Castor Wheels Roll Resistance
5 Factors affecting the Roll Resistance of a Castor Wheel
“Castor Wheel manufacturers use a range of different materials and sizes when producing Castor Wheels. The type of Castor Wheel used will determine the roll resistance of the equipment. Within this article you will find an overview of how to control a Castor Wheels roll resistance, calculating roll resistance and the typical values of rolling friction of wheels.”
Definition of Roll Resistance “ the force resisting the motion when a body (such as a Castor or Wheel) rolls on a surface.
The roll resistance of a piece of equipment mounted on Castor Wheels is related to 7 primary characteristics:
- Castor Wheel Material (Rubber, Nylon, Polyurethane)
- Castor Wheel Size (For example 100mm Diameter Castor Wheel)
- Flooring Substrate (Concrete, Carpet, Tiles)
- Flooring Condition (Level, Uneven, Uphill)
- Castor Wheel Bearings (Plain Bearing, Ball Bearing, Roller Bearing)
- Inertia (resistance to movement, based on equipment dimension)
- Total Weight of the Equipment
Generally a Castor Wheels low roll resistance is a positive attribute for a piece of equipment, providing ease of use for the operator, reduced user fatigue and reduce wear and stress of the equipment. However it should be noted that a high roll resistance may sometimes be preferable, for example to provide high grip and increased control of equipment.
Castor Wheel Friction
Friction is perhaps the main factor to consider when determining the Roll resistance of a Castor Wheel. Friction is the resistance between two surfaces sliding or rolling against each other. There are several types of friction, including Static, Sliding and Rolling Resistance. In this article we will be focusing on Rolling Friction which is most relevant to the Castor Wheel Industry.
The standard equation for resistive force due to rolling friction can be defined as follows:
Fr = Resistance to move i.e. Force (Newton’s)
c = Coefficient of Rolling friction (based on the Materials & Wheel Diameter)
W = Weight of the Equipment
Based on the above simple equation it can be seen that by reducing the Coefficient of Friction of either the flooring or wheel material (or both), the resistance to move can be reduced.
Example Rolling Coefficients of Wheel Materials (For Guidance Online):
- Rubber on Concrete – 0.35 to 0.45
- Nylon on Concrete – 0.03 to 0.04
- Polyurethane – 0.04 to 0.08
Rolling resistance of a Castor Wheel mounted machine or equipment is therefore the measure of the friction between floor surface and the tread of the Castors wheel.
Consider a 102Kg (1000N) Trolley on a set of four Soft Rubber Castors sitting stationary on a rough concrete floor, where the rubber wheels deflect (deform) due to the load put up on them. The rubber deforms onto the uneven flooring surface. At the point of movement, the resistance to move will be relatively high due to the wheel being deflected (i.e. non-circular wheel due to the weight deformation), and secondly due to the soft rubber gripping the uneven floor indentations. In this instance the coefficient of friction will be high, resulting in a high force required to move the equipment. (I.e. High Castor Wheels Roll Resistance)
Example: Fr = 0.4 x 1000N = 400N Resistance
Now imagine the opposite, a 100Kg Trolley on a set of four Hard Nylon Castors sitting on a smooth wooden floor. The Wheel does not deflect under load, and the Flooring is smooth, therefore having low friction. This would result in a very low coefficient of friction and therefore a much lower force required to move the equipment. (I.e. Low Castor Wheels Roll Resistance)
Example: Fr = 0.035 x 1000N = 35N Resistance
The “Resistance” is the amount of energy that is dissipated due to friction instead of being converted into movement of the equipment wheels. Therefore the lower the friction, the less energy absorbed, the more energy used to move the equipment.
The Diameter of the Wheel also has a significant impact on the Castor Wheels roll resistance. The Larger the Wheel, the Lower the Roll Resistance, hence the easier to move. A wheel diameter of 100mm is twice as easy to move compared to a 50mm diameter wheel. Therefore it is always advised to use a bigger wheel as possible, within the height and cost constraints of the project design.
Within the Material Handling Industry, Castors and Wheels are used to reduce the effort (force) required by the operator or user to move a piece of equipment or machinery. The amount of force required has a significant impact on the efficiency of the equipment hence the productivity. Secondly, reducing the force reduced user fatigue and ensures that equipment can be used for longer periods of time without falling foul of manual handling legislation. It is therefore important to consider a Castor Wheels Roll Resistance when designing equipment of selecting Castor Wheels.
Another important factor to consider is the “Inertia” of the equipment. This is the initial force required for move a piece of equipment from a stationary position. The inertia force is always higher than the rolling force, as once momentum has started the force required to continue the motion is less. As a rule of thumb, the initial force to start movement is in the order of 2 to 3 times the rolling force.
The Inertia of a piece of equipment can considerably change the amount of force needed to move an object. The Inertia is calculated based on the size and weight of the equipment. Imagine a 30Kg Box, 10cm x 10cm x 10cm, would be much easier to move or push, compared to a 30Kg Box, 100cm x 100cm x 100cm. Therefore not only the weight of the equipment but also the dimensional size of the equipment has a direct effect on the roll resistance due to inertia.
Castor Wheel Bearings
The bearing in the castor wheel axle can affect the roll resistance of the equipment (note: It does not affect the Inertia). Bearings act to reduce friction between the axle and the wheel.
A Plain Bore Bearing Castor will have the axle rotating against the the bore of the wheel, increasing friction, whereas a Roller Bearing Castor Wheel will rotate the axle against the bearings and reduce the friction. It should be noted however that bearings have the least impact on roll resistance, with low friction roller bearings typically reducing total the coefficient of rolling friction by around 0.01. Therefore Wheel bearings should only be used in the most extreme scenarios, or where extended roll after removal of force is required.
Guide to Selecting the Best Castor Wheel
- Select the Castor based on the Maximum Load per wheel. (consider weight distribution)
- Consider the Wheel Tread Material Type. For example, Rubber tread for Low Noise &/or Floor Protection or Nylon for Wheel Wear Resistance.
- Consider the Environmental Factors
– Shock & Vibration
– Flooring Type
– Fluids, Chemicals & Contaminants
- Choose the maximum possible wheel diameter.
- Choose a wheel with the required coefficient of roll resistance.
- Calculate the roll resistance (Consider initial force will be approx. 2 to 3 times higher).
- Ensure calculated force is within legislation for the operator. For further guidance on UK Legislation check http://www.hse.gov.uk/msd/pushpull/
Bulldog Castors Ltd
Bulldog Castors Ltd are expertise in the Design & Application of Castor Wheels for Material Handling & Industrial Equipment. We offer free technical advice including computer modelling and simulation to calculate Castor Wheel Friction, Complex Load Distribution and Shock and Vibration Analysis, and calculating Castor Wheels Roll Resistance
Contact Bulldog Technical Department on +44 (0)116 2970521 or firstname.lastname@example.org
Bulldog Castors Ltd – British Engineering Excellence