Rubber matters

Tyres are inconspicuous when the crane they support is operating. After all, the work is being done high above rather than at ground level. But with the largest cranes running more than 20 tyres they can become conspicuous immediately. Consider a puncture; that alone would shut down a crane, leading to an expensive repair, complicated logistics, and possibly embarrassment too - imagine a crane, with the manufacturer's and/or rental company's name prominently displayed, given a police escort on its way to the repair shop.

So it behoves any crane owner or user not to underestimate tyre selection and maintenance. Finding the right tyre - one that will perform most reliably - depends on the crane application. These are divided into three general crane types:

• truck-mounted cranes - based on commercial trucks, truck-mounted cranes use normal truck tyres. More aggressive tyres than ones with normal tread patterns, however, are sometimes required

• all-terrain cranes - using specially designed tyres, all-terrain cranes need good performance at relatively high speeds, in some cases 80km/h or more, and also at slow speed off highway.

• rough-terrain cranes - these need better tyre performance than the other two crane types for difficult conditions such as a poor, uneven or yielding surfaces. Tyre sizes are larger and often of low aspect ratio, and generally run at relatively low operating pressures (around 7 or 8 bar).

A tyre's footprint, or contact patch - the area that contacts the ground - is the key to performance. The larger the footprint, the more contact the tyre makes with the ground and thus the better it performs. A larger footprint also improves flotation. With good flotation the tyre rides on the surface of the ground, for example, soil, reducing slippage and soil compaction as well as increasing traction.

The size of the contact patch is largely determined by how the tyre is constructed. A radial construction is considered best for mobile cranes. The sidewall of a radial tyre deflects and lays more of the tyre on the ground, creating a larger footprint (more area on the ground) and thus the many benefits associated with it.

Radial choice

Tyres are constructed as either cross ply (bias ply) or radial ply. Cross ply tyres, which today are only really seen on off-highway rough terrain cranes, have layers of rubber-coated plies with textile cords, placed on each other at 30° angles. Plies are wrapped around the bead wires, which anchor the tyre to the rim, to form the casing. Additional rubber covers them, forming the tread pattern. Tyre manufacturer Michelin says the only way to strengthen cross-ply tyres is to increase the number of plies and bead wires. More plies means more mass, increasing heat retention and reducing tyre life.

Another disadvantage of bias ply tyres is the one-unit casing. When the sidewalls deflect, the tread squeezes in and distorts. This causes the tyre to scrub or squirm, decreasing traction and operator control, and accelerating tread wear. Since the tread is subjected to any flexing that occurs, the footprint, or contact patch, distorts. This distortion causes abrasion from the ground surface which reduces tyre life and increase fuel consumption.

Radial tyres have two parts. A single layer of rubber-coated steel cables arched from one bead to the other to form the casing. Larger diameter steel cables strengthen a radial tyre and thus lessen the chance of punctures, tears and flats. They also help distribute heat, resulting in a cooler-running tyre and better

fuel economy.

The radial tyre's other part consists of numerous rubber-coated steel belts in the crown, under the tread, forming a strong stabilising unit. Combining stabilising steel belts and a single-layer radial casing allows the tread and sidewall to act independently. The sidewall flexes easily under the vehicle's weight and its cargo, while the tread provides even contact with the ground. Greater vertical deflection is achieved as well, a desirable feature because extreme flexing increases resistance to punctures and helps to isolate the operator and expensive vehicle components from vibration and shock from the road surface.

The stabilising steel belts and radial construction also minimise tread distortion. As the sidewalls deflect, the belts hold the tread firmly on the ground (unlike cross ply tyres), minimising tread scrub and greatly increasing tread life. When negotiating curves and encountering side forces, the independent action of the tread and sidewalls keeps the tread flat on the road which allows the tyre to hold to its path.

Studies of highway use demonstrate the effectiveness of radial construction. Independent road tests have shown radials making lane changes on wet pavement 10% faster; stopping in a 21% shorter distance; running 27% cooler and negotiating curves 15% faster.

The right size

Some cranes, particularly those requiring good off-highway flotation, use 17.5 R 25 and 20.5 R 20.5 tyres. These have a lower aspect ratio than standard tyres and provide a bigger contact patch for the tyre without increasing overall height. On some cranes increased tyre height would be a major problem.

Most European mobile cranes have a standard nominal axle load of 12t. This is often exceeded by carrying extra equipment such as rigging tackle, slings and tool boxes. To carry this load on the smallest tyre option, typically 14.00 R 24 or 14.00 R 25 tubeless, a working pressure of 10 bar is needed for a typical standard crane speed of 70km/h. That is this tyre's limit for load capacity - increasing the pressure at this point cannot raise the load capacity.

