The first part of this article is a summary of a study by Paul Fay, from Liebherr-Werk Ehingen’s crane development department, discussing the suitability if independent suspension for mobile cranes. It was previously published in the German publication Kran Magazin. We then take our own look at Grove’s Megatrak independent suspension system, already in use for many years.
Independent suspension systems have been used in high volume automotive applications for more than 50 years. More recently systems have been developed for heavy goods vehicles and vehicles designed for off road performance in harsh conditions. Vehicles based on a rigid box type chassis are particularly likely to use independent suspension systems.
Independent suspension systems can be defined as follows (and are illustrated in the diagram below) :
• Non pendant (without links or control arms) A1-3
4• Single link (top or bottom mounted) B1-2
• Double link B3
• Semi-trailing/leading arm C1
• Swing or pendular axle C2
• Trailing/leading arm C3
A number of other suspension types can be used, depending on the type of work to be performed by the vehicle. What follows is a discussion of the most commonly considered spring types.
Leaf spring
This has been a very commonly used type in automotive and commercial vehicle applications from the outset. It is rigid and performs well even when subjected to the harshest working conditions. Today, however, it has been replaced with more modern systems, even though it is still used in some special carriers. It is not well suited for AT suspensions as it is difficult to establish a suitable axle blocking system. And as the ability to block axles is of paramount importance, for example when travelling with a load, this system has no real application here.
Torsion spring
This type of spring was never really suitable for goods vehicles, as the difference between empty and fully laden is so great that the spring itself would have to be dimensioned impracticably. And the maximum axle loads of a mobile crane would therefore be impractical to anyone considering a suspension of this type. Again a blocking system would only be feasible through extensive engineering measures, and therefore would not be profitable.
Coil spring
Due to its form the helical coil spring is a favourable design, and is therefore the most common type of suspension for cars. Coil springs are used in the most commonly used suspension design, the McPherson strut type, proven on millions of cars and light commercial vehicles.
This type has been studied numerous times to explore its suitability for mobile cranes. A number of variants for HGVs and mobile cranes were developed, but all of them failed in the tough tests performed by mobile crane manufacturers.
Typically, axle blocking, automatic levelling and axle load distribution were all compromised with mechanical springs.
Pneumatic
Independent pneumatic suspension is in widespread use on HGVs and buses because of its ability to effectively dampen vibration, and particularly on HGVs it is ideal for transporting delicate goods. It is also favoured by many authorities as a road-friendly type of suspension (Riding on air Oct00, p35).
For AT cranes this type is not first choice, however, due to the relatively large volume of the air bellows that is needed to support the weight of the crane. Another drawback is the compressibility of the air, which is unacceptable on an AT. Axle blocking would be expensive to achieve as the air must be let out completely and replaced by a non-compressible liquid.
The great advantage, however, is the ability to raise, lower or level the crane. This can be done accurately and is therefore appealing on vehicles hauling containers and other box-shaped goods. It is not strictly necessary to be able to do this on a mobile crane. Another use of this system is the fitting of an extra transport axle. This has been necessary in many countries where axle loads cannot exceed, for example, 10t. An extra retractable axle for road transport is fitted, using the compressed air system for the brakes, already fitted on most cranes that use a dolly to transport counterweight.
Hydropneumatic
The hydropneumatic system is particularly attractive for use on mobile cranes because of its minimal space requirements. This type, fitted with one or two control arms, for example wishbone type links, is the ideal system to fit to mobile cranes. Hydrostatic load equalising and levelling of the crane is easily achieved using this system.
On all types of hydropneumatic suspension it is important that exposure of the cylinders to lateral forces is as small as possible or non-existent as this could seriously shorten the life of gaskets and pistons. This is ideally met with a double, unequal length link or wishbone design, where the lower link is longer than the upper. The suspension rams are mounted on flexible bearings and are not subjected to lateral forces as the suspension operates. Wear on the ram is therefore extremely low and demands on the gaskets are even less. The same cylinder as is currently used on rigid axle designs can also be used for this type of suspension.
A double link design produces an almost vertical stroke of the wheel, thereby keeping track variation to a minimum throughout the full travel of the suspension. The system has good track stability and to increase lateral stability when cornering the suspension rams can be coupled crosswise hydraulically. On vehicles with a high roll centre the rams can be hydraulically connected diagonally to improve stability and reduce pitching under braking.
Even a design with just one overhead transverse link is good for mobile crane applications and produces little track variation. And this type will give the same good ground clearance as a telescopic leg design that does not have links. It also reduces lateral forces on the cylinder which means less friction and therefore good tracking and smoother steering.
