TechArchive

This article was originally written in the period 1995-2000

How do you select the required sprocket combination, belt length and centre distance for belt drives? This feature from GATES looks at some of the construction features for the Poly Chain GT.

Guide flanges are needed in order to keep the belt on the sprocket. Due to tracking characteristics, even on the best aligned drives, flanges will prevent the belt from riding off the edge of the sprocket.

On all drives using stock or made-to-order sprockets, the following conditions should be considered when selecting flanged sprockets:

• On all two-sprocket drives, the minimum flanging requirements are two flanges on one sprocket or one flange on each sprocket on opposite sides.
• On drives where the centre distance is more than eight times the diameter of the small sprocket, both sprockets should be flanged on both sides.
• On drives with more than two sprockets, the minimum flanging requirements are two flanges on every other sprocket or one flange on every sprocket – alternating sides around the system.

Stock sprockets up to 56 teeth are supplied with both sides flanged as a standard practice.

On made-to-order sprockets, flanges must be securely fastened, such as using mechanical fasteners, welding, shrink-fit or other equivalent methods.

In the application of positive belts, fixed centres are not recommended for any drives other than motion transfer (low or no torque) drives. Fixed centre implies exact tolerances. Although length tolerances for positive belts are significantly less than those for other belts, no belt can be manufactured without some tolerance. Sprocket and drive geometry tolerances also contribute to the problems associated with fixed centre drives. Fixed centre drives do not allow for proper tensioning practices, nor do they accommodate accepted maintenance procedures.

Because proper tension principles cannot be used, improper tension will result in reduced belt performance. By using inside idlers, the potential problems associated with fixed centre drives can be corrected.

Idlers

Use of idlers should be restricted to those cases in which they are functionally necessary. Idlers usually are used as a means of applying tension when centres are not adjustable.

Idlers should be located on the slack side of the belt drive. For inside idlers, grooved sprockets are recommended up to 40 grooves, while on larger diameters, flat uncrowned idlers can be used.

Outside idlers are not recommended because they could result in significant fatigue damage to the special polymers in the high performance Gates Poly Chain GT Belt, thus reducing belt life. Because idlers contribute to belt fatigue, idler diameters should not be smaller than the smallest sprocket diameter in the system. All idlers should be securely locked into place during drive startup and operation.

The use of tight-side, spring-loaded, non-locked idlers is not recommended, as the belt can generate sufficient tensions to overcome any reasonable force imposed by a spring loaded system. Any spring force sufficient to impose artificially high belt tensions may be excessive and could significantly reduce belt life.

Slack-side spring loaded idlers are often used successfully, as long as care is taken to avoid resonant vibration conditions and load reversals.

Belt installation and drive alignment

Provision should be made for centre distance adjustment, or change the idler position so that the belt can be slipped easily onto the drive. When installing a belt, never force it over the flange. This will cause internal damage to the belt tensile member.

Positive belts are sensitive to misalignment and should not be used on drives where misalignment is inherent to the drive operation. Misalignment leads to inconsistent belt wear and premature tensile failure due to unequal tensile member loading.

Positive belts typically are made with high modulus tensile members which provide length stability over the belt life.

Consequently, misalignment does not allow equal load distribution across the entire belt top width. So, in a misaligned drive, the load is being carried by only a small portion of the belt top width, resulting in reduced performance. Proper drive alignment is especially important when using Poly Chain GT Belts because of the extra high modulus cords and premium polymers used.

There are two types of misalignment: parallel and angular. Parallel misalignment is where the driver and driven shafts are parallel, but the two sprockets lie in different planes. When the two shafts are not parallel, the drive is angularly misaligned.

A fleeting angle is the angle at which the belt enters and exits the sprockets, and equals the sum of the parallel and angular misalignments.

Any degree of sprocket misalignment will result in some reduction of belt life, which is not accounted for in the normal drive design procedure. Misalignment of all positive belt drives should not exceed 0.25deg or 5mm per metre of centre distance. Misalignment should be checked with a good straight-edge tool. The tool should be applied from driver to driven and from driven to driver, so that the effect of parallel and angular misalignment is taken into account.

Drive misalignment can also cause belt tracking problems. However, some degree of belt tracking is normal and won’t affect performance. On those drives where the centre distance is greater than eight times the small sprocket diameter, belt tracking can be a problem. For those drives, some special adjustment may be required. This amounts to correcting the parallel position of the two sprockets until only one flange guides the belt in the system and the best tracks fully on all sprockets. Regardless of the centre distance of the drive, the optimum operation of the rive will be with the belt only contacting one flange in the system. The worst case is the contacting of flanges on opposite sides of the sprockets in the system. This traps the belt between the flanges and can force the belt into undesirable parallel misalignment.

Improper installation of the bushing can result in the bushing/sprocket assembly being ‘cocked’ on the shaft. This leads to angular misalignment. Be sure to follow the instructions provided with the bushings.

Installation and tensioning allowances

Since fixed centre drives are not recommended, centre distance allowances for a Gates Poly Chain GT belt drive are necessary to ensure that the belt can be installed without damage and then tensioned correctly. The standard installation allowance is the minimum decrease in centre distance required to install a belt when flanged sprockets are removed from their shafts for belt installation.

If a belt is to be installed over flanged sprockets without removing the sprockets, the additional centre distance allowance for installation shown must be added to the above allowance

Operating environments

Poly Chain GT Belt performance is generally unaffected in ambient temperature environments between -54degC and +85degC. Based on lab and field testing, Poly Chain GT belts provide excellent resistance to most chemicals, including some acids, alkalis and petroleum distillates. Actual performance characteristics will be determined by the degree of concentration of the chemical, the time of exposure and the type of exposure (drip, splash, immersion, etc). In addition to possible belt degradation, these chemicals can act as a lubricant in the drive system. As with any positive drive belt. Poly Chain GT drives run where excessive lubrication is present, have an increased tendency to ratchet. Special attention should be given to assure that recommended tension is maintained.

Gates has produced a completely updated belt drive design manual, which details the design possibilities of polyurethane synchronous belts.

• Gates Europe
• Tel: +32 53 76 27 11
• Fax: +32 53 76 27 13
• Contact: Rudy Boeman