Another constraint is the compressor outlet because many struggle to achieve 10 bar, and inflating a large tyre to this pressure takes a long time. It can be tempting to leave the inflation a little low in value, to the detriment of the tyre as well as the machine's performance and reliability. Equipment for filling tyres with nitrogen instead of air can be much faster and provide other benefits (see p51).

If tyre size increases to around 16.00 R 25 tubeless, the required working pressure can safely be reduced to 8 or 9 bar. This lowers the average contact pressure and hence improves mobility and tyre-damage resistance; also, lower pressure is much easier to obtain with the average compressor. Michelin, for instance, designs its tyres to operate at lower pressures - between 14 psi and 43 psi (0.95- 2.9 bar) less than standard pressures - creating the large footprints needed for mobile cranes.

Besides a large footprint, flexibility also contributes to good tyre performance.

A tyre's core strength is its ability to flex (rather than its rigidity and mass) in severe conditions. Because a flexible tyre absorbs shock without internal damage it will sustain less damage when running over sharp objects. When a tyre has a large footprint and is flexible it will operate for up to twice as long as a conventional tyre (due to fewer cuts and punctures). The result is less operating downtime and greater productivity.

New to market

Manufacturers have several new mobile-crane tyres on the market. Goodyear recently introduced the MC-1A radial tyre developed for mobile crane service in high-speed applications of up to 55mph (88km/h). It is available in sizes 385/95 R 24 and 385/95 R 25. Solid shoulder ribs and radial construction improve steering response, ride comfort and fuel economy, the company says, and computer-generated tread design helps minimise tread wear.

Goodyear says that its radial tyre's large rectangular footprint offers even load distribution, better performance in soft underfoot conditions, and less ground-bearing pressure. The aggressive tread pattern has shoulder lugs and angled grooves for traction over all types of soil, sand, rock, hardpack and mud. Goodyear radials also have cut resistant casings which improve fuel economy and allow longer travel between sites, the company claims. A thick nylon casing protects against heat, punctures, and fatigue.

Other Goodyear radials include the GP-2B, RL-2+, RL-2F and AT-2A. Goodyear's GP-2B crane radial has a centre riding rib to improve its on-road characteristics, fuel economy and ride quality. A non-directional tread design is to improve traction and mobility.

The RL-2+ has 25% more tread depth for slower-speed, rough-terrain applications. It has an aggressive lug pattern, 125-level tread depth, and an ultra-wide tread arc width. Also for rough-terrain applications, the RL-2F is the economy radial with directional tread design, an open-tread pattern and open-shoulder grooves.

The AT-2A is for highly mobile cranes capable of highway speeds. It has a non-directional tread design and low rolling resistance.

Michelin developed its XGC radial tyre in the early 1990s specifically for cranes. XGC is replacing the XHC introduced in the 1980s, and the XVC from the 1970s which are being phased out. Also suitable for cranes is the XLB which has an aggressive tread pattern for traction off-road but can also be used on the road. Military cranes use the XLB.

The XGC is available in sizes 14.00 R 24, 14.00 R 25, 16.00 R 25 and 20.5 R 25.

Its non-directional tread pattern reduces noise and enhances handling, resulting in improved operator comfort, Michelin claims. The XGC's aggressive, self-cleaning tread design is to

improve traction on most surfaces, including demanding on- and off- highway conditions.

A special 'C' compound and small block tread design are to allow the XGC to travel at high speeds and remain durable. The tyre's strength helps increase resistance to impacts and cuts, resulting in fewer punctures, and the claimed long-lasting, even-wearing tread pattern and low rolling resistance promotes better fuel economy.

According to Michelin, the XGC is versatile enough to be used in nearly any crane application, while maintaining its durability, strength and operator comfort. The tyre's capacity to travel at higher speeds is an added bonus, providing even more productivity in the field.

Bridgestone's VHS V-steel E2 tyre is for wheeled mobile cranes. It has a heat-resistant compound and casing for high-speed durability, smooth wearing, low-noise tread pattern and, according to the manufacturer, has excellent flotation.

Japanese tyre producer Yokohama offers the RB01 tyre for all-terrain and other wheeled cranes. Its new tread pattern and a new rubber compound are designed to give long tread life. It also has a tough casing construction that lowers fuel consumption and tyre noise, and improves driving stability and high-speed durability, Yokohama says. Four sizes are offered: 385/95 R25, 445/95 R25, 505/95 R25 and 14.00 R24.