The Need for New Generation Conveyor Skirting

KINDER & Co. - Silica is the second most common mineral in the earth's crust and is a major component of sand, rock, and mineral ores. Overexposure to silica dust can cause Silicosis, where scar tissue forms in the lungs, reducing the lungs ability to extract oxygen from the air we breathe. Silicosis is a disabling, nonreversible and sometimes fatal lung disease and the litigation costs associated with it rival that of asbestosis.

Therefore in many of today’s bulk materials handling plants, over and above production rates and maintenance costs, the health and safety of company employees is the number one priority. And in these plants, reducing the dust from conveyor transfer points and paying attention to belt tracking, which is often caused by material escaping from skirts, subsequently reduces the risk of an employee’s exposure to Silica dust.

With ever increasing production rates there is a greater requirement for plants to run 24 hours a day 7 days a week and this is particularly true in the power generation industry, where long run times between short scheduled maintenance windows is often the norm. This means that changing or adjusting skirting on conveyors is a costly job for many bulk material plants across the globe. This is an issue affecting many aspects of a business, where the administration time alone for simple jobs like changing or adjusting skirting can be extreme.

It has therefore never been more important to use the most efficient conveyor skirting available, one with maximum wear resistance and minimal maintenance requirements. But which is the right material to use?

For decades, rubber has been skirting the transfer points of conveyors worldwide, and high wear resistance combined with low cost has made Styrene-Butadiene Rubber (SBR) a logical choice to date.

However, in many cases SBR is not lasting the distance, resulting in spillage, airborne dust, tracking issues, premature failure of idlers, belt damage etc. With safety now being one of the driving forces behind innovation and the risk of silicosis apparent, the demand for highly abrasive resistant skirting material is rapidly multiplying.

While there are materials such as ceramics and heat-treated or hardened steels available to combat wear, these can be unforgiving when used in skirting applications if run close to or contacting the conveyor belt surface. So today, a strain of the high performance plastic polyurethane is now the most economical way of skirting rubber conveyor belts. But even this road is not always clear.

The misunderstanding within the industry in distinguishing the difference between polyurethanes and polyethylenes and witnessing plants use materials such as old conveyor belt, ultra high molecular weight polyethylene (UHMWPE), natural rubber etc, to skirt their transfer points, prompted Kinder & Co to research the most economical and belt friendly skirting material available. Ruling out materials that were hard and or abrasive the company chose 4 materials to analyse.

• SBR Rubber 60 DURO SHORE A (most commonly used)
• Natural Rubber 60 DURO SHORE A
• High Quality Natural Rubber 60 DURO SHORE A
• Argonics Polyurethane 69 DURO SHORE A

Kinder and Co performed three separate tests on each of the selected materials. A friction test, where the coefficient of friction of each material was analysed, an abrasive resistance test and finally, the materials tensile strength was tested. The results of each of these tests, were then combined with specific on site case studies and final conclusions reported.

An overview of each of the tests and the final conclusions follow.

TEST RESULTS

1. FRICTION

The most important of the three tests due to its potential to transfer heat along skirted areas causing premature belt damage, is the skirting materials coefficient of friction value. In this case the lower the value μ the better.

The 200 and 300 psi test data was not graphed as the high quality natural rubber samples pealed off twice due to excessive friction rates during testing.

Reducing friction between skirting and the conveyor belt surface is arguably the most important step in eliminating excessive skirting wear and belt damage. Conveyor belts are often replaced solely due to the skirting wearing a groove completely through the belts top cover exposing the ply carcass and results in tearing, spillage, belt tracking issues etc.

Some rubber skirting is porous which allows dust to adhere to the contact surface forming abrasive grit, which then wears a groove in the conveyor belt.

Polyurethane being non porous restricts the amount of dust being trapped under the skirt’s surface.

- Rubbers high friction = high abrasion and belt damage.

- Polyurethane’s low friction and non porosity = less abrasion and belt damage

2. ABRASIVE RESISTANCE

Test Method: NBS Abrasion (ASTM D1630) Chemtura Laboratory test facility, USA

The above graph shows that high quality natural rubber whilst achieving a high wear resistance is not preferred as a conveyor skirting material due to its high friction coefficient. High quality natural rubber was not used in a case study for this reason.

- The graph shows the Polyurethane sample 326% more resistant to wear than SBR rubber.

This should not be confused with the true wear resistance of the materials tested due to the field situation allowing for friction between the belt and the skirting.

3. TENSILE STRENGTH

Test Method: Extensometer (strain gauge)

 

CONCLUSIONS

High wear resistance combined with a low coefficient of friction is the ideal combination for a skirting material. Polyurethane produced the best results in the laboratory and field tests. Though the abrasive test results show polyurethane is 3-4 times more abrasive resistant than SBR rubber, once the friction factor was added in a real world scenario it was concluded that the polyurethane lasted 6-10 times longer than SBR rubber. This difference in wear rates is due to the polyurethanes very low friction value. The abrasive resistance test ASTM D1630 simulates abrasion by means of grit and not the contact between the conveyor belts.

Further case studies have showed that as the belt speed increased the wear rate of SBR rubber increased exponentially. However polyurethane remained more constant due to the lower friction value.

The table below summarises the suitability of each tested material.

View Suppliers Contact Details: