Tensioning belt drives

Whack the belt with a karate chop, and if it feels firm, the drive is properly tensioned." "Press the belt with your thumb until it deflects about 1/2 in." "Tension the drive until you get a slight bow in the slack side when it is running.

03/01/1999


Whack the belt with a karate chop, and if it feels firm, the drive is properly tensioned."

"Press the belt with your thumb until it deflects about 1/2 in."

"Tension the drive until you get a slight bow in the slack side when it is running."

These old rules of thumb for drive tensioning have outlived their usefulness: It is time to abandon them. They may have worked satisfactorily 20 or 30 yr ago, but they are inadequate for today's high-capacity drives.

V-belts and synchronous belts have been greatly improved over those of a few years ago. They deliver a lot more power in a smaller package. In order to achieve this improvement, it is essential that they are correctly aligned and tensioned. All it takes is a few simple tools and techniques to easily and accurately tension a drive to yield the high performance designed into it.

The consequences of improper belt tensioning are costly. If the tension is too low, V-belts slip and glaze or burn. This action destroys the belts and equipment must be shut down to replace them.

The effects of low tension on a synchronous belt are equally bad. Low tension allows the belt teeth to ride up on the sprocket teeth. This movement places severe stress on the teeth, eventually tearing them loose from the body of the belt. Under heavy loads, the drive can jump teeth (ratchet), which leads to rapid belt failure.

Drive tension that is too high can have other, far-reaching consequences. Undue stress is placed not only on the belt, but bearings and shafting as well. Early belt failure is the norm, because excessive tension over-stresses belt cords. Bearing overload also leads to early failure, and can result in the destruction of a motor and reducer.

Alignment

A prerequisite to proper tensioning is good alignment. Poor alignment makes accurate tensioning impossible, and causes an imbalance of load across the belt span. V-belt drives are inherently more forgiving of misalignment than synchronous belt drives (Fig. 1).

Drive alignment can be checked with a yardstick, machinist's straight edge, or piece of strong cord. The use of a straight edge is readily understood. What is not so apparent is that a good piece of cord can accomplish the same thing. Merely stretch it carefully across the face of the sheave or sprocket and extend it to the other sheave to determine the degree of misalignment. Repeat this measurement on each sheave or sprocket to detect possible alignment problems on either shaft.

Drive tensioning

Drive tensioning can impose a load on the structure that supports the motor, reducer, and other driven equipment. For example, the static drive belt tension between a 100-hp, 1760-rpm motor and the driven shaft can easily exceed 2500 lb. A 20-hp drive running at 50 rpm at the output shaft of a reducer could have a belt tension over 16,000 lb. The mounting structure must be able to support this load without deflection under static and dynamic load conditions. Otherwise, all of the care taken to establish good initial alignment would be fruitless.

The most common method for drive tensioning is "force deflection." A predetermined force is applied to the open span of the belt drive (Fig. 2). If the deflection exceeds 1/64 in. for every inch of span length, the drive is tensioned higher. If the deflection is too small, drive tension is excessive, and must be reduced.

Two values for force are used: A higher number for a new belt (or belts) and lower number for a drive that has been in operation for a while. The higher value is used for a new set of belts, because the drive tends to "settle in" during the first few days of operation. The lower force value is used to check the drive during routine maintenance, after it has been in operation for at least a few days.

The force values, in pounds, may be found in belt installation instructions or software drive-selection programs. Such software calculates force and deflection values for specific drives and is more accurate than using generic tables.

A tension tester is a hand-held spring scale that can be used to apply the deflection force to the center span of the belt. The range of a typical tester is up to 35 lb, which takes care of V-belt and synchronous belt drives in lower horsepower ranges, up to 100 hp. Sophisticated force measuring gauges are available with full-scale ranges up to 100 lb.

Frequency tension testers claim to be especially suited for tensioning synchronous belt drives. They are equipped with a microphone that measures the natural frequency of the tensioned belt, much the same as tuning a guitar string.

The belt is "strummed" while a microphone is positioned to pick up the vibration tone and show it on an LCD digital display. Drive tension is then adjusted to vibrate at the calculated correct frequency.

It is critical that the belt span length (not drive center distance) be used in the calculation of the frequency. Unless this figure is applied, there will be significant error in the drive tension.

Another tensioning procedure suitable for high-horsepower V-belt drives is the "elongation method." It takes advantage of the normal elasticity of a standard V-belt with polyester cords. It is normally used to tension drives using banded belts that require a deflection force beyond the range of conventional equipment. The elongation method is not suitable for tensioning synchronous belts that are constructed with fiber glass or aramide cords that have almost no elasticity.

Drive tensioning methods

The most common method for tensioning a drive is an adjustable motor base or motor slide rails. These methods are available in a variety of configurations, including spring-loaded models that automatically adjust for belt elongation.

For installations that cannot provide an adjustable center distance, the use of an idler is recommended. The preferred location for an idler is always on the slack side of the drive (Fig. 3).

An inside idler imposes less stress on the belt, and should be located near the larger sheave to minimize the reduction in the arc of contact with the smaller sheave or sprocket. If an outside idler is the only option, locate it near the small sheave. This position enhances the arc of contact with the small sheave. It is important that the idler diameter is no smaller than the smallest sheave in the drive. -- Edited by Joseph L. Foszcz, Senior Editor, 630-320-7135, j.foszcz@cahners.com

Alignment precautions

- The alignment of a drive that uses a relatively small sheave or sprocket diameter and wide face width is difficult. This type of drive also imposes a higher overhung load, and is prone to poor belt life.

- High overhung load can damage bearings and seals, and results in bent or broken shafting. To minimize this problem, mount the sheaves or sprockets as close as possible to the motor and reducer faces or use larger sheaves with narrower face widths.

- Ensure there is sufficient clearance for normal belt "flutter" caused by load variations or slight eccentricity in pitch diameter. More clearance is usually required on the slack side of the drive.

Maximum allowable offset

Type V-belt Synchronous belt

Angular, deg 0.5 0.25

Parallel, in./ft of CD 0.1 0.05

Example:

With a 4-ft center distance (CD)

V-belt parallel offset = 4 x 0.1 = 0.4-in. max

Synchronous belt parallel offset = 4 x 0.05 = 0.2-in. max

Elongation method procedure

- The banded belt is mounted on the sheaves and excessive slack is taken up, but the drive is not tensioned.

- A tape measure is wrapped around the belt to measure its outside circumference.

- Circumference is multiplied by a "belt length multiplier," which can range from less than 1% to over 3%.

- The drive is tensioned until the measured circumference of the belt is equal to the calculated elongated length.

High tensioning force values are required for...

1. High-horsepower drives running at motor speeds

2. Polyband belt drives, where the force of the individual belts must be totaled for the whole band

3. High-torque applications

More info

The author is available to answer questions about belt tension and alignment. He can be reached at 864-281-2133.

Key concepts

Incorrect tension can destroy belts and equipment.

Alignment affects belt tension.

Tension can be measured with a simple spring scale or acoustical instrument.





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