Any application can be broken down in to several sub sets which narrow the choice of suitable materials until only a few options remain. A decision can then be made depending on the balance of compromise between benefits and disadvantages.

Determining the application parameters

1. Total load
2. Location of load
3. Running surface
4. Moved by hand or towed
5. Environment/application description
6. Chemical exposure

Additional factors to be considered

1. Overall height
2. Swivel radius interference
3. Safety factor
4. Lifetime expectation
5. Maintenance

Total load

This is the total amount of weight carried by the castors and includes all elements associated. A worst case figure is preferred.

Location of load

Is this central and equally spread over the application and castors or wheels, or is it offset, for example a boat has a heavy stern and a lighter bow. This will help determine the safety factor and correct castor or wheel load capacity.

Running Surface

Exactly what is the running surface, for example smooth concrete, resin, tiles, carpet, tarmac, rippled concrete etc. are there any ramps, details are important to determine the correct castor configuration, thresholds. In many applications several surfaces may be traversed for an application.

Moved by Hand or Towed

If an application needs to be manoeuvred in to place by hand, consideration of the wheel choice needs careful attention. If the application is towed, then care needs to be taken with the bracket design and wheel performance. It is often the case that both situations exist, for example Automotive production lines. Determining whether a directional lock or brake system can be made at this point.

Environment/Application Description

Environmental factors such as location of use, for example inland, or by the sea, onshore, offshore, bridges, waterside, high humidity from weather conditions, temperature range and a full description of the environmental conditions through the applications use or lifetime need to be determined.

Chemical Exposure

Exposure to cleaning chemicals, fuels, hydraulic fluids, mineral oils etc. need to be considered. Chemical suitability/resistance charts are available showing material performance criteria. Always ask your supplier for verification of compatibility. This process can be speeded up by ensuring the relevant CoSh sheets are available.

Factors

Overall height
Height considerations should be made with a view to ensuring access through restricted height doorways, and stability requirements.

Swivel radius interferenc
If a design requires a castor to rotate through 180 degrees, then an allowance for the swivel radius needs to be made to ensure the motion is not interrupted by obstacles including walls or door entrances. In addition, ergonomic aspects such as access to activate a foot brake should be allowed for.

Safety factor
Allowing a safety factor is key to ensuring consistent operation of dynamic products to make sure they do not exceed their design parameters. Often you will be advised that a safety factor of one castor is required. This is true for some applications, but care needs to be taken with heavy load applications as other factors such as load displacement, towing, speed, and running surface may affect the safety factor.

Lifetime expectation
What is the life expectation of the products and/or application, consideration to material choice and environmental factors need to be made.

Maintenance

Implementation of a preventative maintenance programme will greatly extend the life of the product. Attention should be made to adverse factors such as environment, temperature, and chemicals. It is possible to refurbish wheels of certain types, or replace with new, as the castor housings are a lower wear rate component, especially when correctly greased and maintained.

Wheel centres in combination with a tread and bore type determine the overall performance and suitability of a product for specific applications.

Several materials are available for products manufactured in the wheel and castor industry.

Synthetic

These can be polyamide; nylon, polypropylene & polyethylene. The materials have good resistance to chemicals, can be mass produced using injection moulding processes and are a very common in lower load requirements. Other advantages include moulding specific dimensions and forms to meet unusual applications. Generally high volume manufacturing levels are required to amortise the expensive tooling costs.

Aluminium

Often used for heavy duty applications where the cost and performance are critical factors. Aluminium is a versatile material and resistant to many environmental influences. Using a form to create a "honeycomb" shape an increase in load capability and rigidity is created. Applications which are weight conscious for example the airline industry who use Aluminium to reduce costs & emissions.

Cast iron

Cost effective and strong, cast iron is an obvious choice for heavy duty wheel centres. It has been used for many years in combination with rubber and polyurethane tyres to satisfy customer demands for resilient & protective solutions. Hub mounted wheel centres are often made from cast iron especially for high volume production such as in fork lift applications.

