Polyflex Belts Unique Characteristics

Gates Polyflex belts have some unique characteristics. Therefore, special considerations may be needed in designing drive systems and in assessing operating environments:

Liquids act as lubricants – Because of the broad 60° groove angle, belt wedging forces are reduced making the coefficient of friction between belts and sheave grooves critical in transmitting load. Liquid contaminants such as water, chemicals or oils can decrease the coefficient of friction, allowing belt slip. Belt slip can result in belts turning over in the sheave grooves. Heat from belt slippage may also cause belt softening or melting, resulting in catastrophic failure. Adequate shielding should be used when liquid contaminants (either airborne or splashed) are present near Polyflex drive systems.

Damage from debris – The 60° belt sidewall angle and thin cross section both make these belts susceptible to debris. Debris can cause belt instability resulting in belt turn-over, which will result in catastrophic failure. Adequate shielding should be used when debris is present near Polyflex drive systems.

Backside idlers – Polyflex belts running on flat backside idlers can create objectionable noise due to the molded ribs on the back of the belts. Perceived sound level is inversely proportional to the sheave diameter, so using larger idlers can reduce generated noise.

Static conductivity - Polyflex V-belts do not meet the RMA requirements for belt conductivity. Polyflex V-belts should not be used in potentially explosive or flammable environments without the use of adequate static dissipating devices such as grounding brushes.

Measuring A Synchronous Belt

We get a lot of questions from people trying to find a suitable replacement belt. There seems to be a lot of confusion as to what information they need to provide for us to try to ID the particular belt. It’s not as difficult as it seems. The first question that we are going to ask you is to get us any print on the back of the belt. Industry callouts are pretty similar, and if you can find a part number, a lot of times we can ID it from that alone. Now if it comes down to measurements, there are only a couple that we need: length, width, and pitch. We also need to know the shape of the teeth. This is pretty simple, as there are only about 3 different types that we can interchange for power transmission belts; square/trapezoidal, half circle, and curvilinear with a flat spot on the top of the tooth. These shapes help us ID the type of belt, from here, all we need are the other three measurements to really get going.

Length: measure the outer circumference of the belt. If the belt is broken in half, just measure the total length.

Width: measure the width across the back of the belt. This could be in metric or imperial units; it doesn’t matter which one you give us, we just need the best measurement you can get.

Pitch: measure from the center of one tooth to the center of the next tooth. Simple, but sometimes the teeth are small. Getting as close as possible on this really helps – see above about the metric/imperial units issue.

We will also ask you to count the teeth, it’s not that we don’t trust your measurements, but this is verification of the correct length and pitch measurements. I know, I know… There may seem to be a lot of them, but even a belt with 200 teeth will only take a minute or two to count.

That’s it, from here, all it takes is knowledge of our product line to ID the type of belt, and a catalog to see if it’s a stock size. Sometimes we can’t cross a belt because it was made special for someone. In these cases you will have to go back to the manufacturer of the machine that your belt came on – even if we made the belt in the first place.

Fan Drives: High Start Up Loads and Synchronous Belt Drives

Start up loads can be a significant concern when evaluating potential drives for conversion to PowerGrip® GT®2 or Poly Chain® GT® Carbon belt drives.  PowerGrip^® GT^®2 and Poly Chain^® GT^®Carbon belt drives will transmit all of the start up torque, where V-belts may slip if the load is excessive.  Due to the inertia of the fan, start up loads can potentially be 150% to 200% of the normal operating load.  This is obviously much more of a concern when the drive will be operating on a system that frequently cycles on and off.  Drives that run continuously will only see the start up load intermittently, so are not as sensitive to the combination of high start up loads and weak structures.  It is important that the start up load be considered when evaluating a drive.  If the structure is weak, a high start up load will further adversely effect the PowerGrip^® GT^®2 or Poly Chain^® GT^® Carbon belt drive's performance by allowing center distance collapse.  This reduction in center distance results in an under tensioned belt which may wear prematurely from being undertensioned, or even worse, premature failure from ratcheting.  If an electrician or properly trained technician is available, an ammeter can be used to compare the start up amperage to the steady operation amperage.  If the amperage is 1 1/2 to 2 times the steady state amperage, the structure should be carefully inspected to insure that it is robust enough to prevent center distance collapse upon start up.  With the drive shut off and safely locked out, the structural rigidity can be checked by pushing the two belt spans inward toward each other and looking for any relative movement in the structure.

