HastoME490Report1

Hasto 1
Design of Cyclone
Space Mining Drive
Module
ME 490 O
For Jim Heise
Nick Hasto
12/16/16

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Contents
List of Figures...............................................................................................................................................3
List of Tables ................................................................................................................................................4
Executive Summary......................................................................................................................................5
Nomenclature................................................................................................................................................6
Introduction...................................................................................................................................................7
The Case for Tracks..................................................................................................................................7
SCMG and CSM Track Design ................................................................................................................8
2017 Drive Gearbox Requirements...........................................................................................................9
Design Evolution ..........................................................................................................................................9
ARTIE III..................................................................................................................................................9
HERMES I & II ......................................................................................................................................10
Ketchup and Mustard..............................................................................................................................10
Original Concept.....................................................................................................................................11
Harmonic Drive ......................................................................................................................................12
Detailed Design of lead Concept ................................................................................................................13
Design Objectives...................................................................................................................................14
Planetary Gearbox...................................................................................................................................15
Squirrel Cage ..........................................................................................................................................16
Ring Gear................................................................................................................................................17
Motor Mount...........................................................................................................................................18
Trelleborg Sealing Solutions...................................................................................................................18
Silverthin Bearings..................................................................................................................................19
Bearing Spacer........................................................................................................................................19
Motor Coupler.........................................................................................................................................20
Failure Modes and Risk ..........................................................................................................................20
Cost Analysis ..............................................................................................................................................21
Future Work and Final Thoughts................................................................................................................22
Appendix.....................................................................................................................................................24
Appendix A: SL-MTI Data Sheet...........................................................................................................24
Appendix B: Mechanical Drawings........................................................................................................25
References...................................................................................................................................................28

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List of Figures
Figure 1: 2014 CSM Rover 7
Figure 2: Rover with rocker-bogie suspension 7
Figure 3: 2016 Robot “Mustard” 8
Figure 4: SCMG and Squirrel Cage 8
Figure 5: ART-E Rover 9
Figure 6: HERMES I 10
Figure 7: 2016 Gearbox 11
Figure 8: Sketch of Section View Drive Module 12
Figure 9: Harmonic Drive Gearbox 12
Figure 10: Annotated Section View of Harmonic Drive Gearbox 13
Figure 11: 2017 Drive Gearbox 13
Figure 12: Annotated Section View of 2017 Drive Gearbox 14
Figure 13: Versa Load Tables (11) 16
Figure 14: Gearbox Section View 16
Figure 15: Gearbox Mounting to Frame 18
Figure 16: Gearbox Disassembly 19
Figure 17: Tracing of the pinion geometry in Solidworks 20
Figure 18: SL-MTI Datasheet 24

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List of Tables
Table 1: Fulfillment of Design Requirements 14
Table 2: Condensed BOM 21

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Executive Summary
As Cyclone Space Mining begins design of a new rover for the 2017 NASA Robotic Mining Competition,
an opportunity arises for evolution of the robot architecture. In the spirit of the competition the club is
always looking to design a practical, space ready rover. Last season saw the implementation of the single
component metals grousers and full metal, space ready tracks. These innovative grousers help the club
move closer to a fully space ready design by eliminating the PVC belted tracks used on previous CSM
rovers.
Currently, the only non-space ready materials on the CSM drive base are found in the large, heavy, and
complicated drive gearboxes. Previous years drive gearboxes have had excessive power reductions and
made use of heavy chain and sprocket. Historically the drive gearboxes have also needed plastic chain
covers that were sloppily sealed with electrical tape. In the same spirit that drove the design and
development of the SCMG, CSM desires to take a fresh and innovative look at their drive gearboxes.
Like the old PVC tracks, the old drive gearboxes never failed to function. However, there design was
impractical and was not reflective of actual NASA rover design. The team aims to demonstrate their
technology to NASA and as such, wants a design that is well thought out and could be used on actual
NASA projects. That means in addition to being functional, designs need to be lightweight, innovative,
and at the minimum make use of space ready materials.
The design of the new drive gearbox system is lighter and significantly more compact than previous
designs. It houses a 3 stage planetary gearbox insides of the squirrel cage and attaches the output of the
planetary gearbox directly to the squirrel cage body. Sealing is achieved by a single shaft seal and the
need for electrical tape sealing is eliminated. Efficient material use has also reduced the weight to only
082 kg, approximately 40% over previous designs. The overall cost of the system has been reduced
thanks to bearing donations by the Silverthin Bearing Group and seal donations by Trelleborg Sealing
Solutions. This provides for a sleek, innovative design that used zero plastic components and is fully
space ready. This drive gearbox is affordable too, at a final cost of $570 for 7 gearboxes or around $80 a
unit.
Upon completion of this design, the club will move to begin manufacturing. Details on the
manufacturing plan for these gearboxes will be covered in detail in Nick Hasto’s second ME490 paper.
But, the geometry for the new drive gearboxes has been designed with manufacturing in mind and is
layout so important geometric tolerances are easily held. Manufacturing will start during the beginning
for the 2017 spring semester and is expected to be complete before Iowa State University's Spring
Break.

