
o AGMA Grade 2 (Premium Grade)
o Class A5 Steel
o Carburized and case hardened
o Brinell Hardness = 710 (corresponds to 64HRC)
o Qv = 5
Once this material was chosen, the calculations were easily repeated in the Excel spreadsheet and
the safety factors are as follows:
Gear
o Bending Safety Factor (๐๐๐) = 4.76
Pinion
o Bending Safety Factor (๐๐๐) = 3.98
o Surface Fatigue Safety Factor(๐๐๐) = 2.38
These safety factors are more than adequate for our design and from this point we moved on to
designing the shaft and bearings.
Shaft Design for Conveyor System In order to design an acceptable shaft to transmit power from the gear to the sprockets some
assumptions needed to be made. We determined earlier that the chain sprockets have an inside
bore of 1.75 inches. To accommodate these sprockets we decided to choose a shaft that had an
outside diameter of 1.75 inches. The length of the shaft was determined by close inspection of
existing conveyor systems and standard dimensions of the pallets that the conveyor would move.
The width of a wooden pallet is standardized at 48 inches. We decided on roughly a 2 to 3 inch
overhang on the outside of each sprocket. This allows the main supports of the pallet to be
directly over the chain. The remaining distances were found through iteration of the design and
placement of the different components.
We already had the dimensions of the sprockets and the width of their hubs; we also found the
dimensions of the gear. The bearings dimensions were found from a standard manufacturerโs
catalog. This particular manufacturer had one pillow block bearing housing with a very wide
range of dynamic load ratings available for the inserted bearing. So we used this housingโs
dimensions to finalize our locations and would choose our actual bearing based on the dynamic
load rating later in our analysis. Referring to Norton and Mott we tried to adhere to common
design guidelines for locating our power transmitting devices that would be attached to the shaft.
Bearings were placed close to these power transmitting devices to minimize bending forces. The
shaft material was initially chosen to be SAE/AISI 1010 Cold rolled steel. The final dimensions
of the shaft and location of all of the power transmitting elements and bearings is shown in the
appendix.
Once we established the tentative location of all of the elements that will be attached to the shaft
we were able to develop a loading diagram as well as the corresponding shear and moment
diagrams. We used Autodeskยฎ Inventorยฎ to confirm our hand calculations and model the shaft
and location of power transmitting components. Inventorยฎ has a very useful feature called
โDesign Acceleratorยฉโ which allows the user to input the force and support bearing locations on
the shaft. With this information and several dimensions required from the user, Inventorยฎ
automatically calculates the shear and moment diagrams and maximum values for: Shear Stress,
Bending Stress, Torsional Stress and maximum shaft deflection. One can even define the
material to be used in creating the shaft, as well as the modulus of elasticity, Poissonโs ratio and
other metallurgical characteristics of the component. The plots of the maximum forces, moments
and stresses generated by this software are shown in the appendix.
