Friday, July 19, 2013

Fiberglass vs. Aramid Tensile Members In Belts

Inquiries about belts manufactured with aramid tensile members are fairly common. Aramid fibers are known for their high strength and low stretch properties. DuPont’s registered trade name for their aramid fiber is Kevlar®. Both V-belts and synchronous belts can be manufactured with aramid tensile members and have definite benefits as well as drawbacks.

V-belts with Aramid Tensile Cord
Aramid tensile members provide greater tensile capacity and robustness to V-belts. V-belts utilizing aramid tensile members generally utilize premium materials throughout and can carry greater loads than standard belts with polyester tensile members. High tensile strength translates into higher working tensions. High tensile modulus (low stretch) is also beneficial in keeping belt elongation over time low. The Gates Predator® V-belt line is an example of ultra-high performance V-belts with aramid tensile members.

Material costs for aramid V-belts are greater than for standard V-belt constructions. Reduced belt elongation helps keep drive center distance adjustment ranges low, but belt tension levels drop more rapidly with the reduced elongation than with standard belts. In the end, belt re-tensioning intervals for aramid type V-belts and standard V-belts are the same.

Synchronous Belts with Aramid Tensile Cord
Aramid tensile members provide rubber synchronous belts with exceptional shock and impulse load resistance. Fiberglass tensile members used in standard rubber synchronous belts tend to break down when shock loaded as individual glass filaments buckle and fracture. Even though somewhat stronger, however, synchronous belts manufactured with aramid tensile members are not rated for higher working tensions because the load capacities of other belt components remain the same. Even though aramid tensile members may be capable of higher working tension levels, belt teeth limit overall belt load capacity as they become the weaker link.

The length instability of aramid tensile members creates challenges in belt manufacturing processes and in actual belt drive applications. Aramid materials change length as they absorb and release moisture. Changing humidity influences belt manufacturing length targets as well as how belt teeth mesh with sprockets in actual belt drive applications. In addition, aramid tensile members have a negative thermal coefficient causing them to expand and contract opposite that of metal. As machinery warms up in usage and structures expand, aramid belts shrink and become tighter. Aramid belts also loose tension rapidly with structural contraction during cooling as they lengthen. In addition to belt length and tension challenges, material costs for aramid synchronous belts are greater than for standard belt constructions and aramid synchronous belts are all manufactured on a made-to-order basis in minimum order quantities.

Aramid tensile members are beneficial in increasing the load capacity and performance of V-belts and in improving the resistance of synchronous belts to shock and impulse loading. These benefits come along with belt length instability issues that must be considered when making belt selections. The potential drawbacks of aramid tensile members may not be realized in some applications while they may create serious performance challenges in others.

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