Because belt drives are so common, they tend to be neglected. The usual approach is to just install a belt, then forget about it until it breaks. But this approach wastes money and shortens belt life.
Understanding a few basic facts about V-belt design — as well as efficiency factors, failure causes, and typical operating problems — can help you do what’s needed to keep belts running at top efficiency, extend their life, reduce maintenance costs, and increase machine uptime.
What influences efficiency?
V-belt efficiency depends on the load being transmitted, sheave sizes, the number and size of belts, and their temperature. But most of all, efficiency is affected by installation and maintenance factors, such as tension, misalignment, and worn sheaves.
Industry experts generally agree that a well-designed, properly aligned and tensioned V-belt drive is most efficient — about 97% — when first installed (see box on efficiency). With time and use, the belts stretch, lose tension, and begin to slip in their sheaves. The result: efficiency drops to about 94%, provided that the sheaves are in good condition and slippage is normal. To minimize this drop in efficiency over time, keep belts properly tensioned and aligned.
Why belts fail
V-belt failures occur for a variety of reasons, but most involve fracture of the tension member, usually caused by fatigue loads. Though high load is the major cause, high temperatures often contribute to failure.
Load. In a typical V-belt, Figure 1, the elastomer transfers forces from the sheave groove walls to the tension member cords, which carry the load. Historically, tension member cords have been made from a variety of fibers, but today polyester is the most popular.
Under normal operation, the elastomer distributes the load almost evenly to the tension member cords so they are equally stressed, thereby achieving long belt life. But operating conditions such as high temperature and misalignment can cause uneven load distribution.
Even under perfect operating conditions, the tension member is subjected to cyclic loads and alternating stresses — higher as the belt moves around each sheave, and lower as the belt travels in a straight line. If loads are high enough, this repetitive alternating stress eventually causes fatigue failure of the tension member.
As the load (and fatigue stress) increases, belt life decreases much faster. For example, a 50% increase in load cuts belt life by more than 50%.
Temperature. Belt operating conditions, such as the continuous flexing around sheaves, and slipping, may cause belt temperatures to rise. When belt temperature goes up, the elastomer in a belt becomes softer and can’t transmit load as well. The outer edges of the belt cross section, including the outer tension member cords, become more highly stressed, Figure 2, leading to rapid belt failure.
One of the keys to long belt life is keeping the temperature as low as possible. This is demonstrated by a Gates Rubber Co. study (PTD, 10/92, p 49), which shows that belt life is cut in half for every 19 F increase in its temperature.
In addition to high loads and temperatures, there are several common belt problems — loose belts, worn sheaves, and misalignment — that damage both efficiency and life. And they are costly.
V-belts loosen because the polyester cords stretch under load. Some cord materials, such as fiberglass and aramid, stretch very little and don’t require repeated retensioning to maintain their efficiency. However, belts made with these cords cost more, and they tend to have shorter fatigue life when used with smaller diameter sheaves because of the increased flexing. These disadvantages frequently offset the better efficiency. Its best to check with your belt vendor for specific recommendations.
The time-honored way to check for high temperature caused by loose belts is to put your fingertips in the sheave groove (after the drive is shut down and locked out). If the sheave is so hot that you can’t comfortably keep your fingertips in the groove, the belts have probably been slipping. The rubbing or slipping action heats the belt, reducing both its life and efficiency. There are better inspection methods, such as strobe lights, but the “fingertip test” is a simple way to determine if a belt needs to be tightened. Periodic retensioning of the belts on a drive in good condition can increase efficiency about 3 to 4%.
When a V-belt wraps around a sheave, the belt sides bulge out against the sides of the groove, transmitting load from sheave to belt tension member, Figure 3. As a sheave wears, the belt fits more loosely in the groove, so loads don’t transmit as well. This causes an uneven load distribution across the belt tension member, with more load being applied at the belt edges. The effect is similar to that of high temperature.
Because of this uneven loading, worn sheaves reduce belt life by as much as 50%. But their adverse effect on efficiency is even more costly. The efficiency of a belt drive with visibly worn sheaves and loose, slipping belts can easily drop below 90%. To translate this into operating costs, consider a 50-hp belt-driven machine that costs $25,150 (at $0.07/kWh) to operate year-round. The efficiency loss due to loose, slipping belts and worn sheaves costs about $1,300 per year, more than enough to replace the belts and sheaves several times.
A typical reaction to worn sheaves is to ignore the wear and just retension the belts so they don’t squeal (announcing to everyone that they’re slipping). Retensioning boosts efficiency. But if the higher tension exceeds the design load, it may impose too much load on the belts, as well as support bearings and shafts.
Doubling the load on ball-type shaftsupport bearings cuts their fatigue life by a factor of eight (the factor for belts is similar). So retensioning the belts to well-above the design load may increase efficiency, but at the expense of much shorter belt and bearing life. A better solution is to replace the worn sheaves.
As a misaligned belt enters a sheave groove, it rubs against the side of the groove, and flexes, creating friction and heat. The rubbing action also causes vibration in the driven machine, which increases belt load. This combination of higher belt load and higher temperature reduces belt life. Trying to gage the amount of reduction in life is difficult because it depends on the severity of misalignment and the resultant vibration. However, remembering that belt life is cut in half for every 19 F increase in temperature, it is easy to understand that misalignment can significantly reduce belt life.
The amount of efficiency loss caused by misalignment depends on variables such as center distance, load, and belt type. These losses generally aren’t significant (less than ½%) compared to the damage caused by increased belt and bearing loads and by machine vibration. But, if belt life seems too short, make sure the belts are aligned according to the manufacturer’s recommendations — usually within ½ deg.
Here are some other points to keep in mind so your belts will run efficiently:
• Larger sheaves reduce belt tension and bending stresses, leading to better efficiency and longer life.
• Totally enclosed guards restrict air circulation, which can substantially increase belt temperatures and proportionally reduce belt life.
• Dirt, oil, grease, and belt dressings attack belt material or change the friction between belt and sheave, which can reduce belt life.
• Special V-belt materials are available for high temperature and high-vibration applications, or those that require low elongation.
• Raw edge belts run cooler than fabric- covered belts and often last longer for this reason.
• Modern narrow section V-belts carry as much load as classical belts even though the new belts are narrower. They do this by distributing load more evenly over the tension members.
• A V-belt drive designed according to standard methods should operate for 15,000 to 20,000 hr before belt replacement is needed. Sheaves should last for about three belt changes before they’re POWER TRANSMISSION DESIGN JUNE 1995 71 worn out.
• Synchronous belts normally operate with an average efficiency of about 97% compared to 94% for V-belts (after breakin). Frequently, the power savings obtained by switching to synchronous belts will pay for the change-over in less than a year.
• The belt industry continues to upgrade designs and materials. If your belt drive is more than 15 years old, there is probably a better, more-efficient, longerlasting alternative.
Neville W. Sachs is a consultant on equipment analysis and reliability, and president of Sachs, Salvaterra & Associates Inc., Syracuse, N.Y.