Technology Spotlight: Synchronous belts - Sometimes a great motion

The teeth on a synchronous belt prevent slippage, while its shallow depth reduces friction. (Artwork by Washington State University)

About one third of the electric motors in the industrial and commercial sectors use belt drives, the majority of which use V-belts.  One alternative is the synchronous belt (also called timing, positive-drive or high-torque drive belt), which is toothed and requires installation of a mating toothed-drive sprocket. 

Synchronous (sync) belts never slip, and with their shallow depth they have less flexing friction.  This gives them about a 5 percent efficiency advantage compared to V-belt drives in a typical service application of load variation and periodic re-tensioning. The V-belt may be only 3 percent less efficient when perfectly tensioned and exactly at its load rating.  However, it loses efficiency faster than a sync belt when it departs from this ideal. 

A good rule of thumb is that approximately 50 percent of existing V-belt drives would be good potential candidates for conversion to sync belt drives.  Consult manufacturer application literature or a reliable vendor to find out if you have a good application, and to choose the best sync belt for your needs from the many product lines.

Sizing is critical

Sprockets must be correctly sized for the new belt to deliver energy savings. (Photo by Washington State University)

Be careful on sprocket sizing to ensure that you actually get the energy savings you intend.  A sprocket pitch ratio that is the same as the original V-belt sheaves pitch ratio will reduce slip, making the load turn faster. That increase in speed will require a net increase in energy even if the belt drive power transmission efficiency has increased by 5 percent. 

To achieve the same driven speed, the pitch ratio of sprockets (driven over driving) should be targeted for about 5 percent higher than the initial V-belt sheaves pitch ratio.  Sprocket catalogs show that incremental size changes vary, but a 4- or 5-percent size increment is typical.  Therefore, you will not be able to find the exact pitch ratio.  Choose a sprocket pitch ratio at least 3 percent larger than the initial V-belt sheaves ratio to accommodate finite size increments without significantly slowing the driven load, which could be detrimental to the driven load performance. 

When not to convert to a sync belt drive

Sync belts are always more efficient than V-belt drives, but three considerations preclude them from many applications.

1. They make a whirring sound that can be objectionable in some applications.

2. They transfer more vibration and shock load, and are completely unable to slip when driving loads that are susceptible to jamming.

3. They are more sensitive to misalignment, which can cause them to wear unevenly or fail prematurely.  A major belt drive manufacturer states that they should not be used in applications where misalignment is inherent to the drive application such as in any framework (motor mount, structural support, etc.) that flexes under load.  When rigidity is not good, the driving and driven shafts can also be flexed toward each other in high torque situations like starting or jamming, causing the sync belt to jump teeth and be damaged.

Costs and benefits of converting

Sync belts require less maintenance than V-belt drives because they do not require periodic re-tensioning and sheave realignment.  That is partly because the motor and load for which sync belts are suited must be rigid to begin with or modified for greater rigidity when retrofitted with sync belts.  Sync belts are not subject to the loss of tension that occurs in V-belt drives where sheave and belt surface wear allow the V-belt to penetrate deeper into the sheave groove.  The slight tension decrease from stretching or wear does not cause slippage, so it is not necessary to tighten the sync belt periodically to maintain efficiency. 

Sync belts can require more frequent replacement. A good V-belt application has 24,000 operating hours, whereas 12,000 operating hours is more typical of a sync belt application.  De-rating to 60 percent of design rating would extend a sync belt's lifetime closer to that of a V-belt.  Also, better sync belts with Kevlar fiber cords and polyester rubber, in the right application, can be good for 24,000 hours. 

Retrofitting an electric motor with a sync belt should cost between $11 and $22/horsepower (HP) according to a 1995 belt drive technology assessment by Almeida and Greenberg of Lawrence Berkeley Laboratories (adjusted for inflation to 2008 dollars).   The investment covers the cost of the sprockets, plus the more expensive belts and installation. Prices vary greatly, depending on the belt drive rating, with the large drives requiring a smaller price premium per horsepower. Sprockets typically cost 1.5 to two times the cost of belts.  One set of sprockets will outlast three belts. 

Converting to a sync belt drive can pay back in less than a year in heavy-use applications.  Continuous, fully loaded operation of a 100-HP sync belt drive with motor efficiency of 94.5 percent and a $.05/kWh rate will save $1,729 per year.  A sync belt retrofit made for less than $17/HP has a payback of less than a year. Increasing the driven-over-driving sprocket size ratio to reduce driven-load speed at the time of conversion will greatly increasing energy savings.  Speed reduction savings are dramatic in variable torque loads like fans.

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