7 steps for selecting a swivel castor

Swivel castors give mobility to objects of all kinds. Mechanical engineering and plant construction, in particular, often require objects to be transported over long distances. Thanks to swivel castors, this is easy and efficient. This article introduces important selection criteria for swivel castors.

What is a swivel castor?

Swivel castors are versatile components with a wide range of applications. They are usually mounted directly on equipment or machines to give them mobility. Depending on the model, swivel castors can be turned in a limited swivel range or about 360°, which permits flexible movement in all directions. Swivel castors are available in different materials, which are selected depending on the requirements for load capacity and floor conditions. For example, polyurethane offers high load capacity and abrasion resistance, while rubber ensures good damping and quiet movement.

In addition to simple swivel castors, there are special variants such as:

• Double swivel castors for greater stability
• Fixed castors for straight-line movements
• Heavy-duty castors for particularly high loads
• Swivel castors with brake to prevent unintentional movements
• Swivel castors with stop to limit the swivel range
• Lifting swivel castors with locking foot to lift the swivel castor when the unit is parked

How to select the right swivel castor in the face of this variety? The following steps provide assistance.

Preparation - determine the load capacity

The load capacity of the swivel castor decides whether it is suitable for the intended use or whether, for example, several castors are required to carry the load. Determining this parameter is therefore the most important prerequisite for selecting a swivel castor. The load capacity T of a swivel castor is calculated from the following parameters:

• E = Dead weight of the transport equipment
• Z = Maximum payload
• n = Number of load-bearing castors
• S = Safety factor*

The following formula can be used for the calculation:

The safety factor takes into account the operating conditions that deviate from the standard. The calculation takes into account the ratio of the diameter of the swivel castor to the obstacle height and the speed. A distinction is made between manually operated and externally driven. The following safety factors apply:

• 1.0 to 1.5: Manual operation indoors, obstacle height less than 5% of the wheel diameter
• 1.5 to 2.2: Manual operation outdoors, obstacle height greater than 5% of the wheel diameter
• 1.4 to 2.0: Motor operated indoors, obstacle height less than 5% of wheel diameter
• 1.4 to 2.0: Motor operated outdoors, obstacle height greater than 5% of wheel diameter

Step 1 - Select castor by load capacity

The required load capacity T directly influences the selection of the possible swivel castors. For loads up to 100 kg, light-duty swivel castors are generally sufficient. The use of heavy-duty swivel castors is recommended for heavy-duty applications, especially for applications weighing 500 kg or more. These swivel castors are also available from MISUMI in the online shop. Depending on the application and the required load capacity, the swivel castor bearing should also be considered in more detail. For example, high-quality bearings that guarantee uniform movement are essential when the castor is used for heavy loads. Plain bearings and roller bearings are, for example, ideal for this. Swivel castors with ball bearings are in turn suited for applications with higher speeds and ease of movement.

Step 2 - Determine roller diameter

The diameter of the swivel castor determines its maneuverability and navigability. When determining the wheel diameter, the following preliminary considerations should be made: Will the swivel castor be used indoors or outdoors? What speeds is it exposed to and what are the floor conditions? Castors with a smaller roller diameter are for example suitable for smooth and hard surfaces and also indoors at reduced travel speeds. They are generally more agile and reduce the overall height of the transported load, which can be an important decision criterion in space-constrained applications. Larger diameter wheels can overcome obstacles, especially on uneven or rough surfaces. The rolling resistance is generally lower. These wheels are therefore suitable for heavy loads or when used outdoors. Even at higher speeds, they are usually a good choice.

Step 3 - Select design and version

The roller type and type of fastening depends on the purpose for which the swivel castor is intended and on the conditions at the operating location. The following questions provide an indication for an initial classification:

• Is movement a key aspect or are movements only occasional in nature? Smooth-running swivel castors are suited for frequent movement, whereas fixed castors or castors with brakes are suited for occasional movement.
• What are the conditions at rest? Is leveling and alignment necessary or does the equipment only have to be secured against uncontrolled movement? For example, there are castors with leveling feet for uneven at-rest conditions.
• How is the brake actuated? How much space is available? Are there situations where I can only reach the brake on the side or from one side? There are different types of swivel castors with integrated footbrake,
• What are the path conditions? Are there many curves? Is the path at a slope and do many obstacles need to be navigated, such as thresholds, ledges, grooves or grating? Large rolling surfaces are suited for thresholds or inclines; there are shock-absorbing designs for uneven surfaces.