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Nomenclature
ART-E - Astro Robotic Tractor - Excavator
BAG Motor – A small, 150W, 20,000RPM, 12V DC Motor
CSM - Cyclone Space Mining
CIM Motor – A common 330W, 5000RPM, brushed, 12V DC Motor
HERMES - High Efficiency Regolith Mining Excavation System
Material evacuation - Preventing the build up of regolith inside the tracks of the rover. A buildup of
material can cause the drive to seize or the track to fall off.
NASA RMC - NASA Robotic Mining Competition. The annual competition held at the Kennedy Space
Center and attended by approximately 50 different colleges and universities.
Regolith - The material used to simulate Martian or lunar soil. Comprised of volcanic ash and fly ash.
Very fine, extremely abrasive and possess an unusually high shear angle of 80°. Will severely damage or
kill moving components
Squirrel Cage - Refers to the name of the drive cog for the robots tracks and digger. The squirrel cage
has already been the objective of previous independent studies. The tooth geometry is optimized to work
with the rover's tracks and as such, will not be changed
SCMB - Single Component Metal Bucket- Future system that is in development this system utilizes the
same geometries as the SCMB and should be designed for accordingly.
SCMG - Single Component Metal Grouser- Lightweight track system that is designed to be
manufactured using a progressive die. This system is a constant in this design. Appropriations will be
made to use this system. Designed by CSM member Taylor Tuel.
SL-MTI - Brushless Sensored DC motors manufactured by SL Montevideo Technology Incorporated and
donated to the team in 2014. They were first used on the 2016 robot drive train and can deliver around
300W of power running at 19,000 RPM on a 42 volt system.
Space Ready – Avoiding the use of materials that would degrade or otherwise be damaged without the
protection of Earth’s atmosphere. This primarily refers to the use of plastic components, which would be
destroyed by radiation.
Versaplanetary - Planetary gearboxes sold by VEX robotics. Design by former CSM member Aren Hill

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Introduction
Cyclone Space Mining (CSM) builds rover to compete in the annual NASA Robotic mining competition.
This rover’s objective is to mine regolith and deposit it in a collection bin. Points are awarded based on
technical capability and the amount of material mined. Points are deducted for rover power consumption,
weight, data use, and dust tolerance. The spirit of the competition is to simulate an off world mining
experience on mars or the moon. NASA uses the competition to see different ideas in action and as
inspiration for their own designs.
For the last 5 years CSM, has been developing the HERMES architecture. HERMES consists of a tracked
rover, bucket conveyor, and linear actuator dump mechanism. The frame of the robot is a weldment
composed of 6061 aluminum tubing and the hopper is riveted together with panels of 6061-T6. The team
has stuck with this architecture because of its high scoring potential and ability to easily navigate over
regolith. While not always the lightest or most energy efficient rover, the robot compensates by being
capable of mining extremely large quantities of regolith, giving it the potential to easily outscore the
competition.
The Case for Tracks
A key component of CSM design and architecture is the track drive. The team has a long and successful
history designing, developing, and implementing track vehicles. When compared to wheeled designs, the
tracks of the HERMES rover distribute weight over a larger area. Allowing the robot to easily “float” over
terrain and navigate hills and valleys even with a heavy payload. Each year many other teams with
wheeled rovers become stuck in the regolith. Proudly, CSM rovers have never thrown a track, become
stuck or trapped during a competition, proving the reliability and dependability of our track system. An
example of CSM’s tracked rover can been seen in Figure 1.
Figure 1: HERMES rover Figure 2: Rover with rocker bogie

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Many wheeled designs use the rocker-bogie configuration as seen above in Figure 2. This design requires
individual drive motors for each wheel and an additional motor to pivot both front, and both back wheels.
Meaning rocker-bogie drive trains usually need ten motors compared to the two or four motors used on
HERMES drive train. That means fewer wires and fewer motor controllers. Rocker-bogie is great for
exploration rovers that have to carefully transverse unknown terrain and are not required to move heavy
loads. They are extremely maneuverable, and tend to
have less moving parts compared to tracked vehicles.
Tracks on the other hand are generally less
maneuverable and require more power in order to steer.
Tracks are also susceptible falling off of the drive
sprocket or bogies, and can be difficult to repair. The
two systems are simply meant for different tasks.
Due to the nature of the NASA RMC, a more industrial
vehicle is needed. CSM treats their design as an
industrial piece of mining equipment. Choosing to build
a robust and capable rover that is meant to handle
extreme conditions and be used outside its intended
parameters. The CSM rover’s mission is fundamentally
different from that of NASA rovers such as Curiosity.
The team’s goal is to move large quantities of material,
not to gently explore an alien world. The greater
traction and weight distribution provided by tracks is why CSM continues to develop tracked rovers.
Tracks provide additional traction enabling the rover to take more aggressive cuts as it digs through the
regolith. And helps guarantee that the rover can then reach the collection bin even with a heavy payload.
SCMG and CSM Track Design
Last year, CSM saw the debut of the SCMG and new
involute profile squirrel cage. Previous years had used
unoptimized squirrel cages powering PVC belted tracks
with ABS plastic drive teeth. These tracks functioned great.
However, due to the plastic components, the tracks where
not space ready and where very heavy. In the spirit of the
competition the team desired to have a space ready track
design in order to further demonstrate to NASA the ability
of a tracked rover. This lead to Taylor Tuel’s development
of the SCMG and profiled Squirrel cages shown in Figure
4. The new tracks and squirrel cage made their debut on
2016’s Ketchup and Mustard robots. Despite the difficulties
encountered with these robots, the tracks functioned
“flawlessly” and wowed the NASA judges. The team plans
Figure 3: 2016 Robot “Mustard”
Figure 4: SCMG and Squirrel Cage

