CHANGING TO BELT
External gears in a press roll were changed out with a serpentine synchronous belt drive to reduce noise and maintenance. The sprockets and belt were designed according to speed, power, and space requirements. A graphical representation of the belt system, as shown, was used to determine the required belt length. To achieve a standard length, the take-up distance (the horizontal idler placement) was adjusted in the drawing.
Select figure to enlarge.

Synchronous belts are often used instead of flat or v belts for several reasons, the most notable being the near-perfect shaft synchronization they provide. They remain in mesh with their sprockets, while traction-based belts on pulleys can slip.

Synchronous belts are a popular choice for serpentine drives, in which a single belt turns three or more pulleys or sprockets. Besides providing zero-slip, synchronous belts allow a little more “aggressive” belt geometry – the toothed belt can often move the sprocket with a shorter arc of contact than a flat belt along a pulley, and this additional freedom makes complex serpentine arrangements easier to design. Even with toothed belts, however, serpentine configurations often require special attention. For instance, shafts sometimes rotate in opposite directions, in which case a two-sided synchronous belt is required.

The following is an example of serpentine synchronous belt drive design. The machinery involved is a press roll system with two contacting rollers of opposing rotation. The original configuration uses a motor connected to a gearbox, with a spur pinion mounted to the gearbox output shaft. This external pinion meshes with a larger gear attached to one of the rolls. The gear then drives a second identical gear and roller in the other direction. The goal is to replace all external gearing with belts to eliminate the maintenance and excessive noise.

Before selecting the belt, sprockets, and bushings for such an application, the following should be established: XY coordinates of each shaft centerline (with respect to any convenient datum); necessary horsepower for each shaft; the overall horsepower requirement; speed of each shaft; a description of each shaft’s function (type of equipment attached); and knowledge of shock loads and environmental conditions.

In the press roll system, motor speed is 1,750 rpm. The motor puts out 7.5 hp and is mounted to a gearbox with a 20.9:1 reduction. The two larger external gears have 30 teeth, attaching to a 2 7/16 -in. shaft. The 18-tooth pinion attaches to a 1 3/8 -in. shaft. All shafts are aligned, with center distances of 9.55 and 21.45 in. from the pinion to each of the two gears. All of the shafting is there to stay; it cannot be moved or changed. Some level of shock loading is expected.

Knowing the motor and gearbox specs, we could base our design on the maximum driving power. But, even better, the user provides the required power: each roll needs 2.5 hp to operate. Typical service factors for serpentine drives are no less than 2.0, but because of shock loading and relatively tight drive geometry, we choose a slightly more conservative service factor of 2.2.

First off, the design horsepower must be assessed. This is given by:

DH = SH SF η

where
DH = design horsepower
SH = system horsepower
SF = service factor
η = estimated (or known) system efficiency

We use the value of 5 hp (two rolls requiring 2.5 hp apiece) for the system horsepower; this driven horsepower, if known, is preferred over the driving (motor) horsepower. System efficiency is therefore irrelevant because the rolls require 100% of the specified driven power. If, on the other hand, our belt selection were based on motor horsepower, gearbox efficiency would be factored in. If unknown, it would be reasonable to assume a conservative (in this case, deliberately large) efficiency value – the greater the efficiency in the equation, the higher the design horsepower.

With a system horsepower of 5, a safety factor of 2.2, and an efficiency of 1, we have a design horsepower of 11. The belting and driving sprocket must be capable of transmitting 11 hp at the gearbox output speed of 84 rpm. To match the operation of the original system, this output speed must be further reduced by the 30:18 (1.66) reduction ratio of the pinion and gears that are being replaced.

Sprocket designation is a good place to begin. This involves the belt width, tooth pitch, and pitch diameter, parameters associated with horsepower, speed, and space requirements. Based on the speed and horsepower, a 14-mm HTD is the first pitch choice. HTD, or High Torque Drive, has become the baseline rating for power belts. Specs like HT150 refer to a rating that’s 150% of the baseline. “HTD” can therefore also be written “HT100.”

Using the 14-mm HTD sprocket as our launch pad, we go to any standard (HTD-based) ratio and center-distance tables corresponding to 14-mm pitch. A 68 to 112 combination (1.65 reduction ratio) is available at the specified pitch, but doesn’t fit in the 9.55-in. center distance between the driving and the first driven sprocket. A smaller sprocket combination of 38 to 64 is then used, providing a 1.68 reduction ratio.

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