Fixed hydropneumatic without links
In search of cheaper and easier to fit suspensions, a hydropneumatic design without links seemed feasible for a rigid box type chassis, on which fixed suspension cylinders could be fitted throughout the whole vehicle length. In this type of suspension, however, track variation is a problem as the lateral position of the wheel changes through the stroke length of the cylinder. Lateral forces imposed are much larger than those of the double or even single wishbone suspension types. This increases wear on all components. The benefits of having a kit type suspension system are quickly cancelled out by the increased wear in the parts that make up the entire suspension system, as well as in wheels and tyres.
A study showed that of all suspension types examined, this type had the largest track variation – up to 80mm.
There are different types of independent, leg type suspension without links, but they all show the same high degree of frictional resistance in the bearings due to the way they suspend the wheel. This is particularly evident in the vertical movement of the cylinder.
Normally the piston in the cylinder is damped by passing oil through the cylinder. But the piston needs to be protected and covered at all times as normal travel through rough terrain increases the possibility of dirt entering the system which would shorten the life expectancy of the suspension cylinder.
Comparisons
When developing mobile cranes, there are demands for chassis stability, safety, off road ability, simple maintenance and minimal environmental impact. These demands determine the way the mobile crane is built. Before introducing new developments, however, one should always ask whether the new technology has sufficient benefits over existing technology, or whether it is better to stay with the older, proven systems.
To compare the independent suspension one must start out by remembering that, for many years, the old and well known pendular rigid axle with differential has given excellent and reliable service and remains widely used on mobile cranes and commercial vehicles. This type of axle has also undergone developments over the decades that include weight savings and steering improvements.
Comparisons made here are between the latest type of independent suspension without links and the conventional rigid axle design. Both types are hydropneumatic.
Space saving
There is a significant difference in the space requirements of a rigid axle type suspension and independently suspended systems. On and off road ground clearance is different, and gradeability and the minimum clearance height of the machine varies considerably.
Surveys have shown that no significant weight savings can be obtained by using fixed independent suspensions compared with rigid axles. Forces from the wheel in the rigid axle, that have to be absorbed, must by the independently sprung wheel be passed through the connection with the chassis and from there through the chassis itself. This means that extra reinforcement must be added which is not needed with the traditional rigid axle types.
Off road performance
The ability to run a crane on uneven open ground without the need for a prepared surface means cost and time savings on construction of site access roadway. This also applies off site, when driving through rocky or severely uneven terrain to erect, for example, electricity pylons. Important in these situations are ground clearance and suspension travel.
Independently suspended wheels have more ground clearance, particularly when the suspension is raised to the limit of its travel. At this point though, there is no axle equalisation as the rams are fully extended and the crane cannot be levelled. Operating in this condition over uneven terrain results in varying levels of tyre contact and therefore reduced traction.
If the crane is lowered, the ground clearance advantage is lost as the clearance will more or less equal that of the rigid axle. The rigid axle has the same ground clearance at all times.
In summary it could be argued that successfully negotiating rough terrain is more determined by the skills of the driver than the type of suspension.
Fuel consumption
Saving fuel and thereby reducing operating costs and environmental impact are important factors for both the mobile crane buyer and manufacturer. Comparative figures are not yet available to determine any advantages of a particular suspension type.
Comfort and convenience
Driving comfort comes from a suspension system that is well designed, well sprung and well damped. It is important that unsprung weight is kept as low as possible. Problems for the suspension start on an uneven road where the spring forces are no longer enough to overcome any more movement, caused by the weight of the wheel and other unsprung components, to press the tyre against the road. Tyre contact with the road is temporarily lost. Designs with high unsprung weight experience such problems at low travel speeds, and with less unsprung weight, problems only begin at higher speeds.
Tests have shown that at lower speeds, up to about 30km/h, both suspension types give a similarly good driving experience but above that the rigid axle design performs better. The independent suspension could no longer absorb, or follow up, the unevenness in the road. Wheel guidance, due to the friction from lateral forces not taken out by links, causes inertia and a strong damping effect. In addition to reduced driving convenience there is more road noise and a bigger impact on the road surface. Higher driving convenience would be attainable with double link independent suspension because wheel guidance and suspension operate separately.
A double link system, however, is not required on mobile cranes for normal operation and, to date, has not been used. It is questionable whether such travel gear systems are required for mobile cranes.
That is not to say, however, that independent suspension be discounted as unsuitable for mobile cranes because technical progress and development continues. A mobile crane is a tool and so other parameters, such as load capacity and stability, must be considered. Mobile cranes lift loads and sometimes move them over short distances so the suspension system is only a means to an end. Its operating value is not only evaluated by the highest driving convenience. Thus, excessive manufacturing costs for over-engineering are uneconomical.
Conclusion
Every concept is a compromise and each solution has both pros and cons. With today’s technologies, however, it is my opinion that independent suspension does not offer an essential improvement over existing systems. Development of new axle suspension systems is ongoing and we should soon see a new generation but it remains to be seen whether or not these will be independent types. The discussion over rigid axle versus independent suspension will keep design engineers busy into the future.