We calculated all of these values by hand and compared them with the computer generated
model. They agreed fairly well, discrepancies were attributed to rounding differences. The hand
calculations for the bending, shear and torsional stresses are calculated below:
Maximum Bending Stress
๐๐๐๐๐ฅ=
๐๐
๐ผ๐ =
32๐
๐๐๐ 3 =
32(315.17 ๐๐โ๐๐ก)
๐(1.75 ๐๐)3 โ (12๐๐
๐๐ก) = 7188.1 ๐๐ ๐ (Eq.34)
Maximum Shear Stress
๐๐๐๐๐ฅ=
๐๐๐๐ฅ
๐ด๐ โ๐๐๐=
526.88 ๐๐
๐(1.75 ๐๐)2
4
= 219.05 ๐๐ ๐ (Eq.35)
Maximum Torsional Stress
๐๐ก๐๐๐ ๐๐๐ =๐๐ โ๐๐๐ก๐
๐ฝ๐ =
(162.15 ๐๐โ๐๐ก)(1.75 ๐๐
2)
.92 ๐๐4โ
12 ๐๐
๐๐ก= 1850.625 ๐๐ ๐ (Eq.36)
With this information and the assumption of SAE/AISI 1010 Cold rolled steel we were able to
calculate several safety factors for bending, shear and torsional effects on the shaft. It is
important to note that we assumed a constant loading condition; thus, eliminating any alternating
or mean stresses. This idealization would prove to be feasible after we found our safety factors
for bending, shear and torsional stresses to be relatively high. After the actual stresses were
defined we used the Maximum Shear Stress Theory and Distortion Energy Theory to calculate
the safety factor for the maximum shear stress and maximum bending stress respectively. The
information related to the SAE 1010 Cold Rolled Steel is:
o Sy = 44,000 psi
o Sut = 53,000 psi
o Se = 26,500 psi
o Density = .284 lb/in3
o Poissonโs Ratio = 0.3
o Youngโs Modulus = 30x106 psi
The calculations for the safety factors of the shaft under bending and shear are as follows:
Maximum Shear Stress Theory for Static Failure
๐๐๐๐ฅ = โ(๐๐ฅโ๐๐ฆ
2)
2
+ ๐๐ฅ๐ฆ2 = โ(
7188.1๐๐ ๐โ0
2)
2
+ (1850.625 ๐๐ ๐)2 = 4042.525 ๐๐ ๐
(Eq.37)
๐๐๐๐๐ =0.5๐๐ฆ
๐๐๐๐ฅ= 0.5โ44,000 ๐๐ ๐
4042.525 ๐๐ ๐= 5.442 (Eq.38)
Distortion Energy /Von Mises-Hencky Theory for Ductile Static Loading Failures
๐1 = ๐๐ฅโ๐๐ง
2+ ๐๐๐๐ฅ =
7188.1 ๐๐ ๐โ0
2+ 4042.525 ๐๐ ๐ = 7636.575 ๐๐ ๐ (Eq.39)
๐3 = ๐๐ฅ+๐๐ง
2โ ๐๐๐๐ฅ =
7188.1 ๐๐ ๐+0
2โ 4042.525 ๐๐ ๐ = โ448.475 ๐๐ ๐ (Eq.40)
๐โฒ = โ๐12 + ๐3
2 โ ๐1๐3 = โ(7636.57 ๐๐ ๐)2 + (448.47 ๐๐ ๐)2 โ (7636.57)(โ448.47)
= 7870.39 ๐๐ ๐ (Eq.41)
๐๐ท๐ธ = ๐๐ฆ
๐โฒ = 44,000 ๐๐ ๐
7870.39 ๐๐ ๐= 5.59 (Eq.42)
Using the fact that the maximum shear theory is more conservative, our safety factor comes out
to be 5.442. This is more than reasonable for the level of precision required in this type of
machine.
Stress calculations on Shaft Keys
We will be attaching the gear and sprockets to the shaft via profile parallel keys and keyways.
The keys will be made from the same SAE 1010 Steel; however, the ultimate strength will be
used to define the safety factor. This will be done such that the key will fracture and prevent the
other components from spinning in the case of failure, protecting the remaining system from
damage.
From Table 10-2 (Norton) for a shaft having a diameter equal to 1.75 inches the suggested key
width is 0.375 inches. From Table 10-3 (Norton), the suggested height for a .375 inch by 1 inch
long key is .437 inches. The length of our keys will be greater than or equal to the width of the
element they are connecting to the shaft (i.e. โ gear width = 1.714, key length = 1.75 inch.).
