As with everything else in life, the design of a mobile crane is a compromise. The requirements of construction site operation can conflict with road traffic licensing regulations. On the one hand the crane should be as light as possible, while on the other it must support large loads. However, some design requirements are the same for both on and off the highway. On a construction site manoeuvrability and optimum all-round visibility are important criteria for the operator. These require the smallest possible vehicle dimensions which are also desirable for driving the vehicle on highways, as larger cranes may require special permits. Further restrictions are axle load and travel weight limits which may mean the load of the vehicle has to be divided between two or more vehicles.

CT Compact Truck AG, a Swiss company that manufactures in Germany, minimises weight problems by using a single engine and hydrostatic drive. It is not a new concept in drive systems – crawler cranes and tracked excavators have used it for years, as have small wheel-mounted excavators, fork-lift trucks and aircraft transporters. Hydrostatic drives are also used in wheel loaders, harvesting machinery and other agricultural and forestry machinery.

However, these vehicles all have one thing in common – travel speeds are low, to a maximum of about 35km/h. A hydrostatically driven commercial vehicle, that can run at speeds of 60km/h or more, was unheard of until 1993, according to Compact Truck, which claims to have built the first hydrostatic drive mechanism suitable for fast running motorway driving.

Hydrostatics allow freedom of design and open the door to design flexibility. A primary advantage is that the diesel engine or other power source can be placed in any position as no constraints are imposed by fixed mechanical drive trains, for example drive shafts, between the hydraulic pump and hydraulic wheel drive motors. The pump and engine or other power source are connected to the wheel drive motors by hydraulic hoses which can be laid in almost any position.

CT describes its cranes as ‘balanced’ vehicles where weight is more evenly distributed than conventional front-engined cranes. The engine is in the rear section of the superstructure and acts, together with the hydraulic pumps, as a counterweight at the rear part of the slewing structure. The design also prevents the crane from suffering ‘front droop’ which happens on mobile cranes that have the engine and drive mechanism and driver’s cab at the front, in addition to the extra weight of a large proportion of the retracted boom. The centre of gravity on CT cranes is quite central and also low down, as there are no jointed drive shafts that would need vertical space between the drive train and the lower edge of the chassis and thereby create a relatively high centre of gravity.

Considerable weight saving is achieved, largely by the absence of mechanical drive line components. CT managing director Alexander Lutz claims that the components of the hydrostatic drive system are a third of the weight and a third of the dimensions of a conventional system’s gearbox, transfer cases, axles, drive shafts, etc. Much of the weight saved can be put into carrying extra counterweight on board – Lutz claims CT cranes can carry twice as much counterweight as competitors. It can be mounted and demounted hydraulically, without the operator having to leave the cabin.

The comparatively light weight chassis, of minimum height, has “a statically ideal torsion and flectional resistant cross-section”, the company claims. It is not compromised by the intrusion of a mechanical drive line. Despite reduced chassis dimensions, the outrigger base is the same or even larger than that of mechanically driven cranes of the same capacity, the company says. Front outriggers can be placed right at the front of the vehicle as there is no engine in the carrier. Due to the need for torsional rigidity, cranes with a mechanical drive system and a front-mounted driver’s cab need to have the front outriggers behind the first axle as the chassis has to stop short of the front in order to mount the engine underneath the driver’s cab.

For slower speed road travel the front axle is steered by a dual-circuit hydraulic system. For speeds above 62km/h there is a mechanical steering system. Because of this the chassis and superstructure are locked together with a removable bolt. Joining the driver’s cab and chassis in this way allows a sectional steering spindle to be mechanically locked and the system is backed up by hydraulic power.

One cabin

CT cranes have a single cab which functions both as operator’s and driver’s cab, as on other small mobile cranes such as Demag AC 25 and AC 40-1 models. However, at CT this principle is to be continued right through the range, up to the projected 1,000t lifting capacity model CT.8 which is several years away. For crane operation, CT says, this design offers optimum visibility of the operating area while for road travel, the required outward visibility of 12m radius is achieved without problem. Visibility is further improved by the fact that the retracted boom does not overhang the chassis.

The upper section of the cab is removable in case the overall height of the crane needs to be reduced on restricted jobsites. In addition, the driver’s seat can be lowered so that the driver’s head is below the highest point of the vehicle. The crane is operated by two joysticks, and the steering wheel is adjustable, and does not get in the way of crane operation, Lutz says.

Wheel motors with planetary gears

Hydraulic pressure for the travel drive is produced by a variable displacement swash plate pump in a closed circuit. When the pivot angle of the plate that moves the pump pistons is increased, the oil supply rate increases. In the zero position the pump does not supply any oil. The electrically controlled supply output is infinitely variable. Hydraulic motors on each wheel work in the same way but with reversed power flux. As the angle of the swash plate is increased, the flow of oil increases as does the motor speed.

However, this would not be suitable for road travel above 30km/h. So each wheel motor is coupled to a two-stage planetary gear hub mechanism. This combination gives good traction when driving off-road – a climbing ability of 70% – and constant highway driving speeds of 80km/h, all day without overheating, according to Lutz. The maximum speed can be even higher and depends on local road regulations, weight per axle or per wheel, and tyre size – higher speeds are possible with 16.00 tyres than 14.00, for example.

