Week 6 Outside Meeting: Researching Materials

Flat Belts

(the following information was from: http://www.gatesmectrol.com/common/downloads/files/mectrol/brochure/GatesMectrol_Belt_Pulley_Catalog_5_11.pdf )

Features:
Smooth, vibration free operation
Use with small pulley diameters
High strength, low stretch for long life
Sealed edges, no cord fraying
Easily guided with flanged pulleys
Kevlar or steel cord construction
No lubrication needed
No retensioning required

Application Characteristics:
Heavy load lifting or lowering
Allows for "slip" requirement
Smooth uniform motion
Small bending radius for small design envelope
Very low stretch characteristics

As seen from the photos, the best part about flat belts is how it can be used on a smaller scale, the make of a flat belt purposefully reduces stretching and wearing out, and can be used with pulleys with smaller diameters. The charts are helpful when we need to determine the coefficients of friction and weight etcetera when we need to decide what the flat belt's material should be.

Also, for future reference, we can use the following site:

http://gatesmectrol.com/mectrol/brochure.cfm?brochure=5256&location_id=6150#TOPOFPAGE : this chart (materials for belts)

Pulleys

Needs to be:

1. groove to keep pulley in place

2. waterproof (either waterproofed easily or already)

3. rigidity

4. machine-ability

5. light-weight

6. low maintenance

7. able to keep belt on the track with minimal outside help

http://engapps.gates.com/LinearApp/MotionCalculator/Step1 : helps to determine which belts, pulleys, and what materials would be needed based on certain input parameters

https://woodgears.ca/bandsaw/crowned_pulleys.html :

Gives a short history of how crowned pulleys were used with flat belts in addition to more information of flat belts and crowned pulley applications. 

While nowadays we use V-belts, the older technology used flat belts in combination with crowned pulleys. A crowned pulley is a pulley that has a slight hump in the middle and tapers off slightly toward either edge (as shown in the photo below).

This tapering edge helps to keep the belt on track automatically. In order to better illustrate this concept of the pulley helping the belt to stay on track, the author demonstrates with a rubber band:

"The key to understanding the flat belt tracking on a crowned pulley is to look at how a belt flexes when pulled more on one edge than another. I'm pulling the rubber band in the photo on just the right edge. With more tension on the right side, that side stretches more, and the rubber band forms a slight arch. In an actual belt, this stretch is too subtle to be seen by just looking at it.

"To better demonstrate the principle, [the author] built a jig from Lego, using a rubber band and an exaggerated crowned wooden pulley. The higher section of the crowned pulley puts more tension on the rubber band than the narrower edges. As a result, the rubber band flexes into a slight arch towards the middle. As the rubber band winds onto the pulley, this arch causes the band to always wind further up on the conical section than what was previously wound on. The higher point on the pulley always creates more tension in the belt and causes it to arch in that direction.

"With this exaggerated crowned pulley, it takes just a few turns for the rubber band to wander from the edge all the way to up the center hump. Once the rubber band is on top, the maximum tension will be in the middle of it, and it no longer has any reason to arch in either direction...With much more subtle crowning on a typical pulley, the self-centering of the belt happens more slowly." 

The following information was quoted from http://www.niba.org/index.php/resources/technical-articles/0603-crowned-pulleys/

"In order to be effective, a crowned pulley requires that the conveyor system have enough tension in it to force the belt to conform to the configuration of the pulley. Experience has also shown that a crown is most effective when it has a long unsupported span of belt approaching the pulley (3 feet plus, with little added effect over 10 feet) That is, the belt must be free of the effects of troughing idlers, rollers, slider beds, etc., for the crown to offer significant tracking advantages.

"...A crowned pulley should never be run against the coated conveying side of a lightweight belt.

"...Belts with high transverse rigidity require less slope to the face of the pulley to allow the belt to conform to the crown and influence the tracking of the belt."

 Essentially, a crowned pulley needs a lot of tension from the belt in order to actually keep the belt tracked. 
So we next had to research how to calculate the tension of the flat belt we would need.

We will be using the calculator program from http://www.niba.org/resources/belt-tension-calculator/. The math behind this calculator is clearly explained on their website.

From the site http://www.visusa.com/belt_tracking01.htm, we looked up the typical crown radius specifications when used specifically with flat belt pulleys.

Week 6 in class plan: 
1. Group: Continue discussing materials, ask for teacher opinions 
2. Courtney and Jess: discussing electronics, what we need to build or buy, how we're going to do that
3. Sarah and Megan: purchase materials for pulley and belt 
4. Jacque: design and plan to purchase materials for raindrops 

5. Group: Think about prototyping possibilities