Wheel conveyors, roller conveyors and roller strips – Construction, function and advantages

Material handling engineers rely on roller conveyors to efficiently transport products, goods and materials. They are invaluable, particularly in logistics, but also in many other industries and various manufacturing processes. This article discusses how roller conveyors are constructed, how they work and which variants there are.

Terms - wheel conveyors, roller conveyors and roller bars

wheel conveyors or roller conveyors are transport systems that transport goods on several roller bars or connected roller systems. Roller strips or roller bars consist of multiple rollers mounted along a rail. The rollers can either rotate freely or are driven by a motor. Individual roller rails are primarily suited for low loads.

The difference between wheel conveyors and roller conveyors refers to the size and scope of the transport system and the load to be transported: Wheel conveyors transport smaller and lighter objects, while roller conveyors are suited for heavy loads and are wider and larger. The term roller conveyor can also be used synonymously.

Construction of roller conveyors

Roller conveyors consist of several components:

• Rollers: Rollers are the central component. They are typically cylindrical and can for example consist of rubber, metal or plastic. The rollers can be installed in a roller bar or roller conveyor.
• Frame structure: So-called side rails or conveyor frames form the base frame of roller conveyors. They are for example equipped with holes and openings in which rollers, spacer tubes and other elements can be attached.
• Support structure: consists of roller conveyor stands that support the overall structure.
• Drive system (optional): roller conveyors can be either idling or driven.
• Control and safety devices: The control system regulates operation (e.g. speeds) and ensures ideal material flow. Safety devices may stop operation automatically as soon as they detect hazards.

Another design example can also be found directly at MISUMI. A wheel conveyor for body-in-white production (part of the production of car bodies where the individual parts are joined, e.g. by welding) is shown here. It consists of aluminum profiles, roller conveyors, NAAMS components and sensors.

How it works

The rollers allow objects to be easily moved along a path. This means that even heavy goods can be transported along the production chain without much effort.

There are various types of conveyors. Gravity roller conveyors for example are driven by gravity and not by motorized drives. The physical principle behind it is simple: objects are placed on an inclined surface and are then automatically drawn downward by gravity. The rollers support this movement.

Other variants are, for example, motorized wheel conveyors or driven roller bars.

Self-propelled roller conveyors

Roller conveyors can function completely without power supply if the appropriate framework conditions are created. For the system to work, the angle of inclination is of crucial importance. This should be selected so that a gradient between 2.5% and 5% is maintained. The required slope depends on the weight and surface finish of the conveyed goods. The lighter the load, the higher the angle of inclination. Likewise, if the surface condition of the conveyed parts is uneven, it may be necessary to increase the slope. A lower slope angle is sufficient for heavy goods, especially with a smooth surface. If the slope angle is too high, the cargo may be accelerated too much, which may lead to damage or falling. The design must also take into account the maximum load-bearing capacity of the individual roller axles: For example, when conveying goods with up to 40 kg, but the maximum load bearing capacity of the individual axle is only 10 kg, at least 4 axles must be distributed under the total length of the conveyed goods at any time. In the example mentioned, the axle spacing can also be calculated as follows:

However, the following basic rule always applies: there must be at least 3 axles under the conveyed goods, as otherwise the conveyed goods can tip and jam the conveyor, thus bringing it to a complete standstill.

Driven roller conveyors

Especially when conveying horizontally, the gravity-fed self-propelled drive no longer works. Driven roller conveyors are used to overcome upward and downward slopes. Driven roller conveyors are also suited for the following scenarios:

• Precise control of the conveyance speed and direction is required.
• Long conveyance distances must be overcome. Gravity is often insufficient to move the material continuously with self-propelled roller conveyors.
• Very heavy loads must be transported. Here too, gravity may not be sufficient for self-propelled roller conveyors.
• There are additional safety requirements that require controlled motion.

The load-bearing capacity is calculated as on self-propelled roller conveyors.

Belt conveyors

Belt conveyors are an alternative to driven roller conveyors. Belt conveyors are suited, among other things, for overcoming height differences. Different materials for the conveyor belt can address a range of applications:

• PVC belts: good chemical resistance, versatile, cost-effective.
• PU belts: high abrasion resistance, resistant to oils and greases.
• Rubber belts: high tear resistance and abrasion resistance, good damping properties.
• Silicone belts: heat-resistant, good gliding properties.
• Teflon belts: very heat-resistant, chemically inert, non-stick coating.
• Metal belts: particularly durable and robust, heat-resistant.
• Special-purpose belts: developed for specific requirements, e.g. ESD-sensitive applications (the conveyor belt discharges static electricity).

The choice of material depends on the type of transport and the surface of the goods being conveyed. For example, on belt conveyors with plate belts, the friction between the conveyed material and the belt is particularly high, which also reduces the tensile capacity, i.e. the belt can withstand less tension. Which belt to use must therefore always be considered.

For more information on this topic, please read our article on material handling systems - belt conveyors.

Side note: Material chutes

Depending on the application, it may also make sense to use a material chute as an alternative, e.g. for materials that are not sensitive to impact. Advantages of material chutes are: The system is very simple and does not need its own power supply as it makes use of gravity similar to self-propelled roller conveyors.

However, there are also certain practical problems: The conveyance speed and direction of the material cannot be readily controlled and the material can cause jamming. This reduces process safety. Gravity and friction forces are also difficult to calculate. However, guide rails can be installed for better control of the conveyance direction and better material flow.

In general, the use of material chutes is therefore recommended, especially for short and simple transport paths.

Selection criteria for roller conveyors

The roller width is an important property for selecting roller conveyors. The overall dimensions of the conveyed goods are key for making a correct determination. The rule of thumb is: The rollers should be at least 50 mm wider than the conveyed goods. The rollers can also be narrower if the goods are palletized. All cases require that the conveyed material does not come into contact with the frame of the roller conveyor.

When selecting roller conveyors, the slope-induced self-propelled movement of various materials must also be taken into account:

• X-axis: Weight of the conveyed material
• Y-axis: cm of incline per meter of length
• 1 - Material: Cardboard box
• 2 - Material: Plastic box
• 3 - Material: Steel box

The use of 3 steps (4 rollers) is recommended for goods with hard bottom (steel, plastic, etc.). 5 steps (6 rollers) are recommended for conveyed goods with a soft bottom (carton, rubber, etc.).

The diameter and raw material are calculated from the following table with different roller strengths:

Depending on the bottom of the conveyed material, the appropriate material and outer diameter of the conveyor rollers can be determined from the following table, where 1N corresponds to the weight force of about 100g.

Advantages and disadvantages of roller conveyors

Roller conveyors increase the efficiency of material handling or goods movement and play a major role for automating production and logistics processes. Their modular design allows roller conveyors to be easily adapted to different requirements. The maintenance effort is generally low; as a result, roller conveyors generally have a long service life without breakdowns.

Disadvantages may include: The initial acquisition costs are higher due to specialized applications and safety risks can arise due to improper handling (falling conveyed goods, etc.)

Cleaning and maintenance of roller conveyors

If the rollers are heavily soiled, the surface friction also increases and more force must be applied to move the rollers forward. It is therefore important to clean the rollers regularly, especially in heavily contaminated environments. It is important to avoid sharp-edged tools for cleaning. A simple brush or a damp cloth for light soiling should be used instead.

Defective rollers should be replaced promptly.