The force experienced by the shaft at the different locations along the shaft is equal to the torque
at that location over the radius of the shaft at that location:
๐น๐ โ๐๐๐ก @๐๐๐๐ =๐
๐=
162.15 ๐๐๐๐ก1.75 ๐๐
2
โ (12๐๐
๐๐ก) = 2223.17 ๐๐๐ (Eq.43)
๐น๐ โ๐๐๐ก @๐ ๐๐๐๐๐๐๐ก 1 =๐
๐=
81.075 ๐๐๐๐ก1.75 ๐๐
2
โ (12๐๐
๐๐ก) = 1111.89 ๐๐๐ (Eq.44)
๐น๐ โ๐๐๐ก @๐ ๐๐๐๐๐๐๐ก 2 =๐
๐=
81.075 ๐๐๐๐ก1.75 ๐๐
2
โ (12๐๐
๐๐ก) = 1111.89 ๐๐๐ (Eq.45)
From these forces, the corresponding shear stresses can be computed for the keys at these
locations. The forces on each sprocket are equal, thus simplifying the calculations. Once the
actual shear stresses are calculated, the Von-Mises equivalent stresses can be computed and a
safety factor developed. The width of the sprocket and key interaction is 1.25 inches, the width
of the gear and key interaction is 1.714 inches. All keys are 0.375 inches in width.
๐๐๐๐ฆ@๐๐๐๐ =๐น
๐ด๐ โ๐๐๐=
2223.17 ๐๐๐
(0.375 ๐๐)(1.714 ๐๐)= 3458.84 ๐๐ ๐ (Eq.46)
๐๐๐๐ฆ@๐ ๐๐๐๐๐๐๐ก๐ =๐น
๐ด๐ โ๐๐๐=
1111.89 ๐๐๐
(0.375 ๐๐)(1.25 ๐๐)= 2372.03 ๐๐ ๐ (Eq.47)
*Note that the shear stress on the key for the sprockets is equal.
๐โฒ๐๐๐ฆ@๐๐๐๐ = โ3๐๐ฅ๐ฆ
2 = โ3(3458.84๐๐ ๐)2 = 5990.89 ๐๐ ๐ (Eq.48)
๐โฒ๐๐๐ฆ@๐ ๐๐๐๐๐๐๐ก๐ = โ3๐๐ฅ๐ฆ
2 = โ3(2372.03๐๐ ๐)2 = 4108.47 ๐๐ ๐ (Eq.49)
*Note that the von-Mises equivalent stresses on the key for the sprockets is equal.
Calculation of the safety factors for the gear and sprocket keys. (Eq 6.18e pg 368 Norton)
๐๐ =1
๐โฒ๐๐๐
+๐โฒ๐๐๐ข๐ก
๐คโ๐๐๐ ๐โฒ๐ = ๐โฒ๐ (Eq.50)
๐๐๐๐๐ฆ @ ๐๐๐๐=
15990.89๐๐ ๐
26500๐๐ ๐+
5990.89๐๐ ๐
53000๐๐ ๐
= 2.95 (Eq.51)
๐๐๐๐๐ฆ @ ๐ ๐๐๐๐๐๐๐ก๐ =
14108.47๐๐ ๐
26500๐๐ ๐+
4108.47๐๐ ๐
53000๐๐ ๐
= 4.30 (Eq.52)
Bearing Calculations and Design
In order to find an acceptable bearing for our application we first needed to find the forces acting
at the bearings. These forces are reactions to the radial and tangential components of the forces
acting at the gear and sprockets on the shaft. The locations of the bearings, gear and sprockets is
shown in the torque diagram in the appendix. We assumed no axial movement of the shaft
because the shaft is locked in position at the bearings by clamp collars. Also note that the
sprockets have no y-component of force because the only force experienced is that of the chain
acting in the x direction. Calculations of the forces at the gear and sprockets are shown below:
๐น๐๐ฅ =๐๐
๐๐=
162.15 ๐๐๐๐ก
(7.86 ๐๐
2)
โ (12 ๐๐
๐๐ก) = 495.11 ๐๐๐ (Positive x-direction)
๐น๐๐ฆ = ๐น๐๐ฅ๐ก๐๐(โ ) = 495.11๐๐ โ tan(20ยฐ) = 180.21 ๐๐๐ (Positive y-direction)
๐น๐ 1๐ฅ = ๐น๐๐ฅ โ๐๐ 1
๐๐ 1= 495.11๐๐๐ โ
81.075 ๐๐๐๐ก
(7.966 ๐๐
2)
โ (12 ๐๐
๐๐ก) = โ250.85 ๐๐๐ (Negative x-direction)
๐น๐ 1๐ฅ = 0
๐น๐ 2๐ฅ = ๐น๐ 1๐ฅ โ๐๐ 2
๐๐ 2= 250.85 ๐๐๐ โ
81.075 ๐๐๐๐ก
(7.966 ๐๐
2)
โ (12 ๐๐
๐๐ก) = โ6.58 ๐๐๐ (Negative x-direction)
๐น๐ 2๐ฅ = 0
It should be noted that the gear supplies a torque and the first sprocket removes roughly half of
this torque to drive the chain. The remaining torque is transmitted to the second sprocket in order
to drive the second chain. A minimal amount of force remains after the second sprocket. The
bearing reaction forces are then found by taking the sum of the forces about the shaft and the
sum of the moments about one of the bearings. The magnitude of the total bearing forces is
shown below:
|๐ ๐ต1| = 379.15 ๐๐๐
|๐ ๐ต2| = 85.62 ๐๐๐
From these forces, specifically the higher valued force at bearing one, we can calculate the
required dynamic load rating of the bearings to be used.