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to continue the use of these tracks in order to test their longevity and also develop a SCMB for the robot’s
bucket digging conveyor.
2017 Drive Gearbox Requirements
CSM’s rover for the 2017 competition allows for another iteration of the HERMES architecture;
improving upon its strengths and attempting eliminating its weaknesses. With the decision to continue
with a tracked vehicle, it now comes time to design a new drive gearbox to propel the rover and transmit
power to the tracks. Old gearbox designs have been bulky, heavy, and difficult to seal. While they have
functioned as intended, the old designs have had many points of failure. Old gearboxes have had
excessive reductions and power transmission. In particular, sealing the old designs required excessive use
of electrical tape to cover the seams in the gearbox and dust covers. With the addition of the SCMG to the
HERMES architecture, the team desires to move to more elegant drive gearbox design to enhance the
professional nature of the team.
In accordance with the scoring of the competition and the design history previously discussed, the system
must meet the following criteria:
1. Lightweight and compact
2. Function with SCMG
3. Active dust mitigation
4. Low monetary cost
5. Compatible with SL-MTI motors
6. Utilize “space ready” materials
The design for the drive module is to be completed one semester before the competition to allow ample
time for manufacturing and testing. The new design should aim to improve upon old drive gearbox
architecture and present an innovative approach to the problem. But as always, reliability is a must, so
failure modes must be account and planned for.
Design Evolution
ARTIE III
The first iteration of ARTIE III featured modified
drill gearboxes and drill motors from Milwaukee
Fuel 18V drills. These drills featured a 2 speed
transmission and the robot was run on the lowest
speed, giving the robot a top speed of around 1 ft/s.
To avoid excessive pretensions of the drill motors, a
chain and sprocket were used to transmit power
with a 4:1 reduction and allowed the gearbox to sit
underneath the track. As seen in Figure 5, the white
Figure 5: ART-E Rover

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enclosures cover the chain and require electrical tape in order to seal. The dust covers where vacuum
formed and 3D printed out of plastic. They extended outside the track, limiting the overall width of the
robot. While this design in simple, it is very primitive as the drill gearboxes and motors are heavy and not
optimized for performance. The drill gearboxes are not easily compatible with other motors and are not
easily modified.
HERMES I & II
HERMES I made the switch from drill gearboxes to the newly designed versaplanetary from VEX
robotics. Versaplanetary provide a cheap, strong, and lightweight method for speed reduction in a
compact package. The squirrel cage was placed directly on the output shaft of the versa, eliminating the
need for chain. While the drive motor protruded
excessively, it was protected from collision by
the frame. To seal the gearboxes, the
versaplanetaries were wrapped in electrical tape
to keep out regolith. Weight was reduced by
removing the chain and sprocket, and sealing
was made simpler without the presence of a
chain cover.
HERMES I & II used a mini CIM into a two
stage 100:1 (10:1 into 10:1) versaplanetary. The
issue with this setup is that the while 10:1 versa
stages offer the greatest reduction; they are by
far the weakest of the versaplanetary offerings [8]. The small sun gear is highly susceptible to breakages
and HERMES II had issues with the strength of its final 10:1 versa stages. This forced a switch to a lower
reduction versaplanetary and using a chain and sprocket with a 2:1 reduction to bring the overall
reduction back to around 100:1. The change from the original HERMES gearbox design took a step
backward in design evolution by going back to a heavier gearbox that is more complex to seal.
Ketchup and Mustard
The drive system from 2016’s Ketchup and Mustard robot represented a significant shift in drive gearbox
architecture. Because Ketchup and Mustard featured rocker bar suspension, the gearboxes had to change
in order to protect the motors from colliding with any rocks in the arena or the robot frame. Because of
the rocker suspension, the motors could not be shielded by the robot frame as in previous designs. As
such, bevel gears were used in order to tuck the motors and planetary gearboxes underneath the tracks.
Protecting them from any unwanted collisions and allowing the necessary suspension travel. Ketchup and
Mustard also saw many other firsts for the HERMES architecture. This was the first time the team fielded
two robots in the competition arena. And the first use of the SCMG and involute profile squirrel cage [7].
Ketchup and Mustard were also the first robots to utilize the 300W (Appendix A, Figure 18) SL-MTI
motors on their drive base and the first to run on a 40V system. Making them significantly more powerful
than previous CSM rovers that utilized 215W Mini CIMs [10]
Figure 6: HERMES I

