Introduction:

The tow tank is a 120ft long, 8ft wide and 4ft deep tank that is used to test the drag of various objects and shapes as they are towed in water.  The tank holds 25,000 gallons of fresh water and is filtered to prevent any biological growth.  An aluminum carriage runs on tracks across the length of the tank and achieves a maximum speed of about 2.5m/sec.  The carriage is driven by a 5.6kW three phase motor that is digitally controlled by a programmable inverter with a stainless steel tape.  The maximum drag force induced on the carriage is 538N (121lbf) and is measured by a strain gage load cell mounted on the carriage.  The system also has the ability to simulate waves.

Problem:

The major issue with the tow tank which needed improvement was the stainless steel tape belt.  The weight of the belt itself created a large amount of sag throughout the length of the belt.  This sag, in combination with the wet environment of the tow tank created a very problematic and unsafe situation.  The belt would actually stick to itself and would continuously need to be pried apart while the tank was in motion.  The stainless steel tape belt would also slip on the pulley and cause the carriage to become stuck in place, rendering it useless for data acquisition.

Design Objectives:

The focus of our project is to design and implement a new and improved belt drive system for the tow tank.  The new         belt drive system satisfies the following criteria:

• Eliminates belt contact issue.
• Cost efficient with little maintenance required.
• Incorporates a positive drive system to prevent belt slippage.

The specific results of this project are:

• Belt type selection
• Installation of new belt.
• Pulley design.
• Fabrication and installation of pulley.
• Design of modified supports/guards.
• Fabrication and installation of supports/guards.

Design Parameters:

The design of our project was broken down into three major components; the belt design and selection, the pulley, and the guards/supports. 

Belt Design:
Several design considerations were taken into account when choosing the belt type and material:

Corrosion Resistance: 
Because of the damp environment, corrosion was a significant design parameter.  Corrosion resistance is usually determined by the material selection; however, due to formation processes some belt types are more prone to corrosion than others.

Cost:
Cost needed to be kept at a minimum and was a significant design parameter.

Strength: 
The load involved in this project was relatively low (121 lbf), and therefore strength was not a critical design parameter.

Speed: 
The speed at which the carriage travels is relatively low (2.5 m/s), therefore it was necessary to select a belt which would perform well at low speeds.

Elongation/Alignment:
Due to the abnormally long length of the belt needed, elongation was a concern and should be kept to a minimum.  Misalignment was also a concern and we therefore wanted to select a belt which did not require precise alignment.

Durability:
The belt needed to be durable, wear resistant, and require minimal maintenance.

Efficiency:
The tow tank is used infrequently and therefore efficiency is not a critical design parameter.

Oil Bath:
Belt should require minimal lubrication to prevent contamination of tow tank.  In general all belts need some form of lubrication to prolong life.

Maintenance:
The belt should require no more maintenance then is currently needed, which is a very minimal amount.

Belt Type Comparison:

Chain Drives:
Chain drives are generally ideal for high torque, low speed applications.  They are relatively easy to install and are well suited for conveyor type applications. However, chain drives do require some sort of oil bath to run efficiently. Pretensioning is also required for chain drives.  One of the major concerns regarding chain drives is sprocket misalignment.

Roller Chain:
Roller chains tend to be very lightweight but undergo considerable elongation.  These belts are prone to wear as a result of friction between the mating of the pin and bush.  The major advantage of these belts are that they are easily installed and can transmit very high torque levels.

Silent Chain:
Silent chains are heavier than roller chains.  They tend to be very expensive but are easily installed.  These belts are preferable for applications in which noise reduction is a concern.  The belt can bear heavy loads and operate at high speeds.

V-Belts:
V-Belts are used to transmit horsepower and reduce speed.  They are a quiet, low cost product which requires little maintenance.  These belts perform better at higher speeds but are prone to slippage and high elongation.  The v-belt is made in fixed lengths and is not ideal for long belt applications.  The belt must be installed precisely to prolong life.  V-belts are also capable of producing much higher torque than flat belts.

Flat Belts:
Generally used in conveyor type applications a flat belt will take longer to wear when compared to a V-belt.  The belt is not affected by tight bends.  When put in a wet environment slippage becomes a major concern, due to a lack of positive engagement.   The ideal flat belts are ones with a nylon core because they undergo minimal elongation and are virtually maintenance free.

Wire rope:
Wire rope is very light weight, easily installed, and inexpensive.  Based on the material selection the belt can be designed for high corrosion resistance.  Wire rope has the ability to support very high loads with minor amounts of elongation.  The wire rope requires periodic lubrication and is best suited for long length applications.
Belt Selection: Wire Rope:

Wire Rope Design Parameters:

Fatigue: 
Fatigue resistance increases as number of wires increases.

Flexibility: 
Small diameter and core rope have the highest flexibility.

Crushing: 
Especially important when wire rope is used on drums or sheaves.

Lubrication: 
Needed for most wire ropes.

Preformed Wire Rope: 
A process by which strands are permanently formed into the spiral shape they will assume as part of the rope.  Non-preformed wire rope tends to straighten out with tension.

Working Load:
Our working load is 121 lbs and we chose a factor of safety of 8 so that the minimum breaking strength should be 8 x Working load.

Breaking Strength (min): 
Our calculated minimal breaking strength is 968 lbs.
 
