Don’t Procrastinate…Innovate: Reducing Driver-Driven Transmission-System Failures

Walk through any modern manufacturing plant or mechanical room and you’re sure to see power transfer from an electric motor or engine-drive pulley to a driven shaft using either a chain and sprocket or v-belt system. When made of high-quality materials, these common systems are, for the most part, very reliable. But because of that inherent reliability and their forgiving nature, such systems are often incorrectly set up and/or maintained—which can lead to premature failure.

It’s time to tame this driver-driven power-transmission monster. By understanding the needs of these systems and performing basic maintenance in an innovative manner, you can virtually eliminate their unplanned failure, boost asset life-cycle/reliability, increase throughput and reduce energy costs.

Chains and sprockets

The earliest chain-drive transmission is credited to the ancient Greeks in 3rd century BC. They used it on a military device attached to a hand-wound windlass that automatically loaded and fired crossbows. James Watt’s 1763 steam engine later used chain to connect the rocker beam to the steam piston and vacuum pump, and convert linear to rotary motion. Today, most people experience their first chain-and-sprocket experience when they learn to ride a bicycle.

Chains are excellent transmitters of torque, and are not affected by excess heat or excess bearing load. They can be purchased in bulk and easily custom-sized, and can accommodate long shaft-center distances and multiple configurations that aren’t easily achievable with belt-driven systems or direct-drive gearing. On the negative side, if not lubricated or cleaned regularly, chain systems wear rapidly and can fail catastrophically (causing great physical damage at speed).

Under normal operation, the outer surface of the pins and bushings connecting the chain links together “rub” against one other and create friction. This friction causes pin surface to wear and lose its rigidity to the point it could fatigue fail and break

Technically, a chain doesn’t stretch: It elongates through wear on the pins connecting the links. When wear occurs, the chain roller begins to creep up the sprocket teeth as it rolls over them. This, in turn, causes tooth wear and creates system vibration—and eventually allows the chain to “jump” the sprocket teeth. (The situation also increases the drive-energy requirement to overcome friction and vibration.) Consequently, at speed, the chain can come off the sprocket entirely, snap or wrap around a moving shaft, causing significant secondary damage. The allowable rule is 1.5% to 2% elongation before replacement is required.

Maintaining chains and sprockets

1. Lubricate! Lubricate! Lubricate! (And always with clean oil, unless using o-ring, lube-free or plastic chain.)
2. Investigate use of a simple reservoir and brush-applied lubricator to automatically oil the chain.
3. Regularly clean chains with a wire brush and degreaser or with a commercial chain-cleaner attachment.
4. Protect these systems from contamination. (Water and dirt will lead to rust and accelerate chain wear.)
5. Make a simple set of Go/No-Go slip gauges to hold against the slack side of the chain and a fixed reference point to check for minimum tension (see manufacturers recommendation) and maximum elongation (calculate position for 1.5% elongation).
6. Always replace sprockets when you replace the chain.
7. Modify your chain guard with a bottom hinge so the transmission system can be accessed in less than 30 seconds.
8. Precision-laser-align your sprockets.
9. Beware of counterfeits. Stick with quality, name-brand chain and sprockets.

V-belts and pulleys

Industry has used v-belt transmission-drive systems for more than 90 years. These systems experienced a major upgrade in the 1960s when narrower 3V, 5V, and 8V belt profiles were introduced (with a change from a rayon to polyester internal tension member molded in a modern synthetic SBR elastomer for wrapped belts or neoprene for raw edged belts). Modern “high-performance” belts use Kevlar tension members.

A v-belt is a simple device designed to wrap around a number of sheave pulleys to transmit power at a defined number of revolutions per minute (rpm) from a motor-powered drive pulley to one or more driven pulleys. The v-belt “wedges” into the v-shaped sheave pulley and transmits load through its elastomer to the tension member that, in turn, transfers power to the driven sheaves.

During this power transfer, the belt is subjected to fatigue, which can eventually cause belt tension members to fail. Correctly installed and tensioned belts can be expected to perform at efficiencies up to 98%, and deliver more than 15,000 hours of trouble-free operational service.

Premature failures, especially those that occur within the first 1000 hours of operation, can sometimes be attributed to poor belt quality. When equipment uptime is at stake, always purchase quality brand belts from a reputable dealer to ensure you are buying the real product and not a lesser-quality or counterfeit product. The highest percentage of premature belt failures are caused by heat, and are overwhelmingly maintenance-related or induced. They are preventable!

V-belts perform best at operating temperatures between 90 and 120 F. Belts are subject to the Arrhenius rule: For every 18 degree F increase in their operating temperature, a belt’s expected life can be reduced by half. Manufacturers point to elevated belt temperature as the primary cause of failure. Keep these major (and very preventable) contributors to temperature elevation
in mind:

• Belt slippage—As a loaded belt unwraps from the driven pulley sheave, it tends to creep or slip as it releases, slowing the pulley—which is normal. A correctly tensioned belt will slip between 1-3%. Less means the belt is too tight; more means it’s too loose. Both conditions cause belt temperatures to rise.
• Misalignment—Poor alignment causes the belt’s tension members to flex sideward and vibrate, creating additional stress. When a misaligned belt enters the sheave groove, it “rubs” the sheave wall, creating friction and wear of the sheave and belt. Combined, all of these will raise a belt’s operating temperature. Sheaves can be offset and/or angular-misaligned if set up incorrectly.
• High operating loads—When a driven system requiring multiple belts operates with loose, non-matched or missing belts, belt loading will surpass the belt-design factor and create heat.

Maintaining belts and pulleys

1. Slippage, or correct belt tension, is easily checked using a strobe rpm tool to check the speed rpm difference between both driver and driven pulleys. Newly installed belts should be tensioned at startup, again after running full load for 30 minutes, and 24 hours later after the belt has seated into the pulley. With a 1:1 ratio driver-driven system at a measured 1800-rpm driver pulley speed, the driven pulley should run between 1782 rpm and 1746 rpm (1-3% slip) under ideal tension.
2. Protect belts from heat, dirt, water and oil contamination.
3. Use infrared (IR) temperature guns or thermographic cameras to check belt temperature during operation.
4. Purchase a $10 sheave-groove-profile gauge and always check the pulley-groove profile when tensioning and changing belt sets. If more than .030” daylight is detected, immediately change the worn pulley. (Worn sheave pulleys make it difficult to tension belts correctly.)
5. Precision-alignment of driver/driven systems using laser or reverse-dial method is a must to reduce heat, wear and energy loss.
6. Always change multiple belts out as a matched set (i.e., same manufacturing batch or lot number), and check sheave pulleys for wear and change as required.
7. Modify chain guards with bottom hinges so a transmission system can be accessed in less than 30 seconds.

Good luck!