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Figure 7: 2016 Gearbox
On Ketchup and Mustard’s drive gearboxes
power flowed out of the SLMTI through a two
stage 63:1 (9:1 into 7:1) versaplanetary. Power
was then transmitted 90 degrees by two bevel
gears into a sprocket and chain with a 2.375:1
reduction for an overall reduction of 149:1, giving
a drive speed of around 2 ft/s. The club intended
to use a versaplanetary integrated encoder to
place directly off the SL-MTI to measure
position, but this was never utilized or even
plugged in. While these drive successfully
sheltered the motors, they were heavy, weighing
in at 1.4 kg. They were also very complex,
requiring ten custom machined components. 3D
printed ABS dust covers were placed over both
the chain and bevel gears to shield them from
regolith intrusion. As seen in Figure 7, the bevel
gears and chain required dust covers. The dust covers were sealed by taping the seams with electrical tape
and the entire versa planetary and SL-MTI were wrapped in electrical tape. Sealing was extremely
tedious, susceptible to error, and looked terrible. While the seals never failed, there was very high
potential something could go wrong.
Original Concept
With the 2017 competition fast approaching, the team desired a more elegant solution. It became clear the
need for an improved designs that is not only compact and lightweight, but easily to seal. To seal the
reducer from regolith, the reducer was placed inside of the squirrel cage. As seen in the section view
below (Figure 8), a new squirrel cage design envelopes the reducer. Shielding it from dust and debris and
only requiring one rotary shaft seal to properly seal. Inspiration was draw from the in-the-wheel swerve
modules used by FIRST FRC teams such as 3928 and 2451. These swerve modules have the drive wheel
rotated about either the motor housing or a protective sleeve around the outside of the motor.

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Figure 9: Harmonic Drive Gearbox
Figure 8: Sketch of Section View Drive Module
While the squirrel cage will be more complex to manufacture, the design requires significantly fewer
parts and offers better sealing in a compact package. This squirrel cage will rotate on a set of bearings that
sit that rest on a sleeve. This sleeve will secure the reducer and motor, and be used to attach the entire
gearbox to the rover. The next step in the design evolution was to select a reducer. The SL-MTI has a free
speed of around 20,000 RPM but the rover only travels at around 1.5 to 2 ft/s. As such the 2016 robot
needed a 163:1 reduction. Getting such a high reduction in a small footprint would prove to be difficult as
either an expensive off the shelf or custom gearbox is needed.
Harmonic Drive
The first iteration of the drive gearbox design looked at using
strain wave gearing from Harmonic Drive, specifically the CPL-
20-2A with a 160:1 reduction. Stain waving gearing offers large
amounts of reduction in a compact and lightweight package. As
such, it would be a perfect choice for CSM’s new drive train.
Originally the club also considered replacing the gearboxes on
the digger and linear actuator with harmonic gearboxes in an
attempt to save weight. But first, focus was placed on the drive
module.
In order to function properly, harmonic gearboxes need to be run
in a wet housing and concentricity between the ring gear, flex
spline, and wave generator held within 0.0005” [5]. Originally
Delrin journals were considered to support the drive cog. Journals are lighter and cheaper than ball

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bearings however they require a gap between the rotating component and journal surface. Making it
impossible to hold the 0.0005” concentricity needed for the harmonic drives.
The squirrel cage had to be expanded to 10 teeth in order to accommodate the size of the harmonic drive.
Oil fill and drain holes were also added to the front of the cage. The cage was split into two pieces to
allow for easier assembly with an O-ring to retain the oil. A shaft attached the motor directly to the wave
generator of the harmonic gearbox.
This shaft was supported by a
needle bearing to ensure
concentricity. Both the journals
and the motor mount located to the
circular spline with dowel pins. A
radial shaft seal seats in the back
of the squirrel cage and rides along
the motor mount to keep out
regolith. Replacing the 3D printed
dust covers of previous designs.
Ultimately this design fell apart
due to the limits of strain wave
gearing. The harmonic gearboxes are rated for a maximum speed for 14,000 RPM input [5]. The SL-MTI
spins at 19,000 RPM and CSM was hoping to be able to the gearbox at close to double speed. However,
after consulting with harmonic drive’s sales engineer, it was determined that nothing in harmonic drives
offering could safely run at the speeds of an SL-MTI motor. According to their sales engineer the
maximum rated speed of the gearbox was a very high estimate and the gearbox could not handle
continuous operation about 6,000 RPM. Running at such high speeds could result in immediate failure or
an extremely short lifespan. The next issue was cost, at over $650 a unit, using strain wave gearing was
simply too risky and way too expensive for CSM.
Detailed Design of lead Concept
A more elegant solution had to be found, and a far more
elegant solution was found. The final iteration of the drive
gearbox uses a two piece squirrel cage riding on thin section
ball bearings. The bearings sit on the plastic spacer that is
retained by a snap ring to allow for quick and easy
disassembly. Power is transmitted by a three stage
planetary gearbox running inside a custom wire ring gear.
This ring gear also acts as the support for the entire gearbox,
reducing the overweight of the system and making the
design extremely compact. Please see the annotated
section view in Figure 12, for more details and specific
component names.
Figure 10: Annotated Section View of Harmonic Drive
Gearbox
Figure 11: 2017 Drive Gearbox

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Design Objectives
Table 1: Fulfillment of Design Requirements
Criteria Requirement Met (+ or -) Explanation
Lightweight and compact + Only 0.82 kg and small in size
Function with SCMG + Utilized proven squirrel cage tooth
geometry
Active Dust Mitigation + Requires only one seal and zero
electrical tape
Low monetary cost + Significant part donations
drastically reduce cost, cheaper
than previous designs.
Compatible with SL-MTI motors + Improved motor mount and power
transmission
Utilize “space ready” materials + All space ready materials
Please see the Mechanical Drawings and BOM in Appendix B for specific details, dimensions, and part
numbers.
Figure 12: Annotated Section View of 2017 Drive Gearbox

