Power and Precision: Belt Drive Systems Offer Both
James Anderton posted on January 05, 2016 |
A variety of custom belts. (Image courtesy of BeltCorp.)
A variety of custom belts. (Image courtesy of BeltCorp.)

 

Power transfer and rotational timing between parallel rotating elements like shafts has always been a natural application for belt drives.

 

As over head line shafting, leather belts and drum sheaves gave way to fabric and rubber v-belts, old-school engineering professionals stuck to the old rules: chain and gear drives for high torque, power and precision timing applications.

 

The fact is, advanced belt drive systems can deliver power transfer and rotational timing in a low-cost package with performance that matches or exceeds gear and chain drives in many applications.

 

Chains will stretch, require lubrication and are noisy when compared to cog belt systems.

 

For many applications, such as food processing and pharmaceuticals, the need for grease or oil bath lubrication in a hostile washdown environment gives belts a natural advantage. In fact, every food product on the shelves in the United States is packaged by machinery that uses timing belts.  

 

Choose the right belt composition

 

The modern materials used to make belts can withstand significant levels of heat, deflection and shear, making belts superior to chains and gears in many applications.

 

The typical composition for a modern timing belt is neoprene reinforced with fibreglass, or in some cases steel or Kevlar. Polyurethane reinforced with steel or Kevlar is also an option.

 

The ability to choose a belt composition for specific timing/load application is another advantage of belt drives. This gives the engineer greater design freedom to choose from a variety of cost, performance and service/life ratios when specifying a drive.

 

A long life belt product, for example, may reduce the need for detachable or quick access housings on maintenance heavy equipment. Similarly, belt drives are normally much easier to service than equivalent chain or gear drives, allowing easier, faster and lower cost maintenance.

 

For MRO professionals, maintaining spare belts as line replacement items is cost-effective.

 

The old idea that belts age on the shelf is not entirely true. Rubber does age and tends to dry out over time, but ultimately it depends on how the belt is stored.

 

Neoprene belts are UV resistant, so they last a long time on a shelf. However, it’s still worth storing unused belts in black plastic bags and keeping them out of direct sunlight.

 

When asked for advice to give design engineers looking to design a belt drive system, Rich Blais of the Belt Corporation of America (Belt Corp.) said, “I would use polyurethane for efficiency because it’s harder, it works like a gear-rack and it’s typically steel-reinforced. The pulleys can be made in conjunction with the tooth-geometry to be either zero backlash or reduced backlash.”

 

Belt Corp. has the largest inventory of standard imperial and metric pitched timing belts made from neoprene and urethane in the country. They have produced belts large enough for the evacuation gurneys used by paramedics as well as belts with the circumference of a dime.

 

An important factor to consider when specifying a belt drive is the system’s operating environment.

 

A neoprene timing belt rated to operate at 220 degrees Fahrenheit but running at 280 degrees won’t last more than a few weeks before it breaks down.

 

Belt Corp specializes in solving these types of problems by designing or modifying belts to perform specific functions.

 

A selection of belts coated with silicone. (Image courtesy of BeltCorp.)
A selection of belts coated with silicone. (Image courtesy of BeltCorp.)

 

In this case, where the issue is with the material, “we might apply a silicone cover to it for insulation, since silicone is rated for up to 550 degrees,” said Blais. “We use a lot of exotic compounds; we have a full time chemist here that develops compounds for us for specific applications.”

 

Common causes of belt failure

 

Although mechanical belts are not immune to failure, the underlying cause of a belt failure often has less to do with the belt itself than with its environment.

 

If installed correctly, belts are laterally and longitudinally stable. They also offer more flexibility regarding runout than gear drives, though runout can still be a factor in some cases.

 

However, if the pulleys are misaligned they can push the belt in one direction or another.

 

One common cause is bearing wear, particularly thrust bearings. A wandering belt is a strong indicator that axial play should be checked.

 

There are a variety of methods to reduce a belt’s propensity to travel to one side or another.

 

Since the cords on a belt are wound like a guitar string—helically on top of the belt—a dual-cord system can help to offset the belt’s susceptibility to travelling. Winding two cords in opposite directions can counterbalance the belt and reduce its likelihood of travelling.

 

Belt failure can also occur if the equipment on which it is installed is run beyond its design parameters.

 

A company should expect failures if a belt drive running at 250rpm switches to run at 600rpm without receiving the necessary modifications.

 

Again, consultation with a technically competent belt vendor such as Belt Corp is essential if the drive’s operating parameters must change significantly. “We make belts run better and last longer,” said Blais.

 

Heat is the number one cause of belt failure, either from friction or from external heating.

 

Fortunately, in many cases the problem can be solved by changing the belt material. “We had one client we worked with that was only getting about five hundred thousand cycles out of a belt,” Blais commented. “So we manufactured a belt for them out of an alternative compound and now they’re getting three million cycles.”

 

There is no simple way to tell if a belt needs to be replaced, but there are warning signs such as tooth-wear and edge wear.

 

Stretch can also be an indicator of an issue with a belt system.

 

“If you’re constantly tensioning a belt then it must be stretching, which means you need a different belt design,” said Blais.

 

Precision timing belts. (Image courtesy of BeltCorp.)
Precision timing belts. (Image courtesy of BeltCorp.)

 

Since they aren’t meant to stretch, timing belts need an idler or tensioner to ensure the correct amount of pre-tension on the drive. This also means that fixed centers aren’t conducive to using a timing belt.

 

Avoiding belt failure depends on preventative maintenance and operating equipment within design parameters.

 

The next time a power transmission system needs to be replaced or a new one installed, it’s worth considering custom and special-purpose toothed belts.

 

For more information, visit the Belt Corporation of America website.

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Belt Corporation of America has sponsored this post.  They have no editorial input into this post.  All opinions are mine.  James Anderton. 

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