Belt Drive Inspection and Replacement

We sometimes get questions about how often belt drives should be inspected or replaced. This does not have a simple answer because belt wear and life depends on a variety of factors. Belt life depends on the original drive design, the actual loads experienced vs. the design loads, the pulley alignment, the installation tension and maintenance, and environmental conditions such as heat and chemical exposure. As you can see, many of these factors are out of our control, so therefore it is nearly impossible for any manufacturer to give an accurate estimate of the belt life without testing the actual drive. There are also many things to consider when deciding how often to inspect the drive including: Critical nature of the equipment Drive operating cycle Accessibility of equipment Drive operating speed Environmental factors Temperature extremes in the environment Probably the most important factor is the critical nature of the equipment. A belt powering an integral process to a manufacturing line should be inspected much more frequently than a small, seldom used HVAC unit should be. If the belt were to fail and shut down the assembly line, it could cost the operator a lot of money in downtime. Our general recommendations are to do a quick visual and noise inspection every one to two weeks for critical drives and once a month for normal drives. A complete shutdown inspection should be completed every three to six months. These can be adjusted depending on the factors above. For more information about drive inspection, see our Belt Drive Preventive Maintenance Manual available here: www.Gates.com/Catalogs

Selecting the Right Synchronous Replacement Belt

When replacing a synchronous belt on an existing drive, it is important to select a belt that is compatible with the sprockets. Selecting the right belt will optimize the performance and longevity of the drive.

However, the variety of styles available from numerous manufactures increases the difficulty of identifying a replacement belt. To help users select the proper replacement belt, Gates has created the Gates Belt/Sprocket Interchange (http://www.gates.com/index.cfm?location_id=3881). At this site, users can select their sprocket type and determine the best Gates replacement belt.

Rim Speeds and Maximum Diameters

Occasionally I have people asking me what the speed rating in on a specific belt. Here at Gates, we don’t give our belts a maximum speed rating like some of our competitors do. How fast is too fast then? It depends on the metal that you want or need to run. All of our sprockets and sheaves are statically balanced. We rate them to be able to run comfortably up to 6500 feet per minute (feet per minute is based on rpm AND diameter). If you plan to run them faster, you will need to have them dynamically balanced for the maximum speed you are going to reach. So what does this mean for a maximum belt speed? There are unfortunately a lot of things to consider if you’re going to be running at high speeds. If you are looking to run past 6500 ft/min, we recommend that you contact Product Applications so that we can discuss what you might expect when trying to design for high speeds.

Belt Drives and Environmental Temperature

Both high and low environmental temperatures can present problems with belt drive systems.

In general, the maximum recommended temperature for belt drive systems for standard belts is 185 deg. F (85 deg. C). For rubber belts, temperatures exceeding this can result in gradual compound hardening and eventual cracking as belts stiffen. For urethane belts such as Poly Chain GT Carbon, the urethane will may begin to soften and will eventually melt at temperatures exceeding 200 deg. F (93 deg. C). There are a few options for belt drives operating in high temperature applications, but belts made from materials with higher temperature resistance are still limited to a maximum temperature of about 230 deg. F (110 deg. C).

Belts can "overheat" from slippage even when environmental temperatures are not excessively hot. If belts are hard and appear glazed from heat, always check to make sure the tension is at the recommended level, and that sheave grooves are not worn excessively. In most normal environments, belt surface temperatures do not typically exceed 120 deg. F (49 deg. C) or so.

Belts are generally limited to a minimum temperature of about -30 deg. F (-34 deg. C). Rubber belts operating in temperatures lower than this can harden and crack. Cold soaked belt drive starts are especially vulnerable to belt cracking. Poly Chain GT Carbon belts are capable of operating at temperatures down to -65 deg. F (-54 deg. C).