Saving Energy With Timing Belts
By Peter Batchelar
The first successful timing belt drive application, in the 1940s, synchronized the needle and bobbin shafts in an industrial sewing machine. As designers and engineers gained experience, emphasis began to shift from mere synchronization to the tra...
The first successful timing belt drive application, in the 1940s, synchronized the needle and bobbin shafts in an industrial sewing machine. As designers and engineers gained experience, emphasis began to shift from mere synchronization to the transmission of power.
Timing belts, also known as synchronous belts, have teeth that mesh with mating teeth on pulleys. The result is a positive drive with no loss of rpm, unlike the frictional drive of V-belt systems. The drives are found on applications throughout industry, both at the OEM and user level. Today, they have even made their way into the automotive OEM market where they power the camshafts on the majority of four-cylinder engines being made today.
At their inception, timing belts could handle only small loads, but advances in materials and processing have made it possible to use the belts on applications with a design horsepower of up to 700 hp. And the positive engagement of the teeth makes possible a much wider speed range than with V belts.
Timing belt construction is basically similar among all manufacturers. The teeth are covered with a nylon wear surface that also helps the belt mesh smoothly with the pulleys.
The strength of the belt is derived from the helically-wound tensile cord, usually made of glass fibre. Glass is very stable and provides a means to accurately control the length of the belt, a critical feature as the belt pitch must be maintained so as to ensure fit with the pulleys. Some manufacturers will substitute aramid fibre for glass for some applications due to its higher shock resistance.
The tensile cords are encapsulated by a synthetic rubber that forms the body of the belt. This rubber is formulated for the best combination of flex, ozone, heat resistance and adhesion to the other belt components.
The first belts were produced with a trapezoidal tooth configuration. In the 1970s, a big step forward was taken with the development of a curvilinear tooth. This configuration provided more even distribution of stresses in the tooth and made it possible to considerably increase belt power ratings.
How do synchronous belts save energy? V belts depend on friction to transmit power. There is always some slipping action between the belt and the sheave. Any reduction in tension will result in increased slip. While it is possible to limit this slip to a minimum, the high operating tensions required make it impractical to do so. When a V belt is installed at the recommended tension, it will generally exhibit 2% to 3% slip. As the belt seats itself in the drive, this will usually increase to 4% to 5%.
If properly selected and maintained, the overall efficiency of V belts ranges from 90% to 94% due to the combined effects of air turbulence, flexing, creep, centrifugal losses and slippage. Timing belts, by eliminating slippage, and with lower bending and centrifugal losses, can run at 96% to 98% efficiencies. Thus it is fair to say that timing belts are about 5% more efficient than V belts.
Quantifying efficiency by way of illustration, consider a pump representing a constant 50-hp (37-kW) load from an 1,800 rpm motor being converted from V belts to a synchronous drive system. For the V belt system:
DriveN shaft hp = torque x rpm / 5252, or 50 hp = 153.6 ft-lb x (1,800 rpm x 0.95 efficiency) / 5252
DriveR shaft hp = 153.6 ft-lb x 1,800 rpm / 5252 = 52.6 hp.
When converting to a synchronous belt, horsepower at both shafts can be defined as:
145.9 ft-lb x 1,800 rpm / 5252 = 50 hp.
The converted drive at 145.9 ft-lb of torque will draw about 5% less current than the V-belt drive. This advantage tends to increase over time.
Timing belts start out at zero slip and stay that way. V-belt slip, on the other hand, deteriorates in operation due to belt stretch and wear. The fact that V belts operate at temperatures of about 20C to 45C above ambient is ample evidence of the energy wasted by slippage in these drives.
Further cost savings are made with synchronous belts since they require no retensioning or other maintenance. They also require no lubrication, unlike chain drives, and are generally unaffected by airborne particles or wetness.
Additionally, timing belt drives require lower installation tensions than V-belt drives, which means lower overhung loads and longer bearing life. It must be noted that timing belt drives require very good alignment and do not provide the clutching action desired in some drives.
The energy savings when multiplied by the number of drives in an installation can add up to a very considerable annual figure. It has been shown to be enough to convince a major theme park and a large hospital complex, among others, to convert all their drives over to timing belts.
One important point to remember in designing a conversion to synchronous belts is that for drives where the load is sensitive to the driveN speed, such as a fan, energy savings will be possible by an appropriate adjustment in the speed ratio of the drive. The goal is to end up with the same driveN speed with the timing belt as the V belt. The use of a hand tachometer to note the driveR and driveN speeds before dismantling the V-belt drive will indicate the speed ratio required to give the same driveN speed with a timing belt system.
Peter Batchelar joined Jason Industrial in 1986 and spearheaded the company’s entry into belt manufacturing with the creation of Jason’s TBMC timing belt manufacturing plant in Greenville, S.C. He serves as executive vice-president of the Fairfield, N.J., company.