Considerations for Synchronous Belt Drives on HVAC Applications

Some HVAC drives will experience high starting loads if the motor has an across the line start.  The fan will be forced to ramp up to speed nearly instantaneously, and the start up loads can be as much as 150% to 200% of the normal operating loads.

Designers that are considering using synchronous belts on HVAC drives need to be aware of this potentially high start up load.

If a soft start or variable frequency drive (VFD) is used on the drive, there is no need to take any special design precautions as the load is ramped up gradually.

If an across the line start up happens infrequently, simply applying a bit more installation tension will help prevent any ratcheting or start up issues.  

If start ups occur regularly and/or frequently, it is good practice to add .2 to the service factor when designing the synchronous belt drive.  This will provide a slightly more conservative drive design that will avoid any issues at start up. Learn more about HVAC drives and synchronous belts at Gates.com.

Taper Lock Bushings

I’ve been talking to people about Taper Lock bushings for quite a while now. However, the other day, I learned something new that I thought was pretty useful. Taper Lock callouts have always seemed a bit out of left field to me, but I figured there had to be a reason behind it. Turns out there is! A Taper Lock part number is called out by four numbers followed by the shaft bore that it will attach to. The first two numbers represent the maximum bore size, and the last two represent the total length. For example, a 2012 has a maximum bore of 2.0 inches, and a length through bore of 1.25. To properly call out a 2012 bushing to fit a shaft (let’s use 1.5” for example), you call out the whole inch followed by the fraction. This means our 1.5” 2012 bushing would be called out like this: 2012 1.1/2. Metric is pretty simple, you call out the bushing followed by the shaft diameter and include MM behind it; let’s use 20mm as an example size. The Metric convention would look like this: 2012 20MM I know this will certainly speed up my answers, hopefully it will help you too!

Synchronous Belts & Exposure To Water

Light and occasional contact with water (occasional wash downs) should not seriously affect synchronous belts in general. Prolonged contact (consistent spray or submersion) can result in significantly reduced tensile strength in fiberglass belts, and potential length variation in aramid belts.

Prolonged contact with water also causes rubber compounds to swell, although less than with oil contact. Internal belt adhesion systems are also gradually broken down with the presence of water. Additives to water such as lubricants, chlorine, anti-corrosives, etc. can have a greater detrimental effect on belts than pure water.

Poly Chain GT Carbon belts use polyurethane compound and carbon fiber tensile materials that are both very resistant to water. Poly Chain GT Carbon belts can withstand prolonged contact with water without detrimental effect.

Sprockets operating in the presence of water may be vulnerable to rust and corrosion unless resistant materials or coatings are used. Sprocket corrosion can result in accelerated belt wear and tension loss. These can both shorten belt life significantly. For assistance in the design and acquisition of rust and corrosion resistant sprockets and bushings, contact Gates Made-To-Order Metal team.

Gates Drive Design Manuals

Customers often ask where they can find technical information about Gates belt drives. The Gates Drive Design Manuals are a great resource for this type of information.

There are several manuals available at www.gates.com/drivedesign/:

• Poly Chain® GT® Carbon
• PowerGrip® GT2®
• Heavy Duty V-Belt
• HVAC
• Light Power and Precision
• Air-Cooled Heat Exchanger

Below is a sampling of the topics included in the manuals:

• Drive selection procedures (including power rating tables)
• Gates hardware specifications (for standard sheaves, sprockets, bushings, etc).
• Discussion of design considerations such as installation tension, conductivity, alignment, noise, flywheel effect, NEMA motors, tolerances, idlers, etc.
• Standard calculations
• General troubleshooting guidelines

Splicing and Clamping Synchronous Belts

The question is often raised if it's possible to cut a synchronous (timing) belt into a custom length and splice the belt back together when a standard length does not fit the application. To answer this question: yes it can be done, but not very easily. Splicing two ends of a synchronous belt can be done when the belt is made with a particular type of polyurethane construction and additional polyurethane is used as "glue" to attach the ends of the belt. After this splicing process you can expect about 40% of the initial strength of the belt. This is because once you cut the tensile cord you've eliminated the strongest part of the belt. The spliced section is now the weak link of the belt, and like your co-ed softball team, you're only as strong as the weakest link. There are currently no processes to splice together a rubber synchronous belt.

Clamps are often used to attach two ends of a synchronous belt together. If this is done, the clamp must be contained on the belt span between the two pulleys. If a clamp ever attempts to ride over a pulley, catastrophic results will surely follow. If a clamp is used to attach two ends of a synchronous belt, a minimum of 6 teeth in mesh on each side of the belt is recommended. Anything below 6 teeth will lower the belts available working strength (6 teeth = tensile strength of belt), and premature belt failure from tooth shear is more likely to occur.

Synchronous Belt Storage

Did you know that something as simple as storing your belts properly can make a huge difference in your belts life and performance? Here are a few storage recommendations that will keep your belts out of trouble, and performing the way they were designed to.

Whenever possible, store your belts in the original packaging until the moment it is ready for installation. This is the best way to keep from damaging the tensile cords of the belt.

Keep the belts away from direct sunlight and moisture. One way to avoid this is by keeping your belts away from windows.

Keep the belts out of the heat. Do not store your belts near heaters or radiators. Make sure that they are not in the airflow of a heating device.

Don’t store belts on the floor. This may seem a little over the top, but floors are high traffic areas, and a belt could easily be stepped on or rolled over, causing tensile cord damage.

Keep the belts away from chemical exposure. Any place that has solvents in the air could damage your belts. This includes chemicals such as ozone, which could be present near transformers or electric motors.

Belts should ideally be stored in a cool dry environment at a temperature of 85 degrees F or less with lower than 70% relative humidity. If done properly, a belt can be stored for up to 6 years.

Synchronous Drives Operating in Environments with Dust and Debris

Dusty environments do not generally present serious problems to synchronous drives as long as the particulates are fine and dry. Note, though, that particulate matter can act as an abrasive resulting in more rapid belt and sprocket wear. Damp or sticky particulate matter deposited and packed into sprocket grooves can cause belt tension to increase significantly. This increased tension can impact shafting, bearings and framework and can even result in belt tensile failure. Static electrical charges within drive systems can sometimes attract particulate matter, so may need to be dissipated to ground.

Debris should be prevented from falling into any synchronous belt drives by using screens or guards. Debris caught in belt drives is generally either forced through the belt or may result in a stalling of the system. In either case, serious damage will occur to the belt and related drive hardware.