Thanks to advances in rubber technology, the wear life of today’s industrial conveyor belts should be at least two or times longer than it was as recently as a decade ago. Here, Bob Nelson explains the causes and effects of conveyor belt wear and tear and the reasons why there are such huge disparities in operational lifetime and durability between one manufacturer and another.
Causes of wear and tear
The wear and tear that takes place on the outer covers of rubber belts is commonly referred to using the umbrella term ‘abrasion’. However, there is more to the wearing process than simply abrasive wear. Some 80% of wear occurs on the top cover and 20% on the bottom cover, which is mainly caused by friction contact with drums and idlers. Top cover wear is primarily caused by two actions. First is the abrasive action of the materials being conveyed, especially at the loading point or ‘station’ and at the discharge point and is particularly associated with ‘fine’ materials such as sand or gravel, which literally act like a piece of coarse sandpaper constantly scouring the rubber cover.
Cutting and gouging
A second cause of wear is the cutting and gouging caused by materials that have sharp edges such as coarse aggregates and rocks. The heavier and sharper the material then the greater the wear. The ability of a belt cover to withstand wear is not only due to its ‘abrasion resistance’ because much also depends on the cover rubber’s overall strength and its resistance to cut and tear propagation. If that resistance is low then a seemingly insignificant area of damage in the cover can easily increase in size due to the continuous material loading and the relentless flexing around the drums and pulleys. In time, the damage spreads and links up with another area of damage causing small pieces of rubber to be cut from the surface.
Cut damage causes small pieces of rubber to be cut from the surface
Rip, tear and impact damage
The ability of a belt to withstand the forces that cause rip, tear and impact damage is often more important than any other physical attribute. A ‘rip’ occurs when a sharp object punctures the belt and becomes trapped, cutting the belt lengthwise as it is pulled against the object. A ‘tear’ is when a section of belt is pulled apart in opposing directions. Impact damage is caused by heavy objects falling from height that pierce the outer cover and damage the inner carcass. In an attempt to prolong operational lifetime, many conveyor belt users resort to fitting belts with increasingly thicker covers but this is usually a mistake because covers that are too thick can cause other problems. Simply using more of the same material will not provide the solution. The most practical and economical solution is to fit a conveyor belt that has been specifically engineered for the purpose.
An invisible cause of wear & tear – ozone and ultraviolet light
Ozone (O3) occurs naturally in the upper atmosphere. At high altitude, ozone acts as a protective shield but at low altitude it becomes a pollutant created by the photolysis of nitrogen dioxide (NO2) from sources such as automobile exhaust and industrial discharges. This process is known as ozonolysis and causes cracks to form in rubber that is under tension. The dynamic stress a conveyor belt undergoes while in operation is considerable. Ozone attacks the points where the strain is greatest, activating an oxidation process causing the rubber to become brittle, lose strength and increasingly less able to withstand virtually every kind of wear. The second destructive force is ultraviolet light from sunlight and artificial (fluorescent) lighting, which accelerates rubber deterioration (known as ‘UV degradation’) by producing photochemical reactions that promote the oxidation of the rubber resulting in a loss in mechanical strength. In short, ozone and ultraviolet light damage significantly reduces the ability to withstand the forces that cause wear and tear.
Although protection against ozone and ultraviolet damage is easy to achieve by using antioxidants within the rubber compound, laboratory testing reveals that some 90% of belts sold in Europe, Asia and Africa have virtually no protection. The reason is that antioxidants are regarded as an ‘avoidable cost’ by manufacturers wishing to appear price competitive.
The primary cause of poor resistance to wear and tear is the use of low grade ‘economy’ rubber rather than rubber that has been specifically engineered to provide a high level of resistance and a much longer operational lifetime. This is also the reason for the huge disparities between one manufacturer and another in terms of both price and performance. Rubber forms some 50% of the cost of a conveyor belt so it is therefore the biggest opportunity for manufacturers to cut costs and improve price competitiveness.
The biggest single cause of rapid cover wear is poor quality rubber
The solution to all causes of belt cover wear and damage is to avoid buying a belt because it has a lower price, especially those imported from Southeast Asia. There is a direct connection between price and durability and the mathematics are simple. Quality belts last significantly longer and therefore cost considerably less.