How Cost and Performance Direct Drive Choices

When making a drive conversion or designing a new power transmission system, plant maintenance managers and design engineers have several options:

* Roller chain drives

* V-belt drives

* Synchronous belt drives

* Gears

* Couplings

* Shaft mounts.

Of these, the mostly widely used are 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 roller chain drives stems from the ability of these systems to transmit high torque levels at low speeds, 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 these drives 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% 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. This continues in the industry today because initial roller chain parts are low in cost and lost production is not considered. An oil bath, for example, can represent up to 75% of total chain drive system cost. In addition to lubrication, which is petroleum-based, proper sprocket alignment and chain tensioning are critical to increasing roller chain life.

Another maintenance factor with cost implications is drive component replacement. One of the major weaknesses of a roller chain drive is chain wear resulting in stretching or elongation.

Manufacturers recommend roller chain replacement when elongation of approximately 3% has occurred. 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 or less.

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 labour expense of frequent retensioning, which requires shutting down the drive, resulting in production downtime. Also, standard roller chain drives operate at 91%-94% 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:

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

* Ability to create any length of chain with connecting links

* Availability of large selection of chains and sprockets.

V-belt drives

V-belt drives transmit power through friction between the belt and sheave. Even though properly maintained V-belt drives can run as high as 95%-98% efficiency at the time of installation, this often deteriorates by as much as 5% during operation due to slippage. Toothed synchronous belts, however, remain at an energy efficiency of approximately 98% over the life of the belt drive.

V-belt 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-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 a limited operating temperature range of -40*C to 54*C (-40F to +130F).

In a suitable application, the service life of a properly installed and maintained V-belt drive ranges from 20,000 hours 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, V-belt drives lose up to 5% of their efficiency after installation. V-belts stretch as they wear, making slippage worse, which can decrease efficiency by as much as 10% unless corrected by periodic retensioning. Cogged or notched V-belts can increase efficiency by 2% over standard designs.

Synchronous belt drives

Synchronous belts work on the mechanical meshing of the belt and the tooth of the sprocket. Round, trapezoidal or modified curvilinear belt teeth mesh with grooves on sprockets to provide positive power transmission on high-torque applications at high and low speeds.

The components of a synchronous belt drive system typically cost more initially than those of a comparable standard roller chain drive or V-belt drive. By contrast, synchronous belt drives do not have the maintenance costs associated with roller chain or V-belt drives. They require no lubrication and thus 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 centre distance take-up allowances for belt drives can be compared to the centre distance take-up needed for a roller chain in the same length. Assuming a length of 254 cm (100 in.), a roller chain, V-belt and synchronous belt can be compared as follows:

* Roller chain: A roller chain will elongate about 7.6 cm (3 in.) or 3% over its life, requiring about 3.8 cm (1.5 in.) of centre distance take-up.

* V-belt: A V-belt requires 3.8-6.4 cm (1.5-2.5 in.) of centre distance take-up over its life, depending upon the cross section and manufacturer.

* Synchronous belt: A synchronous belt typically requires only 10 mm (0.04 in.) of centre distance take-up over its life, depending upon the belt type and manufacturer.

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

Like roller chain drives, synchronous belt drives are sensitive to misalignment and should not be used on systems where it is inherent to the drive operation. Misalignment leads to inconsistent belt wear an
d premature tensile failure due to unequal tensile member loading.

While a synchronous belt can be 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 alternative.

Due to their high efficiency ratings (as high as 99%), synchronous belt drives can also lower energy costs compared with roller chain drives, which typically operate at 85%-90% efficiency, or V-belt drives, which when properly tensioned, operate at 93%-95% efficiency.

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. A Gates Poly Chain GT belt drive (incorporating the most technically advanced synchronous belt available, made of a polyurethane body and aramid fibre reinforcement), for example, initially costs an average of 30% more than a comparable standard roller chain drive. However, it has many cost-saving advantages for plant maintenance managers and design engineers.

For maintenance managers, synchronous drives can greatly reduce day-to-day operational costs, and increase production output compared to the downtime and lost productivity resulting from the maintenance and replacement necessary with roller chain and V-belt drives.

This article was provided by Gates Canada Inc., which offers a drive cost savings calculator at www.gates.com/cal.