Guidance on the Characteristics of Industrial Castors

Conceptually we are used to a castor as a mechanical assembly which allows us to replace sliding motion by rolling motion, often with an inbuilt turning mechanism to permit both easy forward movement as well as simple changes of direction. It is useful to consider the ubiquitous castor in its elements and to identify the characteristics of those elements as well as the results of their interactions in order to optimally specify a castor installation. We use the following international standards, nomenclature and characteristic descriptions to aid our customer recommendations.

Tread

The tread is the wheel’s outer surface, i.e. the part that comes in contact with the ground. It can be smooth or engraved with raised patterns to increase its grip on the ground.

Covering

The covering, or rolling strip, is the outer ring. It is made of different materials and characterizes the appearance of the wheel. The covering is fixed when joined with the wheel centre body as a single solid piece (using an adhesive or through a mechanical connection) or fitted when mechanically assembled on the wheel centre body.

Wheel centre body

The wheel centre body is the wheel part that connects the covering to the bore. It comes in various shapes and is made of different materials; it can be a single piece or two or more parts joined together.

Bore and rolling actions

The bore is the middle part of the wheel that houses the axle or the bearing surfaces that make rotation easier (ball bearings, roller bearings, plain bearings, etc.).

Depending on the construction methods and materials forming the covering, wheels can be divided into three families: rubber wheels, polyurethane wheels and monolithic (or hard tread) wheels.

Rubber wheels

A rubber wheel covering consists of an elastomer made from natural and/or synthesized rubber. The rubber used to build industrial wheels can be vulcanized or injection moulded.

Vulcanized rubber: special mineral loads and vulcanizing agents are added to the rubber that undergoes a process called “vulcanizing”. During this process, the rubber’s molecular structure changes significantly: the “pasty” material at the beginning of the process becomes a non-fusible product that acquires and, over time, maintains the form of the mould in which the reaction occurs. The ring obtained is mechanically assembled to the wheel centre body. Vulcanized rubber has enhanced elastic deformability properties within relatively broad ranges of applied traction and compression loads.

The physical-mechanical characteristics of vulcanized rubber vary according to the quality of the natural and/or synthesized rubber used, the type and quantity of mineral loads added and the conditions under which the vulcanization process takes place.

Injected rubber: the rubber goes through a process of chemical synthesis. The source material is injected into a mould in which the wheel centre body has already been inserted. The injected rubber maintains its fusibility even after moulding.

Normally, the elastic properties of injected rubber are worse than those of the best quality vulcanized rubber, even though they are comparable to those of medium and low-quality vulcanized rubber. The following are some of the main physical-mechanical parameters relative to the quality of rubber (for the definition of each parameter see the standards indicated next to that parameter):

- hardness UNI EN ISO 868:1999; ASTM D 2240-2004

- specific density UNI 7092:1972; ISO 2781:1988

- impact strength UNI 7716:2000; ISO 4662:1986

- abrasion loss UNI 9185:1988; DIN 53516:1987

- ultimate tensile strength UNI 6065:2001; ISO 37:1994; ASTM D 412c-1998

- ultimate elongation UNI 6065:2001; ISO 37:1994; ASTM D 412c-1998

- tearing resistance UNI 4914:1987; ASTM D 624b-2000

- compression set UNI ISO 815:2001

These parameters are not independent; in other words, changing one of them usually leads to a change in other parameters (to varying degrees). Hardness is the easiest parameter to determine: in general, increased hardness reduces the elastic properties (impact strength, ultimate elongation, compression set) and lowers overall wheel performances. Instead, parameters such as tearing resistance and abrasion loss depend mainly on the composition of the vulcanized rubber and, to a lesser extent, on hardness.

Polyurethane wheels

A polyurethane wheel covering consists of an elastomer obtained exclusively from the synthesis of raw materials. Polyurethanes are chemical compounds obtained from a polymerization reaction triggered by mixing two components, belonging to two different families of compounds (Di-Isocyanates and Polyalcohols), that were previously heated to temperatures that keep them in the liquid state with relatively low viscosity. In general, elastomer polyurethanes do not contain any additional mineral loads. The reactive mix is cast or injected into heated moulds containing the metal or plastic centres. Thanks to the temperature of the mould and of the wheel centre body, the polymerization reaction can be completed inside the polyurethane, while the polyurethane is chemically linked to any adhesive that may be present on the surface of the wheel centre body.

Mould-on polyurethane is no longer fusible, has good elasticity characteristics in addition to medium-high hardness and compression and traction strength.

Injected polyurethane is fusible even after moulding; in general, it has inferior elasticity characteristics but superior hardness with respect to mould-on polyurethane.

The following are some of the main physical-mechanical characteristics of polyurethane (for the definition of each characteristic see the standards indicated next to that parameter):

- hardness UNI EN ISO 868:1999; ASTM D 2240-2004

- specific density UNI 7092:1972; ISO 2781:1988

- impact strength UNI 7716:2000; ISO 4662:1986

- abrasion loss UNI 9185:1988; DIN 53516:1987

- ultimate tensile strength UNI 6065:2001; ISO 37:1994; ASTM D 412c-1998

- ultimate elongation UNI 6065:2001; ISO 37:1994; ASTM D 412c-1998

- tearing resistance UNI 4914:1987; ASTM D 624b-2000

- compression set UNI ISO 815:2001

Monolithic (hard tread) wheels

In monolithic (hard tread) wheels, the wheel centre body and the covering are made with the same material. The physical-mechanical characteristics of the wheel will change depending on the material used.

