Details Needed for Belt Drive Selection

One of the valuable services offered by Gates Application Engineers is assistance with belt drive selection. However, many requests do not contain all of the information needed for proper design and evaluation. When you call or email an application, please include the following details:

1. A description of the application (fan, pump, conveyor, etc.)
2. The hours per day that the drive will operate
3. DriveR rpm and horsepower/torque rating
4. DriveN machine speed or required speed ratio
5. Approximate center distance required
6. Shaft diameters (if available)
7. Any space restrictions
8. Any environmental concerns (oil, high temperatures, etc)

For more complex drives involving idlers, multiple driveNs, or special duty cycles, additional information may be needed.

NHRA Supercharger Drives

NHRA supercharger drives are notoriously hard for belts to survive on.  The application is extremely demanding, and requires a unique belt to perform as needed.   The two top classes that use superchargers are Top Fuel and Funny Car.  In a typical 1000 foot race, a Top Fuel car will have elapsed times in the 3.8 to 3.9 second range.

HP requirements to drive the supercharger are typically quoted in the 800 HP to 1000 HP range.  This is with the driving crankshaft being around 8000 - 9000 RPM.

If it was just a question of handling 800 to 100 HP, the drive (while large), wouldn't be that difficult to design to work properly.  However, the system has a lot of unique load situations that really stress the belt.  The torque  fluctuations from the engine resemble barely controlled explosions.  The speeds are high.  The heavy loads make it likely that shafts will flex and belts will track to the outside flange edge.  During tire spin conditions, drivers have to "pedal" the car.  This is a method of getting off and back on the throttle very rapidly in an attempt to get the tires hooked up and stop the tire spin.  When this happens, the driving RPM drops nearly instantly from 8000+ RPM to a couple of thousand RPM.  The inertia of the supercharger takes over at that point and it tries to become the driving shaft.  This creates havoc in the belt, as the tight and slack side spans in the system are reversed, creating high shock and compressive loads in the tensile cord.  Add in the virtually instantaneous load application when the cars leave the starting line, and you have a very difficult, very damaging application.

An 11mm pitch Poly Chain GT belt was specifically designed for this application.  The 11mm pitch gives more speed ratio flexibility than the previous 14mm pitch belts, and 11mm belts have more strength and ratchet resistance than 8mm pitch belts.   A specially designed tensile cord is also used to handle the loading situations described above.

Add all that together, and you have the situation you'll see on any typical weekend NHRA race.  Top Fuel and Funny Cars going down the track, round after round, reliably.   Its one of the 7 belt wonders of the world.

Belt Profiling

If you know anything about synchronous belts you'll know that belts come in different sizes. This can mean different widths, lengths, and pitches (spacing between teeth). So if you know these three dimensions of a belt it means you can go to the store and pick up a new one right? Wrong. One other important characteristic you need to know about a synchronous belt is the tooth profile.

The tooth profile is going to identify the shape of the tooth whether it's flat, round, or a combination of both. Industry profile standards were created for belt manufacturers to simplify the process of designating a belt type. For example "Timing belts" have flat or trapezoidal teeth where "HTD belts" have round teeth. As technology has advanced, so too has the synchronous belt tooth profile. Multiple variations of tooth profiles have emerged from different manufacturers such as GT, RPP, HTD, and STPD profiles, as seen below.

Because of the difference in shape amongst the profiles, not all belt tooth profiles work in each others sprocket grooves. This is one very common mistake made with synchronous belt drives. A belt is replaced with another one that has a different tooth profile and it fails much faster than the previous belt. This is because the tooth is not sitting into the sprocket grooves properly so it's not transmitting the load as it was designed to.

Now this doesn't mean that two different belts are never compatible with each other. There are actually quite a few that you can use as drop in replacements for one another, but you need to know which ones will work and which ones wont. Gates has published a catalog just for this problem called the Belt/Sprocket Interchange Guide. In the guide you can look up which belts will work in different sprocket configurations and which ones will not with a description of why or why not. Of course its always recommended to use the belt that matches the sprocket grooves, but sometimes this will not be an option. The Gates Belt/Sprocket Interchange Guide can be found online at www.gates.com/drivedesign.

Gates Carbon Drive

In a previous post, I had mentioned Gates Carbon Drive for bicycles. I’d like to give you a little bit more information about this great product. As a quick recap, the Gates Carbon Drive system is two sprockets and a belt. The belt is a version of our market leading Poly Chain Carbon belt. This is a polyurethane belt with carbon fiber tensile members. The bicycle belt has been optimized for operation on bicycles, and the sprockets are very specific, lightweight, machined parts. In fact, the sprockets are made to bolt directly on to existing bicycle components. Now a very important fact to mention is that while the sprockets can bolt on to stock components, most bicycle frames require the chain to pass through a rear triangle to get to the rear wheel. Because the Poly Chain belt cannot be broken and reconnected, there has to be a gap in the frame for the belt to pass through. This makes retrofitting the system almost impossible for most consumers. While this is unfortunate, there is a big light at the end of the tunnel. Because this product is a revolution in this market, bicycle manufacturers such as Trek, Specialized, Norco, etc… have jumped on board, and are building frames made specifically to accept the Gates Carbon Drive. These frames have both the break in the rear triangle that is needed, as well as an adjustment to add proper tension to the belt. Now if you are familiar with belt drives, you probably know that as important as tension is, alignment is just as important for proper operation. This means that the belt cannot be derailed, which makes typical shifting on a bicycle through a derailleur impossible. The good news is that the world of internally geared hubs is growing quickly. An internally geared hub is a rear hub that uses gearing on the inside of the hub to offer several different final drive options. Some of the more common hubs are offered in 3, 7, and 8 speeds. This means that the Gates Carbon Drive system has options for gearing on bicycles.

Now we covered a bit on installation, and a bit on gearing options. Your next question may be “so what is the benefit of a belt to a chain?” Well the answers are not that different from the reasons you would use a belt in the industrial market. The belt is clean, it doesn’t need to be lubricated, it doesn’t stretch - therefore reducing maintenance time, it doesn’t rust, it lasts longer than chain, it’s quiet, smooth, and light weight. These characteristics make it ideal for your average bicycle commuter; think about it, the belt isn’t greasy, doesn’t need to be adjusted once set up, doesn’t have a problem with the elements, and lasts longer. This doesn’t mean that it doesn’t work for other applications. Our patented Mud Ports (holes in the bottom of the sprocket groove) shed dirt, rocks, and snow like nobody’s business. This makes the system viable for mountain bikes as well.

Hopefully this post has given you enough background information to spark your interest in Gates Carbon Drive. To see our product line, and to find a listing of current manufactures and models offering the Gates Carbon Drive, please visit www.carbondrivesystems.com Hope to see you out there!

Bigger is Not Better – Overdesign Impacts Performance

A common perception with industrial belt drive designs is that belt drives with greater load capacity will always outperform those with less load capacity. While this may seem logical , implementing grossly over-designed drives can actually reduce overall performance. “Bigger” is not always “better”. When designing industrial belt drive systems, remember to check rated drive load capacities against design loads. Confirm that over-design ratios are not excessive. Over-design ratios should not typically exceed 2.0 for both synchronous and V-belt drives. For over-designed belt drive systems, re-calculate belt installation tension levels based on rated drive load capacity to improve belt performance or re-design for appropriate load capacity.