Steel

Pressed steel - Used as 2 part centres to form around a pneumatic or rubber tyres wheel instead of direct bonding. Offers a very cost effective product for the commodity high volume market. The centres are joined together by welding, riveting or using a nut & bolt.

Machined steel - Used mainly with a cast polyurethane to offer the highest load possible using a polyurethane tyre. Costly in small runs but versatile and strong.

Fabricated

Made by welding a tube section with discs welded to form a supporting structure, then a hub section is welded in to place. Very cost effective for wider wheels, versatile as many options such as an offset hub can easily be incorporated.

Press on band

Steel rings or bands that have polyurethane or rubber minded to them, they are then pressed on to a steel centre using an interference fit. Cost effective to produce as the polyurethane production process benefits form not heating a large steel mass, thus better bonding of larger wheels is achieved. As an additional possibility several bands can be pressed next to each other greatly extending the possible widths of tyres rollers.

Electrical charges are built up when an exchange of positively charged protons and negatively charged electrons are imbalanced and exchanged . This occurs when two materials make contact that have a charge imbalance.

An object that is neutral has a balance of protons and electrons.

A negative or positive charge is generated when two materials have contact and then separate from each other, one of the materials has high resistance to electric current, also described as an insulator.

An exchange of electrons occurs to restore balance. This exchange causes an electrostatic discharge or shock.

If a person or object has an negative charge from friction on a material that knocks off protons then upon touching a material such as metal which has a positive charge, and acts as a path to ground, an electrical discharge or shock is experienced.

Different materials have varying levels of discharge based on their ease of transferring Electrons &/or protons.

Materials such as metals lose electrons easily so have to be considered when protecting against unplanned static discharges.

Materials such as synthetics have better properties and can be used as insulators.

One standard is currently used in the European wheel and castor industry to offer products with consistent dissipation rates.

DIN EN 12527

An Axial load is a force administered along the lines of an axis.

Axial load consideration is important to ensure the correct load capacity is determined and the most suitable wheel type is selected.

If the application is running at an angle across a slope, encounters ramps or thresholds, or is towed around a corner an axial load will exist.

If the coefficient of friction factor between the wheel is too low then skidding may occur causing loss of control.

Solutions to address axial load include selecting a soft tread material, or using flanges to guide and prevent loss of control, for example steel flanged wheels.

A thermoplastic, or thermosoftening plastic, is a plastic polymer material that becomes pliable or moldable at a certain elevated temperature and solidifies upon cooling.

Materials generally used in the wheel and castor industry include PA (polyamide) described often as Nylon and polypropylene. Other tread materials can be injection moulded on to synthetic centres to create a wide variety of product solutions.

Nylon is a generic designation for a family of synthetic polymers, based on aliphatic or semi-aromatic polyamides.

Polypropylene (PP), also known as polypropene, is a thermoplastic polymer used in a wide variety of applications. It is produced via chain-growth polymerization from the monomer propylene.

Thermoplastic elastomers (TPE), sometimes referred to as thermoplastic rubbers, are a class of copolymers or a physical mix of polymers (usually a plastic and a rubber) that consist of materials with both thermoplastic and elastomeric properties. While most elastomers are thermosets, thermoplastics are in contrast relatively easy to use in manufacturing, for example, by injection moulding. Thermoplastic elastomers show advantages typical of both rubbery materials and plastic materials.

Thermoplastic polyurethane (TPU) is any of a class of polyurethane plastics with many properties, including elasticity, transparency, and resistance to oil, grease and abrasion. Technically, they are thermoplastic elastomers consisting of linear segmented block copolymers composed of hard and soft segments.

Leading

The brake lever and mechanism is part of the swivelling body. The brake lever is on the trailing edge of the castor.
If you push a trolley the brake lever will be at your feet.