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The final iteration of the drive gearbox uses a nine tooth (3.75” diameter), two piece squirrel cage. The
squirrel cage wraps around the internals of the drive gearbox to protect them from dust and dirt. The
squirrel cage rides on two thin section ball bearings that sit on a bearing spacer sleeve. The sleeve is
retained by a snap ring and allows the entire gearbox to be disassembled without taking the gearbox off of
the rover. To remove the sleeve, one first takes off of the front side of the squirrel cage. Them the snap
ride is removed and the sleeve, bearings, and back side of the squirrel cage can be removed.
The sleeve slides over a custom made ring gear for a versaplanetary gearbox. The only parts retained from
the versa are the three planetary stages. Power is transmitted from the SL-MTI to the first stage of the
versa with a custom motor coupler. The third and final stage of the versa is bolted directly to the front
side of the squirrel cage. The custom ring gear is used not only to transmit power through the gearbox, but
also to support the bearings for the drive cog and attached the gearbox to the robot.
Normal versaplanetary gearboxes have their own housings, ring gears, and output bearings. They are
relatively light weight however they are designed to be easily applicable to any task and are not purpose
built. By eliminate the normal versa assembly; a purpose built gearbox can be constructed that allows for
more efficient use of materials in a smaller footprint, bringing the weight down to only 0.82 kg. The ring
gear is used to both transmit power and support the drive cog. The output bearings are eliminated and the
gearbox is placed inside the drive cog to reduce the overall footprint. Making the gearbox fully encased
by the squirrel cage allows sealing to be accomplished with a single rotary shaft seal. Giving CSM an
elegant and purpose built drive gearbox. Detailed information on the design of each component can be
found below.
Planetary Gearbox
The Versaplanetary gearbox is a reliable and battle proven product that is well regarded in the world of
competitive robotics. It is produced by VEX robotics and was designed by Iowa State and CSM alumni
Aren Hill. The versaplanetary CAD model provided by VEX robotics comes directly from the gear
manufacture and are accurate enough to reproduce the tooth profile. The ring gear has 72 teeth and a 0.5
module with a 20 degree pressure angle.
To achieve a 1.5 - 2 ft/s drive speed, the planetary gearbox will be three stages: a 9:1 into a 7:1 into a 3:1
versa stage, for an overall reduction of 189:1 and a theoretical max drive speed of 1.64 ft/s. While this is
slightly over the target, the actual drive speed will be around 1/5 ft/s due to the frictional losses in the
system. The lower reductions stages are placed at the end because they are capable of handling higher
torque loads. The weak point of the planetary gearbox will be the final 3:1 stage. Vex does not provide
specific torque and power ratings, but it does provide loading table with common motors. These loading
tables are based on the maximum torque the gearbox can handle before damage occurs. For a comparison,
a CIM motor with a stall torque of 343.4 oz*in [9] will be compared to our SL-MTI current limited at
10A. With the current limit imposed the SL-MTI has a stall torque of 30 oz*in (Appendix A, Figure 18).

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Based on VEX’s loading table (Figure 13), the
maximum rated reduction for a CIM motor is 21:1(7:1
into 3:1), giving a max output torque of 7200 oz*in.
Compared to an SL-MTI with a 189:1 reduction (9:1
into 7;1 into 3;1) and a max torque of 6520 oz*in.
Giving a reasonable factor of safety on top of the
factor of safety already expressed in VEX’s tables.
The SL-MTI has similar power to a Mini CIM or Two
BAG Motors, both of which are weaker than a CIM
motor, giving confidence that the gearbox will have
ample strength to handle the SL-MTI.
The main concern with the drive gear boxes planetary
speed is their high speed operation, the first stage will
be spinning at close to 20,000 RPM at free speed.
Stock vex planet gears rotate on a hardened steel
dowel pin. At the recommendation of the gear boxes
designer, Aren Hill, the planet gears on the first and
second stage will be drilled and reamed to accept
needle bearings. Having the planet gears rotate on
needle bearings will significantly reduce wear and
friction in the gearbox at high speed operation,
improving both the gearbox’s efficiency and longevity.
Squirrel Cage
Design of the squirrel cage was driven by two parameters: Compatibility with the SCMG and intelligent
part geometry for easier machining. The squirrel cage teeth use the same geometry as the 9 tooth, 3.75”
diameter 2016 squirrel cages, and the contoured surfaces have a thickness of 0.100”. Both sides of the
cage are designed so that all critical geometry
can be machined in the same setup. The dowel
holes are included to aid in fixturing and all the
fillets can be machined with a 0.250” ball nose
endmill.
The front side is designed so the bearing and
output stage of the planetary gearbox can easily
be within 0.001” of concentricity. The bore that
retains the bearing, the bore that retains the
carrier plate of the versa, and the mounting
holes to attach the two sides of the squirrel cage
are machined in the same operation. The cage is
then flipped and located on the dowel hole for
the contours to be machined.
Figure 13: Versa Load Tables (11)
Figure 14: Gearbox Section View

