Calculating Proper Belt Length

We are often contacted to calculate the belt length of a specific drive given the center distance, and pulleys sizes are known.  Of course, we are glad to help in any way possible but if this calculation is needed after hours, or a phone is not readily available to you, it can be made through the equation below.

Where:

Lp = Belt pitch length (inches)

C = Center distance (inches)

D = Large pulley diameter (inches)

d = Small pulley diameter (inches)

For additional information on this or any other topic please contact Gates Product Application Engineering by phone at 303-744-5800 or by email at ptpasupport@gates.com.

Product Application Engineers on YouTube

Are you looking for an easy way to learn more about Gates belts and hoses?

Head over to www.YouTube.com/GatesPT/ for videos created by the Product Application Engineering team.

Once on the site, navigate to the Learn with the Gates Product Application Team and Lessons from Gates Professionals play lists to get quick access to the engineer's videos. Recent additions include topics such as:

• TaperLock Bushing Installation
• QD Bushing Installation
• Gates Super HC XP belts
• eCrimp
• Gates Mectrol – Specialty Belts

Feel free to contact us at ptpasupport@gates.com or +1.303.744.5800 if you have any additional questions.

Belt Drive Design Tension Ratios

As they operate, belt drives use span tension differentials to create a net effective pull. The two belt spans are defined as the tight side span (higher tension value) and the slack side span (lower tension value). The drive loads, as well as installation tension, impact the span tensions.

The tension ratio is defined as:

Tension Ratio = Tight Side Span Tension (lb) / Slack Side Span Tension (lb)

Based on different operating characteristics, belt drives types have different design tension ratios. Assuming 180° wrap on both pulleys, here are some typical design tension ratios:

• 4:1 for Micro-V® Belt Drives
• 5:1 for V-belt Drives
• 8:1 for Synchronous Belt Drives

Assuming other drive parameters to be equal, larger tension ratios result in lower belt pull values.  

Temperature Effect on Belt Storage

To ensure belts maintain full performance capability, six years is the maximum recommended storage time.

To achieve full storage life, belts should be stored at temperatures below 85°F.  For every 15°F increase above 85°F, the recommended storage time is reduced by half.  It is not recommended to store belts at temperatures above 115°F.

For additional storage recommendations, refer to the Belt Drive Preventive Maintenance & Safety Manual: BeltPMManual.pdf

What is Belt Pull?

Belt pull estimates the force a belt exerts on a shaft. It is the force applied to a shaft by the two belt spans entering and exiting a pulley.  Belt pull does not account for pulley weight or pulley location on a shaft.  While related, bell pull should not be considered to be interchangeable with bearing load or over hung load values. 

Belt pull values can be calculated using Gates Design Flex® and Design IQ® software programs, or by using formulas found in the Gates drive design manuals.

Right from the Start - Belt Storage

To get the most from a belt drive, its important to properly store a belt -all the proper installation procedures and design practices can be for nothing if a belt is improperly stored or damaged in storage.

A few of the major belt storage guidelines:

1) Belts can be stored up to 6 years if properly stored at temperatures less than 85 degrees F, and less than 70% relative humidty.

2) Make sure the belts are not bent to diameters smaller than the minimum recommended diameter for that cross section.

3) V-belts can be stored by hanging on a wall rack if they are hung on a saddle or diameter at least as large as the minimum diameter recommended for the cross section. Don't hang from a pin or nail!

4) Don't store in direct sunlight.

5) Don't store near heating devices.

6) Don't store near ozone generating devices such as transformers or electric motors.

7) Don't store belts where they are or could be exposed to solvents or chemicals.

8) Don't store on the floor unless in a proctive container.

9) Don't crimp belts, either in storage or handling.

Complete guidelines can be found in the Gates Belt Drive Preventive Maintenance & Safety Manual.

Water & Synchronous Belt Resistance

Light and occasional contact with water, such as occasional wash downs, should not generally affect synchronous belts seriously. Prolonged contact such as spray or submersion, however, can have detrimental effects.Rubber synchronous belts are considerably more vulnerable than Poly Chain GT Carbon belts.


With rubber synchronous belts, water soaking reduces the tensile strength of fiberglass tensile cords in addition to breaking down adhesion systems between the cord and the rubber compound. While aramid type tensile cords are more resistant to water than fiberglass, they are dimensionally unstable in the presence of water or humidity, so can result in belt length stability problems. Soaking also causes the rubber body to swell, lesser than but similar to oil contamination. This can negatively impact belt pitch fit with pulleys and sprockets in addition to material weakening. Additives to water such as lubricants, chlorine, anti-corrosives, etc. can intensify detrimental affects of water soaking.


Poly Chain GT Carbon belts are quite resistant to water soaking, and have even been used successfully in submerged applications. While urethane can exhibit a small amount of swelling, overall belt performance and the ability to transmit power remains relatively stable.


When considering the use of synchronous belt drive systems in moist or wet environments, the resistance of the hardware to corrosion is also very important. Coatings or treatments used on standard iron based hardware may not have adequate corrosion resistance. Special corrosion resistant coatings such as zinc or nickel plating are available on a made-to-order basis.


Contact our Made-To-Order Metals Group at (800) 709-6001 for further information about hardware corrosion protection options. Contact our Product Application Engineering Group at (303) 744-5800 for further information about applying belt drive systems in adverse environments.

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.