For the heaviest applications (above 500kg/wheel capacity) the usual wheel construction is to mould a polyurethane tyre around a cast-iron centre. This gives a fairly hard tread (approx 95 shore A), which is still resilient (that is, the material returns to its original shape after deformation), unlike rubber (where hard rubbers are not resilient). Unfortunately, some cheaper brands of wheels are troubled by the tyre coming off the centre, which can cause the wheel to jam, or allow the cast iron to damage the floors. Why are the better wheels more expensive?
1. The polyurethane must be correct for the use
Polyurethane is not just one material, but a family of them. It is made from a primary backbone material (the polyol), which is cross-linked by another chemical (the isocyanate). Many different chemicals are suitable as polyol and a number as isocyanate. The various combinations are all polyurethanes, but they have different properties and different costs. Unfortunately, as with plastics generally, the tougher materials of higher strength also cost more. Chemicals used for low strength polyurethanes only cost about 1/3 of those for high strength material, but the low strength polyurethanes become unusable in about 3 months, while the high strength materials often last in excess of 10 years.
It is impossible to tell strong materials from low strength materials by any non-destructive tests, yet once in use, it is easy to tell. If roughly used, pieces from the size of a 5 cent coin to the size of a 50 cent coin chunk out low strength
tyres. If the load on the wheel is increased the low strength polyurethane suffers permanent deformation, and, as the tyre stretches, it breaks the bonding on to the centre, and the tyre falls off. Frequently a whole section of the tyre (maybe 1/3 of the circumference) will break off. If you have such a large section available to you, bend it back on itself – if it is low strength material it will only come back to its natural cast shape slowly (maybe 10 minutes, or not at all), while higher strength materials come back inside 10 seconds, or even ‘snap back’ with the best materials.
The colour of the material tells you little about the quality, In its natural state polyurethane varies from a creamy colour to a high fawn, dependent on grade, It is common industry practice to add a coloured die to give it a distinctive colour. Fallshaw used yellow, other manufacturers red, or brown, or other colours. (Some manufacturers use different colours for different hardnesses). The only colour that may be instructive is that if a tyre has developed a light brown tinge it may have encountered high heat either by use in an oven, or by fast running. And this may indicate misuse. Usually low strength materials fail very early under such conditions (sometimes even before the brown tinge develops).
The preferred polyols for these wheels are ether based, which generally have better low temperature flexibility and higher resilience (bounce) and better oil and heat resistance. Most of the cheaper types are esters, which lack these benefits.
Remember, you cannot tell by simply looking at the material, how good it is. You depend on the manufacturer to specify proper high quality materials, and you only get these for a reasonable price.
2. The bonding must be correct
The tyre is applied by the ‘pour-cast’ technique. The polyol and isocyanate are added together, and agitated, and then poured into a mould around the centre. The chemicals react within the mould, and the tyred wheel can be removed about 20 minutes later. But, to be sure the tyre remains attached to the centre:
– The bonding face needs to be without irregularities or casting flaws.
– The face needs to be totally clean of any oil, or mould release.
– The face needs to be prepared with a bonding agent that will grip to the cast iron, and to the polyurethane about to be poured on it.
If the bonding is not done properly, the tyre will come off.
If the bonding is well done, a tyre, which is forced off the centre, will often break through the polyurethane, rather than at the bond. If through misuse a tyre is off the rim, you should see considerable areas of polyurethane still adhering to the rim.
If the bonding has not been good enough, usually the tyre separates from the centre without leaving any polyurethane behind. Poorly bonded tyres often separate very early in use, (such that no wear shows on the outer side), and the inside has quite a slippery feel about it.
Most competent companies use a bonding agent with a coloured pigment in it (Fallshaw uses blue) to ensure the bonding agent fully covers the bonding face without missing any part. You will often see traces of it at the bond line. You cannot tell how carefully the bond has been made simply by looking at it. You depend on the manufacturer and their staff to carefully and thoroughly do all the right steps if you are to get a good bond.