The bearing housing we assumed earlier in our design is manufactured by Rexnord Inc. We
referred to their lengthy product catalog and found they had a step by step process for
determining the dynamic load rating based on the materials and tolerances they use in their
bearing design. The number of cycles for the shaft is based on the 17 rpm required to maintain a
35 feet per minute velocity of the pallet. We designed the gear to withstand 5 years of service,
and the total number of cycles in this period was calculated previously to be 1.02 x 107 cycles.
Using this value and the instructions from Rexnord, we were able to choose a bearing. The
procedure provided by Rexnord is shown below:
1. Determine the number of hours the bearing is to be used over a 10 year period.
a. โ๐๐ข๐๐ = 8 โ๐
๐๐๐ฆโ
250 ๐๐๐ฆ
๐ฆ๐โ 10๐ฆ๐ = 20000 โ๐๐
2. Refer to Rexnord Table that correlates the number of hours in 10 years, and the RPM to
the C/P ratio. Choose a corresponding C/P ratio.
a. The lowest RPM that Rexnord has values for was 50 RPM, where we only
required 17 RPM. We chose the corresponding C/P ratio for the 50 RPM
b. The C/P ratio was shown to be 3.42
3. Use the C/P ratio from the table and the actual applied load (P) to determine the required
Dynamic Load Rating (C).
๐ถ
๐= 3.42 โ
๐ถ
379.15= 3.42 โ ๐ถ = (379.15๐๐)(3.42) = 1296.7 ๐๐๐
The dynamic load rating required is only 1296.7 lbs. Of the bearings available to fit in our
housing, the lowest dynamic load rating was 20,200 lbs. We chose this double row roller ball
bearing, though expensive, it would last must longer than all other components and require
minimal maintenance. The part number and other information about this bearing are found in the
appendix. We chose the method of calculating the bearing dynamic load rating from the
manufacturer because the typical formula ๐ฟ10 = (๐ถ
๐)
3
does not account for the bearing material
makeup, hardness, revolutions per minute and manufacturing methods. The safety factor of our
bearing is as follows:
๐๐ต =๐ถ๐๐๐๐๐๐๐
๐ถ๐๐๐๐ข๐๐๐๐=
20200
129.67= 15.57 (Eq.53)
Summary of Safety Factors and Allowable Stresses
The design of this drag chain conveyor involves many complex parts that each must be analyzed
with respect to the loads they are asked to transmit. Each component was analyzed and
prescribed a safety factor. This safety factor is a simple ratio of the maximum stress allowed on
that component before it fails to the actual stress it experiences. The higher the ratio the safer, or
more overdesigned, that component will be. Below is a summary of the safety factors for each of
our components. We list the lowest safety factor that was calculated for each component to
remain consistent to a conservative design approach.