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The back side of the squirrel cage must retain both a bearing and the seal. Given the concentricity of the
bearing is the most important it will be machined first. Based on Trelleborg’s technical documentation for
their radial oil seals, run out of the shaft is not a major concern. With the seal able to handle over 0.01” of
runout at slow speeds [4]. Even so, the bore for the seals can still easily be held to within 0.002” of run
out.
Ring Gear
The ring of the drive module is to me made out of 7075-T6. 7075-T6 has a yield strength of 74 ksi and a
Rockwell B hardness of 87 [1] compared to 6061-T6 yield strength of 40 ksi and Rockwell B hardness of
60 [2]. The additional strength is needed to handle the high output torque of the planetary gearbox and
loading on the drive cog. The original versa planetary ring gear is made of 6061 - T6 [6], however the
ring gear is not required to take any additional loading besides for the torque of the planetary stages.
Switching to 7075-T6 gives extra strength as the ring gear is also being used as a structural component to
support the drive cog.
The primary reason however, for manufacturing the ring gear from 7075-T6 was hardness. 7075-T6 has a
significantly higher hardness than 6061-T6. The radial shaft seals from Trelleborg are designed to run on
a hardened steel surface to achieve their lifetime of tens of thousands of hours. A hard material is
required because the seals will eventually erode a grove into the material they run on. The seals also need
to run on a smooth, ground surface to properly seal and achieve their expected lifetime [4]. CSM rovers
do not have such a long lifetime so to save weight the ring gears will be aluminum. Considerations will
also be made to ensure that the surface the seal is running on is as smooth as possible to ensure a proper
seal and prevent regolith from entering the gear box.
From talking with Trelleborg’s sales engineers it was determined that the seals would last significantly
longer running on 7075-T6. Specifically the concern is not the seal, but the seal wearing a groove into the
rotating component making the seal ineffective. A changeable, hardened steel sleeve was considered for
the seal to run on, but the sleeve interfered with mounting the gearbox to the robot. For the lifetime
needed out of these robots, seal wear on the ring will not be a concern. But a Type III hard coat anodizing
(Rockwell C 60-70 [3]) is also being considered to extend the life of the gearbox. The anodize layer will
only be about 0.0005” thick and protect aluminum from wear [3]. The primary purpose of the hard coat
anodize would be to protect the teeth of the ring gear but it would also serve to reduce wear from the seal.
The versaplanetary ring gears have a Type III Hard coat anodize to help protect the ring gears from the
case hardened steel planet gears. If budget allows, the ring gears for the drive module will also receive a
type III hard coat anodize to extend their service life.
The geometry of the ring gear is designed to be machined in a single turning operation in order to ensure
tight concentricity tolerances between surfaces. The cylinder for the bearing spacer, the inner diameter of
the teeth, slot of the snap ring, and the recession for mounting the motor mount can all be turned in one
operation, allowing a concentricity of 0.001” to be easily held. After turning is complete, a wire EDM
will be used to cut the tooth profile.

Hasto 18
Motor Mount
Unlike previous iterations of SL-MTI motor mounts, this mount is designed to hold and secure the SL-
MTI as it was intended. As seen in Figure 12, the SL-MTI is held by the precision boss on its front end
and three screws are used to retain the motor. The motor mount has a circular boss that allows it to locate
on the ring gear, ensuring that the SL-
MTI is concentric with the rest of the
planetary gearbox. Like the ring gear,
the motor is designed so that critical
geometry is machined in the same
operation to ensure tight concentricity
tolerances.
Figure 15 shows the gearbox mounting
to the robot frame. Six 6-32 SHCS are
used to secure the gearbox to the
mounting flange. Clearance holes are
present in the motor mount and the bolts
thread into the ring gear. The mount is
designed to protect the SL-MTI and be
used to shield it from dust and debris.
An end cap with a rubber grommet will
seal the end of the gearbox mount and
allow the motor wires to pass through.
Trelleborg Sealing Solutions
The TRE radial shaft seal with spec’d with the help of Trelleborg’s sales engineers. The seal specified is a
radial oil seal with a dust lip. As the seal ages the mechanical properties of the rubber may change and
the force exerted by the sealing element may diminish. To maintain force, the seal has an internal spring
to maintain a firm contact with the surface [4]. The seal also has a dust lip on the outside to provide an
additional layer of protection.
The seal was originally selected run with the wet housing needed for the harmonic drive and journals.
However when the switch was made to a planetary gearbox and thin section bearings there was no longer
need for a wet housing. The seal would simply be stronger than it needed to be and as such would induce
more friction than a lighter seal. The additional friction is not a concern because the squirrel cage is
relatively slow moving compared to the motor output shaft, meaning there is not a significant loss in
power. Regolith is a highly abrasive and any regolith inside the gearbox has the potential to cause serious
damage so any additional sealing is welcomed.
As mentioned in the discussion of the ring gear design. These seals require a hard and smooth surface in
order to avoid leakage. In most applications the primary concern is oil leaking out. However, since the
seals are no longer retaining a wet housing, maximum sealing potential is not needed. While a smooth
Figure 15: Gearbox Mounting to Frame