Trailing

The brake lever and mechanism is part of the swivelling body. The brake lever is on the leading edge of the castor.
If you pull a trolley the brake is at your feet.

Fixed position

The brake mechanism and operating lever are integral to the top plate of the castor, and not affected by the swivelling motion. The brake lever remains in the same position at all times.

Butterfly Brake

This is a wheel only system and is mounted using the axle as a pivot. The brake lever extends out on one side of the forks and has a rocking lever activation.

Hand Operated

The brake mechanism is attached to the swivelling body. The activation is through the use of thumbscrews to apply as much force as the user deems suitable.

Sprung

The axle hole is slotted allowing movement of the wheel up and down. Springs are mounted either outside the frame or internally, the load applied compresses the spring. A bar or similar then compresses the tyre to create a friction brake.

Many wheel types require load derating for example when travelling at higher speeds, continuously, or with excessive axial forces. Tyred wheels can be heavily affected by these factors.

Ensure the application specification does not exceed the manufacturers stated performance parameters for example DIN EN 12532
If these parameters are exceeded then derating will be required to ensure the solution meets the desired specification.

Tyred materials requiring consideration

Rubber

Soft, Elastic, Virgin, Crumb, Thermoplastic, Pneumatic

Polyurethane

Cast; MDI, TDI, NDI, Thermoplastic

Harder Materials

Polypropylene, Nylon, Acetal

Factors for Consideration

• Speed
• Running time
• Rest period
• Location of wheels C/L
• Centre of gravity of load
• Load position central/offset
• Running surfaces
• Running course (details including bends turns starts stops)
• Axial loads (method of movement Manual/towed/driven)
• Drive/Idler mix
• Known scrubbing
• Axle set up
• Humidity
• Temperature
• Chemicals

The derating result will vary depending on the tyre materials, so consultation should be taken in these cases. Hougen are experts in this field and can assist in reaching the correct technical solution.

Polyurethane Tyred Wheels

Polyurethanes are often used in higher speed, continuous and drive applications, due to the nature of polyurethanes derating needs to be considered.
Stated load ratings from manufacturers are generally based on several factors:

• Intermittent use, a rest period equalling or exceeding the dynamic use of the wheel not exceeding 1 hour of use.
• The wheel runs freely and is not driven
• Smooth running surface of either steel or concrete.
• The running surface is free from chemicals which could compromise the integrity of the polyurethane
• The wheel is in line with no steering or axial load applied.
• The temperature does not exceed 40 - 45 degrees Centigrade and does not fall below 20 degrees Centigrade
• Any loads stated should conform to EN 12532 (4 Kph) or 12533 (6 Kph)

Additional factors which influence derating of the stated load are:

For Vulkollan NDI Polyurethane

• Continuous running (dynamic use exceeds 1 hour) Factor: 0.75
• Speed: 6 - 10 Kph Factor: 0.8
• Speed: 10 - 16 Kph Factor 0.7
• Drive wheel: 0.7

When calculating the derating factors are cumulatively multiplied.

For example:
Wheel is running for 1 .5 hours (0.75) at a speed of 12 Kph (0.7) and is a driving wheel (0.7).
Starting with a stated load capacity of 1000 Kg
1000 x 0.75 x 0.7 x 0.7 = 367.5 Kg of allowable maximum load.

This figure of course does not allow for any additional safety factors that may be required.
It becomes clear that if these factors are not taken in to account tyre failure will occur quickly which can incur costs in downtime.

There are other factors which have to be determined on an application basis. These include; speeds exceeding 16 Kph, high humidity, chemical exposure, uneven running surface. deviation from vertical load (Rotating drums), extreme temperature regimes.

Several factors affect the determination of the desired load capacity of a product.

Application details affecting load capacity

Load & position

Total load of the application and how that is distributed is required. This generates the basic load required for the total number of wheels or castors.

Safety factor

This influenced by the running surface, for example is it smooth or full of pot holes which introduce a shock load. If this is the case the wheel specification may be affected.

