A Technical Primer on Synchronous Drives

Synchronous Belt Drives Restore Energy Savings

A manufacturer of neck-type beverage bottles uses a conveyor that feeds waste glass into a grinding mill for reuse. Frequent peak loads caused the original set of two, four-stranded joined belts to slip, resulting in lost energy and premature belt failure. Gates Polychain GT® synchronous belts were installed on existing equipment and have not required retensioning or any other type of maintenance.

The new drive draws only 9.2 amps -- a 38 percent savings in amperage draw. The company received an $84 rebate from the power company for the energy savings changeover. And, the company is saving more than $60 a month on their energy bill. The drive investment was recovered in four months based on energy savings alone.

At one time, a kilowatt hour (KWH) of power was available to industry for two or three cents. Today, this luxury has disappeared, and plant engineers everywhere are evaluating all aspects of their operations to increase the efficient use of energy. The trend toward synchronous belt drives can translate into significant dollar savings due to increased system efficiency.

Eliminating KWH losses in power transmission systems is a high priority item for any plant engineer. In an era of high energy costs -- more than 250 percent higher than 10 years ago -- a close look at drive system efficiency losses may translate into large dollar savings. Otherwise, an average 35 percent premium paid for energy-efficient motors may be wasted through energy dollars lost due to inefficient drives.

Subsequently, power transmission drive systems designers are tending toward specifying synchronous belt drives to obtain that energy-saving edge over alternative systems.

Belt Basics

Synchronous (or toothed) belts are suited for a range of applications. These belts

  • require no lubrication

  • resist corrosion

  • are basically unaffected by abrasive particles

  • can often operate in wet conditions.

The newer curvilinear tooth configuration provides greatly increased power ratings for these belts. Through improved stress distribution, this design allows the teeth to withstand the shearing action of high torque loads without separating from the belt.

Helically wound tensile cord members impart belt strength and provide exceptional flex and elongation characteristics. A wear- resistant nylon fabric covering the tooth affords extra protection. The neoprene backing insures the tensile cords are protected form oil, grime and moisture.

Testing Proves Synchronous Belt Efficiency

Gates belt test laboratory supports the conclusion that synchronous belts use less electricity. A 3V belt drive was tested next to an 8mm synchronous belt drive under exactly the same conditions. Efficiencies were measured at shaft speeds ranging from 1160 RPM to 3500 RPM. Various pulley diameters and variable loads from 2 to 25 HP were used. The tests measured the relationships between efficiency, torque, speed, pulley size, and single versus multiple V-belts. Here are the major findings:

  • Power transmission efficiencies generally increase with torque. Efficiency increases under increasing torque test conditions with synchronous belts, but declines for V-belts as a slippage increases when torque overcomes the preset static tension.

  • Pulley diameter affects efficiency, with larger pulleys producing greater efficiency.

  • Narrower or fewer belts tend to produce higher efficiency at low torque, whereas wider or more numerous belts offer higher efficiencies at high torque.

  • Underbelted or overbelted V-belt drives become inefficient, while synchronous belts remain relatively constant.

  • There are relatively large variations between V-belt drive efficiencies, while synchronous belt drive efficiencies can be accurately predicted.

Proper maintenance of belt drives can help alleviate the concern of plant engineers over energy loss. V-belts can be made to run with as high as 95-98% efficiency at the time of installation. In operation, this efficiency may deteriorate as much as five percent due to belt slippage. Synchronous belts start operation at zero percent slip and stay that way. In addition, low bending stresses result in minimal heat buildup, allowing synchronous drives to be consistently as high as 98% efficient. Typically, V-belts run at 40 F. to 80 F. above ambient temperatures -- another sign of slippage.

V-belts tend to stretch during their life, causing initial tension to drop. If maintenance and proper tensioning of V-belts are neglected, slippage can increase dramatically, resulting in energy losses of as high as 10% in poorly maintained drives.

Conversion Costs

"Payback" of conversion cost to a synchronous system from a V-belt system can usually be realized quickly. For example, assume a situation where a 75-HP fan drive is converted to synchronous belts for continuous operation.

Here's a before-and-after conversion comparison between V-belts and synchronous belts:

Approximate cost of system with V-belts:

One 4-groove B13.6" sheave


$ 102.00

One 4-groove B18.4" sheave


$ 130.00

Four B136 V-belts


$ 108.00



$ 340.00

Approximate cost of system with synchronous belts:

One P52-14M-85 sprocket


$ 220.00

One P72-14M-85 sprocket


$ 320.00

One 3150-14M-85 HTDR belt


$ 478.00




The synchronous belt costs approximately $678.00 more, but assuming 5 percent slip and $0.08/KWH, the synchronous belt yields a predicted energy savings of $2,149.00 per year. This does not take into account the savings in reduced maintenance or downtime. See Figure 3.

The complete drive conversion cost can be paid for in 0.32 years (just less than four months). This payback time is determined by dividing the synchronous drive cost by the annual energy cost savings.

In addition, recommended installation tension for a synchronous belt is often less than that of V-belts. Shaft bearings will operate under lower average loads, providing significantly longer service life.

In today's economic and energy climates, plant engineers need to utilize drive systems that can get the job done economically and efficiently. The switch to synchronous belts offers favorable financial benefits. Considering a plant whose belt drive systems carry 5,000 HP to 10,000 HP, or higher, annually; money "burned up" in slippage alone can amount to tens of thousands of dollars.


Hours in Operation
Connected Up 40 Hour Week 80 Hour Week 168 Hour Week
10 $ 75.00 $ 149.00 $ 314.00
15 $ 109.00 $ 219.00 $ 461.00
20 $ 144.00 $ 288.00 $ 606.00
25 $ 176.00 $ 352.00 $ 741.00
30 $ 209.00 $ 419.00 $ 879.00
40 $ 279.00 $ 558.00 $ 1,171.00
50 $ 345.00 $ 690.00 $ 1,448.00
60 $ 414.00 $ 827.00 $ 1,738.00
75 $ 512.00 $ 1,023.00 $ 2,149.00
100 $ 682.00 $ 1,365.00 $ 2,864.00
125 $ 843.00 $ 1,686.00 $ 3,542.00
150 $ 1,011.00 $ 2,024.00 $ 4,250.00
  • Estimated annual energy dollar savings per motor using a synchronous belt drive system instead of a conventional V-belt drive system. Calculation is based on $0.08 per KWH and assumes 5 percent slip. (A KWH may range in cost from a low of $0.03 to as high as $0.09.)

Copyright © 1997. The Gates Rubber Co., Denver, Colorado.


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