Gear
o Bending Safety Factor (๐๐๐) = 4.76
Pinion
o Bending Safety Factor (๐๐๐) = 3.98
o Surface Fatigue Safety Factor(๐๐๐) = 2.38
Shaft
o Shearing Safety Factor (๐๐๐๐๐) = 5.442
o Bending Safety Factor (๐๐ท๐ธ) = 5.59
Bearing
o Dynamic Load Safety Factor (๐๐ต) = 15.57
Shaft Keys
o Safety Factor of Gear Key (๐๐๐๐๐ฆ ) = 2.95
o Safety Factor of Sprocket Keys (๐๐๐๐๐ฆ๐ ) = 4.30
It is evident from these values that the first component to fail will be the keys. This is consistent
with the purpose of shaft keys. Keys are designed to lock a power transmitting device to the shaft
and help transmit any rotational forces. If these forces become so much as to exceed the strength
of the key, the key will fail prior to the shaft or transmitting device fails. This is optimal because
the failure of the key keeps the main components of the system from experiencing forces that are
over their design stresses. Keys are cheap to produce and provide a means of protection to the
more expensive components.
Conclusion
In conclusion, the drive shaft conveyor was designed to move a maximum load of 3000 pounds
at a speed of 35 feet per minute. The root of all calculations was the force required to accomplish
this task. Rather than defining our starting torque at the motor and working towards a feasible
load that could be moved, we started with a typical load required in industry and worked
backwards. This process proved successful, as we were able to design components specifically
for the applied load.
The lowest safety factor was 2.38 for the pinion under surface fatigue stresses. This is
understandable because of the high number of cycles experienced by the pinion, relative to the
remaining components. A safety factor of 2.38 is an acceptable conservative number in relation
to the low speed operation of the conveyor. When the pinion โfailsโ according to this safety
factor, it is due to pitting on the pinion teeth. Pitting is not a catastrophic failure and the
component may still be used beyond this point for some time.
As for the remaining components, the keys connecting the gear and sprockets would be the most
likely to fail. This is a desirable result, as the keys act to protect the other components from
overloading. The bearings were purposely overdesigned to accommodate a much longer life of
the conveyor. The shaft had a very high safety factor in order to avoid bending stresses from
repeated rotation under constant torque.
Our initial assumption for the life expectancy of the system was 5 years of 250 days per year and
8 work hours per day. The calculated safety factors are all based on this assumption, with
exception of the bearing which is rated for 10 years. Our final results show that the design should
be successful under normal operating conditions implied during calculations.
References
1. Norton, Robert L., โMachine Design: An Integrated Approachโ, 4th edition, 2011,
Chapters 5,6,10-12
2. Mott, Robert L., โMachine Elements In Mechanical Designโ,1985, Chapters 9-14
3. Taylor et al, โStandard Handbook of Chains: Chains for Power Transmission and
Material Handlingโ, 2nd edition, 2005
4. AGMA, โGeometry Factors for Determining Pitting Resistance and Bending Strength of
Spur, Helical and Herringbone Gear Teethโ, AGMA Information Sheet 226.01, Rev. 908-
B89, 1989
5. Rexnord, โLink-belt, MB and Rexnord Bearings Product Catalog and Selection Guideโ,
Rexnord Spherical Roller Bearings-Engineering Section, pp5-7, 2012
6. Kana Chain, www.kana-chain.com/chain/ansi-chain.htm, ANSI Chain Designations
7. ASM Handbook, Characterization and Failure Analysis of Plastics, ASM International,
2003
APPENDIX
Gear and Pinion Dimension Calculations in Microsoft Excel
Bending and Surface Stresses on Pinion and Gear Excel Screen Shot
Bending and Surface Stresses on Pinion and Gear Excel Screen Shot
Shaft Torque Diagram
Shaft Shear Diagram
Shaft Shear Stress Diagram
Sprocket 1
Gear
Bearing 1
Sprocket 2 Bearing 2
Shaft Bending Moment Diagram
Shaft Bending Stress Diagram
Shaft Torsional Stress Diagram