Slopes pose a particular problem when establishing load capacities. When travelling up or down a slope, the centre of gravity changes and attention needs to be paid to these influences. If crossing a slope then the centre of gravity shifts again putting stress on the castor and/or wheel. Wheels are castors are generally designed for a vertical load.

• Likely chance of overloading, and exceeding the general application designed use.
• Reducing stress and wear on components to extend their lifetime.
• Health and safety factors.
• Industry specific safety demands.
• Castor or wheel configuration.

Generally four positions are used to support a load, however many applications demand a greater number or a varied configuration to suit the requirements of the application. On multiple castor or wheel set ups, a different set of additional factors come in to play. These are based around details of the application and are specific for that requirement.

Directional locks are systems to stop the swivelling action of a castor by using a lock activated by foot or by hand.

This is desirable to enhance manoeuvrability when towing, crossing a slope or other ergonomic requirements.

Directional locks are described by their number of positions. These increments are generally 90 degrees for each position.

1 position - 360 degrees

2 position - 180 degrees

4 position - 90 degrees

Care must be taken to activate the directional lock so the castor is facing the direction of travel. If not set up correctly the directional lock mechanism could fail.

Polyurethane (PUR and PU) is a polymer composed of organic units joined by carbamate (urethane) links.
Polyurethane polymers are traditionally and most commonly formed by reacting a di- or triisocyanate with a polyol.

In the materials handling industry the following polyurethanes are commonly used.

Cast

Cast urethane starts as a liquid that can be dispensed into a mould, post cured in ovens and where required, secondary machining operations can be added.

Types of commonly used cast polyurethanes are:

• TDI - toluene diisocyanate
• MDI - methylene diphenyl diisocyanate
• NDI - naphthalene diisocyanate

Each option has different properties which are desirable for performance.

Temperature range -20 Degrees Centigrade to +60 Degrees Centigrade. Higher temperatures are possible for a limited time and will vary depending on the type of Polyurethane and the application parameters.

Injection moulding

The two parts of the polymer are mixed together, usually by injecting them under high pressure into an impinging mixer. Then the mixture is injected under lower pressure into a mould. The mixture is allowed to sit in the mould long enough for it to expand and cure.

Different materials have varying starting and rolling resistance capabilities.

In the case of standard materials such as nylon these are easily defined. In the case of tyred wheels especially rubber and polyurethane, there are many variabilities between manufacturers and different hardness and elasticity variables that an inclusive statement is not possible.

Rubber is generally used in several forms: Elastic rubber 65 - 67 Shore A typically, Natural virgin rubber 80 Shore A, Thermoplastic/injection moulded rubber 80 Shore .A

There is variability between manufacturers as inclusion of reconstituted crumb rubber and/or bulking agents such as carbon drastically affect the hardness values and elasticity ability of the material.

Polyurethane

There are 3 main types of cast polyurethane: MDI, TDI & NDI. All of these have different properties but share characteristics more closely at lower speeds.

Reducing Shore A values changes the resistance through kinetic energy use through continued motion. A soft polyurethane of around 75 - 80 Shore A will have a lower rolling resistance than a 92-95 Shore A tread. The thickness of the tread has a positive effect, a thinner softer polyurethane tread has the best rolling resistance. The pay off is load capacity and a reduction in protection given by a thinner tyre.

Injection moulded polyurethane is generally more consistent between manufacturers and commonly available options range form around 87 Shore A to 98 Shore A.

For polyurethane treads ranging in hardness from 75 - 95 Shore A values between 1.2% and 3% of the total load can be expected.

Starting resistance changes drastically with harder polyurethanes with low elasticity and rebound resilience properties. If a wheel is left under load for more than hour and less than 8 hours the starting resistance for the wheel only is likely to be around 5% of the total load. If the wheel has been compressed for over 8 hours, then a figure between 5% & 7% of the total load can be experienced.

