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Selection of castors for industrial situations

09 March 2011

Choosing the right wheel Rather obviously any product that isn’t used under the conditions for which it was designed may not satisfy the user’s needs.

It may also damage materials and cause injuries. In order to understand the possible pitfalls here are some examples in which wheels and castors are used incorrectly:
•    using a wheel not suitable for the floor will deteriorate the wheel covering and damage the floor
•    choosing a fixed castor or inappropriate configuration of castors under operating conditions for which a equipment must be very maneuverable will make it extremely difficult to move that equipment
•    applying a load that exceeds the wheel’s rated load capacity can readily lead to wheel malfunctions and premature deterioration.

Therefore it is worth performing a technical analysis of the operating conditions. The most economical solution may then be chosen only after the product has been technically evaluated.

The purpose of performing a technical analysis on an equipment moving solution is to define the operating conditions and any external factors that may affect equipment use.

The following factors should be analysed in order to choose the right wheel:
•    nature and condition of the ground (5.1)
•    environment (5.2)
•    magnitude and nature of the load (5.3)
•    speed and means of traction (5.4)
•    maneuverability (5.5)
•    data analysis and choice (5.6)

The process of choosing the right wheel to match the operating conditions can then be divided into three steps:

Step one: identifying the correct type of wheel based on the floor and the characteristics of the operating  environment.

Step two: calculating the dynamic capacity, static load and rolling resistance required by the specific application and therefore, determining the wheel diameter.

Step three: identifying the correct bracket and checking the dynamic capacity of the castor (wheel/bracket assembly).

If the evaluation of these various aspects generates different data with reference to the same wheel and/or castor characteristic, the final choice should be made based on the most conservative condition.

• Static load [N]

Static load is the maximum load that a motionless (stationary) wheel can support without generating any permanent deformations that may reduce its operating efficiency. A wheel mounted on equipment that is seldom moved and therefore almost always remains in the same position, is defined as being subjected to a static load.

• Dynamic carrying capacity

Dynamic carrying capacity of a wheel is defined as the value (expressed in N) of the maximum load that can be supported by that wheel in conformity with ISO 22883:2004 and UNI EN 12532:2001 that, for industrial wheels, require dynamic testing under the following conditions:
- constant speed of 1.1 m/s (4 km/h)
- overcoming 500 obstacles and 15,000 revolutions of the diameter
- obstacles with width 100 mm and height 5% of the wheel diameter with an elastic rolling strip (hardness up to 90 Shore A) and 2.5% of the diameter for wheels with a rigid rolling strip (hardness greater than 90 Shore A)
- temperature 20 °C (tolerance ± 10 °C)
- non-continuous operation (3 minutes of operation and 1 minute stopped)
- smooth, hard and horizontal floor.

• Rolling resistance

Rolling resistance is the value (expressed in N) of the maximum load that can be supported by each single wheel at a constant speed of 4 km/h with application of a tractive force or thrust equal to 50N (excluding the initial pickup). This value is obtained by applying a tractive force of 200N to a 4-wheeled equipment and measuring the magnitude of the maximum transportable load per wheel during normal moving conditions.

The applied tractive force of 200N complies with the international workplace standard for indoor moving and is universally recognised as the limit of human effort that can be supported for extended periods of time.

5.1 Nature and condition of the ground

The nature and condition of the ground and the presence of any obstacles will have an influence on choosing the right wheel. They are also important factors affecting the performance of the moving equipment as well as the efficiency and longevity of the wheels and castors themselves.

Special attention is required for cases involving uneven floors or where obstacles are present. In this case, the impact of the wheel against an obstacle generates resistance whose magnitude depends on the elasticity of the rolling strip material. In fact, the energy absorbed during an impact is greater in a wheel with an elastic rolling strip than in a rigid wheel, thus partially cancelling the braking effects caused by the obstacle.

For floors that are uneven or on which obstacles are present, with load capacity being equal, a wheel with a greater diameter should be chosen in order to overcome the obstacle. The wheel must be chosen very carefully in all cases in which there are obstacles, chemical and/or organic substances and machining residues. Common types of industrial flooring are:
- tiles
- asphalt
- cement-resin
- not paved floor
- expanded metal floor
- floor with chips, obstacles etc.

The main floor-wheel covering combinations available from Elesa are listed in the following table.
 
5.2 Environment

To choose the right wheel, it’s also important to determine if the wheel materials are compatible with the chemical-environmental conditions, the temperature, the humidity and the inductive electrostatic phenomena that may affect wheel operation.

The standard operating conditions are normally indicated in the manufacturer’s catalogue for each type of wheel.

Chemical-environmental conditions

Because there are so many different types of aggressive chemical agents in work environments, it’s difficult to provide a complete and exhaustive description.

The main chemical substances that a wheel may come in contact with include:
- weak acids (e.g. boric acid, sulphurous acid)
- strong acids (e.g. hydrochloric acid, nitric acid)
- weak bases (e.g. alkaline solutions)
- strong bases (soda, caustic soda)
- chlorinated and aromatic solvents (e.g. acetone, turpentine)
- hydrocarbons (e.g. petrol, oil, diesel oil, mineral oils)
- alcohol (e.g. ethyl alcohol)
- fresh water
- salt water
- saturated steam

Therefore, when choosing a wheel, it’s very important to check if the material forming the covering, the wheel centre body, the rolling actions and the bracket is compatible with the specific features of the operating environment. Caution is required in those sectors in which water, acids, bases, steam and other aggressive agents are often present. For example, a polyurethane wheel should be used instead of a rubber tyred wheel in environments with a large quantity of oils, fats and hydrocarbons, while it is recommended to use stainless steel castors in humid environments and in the presence of high saline concentrations.