For our belt we chose a Stainless Steel full Compliant Rope 7 x 19 class strand core (preformed), manufactured by McMASTER-CARR.  This type of wire rope is ideal for applications that involve a rope being cycled back and forth over pulleys and sheaves and is pre-lubricated.

Diameter:  1/8”
Breaking Strength:  1,300 lbs
Cost Per Foot (100-299):  $1.09

Pulley Design:

For maximum rope life, we choose a pulley that has an inner diameter of 40 x Rope Diameter.

Drive system is over finite length, not a continuous loop, need to design pulley accordingly.

Pulley Diameter (min): 5”

Select pulley and idler diameter:

Pulley Diameter: 6”
Idler Diameter: 6”

Based on 6” diameter, need to determine length on pulley such that wire rope will not overlap and stall out motor:

• Total wire rope travel length: 1440” (120 ft for one way across tow tank)
• Initial assumption: Wire rope only wraps around pulley once
• For every one revolution of pulley, wire rope travels 1/8” in positive x direction:

• Length on wire rope produced by 1 revolution of pulley (pulley circumference):

•  Circumference = π * Diameter= 18.85”
• 1 revolution: 18.85” length of wire rope
• Total length of travel: 1440”

•  Pulley will need to undergo 76.394 or 77 revolutions to produce necessary length of wire rope for carriage to travel across tow tank

•  77 revolutions x 1/8” travel in x direction for 1 revolution.

• For carriage to move from one end of tow tank to another, pulley will need to rotate 77 times and wire rope will travel 9.625” in x direction along pulley.  We round this number up to 10”.
• Need to take into account extra length in pulley needed for pre-coiling.  Pre-coiling is necessary to induce positive drive.  Assume a maximum of 8 pre-coils needed.
• 8 precoils x 1/8” length in x direction along pulley for one coil.
• Find that need an extra 1” of length along pulley for precoils. 
• Total length of pulley needed 10” + 1” = 11” in length.

 


Guard Design:

Because the new pulley is longer than the previous one it was necessary to extend the guards that ran the length of the tank to accommodate this extension in length.  This was accomplished by designing guard extensions from aluminum angle stock and attaching them to the tank using stainless steel threaded rod.  It was also necessary to ensure that the guards did not obstruct the path of the wire rope.


Fabrication & Installation:

The first piece fabricated in this project was the entire pulley unit.  The pulley was made from an 11” long piece of HDPE with two stainless steel 7.5” diameter plates mounted on each end.  The plates were constructed from 10 gauger stainless steel sheet metal and were cut using a CNC plasma cutter.  The plates were then welded to a stainless steel keyed shaft which ran through the center of the pulley.  The plates were also bolted to the pulley to ensure that the shaft would induce a drive.

Once the pulley was assembled it needed to be mounted and attached to the motor.  As the new pulley is longer than the previous one it was necessary to fabricate a shelving support for the pulley.  As in all other aspects of this project it was important to mount the pulley using products which would not corrode or rot when exposed to water  With this factor in consideration the pulley was mounted by using plastic covered metal shelf brackets and a wood-plastic product.

Because the new pulley sits up higher than the previous one it became necessary to come up with a solution which would lower the height of the lateral movement of the wire rope without compromising the integrity of the rope.  This was accomplished by installing a rotating stainless steel conveyor roller which the wire rope would travel under.  This conveyor roller was installed in front of the pulley at a lower height.    

There were several steps involved in the construction of these guard extensions and because it was necessary to fabricate and install 30 of these pieces, this section of the project proved to be the lengthiest of all.

Finally the wire rope was installed.  To induce positive drive the wire rope was wrapped several times around the pulley then ran down the length of the tank and looped around the idler.  The wire rope was attached to the carriage by clamping it between steel plates which are connected to the carriage.  The wire rope ends were then connected using oval sleeves.

Testing:

Once the new belt drive was installed some testing was performed to ensure that the tow tank ran smoothly and efficiently.  A drag test was conducted on a spherical object and data was obtained from this test.

   Tow Tank Test
    Load Cell in operation

Conclusion:

The objective of our project, in short, was to replace the old belt drive system on the tow tank with a new and improved one which runs more efficiently and reliably.  We believe that we've accomplished this goal and are satisfied with the progress that we've made in the ongoing project of making the tow tank a more usable asset to the university.  The tow tank has great potential to be a useful research device for many different facets of the University.  
This project has helped to expand our knowledge of mechanical tools and has tested and improved our problem solving skills.  Overall it has been an excellent learning experience.  

Manual:

Procedure for running the tow tank:

1. On the gray power box, flip the lever to the on position.   
2. Set the desired speed (in Hz) by using the up and down arrows on the Toshiba transistor inverter.        
3. Set the direction of the carriage movement by holding down the read/write button and pressing an arrow (the screen will display forward or reverse, reverse moves the carriage away from the motor).
4. Ensure that all obstructions are removed from tank.
5. Press the green run button and the carriage should commence movement.
6. Be sure to press red stop/clear button before carriage arrives at end of tank and hits bumpers.
7. Change direction of the carriage to return it to home position.
8. When finished with carriage be sure to turn off the power to the transistor inverter by switching the lever on the gray power box to the off position.

Configuration of Strain Gage:

A strain gage is used to measure input from the load cell which is mounted on the carriage.  The strain gage used is manufactured by Omega (Newport) and the model # is DP25-S.  The link below is a brief manual which describes how to configure the strain gage.

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