Abstract The purpose of a drivetrain (or transmission system) of any vehicle is to reliably transmit rotary motion and torque from a motor (input) to an output (wheels, propeller, etc.) that drives the vehicle, using a series of intermediate mechanical elements like shafts, gears, and chains. Typical design objectives involve minimizing losses in transferring power, using gear ratios to decrease angular speed to raise torque (and vice versa), and compactness. For our racing application, it is also a system that is expected to be frequently handled and adjusted, to be in close confinement with other subsystems, and required to pull out the peak performance of those related subsystems. With regards to these objectives we sought to improve on last year’s drivetrain.

In our proposed solution, the internal assembly features a motor plate and rail assembly that is flexibly designed to run one or two parallel motor configurations running on a single roller chain and spring-loaded chain tensioner with all components easily accessible for modifications. Last year’s 1:1.5 and 1:1 gear ratio options has been expanded to include a variety of gear ratios, which is expected to raise the power drawn by the motor controller at full throttle from 250 amps to close to its max limit at 400 amps. In the external assembly, trim and steering are actuated by two cylinder pivoting about a custom mount and the propeller has been switched from a two to three-bladed propeller. Components carried over from the existing system were found to be conservative in sizing with regards to stresses estimated by hand-calculations and predicted by simulations, so these were made leaner and 2014 design flaws noticed post-production were fixed.

Internal Assembly

Solar Racing Electric Boat Drivetrain ProjectKanchan Bhattacharyya, Daniel Huang, Han John Tse, Ting ZhangDr. Anurag Purwar, Dr. Jeffrey Ge, Dr. Thomas Rosati (Advisors)

Stony Brook University, Stony Brook, NY

www.sbusolarracing.weebly.com

Criteria 2014 Design 2015 DesignMaintenance-Friendly

• Two sprocket gears on a common driveshaft were connected to two motors. Was not possible to swap out gears without significant disassembly, limiting gear ratio options.

• Gear ratio adjustment involved breaking the chain, a tedious and time consuming process.

• Motor plate and rail assembly allows for easy change of gears with open end shafts.

• Chain can be easily removed by loosening the chain tensioner and lifting the slacked chain off the gears.

Compactness • Internal drivetrain protruded 26” inwards from the transom. The connection shaft between the propeller shaft and the motor output shaft was too long.

• Internal drivetrain only protrudes 19” inwards from the transom as a result of a shortened connection shaft, for a 25% space saving of 7”.

Trim Control • A static trim angle (vertical angle of propeller) from a fixed length trim rod. Trimming angles influence boat performance – important to have the ability to adjust the trimming angle, between races and during the race.

• Trim control is achieved using an electric actuator. Chosen over pneumatic and hydraulic cylinders because of simplicity, lightweight, and ability to make finite adjustments.

Replacing Parts • The propeller was chosen with little support and research.• The 20 years old brushed lynch DC motors posed a safety and reliability concern

from worn out brushes and loose components.

• A 9.25” diameter, 12” pitch, 15 degree rake, 3 blade bow lifting propeller selected after consulting with Ron Hill Propellers.

• Two new Agni brushed DC motors replace the old lynch motors.

Design Flaws Noticed Post-Production

• Shortening the propeller shaft from the 2013 design to 10.75” did not improve boat performance in handling and navigating.

• The trimming support rod was too weak to support the propeller housing tube and the ¼” rod ends stripped out during the qualifying race.

• Two large and separate rod end mounts on the housing tube contributed more drag through the water.

• Two universal joints connected the propeller shaft and the motor output at different heights, causing timing issues. Custom hex connections in the universal joint were not concentric and led to vibrations and efficiency losses.

• This year’s design utilizes a 23.75” propeller shaft tube.• The trimming actuator and the steering cylinder have stronger rod

ends (7/16” and ¾”) and stronger supports. • The rod end mount on the housing tube is combined into one

compact mount created using a wire electrical discharge machine (WEDM).

• Careful attention placed on CAD models and motion analysis to ensure proper fits, saving machining time and reducing vibrations and losses.

The internal drivetrain assembly (Figure 3) consists of two brushed DC motors, an output geared shaft, a chain tensioner, and a CV joint. Each of the motors are on independent plates and thus the internal drivetrain can feature one or two motor configuration. An ANSI 40 chain connects the two motors and the output shaft.

The chain tensioner (Figure 4) utilizes a 40N-m torsion spring and an idler roller made of UHMW polyethylene for a low coefficient of friction. The chain slides past this roller and the torsion spring of the tensioner presses against the chain. The chain tensioner is lightweight and can be customized to in terms of mounting positon and different springs.

Manufacturing the drivetrain began in March and a finished prototype is scheduled to be complete in late April, in advance of the Solar Splash Competition by 1 ½ months.