Hasto 19
surface is still desired, there is no longer need for a precision ground surface, reducing the cost of the ring
gear.
Silverthin Bearings
Original concepts for the drive gear boxes called for the use of Delrin journals in an attempt to save cost
and weight. Journals require a small gap between the outer surface of the journal and the surface they run
on. If the fit is too tight, lubrication will not be properly distributed and rotating components can seize.
Since the output of the versa is connected directly to the squirrel cage, there was also concern that
because of the same gap, the output stage of the versa would end up taking radial and axial loads,
potentially destroying the ring gear or the versa stages. The Silverthin bearings have a dynamic radial
load rating of over 300lbs [12]. Drastically higher than any loading they will ever see while in service on
a CSM Rover.
Preliminary designs we made for a version of the gearbox that would use thin section ball bearings in
place of journals. When Silverthin Bearing group agreed to donate their product and sponsor the team the
decision was made to do away with the journals. Thin section bearings allow for a tight fit between the
ring gear and squirrel cage, ensuring that any axial or radial load is transmitted to the bearings and not the
planetary gearbox. The additional weight taken on by using bearing is justified by the reliability that
comes with using a proven product. CSM does not have experience with journals and several iterations
would probably be needed to arrive at a functional final design.
Bearing Spacer
The bearing ring spacer is designed to allow the
gearbox to be disassembled and serviced without
removal from the rover. As seen in Figure 16, with the
output side of the squirrel cage removed, removing the
snap ring allows the bearing spacer and both bearings
to be slide off of the front of the gearbox, allowing the
seal and back side of the squirrel cage to also be
removed. The absence of this sleeve would require the
back bearing and back side of the squirrel cage by
sliding them off the back. This could only be
accomplished by removing the entire gearbox from the
robot. An entire sleeve was used, rather than just a
simple spacer because the ID of the bearings is greater
than the ID of the seal and because the sleeve allowed
for a smaller snap ring to be used, saving a small
amount of weight and allowing clearance from the
inside of the rotating squirrel cage. Figure 16: Gearbox Disassembly

Hasto 20
Motor Coupler
The SL-MTI motors did not see implementation until years after their donation because it is difficult to
attach components to the motor's output shaft. The SL-MTI motors are very specialized and designed to
work only with a specific Maxmar planetary gearbox. As such, the sun gear for the first stage of the
Maxmar planetary is cut directly into the pinion of the SL-MTI. Meaning traditional shaft couplers are not
compatible with the motor. Ketchup and Mustard
utilized an ABS 3D printed coupler that press fitted
over the pinion and then attached via set screw to the
input stage of a versaplanetary. During the second run
of the 2016 competition, the set screw in Ketchup’s
left track back out, leaving the robot disabled in the
arena.
In attempt to reduce failure points, the new coupler
does not use a set screw to retain itself. This coupler
will instead combine the geometry needed to interface
with the SL-MTI pinion and the geometry needed to
interface with the versa sun gear into one component,
eliminating the need for a set screw and the need for
two separate parts. The geometry for the versa sun
gear could easily be obtained from Vex. However the
exact geometry of the SL-MTI pinion is property of
Maxmar and could not be disclosed. To obtain this
geometry a picture of the motor pinion was scaled and
traced with the spline tool in Solidworks (Figure 17). The team plans to metal 3D print the coupler. Metal
3D printing lacks tight tolerances and can produce slightly different geometries based off the machines
printing method. Some guessing and checking will be needed to ensure a proper fit with the SL-MTI. If
these methods are not effective, the profile for the SL-MTI will have to be wire EDM’d after 3D printing.
Failure Modes and Risk
The primary cause for the drastic redesign was to improve sealing. Previous gearboxes have needed
excessive sealing and relied on plastic dust covers and electrical tape. Seal failure could result in regolith
entering the gearbox. Short term, the gears and chain can handle a small amount of regolith but over time
this will lead to the destruction of the powertrain and cause moving components to bind or seize.
The 2017 drive gearbox does not require any 3d printed dust covers and will need zero electrical tape to
seal. The only required seal will be the radial shaft seal on the back of the gearbox. Thus significantly
reducing the number of entry points for regolith.
Mating surfaces such as the motor mount to the ring gear and the two halves of the squirrel cages have
potential for regolith leakage. If surfaces are not flat or bolts not properly torque, gaps may open and let
Figure 17: Tracing of the pinion geometry
in Solidworks

Hasto 21
regolith in. This risk has been mitigated by calling out flatness tolerances as seen in the drawings for
CSM-0303 and 0304. Assembly will have to be done with care in order to ensure no leaks. LOCTITE®
5203™ flange sealant may be used in order to improve the sealing between surfaces if it is found that
regolith is leaking into the gearbox.
If sealing is not an issue the primary mode mechanical failure will be wear of the ring gear teeth. The
hardened steel planet gears will eventually wear down the ring gear teeth enough to allow the planet gears
to skip teeth on the sun gear causing the gearbox to seize. To help prevent wear of the ring gear hard coat
anodize is being considered. See the cost analysis for more details.
Cost Analysis
Luckily, expensive components such as the Silverthin bearings and Trelleborg seals have been donated to
the team by their generous sponsors. Table 2 denotes the cost to the team for the 7 complete gearboxes: 4
for the two competition robots, 2 for the practice/parts bot, and 1 for a section view display. The majority
of the system cost are the versaplanetary stages from VEX robotics, the stock required to machine the
other components is relatively cheap.
Table 2: Condensed BOM
Component Material Qty Cost
Bearing Space 2.25” OD 1.75” ID x 12” UHMW $25
Squirrel Cage Front 4” dia x 12” 6061-T6 $60
Squirrel Cage Back 4” dia x 12” 6061-T6 $60
Ring Gear 2” dia x 12” 7075-T6 $75
Motor Mount 2” dia x 6” 6061-T6 $10
Fasteners Various BHCS $20
Snap Ring 7 $5
3:1 Versa Stage 7 $105
Total: $570

