How reinforcement solves a key challenge of monolithic conveyor belt designs

How reinforcement solves a key challenge of monolithic conveyor belt designs

Often referred to as “solid” or “food-safe” belts, monolithic belts are made of a single material, typically thermoplastic polyurethane (TPU). Combining the closed surface design of a traditional fabric belt with the wash-readiness of a plastic modular belt, monolithic belts deliver exceptional cleanability and food safety performance, making them a popular choice for hygiene-intense food processing applications.

However, there is a drawback to constructing a belt with a single, rubber-like material: stretch.

TPU is an elastomer, which is a portmanteau of elastic polymer. Elastomers change shape under stress but return to their original shape when the force is removed. This is referred to as elastic deformation.

However, there is a point where a material will not return to its original shape. This is called the yield point, which marks the end of elastic deformation and the beginning of plastic deformation, a permanent distortion of a material due to stress.

But plastic deformation is not solely the result of overwhelming force beyond a material’s yield point. In applications involving persistent mechanical stresses, such as most conveying applications, a material can deform permanently over the long term, even if the stress is below the yield strength of the material. This is known as creep.

Monolithic belts under regular load can creep, becoming longer and causing performance issues such as mistracking and sprocket disengagement. It doesn’t take much to create costly problems. Just 3% stretch can necessitate re-tensioning, shortening, or even full belt replacement.

To increase the yield strength of a material, manufacturers often reinforce a product by adding a strength member, which is a load-bearing component designed to increase tensile strength. Habasit was the first to champion monolithic reinforcement with its Habasit Cleandrive family of friction and positive drive monolithic belts featuring fully embedded, high-strength Aramid cords spaced laterally every 15 millimeters.

Cleandrive_Aramid_cords_ are_hermetically sealed_within_the_TPU Creep monolithic conveyor belt
Note: the Aramid cords are hermetically sealed within the TPU, creating no hygienic risks whatsoever. Any cut or belt damage deep enough to expose a cord would result in removal of the damaged section or a complete belt replacement, because a severe harbor point would be created in the material, and not because cords were present.

Lab tests comparing competitive monolithic belts have proven that Aramid reinforcement changes the behavior of monolithic belts when under stress.

To illustrate, let’s examine the following curve chart representing the results of a 15-day hanging test when a constant load of 6.25 N/mm – such as experienced in practical applications – was applied to a Habasit Cleandrive and two competitor unreinforced monolithic products. Specifically, we’ll review points marked A and B, which occur within the first 24 hours of the test, and point C, which takes place near the end of the test.

A: The initial load – notice that each belt starts on a different point on the y-axis, exhibiting different percentages of elongation at the moment weight was added. The reinforced belt stretched a mere 0.6%; Competitor 1 stretched 1.6%; and Competitor 2 stretched an alarming 2.9%.

B. The 24-hour mark – during the inaugural load period, a conveyor belt can exhibit more dramatic deformation as it responds to the new stress. Both competitors show steeper curves of elongation in the first 24 hours, representing a greater rate of elongation, while the reinforced belt holds steady.

C. The home stretch – by Day 9, all three belts have achieved consistent curve behavior. Both Competitors 1 and 2 exhibit continuous stretch, illustrated by the steady, upward slope of their curves. The data suggests that both these belts will stretch until they no longer function in a conveying application without some corrective maintenance. Competitor 2, having surpassed 5% elongation at Day 15, would likely require replacement, since the pitch between the belt’s teeth would have grown too large to properly engage with a sprocket. The curve of the reinforced belt, on the other hand, has flattened out—indicating that elongation has ceased.

Habasit_Cleandrive_7_Day_Creep_Test_vs._unreinforced_monolithic_belts_Tension_test, Creep monolithic conveyor belt

Multiple tests confirm these patterns of elongation, from which we can extrapolate some significant conclusions about reinforced belts:

  • Negligible initial stretch when load is first applied
  • No continuous stretch, whereas unreinforced monolithic belt designs stretch until they fail

The overall stretch of the reinforced belt remained under 1%, well below the starting points of both competitors at first load. This small degree of elongation is highly unlikely to cause performance issues. Meanwhile, unreinforced belts often present maintenance problems as they stretch continuously throughout use. This is true for tensioned flat monolithic belts and low-tension positive drive monolithic belts, and even for so-called “zero-tension systems”, a misnomer since all belts experience tensile loading on the carryway.

In summary, Aramid reinforced Habasit Cleandrive belts offer many maintenance-saving benefits:

  • No need to perform maintenance to re-tension or shorten the belt, since its total stretch is negligible
  • Fewer belt replacements and food safety risks due to consistent, optimal tracking, which reduces abrasion between the belt and system components
  • No process interruptions or line stoppages due to disengagement with sprockets
  • No need to waste time maintaining “shoes” or “limiters” used to force belt-sprocket engagement, because of its low-elongation design
  • Increased efficiency and greater output

While the data speaks for itself, you can also watch Aramid cords in action in a recent 7-day creep test recorded at Habasit America headquarters.

Want to know more about solving your monolithic belt challenges? Please contact us to discuss your application needs.

2020 February 24  |  Posted by

Bart Natoli

Bart Natoli is a Chief Application Engineer at Habasit America and has been with the company since 1986. He has a Bachelor of Science, a degree in Mechanical Engineering, Associates in Engineering Degree and Architectural Engineering Technology. Bart Natoli is an expert in fabric conveyor belts and power transmission belts.

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