Temperature

If operating temperatures in an application differ from the standard range of values indicated by the manufacturer then check the resistance of the wheel materials. This not only applies to the rolling strip and the wheel centre body, but also to the type of lubricant used (it may be necessary to contact the 
manufacturer).  For standard Elesa castors the proportions of their carrying capacity varying as a function of temperature are shown in the following table.
 
The above-mentioned variation values refer to the prolonged and continued use (over 30 minutes) of the wheels at the specified ambient temperatures.

5.3 Magnitude and nature of the load

The magnitude of the load is the value [N] obtained by adding the weight to be transported to the equipment weight (tare). The nature of the load, either a liquid or a solid, has a significant effect on the wheel load capacity calculation. The formula to determine the load capacity for each wheel is:
 
where: Q = load capacity for each wheel Pu = weight to transport Pc = equipment tare (equipment weight) n = number of wheels in contact with the ground

SOLID LOAD:

For a solid load, n=3 for a four-wheeled equipment (where three out of four wheels are considered to be in contact with the ground at all times).
LIQUID LOAD:
For a liquid load n=2 for a four-wheeled equipment (where two out of four wheels are considered to be alternatively in contact with the ground).

A thorough analysis is indispensable when the equipment is part of an automated or continuous cycle production unit. In this case, all the forces that act on the wheel must be taken into consideration; therefore, it is recommended to include appropriate allowances and safety factors.

5.4 Speed and means of traction

Equipment speed is an important factor when choosing a wheel. In fact, if the speed is 0, and thus the use is mainly static, it is enough to compare the load capacity for each wheel with the static load indicated in the manufacturers’ catalogues.

If the speed is other than 0, then the means of traction must be taken into consideration. The means of traction is the equipment used to exert the force that moves the body. In industry, traction devices can be manual or mechanical. Manual moving refers to the situation in which the force is exerted by one or more persons, while mechanical refers to the situation in which such force is exerted by a mechanical device (on-board drives or by using towing devices).

• Manual moving

For manual moving, the speed is generally less than or equal to 4 km/h.
 
Choosing a wheel that allows only one operator to move a load should be based on a wheel rolling resistance value determined by the following formula:
 
where: S = rolling resistance Pu = weight to transport Pc = equipment tare (equipment weight) n = number of equipment wheels (maximum 4) The value obtained should be compared to the wheel rolling resistance value indicated in the manufacturer’s catalogue.

• Mechanical moving with towing devices
or towed mechanical moving, the wheel should be chosen based on the equipment’s operating speed. The wheel rated dynamic load capacity normally refers to a speed of no more than 4 km/h (1.1 m/s).

If the speed is higher than 4 km/h, a correction factor must be applied to the load capacity value since the materials forming the wheel undergo chemical-physical changes during operation which lead to their performances decreasing with an increase in operating speed.

The indicative percentages of load capacity variation with an increase in speed for different types of wheels are shown in the following table.
 
• On-board mechanical movement

For equipment with an on-board drive (equipment with drive wheels – self-propelled), the wheels are subjected to particular stress and strain.

In fact, the drive wheels not only support the load, but also must transmit the tangential stress that allows the wheel and therefore the equipment to move. In addition, the drive wheel covering is subjected to even greater stress.

In particular, when choosing wheels and castors for self-propelled equipment, the following factors must also be taken into consideration:
• type of plain or ball bearing applied in the bore
• shaft/bore coupling tolerances
• bore material in relation to shaft material
• start and stopping frequency of the motion transmission part
• direction reversals
• presence of even temporary overloads

Since many factors have to be evaluated, it is recommended that specifiers contact ELESA (UK) Ltd. to choose the correct wheels and castors for applications involving self powered equipment.

5.5 Maneuverability

Equipment maneuverability refers to the ability of equipment to be moved more or less easily during use.
 
The limited space available inside some production departments or particularly winding routes that sometimes connect one work unit to another may require special equipment maneuverability characteristics to make operator tasks easier. Swivel castors allow the equipment to rotate and the greater the castor offset (i.e. the distance between the bracket rotation axis and the wheel rotation axis), the easier the rotation. However, though it does guarantee excellent maneuverability, excessive offset may cause the castor to oscillate along straight routes (Shimmy effect). Fixed castors do not allow the equipment to change direction but do guarantee directionality. In any case, the fixed castors must be mounted so that they are perfectly parallel to each other. The most common wheel layouts along with the relative castors are shown in the following table.
 
5.6 Choosing the wheel

Each of the parameters and operating characteristics outlined in the previous paragraphs is used in one of the three steps involved in choosing the wheel.

Step 1

The type of wheel suitable for the floor and operating environment is identified in step 1. The following graph summarizes the factors that influence the choosing of the type of wheel; “type of wheel” means: materials that form the covering and the wheel centre body; type of anchorage between covering and wheel centre body; rolling actions
 
Step two

The load capacity, static load and smoothness values required by the specific application and needed to determine the wheel diameter are calculated in step two. One of the most important parts of this step is an analysis of the load that the wheel must support. The following diagram indicates what calculations to perform and what values to consider depending on the various operating conditions. These aspects must always be indicated (magnitude and nature of the load and speed), while ensuring that all the values determined are not higher than the rated values indicated in the manufacturer’s catalogue. If the evaluation of any of these aspects generates different data with reference to the same wheel characteristic, the final choice must be made based on the most conservative condition.
 
Step three

The correct castor is chosen in the third step. The step can be divided into two separate parts:
1. Choosing fixed or swivel brackets, depending on maneuverability and directionality needs.
 
2. Checking the compatibility between dynamic load capacity and rated dynamic load capacity of the wheel and bracket. The following table summarizes some general indications for choosing the right wheels according to the application’s features.
 
Further information on Elesa (UK) Ltd products may be found at www.elesa.co.uk




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