As with the hydrodynamic torque converter in large mobile cranes, the operating principle of the hydrostatic drive gives an automatic alignment of the power ratio between pump and engine to the required output. This allows, relatively simply, automatic load-related driving.

An electronic drive system controls the relationship between driver input, hydraulic system output and the resultant road speed. The microprocessor has an operations set that is fine-tuned for rapid reaction to inputs. Pumps and wheel motors have electrically controlled swash plates for smooth travel in both directions.

When the engine is started, the pump and wheel motors are set in the zero position. If the drive pedal is pressed, the pump plate angle is gradually increased which increases the oil flow and the drive motors rotate faster. Once the pump has reached the maximum pivot angle, the electronic system sets the drive motors to a smaller pivot angle. Interaction between the two is always controlled such that a torque reserve is always available at the wheels for acceleration. In addition, the electronic control system ensures that, regardless of engine speed, the engine’s fuel pump operates for maximum fuel economy and minimum exhaust emission.

This is backed up by a load limit sensing control for the engine, where the microprocessor compares the signal from a potentiometer coupled to the injection pump, thus registering the desired output selected by the driver, with the actual engine speed. If this is too high, the pivot angles of the hydraulic motors are reduced, the pressure in the system is increased, and the speed of the engine is reduced to the desired level.

The first planetary gear step on the wheels is constantly engaged, and the second stage is switched in at about 25km/h. This happens without load as the hydraulic motors are set to the zero position at this point. To avoid total loss of power transmission between the drive and road surface, the axles are consecutively engaged within two seconds of each other.

A braking effect, which is used in addition to wheel and motor brakes, is inherent in the hydraulic system: when the crane is driven downhill, the wheel motors act as pumps and the pump on the engine acts as a hydraulic motor. The two crane pumps (for hoist and derricking gear), which are at zero, are controlled so that the oil flowing through them works back to the engine via the gear mechanism, thereby increasing the braking effect.

Conventional mechanical drive systems need differentials in each driven axle between the wheels for cornering, so that inside and outside wheels can travel at the right speed without tyre scrub. These are not needed with a hydrostatic system as everything is regulated automatically. All the wheel drives are interconnected by the common oil flow. If one of the wheels is in danger of spinning, the electronic system reduces its speed. The crane remains fully manoeuvrable, even off road, and performs better than conventional axles with differential locks, according to CT.

Maintenance costs are reduced by the absence of mechanical drive train components which, Lutz claims, account for 60% of all service costs in a conventionally driven crane. Fatigue and reliability are not a problem either, according to Lutz, who says some of the oldest CT.2 models have now clocked up 90,000km. And, in the interests of the environment, CT cranes use biodegradeable hydraulic oil. Lutz says they were the first cranes to use it and that it does not need larger oil tanks for biodegradeable oil than for ordinary hydraulic oil.

Critcs have commented that the exposed hoses of the hydraulic system on CT cranes look untidy. However, Lutz asserts that this makes access easy for maintenance, and cover plates are available if desired. In addition, CT is now using metal piping instead of flexible hoses where possible.

Suspension and steering

Each wheel has independent hydraulic suspension with a maximum travel of 200mm. Vehicle level and ride height can be selected (higher for rough terrain, lower to get under obstacles) and are automatically adjusted by a level transmitter on the wheels that indicates position. The suspension can be locked for stability on pick and carry duty.

For driving on a gradient, the suspension can be adjusted so that, to an extent, the vehicle remains horizontal. When there is a load suspended on the hook, tilting is kept to a minimum. Tilt indicators acting as sensors for the longitudinal and transverse tilt of the crane, plus the evaluation of their measurements by the electronic system, provide a display showing the driver at which wheel the crane needs to be lowered or raised.

On construction sites the all-wheel steering system allows the driver to choose between four different operating modes:

  • once the mechanical steering has been disengaged the wheels of the front axle can be moved electrohydraulically

  • the front axle can be locked so that only the rear wheels can be steered

  • the front and rear wheels can be steered in the same direction

  • they can also be steered in opposite directions.

As the electrohydraulic steering has no mechanically connected jointed shaft to restrict steering movements, steering angles of more than 45° are possible.

Boom system

Slewing, hoisting and derricking gears are driven and operated conventionally but the boom and telescoping gear has a different system. A conventional, multistage, hydromechanical telescopic system with pulley blocks to retract sections was considered too complicated and prone to technical faults. A modular system was developed which operates from a single oil inlet.

Seven or eight telescopic segments, according to model, are extended from the basic segment. Operation is by a two/three-stage and a five-stage plunger cylinder. Telescopic sections are retracted using a rope winch which is continually kept pre-tensioned by the hydraulic motor. Retraction speed is regulated by a valve on the first telescopic segment which releases hydraulic fluid in precise doses.

This comparatively simple design, with its single hydraulic system, has another advantage in that the telescopic section is self stabilising. As the oil pressure which is dependent on the load remains the same within the entire system at any one time, and as the area it meets increases with each lower telescopic stage, the supporting forces acting on it increase at the same time.

Sliding bearings inside each boom segment give stability. There is no need to use large heavy collars for the transmission of force between each boom segment. This means that retracted CT booms do not overhang the front of the crane and they can telescope under load up to the full boom length and even at an angle below horizontal.