Nylon

Nylon can be broken down in to two versions: PA6 which is used for general industrial and light load uses. The resistance figure is around 1% of the total load. The second option is PA66 commonly known as Cast Nylon. Due to the compressed nature of this material, a small improvement in rolling resistance can be achieved, at around 0.8% of the total load.

Polypropylene

This material performs similar to Nylon, however as it is softer a small increase in rolling resistance is expected, but as the loads this material is used for are relatively low, this is not effectively noticable

Acetal

This is used only for special applications, the rolling resistance is similar to Nylon PA6

Steel, Stainless Steel, Aluminium, Cast Iron

These materials have very low rolling and starting resistance characteristics due to their very hard surface and minimal tread deformation, figures as low as 0.1% of the total load are realisable.

Tread Profile & Footprint Shape

One of the biggest factors which influences starting and rolling resistance is the shape of the footprint. This is generated by the hardness of the material and shape of the tread profile.

The effect is most pronounced on softer materials. However this cannot be assumed over all materials. For example, softer Polyurethane materials often have better rolling and starting resistance performance due to kinetic energy storage and rebound resilience.

A flat tread creates a small area of material across the length and a large area of material across the width. This provides the lowest rolling and starting resistance in a straight line, but will scrub when fitted in a swivel castor as the castor aligns to the direction of travel.

If the tread is profiled with a radius, this changes the shape and with a pronounced radius produces a large area across the length and a small area across the width, this is ideal to reduce swivel resistance or scrubbing, but increases the starting and rolling resistance.

An optimised tread footprint which is effectively oval can be achieved by using a large radius profile giving a shallow concave profile. The starting and rolling resistance and swivel resistance from scrubbing is affected. This is often used for softer Polyurethane or Rubber tyred products due to the benefits it brings in optimising all three aspects of resistance.

Improving Rolling Resistance

• Increasing the wheel diameter is the single biggest effect on reducing rolling resistance
• Increasing the tread width can have a beneficial effect
• Bearing choice; Needle roller bearings have a higher resistance than precision ball bearings
• Profile shape; a flat profile has the lowest rolling resistance in a linear motion.

Polyurethane

Changing to a softer polyurethane can offer a lower rolling resistance. Changing to premium Polyurethanes can offer reduced resistance.

Starting Resistance

Generally starting resistance figures are higher. These are impacted by a desired acceleration curve when manually handling an application. The steeper the curve, the more force will be required to start an object in motion.

Harder Polyurethane materials can develop a flat spot on the tread when left under load for an extended period of time. The flat spot can increase the starting resistance considerably and needs to be taken in to account. Generally the flat spot will resolve itself after 5 - 10 revolutions of the wheel.

Swivel resistance (Including scrubbing)

Factors that affect swivel resistance.

• Swivel head design
• Forks offset

Factors that affect scrubbing

• Wheel material
• Material hardness
• Wheel dimensions
• Running surface
• Environment including humidity & temperature
• Speed

In most cases general advice can be given with reasonable accuracy. If specific resistance figures are required for manual handling, then independent or self testing procedures may be needed to generate satisfactory data sets when comparing multiple manufacturers.

The wheel footprint is important to minimise damage to a running surface, to ensure good traction with a drive wheel, or to reach a minimum rolling resistance for ease of movement.

Floor surfaces need to be understood when selecting a wheel. A standard concrete floor can take a load of approximately 25 - 30 Nmm2, however a soft surface such as grass or mud will take considerably less and needs to be allowed for.

The interaction between the running surface and wheel material must be allowed for.

Generally softer materials will generate a larger footprint area when compared to a harder material when the load characteristics are the same.

Often a compromise between increased starting and rolling resistance and traction requirements will have to be realised.

For polyurethanes a softer tread with an optimised tread thickness can achieve reduced resistance characteristics when compared to a harder polyurethane when the load and application remain consistent.

Care must be taken to decide on the priority of the desired performance criteria, ensuring the correct technical solution can be derived.