Hasto 22
Since manufacturing will be performed as a separate independent study, a detailed breakdown of the
manufacturing cost will not be covered. However, since manufacturing is being performed by team
members in Boyd Lab, the club will not see any additional cost.
Unfortunately for the design the part that sees the most wear and tear is the ring gear. The ring gear will
wear from both the planetary gears and the seal. As seen with completer versaplanetary gearboxes, wear
of the ring gear and planet gears will eventually because the teeth to skip and the gearbox seize. The most
likely location of this wear will be at the final 3:1 output stage as it sees the most torque. The addition of a
hard coat anodize should help reduce wear and tear on the ring gear and instead cause the planet gears on
the 3:1 stage to wear. Leading to the just the 3:1 stage needing replacement and not the entire ring gear.
Wear from the seal will erode a groove in the ring gear and allow regolith to enter the gearbox. The hard
coat anodize will offer ample protection until it has been eaten away by the seal. This could be
compensated for with a thicker anodize (0.01’), but such a thick anodize is inherently imprecise and will
compromise the tooth geometry and concentricity of the bearings and planetary gearbox. Discussions with
Trelleborg’s sales engineers concluded that excessive seal wear is not a concern as the robots have a short
lifetime and wear on the gear teeth is greater.
Wear or fatigue of the other components in the design does not present concern. They will easily out live
the rest of the gearbox. Only the versa stages and potentially the ring gear will ever need replacing. While
it is not recommend or in the spirit of the competition, drive modules could easily be ran for multiple
years. While the Silverthin bearings and Trelleborg seals are expensive, both Silverthin and Trelleborg
agreed to donate enough components for 12 gearboxes, giving plenty of backup components in case of a
premature failure.
Future Work and Final Thoughts
Manufacturing of the drive gearboxes is expected to begin at the start of the 2017 spring semester. Details
will be covered in the second ME 490 paper by Nick Hasto.
At the start of my independent study I identified several topics that I wanted to learn more about. These
topics where:
1. Improve 3D modeling skills in Solidworks
2. Improve 3D assembly skills in Solidworks
3. Learn how effectively apply dimensions and tolerances to 3D components
4. Learn how to produce effective 2D assembly drawings
5. Improve methods of sourcing parts and communicating with vendors
Through the course of my study I have used Solidworks in out to layout my ideas and determine if
components could fit within the size constraints I wanted. Due to the significant design changes I had to
make, I had to make use of efficient file managed in order to keep control over my various parts and
assemblies. After this exercise in modeling the one thing I learned was the importance of drawing your

Hasto 23
components so that critical dimensions can be easily changed quickly. Allowing me to try out various
ideas and quickly revert back if they do not work out.
The majority of my learning came from communicating with vendors. Since it is impossible to become an
expert on every sourced component it is important to talk to sales engineers and rely on their experience.
The first dose of this came from Harmonic Drive. As discussed, we would have been pushing the
harmonic gearboxes extremely hard. After talking with Harmonic Drive’s sales engineer it became
immediately clear that the gearboxes could not handle the speeds of the SL-MTI. While the spec sheet
suggested that they might be okay, the sales engineer indicated that the max loadings reflected in the spec
sheet where very high and could the gearboxes could only handle vary intermittent operation at such
speed. My other interactions with sourcing components came with Trelleborg Sealing Solutions and the
Silverthin Bearing Group. Both of which agreed to make large material donations to the team.
Trelleborg’s sale engineer in particular helped me spec a proper seal and design a surface that would
ensure good seal like.
Much of my learning was related to the design of components for manufacturing. By always thinking
about how you are going to machine your components, you can better design them so that it is easy to
hold tight tolerances. Lastly, it is important that you apply appropriate tolerances to your drawing so that
the machinists understand the nature of your part and what geometry is critical. Failure to do so can result
in parts that do not function. However, over tolerances can run up excessive costs. So it important to
identify critical geometry and relax tolerances in areas that are not critical. This study was an excellent
opportunity for me to learn more about the importance of GD&T and intelligent part design.

Hasto 24
Appendix
Appendix A: SL-MTI Data Sheet
Figure 18: SL-MTI Datasheet

Hasto 25
Appendix B: Mechanical Drawings

Hasto 28
References
1.http://asm.matweb.com/search/SpecificMaterial.asp?bassnum=MA7075T6
2.http://asm.matweb.com/search/SpecificMaterial.asp?bassnum=MA6061t6
3.http://www.fortwayneanodizing.com/hardcoat/
4.https://tssstatic.com/remotemedia/media/globalformastercontent/downloadsautomaticlycreatedbyscript/
catalogs/rotary_gb_en.pdf
5. http://harmonicdrive.de/mage/media/catalog/category/2014_12_ED_1019655_CPL_2A.pdf
6.http://www.vexrobotics.com/versaplanetary.html
7. Taylor Tuels Senior Design
8. https://content.vexrobotics.com/vexpro/pdf/VersaPlanetary-Load-Ratings-Rev4-20161121.pdf
9. http://www.andymark.com/CIM-Motor-p/am-0255.htm
10.http://www.vexrobotics.com/217-3371.html
11.https://content.vexrobotics.com/vexpro/pdf/VersaPlanetary-Load-Ratings-Rev4-20161121.pdf
12. http://www.silverthin.com/bearings/thin-section/sa-series/detail/SA020/

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