BRACKETS

The bracket is the part that connects the wheel to the equipment. Normally, all wheels need a bracket to be applied to the equipment; an exception is made for wheels whose axle is built into the equipment. Brackets can be of the swivel or the fixed type.

Swivel bracket

The swivel bracket rotates around its own vertical axis as the running direction changes. The wheel axis is misaligned with respect to the bracket axis so that it is easier to manoeuvre the equipment.

“Maneuverability” is defined as the ability of the equipment to change direction, while “directionality” refers to the equipment’s ability to maintain a trajectory along a specific direction. Excessive offset reduces equipment directionality due to “sliding” of the wheel (the “Shimmy” effect).

Swivel brackets can also be equipped with brakes. The swivel bracket consists of a connecting plate, a fork, a ball race ring, swivel actions, a central pin and, if necessary, a dust seal.

• Fitting plate

The fitting plate is used to connect the bracket to the equipment (usually with four connection holes).

• Wheel support fork

The fork is the piece with the characteristic upside-down “U” shape that supports the wheel. Holes are drilled at the bottom to house the wheel’s axle set, while the swivel bearings are inserted in the top.

• Ball race ring

The ball race ring contains the castor’s swivel bearings. In special cases, it can also be used only as a dust seal or a guard.

• Swivel bearings

Swivel bearings allow the plate to rotate on the fork. They consist of a ring of balls in contact between the plate and the fork (called “ball gyro”) lubricated with grease to protect against dust, liquids and other aggressive agents. The bracket load capacity varies significantly according to the type of swivel bearings being used.

• Central pin

The central pin is the part that joins the plate and the ball race ring. Thanks to the central pin, the plate and the ball race ring form a single piece, while the fork is free to rotate around its own axis. The pin can:

- be incorporated in the plate, through forming and riveting after assembling the parts

- be incorporated in the plate, through hot forming on the plate and tightening with a self-locking nut

- consist of a screw and a nut.

• Dust seal

The dust seal protects the swivel bearings of the bracket against dust and solid and medium-grain aggressive agents.

Fixed bracket

The fixed bracket is designed to keep the wheel moving in a specific direction; therefore, it guarantees equipment directionality. Instead, equipmentmaneuverability depends on the use of swivel brackets. In general, the fixed bracket consists of a single pressed steel plate shaped into an upside-down “U”. Holes to house the wheel axle set are drilled at the bottom, while the equipment attachment holes are at the top.

Swivel bracket with brake

The brake is the device that allows the blocking of the rotation of the bracket around its axis, of the rotation of the wheel and of the rotation of the castor (wheel+bracket assembly).

AXLE SET

The axle set is the piece used to connect the wheel to the castor. Normally, it consists of a threaded pin with nut, washers, tube and, where necessary, spacers. For standard applications, the axle set can be riveted directly on the castor fork.

LOADS, FRICTION AND FORCES

Dissipative forces or friction occur along the contact surfaces between bodies and tend to oppose the movement.

Sliding friction

Sliding friction force opposes the movement between two contact surfaces that slide against each other. This force depends on the type of contact surfaces (materials and finishing level) and on the load applied in the direction perpendicular to the motion direction (Normal force).

In mathematical terms, the sliding friction force is defined as follows:

Fr = br x N

where: br = sliding friction coefficient N = normal force (or load)

If two bodies are initially stationary, the resistance force is called the static friction force and represents the minimum force that must be applied to start moving the two bodies. When the two bodies are in relative motion, a force lower than the static friction force is sufficient to keep the speed constant: this is called the dynamic friction force.

The friction coefficient is obtained experimentally for both static friction and dynamic friction. Rolling friction force is generated when two bodies roll on each other without sliding.

Let’s imagine a wheel with radius r subjected to a load N. As the wheel approaches the contact point, the material is compressed and afterwards, once the contact point has been passed, undergoes an elastic release.

If the material used to manufacture the wheel is not perfectly elastic, some of the energy required for compression is lost in the subsequent return phase – dissipated in the form of heat to counteract internal frictional resistance of the material. If we think in terms of forces, instead of energies, we could say that the distribution of pressure in the contact is not symmetrical compared to the direction of force N.

Mr = bv x N

To keep the wheel turning evenly it is necessary to apply a motive moment identical to and opposite Mr or a traction force F parallel to the forward direction and such that:

F x r = Mr

From the previous formulas we obtained:

F = Mr/r = bv x N/r = fv x N

Where

Fv = bv/r

With fv known as the rolling friction coefficient which can be found with experimental tests.

Tractive force

Tractive force is the force needed to overcome the resistance caused by friction when two bodies slide or roll on each other. Compared to the resistance generated by friction, tractive force has the same intensity and the same sense, but the opposite direction. The lower the force needed to keep equipment moving, the greater the smoothness of the wheel applied to the moving equipment. In the specific case of a wheel rolling on a flat surface, the tractive force must overcome the resistance caused by rolling friction – that arises when the wheel comes in contact with the surface – and by sliding friction – generated by the mechanical bore and axle set coupling.


Post time: 05-07-2017