Selecting the right drive system

When making a drive conversion or designing a new power transmission system, maintenance managers and design engineers have three broad options: roller chain drives, V-belt drives and synchronous belt drives.

Each has its own advantages and disadvantages, along with cost considerations that may not be immediately apparent.

ROLLER CHAIN DRIVES

The popularity of chain drives stems from their ability to transmit high torque levels in a small package, at relatively low cost, while utilizing readily available stock components. While initial costs of standard roller chain drives can be quite low, the cost of maintaining them can be substantial. Proper maintenance is essential for optimal roller chain drive performance and includes the following cost factors:

• Lubrication
• Alignment
• Tension
• Drive component replacement

According to chain industry estimates, roller chain drives operating without lubrication wear approximately 300 times faster than comparable drives that are properly lubricated. And yet, roller chain manufacturers estimate that 90 to 95 percent of all installed drives are either improperly lubricated or not lubricated at all. Determining the type of lubrication method needed is a major design consideration with cost implications of its own. An oil-retaining chain housing, for example, can represent up to 75 percent of total chain drive system cost. In addition to lubrication, proper sprocket alignment and chain tensioning are critical to increasing roller chain life.

Another maintenance factor with cost implications is drive component replacement. A major weakness of a roller chain drive is chain wear, which results in stretching or elongation. Manufacturers recommend roller chain replacement when elongation of approximately 3 percent occurs. Most roller chain manufacturers also recommend replacing sprockets with each new roller chain because the metal-to-metal contact generates severe sprocket wear.

Power rating tables published within the roller chain industry are based on a theoretical design life of 15,000 hours, assuming proper drive design, alignment, lubrication, maintenance, etc. But in a typical operating environment, actual drive life rarely approaches the ideal. Unlubricated roller chain drives operating under harsh conditions can be as short-lived as 100 hours.

The cost of the maintenance requirements noted above, added to the initial cost, approximates the true cost of a standard roller chain drive. However, beyond the cost of lubricant and drive component replacement is the labor expense of frequent retensioning, which requires shutting down the drive, resulting in production downtime. Also, standard roller chain drives operate at 91 to 94 percent efficiency, depending on the application, so energy costs must be taken into account.

Maintenance and energy costs notwithstanding, roller chain drives offer designers and users some advantages over V-belt or synchronous belt drive systems:

1. Versatility (functional attachments can be added to convey products, trip switches, actuate levers, etc.)

2. The ability to create any length of chain with connecting links

3. The availability of a large selection of chains and sprockets

V-BELT DRIVES

V-belt drives transmit power through friction between the belt and pulley. With efficiencies ranging from 95 to 98 percent at installation, these drives use energy more efficiently than roller chain drives, and somewhat less efficiently than synchronous belt drives. V-belt drives are an industry standard, offering a wide range of sizes at relatively low cost, along with ease of installation and quiet operation.

V-belt drive replacement parts may be less
costly than roller chain or synchronous drive
components, but regular retensioning of
V-belts can add to maintenance expenses.

V-belts are manufactured in a variety of materials, cross-sections and reinforcement materials, and are often used singly, in matched sets or in joined configurations. They are well-suited for severe duty applications, such as those involving shock loads and high starting loads. Standard V-belt drives operate best in applications of 500 RPM or greater, speed ratios of up to 6:1, and within an operating temperature range of minus-40 to 130 degrees Fahrenheit. Because V-belts slip when overloaded, they help protect more expensive equipment from load surges. They also allow flexibility in the positioning of the motor and the load.

In a suitable application, the service life of a properly installed and maintained V-belt drive ranges from 20,000 to 25,000 hours. The components of a simple V-belt drive are relatively inexpensive to purchase, install, replace and maintain. After they are installed properly and tensioned to the belt manufacturer’s recommended values, these drives require very little service, except for retensioning during the normal maintenance schedule. Due to belt slippage, they lose up to 5 percent of their efficiency after installation. V-belts stretch as they wear, making slippage worse, which can decrease efficiency by as much as 10 percent unless corrected by periodic retensioning. Cogged or notched V-belts can increase efficiency by 2 percent over standard designs.

SYNCHRONOUS BELT DRIVES

Synchronous belts work on the tooth-grip principle. Round, square or modified curvilinear belt teeth mesh with grooves on sprockets to provide positive power transmission on high-torque applications with high and low speeds.

If drive size is a problem, many synchronous
belt drive systems now have have an equivalent
capacity to roller chain drives in the same width.

The components of a synchronous belt drive system typically cost more initially than those of a comparable standard roller chain or V-belt drive. By contrast, synchronous belt drives don’t have the maintenance costs associated with roller chain drives. They require no lubrication and no lubrication system, only basic safety guarding. While roller chain requires frequent retensioning and V-belts require periodic retensioning, a synchronous belt typically requires no retensioning for the life of the belt.

To illustrate the amount of elongation that can occur in a roller chain, recommended center distance take-up allowances for belt drives can be compared to center distance take-up needed for a roller chain in the same length. Assuming a length of 100 inches, a roller chain, V-belt and synchronous belt can be compared as follows:

Roller chain: A roller chain will elongate approximately 3 inches (or 3 percent) over its life, requiring about 1.5 inches of center distance take-up.

V-belt: A V-belt requires 1.5 to 2.5 inches of center distance take-up over its life, depending on the cross section and belt manufacturer.

Synchronous belt: A synchronous belt typically requires only .04 inches of center distance take-up over its life, depending on the belt type and manufacturer.

Chain and sprocket wear are significant cost factors in a roller chain drive. Synchronous belts and sprockets experience dramatically less wear. In a synchronous drive system, for example, the belt outlasts a comparable roller chain on the order of 3 to 1, and the sprockets outlast roller chain sprockets 10 to 1.

Figure 1. Required center distance take-up due to lifetime elongation (in inches).

Like roller chain drives, synchronous belt drives are sensitive to misalignment and shouldn’t be used on systems where it’s inherent to the drive operation. Misalignment leads to inconsistent belt wear and premature tensile failure due to unequal tensile member loading. And while a synchronous belt is resistant to abrasion, corrosion and the caustic washdown solutions used in the food handling/processing industry, it may not be suitable for certain highly corrosive environments where corrosion-resistant chain may be a better choice.

One misconception about synchronous belts is that they are unsuitable for serpentine drives. Design engineers may think roller chain is the only option when a load must be driven off both sides; however, double-sided synchronous rubber belts offer many of the same cost-saving advantages over roller chain as their single-sided cousins.

Due to their high efficiency ratings (as high as 99 percent on a continuous basis for some drive systems), synchronous belt drives can also lower energy costs compared with roller chain or V-belt drives.

CONCLUSION

When considered on a cost-of-ownership basis, a synchronous belt drive system can be more cost-effective than a comparable roller chain drive or V-belt drive system. While a synchronous drive system may initially cost an average of 30 percent more than a comparable standard roller chain drive, it has many cost-saving advantages for maintenance managers and design engineers.

In the MRO market, synchronous drives can greatly reduce day-to-day operational costs, and increase production output compared to the downtime and lost productivity resulting from the frequent maintenance and replacement of roller chain and V-belt drives. Drive system design engineers who select a synchronous drive can give their products a competitive edge by providing better-performing, longer-lasting, cleaner, quieter and maintenance-free products that operate at a lower overall cost.

This article was written by the power transmission product application department at Gates Corporation. To learn more about this subject, visit www.gates.com.