After the time-tables of the composites, electrical, and mechanical teams have caught up to one another and the drivetrain can be officially installed in the boat, this time will be spent collaborating mainly with the electrical team in conducting performance runs for data acquisition that will help determine the optimal gear ratio, trimming angle, weight distribution, and speed for the different races – sprint, endurance, and slalom. (tight maneuvering between posts)

We want to reiterate our thanks towards our project advisor Prof. Anurag Purwar, our senior design advisors Prof. Jeffrey Ge and Prof. Thomas Rosati, as well as machinists Henry Honigman, Joseph Schurz, and CEAS building manager Bob Martin – without their help this project would not have been possible.

The external drivetrain assembly consists of a propeller, propeller shaft, housing tube, trimming and steering actuators, gimbal mount, and a CV joint.

The propeller shaft is housed in the propeller housing tube and secured with two taper roller bearings at both end of the tube. The free end with the propeller has a locknut to keep the taper bearings in place. The other end of the housing tube mates with the gimbal mount, which allows for this entire assembly to rotate about a given axis of motion. Another taper gear bearing separates the rotating propeller shaft from the stationery gimbal mount. This gimbal mount attaches to a gimbal spider, a circular piece with four holes separated by ninety degrees which connects the housing tube and transom gimbal mounts. The cylinder mount allows the housing tube to be lifted via the action of one cylinder so that motion is not restricted by the other mounted cylinder. As the trim actuator is shortened and lengthened, the rod end allows for the change in angle between the actuator and the housing tube. The joint for the steering cylinder allows enough play to accommodate its change in orientation with changes in trim.

Stress AnalysisAppropriate dimensions for drivetrain shafts and their features (e.g. splines and shoulders) were roughly obtained through conservative hand-written calculations based on loads surmised from motor and roller chain spec sheets, speeds based on desired operating conditions, areas of stress concentration due to local geometry, and choice of material. These were refined and made leaner using Solidworks’s Static Stress Analysis FEA tools to reach the target safety factor of 2.5, appropriate for materials experiencing moderate stresses with well-documented properties from a trusted supplier.

Future Plans & Acknowledgements

Figure 3: Internal drivetrain in two motor configuration with a 1:2 gear ratio

Figure 4: Chain tensioner

Figure 5: Internal drivetrain side profile cross section view revealing details on the output shaft and CV joint

External Assembly

Figure 6: Internal drivetrain. Left end is the gear connection right end is the CV connection

Figure 7: Cross-sectional view of external assembly leading up to boat transom. From left to right: a) housing tube and propeller shaft, b) gimbal, c) CV joint (at the transom), d) CV joint coupler, e) CV joint.

Figure 8: Shaft Design Algorithm (Designing for Weakest Link)

Figure 9: FEA Study - Drive Shaft Static with GearFigure 10: FEA Study - Drive Shaft Static

Figure 1: External drivetrain showing the hydraulic steering cylinder (lower actuator) and the electric trimming actuator (higher actuator) and how the housing tube connects to the transom. The internal drivetrain is visible inside the hull (top left corner of the image)

Figure 2: Entire drivetrain side profile cross section. From left to right: a) propeller, b) housing tube and propeller shaft, c) trimming actuator, d) hydraulic cylinder, e) gimbal mount and CV joint (at the transom), f) CV joint, g) internal shaft, and h) motors.

a b

c

d

e

f g

h

a

b

c

d

e

Problem Definition: Locate features along shaft length Estimate magnitude of weights causing bending () and torsional

loads () from mating components; determine points of application and direction

Choose a material () Choose factor of safety based on how well material properties

are documented & expected conditions (detailed in-lab testing vs. from vendor following standards; low-impact vs. high-impact conditions)

Draw Free-Body, Resultant Moment, Torque Diagrams

Locate feature nearest peak of bending stress on moment diagram

Assume worst-case fillet to bore and/or shoulder ; look up corresponding factors for first iteration;

Calculate overall endurance limit using modifying factors (surface finish, size, load, temp, reliability) and specimen endurance limit

Plug into fatigue failure equation* Obtain ; = 1.5 (by design)(Use next smallest standard size)

Check:Calculate all parameters precisely and verify is met. Obtain fillet radius Find Calculate Recalculate size factor

Plug in into fatigue failure equation Check if n-value regained is equal to or

exceeds of design. If check fails**: Choose a slightly stronger material,

recalculate , and check

Choose next larger standard size for d and repeat entire check

NOTES:*Parameters account for loading (), geometry (, material (, design factor of safety (), and diameter (). If 3 of 4 are known, the other can be calculated.**Design is based on most critical feature. If a check for a feature is failed and must be retried with a new to pass, all other features need to be re-updated using = 1.5 ratio.