Swivel castors can be swiveled and maneuvered easily. For fixed castors, the direction of movement is fixed and they are therefore primarily suited for forward and reverse movements. They have more stable tracking because they do not swing out. The combination of track roller and fixed castor has good maneuverability in one direction with good tracking provided by the fixed castors on the other side.

• 1 = Fixed castor
• 2 = Swivel castor
• 3 = Swivel castor with axial brake
• 4 = Swivel castor with radial brake

Step 4 - Determine wheel material

The material used for the swivel castor has an impact on user comfort, ease of movement and resistance when traveling and steering. The rule of thumb is: The wheels should be softer than the floor because the floor may otherwise sustain damage. Materials for swivel castors can be:

• Rubber
• PU (polyurethane)
• Plastic

The following table provides an overview of the properties of different wheel materials:

Step 5 - Determine wheel bearings

The wheel bearing influences the rolling properties and is important for general mobility. It must meet the requirements for load capacity, duty cycle, environmental influences, start-up and rolling resistances. Various wheel bearings are presented below and their advantages and possible applications are shown:

Plain bearings

Plain bearings are cost-effective, simple, durable and can be corrosion resistant. They are primarily used for light loads, low speeds and medium frequency of use.

Roller bearings

Roller bearings have a small radial bearing clearance, which provides uniform and stable movement. Their high load capacity is advantageous in heavy-duty applications. Roller bearings are often made of steel, making regular maintenance essential as they are more susceptible to dirt and dust.

Ball bearings

Ball bearings are characterized by a very accurate, easy rolling behavior. They are often used as plastic versions for light loads.

Step 6 - Note external influences

There are various external influences that affect the function and service life of castors. These can be, on the one hand, the floor conditions and the floor surface, which affect the rolling, swiveling and startup resistance. The startup resistance is the resistance that must be overcome to initiate the rolling motion. The rolling resistance is the resistance while moving at a constant speed. It is caused by hysteresis (the energy loss automatically caused by internal friction of the material when a tire is rolling in its lane). The swivel resistance must be overcome while steering.

On the other hand, this can also be caused by environmental conditions such as temperature, humidity or exposure to aggressive chemicals. The selection must take these factors into account.

Chemical resistance is relevant, for example, if direct contact with aggressive chemicals is probable. However, not only the chemical itself, but also the duration of contact, the concentration quantity and other influencing environmental conditions must be considered. Acids, for example, can lead to corrosion and pitting on metal parts of swivel castors, which weakens the structure. Alkaline solutions also act similarly on metal parts, but often over a larger surface area. On plastics, they cause the material to slowly undergo chemical decomposition, which can result in cracks. Solvents can cause plastic rollers to swell, subsequently dissolving the material.

Excessive moisture can also lead to corrosion-induced signs of wear in metals. Even on rubber, long contact with moisture can lead to hydrolysis: the material becomes brittle and cracks and loses elasticity. However, there are also hydrolysis-stabilized plastics to avoid this.

Step 7 - Additional properties

At times, swivel castors are used in environments that impose highly specific requirements. For example, cleanrooms are subject to stringent strict hygiene requirements. Depending on the cleanroom class, almost no airborne particles are permitted, as these can lead to contamination on the product. In these cases, it is for example not possible to use wheels made of a material with high abrasion. Wheels for cleanrooms therefore often consist of antistatic material with a smooth surface, thus preventing dirt accumulations. At the same time, they are abrasion-resistant and easy to clean.

The electrical conductivity of swivel castors can become relevant for electronic and explosion protection or in ATEX zones. It is important that the wheel material is not susceptible to electrostatic charging, while also having the ability to discharge static charges/electricity in an ideal manner. Electrostatic charging is a serious ignition hazard in an ATEX environment.

Additional standards, such as ISO 14644-1 for cleanrooms or the ATEX Directive 2014/34/EU, may also need to be observed when castors are used in such specific circumstances.